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Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition (CVD). References : D.M. Dobkin and M.K. Zuraw, Principles of Chemical Vapor Deposition (Kluwer Academic Publishers, 2003) M. L. Hitchman and K. F. Jensen, Chemical Vapor Deposition (Academic Press, 1993)

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Chemical Vapor Deposition (CVD)

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  1. Chemical Vapor Deposition (CVD) References : D.M. Dobkin and M.K. Zuraw, Principles of Chemical Vapor Deposition (Kluwer Academic Publishers, 2003) M. L. Hitchman and K. F. Jensen, Chemical Vapor Deposition (Academic Press, 1993) M. Ohring, The Materials Science of Thin Films (Academic Press, 1992) M.A. Herman, W. Richter and H. Sitter, Epitaxy: Physical Principles and Technical Implementation (Springer, 2004)

  2. Chemical Vapor Deposition (CVD) Thin Film Deposition PVD CVD • CVD : Film species are supplied in the form of a precursor gas

  3. Chemical Vapor Deposition (CVD) Horizontal: Barrel: Pancake: From Ohring, Fig. 4-13, p. 178

  4. CVD Chemistry • Heterogeneous and homogeneous reactions From Herman, Fig. 8.3, p. 173 From Sze, Fig. 19, p. 323

  5. Chemical Vapor Deposition (CVD) • All CVD systems consist of three steps: • gas transport into the chamber and to the substrate • 2) chemical reactions forming the film • aA(g) + bB(g) → cC(s) + dD(g) • 3) removal of reaction byproducts from the chamber

  6. Why CVD ? • Advantages: • No crucible interactions • CVD is more conformal compared to PVD methods which are line-of-sight • No alloy fractionation as with thermal methods

  7. CVD Applications • CVD used to produce poly-Si, SiO2, and SiN in MOSFETs From Ohring, Fig. 4-1, p. 148

  8. CVD Systems CVD LP-CVD AP-CVD PE-CVD VPE

  9. AP-CVD • Viscous flow produces boundary layer at surfaces due to friction From Jaeger, Fig. 6.9, p. 121

  10. AP-CVD Systems • Viscous flow makes it difficult to achieve uniform film growth on a large number of stacked wafers in a reactor • Reactions are “mass transfer limited” requiring flat lying wafers From Ohring, Fig. 4-13, p. 178

  11. LP-CVD Systems • ~ 10 mT - 1 T • Uses low pressures to enhance diffusion and mean free path of gas molecules toward the substrates

  12. LP-CVD Systems • Produces faster growth rates, more uniform deposition, and more conformal deposition • Wafers can be stacked closer together to achieve higher throughput  From Ohring, Fig. 4-14, p. 180

  13. CVD Chemistry • CVD requires that a volatile compound be found for the precursors From Dobkin, Table 5-6, p. 133

  14. Poly-Si CVD • LPCVD at 600 - 650 °C : • SiH4(g) → Si(s) + 2H2(g)

  15. SiO2 CVD • 300 - 500 °C : • SiH4(g) + O2(g) → SiO2(s) + 2H2(g) • 900°C : • SiCl2H2(g) + 2N2O(g) → SiO2(s) + 2N2(g) + 2HCl(g) • 700 °C : • Si(C2H5O)4 + 12O2 → SiO2 + 8CO2 + 10H2O

  16. SiN CVD • APCVD at 700 – 900°C : • 3SiH4(g) + 4NH3(g) → Si3N4(s) + 12H2(g) • LPCVD at 700-800 °C : • 3SiCl2H2(g) + 4NH3(g) → Si3N4(s) + 6HCl(g) + 6H2(g)

  17. W CVD • 250-500 °C: • WF6(g) + 3H2(g) → W(s) + 6HF(g) • W on Si at < 200 °C: • 6WF6(g) + 3Si(s) → 2W(s) + 3SiF4(g) • LPCVD at 800°C : • 2MCl5(g) + 5H2(g) → 2M(s) + 10HCl(g) • where M = Mo, Ta, or Ti

  18. Vapor Phase Epitaxy (VPE) • Epitaxy: a single crystal substrate acts as a template for a film of identical or related crystal structure

  19. Si VPE • Sources include: • silicon tetrachloride (SiCl4) • dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3) • silane (SiH4) • 1200 °C: SiCl4(g) + 2H2 → Si(s) + 4HCl(g) • 650 °C: SiH4(g) → Si(s) + 2H2(g) • Doping: • p-type: biborane (B2H6) • n-type: arsine (AsH3) or • phosphine (PH3)

  20. GaAs VPE From Grovenor, Fig. 3.27, p. 167

  21. GaAs VPE • Source zone: • GaCl3(g) may be produced by passing HCl over Ga : • 6HCl(g) + 2Ga(s) → 2GaCl3(g) + 3H2(g) • Decomposition zone: • As4(g) produced by decomposition of AsH3(g) : • 4AsH3 → As4(g) + 6H2(g) • Deposition zone : • As4(g) + 4GaCl3(g) + 6H2(g) → 4GaAs(s) + 12HCl(g)

