thin film disposition
TRANSCRIPT
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Aspect ratio AR
hw
Note the aspect ratios and the need for new materials. Note also the number of metal layers requiring more deposition steps.
Year of Production 1998 2000 2002 2004 2007 2010 2013 2016 2018
Technology N ode (half pi tch) 250 nm 180 nm 130 nm 90 nm 65 nm 45 nm 32 nm 22 nm 18 nm
MPU Printed Gate Length 100 nm 70 nm 53 nm 35 nm 25 nm 18 nm 13 nm 10 nm
Min Meta l 1 Pitch (nm) 214 152 108 76 54 42
Wiring Levels - Logic 10 11 12 12 14 14
Me tal 1 Aspect Ratio (Cu) 1.7 1.7 1.8 1.9 2.0 2.0
Contact As pect Ratio (DRAM) 15 16 >20 >20 >20 >20
STI Trench As pect Ratio 4.8 5.9 7.9 10.3 14 16.4
Me tal Resistivi ty (ohm-cm) 3.3, 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2
Interlevel Dielectric Constant 3.9 3.7 3.7
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Thin Film Deposition
Quality composition, defect density, mechanical and electrical properties
Uniformity affect performance (mechanical , electrical)
Thinningleads to
R
Voids: Trap chemicals lead to cracks(dielectric) large contact resistance
and sheet resistance (metallization)AR (aspect ratio) = h/w with feature size in ICs.
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Chemical Vapor Deposition
Flat on the susceptor
Cold wall reactor
Methods of Deposition :
Chemical Vapor Deposition (CVD):APCD, LPCVD, HDPCVD
Physical Vapor Deposition (PVD:evaporation, sputtering)
Atmospheric Pressure : APCVD
Cold wall reactors (walls not heated -only the susceptor)
Low pressure : LPCVD batch processing.
Hot wallreactor
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Atmospheric Pressure Chemical Vapor Deposition
Transport by forcedconvection
By diffusionthrough boundarylayer
Diffusion through the B. L
Desorption of by products
Transport of byproducts byforced convection
@ the surface (4): decomposition,
reaction, surface migration attachment etc.
(3) May be desorption which depends ona sticking coefficient (4)
Growth rate for Si deposition N=510 22cm -3
Mole fraction of the incorporatingspecies in the gas phase.
Partial pressure
Total concentration in the gas phase
CT = 1 * 10 19 cm -3 5 * 10 22
V = 0.14 m/min
PG @ 1 torrPtotal = 1 atm = 760 torr
adsorption
transport
reaction
Steady state
Steps in deposition
As in Deal-Grovemodel for oxidation
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Growth Kinetics
Determined by the Smaller of k s or h GTwo limiting cases
1) Surface reaction k s
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Boundary Layer Diffusion to the Surface
Gas moves with the constant velocity U.
Boundary layer (caused by friction ) increases alongthe susceptor, mass transfer coefficient h G decreases,gas depletion caused by consumption of the reactingspecies (concentrations decrease)
Growth rate decreases along the chamber
Use tilted susceptor
Use T gradient 5-25C
Gas injectors along the tube
Use moving belt
Deposition of alloys DIFFICULT various reactions,kinetics (species, precursors)
Use PVD rather than CVD
B.L.
viscosity
gas density
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Doping in CVD for EPITAXY
Intentional and Unintentional
Si source + Dopant(AsH 3, PH 3, orB2H6)
Autodoping: 1 - 4
outdiffusion
autodopingThe growth is faster thanthe diffusion
Dt vt
The dopant sources at thesurface go through:
dissociation of hydride gas
lattice site incorporation
burying of dopants by otheratoms in the film
Simulation very inaccurate :chamber design etc.
In deposition , the doping,
C P i for low growth rates
C P iv
for high growth rat
800-1000C
outdiffusion
autodoping
T&time of CVD
Calculate all distributions(=contributions) to get C(x,t)
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Low Pressure Chemical Vapor Deposition
Operate at the surface reaction limited regime lower deposition temperatures
Larger diffusion @ lower P s
GG
Dh
Y N c
hk hk
v T G s
G s
hG increases ~ 100 X for760 torr 1 torr
Surface reaction controlsthe growth
LPCVD reactors use
Vertical wafer stacking
P = 0.25 2.0 torr T = 300 900 C ( + 1 C) temperaturegradient increase 5 25% to compensatereactants depletion distributed feeding.
