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Deposition PVD, CVD, Electrochem
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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”. 13-Nov-18
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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 13-Nov-18
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Evaporation About 2 decades back Evaporation 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 13-Nov-18
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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 13-Nov-18
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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 13-Nov-18
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PVD: DC sputtering -ve Virtual leak Small -ve bias on wafer
Shield Al Virtual leak Small -ve bias on wafer Strong -ve bias ==> Wafer sputtering Plasma Region Controlled supply of Ar Vacuum pump +ve 13-Nov-18
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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 13-Nov-18
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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 13-Nov-18
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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 for step coverage 13-Nov-18
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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 90 For good uniform deposition Ionized Metal Plasma (IMP) Collimated beam Better step coverage Side wall (pre-dep, post etch) < 90 degress 13-Nov-18
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PVD: Collimated Beam -ve Equivalent to long throw Plasma Region +ve Al
Vacuum pump Controlled supply of Ar Al Shield -ve +ve Equivalent to long throw 13-Nov-18
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PVD: IMP RF Ti ions also present Similar to collimated Plasma Region
Vacuum pump Controlled supply of Ar Ti Shield RF Ti ions also present Similar to collimated Ionizing Coils For liners, IMP Ti is very common 13-Nov-18
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Sticking Coefficient (=1? <1? )
PVD: Comparison Comparison of effectiveness of various methods ©Hynix (Huyndai) Ltd Sticking Coefficient (=1? <1? ) 13-Nov-18
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Optimal location of target
PVD: Comparison Evaporation Sputter Optimal location of target 13-Nov-18
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CVD Reaction occurs on surface APCVD, LPCVD, PECVD, HDPCVD, MOCVD
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 13-Nov-18
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CVD Growing silicon on silicon (for example) Induction Heating Inlet
Outlet Laminar Flow 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 Hot wall not preferred 13-Nov-18
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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 1/T 13-Nov-18
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APCVD Barrel Reactor Vertical Reactor Horizontal
Only few wafers per chamber Vertical (pancake) few wafers per chamber, but no depletion Barrel Reactors more capacity Heating by irradiation; better control than induction 13-Nov-18
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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 13-Nov-18
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APCVD Reactions Etch + Dep Single gas Low temp Single Crystal growth
Growth Rate (m/min) Etch 0.4 Mole fraction of SiCl4 13-Nov-18
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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 13-Nov-18
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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) 13-Nov-18
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LPCVD Silicon Nitride (300 or 700 C). Used for LOCOS Oxide (<500 C)
If the growth rate is very high (for silicon for example), then poly crystalline structure forms. If is is low, we get single crystal structure. 450 C;Quality issue 400+ C TEOS, also used for PECVD, HDPCVD 13-Nov-18
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LPCVD W Dep Substrate reduction ~300 C W-Silicide/ Ti Silicide
Tungsten can be deposited by the thermal decomposition of tungsten hexa fluoride (which releases fluorine and that Will react with silicon) or by reduction of WF6 by hydrogen. Tantalum penta chloride can also be reduced by hydrogen (2 Ta Cl5 + 5 H2 2 Ta + 10 HCl). Aluminum can be deposited by decomposition of Tri-iso butyl Al, but sputtering is cheaper and gives good quality Al. 13-Nov-18
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Controlled temperature (and not necessarily uniform)
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 13-Nov-18
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PECVD 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) 13-Nov-18
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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 13-Nov-18
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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) 13-Nov-18
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Electrochemical Dep Copper properties vs Deposition techniques
Grain size, electromigration 13-Nov-18
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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 13-Nov-18
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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 13-Nov-18
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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 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) 13-Nov-18
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Dep Unit Lab unit 13-Nov-18
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Sample image of Dep unit
Controller for all parameters © semiconductor international 13-Nov-18
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Types of Fill: SEM © casewestern univ 13-Nov-18
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Types of Fill: Schematic
13-Nov-18
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Effect of Liner/Seed 13-Nov-18
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Example of SuperFill SEM Image ©semiconductor international 13-Nov-18
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Example of superfill SEM Image © 13-Nov-18
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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 13-Nov-18
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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 The mass transfer and reaction limitations are governed by the concentration in the solution, flow characteristics (boundary layer) and applied voltage (temperature also has some effect). 13-Nov-18
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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) 13-Nov-18
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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 13-Nov-18
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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 solid content speed 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 © GATech 13-Nov-18
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Silica-titania striation
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 ©univ ariz 13-Nov-18
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Silica-titania striation
Spin Coating: Issues Striation/instability: Hypothesis Silica-titania striation ©univ ariz 13-Nov-18
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Spin Coating: Issues Vacuum Chuck effects Vacuum chuck
Coating on the glass ©univ ariz 13-Nov-18
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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) 13-Nov-18
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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 13-Nov-18
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