Thin film technology and physics of thin-film based ...
Transcript of Thin film technology and physics of thin-film based ...
Thin film technology and physics of thin-film based microstructures.
Sputtering of thin films
Part 1. Physical vapor deposition (PVD) of thin films. Methods and equipment.
Part 2. Molecular beam epitaxy (MBE) of semiconductor heterostructures.
Heterostructure lasers. High electron mobility transistors (HEMTs).
Properties of two-dimensional electron systems.
Part 3. Microfabrication: main steps. Optical and electron-beam lithography.
Coulomb blockade. Single-electron transistor.
Part 1. Physical vapor deposition, PVD
• Introduction. Several important examples of different coatings.
• Thermal evaporation
• In-situ control of the film thickness. Estimations of the film uniformity.
• Electron-beam physical vapor deposition
• Sputtering deposition, magnetron sputtering deposition
• Cathodic arc deposition.
• Reactive deposition
• Laser ablation
• Role of vacuum conditions
• Vacuum systems for the physical vapor deposition. Pumping systems.
• Adhesion of coatings.
• Types (modes) of thin film growth.
S.I.Dorozhkin
Physical vapor deposition (PVD) systems deposit thin films and coatings by a process in
which a target material is vaporized, transported in vacuum, and condensed on to a substrate.
PVD processes include Sputtering, Electron beam, and Thermal Evaporation.
Nanotechnology: metallization in the fabrication of integrated
circuits and photomasks
Metallization is usually the final step in the fabrication of integrated circuits.
(1) Electrical connections of all components of a circuit.
(2) Producing of bonding pads
Thin aluminum films are usually used for the metallization.
Simple photomasks are usually made of glass covered by chromium.
(1) (2)
Thin metal films in superconducting logic elements
Examples of large-area coatings. Thin-film solar cells
CdTe panels mounted on a supporting structure
belong to a second generation of photovoltaic solar sells.
This is one of examples of the largest area thin-film
coatings.
Thin-film silicon laminates being installed onto a roof.
CIGS solar cell on a flexible plastic substrate
Modern thin-film semiconductor materials for
solar cells:
Amorphous silicon (a-Si)
Cadmium telluride (CdTe)
Copper indium gallium diselenide (CIGS)
Produced mainly by methods based on the chemical vapor deposition.
p-type
n-type
Hard and decorative coatings (TiN,…) are mainly produced by the cathodic arc deposition and magnetron sputtering
Several additional examples
1. Molecular beam epitaxy of semiconductor structures for lasers, high electron
mobility transistors (HEMTs), etc.
To be considered in more detail in following lectures.
2. Optical coatings.
3. ….
Thermal evaporation
Thermal evaporation is realized by thermal heating (usually
resistive) of an evaporated material placed in an evaporation
source.
It is carried out in a vacuum at pressure P<10-5 torr.
The thermal evaporation is the most gentle PVD method with
evaporated particle energies ~1500 K (0.12 eV).
This method is the simplest PVD method and needs
comparatively low power consumption.
Main types of simple evaporation sources are metallic
boats (1,2) and heaters made of refractory wires (3) (Wo,
Mo,…) which can be covered by a passive material (2)
(Al2O3)
More complicated effusion cells with a crucible made
of passive material (e.g., boron nitride, BN) and an
external heater are used for precision evaporation
processes like the molecular beam epitaxy.
A simple system for thermal evaporation
Vacuum chamber 1 is evacuated to pressure of the order
of 10-6 Torr via the pumping line. The pressure is
measured by vacuum gauge 2. A material 3 is evaporated
from a metal boat 4 heated by an electric current. The
current flows through an isolated vacuum tight lead 5,
which is usually cooled by water (not shown in Fig.), and
grounded lead 6. A substrate 7 is mounted on a holder 8.
The holder may have options of rotating around vertical
axis and tilting relative to it. The latter option
corresponds to the so named oblique evaporation. These
options help to avoid undesirable shadows. The
temperature of the holder may be varied and stabilized in a wide range. The thickness of
evaporated film is controlled by a thickness monitor based on a quartz crystal microbalance 9
(QCM). QCM measures variation of a mass on a surface of a quartz crystal resonator. For a
high accuracy measurements the crystal should stay at a constant temperature. The evaporation
process is controlled by the current through the boat (i.e., the boat temperature) and the shutter
10 position. The shutter is closed till reaching the desired boat temperature and beginning of the
material vaporization. It closes at the end of evaporation process.
