The Role of Electrons in Sputter Deposition of Thin Films · The Role of Electrons in Sputter...
Transcript of The Role of Electrons in Sputter Deposition of Thin Films · The Role of Electrons in Sputter...
The Role of Electrons
in Sputter Deposition
of Thin Films by Frank Arthur Green
A thesis submitted for the degree of Ph.D at the University of London, and also for the Diploma of Imperial College of Science and Technology.
Department of Electrical Engineering [Science of Materials]
Imperial College May 1979
ACKNOWLEDGEMENTS
My thanks are due to Professor J. C. Anderson for his
supervision of the latter part of the thesis and for the
facilities provided at the Materials Section, Imperial College.
Thanks are also due to Dr. B. N. Chapman for his supervision of
the experimental work and to the Science Research Council for
the provision of a grant.
I am also indebted to Judy, my wife, and to my parents for
the help and encouragement they have given in numerous ways.
Finally, I would like to thank Mrs. Alyson Phillips for
her careful typing of the manuscript and also my friends and
colleagues at the Materials Section, Imperial College, Dover
Grammar School for Boys and Folkestone Grammar School for Girls
for their helpful assistance.
ABSTRACT "The Role of Electrons in Sputter Deposition of Thin Films."
F. A. Green.
Energy and power measurements of the electrons bombarding
the anode in a D.C. planar sputtering system show that a small
but significant number travel in straight lines to the anode
without collision and are responsible for a large percentage
of the power input.
A magnetic field was used in a cylindrical geometry to
alter the electron trajectories and improve plasma efficiency,
measurements being taken to show the distribution of the input
power in this new system. The contribution to the substrate
power input from neutral species was seen to be more important
in this high deposition rate system. Radiation from an uncooled
target was also an important energy source. Modified magnetron
geometries were also investigated as was the anticipated film
thickness uniformity problem resulting from this cylindrical
arrangement.
Over a range of sputtering conditions in the cylindrical
magnetron system, the current-pressure characteristics show an
unexpected discontinuity and current-magnetic field curves ex-
hibit a maximum current. These phenomena seem to depend on the
ease with which the plasma can sustain itself and are related
to the relationship between the maximum distance of the secon-
dary electron initial trajectory from the target surface, and
the target dark space thickness.
The problem of plasma sustainment was investigated by making
theoretical calculations of the main region of electron impact
ionisation together with the predicted electron energy spectrum
in the negative glow. This spectrum was compared to the experi-
mental analysis results.
2 TABLE OF CONTENTS
Title Page .. .. .. .. .. .. .. ..
Acknowledgements .. .. .. .. .. ..
Abstract .. .. .. .. .. .. ..
Contents .. .. .. .. .. .. ..
Chapter One - Introduction to Electron Effects
..
..
..
1
2
7
1.1 Sputtering in General .. .. .. 7
1.2 The Basic Process .. .. .. Of 00 9
1.3 Sustaining the Discharge .. .. .. 12
1.4 Collisional Probability .. .. .. 14
1.5 Efficiency Aspects .. .. .. 15
1.6 Substrate Bombardment .. .. .. 15
1.7 Heating Effects .. .. .. 17
1.8 Nucleation Effects .. .. .. .. 20
1.9 Guidelines of the Work .. .. .. .. 20
Chapter Two - Energy Analysis .. .. .. .. .. 22
2.1 General Introduction .. .. .. .. .. 22
2.1.1 Analysis in the Plasma Region .. .. .. 22
2.1.2 Analysis at the Substrate .. .. .. .. 22
2.2 Energy Analysis .. .. .. .. .. .. 24
2.3 Theory of the Spherical Condenser .. .. 25 2.3.1 Focusing .. .. .. .. .. 00 .. 25 2.3.2 Resolution of the Analyser .. .. .. .. 29 2.4 Experimental System .. .. .. .. .. 29 2.4.1 Introduction .. .. .. .. .. .. .. 29 2.4.2 Vacuum Considerations .. .. .. .. .. 30
2.4.3 Sputtering Chamber .. .. .. .. .. 31
2.4.4 Target Assembly .. .. .. .. .. Of 31
2.4.5 Particle Extractor .. .. .. .. .. 33 2.4.6 Energy Analyser .. .. 00 .. 00 . . 33 2.5 Calibration of the Energy Analyser .. .. 34 2.6 Accuracy of Results .. .. .. .. .. 35 2.7 Experimental Results .. .. .. .. .. 37
3
2.7.1 General Curve Shape .. .. .. .. .. 37 2.7.2 Constant Voltage with Varying Pressure .. .. 37
2.7.3 Constant Pressure, Variable Voltage .. .. 39 2.7.4 Variation of Target/Substrate Separation and
Sampling Aperture .. .. •• •• .. .. 41
2.8 Analysis of Results .. .. .. .. .. 42
2.9 Analyser Acceptance Bandwidth .. .. .. 43 2.10 Heat Input to the Substrate .. .. .. .. 45 2.11 Conclusions .. .. .. .. .. .. .. 46 2.11.1 General Comments .. .. .. .. .. .. 46 2.11.2 Summary of Results .. .. .. .. .. .. 46
Chapter Three - Higher Rate Sputtering .. .. .. 49 3.1 Introduction .. .. .. .. .. .. .. 49 3.2 Factors Influencing Deposition Rate .. 51 3.2.1 Target/Substrate Separation .. .. .. .. 51 3.2.2 The Temperature of Substrate and Target .. 51
3.2.3 Pressure .. .. .. .. .. .. .. .. 52 3.2.4 Voltage Between Target and Substrate .. .. 55 3.2.5 The Current Drawn by the Discharge .. .. 57
3.3 The Concept of Efficient Sputtering .. .. 58
3.4 Plasma Efficiency .. .. .. .. .. .. 61
3.4.1 Triode Systems .. .. .. .. .. .. 62 3.4.2 The Use of an External Magnetic Field .. .. 63 3.4.3 The Penning Cell .. .. .. .. .. .. 65 3.4.4 The Cylindrical Magnetron .. .. .. .. 66 3.5 'Novel' Magnetron Systems .. .. .. .. 67
3.5.1 The Sputtergun .. .. .. .. .. .. 67
3.5.2 The Planar Magnetron .. .. .. .. .. 68 3.6 Experimental Systems .. .. .. .. .. 70 3.6.1 Conventional Cylindrical Magnetron .. .. 70 3.6.2 A 'Modified' Cylinder - Introduction .. .. 72 3.6.3 A 'Modified Cylinder' - Construction Details 74 3.7 Deposition Rates .. .. .. .. .. .. 76
4
Chapter Four - Film Uniformity .. .. .. .. .. 77 4.1 The Factors Affecting Thin Film Uniformity .. 77 4.1.1 Introduction .. .. .. .. .. .. .. 77 4.1.2 Solid Angle .. .. .. .. .. .. .. 77 4.1.3 The Mean Free Path of Sputtered Species .. 78 4.1.4 The Distribution of Sputtered Material .. .. 79 4.1.5 Condensation Coefficient .. .. .. .. 80 4.2 Experimental Investigation for a Cylinder .. 81 4.2.1 Measuring Technique .. .. .. .. .. 82 4.2.2 Experimental Results .. .. .. .. .. 84 4.3 Improving Film Uniformity .. .. .. .. 85 4.4 Split Target Experimental System .. .. .. 86 4.4.1 General System .. .. .. .. .. .. 86 4.4.2 Five Bar System .. .. .. .. .. .. 88 4.4.3 Experimental Results .. .. .. .. .. 90 4.5 Simplified Theory for the Thickness
Distribution .. .. .. .. .. .. .. 92 4.5.1 Extension to the Sectioned Bar .. .. .. 95 4.6 Conclusions .. .. .. .. .. .. .. 98
Chapter Five - Electrons in a Magnetically Supported Plasma .. .. .. .. .. .. .. .. .. .. 99 5.1 Introduction .. .. .. .. .. .. .. 99 5.2 The Role of the Magnetic Field .. .. .. 102 5.2.1 Electron Trajectory .. .. .. .. .. 103 5.2.2 Target/Substrate Potential Distribution .. 104 5.3 Region of Ion Production .. .. .. .. 108 5.3.1 Secondary Electron Coefficient .. .. .. 110 5.3.2 Electron Energy Distribution at the CDS/NG
Boundary .. .. .. .. .. .. .. .. 111
5.3.3 Analysis Conclusions .. .. .. .. .. 114 5.3.4 Energy Distribution Assuming Constancy of Qi 116
5.3.5 Analysis Conclusions .. .. .. .. .. 120 5.3.6 Flux Multiplication Model .. .. .. .. 121 5.3.7 Conclusions on Ion Production .. .. .. 123 5.4 The Pressure/Current Characteristic .. .. 123 5.5 Magnetic Field Variations .. .. .. .. 124
.. 129 5.6 General Conclusions .. .. .. ..
5
Chapter Six - System Performance .. .. .. .. 131
6.1 Optimum Operating Parameters .. .. .. .. 131
6.1.1 Operating Voltage .. .. .. .. .. .. 131
6.1.2 Operating Current .. .... .. .. .. 132
6.1.3 Optimum Condition Conclusions .. .. .. 136 6.2 General Characteristics .. .. .. .. .. 136 6.2.1 Current/Voltage Relationship .. .. .. .. 137 6.2.2 Magnetic Field Variation .. .. .. .. 138 6.3 Substrate Heating .. .. .. .. .. .. 140 6.3.1 Introduction .. .. .. .. .. .. .. 140 6.4 Distribution of the Input Power .. .. .. 141 6.4.1 Power to the Substrate .. .. .. .. .. 141 6.4.2 Distribution of Power to the Substrate .. .. 143 6.4.3 Power to the Target .. .. .. .. .. 145 6.5 Experimental Results .. .. .. .. .. 146 6.5.1 Deposition Rate .. .. .. .. .. .. 146 6.5.2 Variation of Substrate Input Power .. .. 147 6.5.3 Equilibrium Temperature Te .. .. .. .. 152
6.5.4 Magnetron Compared to Non Magnetron .. .. 153 6.6 Distribution of Power to the Substrate (con) 154 6.7 Conclusions .. .. .. .. .. .. .. 157
6.7.1 General Comments .. .. .. .. .. .. 157 6.7.2 Factors Necessary for an Efficient High Rate
System, .. .. .. .. .. .. .. .. 158
Chapter Seven - Conclusions .. .. .. .. .. 160
7.1 Introduction .. .. .. .. .. .. .. 160 7.2 Review of the Major Conclusions .. .. .. 161
7.2.1 Energy Analysis .. .. .. .. .. .. 161 7.2.2 Processes at the Substrate .. .. .. .. 164 7.2.3 Switching Phenomena .. .. .. .. .. 165 7.3 An H Configuration .. .. .. .. .. .. 168 7.3.1 Experimental Results .. •• .. .. .. 169 7.3.2 Switching With Smaller End Plates .. .. .. 171 7.3.3 Concluding Remarks on the H Configuration .. 174 7.4 Conclusions on Switching .. .. .. 174 7.5 The Role of Photons .. .. .. .. .. 177 7.6 Concluding Remarks .. .. .. .. .. .. 178
E
References for Chapter One .. .. .. .. .. .. 179 References for Chapter Two .. .. .. .. .. .. 181 References for Chapter Three .. .. .. .. .. 182 ,References for Chapter Four .. .. .. .. .. 184 References for Chapter Five .. .. .. .. .. 185 References for Chapter Six .. .. .. .. .. .. 186 References for Chapter Seven .. .. .. .. .. 187
Appendix I .. .. .. .. OS .. .. .. . • 188 Appendix II .. .. .. .. .. .. .. .. .. 189
CHAPTER ONE
INTRODUCTION TO ELECTRON EFFECTS 5
1.1 s U'''TERI::G IN GE'.'ERAL
The name sputtering is given to the process whereby atoms
or molecules of a material are ejected from its surface due to
bombardment of that surface by positive ions. Sputtering was
first observed in early incandescent laps which darkened as
the carbon filament material was deposited on the glass envelope.
In this case the necessary ions arose from the residual air left .
in the imperfect vacuum which was achieved in the lamps at that
time. The resultant sputtering was obviously not considered to
be an advantage.
Whilst the earliest reports on the process occur in the mid
nineteenth century(1), more detailed work was produced later by
Fruth(2) and Von Hippel(3)(4) who proposed that sputtering took
place by virtue of momentum transfer between the bombarding ion
and the atoms or molecules of the target material. The general
literature on sputtering has grown immensely in recent years and
has been reviewed in depth by many authors includin 7ehner(5),
Kay(6), Maissel(7), We P and Anderson(8)
, Jackson(9) and
Holland and Priestland ~I. Perhaps the main reason for the
growth in the interest in the process has been its exceptional
versatility as a• deposition technique. This can be illustrated
by looking at some of the advantages of the basic sputtering
technology.
(a) Versatility
It is possible to sputter virtually any material onto any
surface. Whilst sputtering may not be the most convenient sol-
ution to a particular problem, there are times' when it is the
only possibility. High melting point metals such as tungsten
are difficult to evaporate whilst using the sputtering process,
tungsten deposition presents no special problems.
(b) Adhesion
Good adhesion between the film and the substrate can usually
be achieved in sputtering. This is normally attributed to the
relatively high arrival energies of the sputtered species.
(c) Compound Films .
Stoichiometry is preserved with compound targets providing
sputtering takes place in molecular and not atomic form. When
8
ejection takes place in the form of atoms, stoichiometry can
still be achieved by the use of target cooling to inhibit solid
state diffusion in the target material.
Reactive sputtering can also be used for the production of
compound films. In this technique the inert gas commonly used
is either partially or wholly replaced by a gas such as 02, N2,
or H2S. Now in addition to momentum transfer sputtering from
the target, the possibility of chemical reaction exists either
at the target or in the gas phase or at the substrate.
(d) Thickness Uniformity
The angular distribution of the ejected atoms from each
point of the source follows the familiar cosine law for both
• thermal evaporation and sputtering sources. However, the larger
source area available with a sputtering target provides a much
larger area over which uniform distribution is possible.
(e) Deposition Rate
The relatively slow deposition rates obtained in sputtering
mean that film thickness control can be achieved to a high degree
of accuracy. In applications where the low rate is a disadvan-
tage, such as in the production of thick coatings, the rate can
be increased by using an improved configuration such as a
magnetron device which. is discussed in more'detail in chapter
three.
(f) Substrate Cleaning,
A-sputter etching stage can easily be incorporated into
production apparatus. This consists of an electrically insulated
substrate table to which the negative sputtering potential can be
applied thereby making the substrate the target for the ions.
This feature can be of great value for cleaning substrates,before
deposition starts,to improve adhesion.
Another application of this facility is in the etching of a
pattern on the substrate through a substrate mask. Sputter
etching does not produce the undercutting effects usually
exhibited by chemical etching processes.
(g) Co-Suutterino
Several targets of identical or different materials can be •
employed simultaneously to produce films of varying•composition.
(h) Sequential Snutterin
Targets can be utilized sequentially to produce a layered
9
coating such as in a thin film transistor. It should be admitted
here.that radiation damage prineipaily from x rays produced by
high energy elections impacting onto metal surfaces can limit the
value of the process as a means of producing semiconducting
devices.
(i) Emitaxv
There is often a critical temperature above which epitaxial
film growth is possible. This temperature is usually fairly low
for sputtered films. Unfortunately this potential advantage is
nullified by the fact that sputtered semiconductors have very low
mobilities because of damage to the crystal caused by the high
arrival energies of the sputtered species. Sputtered epitaxial
films are not usually thought to be advantageous.
Sputtering in general will not be reviewed in any detail in
this work which is directed at the properties of electrons as
related to the sputtering process and the effects which they can
cause. To this end, the basic sputtering process will be briefly
discussed in the simplest D.C. case with emphasis being placed on
the electron properties.
1.2 THE BASIC PROCESS
When a beam of energetic particles is incident on a target,
material is ejected or sputtered from the target surface in a
momentum transfer process. Since for a given energy E, particle
momentum p is given by p = (2mE) where m is the mass, it might
be thought that heavy ions are most suitable for the promotion of
the sputtering process, but closer inspection reveals that maxi-
mum momentum transfer occurs when the target atoms and sputtering
ions are of comparable mass. Electrons are usually thought to.be
totally inadequate for sputtering purposes because of low mass
although work by Townsend(11) suggests that high energy electrons
can cause sputtering. However, the process is very inefficient
and can normally be safely ignored.
The necessary high energy can easily be imparted to gaseous
ions by accelerating them by means of an electric field. The
sputtering ions can be produced externally using.an ion gun but
it is more convenient to produce them internally by creating a
glow discharge. The requirement for gaseous ions limits the
choice of sputtering environment and argon is often.chosen
10
because it is chemically inert, is a relatively massive ion and
the pure gas is readily obtainable.' Glow. discharge phenomena
have been reviewed by 'baissel(7) and Cobine(12) and will only be
discussed briefly here.
When a high electric potential is applied between two flat
parallel electrodes in a low pressure system (typically 1 to 100
millitorr) a current will flow and there will be a gaseous dis-
charge between the electrodes. Positive argon ions (Ar+) created
in the breakdown are attracted towards the negative electrode
(target) and strike it causing sputtering of the target material.
When breakdown first occurs, a normal glow discharge is initially
established in which the current density at the electrode remains
constant. For a small voltage increase, the current increases at
constant current density as the discharge spreads .to use the
whole of the electrode surface. Once all of the surface is in
use, larger voltages'are needed to increase the current flow and
the discharge is said to be in the abnormal glow condition. This
situation is normally applicable to sputtering applications since
it ensures that all of the target surface is being used. Also,
the abnormal glow is the only situation in which the current den-
sity is controllable and it is only in this region that current
densities are high enough for atoms to be sputtered out of the
target at useful rates.
11
Fig. 1.1 Soati•ai Distribution of Dark and luminous Zones, Electric
Field X, Potential V, and Scace Charge Der sitiec iO+/O in a Glow
Discharge.
1 ASTON D4RK SPACE
TARGET; 2 CATHODE LAYER
/4/ 3
4 3CROOKES DARK SPACE
4NEGATIVE GLOW
1 2
t
I iD DISTANCE FROM I TARGET
The transport of current through the discharge is due to the
axial motion of electrons and positive ions. An electron is
emitted from the cathode by virtue of a positive ion impinging on:
it and gains energy as it is accelerated away by the strong field
12
near the cathode. Since an electron will leave the cathode with
an energy of about one electron volt, initially no excitation of
gas molecules can occur and there is a'dark region (the Aston
dark space) next to the cathode. Further from the cathode,.
electron energies are as high as the excitation potential of the
gas (about 15ev) and there is a luminous layer. Beyond this
area, electron energies are far above the Maximum excitation
potential of the gas and a relatively dark region, the Crooke's
dark space (CDS) results. This region is of funda.r:ental impor-
tance to the sputtering process and elementary texts would
suggest that most of the ions striking the target come from the
CDS. A more detailed examination later in this work does,
however, cast doubt on the assumption.
The distribution of the electric field (Fig. 1.1) is such
that most of the inter-electrode potential is dropped across the
CDS. Beyond the CDS here is a negative glow (NG) region which
contains electrons from two groups:-
(1) Fast electrons produced at or near the cathode which
have not suffered any energy loss due to inelastic collision in
the CDS and therefore have the full fall of potential at the NG
region:
(2) A larger group of slow electrons created in the discharge
which have made many inelastic collisions. Since these slow elec-
trons cannot gain energy rapidly in the weak electric field, they
tend to have energies below the gas ionisation potential but above
the excitation potential and they, therefore, produce many
exciting collisions and a negative glow.
Whilst other light and dark regions can exist in a plasma, they are usually absent under sputtering conditions where the sub-
strates are normally positioned in the NG.
1.3 SUSTAINING THE DISCHARGE
There are several processes which can occur when an ion is
incident on a target in the glow discharge situation.
(a) The ion can implant itself into the target material.
(b) It can be reflected either as an ion or as a neutral
species.
(c) Ejection of the target material can occur either in
neutral or ionic form.
13
(d) Electrons are produced and being negatively charged they
tend to be reseller. from the target area which is also at a nega-
tive potential. It is these secondary electrons which are
responsible for sustaining the discharge since they are able to
create more ions by co?l_Lsion. Since this electron impact o -
ionisation creates another electron (Ar + e--- Ar+ + 2e-) we
have a breeding mechanism by which a small 'number of electrons
leaving the target can multiply by multiple collision to a large
number by the time the GDS/NG boundary is reached..
A criterion for the discharge to be sustained is that each
ion incident on the target must produce sufficient electrons so
that they in turn will produce another ion which is incident on
the target. It should be realised that because of collisions,
any ion created does not have unit probability of reaching the
target.
An electron/gas atom collision need not result in ionisation
and the formation of an ion/electron pair; other processes occur,
notably:-
(a) An elastic collision in which the electron 'bounces' off
• the gas atom with a direction change but no kinetic energy loss
• and no ion production.
• (b) An excitation type collision in which the gas atom is
raised to an excited state and then decays back to the ground
state emitting light at a frequency characteristic of the gas.
This process is responsible for the various glow regions of a gaseous discharge. The electron will typically change direction
and lose kinetic energy in a collision of this type and will
migrate to the walls of the containing vacuum vessel thus being
•lost to the plasma. This represents a serious loss mechanism
since an electron lost to the plasma cannot contribute to plasma
efficiency by causing ionising collisions.
A further important point is that there is a higher probab-
ility of electron/ion recombination on the walls of the containing
vessel. Whilst this process is unlikely to happen in the plasma
area due to the requirement for energy and momentum to be con-
served in the two body problem; the conservation is much easier
to satisfy in the corresponding three body problem which is
relevant when the wall enters into the calculations. This is
14
because the wall allows for the dissipation of kinetic energy in
the form of heat. -
Hence, the wall acts as a trap for the charge carriers'which
could participate in additional useful collisions. In general
the walls ought to be kept as far away as possible from the tar-
get area and attempts have been made to keep the electrons away
from the wall area by using magnetic fields. This will be looked
at in detail in a later chapter.
1.4 COLLISIO_: 4L PROIt3ILITY
So far, no consideration, has been given to the probability
that an electron emitted from the target will strike a gas atom
causing ionisation or excitation. The thickness of the CDS is
often thought of as being approximately the mean distance travel-
led by an electron before it makes an ionising collision. Whilst
this concept is often useful, much of the later work in this
thesis casts doubts of the validity of the assumption. Certainly
the CDS is a fundamental plasma parameter and defines a lower
limit on the pressure which can be used. If the anode (substrate
in the simple D.C. case) is placed inside the CDS, the discharge
will be extinguished because sufficient ionising collisions will
not take place to maintain the discharge. Whilst a discharge can
be maintained with the substrate just outside the CDS, it is
found that ion production is affected unless the target/substrate
separation is at least two CDS lengths suggesting that the NG has
some part to play in ion production and discharge sustairment,
The CDS length should not be confused with the electron mean
free path which is about ten times smaller since it also includes
the probability of excitation and elastic collision as well as
ionisation.
As the gas pressure is reduced the CDS lengthens since col-
lisions of any sort including ionisation become less likely. The
target/substrate separation has, therefore, to be increased to
prevent the discharge being extinguished. In a practical simple
D.C. sputtering system where the aim is an .acceptable deposition
rate, a low pressure limit is imposed by the fact that the elec-
trode separation becomes unrealistic. In the theoretical case
(i.e. a plasma but not suitable for useful sputtering) a low
pressure limit is set by the fact that'the discharge cannot be
15
sustained when the necessary electrode separation is so large
that the consequentially.weak electric field results in electron
loss being excessive. This lowest possible pressure is about 15
microns in the simple D.C. case depending on geometry.
Results presented by Chapman et al( 13) and Ball ( 1) indi-
cate that some electrons strike the substrate with an energy
corresponding to the full fall of cathode potential suggesting
that they have undergone no collision of any type in traversing
the plasma. This is, perhaps, surprising at first sight in view .
of the expected collision cross sections as presented by Massey
and Burhop(15).
Whilst these fast electrons are contributing nothing to the
creation of further ions by collisional processes, they are also
of interest in terms of the substrate heating and damage to the
deposited film which can be caused and this will be reviewed'
later.
1.5 EFFICIENCY ASPECTS
Previous sections have concentrated on how electrons are
produced in the sputtering process, how they interact with gas
atoms causing the discharge to be self sustaining and how they
are lost to the anode and containing walls. One of the major
' disadvantages of the sputtering process is its slow deposition
rate but this can be improved either by creating more electrons'
or by making those present more efficient at causing ionisation.
The two main methods are to support the discharge either mag-
netically by attempting to confine the electrons to the plasma region using a magnetic field or thermionically by injecting an extra supply of thermal electrons. Both of these methods will be
reviewed in chapter three but it is worth mentioning here that it
is the modification of the role of the electron which is respon-
sible for the high deposition rate devices which are now being
developed. •
1.6 SUBSTRATE BO:.:BARD' TENT
The role of the contairing.wall as a loss mechanism has
already been discussed and it is obvious that the substrate to be
coated must also be bombarded by charged species as well as
neutral material. Furthermore, since the substrate is typically
held at anode potential it is particularly susceptible to bom-
16 bardment by electrons. Since an ion current is striking the
cathode, it follows from charge continuity considerations that an
electron current must be drawn from the, anode.
The physical properties of sputtered films are dependent on
the conditions under which the film was prepared. Each particle
which is eventually incorporated into a film must have been
previously adsorbed at the surface and it is, therefore, advan-
tageous to know the charge and energy of the flux of particles
incident on the substrate during deposition since this might help
explain the film properties.
Not all of the material arriving at the substrate is incor-
porated into the film since there is a possibility of
re-sputtering leading to a sticking probability of less than
unity. Thermal desorption of material is also possible. In the
case of a compound target, these processes could lead to the
production of a non-stoichiometric film since the constituent
atoms might exhibit different sticking characteristics on
reaching the substrate. Without information about the incident
particle flux to the substrate, it is not possible to say which
process, if any, is significant.
The energy and mass spectra of positive ions incident on the
substrate in a sputtering system have been investigated by Coburn •
and Kay(16-18). They have been able to demonstrate that the •
Penning Ionisation mechanism is the most important method of ion
production near to the substrate as opposed to electron impact
ionisation which is most significant in the general plasma region.
It should be noted that these ions produced close to the substrate
are unlikely to reach the target because of the relatively large
distance involved. In the Penning mechanism, a metastable gas
(argon) atom produced by collisions itself collides with a neutral
atom of the target material. If the ionisation potential of the
atom is less than the energy of the metastable, then the latter
can de-excite to the ground state by ionising the neutral species
according to a general formula:-
Arm + + +Ar+ e
Work by Gilkinson et al (19) suggests that the following
mechanisms could also take place though the probability cross
sections are expected to be very low.
17
Arm + Ar Ar,+ f- e-
Ar+ + 2Ar ----~ Ar2+ + Ar
Ar+ + X --a Ar + X+
Ar2+ + X 2Ar + X+
Since the plasma potential can be a few volts positive with
respect to the most positive electrode (i..e. anode) the Penning
process provides a mechanism for bombarding the substrate with
positive ions. Coburn and Kay(20)
have pointed out that re-
sputtering would be negligible at the energies liely to be
involved though this might not be the case in bias sputtering
applications. It is difficult to say whether the Penning mech-
nism could represent a significant electron source but this seems
unlikely.
Problems in pumping a vacuum chamber for sputtering appli-
cations have been reviewed by Lamont(21)
and Ennos(22) has
indicated that contamination can be produced by the interaction
of an electron beam with organic molecules which are always
present(23). Whilst this problem will not be discussed in this
work it provides yet another example of the part played by
electrons in the sputtering process.
The effect of electron bombardment on the substrate can,
perhaps, best be considered in terms of the heating effects and.
possible changes in growth mechanism which can be caused.
1.7 HEATING EFFECTS
It is an observed experimental fact that the substrates in
a typical sputtering system tend to heat up unless special
arrangements are made to prevent this. The consequences are many
and varied and have been noted by many workers.; only a few exam-
ples will be quoted here.
Muth(24) has pointed out that the heat received by substrates
during sputtering of multilayer films is often enough to cause
inter diffusion between.film layers. This had the effect in his
experiments of altering film conductivity. Vincett(25) has sug-
gested that there exists an optimum substrate temperature allowing
the correct amount of movement by newly condensed• atoms to promote
good film growth. Riegert(26)
has concluded-that in a high rate
sputtering system, the deposition rate may be limited by the
amount of heat that can be tolerated•iri the substrate. This rate
18
is typically less than that v:hich can be achieved by evaporation,
making conventional sputtering unsuitable for many production
applications.
MarinKovic and .Roy(27) observed that in the sputtering of
tellurium, the film orientation changes from the 100 to 101 plane
as the film gets thicker. This has been explained by assuming
that recrystallisation occurs because of heating during film
growth.
Holland et al(28) in describing an R.F. device with non
grounded electrodes concluded that whilst surfaces at the boun-
dary of the plasma are bombarded by both low energy ions and
electrons, the observed heat levels must be due to energetic
electrons. Brodie et al(29) present figures to suggest that the
total energy dissipated at the substrate in their 'inverted V'
R.F. device was 0.55 watts cm-2. They conclude ( in-agreement
with Holland) that high energy electrons represent the major
source of energy to the substrate. It has been.suggested(30)
that in the D.C. mode with an uncooled cathode, 13% of the
applied power appears as heat at the anode. Whilst electron
bombardment is thought to be the main factor, the kinetic energy
of the sputtered atoms and their latent heat of condensation are
also thought to be important especially at higher deposition
rates.
By using a grid arrangement(30) it is possible to prevent
positive ions and high energy electrons from reaching the sub-
strate and thus demonstrate their affect. The main conclusion concerning film heating is that with electron bombardment during
the entire sputtering process, for a thick film (above fifty
.nanometres approximately) the film temperature is considerably
greater than that of the substrate. As a result, surface
reflectivity of the film can be reduced and, more importantly, a
large amount of stress can be induced into the surface of the
film and this can result in poor adhesion. In thin films, the
usual worry is the mechanical integrity of the, film but stress
can also cause changes in the magnetic and supercond cting
transition temperatures of materials. Stress in thin films has
been reviewed in some detail by Sun et al(31) and Lau(32).
Goldstein and Beliina(33) used a system of grids to prevent
19 electron bombardment of the substrate so that tantalum could be
sputtered onto teflon. In general, delicate substrates can only
be subjected to low input power or the heating effect due to
electron bombardment will cause degradation.
Similar problems have been observed in the coating of
P.T.F.E. with platinum. At 30 microns pressure and target sub-
strate separation of six centimetres, the maximum permissible
power input is 5 watts producing a deposition rate of a mere 50
nanometres per hour. At higher input levels serious blisters
develop on the P.T.F.E. surface. Chapman(34) has observed damage
on perspex substrates and attributed this .to electron heating
effect's.
Apart from causing degradation of delicate substrates, high
temperatures can also affect deposition rate. Davidse and
Maissel(35) quote figures on the variation of deposition rate
with substrate temperature. For example, at 800 watts R.F. power,
the deposition rate of quartz in their system was 120 n m•min 1
at 100°C and only 60 n m min-1
at 500°C. In general the depo-
sition rate decreases with increasing substrate temperature and
it can be concluded that it is imperative to have a uniform
substrate temperature if a uniform film thickness distribution is
to be maintained. Conversely, it might be possible to explain a
deposition profile in terms of a temperature profile along the
substrate though it must be remembered that target/substrate
geometry is also likely to contribute to any effect. Jones et t36i
al have suggested that re-sputtering is also a possibility.
ChapmanC37) has observed a focusing effect in which the pattern
of a composite target is reproduced on the substrate. This is
thought to be due to differing secondary electron coefficients
for the two parts of the target causing differing temperatures on
the corresponding parts of the substrate.
The effect of substrate temperature is reviewed at length by
Vossen(38) who points out that the heat generated by bombardment
has essentially the same effect.as when a substrate is deliber-
ately heated to a given temperature. It is concluded that a
major difficulty is that a hot substrate can cause sublimation
of volatile constituents. As substrate temperature is increased,
grain size can increase causing changes in fila properties such
20
as mechanical hardness. In some applicati.ens, electron bombard-
ment causing heating might be useful whilst in other cases it may
be a disadvantage. This would clearly depend on the individual
circumstances of the deposition in question.
Typical temperatures obtained at the substrate in R.F.
sputtering have been reviewed(30). The temperature obviously
depends on the deposition rate and the materials in question but
• figures of order hundreds of degrees Centigrade are not uncommon.
It has also been noted that cooling the substrate could be dif-
ficult because of problems associated. with obtaining a good
thermal contact between the substrate and the cooling system.
