Section 7: Diffusion Jaeger Chapter 4 EE143 – Ali Javey

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

Fick’s First Law of Diffusion EE143 – Ali Javey

Fick’s Second Law of Diffusion EE143 – Vivek Subramanian

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

Diffusion Coefficients EE143 – Ali Javey

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

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

Constant Source Diffusion Complementary Error Function Profiles EE143 – Vivek Subramanian

Limited Source Diffusion Gaussian Profiles EE143 – Ali Javey

Two Step Dopant Diffusion (1) Predeposition dopant gas dose control SiO2 SiO2 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

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

Diffusion of Gaussian Implantation Profile EE143 – Ali Javey

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

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

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

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

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

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