Ion Beam Lithogrpaphy Seminar New
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Transcript of Ion Beam Lithogrpaphy Seminar New
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Presented bySudama Chaurasiya
Rugmani M
Pratheep P
Shashank Chetty S
Guided byDr. K. Suresh Babu
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MAGIC OF ION BEAM LITHOGRAPHY
LITHOGRAPHY PROCESS
NEXT GENERATION LITHOGRAPHY
INTRODUCTION
HISTORY OF ION BEAM TECHNOLOGY
ION BEAM LITHOGRAPHY APPARATUS
ION INTERACTIONS
IONS FOR LITHOGRAPHY ???
PHYSICAL PROPERTIES OF ION BEAMS
REASON FOR POSITIVE IONS ???
COMPARISON WITH OTHER
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Replication of a master pattern onto a substrate
Coat the substrate with a radiation-sensitive polymer film (a resist)
Expose specific area of film to radiation, which alters properties
(solubility) of film.
Strip
Resist
Develop
Pattern
Positive
NegativeSubstrate
Resist
Ion BeamRadiation
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- Air environment
- Complex maskfabrication ($4k-$12k)- Resists have lowsensivity- High cost X-raysources
-vacuum environment- direct write systems
(software masks)-slow writting over largeareas- very high system cost
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The ion-beam lithographic exposure tool is similar tothat of electron-beam exposure systems.
The major differences being in the source anddeflection system.
The key advantages of ion-beam lithography are highresolution and the fact that it can be used with resists
with poor sensitivities. This implies that minimum energy is delivered to the
substrate, in contrast to high energy electrons used inelectron-beam lithography, which penetrate relativelydeeply into the substrate and lose only a small fraction
of their energy in exposing the resist. Thus, resolution in ion-beam lithography is primarily
limited by the range of the secondary electronsproduced as the ion loses energy in the resist.
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o 1974: Seliger and Fleming recognized potential utilityof focused ion beam for maskless implantation of
dopants into Si.
minimum spot diameter was 3.5mm, beam energy:
60 keV
1975: Krohn and Ringo tested the feasibility of Ce, Ga,Hg as liquid sources
most suitable: liquid Ga ("15nm spot size, effective source
diameter "200nm
1979: Seliger et al. reported a scanning ion probe
system with a beam of gallium ions focused down to adiameter of 100 nm, current density 1.5A/cm2 (high
brightness)
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1982: Anazawa et al. constructed a 35 kV Ga-source SIM(Scanning ion microscope).
surface-sensitive SIM images
1985: Orloff and Sudraud proposed focused ion beamsystem for lithography and implantation with 10nm source
1985: Kato et al. pointed out advantages of the FIBtechnology for the fabrication of sub-micron structures and
other maskless processes, including
scanning ion microscopy
maskless ion implantation
maskless etching and deposition
resist exposure (lithography)
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Ion Beam Source
Ion Optics
VacuumChamber
Ion energies : 20 eV- 200 KeV
Beam Current : up to500 A/cm2
Ion Specimens : H,He, Ar, Hf, Ga, Si, Au,
Co, Pr, P+, BF2+,
etc
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ION-RESISTINTERACTION,
SHOWING
*SPUTTERINGOF
NEUTRALATOMS,
*EMISSIONOF
ELECTRONS, *LATTICEDAMAGE, *HEAT
GENERATION,*IMPLANTATION.
In addition, the
beam can
generate
secondaryelectrons that
participate in bond
breaking reactions
in resist
molecules.
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Ions have two principal features that make their use in
Lithography valuable to considerresolution and sensitivity
The resolution is intrinsically higher than for electrons
because electrons suffer from the so-called "proximity"effect ( developed pattern, is wider than the scanned pattern).
The lateral scattering of electrons and the creation of
energetic secondary electrons both serve to broaden theeffective size of a focused beam as it penetrates a resist.
In addition, backscattering of electrons from the resist orfrom the underlying substrate (If the resist is thin) provides abroad background about the incident beam.