1.8 NUCLEATIO:, EFFECTS
Electron bombardment of substrates has been shown to have
other effects besides those associated with heating.
Stirland(39) working with e ✓aporated gold deposits on a rock salt
substrate has shown that electron bombardment during the depo-
sition increases the nucleation density and produces a single
(001) orientated deposit. He further concludes that electron bombardment results in a higher island density in thin deposits
and a more continuous film in thicker deposits. It is suggested
that these changes caused by electron bombardment are related to
changes at the substrate. Possibly the incident electron flux -
causes dissociation producing nucleation sites in the form of
vacancies or adsorbed substrate atoms which have moved from their
normal positions.
Chambers and Prutte n(40)
in agreeing with Stirland's obser-
vations point out that direct heating of the substrate by electron
bombardment is unlikey to• be the reason for the observed improve-
ment in the orientation of sputtered deposits.
Brodie et a1-(29) concluded that electron bombardment during
the initial stages of growth greatly improves the adhesion of the -
sputtered film. This is attributed to the creation of nucleation
sites on the substrate either by the formation'of surface defects
or by the removal of surface contaminants.
• 1.9 GUIDELI"ES OF THE WORK
As has already been indicated, electrons in sputtering play
an important part in determining film thickness and properties.
21
The starting point of t..i.s work is to look at the energy spectrum
of electrons incident on the substrate during the planar D.C.
sputtering of copper. This is accomplished using an electro-
static energy analyser which is described in chapter two.
Low deposition rate is perhAps the major disadvantage of the
sputtering process and figures already quoted suggest that 'cool'
substrates favour higher rates as well As less substrate thermal
damage.
It is well known that the use of a magnetic field in a
cylindrical configuration ( a cylindrical magnetron) will alter
the electron bombardment of the substrate and at the same time
increase ionising efficiency by confining the electrons to the
target region. This configuration is investigated further since
it is, perhaps, the best example of the modification of electron
motion to improve the sputtering parameters. Most of the work in
this section is directed at three main areas; substrate heating
effects, deposition rate achieved and film thickness uniformity.
If high deposition rate is the major consideration, then the
trend is towards larger power inputs and systems of larger physi-
cal dimensions. This introduces problems for the conventional
cylindrical magnetron since the construction of a large system is
not always convenient. Attempts are made to overcome this by
using a 'modified', cylinder which is essentially a flat rectan-
gular target with the ends rounded into a semicircular cross
section. The construction of this shape is possibly simpler and
it would appear to have advantages in 'on line' continuous feed
production applications. Some surprising results concerning the
role of the electrons were observed and this was investigated in
detail.
This 'modified system' was studied further; the main aims
being a clearer understanding of the part played by the electrons
and the production of a high rate system with reduced electron
loss and improved efficiency.
22
CHAPTER TWO
ENERGY ANALYSIS
2.1 GENERAL INTRODUCTION
Discussion in Chapter One has centred on the general role
which electrons play in a sputtering environment. Various inter-
actions which can occur involving electrons at the target,
substrate and in the plasma regions have been reviewed.
Without information concerning the electron flux density and
energy spectrum at various positions in the discharge, it is dif-
ficult to determine which of the possible mechanisms involving
electrons are significant and which are not Since discharge
characteristics can be modified by altering electron trajectories
and energies (for example by the use of an external magnetic
field) it is obviously necessary to have the basic data available
on electron energies and distributions in a normal D.C. sput-
tering system before modifications aimed at improvement of the
sputtering process can be devised.
2.11ANALYSIS IN THE PL.' S',IA REGION
Electrostatic probes represent a possible diagnostic tech-
nique which cas. be utilised to establish plasma parameters. A.
measurement is made by applying a voltage to the probe and
recording the current flowing from the plasma. Most of the
original wor• was performed by Langmo"ir(1 ) and is reviewed by
Cobine(2). Early results were obtained using a low pressure arc
discharge and are, therefore, not directly relevant but more
recently Ball(3) has performed some measurements for discharge .
parameters in the sputtering region. Summarising his conclusions,
the negative glow of the plasma was found to contain two electron
energy distributions, one of mean energy about 0.6ev and the other
at about 6ev. Both Of these energies are too ]ow to cause
appreciable excitation or ionisation of gas atoms though the
energetic tail of the 6ev distribution should be effective. This •
probe method failed to detect the presence of higher energy
electrons (as discussed by Ball).
- 2.1 .2 ANALYSIS AT S::. :'E
An analysis of the electrons striking the substrate is
advantageous for - two reasons:-
(a) It gives a measure of the electron bombardment of the .
23
growing film; an essential prerequisite if film characteristics,
as discussed in Chapter One, are to. be controlled.
(b) Since the secsndary electrons. from the target must pass
through the plasma region to reach the substrate, a knowledge of
the electron energy spectrum at the substrate should provide
information concerning the interaction of electrons with gas
atoms in the plasma region. This is clearly of interest since
electron impact ionisation - is mainly responsible for sustaining
the discharge and it is important to have an idea of the effi-
ciency of the process (i.e. the collision cross-section).
At the commencement of this project, relatively little work
had been done on the energy analysis of particles hitting the
substrate during D.C. sputtering. Coburn(4) has constructed a
system for determining the mass and energy of particles striking
the substrate in a planer diode system but has, so far, only
obtained results for positive ions.
Various workers have looked at the sputtered neutral species
present in a discharge, some of their conclusions being summa-
rised below. Honig(5) ionised neutral particles with an electron
beam prior to analysis and concluded that neutrals are about one
hundred times more abundant than positive ions on the assumption
that the efficiency of the ionisation process is very low at
about 10-4. Bradley(6) attempted a similar analysis but con-'eluded that his instrument was not sufficiently sensitive to
permit a qualitative study of neutrals,
A novel technique was employed by Wehner(7) in which he deposited sputtered atoms onto a balance pan. Arriving atoms
exert a force MV where M is the mass/second hitting the pan and
V is the average normal velocity component at' the surface. If this force is allowed to displace the pan upwards, the original
position will eventually be restored because the pan is gaining
weight as sputtered atoms condense on it.
Hence MV = ?tg and V can be measured by finding the time t
for the original position to be restored. This method gave an
average ejection energy for sputtered atoms of about 10ev, much
higher than typical values in thermal evaporation.
In another experiment, Stuart and Wehner(8) immersed a
target in a plasma which was pulsed for short periods (,:'i 1 ,t1 s)
24
so that atoms were sputtered from the target as a grotip. The
atoms are ejected as neutral, unexcited particles, but they
undergo excitational collisions with plasma electrons and emit
their characteristic spectra. The atoms in a particular group
become spatially dispersed as a consequence of their distribution
of velocities and this dispersion was observed as a time dis-
tribution of photons emitted by sputtered atoms as they pass an
observation volume a known distance away from the target. This
time distribution was converted to a velocity or energy distri-
bution. Results showed an energy distribution, with a mean energy
of about 10ev (in agreement with the previous ekperiment which
could only measure the mean value) but a most probable value of
about 5ev indicating that the distribution is skewed with a high
energy tail,
Previous work on electron energy spectra at the substrate
was carried out at Imperial College by Guimaraes(9) who obtained
some preliminary results using a 90° deflection electrostatic
analyser. The starting point of this project was, therefore, an
attempt at the repetition of the results of Guimaraes with sub-
sequent extension both in terms of the accuracy of the results
obtained and in their analysis.
More recently, some work has been published by Ball(3) who obtained results on the electron energy spectrum at the anode of a D.C. sputtering system using a 127° electrostatic analyser. His results will be discussed later for comparison with our work.
2.2 E':ERGY ANALYSERS
General principles of energy analysis for charged particle
beams have been reviewed by Steckelmacher(10) who points out that
there are three fundamental methods of analysis:- (1) By the use of a retarding field to allow only charged
particles of a certain minimum energy to reach a collector..
'(2) Deflection caused by electric or magnetic fields.
(3) Time of flight measurements between two fixed points (a method Also available for neutral species).,
As pointed out(10) a general comparison of the merits of
different analysers is difficult if not impossible in view of the
variety of possible, applications each requiring different opera-
ting conditions. As a general principle, retarding fields and
25
deflection types of analyser are simplest in the - case of measuring
charged parti..les. For our specific application, there are advan-
ta?es in filtering out neutral species suggesting that a deflection
type analyser would be most suitable. It should be pointed out
that deflection using a magnetic _field was not thought to be
advisable since a magnetic field significantly alters the plasma
characteristics as will be discussed in a later chapter.
In this work, it was decided to use a 900 deflection elec-
trostatic analyser (a spherical condenser analyser). An advantage
of this type is that there is a focusing, effect in two dimensions
though there are inherent constructional difficulties associated
with the accurate machining of the deflection plates. A major
reason for the choice of this particular analyser was the fact
that the equipment had already been built(9) though it was in need of re-assembly and calibration.
2.3 THEORY 0" THE SPHERICAL C07:DE7SER
2.3.1. FOCUSING
The focusing of charged particle beams by a spherical con-
denser was first studied in detail by Purcell(11). The basic
electrostatic theory required to solve the focusing problem can
be found in any standard text(12); here we develop only the
outline.
Consider (Fig. 2.1) two concentric spheres A and B of radius
R1, R2 (R2} R1) with A carrying a charge of +Q coulombs and B a charge of -Q (i.e. there is a potential difference (electric field) across the plates of the capacitor.
Fig. 2.1. The Spherical Condenser
Since the field strength due to a single conducting sphere
must be zero inside that sphere it follows that sphere B contri-
butes nothing to the electric field in the space between the two spheres. Hence, the field strength between the spheres depends
26
on A only and is given by Gauss's law as:-
£ _ rrE,
0 (r)R1 ) 0
where r is the distance from the centre of the sphere; E is the electric field strength and E is the permittivity of free
space.
Since E. -dV where V is the potential difference
dr between the spheres..
R1
V = Q
dr 4116 o R 2
V - Q 1 -1 4 Tr E0 R1 52
From (1) E r2 4irrE 0
V = E r? 1 - 1 R1
72
• V = E r2( R2 R1
R1 R2
or E = VR1 R2 (R2 - 1) r2
This is an expression for the radial electric field . (r) between the plates.
Now consider the situation of the spherical analyser in
Fig. 2.2 where a beam of charged particles is allowed to enter
through an aperture into the space between the two concentric
(1)
(3)
spheres.
27
Fig. 2.2 Charj e.i Particle Trajectory
Consider two spherical plates radii of curvature R1 and
R2 at potentials V1 and V2 respectively and let the centre line
radius be r1. The arrangement of Fig. 2.2 is essentially.a capacitor and hence the charges on the plates must be equal and
opposite since there is no net current flow. With the above
arrangement, negatively ch ;.rged particles will follow the curved
trajectory between the plates, a reversal of the charges would
favour positively charged species.
Prom (3)
E(r) = Vf R1 R2 ( 4) (R2 - R1) r2
where Vf = (V2 - V1) and is the focusing potential.
From force balance considerations in a circular orbit
radius r1; for a particle of charge e.
2 e C (r) = mv1 = 2Ee
r1 1
where v1 is the velocity in the circular orbit and E is the
energy of the charged particle in electron volts. The neces-
sary force towards the centre to produce circular motion is
provided by the electric field E(r)
28
E (r) = 2E and from (4) r1
Vf R1 R2 (R2 - R1)
1 = 2E
r2 r1 1
Vf R1R2 1 = 2E (R., - R1)
r1
and since r1 = R1 + R2 for the central trajectory 2
Vf R1R2 = E(R22 - R1 2)
or Vf = E R2 - R1
R1 R2
which is the usual way of expressing the relationship.
This can be written as:-
Vf = kE where k is a'constant for the analyser depending on
its geometry and k = R2 - R1
R1 R2
Hence the energy of charged particles passing round the
curved trajectory of the analyser is related to the potential
difference between the spheres and the energy in electron volts
(eV) can be evaluated once the analyser has been calibrated.
A theoretical value for the constant could be evaluated
from the expression for k but a practical calibration.is usually
required because of the constructional difficulties in producing
and assembling two spheres in an exactly concentric manner.
In practice we choose to apply voltages of ± Vf /2 to the
spheres so that the centre line will be close to earth potential.
In fact this arrangement will nOt provide a centre line at
precisely earth potential since the distribution of the potential
between the spheres does not fall off linearly but shows a 1/r
29
dependence:
The reasons for wanting a centre line at earth potential
are associated with the fact that the analyser should not change
the energy of the electrons under observation and the substrate is earthed in a typical D.C. planar system. Setting - 1dr /2 on
the spheres is convenient experimentally and does not rroduce a.
large departure from the requirement of having an earthed
central trajectory.
2.3.2 RESOLUTION OF THE A'"ALYSER
In any deflection type analyser the electric field con-figuration causes charged particles to follow trajectories which
are a function of their energy. It is important to realise that
a band or spread'of eneries will be transmitted and not a single
energy and that there will be an energy distribution about'a
central maximum. This transmission energy band is selected by
the size of the entrance and exit apertures of the analyser.
If the analyser is set to transmit particles of energy E,
then it will allow the passage of particles in the energy range
E - RoE where Ro is the resolution of the instrument defined by:-Ro p E/E where LE needs to be defined in relation to the peak shape of the transmitted beam. Q E is usually related to the width at half maximum intensity. The resolving power /4D of the
instrument is the inverse of the resolution and is, therefore,
defined from AD = E/Q E.
In a practical device, a compromise has to be achieved
between an acceptable resolution and a reasonable number of
charged particles being allowed into the analyser. Clearly a
minimum particle flux must be allowed into the analyser if a 'measurable output current is to result and the consequential
spread of transmission energies has to be tolerated. In the
limiting case with the apertures stopped dour, to zero width, the
resolution would be perfect but no'particles would enter the
analyser.
2. 4 EXPERII..E.:T Ab SYSTEM
2.4.1 I?:TRODU' TION
The system has already been described in detail(9) and it is not intended to 'duplicate that work. A schematic diagram of
GAS HANDLING SYSTEM
ELECTRICAL CONNEC TIONS
TARGET
ENERGY ANA YS£R
CURRENT COLffCTOR
VACUUM CHAMBER
the experimental arrangement is shown in Fig. 2.3
F_.; 2.3 Experimental system
30
1 TO OIL SUBSTRATE AND
DIFFUSION PARTICLE EXTRACTOR PUMP
2.4.2 VACUOL COSI DERATI O?T S
The vacuum chamber consisted of a 45cm diameter cylindrical
glass cylinder with target asse._.bly and energy analyser supported
from the top plate to facilitate easy removal. .The chamber is
evacuated by conventional means using an oil diffusion pump backed
by an oil rotary pump, liquid nitrogen trapping being used. The
best pressure obtainable was about 1 x 10 torr.
SPUT TERING CHAMBER
31 Whilst D.C. sputtering requires an operating pressure in
the range 20 to 100 mtorr, the energy analyser requires a much
lower pressure so that electrons extracted from the sputtering
environment will not have their energies altered by collision
with gas atoms in the analyser. The sputtering chamber is,
therefore, differentially pumped through the particle extractor
(sampling aperture) and, for example, with a 2 m.m. aperture
pressures of 90 mtorr and 4 x 10-1 mtorr can be achieved in the
sputtering chamber and analyser respectively.
The mean free path of an electron is a function of its
energy but at 4 x 10-1 mtorr it is of order 10 ems which is con-
sidered to be satisfactory for these purposes.
2.4.3 SPUTTERI.7G C A :DER
The chamber consisted of a cylindrical glass vessel 15 cm
long and 15 cm diameter with two aluminium end plates held
together by long bol ; and sealed onto the glass by two large
'0' rings. One end plate supports the target assembly and dark
space shield, the other lupi,orting the energy analyser system.
During the preliminary pump down prior to the commencement
of sputtering, this chamber can be pumped directly to the same
• pressure as the rest of the vacuum chamber. Closing a valve
introduces differential pumping so that a pressure gradient can
be set up as already discussed.
2.4.4 TARGET AS SE7BLY
This is shown In Fig. 2.4.
The target can be easily replaced and the complete assembly
including dark space shield is mounted onto a micalex shaft
which can be screwed through the aluminium end plate so that the
target position and hence target/substrate separation can be
varied. Since water cooling is not available, input power to
the target is 1_mited and the'system requires a long time period
to reach an equilibrium temperature. The implications of this
will be discussed later in the chapter.
2.4 Tan et Asse::5ly
MI CAL EX SHAFT
32
TARGET
ALUMINIUM SEPARATOR END PLATE
DARK SPACE SHIELD
))
2 • .1. 5PARTIG1JE EXTR.~C~OR
This is sh01nl in Fig. 2.5. Fi~. 2.5 Parti~le Exfractor
SUBSTRATE SAHPUNG APERTURE
ENERGY ANAlYSER
Insulated from the main portion of the substrate (anode)
end plate so that the a'1alyser has the capability for investi
gating bias sputtering, the sampling aperture is easily
demountable so that the effect of different apertures can also
be studied. In our application, as already discussed, the anode
an~ particle extractor were kept at earth potential wi~h the
centre line of the energy analyser also at earth potential so
that unwanted particle acceleration would not be introduced into
the system."
2.4. 6 E~;ERGY AX ALYSER
Theoretical aspects h~ve already been considered. The
"actual device consisted of two sectors of alwninium hemispheres
10 cm and 11 cm diruneter respectively th~reby yielding a centre
line radius of 5.25 cm. The theoretical nllue of k (the cali-
bratio!1 constant ) -1 of the analyser is, therefore, 0.191 volts av
[k = I R2 - R1 I J
R1 R2
Some of the electrons which Vlould normally strike the anode
in a conventional sputtering system are ~ollected ~y the sampling"
aperture (particle extractor) and pass into the" analyser which is
34 set to transmi a ba=d of energies. The output current associated
with this Und. is measured using a Faraday cage collector.
When the voltage across the analyser plates iS changed, a new
output current results since a different energy band is now-being
transmitted. A spectrum of current collectel against energy
transmitted can, therefore, be built up.
2,5 L L _; ,_~ ; 0, 07 T__F. _... ER"Y A • :,Y` R ~, v. s r:
As already discussed, an experimental calibration is required
and this wee achieved by suspending the tip of a tungsten filament
in front of the analyser sampling aperture. The filament was held
at a negative potential of a few hundred volts with respect to the
earthed sampling aperture and then heated electrically to 'white
heat' (2000 °C) to produce thermionic electron emission from the
tip. Under these conditions, a near moroe.ergetic.electron beam
of energy corresponding to the potential of the filament can be
considered to strike he sampling aperture since at the low
pressure of 1 x 10-6
torr collisions can be neglected.
For these operating parameters kT Pe, 0.1 eV (where k is Boltzmann's constant) which is negligible compared to an accele-
rating potential between' filament and sampling aperture of order
hundreds of volts. Hernie the electron thermal energy can be ignored as can any energy distribution.
The electron beam was energy analysed by varying the poten-
tial across the analyser plates, the following results shown in
Fig. 2.6 being obtained.
Fip. 2.6 Current recorded (I) v Fo^usln? Volt ~..,fe (Vf fpr an
Acre!-erang,Potential of 400 volts.
44 t ' 160 V(Volfsl
35 Hence K = V.f/E = 90.2/400 = 0.226 volts eV 1.
The experiment was repeated with five different accelerating
voltages of 450, 5G0, 550, 600 and 700 volts respectively and an
average vaue of K eva . a :ted. The results were all in the range 1.223 to 0.227 volts eV-1 yielding a value for iC, ., r , of 0.225
-1 nV:~~n.,rF; ..1 volts eV compared to a theoretical value of 0.191 volts eV .
It is worth noting from Fig. 2.6 that the value of I when
Vf = 0 is surprisingly high. This should be borne in mind when
• viewing later energy curves since .it may be that the low energy •
values are unreliable. This possibility will be discussed later
with reference to the energy curves.
2.6 ACCURACY OF RESULTS
Before prese:.ting the observed energy spectra in detail, it
is necessary to discuss the precautions needed to achieve accurate
reproducible results since it soon became apparent that this was
a difficult problem.
Plotting the output current (I) from the Faraday Cage col-
lector against the analyser focusing voltage (Vf) is not satis-
factory since it gives no reliable indication of the input power
setting and would render meaningless any comparison of results
obtained on different days under different operating conditions.
The output current was, therefore, normalised to the total sputter
current (1T) and results were presented as I:/ against V.
Unfortunately the parameter 7, is not a constant and the results
tended to drift with time as discussed below.
(a) The current drawn by the discharge (IR,) is clearly a
function of the sputtering pressure and a small unwanted pressure
fluctuation can have a significant effect on the output current.
It has, in fact, been suggested by some worker s that an alter-
native to using a pirani gauge to monitor sputtering pressure
would be to measure the discharge current, though some of our
subsequent work with magnetic fields might cast doubts on this
suggestion.
Pressure stabilisation was found to take about two hours to
achieve, possibly because as the uncooled target warms up on the
. commencement of sputtering, pockets of occluded gas molecules are
constantly being released.
36
(b) Pre-svutteri:.S has long been recognised as an essential step to remove target surface contamination prior to film depo-
sition. As the surface layer is removed it was found that the
discharge current is reduced because of a difference in the
secondary electron coefficient of the copper target and its sur-
face coating. The following typical results were obtained over
a period of one hour.
Fi? 2.7 Total Shutter Current v Time
It was eventually found to be necessary to allow a warm up '
time of about two hours before steady conditions could be achieved.
Since as TT varies the current I collected by the analyser also
varies it was hoped that I/IT would remain approximately constant
rendering a long warm up time unnecessary but experiments on
these lines did not produce reliable results.
It would be interesting to see if a water'cooled target would
require the same time period for stabilisation but this was not
investigated.
Under the above conditions, reproducibility to within 5% was
obtainable on different days providing the Keithley electrometer
used to measure the analyser current was re-zeroed before every
reading. Pre-sputtering times of this long duration tended to
lead to excessive accumulations of deposited copper on the target
dark space shield and sampling aperture necessitating a frequent '
37 cleaning operation, An even longer pre-sputter period was then
required on the first experimen after cleaning. Whilst this
experimental technique is not convenient, it was found to be
necessary if reproducibility of 5% or better was to be achieved.
2.7 E `'RI=AL RESULTS
2.7.1 GE ``ERAL CURVE SHAPE
A series of experiments for a variety P of sputtering pressures,
voltages and target/substrate separations revealed that the general
shape of the energy 'spectra was of the form shown in Fig. 2.8 and
it is convenient to split the curve intd two separate parts con-
sisting of a low energy region I and a high energy region II.
Fig. 2'.8 General Shape of Energy Soec tra
r
too+
T ' 50-
L
> V (Volts)
The low energy peak (section I) would usually be poorly
defined with a shallow maximum at about 25 eV in contrast to the
high energy peak (section II) which had a much sharper outline
and a well defined cut off at high energy. The analyser voltage
Vf at the cut off was a function of the inter-electrode potential
between target and substrate.
2.7.2 C=TL7T `TOLT.^AGE V,ITH VARYI':G PRESSURE
A series of experiments was performed in which ' the discharge
90 180 Vf (Volts)
—1
IT T
V=1095 Volts P=43ji
-- -V=1095 Volts P= 68p
100
50
38
voltage was held constant and the pressure in the s;; te:: varied
so that a series of energy spectra, each at a different pressure,
could be obtained. Some typical results are shov.n in Fig. 2.9.
Fi , . 2.0 Energy Srestra Pt rsns t sar t V o l to _e Varl alp'+ e Pressure
Fig. 2.9 illustrates the difficulty of presenting information
concerning the - two peaks on the same scale and in future they will
be considered separately.
The high energ y part of the curve is perhaps more striking if
it is replotted (Fig. 2.10) as I/IT against E where E is the energy
in eV of electrons hitting the substrate.
i
1
_ 400
100
133
50
F';r. 2.10 I/IT -r E at Constant Voltage
39
x 10-1 T
v=1095 vat is p=43).1
_ -y_1095Vo(ts p=6624
I I I 800 1067 E. (e0
We can now see that the well defined pea?: of the curve cor-
responds to an energy in electron volts which is the sale as the
inter-electrode potential.
2.7.3 COx'STA:'!T PRESSURE, VARI!t LE VOLTAGE This second series of experiments involved keeping the pres-
sure constant and varying the sputtering voltage to produce a
series of spectra at differing voltages. On both the high and
low energy curves the values of Vf (volts) and E (eV) are shown
on the abscissa. The results are summarised in Figs. 2.11 and
2.12.
15--
10 --
40 Fig. 2.11 Energy Soectra at Constant ?re:; ure Variablē Volta e — Low Enf.,7 Rao,-=e
A / 0
_7 • P 30/u
I t I I I 1 I 2 4 6
• 1 I 1 I I I
I I 1
t 1
I I 1
. I 1
1300 Vol ts ,1
1000 "
I I> V( Volts 1 , 1 1 1 I I
8.8 17.6 26.4 E (e v)
X10-1
_ S
100 — T V4120 Volts
---V-815 Volts
P=40»
50 -- i
I I 30 90 400
1 I { ( 1 >
180 240 V l Vol ts)
800 1067 f E ley)
/
41
TF-14. 2.12 Energy Spectra at Constant Pressure Variab_e Voltage -
H .h_ __: r R to e
The above two graphs are representative of. many in the
pressure range 25 - 100 microns each showing the same general
trend. The low energy peak is not very well defined and does
not strongly depend on either pressure or voltage always occur-
ring at around 25 eV. There is a slight trend for the peak to
move to . higher energies for higher applied sputtering voltages.
The high energy peak does not seem to ne pressure sensitive
.but is strongly dependent on voltage; the energy at the peak
corresponding very closely to the inter-electrode potential.
2.7.4 VA.RI,TIO'. 07 T RGET/SU .STRATI: SEP ARATI .': 1::D SA'"PLING
APERTURE
. All the results presented so far were for a target/substrate
separation of 8 ems and a sa::!plin:; aperture of .1.E m.m. diameter.
Increasing; the target/substrate separation by adjusting the
target position caused a slight reduction in the number of high
energy electrons detected whilst increasing the sampling aperture
42
diameter causei a slight increase in the number of electrons
detected at all energies. ..either change ignificantly affected
the curve shape though the variations were, admittedly, not
studied in any great detail.
2.8 ANALYSIS OF RESULTS
Whilst these experimental curves are perhaps surprising at
first sight in that they seem to imply that a lot of energetic
electrons are striking the substrate, they are also misleading
and it has already been pointed out that care is needed in their .
interpretation.
The value of the parameter Io (the current with no voltage
across the analyser plates) is disappointingly high since with
no voltage across the plates it is not possible for a charged
particle created within the discharge to reach the Faraday Cage
collector.
This Io figure c_nnot be considered simply as a background
count to be subtracted from the detected current to give the net
increase current since once the low energy peak is passed cur-
rents are observed which are below the Io value. Clearly,
however, some sort of background pick up must be represented by
Io though stray signals being induced through the carefully
screened electrometer leads was thought to be unlikely. Obvious
possibilities are either charged particle creation within the
analyser itself, despite the lower pressure due to differential
pumping, or a significant contribution from secondary electrons
being ejected from the analyser walls by primary particles from
the discharge hitting them.
- Despite the reservations introduced by the uncertainty of
this Io factor, the low energy peak does appear to be detectable
under all conditions of pressure and voltage though it was not
observed by Ball(3). Thoughts on its significance, if any, will
be deferred until the acceptance bandwidth of the analyser is
included in the analysis later in the chapter.
The high energy peak which was observed by Ball(3) is,
perhaps, more interesting since it is better defined and shows
that some electrons are striking the substrate with the full
inter-electrode potential having traversed the.discharge without '
43
making an inelastic collision. These high energy electrons are
detrimental for two reasons: firstly because they are not con-
tributing to the enhancement of the spi tterin.g rate by creating
more ions and secondly because they are providing a significant
heat input to the substrate which may cause damage to certain
delicate substrates.
These. two points are crucial to the development of the rest
of the work but before they can be considered further an analysis
of the analyser acceptance bandwidth is required.
2.9 ANALYSER .ACCEPTf?_:c_? BANDWIDTH
As previously discussed, an analyser set to transmit an
energy E will in fact pass energies in the range E ± RoE where
Ro = E/E and is the analyser resolution. Since Ro is a con-
stant for our analyser it follows that as the energy under
analysis is increased in magnitude, the spread of energies which
will pass is also increased and this tends to bias the results
in favour of the higher energies.
If ō F(E) is the flux of particles bombarding the substrate
with energies between E and E + 6 E, then when set to E the
analyser transmits & F(E) particles per unit time.
S F(E) = Q E S F(E) = R I: ō P(E) d E ° S E
and in the limit as 6 F(E) --) 0 we can write:-
& F(E) = RE dF(E) o dE
Since Q = It (by defn) Q = charge
I = current
t = time.
I = Q/t = charge arriving/unit time.
I = numbers arriving/unit time x charge on a single particle.
dI = 8 F(E). e
where dI is the fraction of the total current'
I caused by the energy range dE.
44 dI = R E dF(E). e where e is the electronic charge.
o dE
= dI oe dI dE R.
0Ee E
So far we have presented curves of di (called I/IT in the
experimental results) against E whilst curves of dF(E)/dE against
E or dI/E against E would be more advantageous since they would
show the number of particles actually hitting the substrate from
the discharge.
Hence the curves of current aainst energy.previously pre-
sented need to be modified by dividing the ordinate by E to
produce the following general curve of flux against energy.
Fig. 2.13 ..'.odified Energy Spectra
_ X A.r E
25 1000 E (ev)
In interpreting this curve it should be realised that when E
is sall, I/I, x 1/E is tending towards infinity and any small
error in E will cause very large uncertainties in the value of
the ordinate.
45 When the results are v:.'csented in this way the so called
high e er y peak. ie greatly diminished and the low energy peak
from the previous curves is removed completely to be replaced by
a Slight deviation away from the smooth downward trend. The
previous results could, therefore, be easily interpreted wrongly
since the small low energy peak in fact corresponds to a large
signal whilst the large high energy pear, corresponds to a small
signal.
:,:ore importantly the results show that most electrons
reaching the substrate are of low energy though we must remember
that large uncertainties are to be expected. Bearing in mind
the fact that sputtering is a collision dominated low mean free
path process, this result is not too surprising. There is,
however, still a peak correspohding to the full inter-electrode
fall of potential showing that a small but significant number of
electrons are crossing the plasma without col.lisicn.
It should be noted that bandwidth considerations will shift
the analyser calibration curve (Fig. 2.6) to a slightly lower
voltage maximum producing a lower value for the calibration
constant (K) but the correction needed is very small.
One obvious question still needing to be discussed is
whether the original spectra of I/IT against E with their two
peaks have in themselves any significance and this will be con-
sidered in the next section.
2.10 HEAT INPUT TO THE SUBSTRATE
If SP(E) is the power carried to the substrate by electrons
with energies in the range E to E + S E, then since power is the
rate of dissipation of energy we can write:-
b P(E ) = S F(E).E.
dP(E) = dF(E).E ot dI.E oC dI dE dE L
Therefore, the curves of I/Il against E originally presented
are in fact curves of the 'power input to the substrate caused by
electrons against electron energy.
46 This means that the high energy electrons, although
relatively few in number are largely responsibl:. for the electron
power input to the substrate (the other contributions to sub-
strate power input will be discussed in detail in a later chapter).
Since this power input to the substrate can be large enough to
cause degradation of heat sensitive substrates(13) it is clear
that a knowledge of the electron energy distribution curves is
.useful in the deposition onto these substrates especially.
2.11 CONCLUSIONS
2.11.1 GE7ERAL CO'`"E',TS
At this stage of the work a fundamental decision was needed
on whether to try and reproduce the experimental data more
accurately by improved instrumentation or whether to accept the
results as being a reasonable approximation to the energy spectra
and investigate the implications of these results. The latter
course of action was hosen for a number of reasons but mainly
because it seemed to offer a more interesting line of research
with the possibility of using electron properties to achieve a
higher sputtering rate.
Apart from some work in a later chapter on justifying the
experimental curves by considering the theoretical electron
distribution on the cathode dar: space/negative glow boundary,.
no further effort was put into the energy analysis and a summary
of our conclusions should serve to indicate the dominant lines
of thought.
2.11 .2 SU." .'.ARY OF RESULTS
The electron energy spectra for electrons incident on the
substrate in a D.C. sputtering process show that most electrons
arrive with relatively low energy but some have almost the com-
plete inter-electrode energy (in equivalent eV) and are largely
responsible for the power input to the substrate. The other
departure from the ge::ral trend occurs at about 25eV where there
are more electrons present than might have been expected and a
consequential larger power input than might have been anticipated..