  22. MOVPE = OMVPE = MOCVD • VPE using metalorganic species • It is most commonly used for deposition of III-V compounds From Ohring, Fig. 4-18, p. 187

  23. MOCVD From Herman, Fig. 8.14 & 8.15, p. 180

  24. Metalorganics From Herman, Fig. 8.21, p. 187

  25. MOCVD From Hitchman, Appendix 6.3, p. 383

  26. MOCVD From Ohring, Table 4-5, p. 187

  27. MOCVD From Hitchman, Appendix 6.2, p. 382

  28. MOCVD AsH3(g) + Ga(CH3)3(g) → GaAs(s) + 3CH4(g) TMG From Herman, p. 192

  29. MOCVD From Herman, Fig. 8.23, p. 190

  30. MOCVD From Herman, Fig. 8.17, p. 182

  31. MOCVD From Ohring, Table 4-6, p. 189

  32. CVD Films and Coatings From Ohring, Table 4-1, p. 154

  33. CVD Kinetics • Viscous flow produces boundary layer at surfaces due to friction From Ohring, Fig. 4-7, p. 163

  34. CVD Kinetics From Ohring, Fig. 4-8(a), p. 168 J(x,y) = C(x,y)v - DC(x,y) Boundary conditions: 1. Chemical reaction is complete at the surface  C = 0 when y = 0 2. No net diffusion at the top of the reactor (gas molecules are reflected) dC/dy = 0 when y = b 3. Input source gas concentration is Ci  C = Ci at x = 0

  35. CVD Kinetics From Ohring, Fig. 4-8(a), p. 168 C(x,y) = (4Ci/p)sin(py/2b) exp (-p2Dx/4vb2) flux of gas toward surface (cm-2s-1) = J(x) = -D C(x,y)/y at y = 0 J(x) decreases along the direction x growth rate, R = J(x) / film atom density

  36. CVD Kinetics • Growth rate declines along x-direction • Correct by increasing growth temperature along x-direction or tilting the substrates towards the gas flow From Ohring, Fig. 4-8(b), p. 168

  37. AP-CVD From Jaeger, Fig. 6.9, p. 121

  38. CVD Kinetics • Flux of gas molecules at the substrate surface:  • Js = ksNs • ks = surface reaction rate constant (first order kinetics) • Ns = concentration of reactants above the surface • Flux of gas molecules diffusing from gas stream: • Jg = hg (Ng – Ns) • hg = mass transfer coefficient • Ng = gas concentration in the vapor

  39. CVD Kinetics • At steady-state, • Js = Jg • Growth rate, • R = Js/N = [ kshg / (ks + hg) ] (Ng/N) • N = film atomic density

  40. CVD Kinetics • ks = surface reaction rate constant • hg = mass transfer coefficient • ks >> hg • mass-transfer-limited growth • r = hgNg/N • r is temperature-insensitive • hg >> ks • surface-reaction-limited growth • r = ksNs/N is temperature sensitive 

  41. CVD Kinetics • Desired growth regime is in T-insensitive part of curve Mass-transfer-limited Surface-reaction-limited From Jaeger, Fig. 6.10, p. 122

  42. PE-CVD • In conventional CVD chemical reactions are controlled by thermal energy provided by heating the substrate • The thermal energy provides the energy necessary to break bonds • In PE-CVD, a plasma is used to decompose the gas molecules for film deposition From Dobkin, Table 6-3, p 172

  43. PE-CVD PECVD (N plasma) : 2SiH4 + N2(g) → 2SiNH(s) + 3H2(g) PECVD (Ar plasma) : SiH4(g) + NH3(g) → SiNH(s) + 3H2(g)

  44. PE-CVD

  45. PE-CVD

  46. PE-CVD PE-CVD Capacitively Coupled Inductively Coupled Electron Cyclotron Resonance (ECR)

  47. PE-CVD from Hitchman, Fig. 7.2, p. 392

  48. ECR Plasma • Electrons in a B-field move in a circular path with the Larmor frequency: • w = eB/m • An em field at the Larmor frequency will be in phase with the electron motion and add energy to the electron on each orbit From Dobkin, Fig. 6-11, p 161

  49. ECR Plasma • This effect is known as electron cyclotron resonance (ECR) • Normally w = 2.45 GHz and B = 875 Gauss From Ohring, Fig. 4-16, p. 184

  50. ECR Plasma • Electrons are trapped by the field lines • Increased ionization (10-2 – 10-1) • Lower pressures (~10-4 – 10-2 Torr) • Greater plasma densities (~1012 cm-3) • Lower substrate temperatures (< 300 °C)

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