Transcript Lithography - Chemical Engineering IIT Madras

Overview
 4 processes
 Physical Vapor Deposition (PVD)
 Chemical Vapor Deposition (CVD)
 Electrochemical Deposition (ECD-used for copper)
 Spin Coating (new)
 (Ion implantation is dealt with in FEOL)
 Purely physical processes: PVD, Spin coating
 Chemical: CVD, ECD
 Note: ECD is also used as an acronym to indicate “electrical
critical dimension” or “electrical CD”.
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Deposition (general)
 Uniform step coverage, control
 Adhesion and Step Coverage
 Conformal deposition (may not be what you want!)
 Void free fill
 narrow space vs wide space
 Grain Structure, thin film properties
 Sticking co-efficient
 Contamination free; For alloys, composition
 Extra: Self planarizing
 Metals deposited by PVD: Ti/TiN, Ta/TaN, Cu-seed, Al
 by CVD: W, Ti, Cu-seed
 by Electrochemical: Cu
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Evaporation
 About 2 decades back
 No Reaction Involved
 For Aluminum, or gold
 in vacuum (~ micro torr)
 To avoid oxidation
 To get uniform coating
 Chamber is also heated
 Evaporation source may be crucible heated with coil, electron
beam melting, or just tungsten filament coated with other materials.
 Depositing alloys
 E-Beam, multiple e-beams for alloy co-evaporation
 Hot Plate (with alloy wire flash)
Evaporation
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Physical Vapor Deposition
 No reaction involved
 a.k.a. Sputtering
 Not the same as evaporation
 Operating temperature lower than melting point
 Argon ions, low pressure
 Control of composition
 Dielectric deposition
 Step coverage improvement (planar source vs point
source)
 Rotating wafer holder and heating wafer improves step coverage
 Improve adhesion by sputtering ‘wafer surface’ prior to
dep
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Physical Vapor Deposition
 Dry argon or neon
 moisture == oxidation
 Pressure nano torr
 Argon increases pressure ==> Control amount of Ar introduced
 milli torr pressure during operation
 Ar ionized (using electrodes)
 Low pressure ==> longer l (60 m vs few microns)
 Line of Sight vs uniform
 DC/Triode/RF/magnetron sputtering
 Ion Metal Plasma (IMP) and Collimated
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PVD: DC sputtering
-ve
Al
Shield
Plasma Region
Controlled
supply of Ar
Vacuum pump
Shield
 Virtual leak
 Small -ve bias
on wafer
 Strong -ve
bias ==> Wafer
sputtering
+ve
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PVD: sputtering
 Target sputtering ==> Heat
 Aluminum oxidation ==> Sputtering stops
 ==>Triode sputtering
 Electrons not supplied by target, rather by a separate
filament
 Resulting films are denser (better)
Random direction of electron from coils (or target)
==> low efficiency of Ar ionization
 Confine the field using magnets
 ==> better Ar ionization
 ==> better (faster) deposition rate
==> low Ar flow rate required
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PVD: RF sputtering
 Works mainly if the target is metal
 For insulator, AC voltage (at radio frequency) is used
 Can be used to slightly roughen (etch) wafer surface
==> better adhesion
a.k.a. sputter etch, reverse sputter, ion milling
 Also removes any contaminant ==> better electrical contact
 Target much smaller than wafer+chamber
 Dep Rate monitored with quartz crystal
 Resonant frequency changes with film thickness
 1A/sec accuracy
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PVD: Basic Mechanisms
 Direct Sputtering
 Emitted (desorbed) sputtering (sticking coefficient)
 ions stick well, neutral atoms stick < 1
 Resputtering
 Surface Diffusion (in, out)
 more with more curvature
 filling in of corners is more
 For good uniform deposition
 Increase target width (angle of incidence distribution?)
 Pull target away (long throw) (angle of incidence distribution?)