The reasons for this behaviour are not clear but the effect does
seem to be real rather than some function of the energy analyser
since it is reproducible under varied conditions of pressure and
47 voltage. As a first step, more detailed accurate spectra would
be _'eau re'i to dis , ver the precise energy y rage over which
increased signals ;';ere being detected. Since many of the
ionisation and excitation levels for argon are in the region of
20eV it could be that this is responsible for the observed results
but without more information it is difficult to comment con-
clusively.
The increased signal at hig.n energies is of greater interest
• for our purposes since high energy-electrons are detrimental to
the efficiency of the sputtering process on the following grounds:-
(a) The sputtering process is in general slow in that
deposition rates are often unacceptably low under normal operating
conditions. These high energy electrons having traversed the
plasma region without making an ionising or 'exciting collision
have produced no electron/ion pairs either directly or indirectly
and have, therefore, contributed nothing to the achievement of a
higher deposition rate.
(b) The high energy electrons are primarily responsible in
many cases for substrate heating and whilst this can be beneficial
in certain circumstancesit is undoubtedly detrimental in others
•notably in the coating of delicate substrates such as P.T.F.E. or
perspex. Substrate heating, where required, is usually better
obtained under controlled conditions using an external source and
in some cases substrate cooling may be required because of the
excess heat being generated. Since substrate cooling can be
difficult because of the need for good contact between the sub-
strate and its backing plate, it is preferable to eliminate the
cooling requirement by controlling the incident electron flux.
(c) As higher anode/cathode voltages are tised in the search
for higher deposition rates, X-ray production can become sig-
nificant because of Eremsstrahlen caused by the high energy .
electrons which are produced. Whilst this radiation is unlikely
to constitute a significant health hazard it can cause damage in
certain substrates.
The rest of the wort, was, therefore, devoted to an investi-
gation of the high energy electron contribution to the sputtering
48 process from the point of view of substrate heating and the
production of higher deposition rates by the removal of the
energetic electrons from the substrate. If the electrons could
in some v:ay be trapped in the plasma region then they must even-
tually undergo a collision which has a probability of being
inelastic' thereby causing er.er7y loss to the electron. Since an
ionising collision would produce an extra ion which could eject
more target material there is a pos ibilit;,; of enhancin the
sputtering rate by electron trapping with a chance.t at substrate
heating might be reduced by the resulting modification of the
electron flux.
Whilst this concept was to prove slightly optimistic it
provides a guideline for the rest of the work and is investigated
in the following chapters.
49
CHAPTER THREE
RICHER RATE S U'T':'ERI: G
3.1 ^ROT'."CT -0.r
One of the major problems of the sputtering process is
that power consumption is high in relation to the amount of
material transported resulting in deposition rates comparing
unfavourably with those obtainable in other processes such as
thermal evaporation. Hecht and .ullaly(1) have noted that
because of its versatility, sputtering is in demand for the
fabrication of numerous unique components where thick films
(,'i 6000 microns) are needed but production problems exist because of the excessively long deposition times required to
produce the coatings. Examples include the production of
strong permanent alloy magnets and the production of rocket
thrust chambers.
Further thoughts• on industrial economic lines are pro-vided by Thornton
(2) who suggests the most significant parameter
to be deposition rate multiplied by substrate area (i.e.
coverage) rather than the more usual deposition rate. On this .
basis, sputtering becomes. competitive on a large scale (pro-
vided a reasonably high rate is achieved) principally because
scaling up to physically large production systems is not an
insurmountable problem. Since R.F. power supplies are more
expensive than D.C. and R.F. systems cannot be scaled up as
easily, Thornton further suggests that the most spectacular
improvements can be expected for D.C. systems operating with-
cylindrically symmetric. geometries where the sputtering process
is scale independent in the plane of the substrate.
Apart from the inconvenience of long deposition times due
to low sputtering rates, they can lead to poor quality films.
We have been able to demonstrate this by producing a series of
constant thickness/different deposition rate, copper films on
glass substrates. For example a 1000 oA copper film deposited
at 1000 oA hr-1
(deposition time one hour) appears black,
exhibits very poor adhesion and has a very high resistance
whilst a film of the same thickness deposited in three minutes o -1
(20,000o A hr) ~ h a.. a characteristic copper appearance,
50
exhibits such improved adhesion and has negligible resistance
suggesting a much improved copper. film. Obviously as the arrival rate of sputtered material is lowered, the ratio of impurity species (e.g. oxygen and water vapour) to sputtered
species is increased with an effect on file quality. However,
in the case of reactive sputtering of dielectrics (e.g. S102 in
Ar-02 mixtures) low deposition rates lead. to higher quality
films.
Whilst it might seem reasonable to assume that lower pres-
sures must lead to better film purity in terms of less sputtering
gas (e.g. argon) being embedded in the growing. film, this is not
necessarily (s as was demonstrated by Winters and Kay(3,4) and
Jones et al (5). Winters and Kay were able to show by analysing
the deposited film content of nickel samples that any gas species
can be embedded in a growing film providing the gas atoms' have
sufficient kinetic energy When they arrive at the film surface.
The degree to which inert gas trapping will take place was
thought to depend primarily on the kinetic energy of the incident
gas species and secondarily on the chemical and physical micro-.
structure of the film surface. Film temperature and whether
bias sputtering is employed are other important factors.
At high pressure all of the high energy argon reflected
from the target is likely to be reduced to thermal energy before
reaching the substrate. However, as the pressure decreases, the
average energy of the reflected argon which reaches the substrate
will tend to increase resulting in more argon embedding into the
film in the absence of bias. The main conclusion seems to be
that the purest films are obtained by sputtering at the highest
possible pressure provided that the sputtering gas is itself
pure.
Whilst high deposition rates would appear to be advantageous
on economic and film purity arguments, it should be remembered (1,6) that film structure is also a function of deposition rate '
and it may be that in some applications the structure produced
at high rate cannot be tolerated. This point is not usually of
importance in the deposition of metals and will not be considered
further in our work.
51 3.2 FACTORS I.,F LTJ E7CI..:n UE ^ITT„ . RATE
The choice of sputtering gas has already been considered
in Chapter One, if this is not now considered to .be a variable
parameter there are basically five factors which can influence
the deposition rate:-
(a) The target/substrate separation.
(b) The temperature of the substrate'and target.
(c) Pressure.
(d) Voltage difference between target and counter-electrode
(the inter-electrode potential).
(e) The current drawn by the discharge.
3.2.1. TARGET/SnSTPAT7. L77PARATION
An optimum separation was not evaluated experimentally in
our work but .the .following basic principles(?) were taken into
account in constructing the system.
The counter-electrode (often also acting as substrate
position) must be situated outside the cathode dark space (CDS)
or the discharge will not be self sustaining and as a general
rule it should be at least twice the CDS length away from the
target so that ion production is not significantly affected by
the close proximity of the substrate.
The less the electrode separation the greater the depo-
sition rate, since the substrate is subtending a larger angle
at the target, but the poorer the film uniformity and the higher
the substrate temperature. Since deposition rate is a function
of substrate position and for that matter target size, it is
more realistic in terms of the efficiency of the sputtering
process (i.e. momentum transfer) to consider the amount of tar-
get material removed per unit target area and no further detailed
thought was given to substrate positioning.
3.2.2 THE TEMPERATURE OF SU `;;TRATT A';n TXRGET
Whilst substrate temperature considerations will be dis-
cussed in detail in a later chapter, we should remember here
that since the condensation coefficient of the sputtered atoms
is a function of substrate temperature, this parameter will
affect the deposition rite.
52
The temperature of the target is not usually considered in
a saratus design other than from the point of view that water
cooling is preferable to prevent solid state diffusion in alloy
targets and the melt n:, of seine targets at high power input
levels. an alternative apr,:roach of liquid phase sputtering has (3) who allowed their bee::.investigated by :lr=.zterat and Gesic:: allowed
to melt in order to provide hi,gh.depositon rates in a
combined sputtering-evaporatio:. mode. Little support seems to
have been given to this idea as yet, presumably because of the
inconvenience-of handling a target in this farm.
3.2.3 i R1=RE As the pressure increases, the mean free path of sputtered
atoms will decrease causi_n , them to spread out more because of
enhanced scattering. For example, at 50/u pressure the mean
free heath of atoms is about one millimetre whilst at 5/u it is
one centimetre though this latter pressure is not usually
suitable for unassisted D.C. sputtering.
The disadvantage of increased spread tends to be offset by
the fact that the mean free path of electrons is also reduced
at higher pressure giving an increased probability for electron
impact ionisation and resulting in higher currents and enhanced
sputtering at high pressure.
A further advantage of higher pressures is that positive
ions travelling towards the target will be more susceptible to
collision and will be more likely to strike the target at angles
other than normally. The variation of sputtering yield with
angle of incidence has been investigated by Cheney and Pitkin(9)
who find for a Xe+ on Cu system that the maximum yield (atoms/
incident ion) is when the ions strike the target at an angle
of incidence of about 200 to the normal. This fact can be used
to advantage in the "Sputtergun" to be discussed later.
The reap limiting factor to ' sputtering at high pressure is
the probability of sputtered atoms returning to the target by
virtue of scattering in the gas and it has been estimated that
at 10 pressure approximately 90% of the sputtered material
never leaves the CDS region. Also at high pressure the CDS
53
length is reduced causing problems in the construction of
efficient cathode dark hi.eld. to sunpress sputtering at
unwanted parts of the target (e.g. water cooling connections)
and proper masking of selected areas of substrate becomes
difficult.
At low pressure, the CDS becomes long causing a large
target/substrate separation with a subsequent lowering of
deposition rate and in practice a compromise must be achieved
at an optimum pressure choice.
The distribution of ion energies striking the target of
a glow dischar.e has heel: studied by Davis and Vanderslice(IO)
who obtained an energy spectrum of the form shown in Fig. 3.1.
Fit-. 3.1 Distribution of Ion enemy at the Tar[-et
1.0 — A INTENSITY
0.8
06 -
04 -
02 —
Vo=farget potential V =energy of ion in equivalent volts
r
06 08 10 c
02 04
This distribution curve is thought to be caused principally
by charge exchange interactions of the form:- o +
Ar+ + Ar° '~ Ar + Ar (for argon ions).
The equation represents a moving ion hitting and charge
exchanging with a near stationary atom to produce a moving atom
and a stationary ion which immediately moves towards the target
provided it is in an electric field region of the discharge.
Sputtering at the target is, therefore, caused by a distribution
54
of energetic ions and also a distribution of energetic neutral
species, both di striL'utionr being a func ! cn of pressure since
pressure changes alter the ion mean free path and hence the
charge exchange col_isien_l probability. -
īf sputtering; is truly a momentum exchange phenomenon, we
might expect the yield under neutral atom bombardment to be the
same as that using the corresponding ion.- This was confirmed '
by Bader et al(11)
and ::enKni:..-,t and Wehner(12)
but not by
','ahadevan et al(13) Medved(14) and Hagstrua(1 5) perhaps
illustrating the fact that reliable data in this field is
difficult to obtain.
The work of :redved et al(1 rt ) and Hagstrum(1 5) suggests that neutral bombarding species show two important differences when
compared to ionic species.
(a) Reflection properties are different. There is a
finite but low probability that the bombarding species will be
reflected from the target surface rather than cause sputtering.
This probability would appear to be about twice as high for
neutrals as for ionic bombarding species.
(b) As a consequence of the bombardment of the target,
secondary electrons are ejected as well as sputtered species.
The number of electrons ejected per incident particle is
measured by the parameter Z;i . Since these secondary electrons
are responsible for creating further ions and hence producing a
self sustaining plasma, it follows that the X;, value is critical.
It is observed that ZSz (ions) is greater than Xi (neutrals)
partly because the neutrals are more likely to be reflected and
partly because ions exhibit two possible electron ejection
mechanisms (potential and kinetic ejection) whilst neutrals only
show kinetic ejection. This Xi concept will be discussed more
fully in a later chapter dealing with self sustaining plasmas.
A theoretical calculation of deposition rate as a function
of pressure would clearly be difficult because of the many fac-
tors involved but Laegreid and Wehner(16) have shown that the
sputtering yield (atoms per incident ion ) begins to fall off
above 30 microns pressure as shown in Fig. 3.2
55
?. 2 Spnt ter r:, Yield as a Function of Pressure
YIELD (atoms/ion) 0-5
0.4
03 --
02
D•1 —
10 100 PRESSURE (microns) ( log scale)
This fall off above 30 microns is initially compensated
by the fact that more ions are present at the higher pressure.
The turning point is at about 125 microns and above this value
the increased ion current can no longer offset the reduced
yield and the deposition rate falls. The best compromise is
often found to be around 50 microns since backstreaming from
the diffusion pump often occurs at pressures as high as 125 microns.
Since in practice, film quality may well determine pres-
sure choice, it is usually preferable to choose a suitable
working pressure and concentrate on the voltage and current
parameters for manipulation of the deposition. rate.
3.2.4 VOLTAGE B7-:7;;E:717 T PI T A: 1 SUBSTRATE
The variation of sputtering yield for copper with argon
ions has been studied in some detail by many workers(16-20)~
Since a glow discharge produces an ion energy distribution at
the tar,et(10), most of the above work has been carried out
using ion beam techniques and some care would be needed if
precise information was required for a glow discharge sputtering
application using these data.
YIELD (atoms/ion)
8 -
6—
56
Some of the main points to emerge are that the sputtering
yield is a function of the crystal plane exposed to the ion beam and that the state of the surface of the sample is an
important consideration. Not surprisingly slightly different
results are produced with different copper targets showing that
purity is important.
All samples seem to exhibit the sane' general trend (Fig.
3.3) for yield as a function of incident energy sho '. ing an
exponential rise above a threshold value then rising linearly,
less than linearly and approaching a flat maximum at about 25KeV. At higher incident energ ies the yield falls slightly as
the probability of ion implantation increases.
Fig. 3.3 Sputtering Yield as a Function of Incident fon Energy
10 20 30 ENERGY (KeV)
Summarising these results, it would seem that if high
deposition rate was the sole criterion of the deposition
process then there would be no point in having a target poten-
tial of greater than 20KV and little point in going past about
8KV since the yield does not increase much above this value.
The chosen experimental sputtering voltage is', however, usually
much lower than this for reasons which will be discussed in the
folloviing sections.
57
2,a,1272- : r q n. ; T .7 '.', ; : :'. `. ".. : S C H.: RG E
Since the _,ember of io.ne striking the target is proportional
to the current density, then for ions of the same energy the
amount of material reine ;'e'1 from the target should be proportional'
to the current being drawn by it. We might, therefore, expect
that as the current is increased. at constant voltage the depo-
sition rate will also increase. The conclusion is based on the
usual assumption that spattering yield is not itself a function
of current though work by Jackson(21)
on contamination and damage
effects has led to doubts on the validity of this.
Some interesting work was done by Guseva(22) who studied the
sputtering yield for copper/argon and copper/copper systems in
the energy range 3 25 KeV. The following results were obtained.
Flee 3.4 Sputtering Yields Against Current Density
A YIELD 10 __(atoms/ionl
8 --
100 200 300 CURRENT DENSITY p.AA cm")
For low ion currents the yield depends on target current
density but above 100/& A cm2 this is not true and the curve
is relatively flat. This effect was interpreted by assuming
that a surface layer (presumably oxide or nitride) tends to form
on the target material and that higher current densities are able
to demolish it easily and are successful in preventing reformation.
58
as sputtering proceeds. The build up of oxide layers causing
low sputtering rates in aluminium or some types of refractory
metal is a common problem but is not usually considered for
materials such as copper which sputter easily.
A further interesting re.:ul t from this work is that copper/
coper yields are about a factor of. two higher than copper/argon
yields supporting the theory that S oC :'.1d 2/i ~'1 + ."2)2
n where S is
the yield and Mi M2 are the masses of target and gas atoms
respectively. On this theory, maximum sputtering of copper will
be produced by copper ions.
A reasonable assumption would seem to be that as the dis-
charge current increases the sputtering rate also increases.
The consequence of this is that if high deposition rate was the
only objective then the hig:.est possible discharge current ought
to be drawn.
3. 3 TiiI; CO=PT OP ..-,,, r•i I :T SPUTTERING
Of all the factors influencing deposition rate, current and
voltage are perhaps the most important and most easily understood
and the initial part of the work was concentrated on these two
parameters.
The deposition rate of tantalum films as a function of vol-
tage and current has been studied by Schuetze et al(23) and is
summarised in Fig. 3.5.
1000 2000 3000 VOLTAGE
0.7 -
06 RATE (microns hrl
0.5
0.4 -
03 -
02 -
0.1
15mA
5mA
59
Pic'. 3.5 De^ositicn. Rite c` Tantalum Under Varyin7, Conditions
Consider the effect on deposition rate of doubling the
power input (volts x amps) to the target in various operating
regions. Starting at 5mA and 1000 volts we can see that doub-.
ling the current at constant voltage would approximately double the deposition rate whilst doubling the voltage at constant current would-produce a five fold increase. Whilst the change
in power input is the same in both cases, the resulting depo-
sition rate change is different and it would clearly be
preferable to produce a voltage increase under these conditions.
Repeating this exercise starting at 5mA and 1500 volts we can
see that doubling the voltage or doubling the current will
produce about the same change in deposition rate but at voltages
in excess of this 1500 volt figure it is change of current which
produces the greater change in deposition rate which has now
become fairly insensitive to voltage charge. These figures.
would, therefore, seen to suggest that the optimum sputtering.
voltage is about 1500 volts (considerably lower than the 8KV mentioned earlier) and that if high deposition rates are required.
they would be best obtained by choosing an operating voltage of
1500 volts and then increasing the current as. auch as possible.
60
In attempting to achieve high deposition rates, one pos-
sibility is to build a 1 rge power supply on the assumption
that as voltage and current keep increasing then the deposition
rate must also increase. Whilst this idea would seem to be
valid there are three important objections:
(a) Although the size of the power 2 r sut,ply could theoreti-.
tally be scaled up indefinitely, cost would eventually become
an important 'consideration.
(b) A certain percentage of the input power will be
converted to heat at the substrate. Eventually the substrate
temperature would become unreasonably high thus limiting the
allowed input power.
(c) Large power inputs to a target lead to large thermal
stresses and there is a limit to the heat input Which can be
sustained before damage results. This would suggest that the
power ought to be introduced as efficiently as possible,
presumably by choosing an optimum voltage and then producing
power increase by current increase at constant voltage.
The efficiency of a sputtering system will be measured in
terms of the amount of material deposited for a given input
power. Vie, therefore, use a 'reduced rate' defined by:-
Deposition Rate at the Substrate Reduced Rate = Power Input in Watts
This 'reduced rate' is a convenient parameter by which to
measure the performance of the sputtering system.
If the curves of Fig. 3.5 are replotted as reduced rate
against voltage, the results are as shown in Fig. 3.6.
61
fi x 5mA 15mA 10mA 16
Ve t?. 7 .6 Vc1uon:3. ~?t' Arai..=' Snutter!ng Voltage
2000 3000 VOLTAGE 1000
RA TE/POWER x10-3
24
Whilst these results would seem to support the idea: of an
optimum voltage, it does not follow that the same optimum vol-
tage could be expected for systems with a different geometry.
Since there are several factors which can affect the deposition
rate, a good theoretical model correlating the rate to the input
variables would be di_'ficult to produce and a practical cali-
bration would. be needed for each individual system. Sets of
data for reduced rate against voltage were obtained for a
cylindrical magnetron system which was constructed and the
results will be presented in a later chapter.
3.4 PL 3:A EFT'T (;T F'-r;Y
So far we have discussed the choice oT operating parameters
to obtain minimum wastage of the input power; we now turn to
improving the efficiency of the plasma itself. Since electron
impact ionisation is largely responsible for the creation'of
ions which ultimately cause sputtering, it follows that elect-
rons are the key factor in producing high deposition rates.
There are two obvious requirements, both of which we would like
to satisfy if the plasm?. efficiency is to be improved.-
62
(a) To increase the number of electrons present in the
plasma.
(b) To make better use of those 'electrons already present.
3.4.1 =ODE s' 'S
Electrons having . _:?ro ri_lte energy can be supplied ther-
mionically from an indepen'dent source so that ionisation does
not depend primarily on secondary electrons being emitted from
the cathode. A typical schematic arrangement is shown in
Fig. • .
~.... 3.7 Triode System
TARGET -2 KV
I ANODE
THERMIONIC
+f00V EMIT TER
SUBSTRATE
The anode draws electrons from the heated filament into
the main discharge region and is biased positively with respect
to the substrate which is, therefore, protected from .electron
bombardment. Applying a high voltage across the target/substrate
'electrodes ensures that the target is subjected to a flux of
energetic ions as usual. Deposition rates of 1C00 °A min-I
are feasible using this type of system.
63
~ae~ctic field I, it ex~erie~ces a force K x V so th~t if the
velo::;i ty is p:lral:l.el to t:le :::'!.3!'letic fiel:i it is una:fected
whilst i: it is pe'!"pe:::li<!t.:.l'J.r :~ t:le field it experiC!.8es a
fo!"ce at right angles to both it::elf and the :r;3.r?;netic field
directio:l re:.;ul tir.g in ci.rcul:1.r r:!otion a':Jbut the :.1ClC;1:etic field
line.
In general the path o~ an e~ectron in a m~gnetlc field is a helix but for the special c~se in which the direction of
motion is e~tirely at right a!:slcs to the ma.gn~tiG flelj direc-
tion, this path th~!l sir;-.plifies to an arc of a circle.
If the ~agnetic field is applied perpendicul2rly to the
electric field in a sputterir-g syste;n .'13 in a ma[,;lletron dC'fice,
then seconda'!"y elec trons 1·~8. ving th(~ tarGet surfac e nOrI:1ally t
will follo'lI a curved tr2.jectory (not cll'cular) as L:'licated in
3.8 which assumes the absence of collisions with gas
atoms.
Fi8_ 3.8 Electron Trajectory in n ~~Gnctron Confi~uration
ELECTRIC FIELD
;." /
I
S S \
'~
--'--7 DR! FT DIRECTION
1 8 HAGNETIC FIELD
----., ...... , .... , electron path '" ' '"" ~ " ~ \ / ;' , \, \ I \ I V ,
\ \ \ S S \ S:::s::J
TARGET
64
This curve shape will be analysed in detail later. Once
a collision occurs, the electron nay be knocked out of the
trajectory in the plane at right angles to the electric and
magnetic fields and the path taken would then become helical and would be much more difficult to analyse. 77e could,
however, still say that there would be a general drift in the
direction indicated by the • arrows on Fig. 3.9.
A longitudinal magnetic field (anode - cathode direction)
which is parallel to the electric field is often-used to advan-
tage in planar devices and is successful in reducing electron
wall losses by restraining the motion onto orbits round the
magnetic field lines and channeling the electrons into the
anode. This configuration, however, has no effect on the high
energy electrons discussed in chapter two which move parallel
to the magnetic field without collision and are, therefore,
unaffected by it. Whilst the arrangement is successful in increasing plasma efficiency and, therefore, deposition rate
by reducing electron wall loss, it contributes nothing to the
control of substrate temperature.
A magnetic field transverse to the electric field direction
(Fig. 3.8) would inhibit the high energy electron effect by
deflecting these electrons away from the substrate but unfor-
tunately the drift motion tends to concentrate the current onto
one side of the plane target. The resulting asymmetry is
detrimental to film thickness uniformity since most material
is ejected from one side of the target. A similar effect could
be produced using a longitudinal magnetic field with a target
offset from the centre line axis of the system.
It should be pointed out that the magnetic field strengths
generally used are not strong enough to significantly influence
the ions in the discharge. We assume that the only charged particles to be affected are electrons. It has been noted(24)
that at very high magnetic field strengths, (of order 1T) the potential distribution across the discharge is altered
significantly and the system behaves differently.
CATHODE
65
3.4.3 T: P '...T..0
Perh ns thr' e t known ar_:, u:L.cnt incor oratin'; electric
and magnetic fields is the 'Fermin; Cell which is shown in Fig.
3.9 Fi- 3.9 The
CATHODE ANODE
This device uses two cathodes, one at each end of a cylin-
drical anode. A linear magnetic field is used to trap the
electrons and cause them to gyrate about the magnetic field
lines and reflect successively at each cathode until a collision
is made. The concept is most useful as an ion pump or as a
pressure gauge but is of limited value as a sputtering device
because much of the ejected material is deposited on the inside
wall of the cylindrical anode and cannot be conveniently
deposited onto a substrate.
TOP VIEW
OB
E.= ELECTRIC FIELD J
An alternative anproach is to use a cylindrical post
cathode with an axial magnetic field.
Fi? 3.!0 Electn)n Drift )tion in a Cy1i :ri^a-; acrnetron
66
ANODE TARGET
With this configuration the electron drift motions can
close on themselves as shown in Fig. 3.10, permitting a
uniform current distribution over the target surface. An
electron is, therefore, trapped in close proximity to the tar-
get and keeps returning to its surface not being able to
escape until a collision is made. Furthermore, after the col-
lision the electron will again be confined to some new orbit
and will only make progress towards the anode when a further
collision is made. Ion production is increased close to the
• cathode surface causing a greater current to be drawn by the
device. Since symmetry is preserved, scaling up to large sizes
is relatively easy.
The attractiveness of this cylindrical magnetron device
has been recognised by many workers and in recent years many
'novel' magnetron systems extending the basic • idea have been
constructed. Two of these devices will now be hriefly reviewed
as a prelude to our later work.
- MAGNET
ANODE
TARGET
SUBS TRATES
- - -MAGNETIC FIELD LINES
67 Z r ••il~.N' t -~m:~y• cY TE.,S
The follJi g : evi e:. can be considered together because
they have the basic similarity of being2 devices with the
subsequent modification of electron trajectories and impact
ionisation characteristics.
c. 1 ,-:T S ,,, r, (•rr„
The Sputtergun (Fig. 3.11) uses a per...anent tsagnet bolted
onto the back of the anode to produce a non linear magnetic
field. Electrons are 'trapped' in the magnetron field and are
forced to travel in long helical paths less than 5 m.r;. from
the target surface.
3.11 The S utters u.n
TARGET
One major advantage of this system is that the substrate
is isolated both physically and electrically from the sputtering
source and .goes not at tract high velocity electrons. The 'prob-
lems of having a cool substrate and a high deposition rate are,
therefore, largely overcome whilst uniformity control can be
achieved by altering the angle of the target,' typically to a
cone shape as shownig. 3.12. .
Va : MAGNET
-l.-----ANODE
TARGET (-500 V)
68
VZZ?ZZZZ177ZZZZZ//zjg -SUBSTRATES
rrhis arrangerilcnt h~3 the added adVl1!ltagc tha. t ions tend
to strike the target obliquely incre~sinG the deposition rate
and 'sprayinG' m~terial preferentially in the direction of the
substrate.
3.5.2 THE PLA::AH ~-AG:~E'l'ROH
'rhe prQble:fls of .usinG a planar t~l.rget VIi th a. magnetic
field have already been di~cutsed and led to the idea that a
cylindrical target was p::--eferable. The planar :na~netron
(Fig. 3.13) represents illl attempt at solving the electron drift
problems whilst retaining ~i planar gemcetry.
3.13 T .e Fl:...__ '. ..: etron
69
MAGNETIC FIELD LINES
TARGET ,
The magnetic field which is created by pole pieces behind
the cathode enters and leaves the electrode surface perpen-dicularly but is nearly parallel to the surface at the furthest
distance away. The region on the cathode surface between the curved magnetic field lines is called the area of erosion and
an erosion channel is created by arranging for the field lines.
to form a closed path as shown in Fig. 3.13. Under these con-ditions the E x B electrol drift motions close on themselves and a cycloidal electron path results within the closed loop
created by the magnetic field arrangement. This is shown in
Fig. 3.14 viewing part of the erosion channel only.
-Pic,]. 3.14 Electr3n Path._ in the Planar ..'agnetron
-•,' V
------>-- MAGNETIC FIELD LINE
---)--ELECTRON PATH ALONG EROSION CHANNEL
70
Whilst high deooxitio.. rates are achievable, one
e disadvan-
tage
an-
t_re 1: thatthe target is c Ād prefere: :tiNl_ r in the reg_or
of the closed trac:: and. typically only 255J of it is usable.
There is no obvious way to change to another track once the
first is worn and wastage of target material is, therefore,
high.
E xpi ^T' •T • ,
SYSTE .S •
Some experimental devices were constructed to investigate
several of the ideas discussed in the review of high rate con-
cepts which has been presented. It was eventually decided to construct a co:l:ention:l cylindrical magnetron to test some
basic ideas concerning the removal of energetic electrons from the substrate and a modified cylinder was also devised to see
if the concepts were still valid with a changed geometry. The construction of these devices will now be discussed
briefly.
3.5.1 CONVE':TIC`: L CY=RTCA , 7 AG 'ETRON
The basic design is shown in Fig. 3.15, the final assembly -
having approximate dimensions of 30 cm length and 3 cm diameter.
The target consisted of two copper tubes, the outer one
having one copper end plate brazed in position and the inner
one having two end plates with water connections through one
of them.
11
11
P.T. FE. SPA[ER
TOP PLATE
o RING UPPER COS SHIELD
y-t-t---ALUHI NIUH POWDER
t-trl-+-_INNER COPPER CYLINDER OjTER COPPER > )
L YLINDER I~~~I ~/ / 1:'1 ~- LOWER COS SHIELD
____ TI.J..l.-..--L.L.I---~-e-- BAFFLE PLA TE
The outer cylinder is co~pletely isolated from the cooling
water so that a water leak would not result in water entering
the vacuum chamber but :l good thern~al cont:)ct betVlee~1 the two
cylinders VIas ensured by packing the gap Vii th fine aluminium
powder chosen for its non m~gnetic properties. The inner
cylindp.r is, therefore, .easil.y demountable from the outer but
it was in fact left perm8.::1ently in posit-Lon once the efficiency
of the water cooling h::td bcefi established •
. The co:nplete a~,;:;e:;.bly was ~3upported from the top plate of
the vacuwn systeIrl but :~?:lced fro'::1 it using P.T.F.E. insulation
and 0' ri~gs as shown i~ Fig. 3.15. An axi~l magnetic field
was provided by a c~lr of Helrnholtz coils mounted externally.
72
End effects were ,eliminated by constructing two 'CDS shields,
the upper one being upported by the top plate of the vacuum
chamber and the lower one from a baffle plate assembly on top of
the diffusion pump inlet.
Without the baffle plate assembly, the discharge from the
centrally mounted cylindrical target tended to be drawn down
into the throat of the diffusion pump. .Apart from this possibly
causing crac'_i ng of the diffusion pump oil it seemed likely that
unwanted pressure gradients were being established and it was
decided to remove the effect by using the baffle plate.
Various series of experiments concerning substrate tempe-
rature, efficiency of the target water cooling and deposition
rate as a func :.ion of the adjustable parameters were conducted.
The results
7~will i~be
`
discussed in later chapters.*
.3.6.2 A '::O i/IFIED' rYLI.:Df:R - T'1TRODYJCTIOi:
A method of increasin7, coverage which is likely to become
popular is the 'On Line' deposition system in which substrates
are fed past a stationary target. A continuous deposition
process can be Achieved either by using an air lock to load
and unload batches of substrates or by feeding substrates past
the target in a continuous stream.
Unfortunately a cylindrical target cannot be easily engi- _
neered for either of these two possibilities:-
(a) In the batch process it is not possible to include too
many substrates in the batch since those near to the centre of
the cylindrical target will receive a thicker coating than those
near the ends - this will be discussed in the next chapter.
(b) In a continuous feed process (Fig. 3.16) the shaded
areas of the target surface can contribute little to the coverage
of the substrates as they move along on the conveyor belt.
When a substrate is a long way from the target (having just
been fed into the system) the deposition rate would be low
resulting in poor film quality. Whilst this problems can be
overcome by using a mase to keep sputtered material off the
substrates until a position is reached where the deposition rate
73 Fir-. 3.16 'On Line' re.ociti cn
E--
ARROWS SHOW DIRECTION OF TRAVEL OF SUBSTRATES
would be high, a consequen.e is that about 50% of the target
area is wasted. A better arrangement is shown in Fig. 3.17
using a rectangular target.
Fig. 3.17 'On Line' Deno!:i Lion. Rectangular Taruet
••>. ->, -~- --;. ~-
-E'
- - - ELECTRON TRAJECTORY
74 The wasted area is now clearly reduced. Thilst.the
arreenr7emeht of Fig. 3.17 may appear to be simply two planar
tar:7ets bac: to beca, it does have further advantages. Assuming
that it behaves as a cylinder (a modified cylinder) then the
symmetry problems of the planar system have been overcome since
the electron trajectories will now close on themselves in the
mashetror field as shown by the dotted lines on the diagram. A target as described above was constructed on a small
scale to see -if the new geometry was a feasible alternative to
the conventional cylindrical arrangemeilt.