As a result one feature tend to smear into a neighboringfeature with a loss of contrast.
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Ions scatter much less and produce secondaryelectrons of only very low energy, thus reducing the
spreading of exposure features in a resist to ~
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Forslow heavy ions (e.g.,30 keV Ga+), the interaction
process is that
altering the surfacestructure of a material,
modifying surfacechemistry, and
removing atoms from thesurface through sputtering.
The primary interaction
offast lighter ions(e.g., 100 keV3 MeV
protons) is that
deep penetration into
the material, with aminimal amount ofsurface disruption.
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The diameter of an ion is much greater, of order 10-10 m,
about the size of an atom. Generally, the diameter of a negative ion is somewhat
greater than that of a positive ion.
The radius of almost all negative ions is around (1.32.5)
10-10
m, while the radius of positive ions is about (0.11.7) 10-10 m.
For example, the radius of a potassium ion, K+, is 0.133nm, while the radius of Cl- is 0.181 nm.
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THREE BASIC OPERATING MODES
MAJOR COMPONENTS
TYPES OF ION-BEAM LITHOGRAPHY
FOCUSED ION BEAM TECHNOLOGY
PROBLEMS IN USING FOCUSED ION BEAM
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Emission of secondary ions and electrons
FIB imaging, low ion current Sputtering of substrate atoms
FIB milling, high ion current
Chemical interactions (gas assisted)
FIB deposition.
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IMAGING
ETCHING DEPOSITION
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Upper (condenser) lens : ions are focused and intoparallel beams.
Mass separator: filter out unwanted ion species Drift tube: eliminates the ions that are not directed
vertically.
Lower (objective) lens: helps in reducing the spot size
and in improving the focus.
Electrostatic beam deflector: controls the finaltrajectory or landing location of the ions on the substrate.
Nozzle: provide low energy non-focused electrons to
neutralize the charged substrate.
Multi-channel plate (MCP): recording the secondaryelectron emission and thereby, helps in viewing the
substrate
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All the components are usually placed in a low-pressure
chamber evacuated to the 10
-7
Torr. To prevent interference of the focused ions with particles
in the chamber.
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(a)Focused ion beam lithography,
(b)Proximity ion-beam lithography,
(c)Ion projection lithography.
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Developed in the late 1970s. Utilizes slow focused heavy ion beam. (e.g., 30 keV
Ga+ ions).
The beam itself is powerful enough to etch away
material. Therefore no resist and developing is needed. Used to sputter atoms or structurally modify the
surface of a material to produce topographically orstructurally modified surface patterns.
High-speed fabrication. Imaging resolution with Ga ions is limited to ~5 nm.
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A point beam
can be focusedto a fine spot(10 nm) from abright point
source and
deflected on the
surface, in an
arrangementcalled focused
ion beam
(FIB),to expose
the resistdirectlywithout amask.
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DEFLECTION OF BEAMThat is the time required for beam to find a new location in
response to a new deflection signal more than for electron.
STATISTICS AND MASK WRITING TIME.The writing time increases inversely as the square of the
focused beam diameter.
Both are related to writing speed
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FIB APPLICATION IN TEM
ADVANTAGES
PROXIMITY ION-BEAM LITHOGRAPHY
ION PROJECTION LITHOGRAPHY
ION SOURCES
GAS FIELD IONIZATION SOURCE
LIQUID METAL ION SOURCES
ADVANTAGES OF LMIS
GALLIUM: LIQUID METAL
PAIR OF LIMITATIONS: SOURCE
TWO MAJOR IMPROVEMENTS TO THE
SOURCE
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1. Pt Deposition
2. Coarse milling3. Fine milling
4. Pre-thinning
5. Lift out
6. Transfer of specimen to TEM
grid
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The imaging and high resolution milling capabilitiesof the FIB have made it possible to preselect a region
and mill a TEM specimen with submicrometerprecision.
Because thinning is uniform across the samplesurface are made available for TEM observation.
TEM specimen can be prepared for analysis in 3 to 5hours.
Minimizes artifact.
