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Diffusion in Extrinsic Silicon
SFR Workshop & ReviewApril 17, 2002
Hughes Silvestri, Ian Sharp, Hartmut Bracht, and Eugene Haller
Berkeley, CA
2002 GOAL: Diffusion measurements on P doped Si to complete comprehensive model of Si diffusion by 9/30/2002.
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Motivation• Reduction of junction depth
in CMOS technology
• To maintain shallow profile:– Must control diffusion during further annealing steps– Understand the Fermi level effect on diffusion and its impact on native
defect formation
• Development of a comprehensive, predictive diffusion model that incorporates physically justified parameters
(P.A. Packan, MRS Bulletin, June 2000)
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Our Approach• Use of an isotopically enriched Si heterostructure allows
observation of Si and dopant diffusion simultaneously.
• Determination of dopant diffusion mechanism and charge state of point defects involved.
• Determine contribution of charged native defects to Si self-diffusion.
DSi = f Io CIo DIo + fI + CI+ DI+ + fV− CV − DV − +Kƒ = correlation factor
ƒv = 0.5, ƒI = 0.73
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Extrinsic p-type Extrinsic n-type
Fermi Level EffectHeavily doped semiconductors - extrinsic at diffusion temperatures
– Fermi level moves from mid-gap to near conduction (n-type) or valence (p-type) band.
– Fermi level shift lowers the formation enthalpy, HF, of the charged native defect
– Increase of CI,V affects Si self-diffusion and dopant diffusion
( )−− −−=
−
=
VFF
VFV
B
FIV
B
FIVo
Sieq
IV EEHHTk
HkS
CC o,expexp ,,,
Dopant charge state Native defect charge state extrinsic n-type As
+ I -, V - extrinsic p-type As
- I +, V +
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Approach: Isotopically enriched Si heterostructure• Alternating layers of natural Si (92.2% 28Si, 4.7% 29Si, 3.1% 30Si) and 28Si (99.95% 28Si)
(UHV-CVD grown at Lawrence Semiconductor Research Laboratory, Tempe, AZ)
• Ion implanted amorphous natural Si cap layer used as dopant source to suppress transient enhanced diffusion (TED) (MBE grown at University of Aarhus, Denmark)
• (
• Stable isotope structure allows for simultaneous Si and dopant diffusion studies
120 nm 28Si enriched
FZ Si substrate
120 nmnatural Si
a-Si cap 260 nm
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Extrinsic p-type: B diffusion experiments
• Secondary Ion Mass Spectrometry (SIMS) used to determine concentration profiles of 11B, 28Si, and 30Si (Accurel Systems, CA)
• Computer modeling of 11B and 30Si concentration profiles and comparison to experimental results
Boron dopant source:
Boron ion implantation in amorphous Sicap layer:
30 keV, 0.7x1016 cm-2
37 keV, 1.0x1016 cm-2
Diffusion: samples sealed in silica ampoules and annealed between 850 oCand 1100 oC
11B30Si
30Si depleted layers
Natural Si layers
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Extrinsic p-type: B diffusion results
Si diffusion under intrinsic
conditions
30Si SIMSdata
30Si modelfit
11B SIMSdata
11B modelfit• B and Si diffusion are described
simultaneously• Diffusion mechanism (kick-out)
– Bi0ó Bs
- + I 0 + h
– Bi0ó Bs
- + I +
• Interstitialcy: (BI)0 ó Bs- + I 0,+
not very likely (f[BI] < 0.3 )
1000 oC for 4hr 55min
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Extrinsic p-type: B simulation results• Supersaturation of I0 and I+ due to
B diffusion
• I0 and I+ mediate Si and B diffusion
• Enhancement of DB(p) over DB(ni) under p-type doping
• Enhancement of Dsi(p) over Dsi(ni) due to Fermi level effect
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Extrinsic n-type: As diffusion results• Implant As into amorphous layer: 0.7 × 1016 cm-2 at 130 keV, 1.0 × 1016 cm-2 at 160 keV
• Anneal: 950 °C < T < 1100 °C• Secondary Ion Mass Spectrometry (SIMS)
10 17
10 18
10 19
10 20
10 21
0 200 400 600 800 1000 1200 1400 1600
conc
entr
atio
n (c
m-3
)
Depth (nm)
V2-
V0
V-
Arsenic sample annealed 950 °C for 122 hrs
Reactions for simulation:
AsV( )o ↔ Ass+ + V−
AsI( )o ↔ Ass+ + Io + e−
Vacancy mechanism
Interstitialcy mechanism
V- and Io control Si self-diffusion under n-type extrinsic doping.
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Extrinsic n-type: As simulation results
No supersaturation of Io or V-
during As diffusion
0.1
1
10
0 1000 2000
Cx/C
eq
Depth (nm)
V - Io
10 -19
10 -18
10-17
10 -16
10 -15
10 -14
10 -13
10 -12
0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86
D (
cm2s-1
)
1000/T (1000/K)
DSi
(ni)
DSi
(ext)
DAs
(ni)
DAs
(ext)
Enhancement of As and Si self-diffusion under extrinsic conditions entirely due to Fermi level effect.
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Native defect contributions to Si self-diffusion
10 -19
10 -18
10 -17
10 -16
10 -15
10 -14
0.7 0.8 0.9
D (c
m2s-1
)
1000/T (1000/K)
DSi
(V-)
DSi
(I+)
DSi
(Io)
DSi
(Io+I++V-)
DSi
(ni)
Intrinsic components calculated from extrinsic data
Diffusion coefficients of individual components add up accurately:DSi(ni )tot = fIo CI o DI o + fI+ CI + DI+ + fV − CV− DV− = DSi(ni )
(Bracht, et al., 1998)
(B diffusion) (As diffusion)(As diffusion)
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Current / Future Developments• Isotope heterostructure allows for simultaneous analysis of Si self-diffusion
and dopant diffusion.• Boron diffusion via the kick-out mechanism: Bi
0 ⇔ Bs- + I 0,+
• As diffusion via interstitialcy and vacancy mechanisms:
• Able to determine contributions of various native defect charge states to intrinsic Si self-diffusion.
• Activation enthalpy for self-diffusion due to Io, I+, V-:
• Diffusion measurements on P doped Si to complete comprehensive model of Si diffusion by 9/30/2002.
• Apply technique to other materials systems (SiGe), by 9/30/2003.
2002 and 2003 Goals:
AsV( )o ↔ Ass+ + V−AsI( )o ↔ Ass
+ + Io + e−
DSi(I 0): Q=(4.32 ± 0.15) eV, DSi(I +): Q=(4.74 ± 0.13) eV, DSi(V -): Q=(5.18 ± 0.13) eV