3.6.3 A '=DTT,IED CYII77EF:' - CT:=CTTO7 DETAILS
The basic design is shown in Fig. 3.18. The rectangular
target assembly (8 x 0 x 1 cm) was supported by metal studding
which was screwed onto a vacuum feedthrough mounted through but
insulated from the top plate of the system. Sputtering of the
studding is prevented by a glass CDS shield whilst the top of
the target is protected by a metal CDS shield supported on two
ceramic spacers set into the top of the target.
Fig. 3.18 A 1 7.odified* Cylinder
NON MAGNETIC STUDDING
GLASS CDS SHIELD
DURAL CDS SHIELD
CERAMIC SPACER
8 cm
RECTANGULAR '1AC117 COPPER TARGET
75
Cleaning the glasss after about five lours ru iinig time was
found to be beneficial for r prolonging the CDS shield life to
about thirty `lours. A ceramic shield would probably last longer. Attempts at shielding the studding by plasma arc spraying a
layer of alumina ont it were not successful but were not
extensively investigated since the glass shield could easily be
replaced.
Power was fed into the target along the studding; water
cooling could also be incorporated by the same method in a
larger design.
The first model incorporated a CDS shield supported from
the baseplate for the bottom of the target but this was sub-
sequently removed since satisfactory deposition could be •
achieved without it.
One possible problem was thought to be the sharp edges of
the target shown in Fig. 3.18 and the geometry shown in Fig.
3.19 was used in the initial trials.
Fig. 3.19 Initial r'odiried Target - Too new
8cm
8cm
..j ,''0•5 cm
Experiments showed that the target would sputter satisfac-
: torily whether the edges were rounded or not and subsequent
models used a simple rectangular geometry without the edges
rounded Off; this shape being much easier to construct.
Apart from the possible 'on line' advantages, this rectan-
gular shape may also be easier to fabricate than the conventional
cylinder in some cases.
76 . 3.7 T)EP0SI7T0:: _V'S
Exnerimental results for the two targets will be presented in detail in later chpters, here we should siciply mention that deDosition rates of about 1000
oA tnin
-1 were easily achievable,
though the jstems were usually run at lower levels than this
to avoid the risk of overloading the power supply.
As previously discussed, our intention wra.::: never to aim
purely for high deposition rate at the e.xclusion of other coil-
siderations and no - ei::phasis on the actual rate achieved will be made in the results presented later.
77
! =.._PT F. 'POUR
4.1 T ` __ _S °._ F OT:.71 EI~...
4.1.1 I :TRT 1'; `:'": 0',
One of the usual prerequisites of a successful film is
that it should exhibit good uniformity throughout the coverage
area. ':Whilst this quality may be of little value in reflection
coatings for example it is clearly of vital importance in any
film to be used for electrical conduction purposes since resis-
tance is a strong function of thickness. Although large area
uniformity is unlikely to be of importance in the construction
of a single small device, it is of interest if a batch of
devices is to be fabricated with comparable characteristics.
In device fabrication, it is usual to deposit a uniform film
and then use photolithography and etching to define the required
pattern since this provides more precise patterns than can be
obtained by mechanical masking during deposition. This is
especially true for sputtering applications where tine short
mean free path of sputtered atoms means that sharp line edges
are unobtainable using mechanical masking.
In this section we consider briefly some of the factors
which can influence film uniformity and then discuss hog°, it
could be improved when appropriate.
4.1 .2 SOLI?) A_:GLE
The solid angle subtended by the target at the substrate
is significant since it gives a measure of the percentage of
ejected target material which can be expected to strike the
substrate. Whilst thi amounts to saying that the smaller the
target/substrate separation the greater the solid angle and the
greater the deposition rate, a long cylindrical target (as dis-
cussed in chapter three) is likely to produce additional
problems as shown in Fig. 4.1. •
4.1 IIon Tarr7et
78
TARGET
c<= ANGLE SUBTENDED AT SUBSTRATE EDGE )3 =ANGLE SUBTENDED AT CENTRE
It is clear that p > a. and we might, therefore, expect to
observe more sputtered material at the centre of the substrate
plane than at the end. Whilst this effect is likely to occur
with any target geometry, the variation will be greater using
a long cylinder.
4.1.3 THE :7A7 FREE PATH OP SPUTT=D SPECIES
The conclusion of he previous section is not totally
valid since it assumes that sputtering is a line of sight pro-
cess with atoms following linear trajectories from target to
substrate. Scattering by ar;,on as atoms will in fact modify
the observed dist-f.ibution and is likely to completely dominate
the deposition if the target/substrate separation or the pres-
sure were very large. It is usually assumed that these latter
cases are unlikely to be found in most sputtering systems aiming
at a reasonably high deosition rate and scattering is ignored,
principally because the analysis required becomes complicated.
79
Recently. Westwood(1) has pointed out th7:t this line of
sight `Oie. is _a ):r O 'l at e to tt usual diod e sputtering-con-
ditions where scattering is important and has suggested an
alternative idea which he uses to deal with sputtering through
an aperture onto a substrate. He suggests that the tra.:sport
of atoms to the substrate is controlled by diffusion through
the gas once the sputtered atoms have travelled a Se':! Ci"e.an
free paths from the target and have been scattered by gas atoms. We should, therefore, consider an almost plane front of atoms,
whose dimensions are approximately the same as those of the
target, wite a uniform density at all points in the front. A film thickness uniformity problem would, therefore, not exist
provided the substrate was far enough away so that sufficient
collisions could take place to set up the uniform density atomic
wavefront. Thearetl s'a1 calculations suggest that this minimum
substrate distance is about five centimetres for diode sputtering at 30)U pressure but the number can be varied by a factor of
about two depending on the assumptions made.
Since a typical target/substrate separation for diode
sputtering is about five centimetres, we are just in the region
where this effect is beginning to become important though it was ' thought to be less significant for magnetrons where the pressure
is typically lower and scattering therefore less pronounced.
Westwood also points out that there will be distortion of
the plane front near to the edges of the target because of
diffusion parallel to the front. He therefore, also predicts
ā thinner film at the target extremities but for different reasons.
The expected film thickness distribution profile will be
considered in more detail later in this chapter. 4.1.4 THE fISTRI';UTIC. OF SPIJTTRRET) ?.1TERIAL
The angular distribution of mterial sputtered from tar-
gets has been studied by various workers( 2- 4)
, the results being
summarised in Fig. 4.2.
OVER COSINE
COSINE
UNDER COSINE
N N
80
4. ? ...1R An ;.'.."' Di t.'l ,. ,;tio.'- o: S'_ wt tered :ate-in?
TARGET SURFACE
In general the distribution is found to be approximately
cosine for target potentials in the range 1 - 3 ?V tending to
over cosine at higher voltages and under cosine at low voltages.
In the case of oblique incidence, ejection is favoured in the
direction of specular reflection for the incident Lori. Target
temperature or current density apparently have little affect
on any of the distributions.
4.1 . 5 CO777:S?TT ;_. COE .A'CIP.'T
As discussed in Chapter One, the condensation coefficient
for sputtered species is a function of substrate temperature
and is reduced at increased temperature. Since efficient sub-
strate cooling is usually difficult to achieve, the substrate
usually heats up during deposition and we might expect a thermal
gradient to be established along it under certain circumstances.
Whilst a metal film would not sustain a temperature gradient
(except perhaps when it was very thin) it should be possible for
this sroperty to be exdiibited by insulating films (for which
D. C. s:;uttering would not be successful) and for films deposited.
onto tie .:rate s.., _! 1 substrates which were physically isolated from each other.
81
In this :itu,t 7 or the substrates in the centre May be
hotter than those at the extremities. This will tend to reduce
condensation and hence lber deposition rate in the centre and
therefore oppose the solii anle effet which indicated increased
deposition in the centre. The temperature gradient which the
substrates would typically experience is, however, quite small.
and is unlikely to croduce a significant change in deposition
profile. It is usual to ic;nore this effect and consider a
sticking factor of unity irrespective of temperature.
4.2 EX7EaI=TA". 1.777:7-1A71-7= T-TOr: A CYLT7D7R
In view of the conflicting factors affecting uniformity it
was decided to test the cylindrical target experimentally to see
if a distribution problem existed, no thought being given at
this stage to the extra possible complication of using a magnet-
ron system. . The target geometry used is shown schematically in
Pig. 4.3, the microscooe cLide glass substrates being held in
a gantry which was supported from the top plate of the vacuum
system.
111-. 4.3 The 7.7.2.r.netron. System
E.
SUBSTRATES
TA RGET
82
Early problems were encountered in the achievement of
accirstable repro:luoibility and befbre pxese tirig experimental
results it is necessary to discuss the tech:AqueS required for
the production of satisfaotory readings.
z1.2.1 777,A=7"
Four possible thiekness measuring techniques were Given
cenc,ideration.
(a) A mechanioal measurement of step heiGht using a
Talysurf apparatus: In this techfAque a stylus is mechanically .
trac1-zed over a step ma7le by depositing the film onto part of
the substrate only. The film thickness is LLeasured from a tape chart output. This method is most suitable for hard films which
will not be easily damged by the stylus. A sharp step is
preferable though not essential.
(b) An interferometric technique using a Varian interfero-
meter, the theory of which can be found in any standard text(5)
A step film is produced as with the Talysurf method and the
whole substrate is they: metallised, usually by evaporation; with
a reflecting layer of silver or more commonly aluminium. Since
the step is faithfully reproduced by the aluminium coating,
reflection interfererce patterns can be obtained from light
reflecting at the tor. and bottom of the step. The spacing of
the i_terference fri:fgas is related to the step height which •
can, therefore, be measured.
A well defined step was found to be essential if satisfac-
tory fringes were to be produced. Also the aluminium coating
needed to exhibit good reflectivity for well defined fringes.
This reflectivity could only be achieved if the evaporation system base pressure was better than 5 x 10 torr prior to the com..4=ncement of evaporat4 on.
(c) A resistivity technique in which the resistance of the
film is measured by depositing electrodes onto it. This idea
was not investigated experimentally since it was felt that
electrode contact resistance problems might he difficult to
overcome ani in any case non uniformity of the film would defeat
it.
83
(d) A 1Tethod of direct weight gain in which strate is
wei7hed both 'zefore after depooition. One d'ifficulty here
is that the :7.ht not exhit bulk density though it
should be r: Lf olese Us: C5 tne film “..,s very thin.
Choice of substr-Ae wou1.1 .sesubly be ir:.port'ant though the
idea vis not researche:: in any depth since the interfercetric
technique was found to be If:iving aocel:table results.
The first two fnethis were e:ktessively investigated, both being tried on the sa!:.e fili by making a mecnanic;n1Aep measure-ment and then ::Letallising for interferometry. It soon became clear that the interferoetric Z:echcique was more reproduCible; practice in using the instrument leading to repeatable measuring
to 40 GA 11 5000 eA (an error of 0.9).
This method is most suitable for films in the thickness
rn-nge 2000 °A. 10,000 °A because of the way in which the
interference fringe patern (Pig. 4.4) needs to be analysed.
4 t Inte-Per1::leter Fr!h7e Pat terns
( (41) .
A good quality fringe sycte::: is shown in (i), the deviation of the fringes being proportional to the step heiht of the
film. It is necessary to evaluate the fringe displacement a/b,
diagra,m (ii) showing a systez: where a b and. the step
height is in fact 29470A (hIlf the wavelength of yellow sodium
light).
84
A dislace::.ent of exactly three fringes corresp:onds to a
hei7ht of 2'=41 A hut once a larger step is. used a co!:-Tli-
c ,, tel =- inge t.i ,' ture such as is. (iii) often e:erges and it can
be d'ficu7 t to decdo treisely how larEe the displaceent is.
Thr, calculatel c3ull in fact easily be in error by units
of 2947 °A if the nut1er of fringes displaced was estimated we scull use the Talysurf to give
an approximate thickness and then use the interferolneter, but in practice it is easier to restrict filt, thickness to the
preferrel region Around 5000 °A by adjustis.g'deposition times
accordingly. The following procedures were found to be advantageous:- (a) Using - photo-resist to mask a sharp edge to the film
on the substrate with subse'iuent reoval of the photo-ref3ist layer. :.:echenil makiln;: by putting a layer of metal foil in
close proximity to the cutstrate was found to he unsatisfactory
in agreement with Westv.00d(1)
and !,.leny other authors.
(b) Depositing the film at a suicieutly high rate such
that a shiny 'copper li!te.' aeosit was produced. Liquid nitrogen trapping was also beneficial by preventing backstream-
ing diffusion pump oil fro:n coAaminating the film. Copper
films which appeared .dull did not generally metallise success-
fully for interferoetrte work becnuse the reflectivity was
'too low.
(c) Pre-sputtering usig a large shutter to prevent depo-sition on the substrates us til such time as the plasma operating
variables (voltage, curreht and pressure) had stabilised. This
problem has already been discussed in Chapter Two; presputtering
for at least thirty minutes being required. This plasma eqibriunl was found to be esnential if film thickness .as to
be -n.-produced on successive experi.:tental. runs.
2.2 F=.171=2
The initial experiments were .arried out using the system
shon in Fig. 4.3 but with no magnetic field. Sputtering at
30)u pressure and target potential of 27/N the thickness unifor-
mity curve of Fig. 4.5 was observed with a target/substrate
separation of five centietres.
85
7x7=tal 7-117.,,et
THICKNESS
5000 --
4000 —
3000 —
2000
1000
5 10 15 20 25 30 LENGTH ALONG CYLINDER (cm)
As expected, the film is approximately twice as thick in
the centre (15cia mirk) than at the extremities. When the
experiment was rt7Teted using the magnetron configuration
(magnetic field present) at 500 volts, the thickness distri-
bution at the substrate was not significantly different. This
is not surprising since 955 of ejected sputtered material is neutral and is, therefore, unaffected by the magnetic field
which increases the deposition rate but does not alter the
relative spatil distribution of material.
4.3 I7T7,07171 77,7 1.1';TY3R7.7ITY
Having illustrated in Fig. 4.5 that the anticipated
distribution problems does exist for the qTliildrical arrange-
ment, we now need to look at waysof improlAng the distribution
profile. The following possibilities merit consideration:—
86
(a) :i:ov1 1:g the substrate further away from the target.
This is not a very elegant solution since a reduction in
deposition rate wou?_d h:/e to be tolerated.
(b) ovin the target tst the deposition is in orogress
so that the substrates are in effect 'sprayed' with sputtered
material. ProbleJ,s would .:learly h.- ve . to be anticipated
because of the need to move target and water cooling connections whilst they are electrically live. Since the target would
require movement in a vertical plane, vacuum feedthrough diffi-
culties might also be anticipated further suggesting that this is not a very attractive cozsibility.
(c) :,7oving the substrates whilst the deposition is in pro— .
gress. Whilst this would be preferable to target movement, it
would still lead to wasted space since vertical movement would
again be required. This would still present difficulties for •
bias sputtering applications because of the need to move
electrically live parts.
(d) The use of a split target with differing power inputs
to each section as discussed briefly by Hajzak(6). The thick-
ness distribution of Fig. 4.5 would be reproduced approximately
with equal power inputs to each section but the distribution could, presumably, be made more uniform by supplying more power
to the outer sections of the split target. This idea was investigated in some detail using a thin straight bar rather
than a cylindrical target since a sectioned cylinder seemed to
provide unnecessary constructional problems.
4.4 SPLIT TARGET IVI.P7PT.77=1. SYSTFA
4.4.1 G=HAL SYSTFd
The first experiments were carried out using a single bar
(one piece) to ensure tht a similar distribution was produced
to that using the cylindrical target. The most convenient method was to support the target from the vacuum system base-plate using cefamic spacers (Fig. 4.6) and to sputter upwards
rather than horizontally as with the cylir.drical target.
CERAMIC SPACER
SUPPORTING PLATE
TARGET BAR (later split into
sections )
BASEPLATE
TO DIF FUSION PUMP
87 Pig. A.6 Ex7-my..imnt:,.1 Target
SLIDE SUBSTRATES -GANTRY TO SUPPORT MICROSCOPE
(adjus table height) 1
The thickness of thc.ceramics was chosen to be less than the CDS length under the chosen sputtering conditions so that the supporting plate could act as a CDS shield for the under-
side of the bar. This arrangement of coutterisg ucwards has the advantage
that the target geometry can be changed quickly since a per-
manent supporting gantry is not required. Whilst a 30 cm bar would have been preferable for a
direct comearison with .the cylinder, a bar of this length when
arranged horizontally would only just have fitted into the vacuum chamber and it was felt that the side walls might intro-duce spurious edge effects. A 20 cm bar was used to eliminate this possil:Hity.
Using 30/A pressure, 2KV target potential and a target/
substrate separaton.of five centimetres, the result given in Pig. 4.7 was obtained.
68
.71-7. 4.7 Sir 1e :a-- - 7z7e-imental Results
THICKNESS ;4) 5000
4 000 -
3000 7
2000-
1000-
I I i I
5 10 15 20 LENGTH ALONG BAR
This result shows the bar system to be a reasonable analogue
of the cylinder (Fig. 4.5). The end thickness is 42 of the
central thickness for the bar and 46% for the cylinder.
4.4.2 FIVE TIR SYSTE77,
Preliminary experients in building up a split bar of total
length 20 cm showed that five sections was the optimum choice.
Whilst it would be desirable to have as many small bars as
possible to maximise the variability of the system, it was found
that the individual sections became too difficult to handle if
they were much smaller than 4 cm in length.
The bands system c3ncisted of five bars each of s length
3.9 cm laid close to bat insulated from each other. Five poten-
tlometePs were used to vary independently the potential on each
bar and, therefore, the current drawn by it. A schematic
diagram of the electriCal arrangements is shown in Fig. 4.8.
89
PLASMA
ANODE AND SUBSTRATES
_ indicates limits of vacuum chamb©r
POWER SUPPLY
The followig point were fJund to te iu..portant:-
(a) The of !putteri ,at thr vacuu feed-
throuchs and alon th,) cor.necti.v the feedthrouhr3 to the
cthcr.rice th, curre:1t
reaiins be tc,c jh I the pote;:tLreter setcs incor-
rect. The c;onneti.i:j wire oLto them
to act a a QDS the fee,Ithrohs ';re fitted with
T7;C sh!.]ds.
90
(b) The target/substrate separation must be the sae for
all f'7e bars or %r. acyetry de-velcs in the unif:Jrmity
It was, threfore, ncessary to ensure that the tet
all. the :=1 ht off sup:)rti. plate and tne substrate
bar was narliel to the surfs..::e of the tar:ets. This was
checked with a travelling microscope.
(c) Exneriments on altering the potential and hence cur-
rent on one bar o.:.e1 tnat this did not sg -ifi(:antly afect
the sae parameters . oc the other four bars. The targets,
there.'ore, work :7 :1decenently simplifying any adjustment.
Since ',111 five bars are linked to one plasma it mi;7nt be expec-
ted that change:! in ccc part of the cyte:: would affect the
behavioar in another but this plasma coupling affect does not
see;:. to be strong.
(d) As the supply vo'ltage is increased, the targets all
draw more power but in the sa:::e proportion. The thickness
distribution does not appear to be altered significantly by the
choice of supply voltaze wnch is net a critical parameter.
(e) Pressure chnii:e did not significantly affect the uni-
formity results though the range studied (30/u - '72.1) was
admittedly small.
4.4.3 7177":77=T A7. pn!--,ulTs
Consider a series of experimental runs, all at 39/4 pres-
sure, 5 centimetres target/substrate separation and a power
supply settinz of 2.5K7 (though not all of this will appear
across the discharge 25...Ice some will 'se dropped across the
potentiometers). The table on the next page shows the currents
drawn by the five bars for five different experiments. The
unifority results are shown in Fig. 4.9, the positions of the
bars also being indicated. Deposi tior. times were adjusted to
orAuce films ;:f about the preferred thickness (5000 o
5000 —
4000 —
3000
2000 --
1000 —
I
8 12 16 4 20
91
RUT:: ER
7AP 1 (NIA)
.7.,..F: (,:::. A,)
2 '.. AR 3 (::, A)
T.':1/ (--n A)
4 2AR 5 (rr, ;,)
TOT A.:, 01.7.RENT (7': A)
a 3 3 3 3 • 3 15
1-, 3.1 3 . 2 .9 3 3.1 15.1
, 3.2 2.9 2 . 8 2.9 3.2 1 5
d 4.2 2.9 2.8 2.9 4 . 2 17
e 3.5 2.35 2.75 2.35 • ..) , •) r 15.45
^ THICKNESS A
BAR 1 BAR 2 BAR 3 BAR 4 BAR 5 . 3
LENGTH ALONG BAR (cm)
92
(a) ',7-1.07 the e'fect of runz.inr-- all tho bars at equal
ou.:rent and iz; thercfre, siu larto the earlier rezolts pre- 2:e:Ited for a s. -inF,le ('c) ani (c) sh,; the result of
radually th., current to thc cuii bars. (d)
shov:s better unifor:Lity touj-h over-correction 7,1th the ext 7-elr.ities noN cotributiliT, too mush. Thee c::.rves :,hov, that .
in -..)rincinle it should be possible to achieve improved unifor-mity with a sectioned target, the be::+. attet (e) ;:howi5L2; a uniform film to within 4% over 75% of the substrate and 10% over all the substrate.
.Unfortunately, the improvements madefl.om (a) to (e) were
achieved totally by trial and error and we hn.ve no way of knowing
boa to use the inFormation ;ailed on a bar of differe -ct length.
It would obviously be an advantage to possess a theoretical
model for the observed distributions.
4. ST=LTFIED TT.:07.Y 1-20 TT.E. MTTK7= TTITPI7T5TION
Consider a bar L cm long situated a distaLce T cm from the
substrate (Fig. 4.10). F17. 4.10 TheTlry of ti1,7, Distribution
17ti TARGET
I I
dx I -x)
SUBSTRATE
93
If we assume:-
(a) A unifDrm c'_12rent dictritution over the tar,.;et area.
(b) A cosine dietr:bution of ::.aterial sputterel from each
point on the target surface.
(c) ro csilie:o:is of <ecte'l material with gas soiccuies. (This is not a very assumptiin since the mean frt:e path
at even a low pressure such as 5 micr7)nsis still only of
order 1 cm.)
We can show (Acoendix 1) that the ncnt of material
deposited (D) onto a small element of substrate of area dx is
given by:-
D 2T [ Kdx tan-1 (1-xl + (1-x) T
where K.is a constant ani I can take values from 0 to L.
Choosing some suitable values of L 20 cm, T 5 cm and evaluating the part of the expression in square brac'nets, the
following results arc obtained where x is the distance from
one end of the bar in centimetres and the thickness is in
arbitrary units of Kdx/10.
POSIT= EALONG
SI_TrTRAT. :,:. (cm) 0 2 5 7 10 13 15 18 .20'
xKqx 1.57 2.28 2.84 2.96 3.02 2.96 2.34 2.28 1.57 THICKNESS --- 10
If we normalise these results to fit the experimental
results of Pig. 4.7 at the mid point of the bar then the resul-
ting theoretical curve calculated from the table of values above
is shown by the dotted line in Fig. 4,11.
4.1
0
94
Ti7.. 4.11 Comrariscn 7etw,aen F.,::-:ri7ental Result and Theoretical
Prediation
0 THICKNESS A EXPERIMENTAL
-- -THEORETICAL 5000
4000
3000
2000
1000
4
16 20 DISTANCE ALONG BAR (clli
It ls seen that the two curves correspond very well though the experimental uniformity is slightly worse than the predicted theoretical result.
This comparison is surpri2ingly good bearing in mind the
simplifying no collision assumption which was made. Another
factor which has been overloked(7) is th.:tt the cathode dark
space 11-,s a focusing effect on ions ncar the edge of the tar-
get casing an increase in current deity in this region and consequential enhanced suttering.
Film temperature nas measured at different points in the substrate plane ucing therocouples. The method of measurement
is discussed in detail in Chapter six. Any temperature
varlatioa was found to be negligible and Could not, therefore, be reson,:ible for modifzTing the tir•-ess distribution.
0.6
0'2
BAR 12 16 20
DISTANCE (cm)
1.4 THICKNESS
(arbitrary units K (ix )
10 10
95
1 ''-lrr".7",- =T 77T-7 7-.-7,
Sinee the. tical correlation to the experimental
res1;_its ap::ears to 'oe quite 4 -ood, it seems wcrthwhile trying
to tend the tIlecry to ftvebnrs des:.ite reservations about
the validity of the basic a=”rptions.
Given that we can calculate the thickness distribution from
P sin1e bar, it should be ^cosible to ue the p:sinciele of
superposition to add the five individual contributions and pro-
duce a resultant curve. Since the bars are drawing different
currents, they will be eaiittin different amoants CI material;
this can be allowed for by using the current an a weighting
factor in the theory.
Tne fact that the Lors are at different volta,;es and will
produce slightly differing sputtering yields can be ignored
since the distribution of ion energies caused by Th,lr;e exchange
collisions (as discussed in Chpter three) will be fairly
similar.
If we use equation 4.1 to calculate the distribution pro-
duced along the whole substrate by one of the five bars only,
the shape given in Fig. 4.12 is produced:—
Pig. 4.12 Unifurmity Distribution From! A Single Bar
96
of 7alues
S-_,atrate
(cm)
Thic',:ness x7f7dx 10
0 1.16
2 1.45
4 1,16
o 0.643
8 0.3
10 0.139
12 0.07
14 0.037
16 0.021
13 0.013
20 0.007
If all of the five bars vere drawing the sa;:le current, they
would all produce the came distribution (symmetrical about the
maximum) only shifted qlong for each bar such that the maximum
of the distribution alv;ayc corresponded to the centre
bar v:hich prodsced it. We can, therefore, produce
table of values to show the relative contribution of
on the suttrate from each of the five barn separately.
Substrate Position flAR 1 3AR 2 BAR 3 1-',?.R 4 :BAR 5
(cm)
of the
the following
any point
Total xKdx 10
0 1.16 0.3 0.07 0.021 0.007 1.56
2 1.45 0.643 0.139 0.037 0.013 2.28
4 1.16 1.16 0.3 0.07 0.021 2.71
6 0.643 1.45 0.643 0.139 0.037 2.91
8 0.3 1.16 1.16 0.3 0.07 2.99
10 0.139 0.643 1.45 0.643 0.139 3.02
12 0.07 0.3 1.16 1.16 0.3 2.99
14 0.037 0.139 0.643 1.45 0.643 2.91
16 0.021 0.07 0 1.16 1.16 2.71
18 0.013 0.037 0.139 0.643 .1.45 2.28
20 0.007 0.021 0.07 0.3 1.16 1.56
97
b%sio LLe no.:: only eoh to be modifie,d by scaline,
u- '0 'Is the b -,r is
drain/. Con:..ider thi eu fexerturt e on
4.9 where birc one T:_a draW 3.5 mA, two and four
draw 2.C5 :nA rindtr Jr...,.ws 2.75 71A.
Havi:u fol:nd the r.,:odified co...trbution from eacl', hr to
each point on the :::ubntrate plane, the t'ive contri-
' butionL: are then adled to Jive the r.;itant i:..-:trution and
the re cult tJ the exi_eri:ente.1 curve at
the centre.
The re2u1',11.. copari::on betweerl the e.7r:,ntal read.;nzs
and the theoretical ;:ode1 :thown in Pig. 4.13.
Prc:diotion
EXPERIMENTAL - THEORY
I 4 8 12 76 20 DISTANCE
(cm)
5000
4000
3000
2000
1000
98
.::h1lct there ar,e i,oints of :7imilarity t the t'::o curves
it is ovus thtf'it is no: :7articul-?.rly 27cod. This is
not sur-:riin oicc, the thc-retial mei i: 1-7.:ic7.n to be
oversim?lifiel.
}-.e a::::arel-,t reaLonL7e fit between theoretical and
excerimental re ,ults fc.. acin.j:le bar doen not, in fast, T':.ean
that th theoreticc:. m.)ael Ic f:tfactol-y. The re it v:hen
the theory is extended to a multi2le bar system are not co ecouraing. A better ::.odei i_ccrporatin :c tein ideas
would be reqred before the cu-rehts nee..:ed to produce unifor-
mity.could bc calculated theoretically. 'hilot it is c.fortunate tht thf,oreticl 1-,redictions were
of little value in solving the. Ilhiformity roblem, it was
nevel-th,-le:: cs:-Jible to achive imnroved uniformity ging ,3
Solit taret cystem in a planer co_figuratin. Since results with a planar target and a cylindrical taret show a si:cilar
trend and intreducinG a ::i.Agnetic field did not appear to sig-nificantly alter the thickness distribution, it seen reasonable
to conclude tl'at a :-:plit target cylinder could be used to improve
uniformity in a manetron configuration.
The main purpose. of these experiments vas to demonstrate
that the unifo=ity prol;lem could in principle he overcome. It
wai' not inteded that this shoul be a major part of the work. Altelmative aoi.roacher: for future work are discussed in the final chapter.
99
7:71T
7.7 :POT' ) ?':A
A series o° tests wa:- :initiated ts.compare the conventional
r tr try to the f3J.) celled 'modified
cylinder' az discunsei in Ch.:,.pter three. This latter configur-
ation was thou=7ht tc have possible alv”.ntages over the
con_ventional arraemest in ter; of its m!pre convenient con-
stv'uction 3,nd 'on line' 1,1re scale benefits where the uniformity
problems as discussed in Chapter four would not he relevailt.
Initial results were encourac;ing in that the system
exhibited similar results to those observed usin a conventional
ma6netron. However, in an experiment to monitor the effect of
pressure chn,,e o: dischae current, the- characteristie.given
n Fig. 5.1 war oh rvo. 'ins a chart r3corder.
I Nal —3
B= 8x10 T
50-r
I I I 5 15 25 P(microns)
Fin . 5.1 Curreni: 1 v. Prersure n Fiela B S x 10-3T
100
I (ma)
100
/ /
100
Start._: at high pressure, as the pressure was reduced the
discharge current initially fell teadily as might be expected
:;til at some p Yes ?~:( t re was r sharp decrease w2 current re_t
for a neraigible re: sare change s'J.g4estintg that the plasma had .
in scne way s itc_._3 fr...:,m cre stable state to another. Further
investigation showed that this transition was both reversible
and repro...uci :le end a series of curves (Fig. 5.2) was t-l::en
for differingma.e ie field strengths. s.
Fil:. 5.2 Current T . v Pres,,ure s at m_f rerc" :t xraq, ..t_ic Field
StLp: Lh
-3 B2.$,10 T
, B = 7x10-3T _3 f3=6x10 T
' B =4x10- 4x 10-3T / / _3
/ B=240 T /
50
15 25 P (microns)
The magnitude of.the current change on switching with con-
stant field B increased as B ;r.:s increased but the pressure at which switChng occurred was not a strong function_ of B. :The
range of ca gnet.ic field variation was admittedly small being
.limited by the bzax:i sum permitted current to the Helmholtz coil configuration which was providing the field.
101
The results at 8 7_ I0 3T show the current approximately
halvin for negl;Ible :-,ressure change and it was observed
this the rlasma colour switched fro::: 7reen (cop; er dominated)
to blue (argon doriinatel), drawing less current and appearing
less intenf-e. This in the ourrent/prossure
characteristic is accompanied by funda,„..ental change in CD3
len;:th at the switching pressure (Fig. 5.3); these results
havin been Obtained by observing the discharge with a kathe-
tometer.. This is essentially a travelling :ilicroscope with a
long focal length objective lens so that the target edge and
CDS/ITG boundary can be observed from outside the discharge;
the CDS length being the diffese-ce of the two readings which
were noed on the vernier scale of the irstrument. •
Fir-r.. 5.3 crY3 Distance d v Presure n with 7 = 8 x 10 3T
d (cms)
2-0
15
1•0
0.5
1 1 5 15
4
25 P (microns )
102
Larger errors have to be tolerated at low pressures because
the definition of the ODS/::G ede'is not as clear. The error
of about 1mm at higher pressures is (aused by the fact that the
CDS/1.71 edge is not quite parallel to the target edge resulting
in variable readings depending on which part of the target the
observation is made. •
The variatien of d with pressure has been studied by
.kston(1) Who derived the empirical relaticnship.
A C • d = 4. (j)i
(5.1)
where A and C are constants and j is the current density
at the cathode. Sino C is usually small, the second term can
be neglected leaving a relationship suggesting the increase of
d as p is reduced but predicting a smooth transition rather
than -a discoetine;j • A relationship of this form was observed
experimentally for •a planar diode non magnetron arrangement and
at first sight there is no obvious reason why the introduction
of a magnetic field should produce a discontinuity in the
relation between d and p. •
Similar results Were observed using a conventional cylin-
drical magnetron indicating that the effect W9.2 not solely a
function of the unusual target geemetry. Extensive tests over.
a variety of current/pressure regions revealed that the switching
effect was totally absent in the absence of on external magnetic
field confirming that en investigation of the role of the mag-
netic field was necessary.