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The second way to employ ion beams in lithography is to
place a stencil mask in close proximity to a surface andirradiate the mask with a collimated beam of ions.
This is referred to as proximity ion-beam lithography, and it
is a 1:1 shadow mask printing.
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The third way to employion beams in lithography
is to combine the firsttwo techniques anduse an ion opticalcolumn to project theimage of the pattern of
a stencil mask onto awafer.
This is called ionprojection lithography(IPL), which was
pioneered at IonMicrofabrication Systems(IMS) of Austria in thelate 1980s
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In any finely focused Ion beam system the ion source is thekey element - brightness must be high, since it limits themaximum beam intensity that can be brought into a focused
spot. Energy spread of ions from the source must be small
because of chromatic aberration in focusing lenses.
Metals that have relatively low melting temperatures andlow reactivity.
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The ion sources that are currently available include Al,As, Au, Be, Bi, Cs, Cu, Ga, Ge, Er, Fe, In, Li, Ni, Pb, Pd,Pr, Pt, U, and Zn.
Among these, Ga is the most popular ion species usedin IBL.
In orderto lower the melting point and to control thereactivity, alloy sources, such as PdAs, PdAsB, AuSi,and AuSiBe, are frequently used to deliver thedopants for semiconductors.
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The source utilizes tips comparable to those used in fieldion microscopy.
A sharp tip is held at a potential a few kilovolts positivewith respect to an aperture.
The electric field at the tip is ~V/A hydrogen moleculefrom the gas that enters into the region of high field isboth attracted to the tip by polarization forces andionized.
when it is very close to the tip (~A) and then repelled inthe repulsive field.
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The source depends upon a liquid metalwhich wets but does not react with asolid needle.
In this case, a tungsten needle welded to atungsten wire hairpin (for heating) holds amolten gold ball at the crossing.
The needle is sharpened to a radius of afew microns (a rather blunt pointcompared with the gas field ionization tip).
Liquid gold wets the needle. When an appropriate voltage is applied
between the needle and an aperture theelectric field forces on the liquid can exceedthe surface tension force and the liquidmetal is pulled into a cone shape.
At the very tip of the cone the field thefield is high enough to produce fieldevaporation and ions of the liquid arepulled off at currents ~1-100A
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LMIS has high emission current density (highbrightness) and extremely small size of emissionarea.
These are favorable characteristics for achieving finefocus of ion beam.
Stable emission process
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Liquid metal sources have been made to yield ions of
many different species in Al, As, Au, Be, Bi, Cs, Cu, Ga,
Ge, Er, Fe, In, Li, Ni, Pb, Pd, Pr, Pt, U, and Zn. provide a source ofions of ~ 5 nm in diameter
its low melting point ( 29.8 C)
Its low volatility at the melting point conserves thesupply of metal and yields a long source life.
its low vapor pressure
it has excellent mechanical, electrical, and vacuumproperties.
its emission characteristics enable small energy spread.
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The majority of the ions generated are blocked by thesides of the aperture, as their velocity vectors are not
pointed along the direction of the beam.
The ion emission current is strongly dependent on thetip radius and on the tip surface condition.
The sharper the tip is, the higher the field; and the higher
the field, the stronger the ion emission.
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Hanson and Siegel have introduced Major Improvement
(Cold field emission guns). First, by running the tip at 4 K and introducing H2 at ~10 -2 Torr,
H2 condenses on the tungsten needle.
The H2 moves along the needle to the tip where it is field
desorbed as a molecular ion.
This flow along the tungsten needle greatly increases the
available H2 at the tip and hence the current.
The system is differentially pumped to prevent Ion molecule
collusions after ionization and acceleration
Secondly Its very important that that the ion beams areproduced from very end of the tip ( which has leastatom).
This greatly enhances the axial ion density.
