Chemical Vapor Deposition CVD References D M Dobkin

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)

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

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

CVD Chemistry • Heterogeneous and homogeneous reactions From Herman, Fig. 8. 3, p. 173 From Sze, Fig. 19, p. 323

Chemical Vapor Deposition (CVD) • All CVD systems consist of three steps: 1) gas transport into the chamber and to the substrate 2) chemical reactions forming the film a. A(g) + b. B(g) → c. C(s) + d. D(g) 3) removal of reaction byproducts from the chamber

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

CVD Applications • CVD used to produce poly-Si, Si. O 2, and Si. N in MOSFETs From Ohring, Fig. 4 -1, p. 148

CVD Systems CVD AP-CVD VPE LP-CVD PE-CVD

AP-CVD • Viscous flow produces boundary layer at surfaces due to friction From Jaeger, Fig. 6. 9, p. 121

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

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

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

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

Poly-Si CVD • LPCVD at 600 - 650 °C : Si. H 4(g) → Si(s) + 2 H 2(g)

Si. O 2 CVD • 300 - 500 °C : Si. H 4(g) + O 2(g) → Si. O 2(s) + 2 H 2(g) • 900°C : Si. Cl 2 H 2(g) + 2 N 2 O(g) → Si. O 2(s) + 2 N 2(g) + 2 HCl(g) • 700 °C : Si(C 2 H 5 O)4 + 12 O 2 → Si. O 2 + 8 CO 2 + 10 H 2 O

Si. N CVD • APCVD at 700 – 900°C : 3 Si. H 4(g) + 4 NH 3(g) → Si 3 N 4(s) + 12 H 2(g) • LPCVD at 700 -800 °C : 3 Si. Cl 2 H 2(g) + 4 NH 3(g) → Si 3 N 4(s) + 6 HCl(g) + 6 H 2(g)

W CVD • 250 -500 °C: WF 6(g) + 3 H 2(g) → W(s) + 6 HF(g) • W on Si at < 200 °C: 6 WF 6(g) + 3 Si(s) → 2 W(s) + 3 Si. F 4(g) • LPCVD at 800°C : 2 MCl 5(g) + 5 H 2(g) → 2 M(s) + 10 HCl(g) where M = Mo, Ta, or Ti

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

Si VPE • Sources include: silicon tetrachloride (Si. Cl 4) dichlorosilane (Si. H 2 Cl 2), trichlorosilane (Si. HCl 3) silane (Si. H 4) • 1200 °C: Si. Cl 4(g) + 2 H 2 → Si(s) + 4 HCl(g) • 650 °C: Si. H 4(g) → Si(s) + 2 H 2(g) • Doping: p-type: biborane (B 2 H 6) n-type: arsine (As. H 3) or phosphine (PH 3)

Ga. As VPE From Grovenor, Fig. 3. 27, p. 167

Ga. As VPE • Source zone : Ga. Cl 3(g) may be produced by passing HCl over Ga : 6 HCl(g) + 2 Ga(s) → 2 Ga. Cl 3(g) + 3 H 2(g) • Decomposition zone : As 4(g) produced by decomposition of As. H 3(g) : 4 As. H 3 → As 4(g) + 6 H 2(g) • Deposition zone : As 4(g) + 4 Ga. Cl 3(g) + 6 H 2(g) → 4 Ga. As(s) + 12 HCl(g)

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

MOCVD From Herman, Fig. 8. 14 & 8. 15, p. 180

Metalorganics From Herman, Fig. 8. 21, p. 187

MOCVD From Hitchman, Appendix 6. 3, p. 383

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

MOCVD From Hitchman, Appendix 6. 2, p. 382

MOCVD As. H 3(g) + Ga(CH 3)3(g) → Ga. As(s) + 3 CH 4(g) TMG From Herman, p. 192

MOCVD From Herman, Fig. 8. 23, p. 190

MOCVD From Herman, Fig. 8. 17, p. 182

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

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

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

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) d. C/dy = 0 when y = b 3. Input source gas concentration is Ci C = Ci at x = 0

CVD Kinetics From Ohring, Fig. 4 -8(a), p. 168 C(x, y) = (4 Ci/p)sin(py/2 b) exp (-p 2 Dx/4 vb 2) flux of gas toward surface (cm-2 s-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

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

AP-CVD From Jaeger, Fig. 6. 9, p. 121

CVD Kinetics • Flux of gas molecules at the substrate surface: Js = ks. Ns 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

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

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

CVD Kinetics • Desired growth regime is in Tinsensitive part of curve Mass-transfer-limited Surface-reaction-limited From Jaeger, Fig. 6. 10, p. 122

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

PE-CVD PECVD (N plasma) : 2 Si. H 4 + N 2(g) → 2 Si. NH(s) + 3 H 2(g) PECVD (Ar plasma) : Si. H 4(g) + NH 3(g) → Si. NH(s) + 3 H 2(g)

PE-CVD Conventional CVD Thermal Energy High Substrate Temperature Small Substrates (for Uniform Heating) PE-CVD Plasma Energy (electron-atom collisions) Low Substrate Temperature (Ts < 300 °C) Large Areas

PE-CVD Evaporation PE-CVD Alloys Fractionate No Fractionation Crucible No Crucible Interactions Line-of-Sight Not Line-of-Sight Sputtering Limited Composition Control Line-of-Sight PE-CVD Excellent Composition Control Not Line-of-Sight

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

PE-CVD from Hitchman, Fig. 7. 2, p. 392

ECR Plasma • Electrons in a B-field move in a circular path with the Larmor frequency: w = e. B/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

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

ECR Plasma • Electrons are trapped by the field lines • Increased ionization (10 -2 – 10 -1) • Lower pressures (~10 -4 – 102 Torr) • Greater plasma densities (~1012 cm-3) • Lower substrate temperatures (< 300 °C)

Afterglow/Remote Plasma afterglow metastable neutrals plasma decaying plasma substrate additional gas input

Inductive-Coupled Plasma (ICP) • Another method of exciting a plasma is to use inductive methods rather capacitive methods • A time varying magnetic field from a solenoid will create a time varying electric field • This em field can be used to excite electrons and sustain the plasma From Dobkin, Fig. 6 -13, p. 163