.2 TF 0L5 OF THF. !.7AG7777TC FTrLD
Consider an electron leaving the target in a magnetron
arrangement and being influenced by the perpendicular electric
and magnetic fields (Fig. 5.4). •
103
717..1_5.4 Elt'ctrJ .. "ect2rv
SUBSTRATE
NG
T •1 d WS
I .~ Yin , tg i .
i TARGET 1
E. = electric field strength B =magnetic field strength
The electron (charge e) will experience a force e. in the
direction perpendicular to the targ et/substrate plane and
e(v x B) from the magi:otic field ;there v is the velocity at
any particular in:Aant along the trajectory. A curved path
will, therefore, result and in the absence of collisions with
gas atoms the electron will return to the target surface pro-
vided the magnetic field is strong enough. In the case of a
weak magnetic field, the substrate or vacuum vessel containing
walls will intervene before the orbit can close on itself.
This situation is fundamentally different from the non magnetron
case where in the absence of collisions the electron is confined
to a line between target and substrate and has no chance of
returning to the target.
5.2.1 E=»_„m7.37 TDAT^-':'RY
Sup_ oce Y:7:0 is the maximum distance of the curved trajec-
tory from the target assu! ir.g no collisions and that an electron
has fallen through V volts to reach yMAX'
x>
104
Potential energy lost = eV joules ac.~d this will be true
irrespective of how the voltage drop i distributed between
target and substrate. This loss in potential energy will
appear as kinetic encrg7 of the movin electron.
2 L^ =
.2 -.eV
V _ _~t since at y ,, the electron of mass m MAX
is moving in the x direction only.
Applying Newton's second law in the x direction we can
write:-
mx = eyB.
Integrating with respect to time produces:-
mx = eyfl + K where K is a constant.
If the electron leaves the cathode normally then x = 0 at
y = 0 and we can set K = 0 simplifying the expression to
mk = eyB (5.2)
m e.2 ( Y, .A )2B
2 •
. . eV = mj
iii
Pr:V] 1 and Y"TAX =
If Vt is the target potential and VxM is the potential at
)( MAX, then V = Vt - Vyt and we can write:-
y MAX _ 1 ?2 (Vt - VrT) 1.
Before any further progress can be made it is essential
to know the potential distribution across the discharge so that
a value forVrm can be estimated.
~ S f.~ .T T<<. TTS TE r T TT Is T .T 5.2.2 ,1F:i S 7 :~:. 0 .. ~h D ~T T":U_IO.,
Aston() has le7onstr tei that the electric field strength
across a glow discharge falls linearly with distance from the
target and that the negative glow region is essentially field
free (i.e. nearly all of the voltage dro_ ned 'between_ target and
substrate is in fact dropped across the ODS with no drop across
the rest of the discharge to a first approximation).
(5.3)
105
The, linear reduction in field strength E. with distance y can be expressed as:-
E = - y) _ v dy
where C is a constant and d is the CDS length. This
expression correcp -Jnds to the field being strongest in close
proximity to the target and filling to zero at the CDS/`:G edge.
Integrating with respect to y yields the expression.
(vt - v ) _ t (yd - y2/2) (5.4)
where Vt is the target potential (y = 0) and (Vt - V,) J
represents the potential through v.hich the electrons ons have fallen
to reach the po.i,lt y. Putting iii typical spattering values of say V1 . -1KV and '
d = 2cm allows us to plot the electron energy (e [17 t - Vy] ) as a function of distance from the target assuming no collisions
to change the energy.
D1.-tnce across TABLE OF Electron Energy discharge e (y)(cm) VALUES (ev) (iĪt - V )
0 0
0.2 190
0.4 360
0.6 510
0.8 640 1.0 750 1.2 840
1.4 910 1.6 960 1.8 990
2.0 1000
000
Soo
600
4co
Zoo
EL ECTRON NERG Y (ev)
1 COS NG
2.0 DISTANCE ACROSS DISCHARGE (cm)
10 05 1.5
106
Fig.. 5.5 Electron Ener:*'r ncreass Across the Discharpe
Fig. 5.5 shows the electron at first gaining energy rapidly as it is accelerated in the strong field close to the
target but this acceleration, tending; to zero by the time the
relatively field free NG is reached.
The fore. oing work on the significance of the para..leters
y',iAX and d has been published (4) and the papers e.re enclosed
in Appendix %i. The problem will now be considered in more
detail.
Since the NG is essenti=J1y ficid free, then provided
_.~ d we will be justified in setting Vni = 0 and writing
equation (5.3) in the fōrm:-
1 201 B e .. (5.5) .
Since y is pressure independent and the CDS length
dcc1/p, then provided the pressure is high enough, we should
always he „ale to achieve the condition y > d when the
107
trapped electron trajectories are outside the CDS.
In the limit ..he : )I ., ~Y - d We will obtain the limiting A
mag:etic field Blim such that for B $h T we will not be
justified in us_ equation (5.5) since 'with the trajectory
inside the CDS we could no longer set V. = 0. Putting in the values of B = 8 x 10~ 3T and V, = .1KV e
.
predict y-- 1.3cm using equation (5.5) . Referring to ? :AY.
Fig. (5.3) this suggest; that at pressures above the 'switch'
whilst at lower pressures .`/ ..,AX Z d and that in moving through the switch region they ,AX = d relationship is satisfied at some time.
The significance of the y ,'Ax
:d ratio is not only of
importance theoretically, giving, as it does, information on
the region of ion production in the discharge, but also
experimentally since the y d resion should be avoided
in the interests of plasia stability and the consequential
non controllability of deposition .rates.
5.3 REGIO".' OF TO: PfOD:3CTTON
Before any further progress can be made it is essential
to know the main region of ion production in the discharge
or more precisely the origin of the ions which strike the
target since it is these ions which are ultimately responsible
for sustaining the .risen, .rge. Various diverse ideas appear to
exist notably Druyvesteyn and Penning(5) and Brewer and
We s th_aver(6) who suggest th _t most ions come from the negative
glow and Little and Von Engel(7) and Holmes and Cozens(8) who
conclude that ion production is mainly in the cathode dark
space with a significant contribution to target secondary
electron emission from photons. in the negative glow. An
important point is that both ideas agree on the need for an
NG to exist, agreeing with the erperi~..er:ta_i observations that
ion production ie affecte:1 if the, target/.substrate separation
is less than two CDS lengths.
Cor_. ides 3. secondary electron bei.:; libe_rwted 'from the
target acel beln j accelerated acroe.s the di :rhar'ge by the
103
electric field. If Q. is the effi~iency of the ionisation.
. e'=.:''.1.:':3'.. 't s r u:JJers of i7ositive charges Jr'aluced per
electron rer cen i etr e path r_ re:s rr, then in passing
through an e'_ aLLe ,a. .. i;_;1h S s, the nu..ber :f ionisati..n.s c,
occurri is.
Sn =Q_ p Ss (5.6)
rovide d S s < <X where X is the me1n free path for
ionisation.
The para:_:eter Qi is a function of e.:.ectr:;n energy and has
"been measured by Smith(9). The basic .results are shown. in
Fig. 5.6.
5.6 Efficiency of Ionisation sation for Electrons in Irgon .
90 ENERGY (e v) The main point to emerge is that 90eV i7 the most efficient •
ionising energy for electron imp=-cting into argon when
0. = 13 per cm per m.m. pressure. It should be realised that
this figure of 90eV is rel::.tively low when compared to the
energies obtained in the high field regions close to the target
of a glow dischrge and the optimum ionisation energy is soon
exceeded as the electrons accelerate away from the target:
Hence, a.t a typical sputtering : ressu_,e of 30 microns
.(3 -Z ) we can expect 0.39 collisions 1~J ia..~,. we ,9 ia..isii_g col.~isia..s per
centimetre implying a mean free path for io.:isation of about
2.5cm for 90eV electrons. Since the mean free path at other
109
energies is longer, thc, neglecting of collisions in Calculating
the electron trajectory in a magnetic field would appear to be
reasonable, since excitation collisions in the CDS are relatively
rare and elastic collisions with neutral atoms would not produce
a oh.-_nge in energy.
Usin: the data of Fig. (5.6) it is possible to calculate . .
the nu_Lber of ions produced by one secondary electron during
transit from the taret to the edge of the CDS. The cal-
culation is complicated Ìy the fact -Ulat each ionising collision
produces an extra electron which can itself cause further
ionisation (a breeding effect).
Examinationoftheparal%eter Q, (Fig. 5.6) reveals that 1
it is constant to within a factor of about three once an energy
of approximately 20eV has bechl exceeded. If we make the.simp-
lifying assumption that Q. is cnztant for all electron energy
values, it is possible to use integration to calculate the
number of electrons reaching the CDS/NG edge for every electron
leaving the target.
Electron Proiucton Across a CDS of Lerith 2cm
te-- CDS
NG —
TARGE T
I I I I I I
n 1 1
I I I
X l")
X=0 6X
Every electron leaving the target will have multiplied to
n electrons ly the time .ah ele:.:ental length Sx is reached a
X=2
110
distance .x from the target. Since each electron entering S x
will producce Q r, C x electron in the element then the total - i.
number of electrons. produced in the element (8n) oan be
expressed as:-
Sn - nQp Sx i. w dr = Q pd.x
n ~ a
loge,i 2Qip
N = e2Qip
(5.7)
If we put Q. equal to its maximum value of 13 and, there-
fore, deliberately over-enti:date the number of electrons '_`1
reaching the CDS/`IG boundary we predict N = 2.18 at 3~
pressure using equation (5.7). This would .:u Best that at
most, every electron leaving the target creates 1.18 new elec-
trons and, therefore, 1.18 ions whLch will be attracted towards
the target to cause further sputtering. A more realistic
average value for Qi is, perhaps, 6.5 (Fig. 5.6) when N = 1.48
suggesting the creation of 0.88 electrons and ions.
5.3.1 S C _P Tv r. CT._ )' COEFFICIENT
Since we are interested in whether the CDS alone can
produce sufficient ions to sustain the discharge, a knowledge
of the secondary electron coefficient of the target (ō )
(the number of electrons emitted/incident ion striking the
target) is clearly essential.
Following the early work of Oliphant(10) most of the recent
work has been by Hagstrum(11-15)
Some of the major conclusions are:-
(a) Dirty surfaces give much higher 'ā values than clean
ones.
(b) iii depends on the actual sample condition and past history.
(c) ii i increases vjth_ increasing ion energy but this is not a strong function for clean surfaces. No value of ō is i
111
greater than unity within- the energy range 0 -- 1000eV
irrespective of sputtering gas or target condition. A figure
of less than 0.3 electrons/incident ion would seem most likely
forO_r copee_' target.
(d) Two distinct electron ejection mechanisms are possible:
(i) Potential ejection requiring a charged particle,
the ō values for which have already been i di:cussed.
(ii) Kinetic ejection in v.hich the energy source is
the kinetic enerEy energy of the bombard' r particles.
As expected, kinetic ejection by neutral species tends to zero with d creasing energy and is too small to measure accurately below 300eV. Since
energetic neutrals with energy in excess of this figure could only be produced by charge exchange
collisions rrit'h ions in the CDS, it seems
reasonable to ignore this mechanism and rely on
111 = 0.3 electrons/ion as the figure for cal-
culating whether the plasma is self sustaining.
If the plasm:. is to sustain n itself, each electron leaving
the target must create sufficient ions striking the target so
that they in turn will cause a further electron to be ejected.
If '. = 0.3 electrons/ion we require 3.3 ion:, to produee one electron and since there is a maximum of 1.18 ions produced in
the CDS with 0.48 ions thought to be a better estimate, it is
reasonable to conclude that ionisaticn in the CDS cannot alone
produce sufficient ions to sustain the plasma. This conclusion
eme (5-8)
is in agreement with earlier v:or;:
5.3.2 ... rp''TaD 77177- Y DIST2 7=1?? AT Ti.. r?12 ,.0 r0J"DARY
Sir.:L, one electro:: leaving the target only multiplies to about 1.48 electrons at the CDS/':;G boundary it follows that at least 50(1., o t the electrons leaving the target undergo no ionising collision in the CDS an1 their detection as high energy electrons in energy analysis is eot so surprising as was at first thought.'
We can obtain furthe ^ information on how many electrons undergo
no iori.. e J -I -ion by allowing Q1 to vary (Pig. 5.6) and using an iterative technique to chart the progress of the electrons
across the ';DS,
112
C=nsider our operating parameters 3' pressure and
targetpotential of -1kV when ~i' r~ external i•~ ~-gnet is field
the CDS length is 2cm. if the CCD S region is split into ten
equal segments each of length ā s = 0.2cm then the necessary C
c ndi. Bio-_ O ei > .rill be satisfied and we can obtain an
estimate of the electron multiplication by assuming that all
of the ionisation within a segment occurs at its .,:id point
which can be givee a characteristic energy using Fig. 5.5.
Using the CDS potential distribution of Pig. (5.5) we can
assign an average electron en ergy of 97.5eV to the first seg-
ment and obtain a value ofQ. = 13 from Fig. (5.6). Then this
value is inserted into equation (5.6) we predict 0.039 ionising .
collisions in the first segment. Therefore, 0.039 ions and new
electrons are created in this ee;mont by one electron leaving
the target surface and 1.039 electrons enter the :second segment
of the CDS.
Since electrons will, in fact, cause.ionising collisions
anywhere in the first segment with energies in the range from
the ionisation threshold to 190eV we have over estimated the
number of icnisations since the representative energy chosen
(97.5eV) happens to co •respond. to the mo:,t efficient energy
for electrons ionising argon atoms. The prediction of only
3.9% of the electrons causing ionisation in this first segment"
is, therefore, high but illustrates the point that this type
of collision is a relatively rare event close to the target.
Leaving the first segment we hive 0.961 electrons of
energy 190eV and 0.078 electrons whose energy is uncertain
since we would require precise information concerning the col-
lision. If we now repeat the calculation for the second segment
there are 0.961 electrons of representative energy 278eV giving
Qi =9 and producing 0.026 new electrons together with 0.078
electrons of unknown energy. Even if we deliberately over-
.estimate the contribution from these. 0.078 electrons by assign-
ing the;:: the maximum possible Qi value of 13 _they will still
on'y produce 0.003 new electrons which is about 105 of the '
electrons formed in this segment. Even in the ioost optimistic
113
caee the breeding effect 'is, therefore, relatively Unimportant in this region of the disellarge a:.1 the choLce of energy for
electrons affected by a collision is not critical, especially
as Q. is not a rapidly varyieg funtion of energy.
To simplify the caecdiation, let us aeoeme that any col-
lision affected electron has zero energy after the collision and then gains energy ae it accelerates in the electric field of the CDS. • These 0.076 electron:, therefore, started with
zero energy at the mid point of 'the first segment corres;onding.
to an energy of 97.5eV and should be assigned an energy of
160.5eV(278-97.5)and Q.=11 at the mid point in the second a
segment. They, therefore, create 0.0026 extra electrons which is 10% of the electrons created in the cegalent.
Three electron energy groups need to be carried into the third segment:- (a) those.electrons ehich have not yet collided. (b) Those formed in the first segelent and have not collided
since. (c) Those formed in the second segment. The calculation
becomes progressively more involved until by the end of the
tenth segment we have eleven groups of electron energies, the
relative abundance of which is shows in the table below.
Electrons Created Pr One Electron Leavine The Target
Energy Number of (eV) Electrons
1000 0.835
903 0.066
723 0.049
563 0.040
423 0.038
303 ' 0.032
203 0.032
123 0.031
63 0.031
23 0.034
3 0.036
114
The following points should be noted:-
(a) The main conclusion from these results is that about
80% of the electrons le•ivi the target cross the CDS without _.c;iY g an ionising collision and t1a; the number of electrons 2
with the maximum possible energy is far greater than that at
any other energy.
The excitational collision probability has been ex.4.mined
by Zapesochnyi and Feltsan(16) who found an energy/collision
probability relationship of similar shape to the ionisation
results of Smith(9) but with a maximum at about 15eV. This
explains why the CDS region is relatively dark since in a high
electric field area, 15ev and :maximum excitation is soon
achieved and exceeded by the accelerating electrons. Since it
is usually assumed(S) that the cross section for excitation by
electron collision . s significant only for e.iergie s below 30eV
it follows that excitation in the CDS will be even less sig-
nificant than ionisation.
Given that elastic collision will not change electron
energy, it follows th.t a large percentage of electrons released from the target will reach the C';S edge with the full
.inter-electrode drop of potential.
The ener:;y distribution at the CDS/NG edge therefore shows a large number of high energy electrons and a much smaller
almost constant number of electrons at all the other lower
energies. Since the relative nu:_.hers of these lower energy
electron_, are so small it is doubtful whether the variation in
their number is significant and the only firm conclusion we should draw is that the highest possible electron energy value
is by far the most heavily populated. .
(b) Allowing Qi to vary predicts 1.22 electrons at the
CDS/NG edge for every electron leaving the target once again
suggesting that ion production in the CDS cannot alone sustain
the plasma.
(c) The poor comparison between this predicted energy
distribution and the energy analysis curves of Chapter Two
which shoved most electrons with low energy and a few at the
115
maximum possible energy is not surprising since they refer to
different regions of the discharge. The above analysis refers
to the cps/rG bound'ary whilst the experimental analyser results are taken well into the G.
In the nearly field free rG region, any new electrons
produced by ionising collisions will heve no opportunity to
increase their energy significantly since the electric field
streegth is low. These electrons will, therefore, retain low
energy which faveUrs further ionising aad excitational col-
lisions and the breeding effect will be lore significant. This NG region should be an area of colour, as is observed by looking at tiie dischare, and ion production. We would, therefore, expect an increase in the number of low energy electrons and a reduction in the percentage of high energy electrons detected as we move through the NG region away from the target. The experimental rerealts of Chapter Two chow the extent of this restructuring of the distribution. The change is admittedly large and no close correlation could be claimed between the
theoretical CDS/NG boundary distribution and earlier experi-mental results though some broad agreement is evident.
(d) The number of segments chosen for the prediction of
the number of ions created in the CD3 is not an important
factor. A simpler model using only r'our segments predicts 1.4 electrons arri7ieg at the COSPIG boundery for every one leaving the target as compared with 1.22 using ten segments. This is a
result of the breeding effect not being very important in the nsa-Aalsolererelat-ivecelleteeicyof Q. over the energy 1 range 20-1000eV. This seems to suggest that we might obtain a
satisfactorVelectrollelleriD'clistributiollbykeeping Q.con-stant; the great advantage of this being that the analysis can now be extended into the rG so that the theoretical distri-bution produced could be compared directly with the experimental
-results of Chapter Two.
116
Qi
S}inoe, the nw-.ber of el€trons n at a position in the dis-
oha_ ;e a distance :c from the target is given by:-
n _ `ioix x
We can use this e1:pre:_ si,_. to see ho.: the number of elec-
trons increases a :ross the CDS. Assu:::ing Qi = 6.5 and p = 3` 1
the results are .,hov.n in Table 5.1 where S n x refers to the
number of electrons formed in the segen t of the ddisch tr;e and E shows the enervy which the newly for.::ed electrons would
possess assuming they 7ere13rmed in the centre of the segment
and reached the CDS/::G boundary witho'wt further collision.
Table 5.1 .'ultinli c:.ti.en of Electrons Across the CDS
X(cm) nx Snx Ex(eV)
0 1 1000 0.2 1.035 0.035 903
0.4 1.081 0.046 723
0.6 1.124 0.043 563
0.8 1.170 0.046 423
1.0 1.216 0.046 303
1.2 1.264 0.048 203
1.4 1.314 0.05 123
1.6 1.365 0.051 63
1.8 1.420 0.055 23
2.0 1.477 0.057 3
This analysis can be extended well into the VG (to
x = 10cm say) but unfortunately we cannot assign a value to Ey
since at the moment we have a r;ode1 where all the inter-
electrode potential is dropped across the CDS which would
require E = 0 in the G. S:..ce the .:G is in fact only relatively field free( 2) we should assign a ::.call potential to
each segnent in the ."G. Table .2 shows the result where we assume Q. = 6.5 and the proule:.. of the value of Ex has been i ignored for the r.•,oment.
117
T -11 "';.2 l atir. Electrens Acro: the 7G
7(=-.)
3 1.795 0.07
4 2.192 0.134
5 2.651 0.103
6 3.222 0.126
7 3.916 0.153
8 4.759 0.186
9 5.784 0.225
10 7.029 0.274
In table (5.2) Sn refers to the number of electrons
created in a segL.lent 0.2cm long centred on x. Table (5.2)
shows the number of electrons formed in each seg.:.ent of the
discharge; we require the number which would actually reach
an energy anabs(ar situated in the G. If L iz the mean free
path for an ionising colltsionand Xi is the distace which
an electron has to travel to reach an energy analyser in the
NG, then the probability o arrival without further collision
is :71.ven by exp(-2:1/1), and the bn values in tables (5.1) and
(5.2) should be multiplied by this factor. This correction
will tend to discriminate against the high energy electrons
since they have the furthest distance to travel. From table
(5.1) we expect to create 0.035 electrons in the first 0.2cm
so for every electron leavig the target we expect 0.965 to
trave:L 0.2cm without colliflion. The probability of arrival
(exo -X1/L) is therefore equal to0.965 and putting X1 = 0.2cm
allows us to calculate the mean free path L as being aoprox-
imately 5 cehtimetres. Table (5.3) shows the reconstructed
energy distribution in the case when the analyser is 10cm
from the target (8cm into the NG).
118
T7Ible r.3 oner?, the : rrr,-Y Ar ,: °er
Z(co) X (c.-) .Snx
exp(- il )
' ~~ (' v ) x
0.0 10.0 0.135 1000
0.2 9.8 0.005 903
0.4 9.6 0.007 . 723
0.6 9.4 0.007 563
0.8 9.2 0.0073 423
1.0 9.0 0.0076 •303
1.2 8.8 0.0083 203
1.4 8.6 0.0089 123
1.6 8.4 0.0095 63
1.8 8.2 0.011 23
2.0 8.0 0.012 3
3.0 7.0 0.017
4.0 6.0 0.025
5.0 5.0 0.038
6.0 4.0 0.057
7.0 3.0 0.084
8.0 2.0 0.125
9.0 1.0 0.184
10.0 0.0 0.274
In order to plot this distribution we must m: Ike an
assum?tion concerning the value of Ex in the NG when
2 < x < 10. Let us assume a constant pote.,tial gradient in
the NG of 10 volt cm-1 which would give 1000 volt across 2cm
of CDS and 80 volts across 8cm of NG. This is in line with
the idea thi.t the Z'G is essentially field free but it must be
stressed that the estimate is purely convenient, though hope-
fully realistic, to enable the energy characteristic to be
plotted. The theoretical distribution is shown in Fig. 5.8.
C: r Distribtion
—~--;— - -
the G
119
27
21
15
9
T — — — — - a > 2 0 50 00 1080
ENERGY (ev)
Two important points should be noted concerning Fig. 5.8.
(i) The rate at which the predicted number falls off from
its low energy value is determil:Ol i ari:eiy by the assumption of
a potential gradient of 10 volt cm-1 in the ';G. An assusi:ption
of say 5 volt cm-1
would produce a sharer decline and the only
firm conclusion we should draw is that there would be a large
low energy signal from the analyser.
(ii) Fig. 5.3 is a. plot of the energy distribution of the
electrons created in the discharge together with the predicted
signal for the electrons which were not involved in an ionising
collision. The electrons which collided with gas atoms to
cause the ionisation are nos, at the i:.omen`:, included. Consider
again Table 5.1 where for x = 0.2cm, .. nx = 0.035 electrons
created. We should also i:.c1u.e a further 0.035 electrons which
must have collided and lost energy in the ionisation. If we
120
made thesimplifyinp. assumption that these electrons had their
energy reduced to zero in collidiag wi'Gh heavy gas atoms we
should then double all of the S.nx values producing a theor-
etical distribution e.ith an even larger lo'.': energy contribution.
Alternatively, we ceuld assue.e thet the colliding electron loses only the energy equivient to the ionisation poteatiul of
argon (approx 15eV). In this case we would expect a slightly greater high energy sijaal but most of the additional electrons
would still be low energy having been created in the NG and not ,
increased their energy significantly before themselves colliding
to create further ionisation (nx=1.48 at x=2cm rising to nx=7.03 at x=10cm).
221t5 ) The following points should be noted:- (a) Most electrons hitting an analyser situated in the NG
themselves ccice from the 7'7G and are, therefore, of low energy.
Since the excitation anl ionisation collisional probabilities are similar functions of electron energy(9)(16), we might expect that the NG would be the main region for the production of ions and electron:, since observing the discharge shows that the NG
is the main colour area where most excitation is occurring. (b) Comparison between Fig. 5.8 and the experimental
results of Chapter Two shows some agreeieent in th - t both curves
exhibit the maximum signal at low energy with a euhsidâå'y high energy peak corresponding to electrons v.hich heve traversed the discharge without collision. The main differences are that the theoretical curve (Fig. 5.8) falls off more steeply at low
'energies and shows a larger, better defined, signal at the high
ene= peak. ' Discrepancies are to be expected in view of the
simplifying assumptions made in the theory and our uncertainty about the experimental results at low energy when the energy analeeser acceptance bandwidth (SE) becomes very small.
(c) The fact that the theoretical energy distribution is
similar to our experimental results 'from Chapter Two suggests •
that the assuleptions msde in the derivation are reasonably well justified. Attempts at an exact carve fit were not thought
121
to be worthwhile, however, since a very much simrilified model
has been used. ePore •su,.'ther pro6-ess could be made, a
vriable colliLion cross section Q. would be required and a
means found or ihcludih:2; excitation of. atoms in, or due to,
the nezative This Intecfacto-?- would -L-_erease the num-
bur of low eery electro,..s r.h1h ;oul'I be consitc.:t with the
experimental results which. showa large low encry
Furthermore, ae would require infor:Lation oonerning the
snali pote:.tial.op in the :1% (d)- Whilst eomprison with experimental energy analysis
res-.11ts has been considered at some length, the main conclusion
as fir as the rest of the work in this chapter is concerned is
that i'nisatio I. the CDS anot alone sustain the discherge.
This conclusion largely depends on the accuracy of the O. '1 values as oresent0 by Smith(9); fortunately we can obtain an
check by considerin a 21:htly different collision
model.
7=T7IC;Tin 700EL
Consider now a similar altern2tive approach by looking at
the increase in the iux of electrons (number/area sec) as they
move :,eross the CD.
0 TTIN o lectron ovin Across thr, CPS
I I I I I I I I I
11,2Ve I I I I
I I
X X+SX
• X =0 TARGET
x=cl CDS/NG EDGE
122
If ne any Vo are the e'_cctr. r. density oili velocity at
_` amiel er _ni t :c a the flux o.. electrons moving across the
di.._'iir„e at that point (the nue.ber crossing unit area per
second) will be "z.en r ii V • As the electrons accelerate e c
aero—, the di,..her,ee, then .in. the absence of colli:..:._.- ,flux.
(n rt)remaine uY:cr'iar'e ed. Collision e, however, can lead to
flux generation as discussed below.
1-len the following simplifying assumptions are made:
(a) No density gradient in the g-s of density n0.
(b) The electrons are available to produce Ionisation
whatever their energy, i.e. ignore the small threshold energy.
(c) The cross-section for ionisation (Q ) is not a func-
tion of electron energy. In fact the cross-cect'.on ices vary
with energy but in cor:'tnna t to within a factor oftiYo(17) for
our experimental rare of .0-1kV.
Consider the electron flux api:rc :cii'.. a section of the
CDS of thickness S x.
Flux out = Flux in + Flux generated in S x.
(neVe)x + c x = (neVe)x + nOQ.Sx(neVe)
i.e. n0Qi S x(nee) represents the extra flux created
since , by definition, n0gi S x represents the fraction of the flux colliding in the element.
(neVe)x + dd(neVe)
S x = (neVe )x + n0QiS x (neVe).
and since we have set ai constant it will be possible to form
and solve an integral:-
d/dx(neVe)
(neVe)._
ilog (neVe I P 7
n00.
d
0 n0Q.dx
where d represents the CDS length and is the limiting value •
of x.
(ne'Te)d = (neVe)0 exp(n0Qid) (5.8)
This expression for the flux multiplication across the dis-
charge has a tiimi.ar form as equation (5.7) for the increase in
the actual number of electrons across the discharge.
Put tin- in soa' typical figures of n0 = 1.06 x 1021
m_323
(%d ;'re.".....re) d = 2x10-2m 3n•j ~• = 3x10
-20m2
as given by
I.7aseey and _urho:~( 17) which shows Q 'to be with_., a~ factor of Q.
two of 3x10 2' m2 for electron energies between 40 and 800e7;
we then find the flux nulti:e i c''t icn to be 1.89.
Whilst this figure is high enough to ;'un gest that a
significant number of electrons did actually undergo an iori ting
collision, it in not high enous-h to provide a self sustaining
plasma bearing in mind a X value of about 0.3 electrons/
incident ion.
5.3.7 ?'C:.0"IO'"`. 0' I0': P':0T`;7CTIOT
The rain conclusion to be drawn is that the two models
presented both lead to the name result th::t the CDS does not
seem to be able to produce sufficient ions on its own to main-
tain the ditch_.rge and the ;;G is needed as an extra source of
ions whether they be created by electrons or photons.
Whilst conditions for producing ionisation in the NG are
favourable since any new electron formed is not immediately
accelerated to energies in excess of the most favourable energy,
it should be remembered that any ion formed has a reduced chance
of ever reaching the t«r?.et since it will 'drift' randomly in
the relatively field free ':'t region. In the CDS, conditions are'
contrasting in thet whilst ion production is not favoured, the
electric field ensures that any ion produced has a high probab-
ility of reaching the target. (Since charge exchange collisions
would still produce an ion reaching the target).
5.4 '1 - °.7:)r/CU: E..,
Since it ht's been shown that the 'switch' in the pressure/
current ch"racteris tic of Fig. 5.1 is associated with the ratio
of y nx :d and that the CDS is apparently incapable of sus-
tair!in . the discharge alone, we can speculate further on the
reasons for this switching behaviour.
Consider the plasma in a state where the pressure is suf-
ficiently hieh that )1 :14:X> d (say 30/1 ) and ion production is
predominantly in the NG.. As the pressure. i s reduced, d increases
whilst V ,,AX remains unch'.nged provided y ; AX > d. The equality
124
= d which eerresponds to the 'switch' in eurreet (Fig. 5.1)
veuld, therefore, be achieved evehtually. We now consider the
mee-Aaniszis operatin'e ih the :.itching re( ion.
When the i:reseure is reduced, fewer ieniein.1 and exciting
collisiens wi21 be nede and the dih-charge might be expeoted to
extinguish. However, with the electron trajectory wholly in
the CDS region moct ions created will reach the tar:et, in con-
trast to the situatien in the ITG region where m -eny ions will
be lost. Thus we nove into a 'different regime in which fewer
ions are created but the majoritj will be able to produce
further electron ciuiscion from the target. Clearly the most
effeCtive situation will occur when maximum height of the
trajectory (1/.. ,) is just inside the CDS since an ion formed /
in this regien is subject to the full accelerating potential
before hitting the cathode and therefore, produce the
maximum possible number of new electrons. In addition, any
new electrons generated near the CDS/NG boundary are in .a
relatively weak field and, therefore, remain at relatively low
energies, increasing their ionising efficiency in collisions.
with gas atoms.
In the magnetron configuration, the magnetic field acts
to confine these electrons to the CDS further increasing the
overall ionisin:7 efficiency. Thus we argue that, when y AX passes from being greater than to less than the CDS length the
plasma changes from being etaihtaihed mainly by copious ion
pro.iuction in the 71G region to a relatively osriller ion pro-duction almost entirely in the CDS region -which is nevertheless
capable of maintaining the discharge. in view of this change
in mechanism, it is not surprising that there is an abrupt
change in the plasna ourrent.
5.5 7.7A777Tir.! 7=7) VARTVPI07S
As the magnetic field strength is increased it is observed
eveat.bothy.arid d decrease and we might expect greater
difficulty in observing a characteristic switch aeseei.,,.ted with
d moving through y 77 since hoth parmeters are changing. A series of experieLentAl curves of target current against
nar;netic field streegth at constant voltage of 1000 volts are
given in Figs. 5.10 and 5,11.
