1. Fermi Level Dependent Diffusion in Silicon
SFR Workshop
May 24, 2001
Ian D. Sharp, Hartmut Bracht, Eugene E. 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
Downscaling of device size is, in part, limited by diffusion of dopants. (P.A. Packan, Intel Corp., Symposium X3.2, MRS San Francisco, April 19, 2001)
Si interstitials promote self- and dopant diffusion.
The Si self-interstitial concentration is expected to be Fermi level position dependent.
Need to develop a new, generalized diffusion model that incorporates physically justified parameters.
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3. Silicon Interstitials
Below ~850 oC Si interstitials are the native defects controlling self-diffusion and the diffusion of most major dopants (e.g., B, P).
To study equilibrium diffusion, it is necessary to avoid any excess Si interstitials, which may be generated by ion implantation, oxidation, or impurity clustering.
Si interstitials have never been directly detected and their charge states and energy levels in the band gap are not known.
The Si interstitial concentration can be determined from dopant diffusion coefficients and from Si self-diffusion.
Si isotope multi-layer structures can be used to directly measure Si diffusion: highly enriched 28Si layers (99.95% 28Si), alternating with natural Si, reveal diffusion of 29Si and 30Si into the enriched regions.
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4. Experimental Approach
Growth of isotopically enriched Si multi-layer structures.
Deposition of a 260 nm thick amorphous Si cap layer by low temperature MBE.
Boron dopant source:
Boron ion implantation: 30 keV, 0.7x1016 cm keV, 1.0x1016 cm-2
No implantation damage enters the isotopic multi-layer structure.
Diffusion: samples sealed in silica ampoules backfilled with 180 Torr of Ar and thermally annealed in a temperature controlled tube furnace.
Secondary Ion Mass Spectrometry (SIMS) used to determine concentration profiles of B, 28Si, and 30Si.
Computer modeling of B and 30Si concentration profiles.
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5. As-Implanted Si Superlattice Sample
11B
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6. SIMS Profile: 15hr 57min Annealing at 950 oC
B spike is due to oxygen at amorphous Si - crystalline Si interface
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7. Modeling Results for B and Si Concentration Profiles
30Si model fit
30Si SIMS data
Si diffusion under intrinsic conditions
11B model fit
Si diffusion under intrinsic conditions
11B SIMS data
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8. Self Diffusion in Si
Self diffusion is enhanced as Fermi level is brought towards the valence band. T ( o C ) . 7 8 9 1 4 2 1 - 9 7 5 3 . 6
This work
DSi(p++) ) 1 - s 2 m D c ( B D
Bracht et al., Phys. Rev. Lett. 81 (1998) 393 D ( p ) S i
DSi(ni) A n t o n i a d i s e t a l . , J . E l e c t r o c h e m . S o c . 1 2 5 ( 1 9 7 8 ) 8 1 3 . D ( n ) S i i 1 3 / T ( K - 1 )
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9. Summary and Outlook Milestones
First Fermi level dependant self-diffusion experiments in Si have been conducted.
Si self-diffusion is enhanced by moving the Fermi level towards the valence band (p-type).
The kick-out mechanism accurately describes the measured SIMS 30Si profiles assuming:
Neutral boron interstitials:
Singly negatively charged B on substitutional sites:
Singly positively charged interstitial:
The quantification of Si self-interstitial concentration and diffusion will allow for advanced predictive modeling of boron diffusion.
Milestones
Model diffusion of dopants in isotopic multi-layer structures. Study interference between diffusion and native defects by 9/30/2002.
Comprehensive model on dopant- and self-diffusion in Si. Properties of dopants and native point defects in silicon by 9/30/2003.
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