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Section 7: Diffusion

Section 7: Diffusion. Jaeger Chapter 4. Dopant Diffusion Sources. (a) Gas Source: AsH 3 , PH 3 , B 2 H 6. BN. Si. BN. Si. (b) Solid Source. (c) Spin-on-glass. SiO 2 +dopant oxide. (d) Liquid Source. Fick’s First Law of Diffusion. Fick’s Second Law of Diffusion. 10 -6. Cu. Au.

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Section 7: Diffusion

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  1. Section 7: Diffusion Jaeger Chapter 4 EE143 – Ali Javey

  2. Dopant Diffusion Sources (a) Gas Source: AsH3, PH3, B2H6 BN Si BN Si (b) Solid Source (c) Spin-on-glass SiO2+dopant oxide (d) Liquid Source. EE143 – Ali Javey

  3. Fick’s First Law of Diffusion EE143 – Ali Javey

  4. Fick’s Second Law of Diffusion EE143 – Vivek Subramanian

  5. 10-6 Cu Au Diffusion Coefficients of Impurities in Si Substitutional Diffusers Interstitial Diffusers B,P As EE143 – Ali Javey

  6. Diffusion Coefficients EE143 – Ali Javey

  7. Diffusion Mechanisms in Si (a) Interstitial Diffusion Example: Cu, Fe, Li, H Fast Diffusion Cu 10-6 cm2/sec Au EE143 – Ali Javey

  8. Diffusion Mechanisms in Si (b) Substitutional Diffusion (c) Interstitialcy Diffusion Example: Dopants in Si ( e.g. B, P,As,Sb) EE143 – Ali Javey

  9. Constant Source DiffusionComplementary Error Function Profiles EE143 – Vivek Subramanian

  10. Limited Source DiffusionGaussian Profiles EE143 – Ali Javey

  11. Two Step Dopant Diffusion dopant gas (1) Predeposition SiO2 SiO2 dose control Doped Si region Si (2) Drive-in Turn off dopant gas or seal surface with oxide profile control (junction depth; concentration) SiO2 SiO2 SiO2 Si Note: Predeposition by diffusion can also be replaced by a shallow implantation step. EE143 – Ali Javey

  12. Normalized Concentration versus depth Drive-in Predeposition EE143 – Ali Javey

  13. Diffusion of Gaussian Implantation Profile EE143 – Ali Javey

  14. Successive Diffusions: Thermal Budget Example Dttotal of : Well drive-in and S/D annealing Temp (t) Temp (t) well well S/D S/D drive drive - - in in Anneal Anneal step step step step time time For a complete process flow, only those steps with high Dt values are important EE143 – Ali Javey

  15. Solid Solubility Limits • There is a limit to the amount of a given impurity that can be “dissolved” in silicon (the Solid Solubility Limit) • At high concentrations, all of the impurities introduced into silicon will not be electrically active EE143 – Ali Javey

  16. Log C(x) High conc. profile: D gets larger when C(x) is large Log C(x) J large Low conc profile: Erfc or gaussian J small x x * C(x) looks “flatter” at high conc. regions High Concentration Diffusion Effects • E-Field Enhanced Diffusion • Charged point defects enhanced diffusion EE143 – Ali Javey

  17. Electric-field Enhancement Example: Acceptor Diffusion Na(x) p(x) Na(x)=Na-(x) hole gradient Hole diffusion tendency E build-in x At thermal equilibrium, hole current =0 Hole gradient creates build-in electric field to counteract the hole diffusion tendency Complete acceptor ionization at diffusion temperature EE143 – Ali Javey

  18. Electric Field Enhancement holes tends to move away due to hole concentration gradient +[p] B- Ebuild-in B- acceptors experience an additional drift force Enhanced Diffusion for B- acceptor atoms EE143 – Ali Javey

  19. Electric Field Enhancement – Substrate Perturbation As diffusion caused by As conc gradient Uniform B conc in substrate B- EE143 – Ali Javey

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