125
r. .10 Ivt 71.-Te'd 7 f-.1r D4fferent'Precs.Ares
I (ma) A
100 -
p.2u
P=201A
50 -
P-18/u
40 65 90 B (Tx10-"I)
1 26
Pi. 5.11 a=ent T v ielY! 7 DiffPrent Prer:cures
10
P 19/A
P
P =19.1
P =13,A P=10ju. ).
B Tx10-4
30
60
127
The most striking portion of these .=agnetic field charac-
teristics, which are different eren t shape at different pressures,
is that there is, in Pie. 5.11, a region ,.here increasing the
magn.etic field ; tre' t lowers the discharge current. This
rest is at varinnce with the u._u al ideas on th• .-agnetic
{ield role which state that as the field strength _s increased, f
electron trapping is __. . .roved and increased ionisation results
in a larger Cur e t 112s can ~e explained ed 1% the following
r Since y :„Al. = ~'' for y .., d we c3:. cal-
culate ~/ Y. for different values of B and compare the results• with the e_Yneri cn aI values of d. If thi is done for 35
pressure (well above the observed switching pressure) and
1000 volts across the dischrge, the following results e.orge.
B(Tx10-4) d(om)
41 0.80 2.32 3.53 68 0.57 1.7 2.98 89 0.48 1.3 2.7 110 0.40 1:05 2.6
Unfortunately higher magnetic field strengths were not
attainable with the Helmholtz coils available, although in any
case the s:i.all d values which would then be observed would be
difficult to measure accurately. It would seem that y /d
is tending to unity only very slowly implying that yL. TAX < d might not be achievable at 35/ul. The electron trajectories
are, therefore, into the NG which is the main region of ion
production. This is in agreement with the experimental obser-
vation that at 35/43. and 1000 volts, a typical plasma
in our system is easily self sustaining and strengthens as the
magnetic field strength increases.
way.
126
A similar a :ly.:i:- at 18,U pre"'su:e, 1000 volts, in the
unstable region of t::e plasm?. reveals the_ following result s .
B(Tx10-4) d(cn) Y,.,,x(cra) YL: Axid 41 2.51 2.82 1.12
68 2.35 1.7 0.72
89 2.10 1.3 0.62
Whilst it should be reuemt e.!'ed th t the latter tw ' values
of y MAX are not valid since once d > y'~nX we are no longer able to use y _ 1
2mV/e , these figures show that at
MAX 13 18" pressure and 1000 volts it is possible for y to
approach d. We can mase an improved attempt at calculating
y "AX by using the more general expression:-
y MAX
= [2m(Vt_V.0!)]
Since V 7 0, the value f or y MAX when ~/ MAX d will be smaller than the above results suggest and can be found
using:-
V -V = 2Vt (d
y:. x y:, AX 2/ ) t Yr d2 2
( 5.4)
Rearranging equation (5.3) then gives:-
4mVd MAX• , r. B2ed2+2rV
The recalculated table of values then becomes for n=18,M:-
B(Tx10-4) d(cm) y;;,AX( cm) slfLIAX/d
41 2.51 2.82 1.12
68 2.35 1.45 0.62
89 2.1 1.03 0.49
Thus at 18AA pressure, the magnetic field change can pull
y ,,AX inside d. yL?AY then becomes inversely proportional to B so that further increase of.B moves the electron trajectory
rapidly closer to the target within the CDS.
(5.3)
129
If we supose thet = d represents the optimum con-
4 tion for sustaining the discharge when the trajectory does
not go through the negative glow region, the reduction of
current as B is increased, beyond the value for which
y = d, can be understood.
At pressures less than 1,4, the discharge is weak ren- .
dering accurate estimten of d difficult because of poor
definition of' the CDS/N1 edge. The results, though admittedly
inaccurate, are consistent with the model described above and • correspond to the situation for which y_ . d for all
(:,AL pressures. The maxima in the curves for 13, 14 and 1tVA
correspond to the y,„ = d condition in each case. As the
pressure is reduced, d increases and the maximum will move to
a lower value of 13 as shown in Pig. 5.11.
5.6 GEAT, CT7CLUS707S
It is seen that our Liagc.etren assisted discharrre is
characterised by three regions.
(a) A high pressure region where ion pro- duction
M ) d ape: - , AX duction is predominantly in the NG. As the magnetic field
strength is increased )'X and d both decrease but the
situation y
= d is not easily attainable. Furthermore, mAX
increased magnetic fields introduce stronger electron trapping
in the relatively electric field free NG region and the disL
charge current increases. Since the discharge is continually
strengthening as the magnetic field increases, pressure
reductions ought to be possible at high field strengths if
this was thought to be advantageous.
(b) A transition region where y mAx z d and the discharge is unstable and likely to switch from y ) d toy Lux d
as a way of improving ion production. (c) A low pressure region where the discharge appears to
consist primarily of a CDS and the rG is not very pronounced, This would net normally represent a significant sputtering
ree.ion since the discharge is very weak and is characterised
by an optimum magnetic field such that ion production is maxi-mised by having the electron trajectories just inside the
CDS/NG boundary.
130
cc our e this 'transition' ln• pressure is in the
l~::il.. _.. r, r ~,~. te... isis tr .. iti~
region of 1 ~
, :a„:etIe^ systems Cry.. be operated successfully Llf
at low pressures as demonstrated by commercially available
P? r-er Yagetrcn an_ etter in systems. Both of these devices
use much hig er non u_ if - magnetic fields created by permanent
magnets. It seems likely that, in a higer mai•=e'tic -field,
electron trapping is s efficient that the plasma can adopt a
mode in which ion production is satisfactory even at low pres-
sure. This is supportei by the work. of Thornton~ 18) who used •
much higher :, ag _etic field s tre:.g ths of order several hundred
x 1 - ~0 'T.
In the next chapter we consider some experimental results
aimed at 1rO'iuoing an efficient s uttering system. Bearing in
mind the different regions of the discharge which appear to
exist, pressure values must be cro::e.. ..uch thatoperatir_g para-
meters are in the high re ere region and the discharge is
drawing large currents. Under these conditions, a high magnetic
Field strength will produce the intense plasma required.
131
SIX - •• •
.1 C-7.=7.77 7.-2A77e7.7aS
As diseesed 5n Chapter Three, it is useful to consider the
scutteeing e-ecess in terms of a reiusel rate defined as Depo-
sition Rate at the Substrate/Power Input to the System. since
high power inputs will dueage a heat senSitive substrate, it
is important to apply the power as efficiently as possible by
maximising this so-called reduced rate. A series of experiments was, therefore, initiated to try to
find the optimum operating parameters for the system, the 'main
aim being to check that the anticipated voltage optimum choice does exist whilst there is nc equivalent conditicn Par the current parameter.
G.1.1 OT- VG VO -YE
A series of exeerients was carried out at constant pressure to determine the variation of deposition rate with applied vol-tage, the current being kept constant by varying the applied
external magnetic field. Here we present some results taken
with the water cooled cylinder; a similar analysis was also
underta!:enn with the so called 'modified' cylinder.
The deposition rate was measured by interferometric tech-
niques using the Varian interferometer. Since this method is -
time consuming because the vacuum system needs to be opened
up after every experimental run to facilitate the removal of the test slide s some measurements were taken using a quartz
crystal rate monitor. Crystal oscillators are not usually used in sputtering because the plasma interferes with the oscillating crystal and the results cannot be trusted. This problem was found to be predomieent, results over different trials proving totally unreliable, an -I causing the technique to be rejected in
favour of the much slower interferometer method which gave reprodueiLility to better than 2%.
The graph of deposition rate against power input at con-
stant current and pressure is shown in Fig. 6.1.
9000 0 _I _RATE A HR
6000- I= 20mA
P=30)14
3000-
132
(") 1 De7)csition Rate 1 Power Suv::7 ier! at Constant Current
10 20 30 40 POWER (WATTS) - — I— — — I— — - -- --I —
500 1000 1500 2000 VOLTS.
This il,formation car. be replotted to show reduced rate
(deposition rate watts-1
) against voltage.
. 6.2 Reduced Rate v Volt9:e at Coitant Current
REDUCED RATE 1=20 mA
P = 30)A
340
220
60 500 1000 1500 2000 VOLTS
133
As exne.-..ted, folloveing the discussions on sputtering yield
and appliel voltage in.Chapter Three, an opti:nue, voltage does
exist and whilst deposition rate could be increased further by
raising the applied voltage, the reduced rate falls es the per-
centa.7e wa-.1, -te of input power increases. The voltage ohoice is
not, however, too critical providing it in below say .1400 volts,
the optifue volt approxiffitely 8C0 volts.
A eiIL,tiar curve shape was obtained uf:ing different constant
currents e.fel pressures and was also noted for both tynes of
copper target.
6.1.2 (:177'17:7(1 CTJR17.71'
A sifIviLr series of results was taken et constant pressure
and voltae to find the:relationship between (19.o:A.tio:i rate and
::Lpplied current, the current being changed at eonstant voltage
by regulating the magnetic field strength. The results are
surcnaried in Figs.' 6.3 and 6.4.
Fir. 5.3 lieeesition Rate v PowerSunnlied at Constan/ Volta=
A DEPOSITION RATE 6000 - 0 -1 A HR
3000 P =30jA -1200 VOLTS
12 24 36 • 48 60 - - -1 - - -I - - 1- POWER (WATTS) - - 1- ›- - f --
10 20 30 40 50 CUR R ENT (mA)
REDUCED RATE A HR-I W-
134 ir. 6.A .e . C...2rent at Constant ';cL-,a e
10 20 30 • 40
CURRENT (mA l
As the current is increased, more ions per seconJ are
striking the target and we should, therefore, expect the
deposition rate to rise. Since the voltage distribution of
the increased current flux is un1hsnr-eol, the sputtering yield
(atoms/incident ion) should remain constant and doubling the
current should double the amount of material ejected per unit
time from the target and, therefore, double the deposition
rate. However, at constant voltage, doubling the current also
doubles the power input and the reduced rate should, therefore,
remain unchanged as shown in Fig. 6.4.
Two further points merit cnsideration -.
(a) The initial rise in reiu.ed rate (Pig. 6.4) is attributed mainly to the fact th .t as the deposition rate
increases, the ratio of copper atoms to argon atoms increases
ire the space between the electrodes (i.e. the partial pressure
of copper rises from its initial value of zero). The incident
ion flux therefore, contains an increasing number of copper
ions.
100
60
135
This is easily observei visibly as the plasma colour becomes
an intense green shade iniioatiue of copper at higher power input ]evels sho:.ing improved copper excitation. and also ioni-
sation since tne two prcce:ses retuire similar conditions.
Since utteri;; is a momntam exchnge process, the ion with
the highest sputtering yield is that of the sa::.e species as the
target material. An increased copper ionic flux to the target
should, therefore, improve not only the deposition rate but
also the reduced rate.
(b) The above discussions assume that the sticizing coef-
ficient for sputtered atoms is not a function of substrate
temperature. In fy,ct as the power input is increased, substrate
temperature will also iserease and the reduced rate would
decrease if all the other operating parameters were kept.con-
stant. As discussed briefly in Chapter One, this effect should
not be too serious provided the substrate temperature is not
allowed to rice too high.
Fortunately from our point of view these two fe.ctors of
substrate temperature and a copper ion contribution are either
small and/or cancelling each other out and Fiats. 6.3 and 6.4
sugest that we are justified in assuming that deposition rate
is directly proportional to current at constant voltage and
reduced rate is constant.
Since Fig. 6.3 shows discharge current directly proportional
to deposition rate, we could, perhaps, sensibly take discharge
current at constant voltage as a r_Aigh measure of deposition rate and eliminate the need to open up the vacuum system for
interferometric measurements. ',lie should not, however, expect too great an accuracy from this assumntion sin.;e it has been
pointed out by Jackson ' that stulies of contamination and
damage effects cast doubts on the validity of our assumption
that stutterins yield is not a function of bombarding ion cur-
rent density. In a later section on substrate heating it was
found to be possible to use Fig. 6.1 and 6.2 as calibration
curves to calculate the .t7pected deposition rate under different
operating conditions but the results are only accurate to within
about 15%.
136
6.1.' 07)7T77- 7 '7:: -=s70rs
The experimeetal fe.:ol.,;s show an eeti= eperating voltage
of areend .T.00 volt bet no oetimum cureent eli can be explained by assuming the: seuttereng yield -1 a funetion of voltage but not current. Hie rote efetent seutterhg iu, therefere, most likely to be achieved with a high current low voltage device though voltage choice is not too critical providing 1400 volts is not exceeded. This eonolueion is similar to those reached with the Sputtergun and. Planar Mageetron whioh
operate in the 600-700 volt region.
Similar conclusions were reached with the modified cylinder
though the reduced rate figures are slightly different because
the sputtered material is ejected in a different distribution Profile from the new target.
The limiting fee.tor on deposition rate in our systems seems
to be the magnitude of the magnetie field achievable with the
Helmholtz coil arrangement. A rough comparisch at 700 volts
across the discharge between the planar magnetron and our system shows the planar magnetron using a magnetic field
strength of 3 x 10-2T aria a resulting power density of 6 Wera-2
to produce a deposition rate for copier of 10,000A min-1 -2 whilst our system draws 2 Wcm-2 at 1 x 10 -T to produce
1500A min . The production of higher deposition rates at
the optimum voltage would require either 'heavier' Helmholtz
coils tapable of carrying a larger current or the use of per-
manent magnets fixed in close proximity to the targot asseMbly.
6.2 rr7E2AT. r:H=TEi:T.S7TC3
Having eetablished that high rate deposition is possible,
it is necessary to look at the consequential substrate heating
to ensure that unacceptably high temperatures are not being
produced by virtue of the power input becoming unreasonably large. The general characteristics of the water cooled cylin-
drical target Je.ere, therefore,. investigated as an introduction
to the heating studies.
137
6.2.1 C'77.272:T/70ITAGF 7=707S1=
The relatihip beteel. current an l volte at constant
!:Lan:netic field str.,L7th and pressure is shown in Fig. 6.5; the
pressure of "vAA tc ';:ep well clear of the dis-
continuiV licc-,.;:ss.1 in Chater Pi7e.
at Const-,.t :arnaetic P4 c11 and Pressure
300
200
CURRENT (mA) B=9x10
3 T
ß=4x10-1T
30),A
100
B=0
1000 2000 3000 VOLTAGE(VOLTS)
Comparison between the magnetron case (B = 9 x 10-'T) and
the non magnetron (B = 0) is difficult since they operate in
totally different ways. When the power input is increased the
non magnetron voltage rises stead ly as the impedance changes
from 51JV to 33 Awhilst the magnetron tal:es in more current
at almost constant voltage and its impedance falls from 7J\. to
2.A. The impedances are, therefore, an order of magnitude
different as indicatel by the different gradients of the curves
of Fig. 6.5.
A.rNA) V(VOLTS)
2401- 2400
120 -- 1200
CURRENT
P=.3p1
VOLTAGE
139
Current and vo2t-1.,-,0 were investigated as a function of
aplted T'a:7netic fie2. 1, the power sunly output setting remaining ur.chen]:ed throut the ex7-:criment. The resalts are shown in 6.6.
7i2-. 6.6 Currcnt anl Volta::e Af.ainst 7ep::letic Field
20 40 60 80 MAGNETIC FIELD •
(Tx10-4.)
The fact that the output voltage reduces sharply as the current increases at higher ma'fnetic field strengths indicates
that the power su:2p1y regulation is not very effective. Since
the plasma impedace chan7es for differeat operating conditions,
we should expect the 'maxi:num power theorem' to apply and the
discharge to accept most power when its iLlpedance is matched to
the internal resistance of the supply.
Replotting the inforl.:ation on Fig. 6.6 shcws no maximum in
the power against magnetic fieli curve but most power being
drawn at .zero magnetic fl:ld. This suggests that the supply
resistance is never matched to the plasma and that the impedance
mism-tch gets worse as the magnetic field strength is illolneased resulting in less power being drawn. ,
139 The nower aga__n'et !naC: eti. field curve is, therefore,
largely governed :y the i._ pedaece os the power supply end care
is needed in interpreting t, e re lts. For exa• ,l o some .0 L_ or 1. „~_ ,: in orte
regimes of the discharge it is possible to increase the meee-
netic field streegth at cen , ta_nt power supply setting yet
reduce the deposition rate since the ch ,Ing plasma impedance
significantly v.oreens the mie match causing machh less power to be drawn by the discharge. •
For a fixed power supply setting, the following result is
obtained at 30/1 pressure.
?'agnetic Field Current Voltage Power Input Deposition Rate (T x 104) (mA) (volts) (watts) °A hr-1
0 60 2600 156 7,500
85 200 510 102 12,500
As exeec ted, the magnetron is showing the greater depo-
sition rate for lower power input. The power is being used
less efficiently in the non magnetron case and more has to be
dissipated as heat in some part of the system. It would be
more relevant to compare the two cases at the same power input
i.e. with the magnetron drawing 156 watts. This would lead to
it taking in more current at almost constant Vol ag e and the
deposition rate would rise even further.
As the systems are pushed to higher power levels, the
magnetron will draw a continually increasing current at almost
constant vo' tage and the deposition rate will rise; the non
magnetron will require an increasing voltage and the deposition
rite will increase only slightly since the sputtering yield
increases less the e-1 linearly with voltage increase. The mag-
netron eyste ;l is, therefore, iecreasir.giy advanta' 'eous as the
power in; ut is increased.
It is now neses5ary to examine the substrate temperatures
to determine where the power input is being distributed.
140
.3 s!..77!:7a,,,7,,: H7:-1
3.1 1.7=777.7;7. Seutterine. traditienally been vewed as a hot process
and many v.ores have studied the extent of the rero-e1F-- --.. The
heating revew in Chaeee- One dieeu-sed briefly -ueh ideas as
heat input limiting the depozition rate, stress being induced
into a film, delicate sebetrateS beine dameged and film growth
being affected by temperature; we nasi lose',: at the distribution
of the input power in sreater detail.
The sources of :ower input to the substrate in an R.F.
sputtering system (7:.aterials Receoreb Corporation multi-target
sputterins module) h:ve been studied by Lau(2). By using
thin film thermocouples at the substrate an: measurine .water
flow rates and inlet/outlet temperatures at the wetrr-cooled
target it was possible to anlyee where the input over was
being dissipated. The mein eonolusion was that approximately 55% of the input power ie dissipated at the target, 5% at the
substrate and 405 by other sources such as ejection of sput-
tered atoms and secondary electrons, ionisation end excitation
of the gas and he it loss in Lne R.F. matcning network. Using
a planar D.C. target which wee not water cooled, 135 of the aeplied power appeared at the substrate suggesting that
radiation from the uncooled target might be en important con-
tribution to 'substrate heating in the particular geometry used.
Considering solely the power at the substrate, 805 of this was
thought to be caused by electrons with the kinetic energy of
the Sputtered atoms contributing 15% and their latent heat of
condersatIon a further 55. Radiation either from the plasma
itself or from the water ee)oled target was not thought to be
significant. The power input to the system was surfi3iently
high to cause the relatively high substr.:Ite temperatures of
550 00; the greater the power input the greater the temperature
produced.
A similar type of anal,sie of the distribution of the
input power was attemeted by :Ill for a D.C. plasar system.
Perhaps not surprieingly since thL confl :uratioh is ii:ferent,
his results do not igree closely with the results of Leu(2).
141
Approately r•J:5 of theapplied power was attr5buted to the
target with the ee-cetr%te ta::in: Up the•ree,ainieg 4C5.
all of this elbetrate inpet is eroviied by electrons with the
kinetie energy of the eputterel species and their coedensation
energy gieing only 1'% depoeitien rate wae, however, very ei
low at a'eeut 70 oA zin and it was thou-let like' that this
centribution frore neutral eplc e would rise at higher depo-
sition rates. A further important at iL that the 7eltages
used were in the reeion 3-5:(71 where .the Chapter r'wo energy,
analysis curves of current (i.e. power) against electron energy
are suite sharp. It should be reeeembered that these curves
showed a relatively s,nali high energy signal being largely
recoonsible for the electron power input to the eutetrate
especially high voltages wheee the sharpness of the curves
increases. (7 Ileither Lau -' nor Ball(3) have attempt e. an analysis of
the newer distriHtion in a cylindrical arrangee,ont or, per-
haps more importantly, s a maenetron arrangeeent. Since both
seem to believe that electrene are largely responsible for the
power input to the su'eetrate and the electron traj cc tories are
significantly affected by the magactie field; an analysis in
the eageetrer eystem.seeme te be required and will be discussed
in the next section.
■.! A
' • r .!' r ," '11' T•Tp `ln 1,0.
)pp 0 a .
Thie cae be conveeieely subdivided into two sections,
namely the power to the substrete and the power to the target
and these will no be eoneiderel seperately.
6.L.1 '777 ;1,, TRT!
The eystee. ueed con5.ietei of the water cooled cylindrical
target wi-n a cepi,ee diee of 1.5em diameter suspended in
fro L ths eebetrefc to aet as a seeeer for the incoming
eeergy flux.. The use h-A a ehromel/alteeel thermocouple
attashci with ds; ti rain: to the side ehielc!edfrora the
inc4 1e: t flux eo Ul lt the eeeeoe temperature 'could be monitored,
the .er.ocourle wires nroviYi.eg the neceese-ry support to hold
the zeneor in position. Since the sensor has no cooling
142
facility, radiation iii be the principal heat loss mebhanism
end the thermocouple . 11 measure -the tenperature
T
”,,ich will be e.^_':a'„l b_ed after %a suitab_e time period. P
The main problem in readingd_r Te stray electrical pick-
up from the discharge. Whilst this was reduced by plactn% , a
capacitor across the dig tai vol t.._eter which monitored the thermocouple output, the effect could not beeliminated sh o:ing
that low frequency radiation was still being picked up. The
only satisfactory me thod was to allow the discharge to run for '
long enough to establish equilibrium (approx. 20 minutes) and
then to switch off the discharge and read the thermocouple
im5..ediate2y before cooling of the disc sensor could commence.
Freli:.;inary ex e: i_nents suggested, as expected, that the substrate heating (equilibriu::s te:_.perature) is largely con-
trolled by the applied power density and is directly propor-
tional to it.
I,et us assume that the power input to the target is Vt
watts and that a fraction K of this power arrives at the sub-
strate as incident energy flux. Whilst X will be constant
for a particular set of operating conditions, we might expect
it to change once those parameters are altered since the effic-iency of the plasma only have been affec ted.
For a target of length 1 and a target/sensor separation r, the total energy K?! will be distributed into a cylinder of area
2lTrl at the sensor ignoring the e':fect of the end of the
cylinder which is not covered with a cathode dark space shield.
The power density at the substrate, is, therefore, K',7/2'rTrl
watts m-2 and a copper disc sensor of radius X metres will receive (K';/2 Trrl)(1TX2) watts provided that X «2'R'r.
By Stefan's radiation law, the disc v.i.il lose energy according to its emi cci'. ity . e, the fourth power of its abso-
lute temperature Te4 and its surface area 21TX2 (since it
radiates from both faces). If we assume that the conducting paint does not alter the radiating characteristics in the small
region where it is in contact with the disc sensor and that conductive heat loss along. the thermocouple wires can also
that
neglected, we can express re. s the power loss P as:-
143 Y P = e . 2 ITĪ.2d Te
'r1a r t."i
6 r _ '2:1-4
When e y:..._ iorlu.. is a..:_eve't, power losses and gains must
balance for the disc and we can write as a first approximation:-
KW. 2 = 2 'NX2 ā eT 4 2 lT rl e
Prevost's theory of heat exchanges states that when a
body is at a constant temperature, it is losing heat by
radiation and gaining it by absorption from the surroundings
at equal rates. Therefore, when the system is at a temperature
T1, before sputtering commences, the disc will radiate
e. 2 TrX26 T14 Watts and receive the same amount from its sur-
roundings. Once sputtering is in progress, we are measuring
the energy radiated correctly by noting Te but ought to include
in the power equati^n the 'background' input and write:-
K = 4lTrl 6 e(Te4 - T14) (6.1)
If we put in some typical values of 1 = 3 x 10-1m,
r = 2.5 x 10-2m (target/sensor separation),
6 = 5.67 x 10-8Wm-2K-4, Te = 41.5K, T1 = 298K, e ='0.1(4)
'yI = 121 watts we then calculate K = 0.097 implying that 9.7% of the input power is arriving at_ the substrate. This result
is broadly in line with earlier conc.Lusions(2)(3) for a
planar disc target configuration suggesting that the analysis used for the cylindrical target is valid.
The variation of K with change of operating conditions will be discused later.
6.4.2 DISTRThU O'r OP PO'..17.R T3 THE S fl3 TRATE
Having estbli.hed t:c_t about 105 of the input power is
being dissipate:. at the substrate, we net attempt to estimate the fraction lue to electron bombardment and that due to the
kinetic energy of the sputtered species and their condensation.,
Since the expression depends on the fourth power of the
absolute temperature, the correction for the input from the surroundings becomes insignificant at higher Te values when
T4» T 4. e 1
144 Consider thLe energy flux (energy/area sec) caused by
sputtered atoms impacting onto the substrate. .Sup: ose each
atom occupies a vorwse V when it is incorporated into the
growing film and that N atoms arrive per second. The increase
in atomic volume per second io , therefore, NV. If these atoms
were deposited onto an area A, the thickness increase would ce
(NV/Alms -I and this is the deposition rate D which can be
measured experimentally.
Since the energy delivered to the substrate. per second is
equal to the number of atoms arriving 'per second multiplied by
the energy of each arrival, we can write:-
Energy/sec = N(E + All) (6.2)
where E is the average energy of the impacting atoms and
DH is the latent heat of condensation. This expression
assumes a sticking coefficient of unity and neglects surface
mobility of adatoms. Since I) - N /A we can write:-
Energy/sec = DA (E + H)
and the energy flux (J) due to the atoms is given by:-
D(E + QH) (6.3) V
where J is the energy arriving/area sec.
The emission energies for atoms sputtered from a copper
target have been measured by Stuart and Wehner(5) who observed
that the energy distribution peaks at 2.9eV though some atoms
have energies in excess of 50eV yieldi n'; an average value of
9.25eV. The results are slightly different for different
sputtering gases and different crystallographic faces of the
target. As discu._sed in Chapter Four, however, Westwood(6)
has pointed out that arrival energies at the substrate will be
less than this quoted figure of 9.25eV and that in certain
circumstances we should put E 0 since the majority of atoms
will arrive with thermal eher ies due to collisions.
The heat of condensation .AH can be found in standard
tables(7), a typical value for copper being 3.2eV.
145
The atomic velume of an element is defined as being the
volume occupied by one mole of that ele-ent and is obtained by
divng the atomi& weight by the density of the solid. A rp)
typical figure for ceeeer' is 7.1em3
mole-1 though this is
based on the assumption that bulk density is being exhibited,
an approximation which is probably satisfactory providing the
film is a certain mini:em -thickness, say 200°A. The volume V
occupied by a sinele atom is then calculated to be 1.2 x 10-29
m3
Since the deposition rate D can be measure& experimentally, the energy flu:: J due to neutrals can be calculated using
equation (6.3) which shows, as might be .expected, that the
enerTef flux is directly proportional to the deposition rate.
If the power input to the target is known, we can use the
parameter K to evaluate the power reaching the substrate and
compare this to J to find how much of the power is- due to
neutrals and hence what fraction is caused by electrons. This evaluation will, however, need to be postponed until the
variation of K with the operating parameters has been considered.
6.4.3 P0WE7 TO T= TAT
Consider a water cooled target where the inlet and outlet temperatures T1 and T2 can be measured by inserting a thermometer
into both flow lines. If m is the mass of water flowing per
second measured by noting the time to collect a litre and using
the fact that the density of water is one gm cm-3 we can write:- Heat energy removed/sec = mc(T2 - T1) (6.4)
where c is the specific heat capacity of water. We can,
therefore, calculate the power removed as heat by the water
using equation (6.4) and compare it to the input power to the
system.
This experiment was carried out at 3C,)a pressure for a variety of magnetic field strengths including the B = 0 non magnetron configuration. In every case the result was that
approximately 80% of the input power was being dissipated as
heat at the target, no significant change becoming apparent
with change of operatin.para;:.eters. ';;hilst this figure is
higher than earlier results(2)(3) it is comparable and illus-
trates the point that sputtering is a 'het' inefficient process.'
146
Sinee so such power is appearing at the target, this would
seen to su;gest that 7,ater cooling is an important feature of a
system minimising substrate teee2erature since .raliation from an
uncooled target ni;;ht be e::petted to contribute significantly.
.5EX-72T-':-',J. R:.S=
A progra^me of experiments was carried out to see how the
sensor equilibrium te'noera tu: e Te and the narameter K (the
fraction of the input pou.er reachir: the substrate) varied with
operating co n itions. The effects of varying target/substrate
separation, magnetic field strength at :onst=ant voltage and,
voltage at constant magnetic field were studied. The pressure •
was kept constant at 30,1 in each case to eliminate one of the
possible variables.
Before each reading the magnetron system was run for
thirty minutes to allow the plasma to stabilise and to give the
sensor time to reach its Equilibrium temperature T e which was
then recorded. K was then calculated using the theory given
earlier.
In all, two hundred and forty experimental runs were con-
ducted split into forty series of six, each series being aimed
at varying one of the parameters whilst keeping the others
constant as far as possible. In all cases `f (the electron MAX trajectory maximum assuming no collisions) was calculated _for
comparison with the target/substrate separation.
6.5.1 DrpJSTfI07 PATE
Since we require the relative contributions to substrate
heating from electrons and neutrals, the deposition rate is
required so that the neutral energy flux can be calculated
using equation (6.3). is previously discussed, the accurate
measurement of film thicinness requires an interferometric
technique but the need to o, ee up the vacuum system considerably
lengthens the time for each experimental run and renders imprac-
tical a deposition rate measurement for two hundred and forty
specimens. A simpler method was required to give a quick,
reliable, estimate of the deposition rate.
147
Consider again Pigs. (6.1) and (6.3) of deposition rate
against voltage and eureeet. If we take as e reference from
Fig. (6.1) 500 volt equivalent to 3400 OAhr
we can une the
graph to calculate a ':zrk up' factor for any other voltage.
For example 1200 volt oro laces 7800 oAhr
1and a scale un of
2.28.
Using as an exaeple the experimental results, quoted
earlier, from a magnetron drawing 200 mA at 500 volt where the
deposition rate, measured ueing the interferometer, was 12,500
0Ahr-1 , - the following result emerges. Fig. (6.3) shows that
say 4OmA at 1200 volt gives 5600Ahr-1 so 200 mA at 1200 volt
will give 28,000 oAhr
-1 if we assume deposition rate is pro-
portional to current. This now needs scaling do..n to 500 volts
(factor of 2.28) to give 12,300 oAhr_l .
The agreement between the observed experimental result and
the very :Ample roo. 1 predietion is about 2 in this case. The
deposition rate was actually measured on ten of the experi-
mental runs using the interferometric method so that the .
accuracy of the model could be checked. The example quoted
did, in fact, exhibit the beet fit; all the others being in
agreement to within 15% (most better than 10;) which was con-
sidered satisfactory for our purposes. It was, therefore,
assumed that deposition rate couli be evalueted to within 15% •
on any of the two hundred and forty experimental runs.
6.5.2 VARITIT: OF SUPT2AT7 777PUT PUNER
The paraTeter K was evaluated from each experiment and
the results analysed to see if any trend emerged. The fol-
lowing general observations were noted:-
(a) The parameter rices slowly as the input power rises
in the non megeetron care. This risireg trend is also noted
for the megeetron case (Pig 6.7) with the K value being
slightly lower for a:et eiven poner input.
K Innut Po7:er .7 Tart tior. Cf
148
100 200 300 4-00 POWER (WATTS)
This reduction in the K value suggests that magnetrons
are more efficient since leLs of the input energy is now
appearing at the sustrate. There are, also, other important
differences which will be dicussed later. The rise in K
flattens out at higher power inputs and at no time exceeds
0.16 with a wter cooled system. This non constancy of K, even in the non magnetron configuration, is inconvenient
thouFh not unexpected since not only can the plasma efficiency
vary as already discussed but also simplifying assumptions
have been made in calculating K. The sticki:ng coefficient for
sputtered atoms is probably not unity but perhaps more impor-
tant, radiation is not the only cooling mechanism for the
temperature sensor since there will be a small contribution
from ooeductioh via the as and the thermocouple wires. •
The fact that he K valuil flattens off at higher power • inputs and, therefore, higher Te values may be due to the
background inieut correction which is significiant only at lower
temceratures especially since the fourth power law is operative.
149
This oorre,ton was used in all the calcIllations for Fig. 6.7
anI may e respc:-.2ile for the K variation at low Te value
where =an error in the background input would besii7nificant.
