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

Heterogeneous and homogeneous reactions 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: 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

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

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

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

Reactions are “mass transfer limited” requiring flat lying wafers 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 mT - 1 T Uses low pressures to enhance diffusion and mean free path of gas molecules toward the substrates

Wafers can be stacked closer together to achieve higher throughput 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 requires that a volatile compound be found for the precursors 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 : SiH4(g) → Si(s) + 2H2(g)

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

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)

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

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 (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)

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

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)

VPE using metalorganic species 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 TMG AsH3(g) + Ga(CH3)3(g) → GaAs(s) + 3CH4(g) 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

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

CVD Kinetics J(x,y) = C(x,y)v - DC(x,y) Boundary conditions: 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

CVD Kinetics C(x,y) = (4Ci/p)sin(py/2b) exp (-p2Dx/4vb2) 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

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

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

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  

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 = hgNg/N r is temperature-insensitive hg >> ks surface-reaction-limited growth r = ksNs/N is temperature sensitive 

Desired growth regime is in T-insensitive part of curve 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

The thermal energy provides the energy necessary to break bonds 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) : 2SiH4 + N2(g) → 2SiNH(s) + 3H2(g) PECVD (Ar plasma) : SiH4(g) + NH3(g) → SiNH(s) + 3H2(g)

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

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

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

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 = 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

This effect is known as electron cyclotron resonance (ECR) 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 – 10-2 Torr) Greater plasma densities (~1012 cm-3) Lower substrate temperatures (< 300 °C)

Afterglow/Remote Plasma 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