Less autodoping (at lower P)
Fewer particulates.
Possible disadvantages:
Deposition rates may be too low, Film quality decreases
Shadowing (less gas-phase collisions) due to directionaldiffusion to the surface deterioration of the step coverage andfilling.
transport less important
low pressure
DG 1/P total
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Plasma Enhanced Chemical Vapor Deposition(PECVD)
Used when :
Low T required (dielectrics on Al,metals) but CVD at decreased T givesincreased porosity, poor step coverage.
Good quality films energy supplied by plasma increases film density,composition, step coverage for metaldecreases but WATCH for damage and by
product incorporation.
Outgassing , peeling , cracking
stress.
P 50 mtorr - 5 torr
Plasma: ionizedexcited molecules,neutrals, fragments,ex. free radicals very reactivereactions @ the Sisurface enhanced increase depositionrates
High Density Plasma CVD dense layers ( SiO 2) at low T (150 C) and low P ( 1- 10 m torr); T increases to 400C by bombardment
Separate RF (gives substrate biasing bombardment) from plasma generation (Electron Cynclotron resonance ECR andInductively coupled Plasma ICP)
Controlled bombardment (angular -> sputtering) preferential sputtering of sloped surface improved planarization andfilling
200- 350 C
13.56 MHz
Ions, electrons,neutrals =
bombardment
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High Density Plasma (HDP) CVD
Remote high density plasma with independent RF substrate bias. Allows simultaneous deposition and sputtering for better planarization and
void-free films (later). Mostly used for SiO 2 deposition in backend processes.
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
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Physical Vapor Deposition (PVD) no chemical reactions (except for reactive sputtering)
Evaporation
Advantages :
Little damage
Pure layers (high vacuum)
Disadvantages :
Not for low vapor pressure metals
No in-situ cleaning
Poor step coverage
Very low pressure (P < 10 5 torr) - long mean free path.
purer no filaments, only surface of thesource melted
X-rays generated trapped charges in thegate oxides anneal it !
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Evaporation
k 2 cosr N vop Re
v
k i2 coscosr N vop Re
v
Ideal cosine emission
Wafer holders to increaseuniformity of deposition
Projectedarea
Knudsen cell like behavior
The largest fluxfor the
perpendiculardirection
Affected by a crucible(melt,)
l rand cos k
v
Practical cases
Use sphericalholders & rotatethem in a
planetaryconfiguration
Deposition rates:
nonuniform deposition
lower because ofcos i emission
source here is ||to the wafer
Emitted fluxes from crucibles
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Evaporation
Partial Pressure (P e) of the source(target)
e
21
s2
evap P T m
A1083.5 R
Needed forreasonable v 0.1 - 1 m/min
No alloys partial pressure differences
Use separate sources and e-beam
incident
reacted c
F
F S
Step Coverage Poor :
Long mean free path (arrival angle not wide = smallscattering) and low T (low energy of ad-atoms)
Sticking coefficient high (@ T) no desorption andreadsorption poor step coverage
Heating can increase S c but may change film properties (composition, structure)
Rarely used in IC fabrication
Sticking coefficient
1-10 mtorr
Deposition
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Sputter Deposition
Higher pressures 1 100 mtorr ( < 10 -5 torr in evaporation) -> contaminations!
Use ultra clean gasses and ultra clean targets
Alloys (TiW, TiN etc)
good step coverage
controlled properties
DC Sputtering (for metal)
Conductive
Al, W, Ti
Ar inert gas at low pressure.
No free radicals formed by Ar (ex. O, H ,F as wasfor PECVD)
Major Technique in Microelectronics for:
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DC Sputtering
Low concentrations of electrons ->few collisions -> large voltage here
0.1 10 mm
Ar ions (+) strike the target and sputterions electrons do not have highenergy yet dark glow anodesheath.
e-
I+
e- collide with Ar atomsexcitation=glow
(Crookes dark space)
positive potential (form next to eachsurface = anode)
I+ and e - strike the surface:
e larger (smaller mass than for ions)more electrons than ionsE field due to charge imbalance V p (10 Vdevelops) to decrease e - accumulation
+ potential
secondary electrons!They sustain the plasma
ions
The source of material to be deposited
Conductive !
low e --conc.