Pumping
line
Features of the thermal evaporation method
• (+) The most gentle PVD method, due to the lowest possible energy of
evaporated particles (~1500 K~0.12 eV).
• (+) Atomic or molecular deposition from the vapor.
• (+) Appropriate for deposition of a large number of materials.
• (+) Appropriate for ultra-high vacuum conditions.
• (+) In-situ control of the deposited layer thickness.
• (+) Wide range of the evaporation rate.
• (?) Nearly line-of-sight motion of evaporated atoms. Shadow.
• (+) Estimation of the film thickness uniformity.
• (-) Potential contamination of an evaporator material (in simple systems)
About thermal evaporation of particular materials see
1. Thin Film Evaporation Guide http://www.vem-co.com/guide
2. Напыление пленок в вакууме (Physical vapor deposition, PVD)
by С.И. Дорожкин, МФТИ (2012)
Thickness of deposited films: control and uniformity.
I. Thermal evaporation provides the best conditions for in-situ control of the average film
thickness.
A real – time film thickness monitor can use different sensors: (i) A quartz crystal microbalance,
(ii) Reflection high-energy electron diffraction (RHEED) (in the molecular beam epitaxy), (iii)
optical methods including ellipsometry.
A simplified geometry of the quartz resonator,
produced by a cylindrical quartz plate 1 and metallic
films 2,3 on opposite sides of the plate. To excite the
mechanical oscillations in the plate the ac voltage Vf is
applied between films 2 and 3. The resonator is a part
of an oscillator which frequency f is determined by the
resonant frequency of the mechanical resonator
(typical value ~5 MHz). The Q-factor of the
mechanical resonance can be as high as 106.
Evaporation of a thin film with mass Dm shifts the resonant frequency by mS
ff DD
00
2
02
661.10 NHere is the resonance frequency without evaporated film,
the quartz AT cut; n - is an integer numbering different resonant overtones, D-the plate thickness,
S- active area of the resonator.
DnNf /00 MHz*mm for
648.20 g/cm3 – density of the quartz , 11
0 10*947.2 g/cm s2 - shear modulus for the AT cut
of a quartz crystal.
(Q1)
Quartz crystal microbalance
For Au ( =19.3 g/cm3) film, one monolayer (d~3A) corresponds to Df=33 Hz.
Temperature stabilization is important condition for precise measurements !
.
Equation (Q1) is valid for 03.0/ 0 D ff
For a wide frequency range (Df/f0<~0.3) a more complicated equation is used:
D
0
000
f
ffZtgarctg
Zf
N
S
m
00ZHere , and are the density and shear modulus of evaporated film.
Factors limiting the accuracy of the thickness measurements:
1. Different positions of the substrate and the sensor needs calibration for particular geometry
and evaporator.
2. Heating of the quartz resonator can be eliminated by a cooling system.
3. Deposition of different materials on the same sensor van be eliminated by the use of several
sensors: one for each material.
Estimate for the thermal evaporation rate
ev
aev
kT
mTPdtdm
2)(/
This formula is equivalent to the well known equation of the molecular kinetic theory for the
particle flow Ns /t on a unit flat area: Ns /t=nV av /4 where n is the particle density and
Vav=(8kT/ma)1/2 is the average particle velocity determined from the Maxwellian distribution.
Here dm/dt – the mass rate of evaporation from a unit area, ma – mass of evaporated particle,
P(Tev) – the vapor pressure at evaporation temperature Tev .
VT
Q
dT
dP
DP(T) is usually described by the Clausius–Clapeyron equation
,
is the specific latent heat of the phase transition (vaporization) Q
DV is the specific volume change of the phase transition (the difference of specific
volumes of a liquid and vapor).
RT
PQ
dT
dP2
For an ideal gas and dQ/dT=0 TBAATR
QP /ln which gives (Q2)
Temperature dependence of the vapor pressure
Temperature dependences of the vapor pressure P on the
temperature T (horizontal scale is inversely proportional
to T) for Au and Ti. Solid lines were calculated in
accordance with Eq. (Q2) with the use of parameters
AAu=20.84 and BAu=4.26x104 K (liquid-vapor).
