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Lecture 9.0

Lecture 9.0. Silicon Oxidation/Diffusion/Implantation. Silicon Oxidation. Reactor Furnace at T=850C Pure Oxygen Si + O 2  SiO 2 Kinetics BL-Mass Transfer J=K g (C A -0) SS-Diffusion J=D O-SiO2 (dC/dx) Heat Transfer BL, q=h(T 1 -T) Solid, q=k SiO2 (dT/dx) J=q/  H rxn.

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Lecture 9.0

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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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