Fortunately, the variation of is both slow and regular
allowin..; comparisons to be made between the different experi-
mental series,
(b) An average value for K over all the experiments is
about 0.1. The experimental results suest that K is higher when the target/sensor separatioh <y so that energetic
AX electrons stri--le the sensor, but the improvement made is not as significant as the energy analysis results in Chapter Two
might have suggested. These results are shown in Fig. 6.8
where the so called N series has a relatively high magnetic field strength of 7 x 10-3T corresponding to an electron.
trajectory which does not -reach the substrate in the absence
of collisions whilst the C series (2.5 x 10-3T) has an electron trajectory which intersects the substrate.
- F17. 6.8 K v Ainlied Power For A ',:agnetron
012
0.06
0 SERIES B=2.5x10- T
N SERIES (3-740-3T
50
100
150
200 POWER(WATTS)
150
This seems to sugest that keeping the ellergetfc electrons off the sensor produccee apercximately a 20;-:, reduction in the
value cf X for a rieen power inut though we must be careful in
interpreting this result sinus the energy contribution from the
neutral flux has yet to be ceeside-ed. This result correlates fairly well with the energy analy-
sis of Chapter Two which snows most of. the power input to the
substrate coming from tne relatively few high energy electrons
in the incident flux. We should not, however, expect the sort
of improvement which the Chapter Two results might have pre-dicted since most of the curves then taken were at voltages in
excess of 1KV and had sharp, well defined maxima. (Correspon-
ding to fem high energy electrons producing most of the power
input.) As the vol tage was reduced the maxima became shallower
(corresponding to the power input being more evenly distributed
throughout the electron a:erival energies) aed it is, therefore,
less important to keep the energetic electrons off the substrate
in a magnetron operating at 500 volt than in a conventional
syste,A at say 2KV. Also, keeping the energetic electrons off
the substrate will mean that they will produce, by ionising
collisions, further electrons which may reach the substrate.
The energy input from an increased number of low energy elec-
trons is likely to be comparable to the input from fewer high
energy electrons and increase of magnetic field strength to
control the electron trajectories may not, therefore, control
a substrate heating problem.
(c)Sofarwehavecomparedtheconditions LAX Ygreater
than and less than the target/sensor separation in different
experimental series; consider now a set of results (n series) in which y is changed by altering B at constant voltage
7..AX such the.t we treverse the y mxx = target/sensor separation at
some point in the series.
151
75;. 6. . v Atrelled. Power
K
042 - 11 SERIES
V-500 VOLTS
ao6
1 50 - 100 150 200 POWER (WATTS)
Now the K v power curves show a region where K can be
reduced as the power input is increased. This behaviour is in
the part of the characteristic where V. , target/senser f
Separation once again sug;;esting an advantage in keeping the
energetic electrons off the se:.sor. It should be ad:ilitted,
however, that the effect, whilst always detectable, was not as
significant as had been expected and the posible reasons for
this will be discussed later.
(d) Providing water cooling for the target has a signifi-
cant effect on K s7Iggestinc: that radiation from an uncooled
target would be an important source of power input to the sub-
strate. Under identical operating conditions, the provision
of water cooling can reduce K by about 33., (roughly 0.15 to 0.1). • ( This result is in agree:nent wUth those of Lau who concludes
that in his planar D.C. non :r.agnetron syE.te::J, target radiation
can account for 6C-/ of the enerzy flux to the sUbstrate when
the target is uncooled.
152
6.5.3 FUT=RIT.J7 T=E72=77.] T
Since fror.., (6.1), Te4 is proportional to the •
pro,luct of K a:1d the rower in. t (W), we exp:t that Te
will rise as the power inp,-,zt ees of the trends
of the K
.' power viiritio curves will be exh4 bitPd in the Te
curves. This expected trend is observed and noted in Figs.
6.10 and .6.11.
Fig. 6.10 T4 v Applied Power
1
0 SERIES
N SERIES
100 200 POWER (WATTS)
As anticipated, the sensor te!;:perature rises with applied
power inr,ut and there is scfne alvastage in keeping the ener-
getic electrons off the sensor as f:Aicated by the frlot that
the N series temperatures are lower. The fact that the fourth
power of the sensor te;:.perature is proportional to the input
power sug:ests that assuming radiation to be the principal heat
loss mech'anism is reasoaille.
153
6.114 v Applied Power
4
3
1
. Te
* (K -x10
IV SERIES
100 200 POWER (WAT TS) •
It will be seen that the maximum in K of Fig. 6.9 corre-
sponds in Fig. 6.11 to an inflexion in the Te4 characteristic .
end at no time does it seem to be possible to Llorease applied
power yet reduce sensor temperature, a result which the energy
analysis in Chapter Two had su:gested might be posible.
6.5.4 7ATT77F07 CO7PAP,7D TO :m n.(37:ETRO7
Whilst Fig. 6.7 shows K being redued in the magnetron
compared to the non magnetron, both cohfigurations have roughly
10% of the input power appearing at the sensor.
The obvious question is therefore, whOsher the ma,-netron
is a significant improvement on the non magnetron if a com-
parable amount of the incut power is arriving at the substrate in each case. The ansv.er lie partly in the deposition rates being achinved and these will now be discussed further.'
154
6.6 Iff777.777:0- 12, 'CWER TO T7.7 f7:7'..:277'...TE (Con)
Ap:1;finL; the eciutiol. j = DIV (E + AH) to the cal-
oulatich:, of the enercy flux due to the sieuttered at=s in the
two previo=ly quoted exa_ples we use the values as follows:-
(a) ':'a.,7::etrer:
Power input 102 Watts, Deposition rate D = 12,500 °A ho--1
-10 -1 (3.47 x 10 ms ), average arrival eaerify "E .0 since
Westwood(6) has indicated that the eery of the sputtered
atoms will h,,ve been almost reduaal to thermal levels under
these operating coaditions; condensation energy All 3.18eV
(5.09 x 10-19Joules) and I = 1.2 x 10-29m3 giVing the energy
flux due to - the neutrals as 14.7 Wm-2. This value will be
slightly low si cc E will, in fact, be slightly greater than
zero but serves as a reason:..ble first approximation.
If we assume thi t 10% of the power input is dissipated at
the substrate then the total energy flux is calculated at
108 Wm-2 at the substrate 5cms away. The energy breakdown is,
therefore, that the sputtered neutral atoms contribute approx-
imately 14% of the substrate input energy with electrons
providing the remaining 86% on the assumption that the water
cooled target is not radiating significantly.
(b) Non Ma7,netron
If this calculation in repeated when P = 0 using
D = 7500 elA hr
and an input power of 160 watts, the corre-
sponding energy input brea.alown is:- sputtered neutral atoms 5% with the e]ectroas contributing the remaiaing 95%.
Whilst this non roagaotro analyais is in agreenent with (2)(3). previous results ' in that the electron contribution is by
far the most important, vie can see that the magnetron results
are differeat with the 1.1 -atra3. contribution having been
increased by a factor of about three. This trend of the .
neutrair contributing 5 in the non manetran and 15% in the
magnetron case is repeated over a variety of operating con-
ditions though it is not Irossible to be too specific about the
actual values slace there is a possible uncertainty of 15% in
the deposition rate measureaent and, therefore, in the calcu-
lated neutral eacz-vgy flux at the substrate.
155
These results est that as the magnetic field strength
is increased to eee; tnc eher eti.. electrons of the subs urate,
these electrons eroduce more _onisation collisions because of
improved tr _ cinq and __here are the deposition rate thus causing
more energy to rech the substrate when the increased atomic
ffluxC;.rt~.:~ n:'eS onto 1 ~a .
As the magnetic f ld strength_ is increased, the energy
distribution to the : lb ~trĀte , therefore, c.aYgas from eiectran
predoeinance at zero .a xle tic field 'strength to a distribution in which e:'.ectro .s are still the main energy source but the
neutrals are increasingly important. In the above example, the
neutral contribution at 15% in the magnetron case is probably
an under estimate, partly because E is slightly higher than
zero and also because both calculations use K = 0.1 when, in
fact, the meL4netroa. K value will be less than the non :magnetron
value.
It is interesting to repeat the calculations assuming
K = 0415 with no targ:t water cooling, the energy breakdowns
are then:-
Tegnetren: Neutrals 9%, electrons 58%, radiation from an
uncooled target 33%.
„on ae; ecron. neutral s 3%, electrons 64%, radiation from
an uncooled target 33%.
This provides a striking illustration of the need for
efficient target cooling so that the radiation contribution
can be eliminated.
Consider now the energy breakdown for the K v applied power
curves of Figs. (6.8) and (6.9) for the 0, N and M series. Com-
Daring the ': and 0 series at 100 watts power, both give
aoproxim:etely the sane deposition rate, though we should re-
me:eber that a large error is possible since we are in a region
where deposition rate is a function of volt' ā (Figs. (5 1) and ti. ~ o „~. -e .
(6.2)) and the v it age choice .is not too i,.Dortaat as far as
efficient sputtering is concerned. (N sup _lies 100W at 530V, 189mA and 0 supplies 100"; at 900V-and 111mA. )
156
The neutral energy flux -,';hich is proportional to deposition
rate is, therefele, the sa.;e f:r 'esoth examples and calculated
to be ap.oroximely.13 L-e Since the values of K are dif-
fseeent, di"'erie; aunts of the total input power are reaching
the substrate and the equilibrium sensor temperatures are
differert. Tne ca'oulate-1 total energy fluxes are 95 1cm
-2 (N series) anl 120 Wm (0 series) and the electron fluxes are,
-2 therefore, 82 Wm " (N series) aad 107 Wm
-2 (0 series), giving
a roueh idea o the effect of keeping the energetic electrons
off the sensor. Both systems are producing the same deposition
rate from the same nower intut but the N series has a cooler
sensor temperature (Fig. (6.10)) because the energetic elec-
trons do not strike the sensor. The electron flux is reduced
by approximately 23% wl,ich is a sLeeificant improvement hut not
as much as had been orieinally hoped for.
If this analysis is repeated on the 7 series (Fig. 6.9)
for two readings, 1.11 (energetic electrons hitting the sensor)
and. M2 (energetic electrons kept off the sensor) the following
results emerge.
Run. r1 M2
Power (r:atts) 57 82.5
Current (IA) 114 165
T (°c) 83 103
K e
0.095 0.078
Total flux
(Wm-2) 57.1 68
Neutrel flux 7.5
(Wm-2)
11
Electron flux _n
(W2) 49.6 57 m
q of inrut from fieutruas 13 16
The contrIbution from the neutrals rises steadily as the
power input is increased and this teL;ether with an increasing
number of lower energy electrons is sufficient to keep the
sensor temperature rising.
157 ; . 7 7,0777
1 GF-E7A: C3777:73 The nest important coeclusion to emerge from this part of
the war.: is that in mageetrcn sputtering, the contribution to nu'estrate heating Cr.:a the neutrae sputtered atoms in much more significant than was at first thought. ':;hilst in our system the pressure choice o± :10/A effectively reduces the atoms to
thermal enersies by the time they reach the sensor, this will
not be so in magnetrons operating at lcwer pressures when the .
parameter E could not be set to zero. Since the average atom
ejection energy is 9.28 eV, tnsn in the case at low pressure of an atom reaching the substrate without collision we should
set E = 9.28 eV and since A H 3.18 eV the neutral contri-
bution would need to be increared by a factor of three and
would be approaching 50?; of the total energy flux to the sub-strate in some cases. Whilst this is an over estimate since
some collisions will occur, the heating contribution from
.neutrals will obviously be even more important in lower pres-
sure syste.es.
In our magnetron system, approximately 80% of the input
power is removed by the target cooling water, 10% appears as heat at the substrate and the remaining 10% is presumably used
in a variety of ways including the ejection of target atoms
and secondary electrons. There are two main contributions to the input rower to the substrate, the neutral sputtered flux
contributing approximately 15% with electrons the remaining -
85% on the assumption that target cooling is effective.
Comparing magnetrons and non magnetrons at constant power is difficult because their operating characteristics are not similar but significant differences emerge at higher power
levels when the non masnetron voltage rises quickly to figures
in excess of 1400 volts. Under these conditions, as the power
input increases, the Deposition rate does not rise as quickly
and the neutrals play a less Significant part in substrate
heatng. In our system at 2.6KV and no magnetic field, the
electrens contribute aroroximately 95% of the substrate input power, a figure whioh might be expected to rise further at
higher inputs and, therefore, higher voltages.
158
The ma,e,netron behaves rather differently at higher power
levels dra•oing in more. current at edmost cer.stant volta7e,
The deposition rate, therefore, rises steaJily and the neutrals
play a more important part in substrate heating.
At constant sower, the substrate is cooler in the mar;rnetron
case with the energetic electrons removed compared to the non
magnetron but the effect was not as significant as had been
expected. The important difference is that at high constant
power, the marnetron deposition rate is muoii greater than that .
achieved in the non magnetron case. As the ma-netic field is
applied it provides no power itself but shifts the energy in-put distribution to the substrate nuob that the neutral
contribution rises. A high magnetic field strel:gth will keep energetic electrons off the substrate but replaces them with . a greater number of lower eherPy electrons and also an inc-
reased neutral flue. Therefore, the deposition rate rises at
constant power and the substrate temperature also increases. It would only be true to say that magnetrons provide '
'cool' sputtering if we looked at constant high deposition rate in an area where Ane non magnetron was having to operate
at an inefficient high voltage. The magnetron would then
achieve the required. rate with less power input than the non
magnetron and the substrate would, therefore, be cooler.
To summarise these comments; it would seem that the magnetron can provide increased deposition rate at constant .
substrate temperature or reduced temperature at constant rate
but it does not seem to be possible to increase the rate and
lower 't he temperature partly because of the increased neutral
flux contributing more to substrate heating.
6.7.2 FAT7eRC 77r7ESS'tv FCR A7 17'7I'3'177'7T HIGH R'‘TE =IPM
Depositioe rate can always be increased if the power input
level is allowed to rise indefinitely by using a mare capacity power supply. Problems will, however, arise when the substrate .
can no longer tolerate the heat input. The following factors appear to be desirable if the system is to be classed as
efficient.
159
(a) Coolinj the target is essential, water cooling being
the obvious choice.
(b) The cyste: should te run at the optimum voitage, the
power increase t:) impebve deposition rate being achieved by
using ::.axi:io.s:, current at this chosen voltoge. In 3:10 case, the
preferred voltage is about 800 volts but any choice in the
range 500-1000 volts 7,oul1- .:e. acceptable. Efficiency reduces
significantly above approximately 1400 volts explaining why
conventional D.C. 'sputtering cannot achieve high. deposition
rates successfully.
(c) Keeping energetic electrons off. the substrate isuse-
ful bat by no means eliminates the electron contribution to substrate heating. Ideally electrons shoull be kept off the
substrate altogether meaning that the substrate should not be
the counter-electrode of the sputtering system;
One possibility for reducing the substrate electron flux
is to introduce a biased grid between the target and substrate. Unfortunately, this is likely to cast a shadow giving non-
uniform deposits. Another approach(9) to control the secondary
electrons has been to incorporate a series of biased grids.
Whilst substrate heating is reduced, shielding is, unfortunately,
required to prevent the electric field associated with the grids from interfering with the plasma ani the resultant complex
geometry produces not only a non-uniform deposition but also a
lower deposition rate.
Perhaps the best solution is incorporated into the
Sputtergun which utilises the fact that most sputtered material
is uncharged to 'spray it' in a partioalar direction whilst
the electrons are drawn away from the substrates to the counter-
electrodes.
160 •
7.1 1-=D77.7707
In this, the final nheeter of the work, it is approrriate
to look back on and to so:;le extent restate, the major con-
clusions of the preceeding chapters and to consider problems
which have emerged. nilst some problems have been elucidated,
others have been identified as a consequence of the work and
this should be seen not as a disappointment but as the expected.
result since scientific progress throughout the centuries has
led to the commencement of further work.
The title of "The Role of Eleetrons in Sputter Deposition
of Thin Films" is a broad one and many of the basic facts have
been long established. The role of the electron in its simplest
senee, namely electrons causing impact ionisation to produce
ions which bombard the target, thereby releasing sputtered atoms
and further secon1ar electrons, can therefore he assumed. The
main function of this work should be seen as a more detailed
comment on the precise mechanism by which the discharge is
achieved and maintained. At the time of the eommenceent of this work, magnetron
sputtering was not the well established teehnoloPy ehich' it now
undoubtedly is, but the firdiegs of this work have not, to far
as the author is aware, been published elsewhere. Lluch of the published work on magnetrons has, so far,
tended to concentrate on the technology of producing thicker,
improved quality films, at a greater rate, the role of the electron often being summarised only as a brief statement to
the effect that ionieatien is enhanced by the trappirg of elec-
trons within the plasma region by the magnetic field.
One of the aims of this thesis has been to attempt to
maintain a balance between the theoretical approach based on electron trajectories pn the One hand and the practical aspects . of the technoloy of -„er.odu:...ing useful thin films on the other.
161
The-e is, however, little chance of producing satisfactory
thin films without se.::,c knowleds-e of the theor:: involved in
the deposition process and it was with this in Mini that the
work wa:: undertaken.
7777-77: 07 =7 -=
7.2.1 :7E7.7! A-A7.YEI3
The nair point fron Chapter Two was that a relatively
-mall .ruber of high energy electrons cause a large part of
the 'Cower inrut to the substrate in D.r.;. sputtering with a
planer target in the absence of a mag-etic fiel I, the analyser
curves of current against electron energy having been shown to
be equlvalent to power input against energy.
This enery ana'iyis was carried out relatively early in
the project and the point was made in Charter Two that a
decision was required on whether to si.)end more time accurately
producing more UUrVeS or whether to accept the broad trends of
the results as outlined, above and to observe the characteristics
of the discharge when aHri17,netic is introduced to modify
the elcct,.on trajectoris. In future work, further energy
analysis would be of value, in particular to extend our know-
ledge of the following ncints:-
(a) The energyanalyis curves of current against energy
were shown to be equivalent to plots of power input against
electron energy and po!,:sensed the general chase shown in Fig.
'1.1.
P1::. 7.1 Ene:-r--, Srectrum
PROPORTION OF TOTAL CURRENT
IT
ENERGY (eV)
162
The lo: ,,nerny -J.--xinum was found to occur at about 25e7
irrespeetive on' onerat.':ng condition- and was thought to be
relatively uni:nnoriant so far as substrate heating was concerned
when compared to the larn.er peak at an energy correspohnling to
the inter-eleotrode poteettal.
lost of these curves were obtained for inter-electrode
notentials 4 11 excess Of 1Y:V,whilst a typical magnetron operates
at lower voltages. Since the high energy cc-al: tends to be
better defined as the inter-electrode potential -rises, it is
necessary to study the distributions more carefully for tyrical
magnetron voltages in the region 500-1000 volts. A better
!tnoledge of the peal: definition would provide more information
on the advantage of kecping the energetic electrons off the
substrate and this could be compared with the substrate heating
results of Chapter Six. The energy analysis results which are
available at lower voltage would seem to suggest that the
eliminaticn of energetic electrons at thee low voltage's would.
be less advantageous than at a hieher operating voltage from
the point of view of substrate heating.
(b) One of the more interesting features of Chapter Five
was the approximate correlation between the theoretical pre-
dictions of the electron energy distribution at the substrate,
assuming a knowledge of collision cross sections, and the
experimental energy analysis of Chapter Two. Two points
emerge from the theoretical predictions which could now be
checked by further enerny analysis.
(i) The detected energy distribution should be altered by
placing the analyser at different positions in the NG and
should be changed slightly by pressure change. Neither of
these changes were extensively investigated. It would be of
particular value to position the analysar as close as possible
to the CDS/.70 elge to see if a greater number of high energy
.electrons could be detected; theory having predicted a rela-
tively large high energy signal in this region of the plasma.
Unfortunately this boundary cannot be appro,.ched too closely
or ion oroductlon is affected.
163
(ii),Hays estalichel that there is a peak in the enerz,y
distribution curve cr.:Tres-e'en:ling to electrons reech'n: the
ana7yser without co1741-ion, we might expect that there will
also be a pea':: corrennonding to electrons which ma:e a ningle
ionising collision in passing from target to ru.':estrate. These
electrons would lose nem? energy Curing the collision and sncuri,
therefore, give a pea'..: in-the energy distribution at an energy
below the maximum possible. It would be necessary to scan the
enerev distribution curve more closely looking, for fine
structure and subsidiary peak::, in order to confirm the nresence
of such a oeak. The present analyser would not be suitable for
this work since its acceptance bandwidth is too large but
imeroved instrumentation miht be succes-zful.
(c) So far no eaercy analysis experief:te have been per-
foried on a magnetron configuration and vie, therefore, have no
information on the r, recice energy distribution of the electrons
which strike the substrate in this type of system. At the
moment it in only possible te nay whether the co called 'ener-
getic electrons' are reaching he substrate or not and in the
cases where the magretic field is sufficiently strong, we cannot assign an energy value to the most energetic electrons which arrive at the substrate since the outcome of the necessary
collision is uncertain.
In addition to studying the "eicetron distribution at the
substrate, we also need the spectrum of electrons appearing at all the earth ;lanes within the system geometr;e to see if these
surfaces receive any of the higher energy electrons. Geometry
variations directed at keeping electr)ns of the substrate could then be investigated, as well as the results of ihtroducing.sub-strate bias.
Ne conclude tnat t'ne c:Ier,:y analy,As, whilst contributing much te our present understan:ing of the basic.seuttering pro-cess, could provide further inform-ition if a detailed programme
was Initiated aimed at different geometries and the effect of
varying them. This might '._e especially true in low pressure
magnetrons where repeated collisions with gas ate= would be
less likely to modify the observed electron energies.
164 1.2 .2 .7,2-.
This Vik VC split into teo. eeetiene concerning substrate
he-ting film unferity. Ele-tve:-.o were shown, as expected,
to be t principal energy input to substre but the energy
flux from c:.uttered neutral atoms was found to be more signifi-
cant in high depoelion rate eystem. This neutral contribution
is likely to be even mere important in low pressure systems
where the eollie'on probability in reduced.
Speelfic conclusions on ...ueh cf the film uniformity section
of the Work are difficult to fore:ale:to ei_ce the importance of
ac}eievieg a given uniformity tolerance clearly depends on the
use to be made of the fabricated film. All we should say is
that cylindrical magnetrons h.ve an unfavourable terget geometry
as far as film uniformity is concerned and this aspect of the
deposition process could be improved by using a split target
system with differ power input densities to each eection.
Whilst this concept is not cohvenient experimentally, the
alternatives are not convenient either and it does mean that
uniformity problems are not insurmountable.
The alternative uniformity solution, discussed in this
work, of using a continuous production system with the no
called 'modified cylindrical target' has demonstrated, on a
small scale, that such a target can operate suocessrully as a'
magnetron.
The most important aspect which merits farther investi
gation concerns the arrival energies of sputtered neutral atoms
and the idea, as ihtroluced by ';:estwood(1) that neutral ener-
gies will be thermaliced after the atoms have -progressed a
certain number of mean free paths from the target. 'A knowledge
of the energy distribution of the sputtered neutral atomic flux woula be alveetaeous for two separate reasons:-
(a) It would pro; ide information on the accuracy of the
predictions(1)
concerning the ther:Ialisation of sputtered
neutral atom energies. At the moment it is clear from experi-
mental results that a uniformity problem exists yet there is no totally satisfactory theoretical model, since the affect of scatter!_ng is not certain, especially in magnetrons winich can
operate at I ressure. stuly of the at.1 c ar2iv1. en6rT:42r;
,f cress7-:..e a better r1.C. de to the
theoretical s.hould be
(b) In substrate '- c''4"ir"stnh.cs e re:aire E (the a-rer-,-e
arrival ener.7y of spattered at;:.:.$) so th,t
signli:".icart) fre, atc,z striking
ths, bstrte cah be ccalatel: As disCu.:sed in Chapter Six,
the est inf7.-)rr_ation awallabl,:: favours :iattin E= 0 though
this ass_::ption is -less hiy"Lj ;Je true for z.agnetre:_s ol-,er-
atinz at low pressure. Whilst it is c.crtin thlt thc neutral
shery L:1at has under-esti:aatel by setting E =0, an
ex7eri.-..ents.1 value of E as a fu-:stior_ of pressure would indicate.
the significance cf the error. Einoe the neutral ato:.: energy
input contribution to the substrate heatin is more i.::.nortant
at ihrates, It Liay he that low pressure netrs)li.: have a
:ore si6nificant nutral eontri'l,ution th n that estii%ated in
Chapter Six.
The proble:a in measuring the energj of neutral species(2)
is th.tt only syste.:,: b..:xed on ;:eaguring the ti!: of flight
between two points an; avail,ible. Coburn(3) has discussed the
idea of ionistng t neutrals and then using deflection methods
but the obvious diffi(taity here is that the energy of the sput-
tered atom aunt not be altered in the ionisation T)roce-e.
S7:77t1T;
The discovery f.!..4.planation of the sharp switch of plas:na
characteristis v‘ith 2resure cln,r1:.:e perhaps represents the
major feature of +,he beil,-orLour does not seem to
have been hotel in othcr publi:-:hel work. The moot striking
oh-,rcteristio is the sharpness of the switch, the change in
oasa n;---1-.20ristic:7 being so simificant tht, when the effect
sac 7J1):- rved, the i=ediate diagnosis wan to suspect a
fault in the eTectri.o'l system. Passing references to plasma
.tran=it;ons bee:. male by ?-ra3is(4) ani Wasa and Hayakawa(5)
and will he di!:cur=srl later. 'They have ot,however, mentioned
changes in plasna nmJeters which are as large as our observed
transitions or under the same operating conditions.
166
Wh'ist we hve been 'able to :7 -low th: the switch is close-
ly lihf:ei to the y „,,:d ratio v:here ›/„., is the e.axi:..um dista .la from the target of the eleotrpn tractories before
collision anl d is the S len:;th, it SCEs unlikely that this
the sole criterion determining the state. Further
ex7e-riental :r;1 theo,-etical ::or !.: is re,luired if the m:lerctan-
:n of this switcl.n; behavicur is to be improved and two main
asoects of the proble.:1 seem, at this st-19e, to merit investi-
gation:-
(a) The so c,!3led switch in plasm% ch.:.racteritics has
been explained in tern,; of a pos.-:ile m,,ichanis:ii by which ion
production can be proved to prevent extinction of the dis-
charge. The traition oressure hrbe,h shown to correspond
to the electron trajectories, in the lbsence of collisions,
just touch-!.nL; th.7. CflS/TT1 edc. Further wor should establish
whether this transition preesure is a fuction of ;;.anetic
field stren3th,.in particular at .r.gher Field stren-. In
the present work, the viailable raLge of variatio of field
strensth was such that ohiindes in transition pressure were not
detectable.
It seei.ls surprisin that the trai.sition pressure is as
high as a:Troximately 1!>il since Many cylindrical masnetrons
operate successfully at lower Tressures. Commercial equipment,
however, tends to use much hiner mIE;netic ficl ri strenL;ths than (6)(7), for example, those available in this work. Thornton
has used ;:iagnetic field strengths about five times hij;her to
produce thick s:.:utterud high rate copper coatihEs for operating
pressures in the ral.ge. 1-39,LA.
The switching phenOmonc,:l, as discussed in Chapter Five,
occurs presure is alters.1 (thus chang.n6 d but not y lax) but not whe,.. st.rengtn in altered ch-i.n;in,E.both
and d. StartL.L: at say 39AA pressure and y d, then
as the L:2.-netic field strn.:eth (2) is increased, 'oeth y and
d reduce u:.1 the ratio y ,» fi tends to unity only very slowly.
Since the plasma b.-es 1..-2orensively str.)nger as P is ihoreased,
as indicated by increased current and colour at constant power
input, ne ought to be reduce the pressure to below 1y1.
Shaded areas indicate
colour regions
167
2nd still retain a -,trosg nlasma provded the magnetic: field
stregth has inorec..seacufficient:y. &n important poiht tyo
note 12 that v.:,e:-_ the plasma is :e- :n, the CDS length
shrdn'.;.2 to a very o:-l1 value wn:::h ws,.11 not be lable
by the .,:theto::-.eter teohni;ue wn!_h :3-s used an'l the plasma
ap:.eare_sce is totally dominated by tl-,e intense oolouration of
the 771. This wo.lii seem to indidate that the !77/ is the Lost
important region of the discharge under these oirou::.stances'
and the role of photons :Light be cloghi;:ica.nt anl nil be
briefly diricussed lter.
(b) Wtilz;t the switch in plasma -para:leters is detectable
for both the oonehtional and 'modifiedi . cylindrioI geometries,
the i_fluence of target ge3L:etry re,:uires further in7estigtion.
In particular there is a significaht power loss to the earthed
base plate of the vacuu;ii yte..,1 from the bottom of the targets
when COS shields are- not fitted. Confirmation of this loss is
pro7ided by tne existence of a glow region in the plasma from
the bottom of the target to the earth plane (Fig. 7.2), the
spreading of the plasma below the target being controlled by
the magnetic field.
Pie-. 7.2 C-)loar Renin- 11 the Cy'l!:!al_ 7,,;:net -oon - Sche:ratic
18,9
c'..: eh 4 7 related to the efficiency of icn pre-
tke chara-..terstics migt 'oe altered by
provi a ADSchiell elimir.ate this 52ov,er loss an, improve
efficiency. point was
ea'nstructci and will ed in the section.
7.3 A- H C.:-.777TFAT-TT
One target geon.e:ry which is ',,:nown to be 13::rticl_ziarly
effective in pro -lcin ir:nisation is the "Hollow Cathode" dis- (3) cused by Little and Von Engel . This f.uste.::: involves the
use of two parallel plates both acting as cathodes placed op-
posite each other a'::1 keft at the slme iotentil. Electrons
leaving one target and failing to collide with a gas ato:r1 are
therefore, reflected back into the p1,4sma by the other target
and losses are significantly reduced. In this geometry The
cathode separation ec.'tro Ic the width of the d-r% space:; and
'when this seE;aratlonis :Alfficiently reduced the two negative
glows coalesce and the lip-ht e:aitted as well cc the cathode
current density rises . greatly. Under these con,ftiticno with
the olas:.ias ccupled together, the device is extremely efficiAsa
et sustaining ionisation and the glow can be male sufficiently
intense for use as a spectroscor,ic cource.
We can, perhaps, coLthine the efficient ionin::ation of the
hollow cathode with the nigh rate depoeition of the magnetrOn
by constructing a tarF,et to the desigh of Pig. 7.3. In this
system the end :plates constitute a hollow cathode and will tend
to reflect electrons back into the main plasma region.
F1F. 7.3 An H CohfiUration
7cm
CERAMIC SPACER CDS SHIELD
END PLATE
ORIGINAL TARGET
Elcd 8cm '
::'i::-. 7. I.
ATTACHING
SCREW
169
-1------ MAIN TARGET
COS SHIELD FOR SCREW HEAD
END PLATE
ellS SHIELD
INSULATOR
SCREW HEAD
Si~~e ~~e scrc~ is ut tar~et pote~t:al it requires insu-
The p\ost s~ri~-:i;};; i~;.itial obscrv~ttio~: ·j:~:'t: that for a fixed
pressur~, voltu~e t~e introduction
Q:J~o·.~e 2SfA fer o[Je:r:-'.t:~~:~ volt:..'..;;C]s iYl €:':G(-,;:;~ 0: 500 '!(ll ts, the
·~::yste.:l dr3:;;S ~,o l;~'lCL ~~')'.·;(~r t!'":'11: th~? gln.::::::· eDS s;lielj, sur-
too A£ter
1 ,--:„.............----
BLUE ..------------, - ' COLOURATION
_-- -END PLATE
MAIN TARGET
. .-----". \
----°".
I
CDS/NG BOUNDARY
170
extin -Inin.: th:: !.1.= the t- rzet wIlic 1. not w.,,to,- cooled
.'.-:a.: a d',:17. r,---]. :, r:c a:_o :=.1:sti, excesive heat- L.:.
The :?la . ly ve-:: :tro.-1,- even at rresure:. ag low az: ;e4.J. and
as a rou:h .-u-YIc, the -::::-..te.:. '::' -th e.:d plater.: exli.1)its approxi-
r:lately the sa.:.e tar„,,e orent de:ILIty at 7/A _12 the syste:.%
without ei plates drfx at 33,A for a fixed value of voltae.
and -anetio flold strencth.