ATi=23.81 and Bti=5.67x104 K (solid-vapor).
The solid signs are taken from handbook “Physical
quantitites” edited by Grigor’eva and Meilikhov (1991).
The melting points Tm are marked by vertical by arrows.
Equilibrium vapor pressures of selected
materials. The slashes indicate the
melting points (MPs).
Typical for the thermal deposition value
of the vapor pressure 10-2 Torr is marked
by the horizontal arrow. Some of the
materials are evaporated and some are
sublimated.
Sublimation of metals is possible by
their direct heating by passed current
(e.g., Ti).
Angular distribution of atomic flow
A general geometry of evaporation. q is the angle between the
normal to the surface element dS1 of the evaporator and a direction
R to the surface element dS2 of the substrate. R is a distance between
these elements. The angle between R and a normal to dS2 is b.
Projection of the dS2 on the plane normal to R is dS0= dS2cos(b) .
A part dw of a total flux w from a flat source into a small
solid angle dW in the direction determined by angle q is given by
equation dw=(w/) cos(q )dW . This relation is the result of the
maxwellian distribution and an analog of the Lambert law in the
optics.
For evaporated mass m, the film thickness is d= m cos(q )cos(b) /R2
If the source is spherical, dw=(w/4) dW and d= m cos(b) /4R2.
Coordinate dependence of the thickness of the deposited films
Consider thermal vapor deposition from a small flat source on parallel
substrate . This corresponds to q =b , cos q =H/(H2+X2)1/2 , R2=H2+X2.
source
substrate
222
2
)()(
XH
HmXd
2max
1
H
md
2/322 )(4)(
XH
HmXd
2max
1
4 H
md
Spherical source
Flat source
0.0 0.2 0.40.7
0.8
0.9
1.0
1.1
(b)
d/d
max
X/H
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.0
0.2
0.4
0.6
0.8
1.0
d/d
max
(a)
Flat source
Spherical source Thickness distribution along the substrate for a small flat (black
lines) and spherical (red lines) sources. Panel b shows initial part of
panel a.
These data show that condition of thickness uniformity leads to
requirement of a large distance between the source and the substrate,
i.e., to a large size of a vacuum system.
Theoretical calculations may be inaccurate due to a finite size of the
source and redistribution of the temperature along it. However some
of these factors may even lead to a better film uniformity. In fact, in
practice the optimal conditions are chosen empirically and
reproduced from one evaporation cycle to another.
Electron-beam physical vapor deposition (EBPVD)
A focused electron beam is emitted from an electron
gun and deflected to the evaporated material by
magnetic field. Energy of electrons is determined by the
acceleration voltage of 1-10 kV. The electron beam
diameter is usually about several mm and the total
power can reach 10 kW. At such conditions, local
temperature of the evaporated material can be in excess
of 3500 K, which allows for evaporation even very
refractory materials. The crucible is kept rather cold due
to the cooling by a water flow.
High vacuum is necessary not only for evaporated atom
transport but also for long life of the cathode of the
electron gun. Typical pressure P<10-5 Torr.
Magnetic field helps to avoid the direct flow of desorbed atoms from the electron gun area to
the substrate and the appearance of the evaporated material and ions in the gun area.
A local pressure at the heated surface may be rather high, so that the evaporated atoms first will
move diffusively. Ballistic motion begins at some distance from the source surface.
Electrons may also ionize the atoms of residual gas and of evaporated material.
Electron-beam physical vapor deposition (EBPVD)
Electron beam evaporators can be rather compact due to
the use of the magnetic field H
The electron beam radius is given by formula
H
c
e
UmR e
||
2 ][/][8.1][ kGHkVUcmR
Features of the electron beam evaporation method.
• (+) A very wide range of evaporation rates (0.01 nm/s – 0.1 m/s)
• (+) A large number of metals including refractory ones.
• (+) Atomic deposition from the vapor.
• (+) Evaporation of source material only.
• (+) Appropriate for high vacuum conditions.