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MASK AND RESIST
ADVANTAGES OF ION BEAM LITHOGRAPHY
ION CHANNELING
EFFECT ON MATERIAL REMOVAL RATE
EFFECT OF ANGULAR SPREAD
DRAWBACK OF ION BEAM LITHOGRAPHY
CHALLENGING INTEREST
CONCLUSION
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The mask needs to be made from an ion-transparent
substrate with a pattern made up of an absorbersurface (Vive versa).
The silicon acting as the absorber.
Polymethyl methacrylate (PMMA) is the most popular
type ofIBL resist. They have the better resolution, sensitivity, and etch
resistivity.
RESIST IN ION BEAM LITHOGRAPHY
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RESIST IN ION BEAM LITHOGRAPHY
Typical process flow for resist-based IBL
The pattern definition is performed by the physical modification of the
resist irradiated by ions.The key elements in the process are thus the employed resist and its
interaction with the beam employed for exposure.
Resist material behaves under ion beam irradiation largely depends on the
form of energy deposition.
MASK PREPARATION IN ION BEAM LITHOGRAPHY
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The main property required for
membrane Preparation is Coefficient
of Thermal expansion and Youngmodulus:
For this most preferred is Silica,
Alumina and Gold.
Alumina exhibit very low etch rate with
favorable Coefficient of thermal
expansion and young modulus.
MASK PREPARATION IN ION BEAM LITHOGRAPHY
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Comparison of limiting resolutions
Line Width [m]
0.01 0.1 1
con
tras
t
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
multilayer resistsingle layer resist
ION
X-RAY
OPTICS
E-BEAM
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When ions are implanted along the principal crystallineaxis, they may collide with lattice atoms in a similarway.
At each collision, the ion suffers only small-anglescattering and can penetrate the solid to deeperdistances. This phenomenon is called CHANNELING.
since the diameter of a positive ion is 0.02 0.34 nm, if a
channel presents an opening greater than the iondiameter then the ion can be implanted in the opencrystalline direction to a greater depth.
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Ion dose:A threshold valueexists.
48
3 x 1016 cm-3
Threshold
More (=>swelling)
Less (=>net MRR)
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Material removal rate m
Particle velocity V (known)
Particle diameter d (known)
Particle indentation depth w (known)
Semi contact radius of the indented surface a
Angle subtended by the arc at the center of the particle2
Area ploughed by the particle in the direction A.
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Distance from slit center [m]
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
numberofeventsa
twafer(=protondo
se)
0
5000
10000
15000
20000
25000
slit width = 1m
0.8
0.60.4
0.2
0.1
0.05
0.02
slit patternmembrane
wafer
400keV proton
10m
develop untilthis dose region
TRIMsimulation
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Ion-beam lithography is not without its drawbacks,
especially when compared to electron-beam lithography.
Shot noise is a major issue for ion-beam lithography, justas it is for electron-beam lithography.
Since energetic ions carry enough energy to displace
crystal lattice atoms, they can also cause considerabledamage to the substrate when subjected to verythinresists.
Due to interaction of ions with each other within the ion
optical column leads to resolution limit issues.
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New technology with no commercial instruments
available as yet
Difficulties in focusing MeV ions to sub-100nmdimensions
With the advent of compact magnetic quadrupole lenssystems, these difficulties have recently been overcome
first prototype p-beam writer has recently been designedand constructed at Centre for Ion Beam Applications
(CIBA), Singapore.
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Ion beam lithography is not yet a technology that isneeded at the feature sizes ofintegrated circuits currently
being produced.Its used in various applications like doping,photomasksrepair, material addition, (re)deposition, etc.
It is very low on the learning curve in comparison to
other advanced techniques such as e-beam or X-raylithography.
Its major advantage of high spatial resolution becauseof the absence of a proximity effect will become
important when feature sizes drop.
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Nanofabrication: Principles, Capabilities and Limits by
Zheng Cui (2008).
Micro-Nanofabrication Technologies and Application by
Zheng Cui (2005).
Principles of Lithography by Harry J. Levinson (2010).
Focused ion beam system: Basics and Applications by
Nan yao (2007).
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6 microns
8 microns
line width in the letters of the word Washington is 100 nm(close to the resolution limit)