To s7,itohin wa ol-...T,erved In the 7recf.,3ure race ;AL-3;AL
which was
'..ted and it i'.. cear that the hollow cathode
e:feOt proll:,oel by tu e.11 plte:: d... do:-.:in-ltinE; the mode of
disce c-;:cration. The ai2:7:aarar.oe of th,_; plasr.a (7ig. 7.5)
is also oh ,z.ged With the CDS len,th hocor:.in,,L; 1-,21 uniform and
the intenLe plasL:a col,Dur 1-:1:1,1; 1)1 u,, rather th:u croon. A
green cop:;er .;low wa:: ab:7er.t In the central region of the tar-
got an,..: .w:e only observed in the shaded vo;;;ons of FiL:. 7.5.
,.,.g. 7.5 Pla:Ana Appearance '1 '''" 1 Pate: - at 7bt, p7:-(=ure
GREEN COLOURATION
The re:aoval of the switchin,! effect in an iw,proved
ionisation environ7est sui:;,:.,e2ts that sItching and efficiency
of ionisation are rell-..ted and we can clearly conclude that
tr7et Ewo:-.etry ifJ a[. -1:2.ortant factor. U.ifortunately, e
cannot now c.::::zare y,..., to J. since neither y 7ax nor d are -AA
constant'_.:: this gee:ietry. The :)ara::Icter d varies as shown in
17 . (7.5), no obvio....s CTS rc,:::i3n 1)eing n.:.Fo'.:iated with the
,,L,..! eleotrons coming off en 1.Dlates. r11/"(' ''O'''''-'1 -(' for
CURRENT (mA)
171
the el-:"1. pl:Ite7 i2 not e.:-.,roprate rice they w4 71 be channelled
aloc.: .'1. ,.:H 1:ie.: in.tne :,_1...tence of col1i.7.1oqz.
It ie n)t i -)e :-_.-1r. that th-; :-.1 ::.1 .1te lel:linate the
:laf:.:.a 1-)eh7.vi f..- i_er :he: '.ontril;z:.t.a'-)eut tw, t':- ir -1:: of the
f:,: .7tteLTI1.
he 1r(), :):_ j. :;1-at!:.: .;.ere rl eel . 1.th r.11er plater of
dieil:on e=xie:' so that the e%dr-: no co.f.tri.c.l.ite aroxi::lately
4: , Jr the tLtr,7t .area avail7,be for c-_utte--1:::,. The p1ast-2a is
not nov as intenee and coc:pari2on of operatini: currents' at 13U
(Fl;. 7.6) shows the :Laz:_Ltuic of the Cfect w...i.c11 the different
end ':,lateL prol- zee on the curront/mnetic field chracteri!;tic.
The a-eas give:. ,,-) the fl-::re refer to the :i.re"..,, o tarc:et cur-
face boil.:: :.puttered, thes'e f'..uree hein„: lower than a calci- on bd cir:.ply on th:. target din'n:ionc :11i.ht ::. ua;est since
the (nS slliel]. -1AibLt 2:2utte2.ing on eertaLl parts ();": the
target surface.
F.L. 7.G eI'Ispn '-".TLI=IlEilLL144-11 P,C0 volts
200
100
LARGE END PLATES ( 235 cm'. )
SMALL END PLATES ( g6cml )
1
NO END PLATES ( 81cmi)
2 3 4 5 MAGNETIC FIELD ( Tx101
CDS
NG
172
7de: thece cc::11t1:72.. the c):inal wth end
v.Lt71 an1 -2::_ectrDn
tr:.jectorie:: tlIc; a -,, cc: G7 collfL.Lo: the
This T:_ew wt Ea::cess-
fu.ily witht ".7DS s'L-11-: at 39A1 pressre
the 1-.er draw is n..;t but at Lower pl.e.;I:urec the plasi;la
intel-:sity is nDticeahly the is
aE:ain in evllee at :,-.1-1d Pl,;. (7.7) sho.:. the CDS
a:Tearance i. tho . '07'ht c:rre:It state the 'OFF' low
curre;lt state where the 8reel: ce or rolourati alet.
7 7 """4'''' 4'
MAIN TARGET
CDS
NG
ON OFF (high pressure) END PLATE (low pressure)
The follovAnz noiLts 7.noul.i. be noted:-
(a) The switch is now pre:;eLit but at a lower precure than
is ol.,served without ea- ] 2:lates. This that since
io'isation is improve by the end ;dates, the rlo.c.ma has less 1:;self until the pressure is further
decreased whe:-. fewer col:Li:I-Lens nul.iify the v1ae )f
improved ionisation.
(b) The 'switch C! and 'cwitch off' doot exhibit exactly
the (FiG. 7.8). The reaspns for this
a:parent hysteresis are not clear.
173
50
I (mA)
SWITCH OFF (pressure decreasing)
--- SWITCH ON (pressure increasing )
I III I I —›—
4 6 8 10 PRESSURE (microns)
The switch off teildz to he relatively olow takin;J: ap-
prox'7:1'2.tely one E,";'30:1 to co:nDlete drin. which time an
teri:lediats CDS shpe (Y3... 7.9) is trinie as the plazma
rove s •fro:: '.he O :;t::Lte toth 070 state.
7.9 T:Itelyr.E3' ,te Sta:-e of fli ir Sv.ttr!h Off
CDS
NG
MAIN TARGET
END PLATE The :.itch o lE ter..I t occur at hilicr
m1792surcs :J.:: chow_ (7.E); no interediate sae (Fig.
(7. (1)) iL
174
(c) In V..e 'CF:' state, the CL:S length lc larly' control-
by thc :imencina 3f ,:nd plates an1 CnS hncead
(7.7)) so ht hire.oentral t - - - et
d.:rk. s:.acc electrchs coin; off the end
late:' are pro.jec:.,_sj t - .-i ,- CDS. It shou"_: be %otel that
the CD3 has spread out;.ard:,. from the crigi:al central tar,:).et
ani not oatwards from thi enl plate anitIlere is no sign of
a separate ODS as.po.:ited with the e:Id. p17,.te.
on therercre, the system adopted a mode of
oPeration where icnisaticn in ar, en?argei CT)S re Lon is
an2-irentTy fave,.:red in a weak nl%Pma.
C -77.7!ri7-7 7=
If tni. ::y:.tem was to be I ves',IL:ated in donth,
we woul'i req-Are a better nyste design incorporating tar;et
water oin and impruvel. CDS shields so that ope2ation would
be possible at h1i.r power levels. These preliminary re-ults h.,.ve, therefore, been presented in this ch pter rather than Ch'ti:te.c Five since no firm conclusio.,,s c'.!.11 be drawr„ without
"urther work. The ,-eslts 20 far support the idea that switchin6
Ic anocia,.te'J. with plana sustai.:.ment thoi„h the idea of oquatin:,
)(..„7. and d early too simple ;t: the case 0-7 this rorc com-
plicated geometry.
7.4 P077.1.77.= It is n..t the intention here to state again the main con-
clusions of the worn oh switchi,,g but to T.:.ake a few concluding
remar.K-s. (A) who who usel an R.F. soter:-., briefly di!:cusses
"The rather a'orupt ch.:-.gcs occurring in electron imninge-
ient L:rofileat the :-vicstrate wi.th sm-1- --_ec in the magnetic
field ctroH,Ilth" and p:JJ,gEsts- the are in(lict -;.ve of the plass,a nw. toi:in t.j sligtly di.fferent oeratin modes. He further suagests that the energy distributins of the charged particles are beil:L; severely affe;ted at times by the strength of the lonitudinll magnetic field. Since thin is a small s.::itch
from one strong state tf another jn R:F. lonL;i-
tudihal ficl], it is not the trancition to which we
refer but 1...,..'ieates th'2.t the switc.:1LI_L o,r:ce:;t to an alter-
native
o-i_e17..-Itin2 mode is not new.
, '
(V. ) 1.
:J·1~~~· .. eti(; : ':'cl.l.
175
clo:Je
of
Tr:e o.;,ti::lI.lf:1
Vi,' l'ec.'"lced 8
The
176
The only tra,F_It!:::. -tween the p;:sltive .J.cl
-..riti.o.:1 val.:.e of magneti:: fiell w?.s re and -,,hc ,lischarge
be^ame u-i..t.,le a:-.d t':_ei. clr.g,..-:: a:.pear%::-.:e wiLth a sl'.- i-, chanre
in nt. Th:_, culo-..1.r w-.:..: 01;270:: tu ch-,.:.re be-
twe,:r. blue -.1..1 red in ..:-: arcon ,>.7.1copper tar:et syste:— Zinoe
the 1-,ress1;re. was 5 x 10-'' torr (:earlytL) order:: of :_ ..,:. -..1.tude
lower th-In ours) is .2...: ivertel r:..:.netron reome ry v;ith a
cr...tic,, mag..eti_ f-ii-:'d stmhgth of 3 x 10-2T (fivo til:.es hirher)
aLd 200 volts letwe:.n the eleotrodes (three. times hil7her) with
a switch fro:.. blue 1,o rel (rather th...n green) it sees unlil:ely
that ti.:- n;itch i, the s,,....e as our el7served transition..
. olr,f- i.-, -h( a'.,ocLe of a'. etorn, Li-,rnet! field
e7,-..hibits te u.:ui.1 CM a:.d. 7..1 regions of 1:-1. discharre as
described a previos :.hters a.:.3. is in the 'i-ositive. Hpao.e
crre mot. Sir,oe our p:..as,..as ol.erating at relatively high
pressure arld low m-.:..rT.etio fieil trenrtir also show there
characteristic rerions on both si,ies of the transition, it
seems rousonabls to as..:r::e that they are al!,o in the positive
space charre mode -.-:.(1 the switch 1:: from o!.e positive sp-ice
cho mode to another. Farther evidc,;,oe that our niass are
LI the positive ::::-!.ce cargo mode comes fro::. Gill and Kay(9)
themselves who are 1.51s to operate their Inverted li:ar..etron. at
1Ik -presslIre,650 volts in either the !:o:sitive or negative space
charge modes by ch2ncre of 1:-.:;:,eti-:; field strerth. These
operating par...cters 'ire similar t our o:in, though the pressure
is lower, and the positi -:e spaee cl.rre mode is prolu,:ed at
9.5 ..x. 10-3T (our :Larnit;,:.de of ma..etio field =.trength) whilst
5.7 :: 10-2T (a 512. foll'Llcrease) is required to produce the
ner,Itivc sac -=:har!._:e ::sa.-_,. An inverted mag_etron is, there-
fore, in tIv. !?:.;sltive -,:aco ch-re mole anlor our type of
co::.ditions, but it should be noted that the operating conditions
of the nor ni and invef.ted ma.-letro: are different.
iasa an! Eaya.ca72(5) operated :,. cor.vconal m:-Lgr.ctron at
hi,:,h ::1 -irnetio fi,,11 .stre.rths of order 1T and 1/(1 pressure to
produce a so railed -o-21.7ied ne:,7:it -ire space charge mode in
177
which there -Is an are.!' ,ble :oie a:-,: :tathode fai -. both of '::hich are cacle of prol..-.7.1 electron 7Imi-a-tt ienizal_on.
Th,:y f=ther calc_,I:.to that a ch:c L their :-:;-:.- te:” fro:.: the :.,oeitive tc :ti.-e s . ce ch..re mode (,-fc,aracteri: V he ) :::,..,1a, toretically, 1- e at a Llaneti-; ld fie streth as
e
2 low as 1.4 x 10- T. Thi: trarslt , n.1 ficli is about twice as hi L.:h as our :aximm f' ,:,1,1 .7-12.:-:--t':. ohe aiin tht our 6ystem is 1.-. tIle .:.cLiti-le s: cc chir;:e mode. Furthermore, si_ce we
havr_. h -7,her 2rescures .;:hieh r?.vou increased electroL radial velocity, a ,:,.;:-,itIT.ve :,7-,:ace charge node seeL.s even ::ore likely.
All the evidene -av:il.t.le ;_oin:ts to a positive r.itace charce mo.lo r1.1_ i thca_na.lynio in Ch.:..:to.r. Fivc.is, th:Irerore; 1,--.1id. The e;:=.1.r, cocc:111- lon from this di:-.cuscio:. i:. tt the
with ,; -1:-J::or:leno.1.: i? too coftl21icateq to -,c., c=1)letely
exp1ai7_ed ',y e,.3uatin.:. y to d. .. ion,: term aim should,
threfore, he t:, estilDn:1-1 accul-Ltely the potentia] distri-
but'on 2r)::!-: the di:,che for a varet:T of ,nac!:etror.-
operatin,:; oonditione ani ,:::eoc:trie:7 and to ::ursue e-leri;y
analyis in manctror:; to sec if less si.:_ific:Int re . ]ictri-butins of electron eheri.iec (i.e. tranitions) are, in fact, rez7:ular evets. , 7."1, TH7 r= "Y: DTC'7S.
One acne _:t of this won: which has bee:: almost completely ignored is the role whioh 1:hot_;:- from the 7G play in the ow;- t:linLnt of the plas. Little :2.n:1 Von EnL.el(8) in discussini; the hollow cathoe e'ffect c'Jg::cot th::t .:to-electrio e;Lission at the tat causel by ultra violet ql:lat from the :TG is an
i_oortal: fiotor in_ ni., 51:ooL,etry. Thej ''u.rther s.:Lest that
!,.:e ion cdrrL..t fro:.: the 73 is small compred with the current
from the (2D'.7, ani se:. thr O as a :.rovider of photons rather
the:. JL L ..on:-. The 1.ol,-. of the 73 in a normal lischarge
s, however, leas o1.,-r since the !TG il.tohL:Ity is very ::.uch
reduce,I co-,:ared wich the holl.o...- Cathode G.
Am2.1L.....;i: is Cha-2ter Five s....owed that the CDS could not
1:1-o7-1 :1e ,.;:-., -c..1. he
sropniary el;:ctron coeffd._:i.:nt fc--; ion imponot) ia too small.
17, the 7-..! a.: to ef-
fc.;cti7oly a; adlitional
wo;.1d Le . . he d.Lff.:.)1t, to
accenz I ;.;.
intc::::c ultra vi-)1,-_.t .t-A.1 as a 1.:rce
of ailitional • a.-.1 t cervc the :)ffert, " any, of
j thlo
7ro:. the poi.lt of view of the role of eloctr3)::,:, ionisation
and e::citation th and it is
not too i.:.,2ortIl.at .'h oh of the t'..o pos:dble 73 role:-, is do:.irart
sinoc they both prol- oe ea.2. :.ro.3uct, enterin
the CDS fro::: the 70 will h..:11 to sasti:: the piaL:::.a ,::111.1st enterin::, the CDAwill ctf.ie Lii. trot ro)_eacj.::.L: elec-
tron:: azId caaLe thor iJ aticn Ii the; CDS which
help to sustair. the --itcl- 'nj • hov:ever, benefit a clearer ,Inderc.:. tanding
of the 1:recicc 70 role.
7.6 Cfl7=T"r1 777 777S SihoP the ,:h:),ters 'nave all colot:entrated on
the which le. in: , it wa._ folt to Le al)proprLate
that this fir.al chiu Çj sho :eoulate on future- work, bath'
expe-iz:et:-.t1 a-2i theoretical , and on the proble which are
as yet, unrecolvc:d. Ulldoabtedly :;:ajor understan a plasma
discharc oompl.:tely is thc fact tat thel-e are :::any co:ncting,
inter-related 1.for:)e sec, 11 of which ',lave so:he probability of
occur-in,:; 'Ind it is diff'.oalt to an.?.14:e which are
and which are Lt. The r.)le of re'atc:1 to sputter Cieposition is
clearly a ,:!iar, one are all areas of the
sputtc2in„; iLeo:Let ,-j it i2 hopet th7).t this work has :Jade
scar contri:,:_ti_on to '.:r.clod,L;c of tlie sabject.
179
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187
SUBSTRATE
dt TARGET
L
T
E.74E14E- -
X dx
Substituting for e gives:- D = KT2dx
dl
[(T2 + (l-x)21 2 0
188 Appendix I
Assume:- (1) Uniform current distribution over the target area.
(2) Cosine distribution of material sputtered from each point on the cathode surface.
Total deposit (D) from the target strip onto the substrate
element dx is given by:-
L dldxcos2 6 D = K R2 R
0
where K is a constant
(The solid angle subtended by the substrate element at dl is dxcos 6/R2 , the cosine distribution introduces an extra factor
of cos e. )
and this integral solves to give the solution quoted in the
text.
50
10 11 a II
5 15 25 35 P(microns)
FIG. 1. Current (I) vs pressure (p) with magnetic field of 80 G.
189
Current-pressure transitions in a magnetically supported dc sputtering system
F. A. Green and B. N. Chapman Materials Section, Department of Electrical Engineering, Imperial College, London, SW7 2BT, England (Received 28 April 1975)
Sputtering in a cylindrical magnetron configuration, current-pressure characteristics show an unexpected discontinuity and current-magnetic held curves exhibit a tnaximum. These characteristics seem io be closely related to the ratio of the height from the target of the secondary electron initial trajectory compared to the cathode dark space distance.
PACS numbers: 79.20., 81.35.11
Secondary electrons emitted from the target during sputtering interact with the sputtering gas in three main ways causing ionization, excitation, or elastic impact, and are vital for maintaining the glow discharge. It has been indicated" that the major source of power input to the substrate is from secondary electrons, and en-ergy analysis''{ has shown that some electrons hit the growing film with the full interelectrode potential dif-ference and are primarily responsible for the often undesirable heating effects which occur.
It has long been recognized that the use of a magnetic
189 Applied Physics Loners, Vol. 27, No. d, 15 August 1975 Copyright © 1975 American Institute of Physics 189
FIG. 2. Cathode dark space B=60G iss distance (:(1 vs pressure (p)
with magnetic field of 80 G.
5 10 15 20 25 30 35 P (m<roM )
field enables gas discharges to operate at lower pres-sure and/or increases the deposition rate. Under the influence of perpendicular electric and magnetic fields (a magnetron configuration) the electron path to the earth planes is increased and the probability of a colli-sion causing ionization also increases. It is therefore expected that increasing the magnetic field would in-crease the current drawn by the target, and traditional arguments follow.along this line. In aiming to produce higher sputtering rates, various magnetic field confi-gurations have been investigated including quadrupole fields, S "planar magnetron", 6 "sputter ;run", ° and the cylindrical magnetron.8 Perhaps the main advantage of the magnetron concept is that it can both increase the ionizing efficiency of the sputtering process and also keep high-energy electrons away from the substrate (compare parallel electric and magnetic fields).
In this letter, some preliminary results are presented which are at first sight surprising. Their explanation may lead to a better understanding of ion production in a_dc sputtering discharge.
Using a conventional cylindrical magnetron arrange-ment with magnetic field parallel to the cylinder axis, it was noted that at current input levels of 2 mA cm" 2 the plasma was a typical bluish argon color next to the copper target but an intense green color beyond. This coloration was also present using nitrogen as the sput-tering gas but absent at comparable inputs using an aluminum alloy target. A flame test on copper gives a similar green coloration though it should be remem-bered that different types of emission spectra are now being compared. This green coloration has also been observed with the "sputtergun". g
As the pressure is varied, there is a sharp transition in the current-pressure curve as shown in Fig. 1. Our results were obtained with a modified "cylindrical" magnetron arrangement designed for high-rate sputter-ing production applications though a similar effect is
d(cros) 20
15
10 -
05,
p:75µ
p= 3µ 1- -p=1ov
10 50 90 B (Gauss I
FTG. 3. Current Cl) vs magnetic field (B) for different pressures.
I(mA) 10 9 a 7
6 5 4 3 2
p.1SJµ
• CATHODE DARK - ----1 SPACE
TARGET
190
SUBSTRATE
NEGATIVE GLOW
FIG. 4. Electron trajectory in perpendicular electric-magnetic fields.
seen with a conventional cylinder. The transition mani-fests itself as a sharp switch, possibly from one stable state to another, and therefore it is possible to sharply increase the current drawn by the target, and hence the deposition rate, by only slightly varying the pressure.
As the "switch" occurs, the cathode dark space length , also changes sharply as indicated in Fig. 2. •
If the current drawn by the target is plotted against the magnetic field as shown in Fig. 3, it can be seen that again the pressure choice has an important bearing on the current characteristic. In some regions, the curve exhibits a maximum and lowering the magnetic field can increase the current drawn; this fact appears to contradict, usual thoughts on the magnetic field role.
Calculations suggest that at the pressure correspond-ing to the transition, the maximum height from the tar-get, ymu (see Fig. 4), of the electron orbit (assuming no collisions) in the perpendicular electric-magnetic fields is equal to the cathode dark space distance d. With reference to Fig. 2, at 15-Am pressure ymu<d, while at 30 pm ymu> d. This seems to be connected with the important question of whether ion production is predominantly in the cathode dark space or negative glow region of the plasma.
As the magnetic field is reduced, the switching phe-nomenon becomes less sharp and is totally absent with no magnetic field. This would seem to suggest that the effect is concerned Ivith electron motion in the plasma and not with target geometry, especially since similar results have been seen on modified cylindrical configurations.
Further work is in progress in an attempt to fully explain the effect.
11. Brodie, L.T. Lamont, and D.O. Myers, J. Vac. Set. Technol. 6, 124 (1969).
2 L. Holland, T. Putner, and G.N. Jackson, J. Set. Instrum. 1, 32 (1968).
313.J. Ball, J. Apps. Phys. 43, 3047 (1972). 1B.N. Chapman, D. Downer, and L.J.M. Guimaraes, J. Appl. Phys. 45, 2115 (1974).
5E. Kay, J. Apps. Phys. 34, 760 (1963). 6J.S. Chapin, Iles. Dev. No. 1, 37 (1974). 'R.P. Riegert, Res. Des. No. 2, 64 (1973). B.T.A. Thornton, Trans. S.A.E. Detroit 1973, Paper No. 730544. '
3P.J.. Clarke (unpublished).
190 Appl. Phys. Lett, Vol. 27, No. 4, 15 August 1975 F.A. Green and B.N. Chapman 190
Iectron effects in magnetron sputtering F. A. Green and B. N. Chapman Department of Electrical Engineering. Imperial College. London SA7 2BT. England
(Received 18 August 1975; in final form 9 October 1975)
Over a range of sputtering conditions in a cylindrical magnetron system, current-pressure. characteristics show an unexpected discontinuity and current-magnetic-field curves show a maximum current. These phenomena seem to depend on the relationship between the maximum distance of the secondary electron initial trajectory from the target surface, and the target dark space thickness.
PACS numbers: 79.20.M, 81.20., 52.80.H
191
° INTRODUCTION
Considerable interest has been shown in recent years in the use of sputtering devices which can achieve a high deposition rate. Since sputtering is due to target iirabardment by ions and high-energy neutrals, higher rates are usually achieved by increasing the electron ar:d hence ion populations in the plasn' Two obvious ; cthods of achieving this are to inject extra electrons il:to the plasma region (triode system) or to use a mag-rctic field to increase the number of ionizing collisions et the existing electrons.
In this paper we report and discuss some results which have been obtained with a magnetically sup-ported dc sputtering system. Some preliminary results 'Inc already been published.'
Ft SULTS
i sing a conventional cylindrical magnetron arrange-` T-':nt (Fig. 1) with a magnetic field parallel to the axis
cf, a copper target, a sharp and reversible increase in `•i:rrent was observed with increasing argon pressure (I'd. 2). The current increase is accompanied by a '".responding decrease in cathode dark space (C DS) tl•ct'kness (Fig. 3), and the appearance of an intense
green glow in the discharge. Further measurements were made of target current against magnetic field (Figs. 4 and 5). Contrary to expectations, in some pressure regions these results showed the current reaching a maximum and then decreasing for further increases in magnetic field strength.
DISCUSSION
We are unsure about the mechanisms involved in the circumstances described above, and the main purpose of the following is to promote discussion.
To understand the effects of a magnetic field on a glow discharge, we first need to understand the.mecha-nism of the basic glow discharge. However, this mecha-nism is still unclear, particularly under sputtering con-• di tions, even though research on the subject dates from the last century.
In qualitative outline, the basic mechanism of the sputtering glow discharge is understood and is described in most introductory texts. However, as soon as we examine the theory in more detail, problems arise such as knowing the region of origin in the plasma of the ions which strike the target. Druyvesteyn and Penning' and Brewer and Westhaver3 suggest that most of the ions come from the negative glow, and Davis and Vanderslice' using energy analysis at the cathode,
B
I
Axial Magnetic Field
Chamber and Substrate•
90
j 80 (MA)70
60
50
. 40
30
20
it5
9.80 Gauss Fin. 1. Cylindrical magnetron splatter-ing arrangement.
10
5 10 20 30 p (microns)
FIG. 2. Current 1 vs pressure p with magnetic field of 80 G.
J. Vac. sei. Technol., Vol. 13, No. 1, Jan./Feb. 1976 Copyright © 1976 by the American Vacuum Society 165
;a.
192 left
(mA)t .0o - 90 -80 -
70 -60-50 -40 -30 20 10
40 65 90 B (Gauss)
p•18p
1000 Energy(&) I
FIG. 6. Efficiency of ionization for electrons in argon.
9
7 6
5
4
3 2
1
0
p.190
p•15p
166 F. A. Green and B. N. Chapman: Electron effects in magnetron sputtering
.d (cms)
2.0 5.50 Gauss
1.5
1.0
0.5
1 1 t I I t t —s 0 5 15 25 35
p(microns)
FtG. 3. Cathode dark-space distance d vs pressure p with magnetic field of 80 G.
conclude that ions originate in or near the negative glow. However, it has alternatively been concluded by Little and Von Engel' and by Holmes and Cozens' that ion production is mainly in the cathode fall, and further that there is a significant contribution to second-ary electron emission from the target due to photons from the negative glow. It should be pointed out, however, that both models have a point of similarity in that they both require the negative glow to sustain a plasma, in one case to provide additional ions and in the other to provide photons for increased secondary electron production. • Holmes and Cozens' have further suggested that a pressure gradient can exist in the negative glow with gas pressure increasing towards the cathode due to the ion flux. Their figures suggest that this would be very significant in high-rate sputtering where current densities may exceed 10 mA cm-2. They have, however, not considered the flux of sputtered atoms and reflected high-energy neutrals which would oppose or may even reverse the pressure gradient due to the ion flux.
In spite of these reservations about our understanding of the glow discharge, we are still able to suggest some qualitative interpretation of our results. Sputtering
(mA) 10
p•13y p•1Ou
1 1 I 1 1 t 1 1 1 10 20 30 40 50 60 70 eo 90
B (Gauss)
FIG. 4. Current 1 vs magnetic field B for different pressures.
J. Vac. Sol. • Technol., Vol. 13, No. 1, Jan./Feb. 1976
FIG. 5. Current I vs magnetic field B for different pressures.
conditions produce very high ratios of electric field tn, pressure so that electrons rapidly reach the ionization' threshold for argon with little chance of collision.;: The efficiency of the ionization process (defined numbers of positive charges produced per electron per cm path per mm l Ig pressure at 0°C) has been 1ne )surLj: by Smith' and is shown in Fig. 6. Data in Massey an; Hurhop'shows that the ionization cross section remains• within a factor of two of 3 X 10-2° m2 for electron energic . between 40 and 800 eV, and we may thus take it aL•• sensibly constant in this range. A simple collision mood then predicts
Fd/ Fo = exp (noq,d) ,
where Fo is the electron flux at the target, Fd is th electron flux at the end of the CDS, no is the ga molecular density, g; is the ionization cross section, an.i. d is the CDS thickness. For typical values of 30 pm gas pressure and d=2 cm, then Fd /Fo= 1.89. Then figures are in agreement with data from Von Engel9 ar": an analysis using the results of Smith.' Since secondar. electron coefficients due to ion impact are generally lc ,; than 0.3,2•'°.'I this would suggest either that more originate in the negative glow (charge exchange in tl: CDS does not produce an additional ion) or that a ic'• ., of secondary electrons are due to photon bombardreep• or from high energy neutrals.
Efficiency
193 F. A. Green and B. N. Chapman: Electron effects in magnetron sputtering 167
Since the electric field becomes small close to and in •=,y negative glow, electrons tend not to pick up energy
rapidly after collision, and hence their mean energy :ie,:reases; this will at first increase both, the ionization "r,j excitation cross sections, causing the negative glow.
ionization cross section decreases below 100 eV ; king excitation increasingly important until this also
ch a ppears below about 11 eV (the threshold for excita- • tion in argon) marking the end of the negative glow
al .ough the anode may have intervened first.
ELECTRON TRAJECTORY
The action of a magnetic field B (along a c axis) ;perpendicular to the electric field (along the y axis) is ro produce electron motion along an x axis (Fig 1).
The equations of motion are
9_ (e/m)CE(y) — Bx], x= (e/m)B y ,
where B is the magnetic field strength and e (y) is the electric field.
By considering the gains and losses of potential and khietic energy, we can show that the maximum height y._.„ of the electron trajectory (assuming no collisions)
- is given by 127r:
ymax= B[— e
(v,—v)] ,
where v, is the (negative) target voltage and v is the - Potential at y,„ax. This expression holds both inside and eitside the CDS and is a better approximation at lower pressures when there are fewer collisions to alter the trajectory.
In the negative glow, the electric field, and hence v, ns a weak function of y and is almost constant as, therefore, is ymax ; on the other hand, because of the M'rong field in the CDS, v, and hence ym „a, is a strong function of y.
Consider a situation where, for a given constant clagnetic field, the pressure is high enough that ymax
in the negative glow, i.e., ymax>d. As we now reduce the pressure, ymax remains sensibly constant until
d being a strong function of pressure. A further r'.x:uction of pressure will now cause a rapid reduction
ymax as ymax and v enter the strong-field CDS region '-i the discharge.
BY comparing the value of ym„x with the data of 3, it can be seen that the sharp change in d at •
20 µm corresponds closely to the ymax =d situation. 20 µm, ymax> d while below 20 µm, ymax<d.
.Phe CDS thickness is a fundamental parameter of discharge. By comparing the magnetic/nonmagnetic
•'"'d situations we see that to a first approximation, the CDS distance will be the corresponding y'displace-
°nL when the path length is equal to the previous `-i'S thickness, i.e., the addition of a magnetic field "creases the CDS thickness, as observed.
Vac. Set. Technol., Vol. 13, No. 1, J.an./Fab. 1976
Since ionization is more efficient in the negative glow than in the CDS, we might expect a significant change in
glow characteristics when we alter the electron popula-tions by changing from a ymax>d to a ymax<d situation.
INTERPRETATION OF RESULTS
A quantitative solution does not look hopeful. But consider qualitatively the situation at say 30 Am Hg pressure and y,aax>d with a suitable magnetic field. As the pressure p is reduced, d increases towards yn„x, and fewer collisions occur. The negative glow is now producing fewer ions and photons and hence the current falls. A situation eventually arises where the negative glow fails to provide sufficient ions or photons: and the glow would be extinguished in the absence of a magnetic field. However, with a magnetic field, the glow now adopts a different form as d goes to ymax and the CDS lengthens to increase ion production within the CDS, an important point being that for each ion created in the CDS, there is unit probability of an ion striking the target, while ions or photons formed in the negative glow are in a very low field region, are further from the target, and are more likely to be lost before reaching it.
Let us now turn to the y,,,ax <d situation (i.e., 10 to 19 µm) in Fig. 4; under these cōnditions the discharge is more dependent on the CDS than with ymax >d. Now, as B is reduced, ymax increases towards d and ionization will in general take place nearer to the CDS/negative-glow boundary. This will produce a higher current because the new electrons formed will be in the weaker electric-field region, will accelerate more slowly, and will therefore be more likely to cause further ionization within the CDS. This gives a possible mechanism for current increase with magnetic field reduction, and at lower pressure (larger d), the maximum current would occur at a lower magnetic field as observed in Fig. 4. As B is further reduced, electrons escape more easily from the glow causing the current to fall again (as it eventually must do since B= 0 at, say, 10 Am Hg pressure, is not normally a selfsustaining situation.)
The results in Fig. 5 also show that pressure choice is critical to the current characteristic, the switch from one state to the other being induced by the magnetic field rather than pressure in this case.
CONCLUSION
The current drawn by the discharge is a strong function of magnetic field and perhaps more surprisingly shows a sharp change in a specific pressure range. Whilst high pressures and magnetic fields are compatible with high deposition rates, a low operating pressure may be required on grounds of film purity and accurate masking of specific substrate areas. In the light of the above results, the parameter choices should be arranged such that the 'ym„x
=d. situation is avoided in the
interests of plasma stability. An important aspect of electron behavior in a
sputtering discharge is the important role electrons play
168 F. A. Green and B. N. Chapman: Electron effects in magnetron sputtering 194
in substrate bombardment." We expect this behavior to be modified in magnetron situations. Preliminary observations confirm this and will be investigated in more detail.
ACKNOWLEDGMENTS
Thanks are due to Dr. J. Cozens for his interest and advice, and to the Science Research Council for provid-ing a grant to one of us (F.A.G.).
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