• (+-) In-situ control of the evaporated layer thickness (small thicknesses).
• (?) Nearly line-of-sight motion of evaporated atoms. Shadow.
• (-) Poor estimation of the film thickness uniformity.
• (-) Charging of the substrate. Possible damage of the evaporated film or a substrate
material by high-energy electrons.
Deposition of a material sputtered by ions
Sputtering by ions is a very effective method due
to high possible ion energy and a large mass. The
sputtering normally leads to extraction of atoms
from a source material. The extracted atoms may
have energy of several eV.
While the high energy ions can be produced by
separate ion sources, usually a scheme with
ionization of the incoming sputtering gas by high
energy electrons directly in the coating chamber
is used. Inert argon is frequently used as a
sputtering gas.
The method is suitable for a reactive deposition of different compounds of the sputtered
material with the use of an appropriate sputtering gas
Sputtering process
One ion usually transfer its energy
to many atoms of a solid target and
can sputter many atoms if its energy Ei greatly exceeds the
binding energy Et. It also leads to
the fact that the threshold ion energy Ethr >> Et (Ethr~20-50 eV
while Et~3-6 eV). The sputtering
coefficient
gsput depends on the ion energy and
mass.
for Ar+ at Ei=600 eV
Theoretical energy distribution of sputtered atoms has maximum at E=Et/2.
Here c is the angle to the target normal.
Magnetron evaporation
Schematic of the magnetron operation. The cathode is made
of evaporated material. It is bombarded by ions produced
due to collisions of a gas (usually Ar atoms) with electrons.
Both ions and electrons are accelerated by electric field
applied between cathode and anode (typical electrical
voltage is several hundred volts). The gas is supplied
through the fine control needle valve to a typical pressure
~10-3 Torr. To enhance the probability of ionization of the
gas atoms by electrons, the magnetic field (0.2 – 2 kG) is
applied which causes electrons to move along spiral orbits
centered on the magnetic field lines instead of along short -argon ions H
EAre
ee
Ar+
ArAr e
+
S
SN
N
cathode-source (-)
substrate
magnetic system
N
S
film
anode
+
e
H
EAr
plasma+
+
+
+
-electrons
-sputtered atoms
-argon atomslines of
electric and
magnetic fields
straight lines parallel to the electric field. The magnetic field only slightly effects motion of ions
due to their large mass. The plasma arises in the region with the largest magnetic field. The
cathode material is sputtered by the ions. The atoms of the sputtered material move in different
directions and, in particular, cover the substrate.
Typical parameters of the process.
•The gas pressure 10-3-10-1 Torr,
•Electrical voltage applied between cathode and anode 300 – 1000 V;
•Magnetic field at the cathode 0.2-2 kG;
•Current density on the cathode up to 10 А/cm2;
•The supplied electric power up to 10 kW.
Uniformity of the film thickness produced by the magnetron
deposition method
Coordinate dependence of the relative copper film thickness produced by the magnetron
deposition method. Diameter of the source material on cathode is 76 mm. The data are
obtained for two different depositions (different symbols) for two different distances (102
mm, red symbols) and 152 mm (blue symbols) between the cathode and the substrate.
(Data from PVD Products Company)
Features of the magnetron deposition method.
• (+) A high evaporation rate.
• (+) Rather good adhesion of the coating due to high energy of sputtered atoms.
• (+) A large number of materials including refractory ones. Dielectrics are deposited with the
use of the RF voltage.
• (+) Atomic deposition from the vapor.
• (+) Possibility of a reactive deposition with the use of an appropriate sputtering gas.
• (-) Inappropriate for high vacuum conditions.
• (-) Not very clean coatings.
• (-) Difficulty to control uniformity of the film thickness.
• (+-) In-situ control of the evaporated layer thickness is problematic.
• (?) Essential deviations from line-of-sight motion of sputtered atoms. Poor shadow.
• (-) Possible damage of the evaporated film or a substrate material by high-energy electrons.
Cathodic arc deposition
Electrical arc in a vacuum is a region of plasma produced by
positive ions of a cathode material and electrons. The arc current
is concentrated in small cathode spots about several microns in
diameter . The current density in the spots is of order of 106
A/cm2. It produces local temperature up to 15000 K, which is
accompanied by intensive material evaporation, emission of
electrons and vapor ionization. The spots disappear in a one place
of a cathode and appear in another shifting along the cathode.
