ch5 oxidation diffusion -...
TRANSCRIPT
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Ch5 Oxidation and Diffusion()
Introduction to Semiconductor Processing
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Overall View
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Ch5 -1 Oxidation
Si Si
SiO2
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Horizontal Furnace
0.5 C1000 C 0.05%!
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Vertical Furnace
Dummy wafers 1 Test wafer Production wafers 1 Test wafer Production wafers 1 Test wafer Dummy wafers
Why vertical furnace: Smaller footprint Better contamination control Easier wafer handling
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Furnace Material Considerations
High purity quartzPros
stable at high TCleanliness
ConsFragileContain metallic ionsNot a sodium barrierSmall flakes at T>1200 C (glassy substrates crystalline solids)
SiCPros
Better thermal stabilityBetter barrier
ConsHeavyExpensive
Only for critical parts, but not the chamber body.
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Oxygen Sources
(Si+ O2 SiO2) (Si+ 2H2O SiO2 + 2H2
Flash systems, H2 + O2 H2O
HCl (TCE, ), (TCA)
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Si
():
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9Si
SiO2 SiO2
(field oxide;)
():
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Special Topic: Isolation Techniques
Goal:Wafer
Device 1 Device 1ElectricalInsulation
Field Oxide(Early 80s) Si Si
SiO2Wet
oxidation Etching
SiSiO2 SiO2
Thickness:
Problems:1.2.
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Isolation Techniques (LOCOS)
LOCOS (Local Oxidation of Silicon), ~ mid 90s
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Isolation Techniques (STI)
STI (Shallow Trench Isolation), mid 90s~
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RCA ()
Kern Puotinen1960 RCAICSC-1-- NH4OH:H2O2:H2O1:1:51:2:7 7080 C SC-2-- HCl:H2O2:H2O1:1:61:2:87080 C
HFHF
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Cleaning
Cleaning before oxidation, removeParticulates, organic and inorganic residues, and native oxide
Sequence of not well-cleaning:____________, ____________
DI water: ultra pure water (UPW), ultra high purity, high resistivity > 18M-cm Total organic content < 5g/LOxygen content
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Silicon Oxidation Model
CG
CS
CO
CiXOX
Oxide Silicon
CG: concentration of O2 in the gas phase, =PG/RT
CS: Concentration of O2 at the gas-solid interface
C0: O2 solubility in SiO2(S); C0=H*CS; H: Henrys Constant
Ci: O2 concentration at SiO2-Si interface
r1=hG(CG-CS),
if define CA=H*CGr1=(hG/H)(CA-C0)=h(CA-C0), where h=hG/H
r2=D*(C0-CX)/XOXr3=ks*CiAt steady state, r1=r2=r3
DXkhkCC
OXSS
Ai //1 ++=
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Silicon Oxide Model Oxide Growth Rate
DXkhkCkCk
dtdXN
OXSS
Asis
OXi //1 ++
==
Define N1= number of oxygen atoms per unit volume of SiO2 (4.6*1022 cm-3)
At t=0, XOX=Xi (initial oxide thickness)
++=
+=
=
+=
+=+
14/
12
/)(
/2
)11(2
)(
2/1
2
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2
BAtAX
BAXX
NDCBhk
DAwhere
tBAXX
OX
ii
A
s
OXOX
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Oxidation Model Limiting cases
++= 1
4/1
2
2/1
2 BAtAXOX
Case 1: For short time, t+
Linear growth regionLinear growth rate constant:_______
Parabolic growth regionParabolic growth rate constant:_______
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B/A: Linear Rate Constant
Source: Microelectronics: processing and device design, R. A. Colclaser, J. W. & S. 1980
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B: Parabolic Rate Constant
Source: Microelectronics: processing and device design, R. A. Colclaser, J. W. & S. 1980
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2 4 6 8 10 12 14 16 18 20
0.2
0
0.4
0.6
0.8
1.0
1.2
()
(
)
1200 C
1150 C
1100 C
1050 C
1000 C
950 C
900 C
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Example: Silicon Oxidation Rates
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2 4 6 8 10 12 14 16 18 20
0.5
0
1.0
1.5
2.0
2.5
3.0
()
(
)
1150 C
1100 C
1050 C
1000 C
950 C
900 C
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Oxidation vs. Diffusivity of O2 and H2O
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Si-SiO2
SiO2
Si
Si-SiO2
SiO2
Si
P N
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Conventional oxidation process is a multi-wafer batch processhard to precisely control the temperature at every points
Rapid Thermal Oxidation, RTOProcess features and applications:_____________________________________________________________________Integrate with HF native oxide removalO2 + HCl
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n1, k1, t1
n2, k2
sp
Film thickness measurements:
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http://www.ee.byu.edu/cleanroom/color_chart.parts/sio2_chart.jpg
SiO2 Titanium
But, DONOT always trust your eyes!!
Empirical Thickness Estimation
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C-V
Test for:
1:
2:
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Ch5 -2 Diffusion
N-Silicon
Masking Oxide
N-Siliconp+ p+
Masking Oxide
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(Pre-deposition): B2H6 + 2 O2 B2O3 + 3 H2O2 B2O3 + 3 Si 3 SiO2 + 4 B
Thermal oxidation: remove un-reacted gas, oxide formation that covers the dopant. (Drive-in)
N-Siliconp+ p+
Masking Oxide
Major concern:
________________________________
Materials disperses from high concn. to low concn. SiO2 as the diffusion mask Was widely used for doping
Replaced by ion implantation for better control from mid 70s Still being used in drive-in for ____________.
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Si
SiO2
Si
SiO2Drive In
Si Substrate
SiO2Cap oxidation
Pre-deposition
Major Steps
Determines:
______________
Determines:
______________
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, xj
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MFC
MFC
MFC
O2
POC
l 3
MFC
Regulator ():
MFC():
Control valves():
:
:
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thermal budget)
D = D0 exp (EA/kT)
S/D
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1 m
Thermal Budget
1
10
100
1000
104/T (K)7 8 9 10
10001100 900 800
2 m
0.5 m
0.25 m
Ther
mal
Bud
get (
sec)
T (C)
Source: Chang and Sze, ULSI Technology
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Diffusion Model
Diffusion1-D diffusion problem without bulk flow
*Same governing equation for pre-deposition and drive-in steps
2
2
xND
tN
=
1. Pre-deposition: Constant (Saturated) surface concentration
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Diffusion Model - Conti.
2. Drive In: Constant (Saturated) surface concentration
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Diffusion: Spatial Profile
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REF 1
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4141
REF 2
Source: VLSI
4242
REF 3
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REF 4
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REF 5
1000
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Special Topic: USJ Formation
Diffusion for Boron USJ formationSmall devices needs ultra shallow junctionBoron is small and light, implanter energy could be too high for it goes too deepControlled thermal diffusion is used in R&D for shallow junction formation
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Surface Clean
Si SubstrateSTI STI
Silicide
Sidewall Spacer Sidewall Spacer
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BSG CVD
Si SubstrateSTI STI
Boro-Silicate Glass
Silicide
Sidewall Spacer Sidewall Spacer
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RTP Dopant Drive-in
Si SubstrateSTI
Boro-Silicate Glass
STI
SilicidePolysilicon
Gate Oxide
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Strip BSG
Si SubstrateSTISTI
SilicidePolysiliconGate Oxide
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