 Rotate wafers around the target (similar to increasing target
width)
 Desorption (Sticking coeff < 1) more important than surface diffusion
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for step coverage
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PVD: Basic Mechanisms
 Sputtering Yield
 No of Aluminum / No of Argon ions
 Depends on angle of incidence and energy of ions
 can be > 1
 Minimum energy needed for sputtering
 RF Sputtering - Reduction of necking
0
90
 For good uniform deposition
 Ionized Metal Plasma (IMP)
 Collimated beam
 Better step coverage
 Side wall (pre-dep, post etch) < 90 degress
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PVD: Collimated Beam
-ve
Al
Shield
Shield
 Equivalent to
long throw
Plasma Region
Controlled
supply of Ar
Vacuum pump
+ve
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PVD: IMP
RF
Ti
Shield
Ionizing
Plasma Region
Shield
 Ti ions also
present
 Similar to
collimated
Coils
 For liners,
IMP Ti is very
common
Controlled
supply of Ar
Vacuum pump
RF
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PVD: Comparison
Plot of coverage vs Aspect ratio
for IMP, Long throw and
standard PVD
 Comparison
of effectiveness
of various
methods
 ©Hynix
(Huyndai) Ltd
Sticking Coefficient (=1? <1? )
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PVD: Comparison
Evaporation
Sputter
Optimal location of target
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CVD
 Reaction occurs on surface
 nucleation occurs only on surface
 similar to catalysis
 Relatively higher pressure compared to PVD
 lower mean free path
 Anisotropic dep
 better step coverage
 conformal dep
 APCVD, LPCVD, PECVD, HDPCVD, MOCVD
 Epitaxy: VPE, LPE, MBE, ALD
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CVD
 Growing silicon on silicon (for example)
Induction Heating
Inlet
Laminar Flow
Outlet
 Atmospheric Pressure (APCVD)
 Cold wall. Only the wafers and wafer holders are heated
 Induction heating, radiation heating
 Reaction (Deposition) will occur only on the wafer
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 Hot wall not preferred
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APCVD
 Mechanism:
 Boundary Layer Diffusion in gas phase is important
 decrease press, increase diffusivity, increase BL thickness
 Mass transfer in gas phase, adsorption, surface
reaction, surface diffusion to ‘nucleating sites’ and
desorption of other products
 Wafer sets are not horizontal
Film Dep Rate
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1/T
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APCVD
Vertical Reactor
Barrel Reactor
Heating by irradiation;
 Horizontal
 Only few wafers per chamber better control than induction
 Vertical (pancake)
 few wafers per chamber, but no depletion
 Barrel Reactors
17-Jul-15  more capacity
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APCVD
 Tilted base (susceptor) vs horizontal base
 Gas velocity, boundary layer thickness
 Diffusion - BL thickness, Kinetics-Temp
 High temp operation (1000 C) for single crystal
growth
 APCVD: Tilted base
 Low pressure CVD (LPCVD)
 Kinetics controlled
 Keep wafers close by, change temp
 ==>Reactor design based on kinetics/ mass transfer
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APCVD
 Reactions
SiCl4  2 H 2  Si  4 HCl
Etch + Dep
SiH 4  Si  2 H 2
Single gas
SiH 2Cl2  Si  2 HCl
Low temp
 Single Crystal growth
Growth Rate
(m/min)
Growth
Etch
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0
Mole fraction of SiCl4
0.421
LPCVD
 In APCVD, sometimes, reaction may occur in gas
phase, forming particles that fall on the wafer
 In LPCVD, gas phase reaction is less likely to
occur
 Lower temperature (i.e. Compatible with Al)
 Pressure = bit less than a torr
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LPCVD
 Hot wall (vs cold wall for APCVD)
 Reaction Limited (vs Mass Transf Limited for
APCVD)
Used for poly, oxide, nitride, W
 Poly (600 C) SiCl4  2 H 2  Si  4 HCl
SiH 4  Si  2 H 2
SiH 2Cl2  Si  2 HCl
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LPCVD
 Silicon Nitride (300 or 700 C). Used for LOCOS
3SiH 4  4 NH 3  Si3 N4  12 H 2
3SiH 2Cl2  4 NH3  Si3 N4  6HCl  6H 2
 Oxide (<500 C)
SiCl2  2 NO2  SiO2  2 N 2  HCl
900 C
SiH 4  O2  SiO2  2 H 2 450 C;Quality issue
 C2 H 5O 4 Si  O3  SiO2  Others 400+ C
TEOS, also used for PECVD, HDPCVD
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LPCVD
 W Dep
2WF6  3Si  2W  3SiF4
 Substrate reduction
~300 C
WF6  3H 2  W  HF
 W-Silicide/ Ti Silicide
WF6  2SiH 4  WSi2  6 HF  H 2
TiCl4  2SiH 4  TiSi2  4HCl  2H 2
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LPCVD: Reactors
Controlled temperature (and not necessarily uniform)
Gas IN
Vacuum
 Horizontal tube reactor
 Deposition of oxide on Al
 Temp of 600 C causes alloying
 Need to go for PECVD
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PECVD
 C2 H 5O 4 Si  O3  SiO2  Others
400- C
 Plasma can also be used for etching
 Used for scrubbing (cleaning) the wafer, before dep
 RF used for creating and sustaining plasma
 dep rate can be high, but too high a dep rate is not
good (film stress, grain structure etc)
 BPSG, PSG by introducing phosphine / diborane
(B2H6)
 Introduction of Ar in plasma causes simultaneous dep
and ‘little’ bit of sputtering
 Deposited film is dense (high density plasma chemical
vapor deposition= HDPCVD = HDP)
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CVD: Other
 MOCVD (eg tri methyl gallium / Arsine)
 MBE (like scanning)
 Easy to form hetero-junction
 slow, good control
 growth (along with doping)
 VPE
 Introduce doping gas in the CVD; obtain epitaxial
dep with doping
 ALD
 Brief Mechanism
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Epitaxy Issues
 Epitaxy: Issues
 Haze (oxidation)
 spike (accelerated local growth)
 stacking fault, dislocation, slip,
 out diffusion/ auto doping (reverse of out
diffusion)
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Electrochemical Dep
 Copper properties vs Deposition techniques
CVD
PVD
Resistivity(m  cm)
>= 2
1.75—2
Impurities
C,O
Ar
—
Seed layer
—
>=100
—
<=100
~200
Deposition rate(nm/min) ~100
Laser
reflow
Electroless
2.6 ~2
Electrolytic
~2
Process temp. ( C)
~250
RT
Melt
50—60
RT
Step coverage
Good
Fair
—
Good
Good
Via-filling capability
Good
Poor
Good
Fair-Poor
Fair-Poor
Environmental impact
(waste)
Good
Good
Good
Poor
Poor
 Grain size, electromigration
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Electrochemical Deposition
 For copper
 Related technique: Electroless deposition
 Catalyzed on the metal surface
 may occur on other surfaces (bad)
 may occur in solution (very bad!)