Very small sputtering at the anode can be used for biassputtering and ionized sputtering
Ions get neutralized by e - anddiffuse to the wafer surface
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Sputtering
10-20 eV
secondary electrons
electronssustain the
plasma -
ionization ofthe gas
Bombardment by I + ,e- charges and neutrals
adsorption
migration
desorption (small)
Cathode DC Sputtering
heating
can beused inhot sputter
reflow
Reactive SputterDeposition = add gas:. N2 Ti N. O2 Ti O 2
Can beincorporated
in the film
CONDUCTOR
WAFERS
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SPUTTERING YIELD
Effect of mass (gas) & EnergyTiWsteady statesputtering
Effect ofmass (target)
implantation
Target
Target atom
gas
muchsmallerdifferencesin sputteringyields thenin partial
pressures ofcomponents(target) inevaporation
+
Neutral
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RF Sputter Deposition Dielectric
Instead of DC: 13.6 MHz RFcoupled capacitively to
plasma
several 100V wafers
DC sputter cannot be used for dielectrics
secondary e - plasma extinguished (V Z ) in 1-10s
More on the walls charge built-up
potential V P
potential
@ the target ( area)
-
= NON-CONDUCTINGOscillating (with RF) e -
ionization yield pressure
+ magnet e - trajectoryMagnetron Sputter Depositionhave better ionization yields
deposition rates (10-100X)
better film quality (Ar needed)
use in DC & RF ( heating of thetarget since I + )
large A1 area
A2
A2 A1
e - charge onelectrode (e - are fast sothey keep upwith RF)
e - tenths of volts
faster, smaller
can be used
V1/V2=(A 2/A1)m m=1-2
For A 1=A 2 Ions would bombard the target and thedeposited layer
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BIAS SPUTTERING - Small (-) Bias @ the Wafer Chuck50 300V on wafers 700 2000V on target
For :
precleaning= sputter etch
better planaritystep coverage
properties(stress, compos.)
Not used much particulates from flaking
Use High Density Plasma CVDinstead or
Collimated Sputtering and Ionized Sputtering
smallWider arrival
in
extendedsources = not
point sourcesevap;
in point sources
Collimate the beam by using holes to direct the ions to the wafers : that u
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Ionized Sputter Deposition or HDP Sputtering
In some systems the depositingatoms themselves are ionized.An RF coil around the plasmainduces collisions in the plasmacreating the ions (50-85% ionized).
This provides a narrowdistribution of arrival angleswhich may be useful when fillingor coating the bottom of deepcontact hole.
a) b)
Highlydirectional fluxBetter solution
than a collimator(holes)
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
VARIOUS DEPOSITION TECHNIQUES
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VARIOUS DEPOSITION TECHNIQUES
3 3 2 6AsH ,PH , B H
give better step coverage
than sputtering
0.5 2 torr80 200 torr
Barrier
plugs
400 700 0C
VLSI!
mass transfer regime
T oxide evaporation
Cl- cleaning (HCL etc)
& better doping control Selectivity with Cl, poorfor SiHCL
porousBPSG 4-8% B, P)
reflow for planarization, steam, N
2
! Al TIBA or DMAH Al-H
C - better Cu or Al-Cu sputtered on
CVD Al diffusion @400 0C
= salicide
Low T: -Si instead of poly_Si
Conditions: density of SiO 2,step coverage, contaminations(C, H, N) all that determinesT, pressure and gas used.
TEOS
LPCVD (conformal, 650-800C)
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Polysilicon@ low T amorphous Si
columnarstructure
As & P deposition rate of poly Si use dopingafter poly deposition
B V poly - Si
As , P segregate @ the grain boundaries
( B does not ! )
625 C
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Models and Simulation
Ar +
Direct flux,neutrals
Al
Resputtered flux
Desorbed(emitted)
flux
Redeposited fluxes
Al
Al
Surface diffusion flux
AlAl+
Direct flux, ions
Fneti Fdirect (neutrals )
i Fdirect (ions )i Fredep
i Fdiff .ini
Femittedi
Fsputteredi
Fdiff .outi
Within the past decade, a number ofsimulation tools have been developed
for topography simulation. Generalized picture of fluxesinvolved in deposition. (No gas
phase boundary layer is included, sothis picture doesn't fully modelAPCVD.)
Essentially the same picture will beused for etching simulation (inChapter 10).
(14)
To simulate these processes, we need mathematical descriptions of the variousfluxes.
Modeling specific systems involves figuring out which of these fluxes needsto be included.