The arc starts by touching a cathode by an ignitor electrically
connected to anode.
A low pressure gas, if present in the chamber, is also ionized and
produces a compound with cathode material (Superhard coatings
by nitrides like TiN, etc).
Steered cathodic arc source
Random cathodic arc source
(photo)
Features of the cathodic arc deposition
• (+) Very high evaporation rate.
• (+) Ability to produce extremely hard coatings in the reactive process.
• (+) A large number of materials including refractory ones.
• (-) Difficulty to control uniformity of the film thickness.
• (-) In-situ control of the evaporated layer thickness is problematic.
• (+-) Essential deviations from line-of-sight motion of vaporized atoms. Poor shadow but
incomplete.
• (--) Presence of small drops in the ionized vapor of the cathode material. Needs special
arrangement to be avoided.
Laser ablation
Under high power laser irradiation the target injects a
vapor formed by individual atoms, ions, atomic
clusters and even small drops. A pulsed regime is
usually used.
For example: Nd:YAG laser (neodymium-doped
yttrium aluminum garnet; Nd:Y3Al5O12) , ~10 nc pulse,
~1J/pulse, 10 Hz repetition.
Particular composition depends on the evaporated
material and laser power.
The method is compatible with ultra-high vacuum
conditions and allow to keep a complicated
composition of a compound in the course of deposition
Role of vacuum
Three main factors:
I. Purity of a deposited film.
II. Ballistic (good shadow) or diffusive (poor shadow) transport of atoms from a source to a
substrate.
III. Ability of producing ions (in sputtering processes).
Several estimates.
I. A flow of residual gas molecules incident on the surface (dN/dt)/S=nVav/4=P/(2makT)1/2
gives for P=10-6 Torr of O2, T=300 K, (dN/dt)/S=3.4x1014 cm-2s-1 or about half of a
monolayer per second. This estimates shows that one needs extremely high vacuum
conditions to deposit very clean films.
II. For an ideal gas of particles with cross section s and the Maxwellian distribution of
velocities, the free path reads as l=(2)-1/2 /ns(2)-1/2kT/Ps , which gives for s = 16x10-16
cm2, T=300 K and P=10-4 Torr l~0.5 m.
III. Typical pressure in vaporization by ion sputtering (e.g., magnetron sputtering) is of the order
of 10-3 Torr.
Pressure dependences of characteristic parameters (an estimate)
A unique line correspond to either P (left scales) or P-1 (right scales) dependences. Some absolute
values depend on the molecular mass and cross-section.
Measurements of the pressure
Three main physical properties:
1. Mechanical deformation allows measurements of a differential pressure. If the reference
pressure is known constant an absolute pressure is measured.
1000 bar – 0.1 bar – usual dial bourdon gauges with atmospheric reference pressure (1).
100 Torr – 10-1 Torr – precision mechanical manometers with evacuated reference volume.
1000 Torr – 10-3 Torr – precision capacitance manometers (2).
2. Measurements of the gas thermoconductivity allows for measurements in the range 1000
Torr -10-5 Torr. (Modern Pirani vacuum transducers (3))
3. Measurements of ion current with the ion ionization being produced by a high-energy
electrons allows measurements from 10-2 Torr down to 10-12 Torr. (4)
(1) (2) (3) (4)
Comparison of different PVD methods.
Thermal evaporation.
Low energy atomic beams, compatibility with ultra-high vacuum conditions, stability and precision control of deposition rate, possibility of simultaneous deposition of several materials from different sources.
Inappropriate for some refractory materials, special cautions to improve the adhesion to some substrates.
Electron beam evaporation
Appropriate for many materials including refractory ones, compatible with ultra-high vacuum conditions, wide range of deposition rate, heating of the evaporated material only.
Inappropriate for films and substrates sensitive to the electric charge, problems with stabilization and control of the evaporation rate, limited target utilization.
Ion sputtering.
Appropriate for many materials including refractory ones, possibility of reactive deposition.