 may need activation
 temperature, fluid flow patterns
 Electrochemical Dep
 dep in pattern: proprietary solution
 need liner/ Cu seed layer
 field in corners
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Electrochemical Deposition
 Fluid flow pattern/ mass transfer limitations
 kinetics controlled by potential, temperature
 Film quality, temperature , anneal
 Role of additives
 Fill, void, Super fill (next few pages)
 suppressors in solution
 depletion in small trenches
 Grain Size, growth in room temperature, at higher
temperatures
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Electrochemical Deposition
 Usually < 50% of max current
 max current at mass transfer limited region
 operation at tafel region (reaction limited region)
 uniform voltage distribution is necessary (seed
layer)
 convection is key
Current vs Volt
for Copper
Electrochemical
Dep
 DC or wave form
 pulsed
 can etch ‘sharp’ regions
(more planarity)
 need sophisticated control
 acidic pH (sulfate, pyrophosphate etc)
 basic pH (cyanide and other solutions)
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Dep Unit
 Lab unit
Image of a lab scale dep unit
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Sample image of Dep unit
 Controller for
all parameters
©
semiconductor
international
Image of a
production scale
control unit
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Types of Fill: SEM
© casewestern univ
SEM images showing
conformal, anti conformal
and superfill dep
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Types of Fill: Schematic
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Effect of Liner/Seed
SEM images showing effect of good vs poor seed layer
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Example of SuperFill
SEM Image ©semiconductor international
SEM images showing
conformal, anti conformal
and superfill dep
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Example of superfill
SEM Image
 ©www.future-fab.com
SEM images showing
conformal, anti conformal
and superfill dep
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Electrochemical Deposition
 Organic additives used
 mechanism not very clear
 could be steric effect
 some of them consumed in reaction
 sometime incorporated in the film
 Suppressors (levelers, carriers)
 carriers: polymeric surfactants (poly ethylene glycol PEG)
 in presence of Cl-, adsorb on surface
 levelers: multiple charged structures
 adsorb on corners/edges etc, level
 large size, cannot go inside trench
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Electrochemical Deposition
 Change additive concentration: plating voltage changes
 Boundary layer >> feature size
 Diffusion is important
 additive must be consumed (otherwise, surface will be saturated)
Accelerators (brighteners)
 unsaturated compounds with polar group
 sulfur, oxygen, nitrogen based groups
 may help nucleation
 many small grains formed (brighter surface)
 more growth at the trench/via bottom
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Electrochemical Deposition
For good dep
 monitor concentration and replenish
 remove products (oxidize in presence of uv)
 photochemical catalysis to enhance oxidation
Summary
 Main dep technique for copper
 superfill
 need to control composition well
 Environmental issues (waste disposal)
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Spin Coating
 Similar to photo resist coating
 used for organic / mix of organic+inorganic ILD
 DVD, CD...
 mainly used for low-k materials
 low temperature
 better flow characteristic / self planarizing
 adhesion
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Spin Coating
 Form solvent puddle at the center
 spread it by low rpm spin (500 rpm)
 thinning by high rpm spin (2000 rpm). Dep rate,
uniformity affected by
 viscosity
Image of lab
 solid content
scale spin
© GATech
 speed
coating unit
 time
 remove the ‘bead’ formation at the edge (due to surface
tension)
 using solvent back wash
 evaporation of solvent (drying)
 also depends on airflow over the wafer
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Spin Coating: Issues
 Presence of large solid particles
 comets
 variable solvent evaporation rate in multi solvent
solutions
 Striations
 due to surface tension effects and instabilities
Silica-titania comet
Silica-titania striation
Images of
various
defects
©univ
ariz
Images of
various
defects
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Spin Coating: Issues
 Striation/instability: Hypothesis
Silica-titania striation
©univ ariz
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Spin Coating: Issues
 Vacuum Chuck effects
Vacuum chuck
Coating on the glass
Image of
coating
Image of
vacuum chuck
©univ ariz
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Spin Coating: Issues
 Note: outside the O-ring variations
 Near the ‘grooves’ clear variation
 Hypothesis:
 heat from from chuck compensates for cooling by
evaporation
 when there is no contact, cooling is not ‘prevented’
 ==> lower evaporation when there is no contact
 more pronounced in glass vs silicon (better heat
conductor)
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Spin Coating: Summary
 In R&D
 Similar to photo resist coating
 Goal is to use it for low-k dielectrics (insulators)
 Issues yet to be resolved
 Most of the issues in low-k are not with spin
coating. They are with subsequent processes /
integration
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