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Lecture 9.0: Silicon Oxidation/Diffusion/Implantation

This lecture covers topics such as silicon oxidation, diffusion, and implantation. It discusses the kinetics of silicon oxidation, heat transfer, and the different controls that can affect the reaction rate. The lecture also covers the thickness of oxide layers, slow solid-state diffusion, and diffusion of dopants. Additionally, it explores implantation techniques, including stopping power and implantation depth, as well as diffusion of implanted dopants through heat treatment or laser annealing.

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Lecture 9.0: Silicon Oxidation/Diffusion/Implantation

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  1. Lecture 9.0 Silicon Oxidation/Diffusion/Implantation

  2. Silicon Oxidation • Reactor • Furnace at T=850C • Pure Oxygen • Si + O2 SiO2 • Kinetics • BL-Mass Transfer • J=Kg(CA-0) • SS-Diffusion • J=DO-SiO2 (dC/dx) • Heat Transfer • BL, q=h(T1-T) • Solid, q=kSiO2(dT/dx) • J=q/Hrxn Grxn<0, Spontaneous

  3. Kinetics • Thickness • Linear Rate • Reaction Control • First Order • BL-MT Control • BL-HT Control • Parabolic Rate • Product diffusion Control • Product heat transfer Control • J =(dx/dt) SiO2/MW SiO2

  4. Thickness Experiments • Parabolic Rate • Derive it! • dx2/dt=2K • K=Ko exp(-Ea/RgT) • x=o @ t=0 • x=  at t=  • Very common!! • Slow Solid State Diffusion • Slow Heat Conduction

  5. Field Oxide • Thick oxide • Oxygen • Steam • High Temperature Reaction

  6. Diffusion • Deposition of B or P on surface • Heat and Hold for period of time • Solid State Diffusion • dC/dt=D d2C/dx2 • C=Co at x=0 • C=0 at x= • C=Co(1-erf[x/(4Dt)]) • Etch excess B or P from surface

  7. Concentration Profile time

  8. Diffusion Coefficient • Self Diffusion • D*=Doexp(-Ea/RgT) • Diffusion of A in B • Depends on A and matrix B • DAB =(D*A XB + D*B XA) (d ln [aA]/d ln [XA]) • d ln [aA]/d ln [XA] = 1+ (d ln [A]/d ln [XA]) • d ln [aA]/d ln [XA] ~ 1 for ideal solutions • And • DAB =(D*A XB + D*B XA) = (D*A (1-XA) + D*B XA) • Note Concentration dependence!! • DAB ~D*A when XA ~0 , the dilute solution limit • Good for dopants

  9. Implantation • Energy Loss • Stopping of Ion • Nuclear cross section, Sn(E) • Electronic cross section, Se(E) • ρT = atomic density of target (#/cc)

  10. Average Range • Integration of Energy Loss equation

  11. Implantation • Create Ions in Vacuum • Accelerate in Electric Field

  12. Implantation • Impinge onto Silicon Surface • Knock out Si ion(s) • Charge Balance • Travel deep into Silicon

  13. Implantation • Effect of Ion Mass Mi>MSi Mi<MSi

  14. Implant Depth Depth Increases with Energy

  15. Implantation Straggle Increases with Energy

  16. Implantation Concentration Profile • Probability Based • N(x)=Nmax exp[(x-xave)2/2x2] • Nmax=(Ndose/[(2) x])~(0.4 Ndose/ x) • Ndose=Qdose/e • Qdose= current applied/cm2 • σx = standard deviation of projected range

  17. Implantation Through Slit • Slit opening = a • N(x) =projected range formula • ΔR = transverse straggle

  18. Mask Thickness • To effectively prevent ions penetrating in thick zone • Relatively thick Oxide Protection layer • Patterned • Thinning (etching) of Oxide Protection layer over implantation zone • Remove oxide layer with impurities inside

  19. Mask Thickness • Transmission through mask • T=1/2 erfc[(x-xave)/2 x] • To stop 99.99% of implanted materials, T=10-4 • Solve for x, the thickness to stop 99.99% of ions.

  20. SiO2 Mask Thickness

  21. Si3N4 Mask Thickness

  22. Photoresist Mask Thickness

  23. Implant Depth Depth Increases with Energy

  24. Diffusion of Implanted Dopants • Diffusion Furnace or Laser • Heat Treatment • Solid State Diffusion • dC/dt = CT d/dz(DAB dXA /dz) • C=Co(z) = CT XA(z) at z=0 • C=0 at z= • DAB =(D*A XB + D*B XA) (d ln [aA]/d ln [XA]) • Interdiffusion or mutual diffusion coefficient

  25. Laser Annealing

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