Incompatible with high-vacuum conditions, possibility of undesirable contamination of the sputtering gas atoms in the deposited material, limited target utilization (~30% for magnetrons)
Laser oblation.
Appropriate for many materials including refractory and compound ones, compatible with ultra-high vacuum conditions.
Complicated composition of the vapor: atoms, molecules, ions, droplets.
Pumping systems
Dry pumps
Wet pumps
Two main mechanisms of the gas evacuation:
(I) Compression and pumping out.
(II) Adsorption . Usually applicable under high vacuum conditions in the absence of a gas
flow.
Rotary vane vacuum pump Leibold Trivac B
100m3/h= 27.7 l/s; 1 mbar=0.75 Torr
The Principle of Operation is transfer of a mechanical pulse from oil vapor molecules to gas
molecules resulting in their average motion from a pumed area to forevacuum area
Oil diffusion pump
The principle of operation is transfer of a mechanical pulse from oil vapor molecules to gas
molecules resulting in their average motion from a pumped area to forevacuum area
Dry preliminary pumps: diaphragm pump.
Scroll pump
Screw pump
Pump Model SP250 SP630
Disp. CFM @ 60 Hz
(m3/hr @ 50 Hz) 177 (250) 371 (630)
Ult. Pressure Torr
(mBar) 7.5 x 10-3(1 x 10-2) 7.5 x 10-3(1 x 10-2)
Motor HP (kW) 7.9 (5.9) 20 (15)
Dimensions
LxWxH - in. (mm)
5.1 x 20.9 x 34.6
(1,350 x 530 x 880)
64.2 x 26 x 34.6
(1,630 x 660 x 880)
Weight lbs (kg) 992 (450) 1,166 (530)
Roots pumps
Roots pumps are usually used at the pressure range between 10-1 Torr and 10 Torr, where
they are very effective
Turbomolecular pumps
Turbomolecular pumps have very different rotor diameters covering pumping rate range
from tens to several thousands l/s.
Turbomolecular pums may have either mechanical or magnetic rotor suspension
The heart of a turbomolecular pump is a turbine rotating with a speed of the order of 50,000 rpm
(turns per minute).
The principle of operation is transfer of a mechanical pulse from turbine blades to gas molecules
resulting in their average motion from a pumped area to the forevacuum area.
High vacuum turbomolecular pumps
The typical ultimate pressure of the turbomolecular
pumps is 10-8 Torr but may be as low as 10-10 Torr.
Sputter ion pumps
The basic principle of operation is adsorption of residual gases by Ti film sputtered from Ti
cathode due to the ion bombardment. Other mechanisms are also involved. Magnetic field
increases probability of ionization by high-energy electrons.
Pressure-current dependence
Dependence of the current through the ion pump is a measure of the pressure .
However this dependence should be calibrated for a particular pump and is different for
different residual gases.
Titanium sublimation pump
Model PGT-3F PGT-6F
Pumping speed *1 24m3/(s·m2) <20ºC> / 64m3/(s·m2) <-196ºC>
Applicable pressure
range Ultra-high vacuum of between 10-1 to 10-9Pa
Applicable gases H2, N2, O2, H2O, CO, CO2
Inapplicable gases He, Ne, Ar, CH4, C2H6 , other organic gases
Leak volume 1.3 x 10-11Pa·m3/s max.
Baking temperature 250ºC
Power consumption 270W
Filament material Titanium alloy
Filament life span
Rated continuous operation time *2 : About 75
hours/filament
Continuous ON time : About 25 hours/filament
Number of filaments 3 6
Connection flange *3 UFC070-FH UFC114-FH
Weight 1.0kg 2.5kg
The principle of operation is adsorption of residual gases by thermally evaporated Ti film.
The pumping becomes very effective when Ti is evaporated onto a cooled (liquid nitrogen or
helium) panel.
Cryo-pumps
The principle of operation is condensation and physisorption of residual gases on a surface
cooled by liquid 4He (T>~4K). Porous materials (charcoal, zeolite, molecular sieves, etc) highly
enhance the sorption capacity and allow effective sorption of light gases (H2, He, etc) at
temperature ~4K.
Zeolites cooled with liquid nitrogen are frequently used as dry preliminary pumps (P=1000-
10-4 Torr). They should be backed up after each pumping circle.