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
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Finally, ions striking the surface can sometime enhance the deposition rate (bysupplying the energy to drive chemical reactions for example), so that
Fion inducedi K i Fions
i (22)
LPCVD Deposition Systems
Fdirect(neutrals)i
Yes
Fdirect( ions )i
No
Fdiff (net)
iF
diff ( in )
iF
diff (out )
i No
Femittedi
Yes
Fredep(emitted)i
Yes
Fsputteredi
No
Fredep(sputtered )i
No
F ion inducedi
No
Furnace - with resistance heaters
Standup wafe rs
Directflux,neutrals
De sorbed(emitted)
flux
Redepos ited fluxes In these systems there are no ions involved andhence no sputtering. Surface diffusion also isusually not important.
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
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PECVD Deposition SystemsRF power input
Electrode
WafersPlasma
Gas outlet, pump
Heater
Gas inlet( SiH 4, O 2)
Direct flux,neutrals
Desorbed(e mitted)
flux
Redeposited fluxes
+ Direct flux, ions
Fdirect(neutrals)i
Yes
Fdirect(ions )i
No
Fdiff (net)i
Fdiff ( in )i
Fdiff (out )i
No
Femittedi
Yes
Fredep(emitted)i
Yes
Fsputteredi
No
Fredep(sputtered )i
No
Fion inducedi Yes
In these systems an ion flux can enhance the depositionrate by changing the surface reactions. Sputtering isusually not significant because the ion energy is low,
nor is direct deposition of ions significant.
rateS c K d Fd K i Fi
N Thus (25)
where K d and K I are relative rate constants for theneutral and ion-enhanced components respectively.
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
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Ionized PVD Deposition Systems
+Direct flux,
neutrals
Resputtered fluxDesorbed(e mitted)flux
Redeposited fluxes
Direct flux, ions+
DC targe t bias
RF substrate bias
Al Al + + e-
Al target
Fdirect(neutrals)i
Yes
Fdirect(ions )i
Yes
Fdiff (net)i
Fdiff ( in )i
Fdiff (out)i
No
Femitted
i Yes
Fredep(emitted )i
Yes
Fsputteredi
Yes
Fredep(sputtered )i
Yes
Fion inducedi
Yes
These systems are complex to model because bothions and neutrals play a role.
They are often used for metal deposition so that Ar + ions in addition to Al + or Ti + ions may be present.
Thus almost all the possible terms are included
rate S c Fd Fi K sp YF i K rd Frd
Nwhere F d includes the direct and redeposited (emitted)neutral fluxes, F i includes the direct and ion-inducedfluxes associated with the ions, and F rd models
redeposition due to sputtering.SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
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High Density Plasma CVD Deposition Systems
gas inlet
plasma
magneti c coil
RFbias supply
(13.56 MHz )
wafer
gas outlet,pump
Microwavesupply
(2.45 GHz )
Resputtered flu
Redeposited fluxes
Direct flux, i ons
+
+
Very similar to IPVD (except neutral direct flux not as important):
rate K iF i K sp YF i K rd Frd N
(29)
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
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Models in SPEEDIE
LPCVD:
PECVD:
Standard PVD:
High T PVD:
Ionized PVD:
HDP CVD:
rate S c Fd
density
Fd Fdirect(neutrals)i F redep(emit)
i
rateS c K d Fd K i Fi
density F i Fions
rate S c Fd
density
rate
S c Fd D skT s
u2 K
s2
density
rate
Sc
Fd
Fi K
spYF
i K
rdF
rd density
rate
S c K i Fi K sp YF i K rd Frd density
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
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SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
OVERHANG TEST STRUCTURE
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DIRECT DEPOSITION
SURFACE DIFFUSION
DEPOSITION PRECURSORS
POLY-Si OVERHANG
SILICON SUBSTRATE
OXIDE
16 m
1-4 m
RE-EMISSION
OVERHANG TEST STRUCTURE
INDIRECTDEPOSITION
BY OBSERVING DEPOSITION PROFILES IN THE CAVITY CONCLUSIONS CAN BEDRAWN ABOUT THE DEPOSITION MECHANISMS
* TAPERING OF THICKNESS ON TOP SURFACE
* INFLUENCE OF CAVITY HIGHT ON DEPOSITION ON THE UNDERSIDE
J.P. McVittie, J.C. Rey, L.Y. Cheng, and K.C. Saraswat, "LPCVD Profile Simulation Using a Re-Emmission Model", IEDM Tech. Digest, 917-919 (1990). L-Y. Cheng, J. P. McVittie and K. C. Saraswat, " Role of Sticking Coefficient on the Deposition Profiles of CVD Oxide, "Appl. Phys. Lett., 58(19),
2147-2149 (1991).
