Section 5 Thin Film Deposition Part 2 Chemical Methods Jaeger Chapter 6 EE143 Ali Javey Chemical Vapor Deposition CVD source chemical reaction film substrate More conformal deposition vs PVD t Shown here is 100 conformal deposition t step EE143 Ali Javey LPCVD Examples a SiO2 gas solid SiH 4 O2 SiO2 2 H 2 gas 350oC 500oC b PSG phospho silicate glass P2O5 SiO2 4 PH 3 5O2 2 P2O5 6 H 2 SiH 4 O2 SiO2 2 H 2 350oC 500oC c TEOS tetraethylene orthosilicate Si OC2 H 5 4 SiO2 C X H Y OZ EE143 Ali Javey LPCVD Examples d Si 3 N 4 3 SiH 4 NH 3 Si 3 N 4 12 H 2 e Poly Si SiH 4 Si 2 H 2 600 o C f W WF 6 3 H 2 W 6 HF EE143 Ali Javey CVD Mechanisms reactant 5 1 surface diffusion 2 3 substrate 1 Diffusion of reactant to surface 2 Absorption of reactant to surface 3 Chemical reaction 4 Desorption of gas by products 5 Outdiffusion of by product gas EE143 Ali Javey 4 stagnant gas layer CVD Deposition Rate Grove Model film D Si F1 ks koe F3 F1 F3 hG E kT thickness of stagnant layer D CG CS kS CS EE143 Ali Javey F1 F3 Grove model of CVD cont d 1 F3 CG 1 1 hG k s Film growth rate F3 N N atomic density of deposited film dx F3 contant with time dt N Note This result is exactly the same as the Deal Grove model or thermal oxidation with oxide thickness 0 EE143 Ali Javey Deposition Rate versus Temp log scale Rate gas transport limited 3 R T 2 surface reaction limited 0 1 T high T low T EE143 Ali Javey Growth Rate Dependence on Flow Velocity EE143 Ali Javey LPCVD Features 1 More conformal deposition if T is uniform Wafer topography 2 Inter wafer and intra wafer thickness uniformity less sensitive to gas flow patterns i e wafer placement EE143 Ali Javey Comments about LPCVD 1 Sensitivity to gas flow pattern Furnace tube wafers 2 Mass depletion problem in more less out EE143 Ali Javey Plasma Enhanced CVD Ionized chemical species allows a lower process temperature to be used Film properties e g mechanical stress can be tailored by controllable ion bombardment with substrate bias voltage EE143 Ali Javey Atomic Layer Deposition The process involves two self limiting half reactions that are repeated in cycles Unlike CVD in ALD pulses of precursors are introduced in each cycle ALD is highly conformal and enables excellent thickness uniformity and control down to nm scale EE143 Ali Javey
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