Temperature (K)
Elements of PVD systems
1. Vacuum chamber of a large volume to guarantee a uniform coating
2. Pumping systems.
3. Systems for evaporation (boats, electron beam guns, lasers, etc) and sputtering (regulated
needle valves, ion sources, magnetrons, etc.)
The PRO Line PVD 75 is compatible with the following
techniques:
Thermal Evaporation (up to four 4" individual boats, or
six 2" boat assemblies)
Torus® Magnetron Sputtering sources (up to six 2" or 3"
sources)
Electron Beam Evaporation Source (4 pocket 8cc, 8
pocket 12cc, 6 pocket 20cc)
LTE10 Organic deposition sources (up to two)
Combinations of the above techniques are also available.
Pumping system
Pfeiffer 790 l/s turbomolecular pump with KJLC RV212 oil sealed roughing pump. Base pressure
for a properly conditioned chamber is 5 x 10-7 torr (6.7 × 10-7 mbar).
Wide range gauge reads from atmosphere to 10-9 Torr.
Magnetron system
0 to 800V @ 0 to 2.5A
0 to 400V @ 0 to 5A Output Assembly of three magnetrons
Evaporation Sources
Tungsten tapered helix coil. Wire diam.=0.76 mm; Max diam 20 mm;
2.5 V, 23 A , 1800oC
Tungsten flat trough 0.25mm thick; 19 mm wide; 3 mm deep
1.47 V, 158 A, 1800oC
Boron Nitride Crucible inner Diam.=11.7 mm
Molibdenium heat shielded crucible heater
1.24 V, 273 A, 1600oC
Quartz sensors
Gold recommended for low film stress deposition,
such as Aluminum, Gold, Silver, etc. Silver or Alloy
recommended for high film stress deposition, such as
Chromium, Nickel, Inconel, etc. Alloy recommended
for dielectric material deposition, such as Magnesium
Fluoride, Silicone Monoxide, etc.
Control quartz crystal, plano-convex, 6 MHz, 0.55
inches (1.4 cm) diameter. Compatible with all 6MHz
crystal sensors. Choice of coating, silver, gold, or alloy
determined by the application.
Twelve crystals contained in one sensor
Thickness controller
A monitor measures only deposition rate and thickness.
A controller measures deposition rate and
thickness like a monitor, but also provides an
output signal to control source power supply
and deposition rate.
Radio-frequency oscillator
Adhesion of coatings.
Adhesion determines the quality of a mechanical contact between the substrate and the coating.
1. Inter-diffusion of materials in contact as well as chemical reactions between them provide
the strongest adhesion. Corresponding energy usually exceeds 1 eV per particle.
2. Van der Waals forces lead to so-named physisorption (10-100 meV).
Vacuum cleaning and ion (electron) bombardment can remove adsorbed atoms/molecules from
the substrate surface and highly improve the adhesion.
The best adhesion to a metallic substrate have metals producing alloys with the substrate
material.
A good adhesion to the oxide surfaces (e.g., glass) have an oxygen-active film materials such as
Ti, Cr, Mo, or Zr. E.g., the Cr films are usually used in photo-masks for optical lithography.
These materials are frequently used as an intermediate thin layer (~10 nm) between a substrate
and a main film.
In general, the higher energy of evaporated (sputtered) particles leads to their better adhesion.
As a result, one gets better adhesion in the cases of cathode arc deposition, magnetron
sputtering and electron-beam evaporation. It may be related to desorption of extrinsical
adsorbates from the substrate surface.
Some materials (e.g., Zn, Cd) may have very poor sticking coefficients to many substrates.
Film growth modes
There are three primary modes of thin-film growth for mutually insoluble materials:
(a) Volmer–Weber (VW: island formation), (b) Frank–van der Merwe (FM: layer-by-
layer), and (c) Stranski–Krastanov (SK: layer-plus-island). For each mode, the layers are
shown for three values of surface coverage characterized by number of monolayers (ML)
Θ with the same mass.
In the most common VW mode, individual islands coalescence into a continuous
polycrystalline film above some threshold thickness which depends on the substrate
material.
E.g., a gold film on a glass substrate gets continuous at average thickness about 7 nm.