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
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PECVD LPCVD
J.P. McVittie, Test Structure and Modeling Studies of Deposition and Etch Mechanisms, Talk TC1 -WeM6, AVS mtg in Orlando, Florida, 1993
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
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Parameter Values for Specific Systemsn
(exponent in cosinearrival angledistribution)
SC(sticking
coefficient)
Sputter deposition-standard ~ 1 - 4 ~1-ionized orcollima ted
8 - 80 ~1
Evaporation 3 - 80 ~1
LPCVD silicon dioxide- silane 1 0.2 - 0.4
-TEOS 1 0.05 - 0.1LPCVD tungsten 1 0.01 or lessLPCVD polysilicon 1 0.001 or less
PVD systems - more vertical arrival angle distribution (low pressure line of sightor field driven ions). n > 1 typically.
CVD systems provide isotropic arrival angle distributions (higher pressure,gas phase collisions, mostly neutral molecules). n 1 typically.
PVD systems usually provide S c of 1. Little surface chemistry involved. Atomsarrive and stick.
CVD systems involve surface chemistry and S c
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Topography Simulation (Using SPEEDIE)
SPEEDIE simulations for LPCVD deposition of SiO 2 with S c = 1 (which is more typical of PVD than LPCVD)and varying values of n, the arrival angle distributionfactor: (a) n=1; (c) n=10.
Worse step coverage results as n increases (the arrivalangle distribution narrows).
Even for n = 1, conformal coverage is not achieved.
-1.00 1.000.0-0.5
0.0
0.5
1.0
1.5
2.0
microns
microns
SPEEDIE simulations for LPCVDdeposition of SiO 2 in a narrowtrench with the same isotropicarrival angle distribution (n=1) butdifferent values of S
c: (a) S
c = 1;
(b) S c = 0.1; and (c) S c = 0.01. Reducing S c is much more effective
than changing n if conformaldeposition is desired.
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
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Simulations
Si l i CVD
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90 85 80
Results of SPEEDIE LPCVD simulations with the sidewall angle changed. S c = 0.2 and n = 1.
Note the improved trench filling.
SPEEDIE simulations comparing LPCVD andHDPCVD depositions. (a) LPCVD depositionof SiO 2 over rectangular line. S c = 0.1 and n = 1.(b) HDPCVD deposition, with directed ionicflux and angle-dependent sputtering, overrectangular line showing much more planartopography.
CMP might still be required in the HDPCVD
case to fully planarize the surface.
Simulations - CVD
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
Si l i CVD
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SPEEDIE simulations comparing LPCVDand HDPCVD depositions. (c) LPCVDdeposition in trench, showing void formation.S
c = 0.2 and n = 1. (d) HDPCVD deposition in
trench, showing much better filling. HDPCVD has a strong directed ion
component and any overhangs that form aresputtered away.
Actual SEM images of HDPoxide deposition.
Simulations - CVD
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2000 by Prentice HallUpper Saddle River NJ
S f K Id
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Summary of Key Ideas
Thin film deposition is a key technology in modern IC fabrication. Topography coverage issues and filling issues are very important, especially as
geometries continue to decrease. CVD and PVD are the two principal deposition techniques. CVD systems generally operate at elevated temperatures and depend on chemical
reactions. In general either mass transport of reactants to the surface or surface reactions can
limit the deposition rate in CVD systems. In low pressure CVD systems, mass transport is usually not rate limiting.
However even in low pressure systems, shadowing by surface topography can beimportant.
In PVD systems arrival angle distribution is very important in determining surfacecoverage. Shadowing can be very important.
A wide variety of systems are used in manufacturing for depositing specificthin films.
Advanced simulation tools are becoming available, which are very useful in predicting topographic issues.
Generally these simulators are based on physical models of mass transport andsurface reactions and utilize parameters like arrival angle and sticking coefficientsfrom direct and indirect fluxes to model local deposition rates.
SILICON VLSI TECHNOLOGYF d l P i d M d li 2000 b P i H ll