1. Silicon Self-Interstitial and Dopant Diffusion
SFR Workshop
November 8, 2000
Cynthia Nelson, Hartmut Bracht, Eugene Haller
Berkeley, CA
2001 GOAL: to conduct and analyze experiments on boron diffusion in silicon isotope hetero-structures. Develop a predictive boron diffusion model for various experimental conditions by 9/30/2001.
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2. Motivation
Down-scaling of device size is, in part, limited by diffusion of dopants. Under certain circumstances diffusion is faster than normal.
The presence of excess Si interstitials is believed to promote an increase in diffusivity.
Need to develop a new, generalized diffusion model that incorporates more parameters.
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3. Silicon Interstitials
When present in greater-than-equilibrium concentrations, Si interstitials cause accelerated diffusion of dopants.
To study equilibrium diffusion, it is necessary to avoid any silicon interstitial wind, caused by implantation damage or oxidation.
Si interstitials have never been directly detected.
The Si interstitial concentration can be determined from dopant diffusion coefficients, or from Si self-diffusion.
Si isotopes can be used to directly measure Si diffusion: highly enriched 28Si layers (99.95% 28Si), alternating with Si, reveal diffusion of 29Si and 30Si into enriched region.
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4. Simulated Diffusion of B and 30Si
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5. First Experimental Results: SIMS concentration profiles of boron (B) and 30Si
In-diffusion from boron-implanted poly-Si layer. Annealed for 15 hours at 950° C.
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6. Comparison of Simulated and Experimental Data
Simulated boron profile agrees well with experimental profile.
30Si profiles:
profile rounds out as expected for diffusion anneal.
deeper 28Si layers diffuse faster than outer layers -- opposite of predicted diffusion.
It is possible that knock-on contributes to rounding of SIMS profile.
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7. Previous Models for Boron Diffusion
Model based on the kick-out reactions (h = hole):
Bi0  Bs- + I+
Bi0  Bs- + I0 + h
Model does not incorporate interstitial transport coefficient (CIDI).
M. Uematsu, J. Appl. Phys. 82 (1997) 2230
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8. N-Type Dopant Diffusion
Diffusion studies of n-type dopants will follow boron diffusion studies.
For intrinsic concentrations, it is known that P diffuses via the substitutional-interstitial (S-I) mechanism, whereas As diffuses via both vacancy and S-I mechanisms.
At high concentrations, As diffusion is nearly in thermal equilibrium, while P diffusion leads to a nonequilibrium, “kink and tail” concentration profile.
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9. Future Work Short Term Goals:
Find mechanism causing increase in 28Si diffusion in deeper layers.
Continue studies of dopant diffusion in multilayer structures.
Goals:
Model diffusion of dopants in isotopic multilayer structures. Study interference between the diffusion and native point defects, by 9/30/2002.
Comprehensive model on dopant- and self-diffusion in silicon. Determine the properties of dopants and native point defects in silicon, by 9/30/2003.
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