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2. Chemical Vapor Deposition (CVD)
CVD is the process of chemically reacting a volatile compound of a material to be deposited, with other gases, to produce a nonvolatile solid that deposits atomistically on a substrate. For Metals, Semiconductors, Compound Films & Coatings 2.1 Reaction types 2.1.1 Pyrolysis thermal decomposition of gases on hot substrate SiH4(g) Si(s) + 2H2(g) (650℃) Ni(CO)4(g) Ni(s) + 4CO(g) (180℃) Hydrides, Carbonyl, Organometallic compounds
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2. 1. 2 Reduction 2. 1. 3 Oxidation Hydrogen as reducing agent
Halide, Carbonyl halide, Oxyhalide, Oxygen-containing compounds SiCl4(g) + 2H2(g) Si(s) + 4HCl(g) (1200℃) : Si Epitaxy WF6(g) + 3H2(g) W(s) + 6HF(g) (300℃) MoF6(g) + 3H2(g) Mo(s) + 6HF(g) (300℃) WF6(g) + Si (s) W(s) + SiF4(g) (selectively fill contact hole) Oxidation SiH4(g) + O2(g) SiO2(s) + 2H2(g) (450℃) 4PH3(g) + 5O2(g) 2P2O5(s) + 6H2(g) (450℃) 7% of P in SiO2 “planarization” (glass film) SiCl4(g) + 2H2(g) + O2(g) SiO2(g) + 4HCl(g) (1500℃) optical fiber for communications purposes soot particle silica rod by sintering fiber
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MOSFET Dielectric SiN Al(Cu, Si) Poly-Si SiO2 Gate oxide N+ p
Source Drain Barrier metal
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Compound Formation Carbide, nitride, boride, .. films of coatings (hard, wear-resistant) SiCl4(g) + CH4(g) SiC(s) + 4HCl(g) (1400℃) TiCl4(g) + CH4(g) TiC(s) + 4HCl(g) (1000℃) BF3(g) + NH3(g) BN(s) + 3HF(g) (110℃) 3SiCl2H2 + 4NH3(g) Si3N4(s) + 6H2(g) + 6HCl(g) (750℃) Precursor gases should be sufficiently volatile and reactive in the gas phase Disproportionation Disproportionation reactions are possible when metals can form volatile compounds having different valence states depending on the temperature 300℃ 2GeI2(g) Ge(s) + GeI4(g) 600℃ Ge, Al, B, Ga, In, Si, Ti, Zr, Be, Cr halides lower-valent state (stable at high T), metal transport single crystal In systems where provision is made for mass transport between hot and cold ends
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Reversible Transfer In the reaction equilibrium at source and deposition regions maintained at different temperatures within a single reactor GaAs epitaxial films by “chloride process” 750℃ As4(g) + As2(g) + 6GaCl(g) + 3H2(g) GaAs(s) + 6HCl(g) 850℃ “Chloride VPE” (Vapor Phase Epitaxy) In the hydride process, AsH3 and HCl “Hydride VPE”
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For optoelectronic devices by hydride VPE
Binary : GaAs, InP, GaP, InAs Ternary : (Ga, In)As, Ga(As, P) Quaternary : (Ga, In, As, P)
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Gas-phase reactions 2AsH As2 + 3H2 2PH P2 + 3H2 2HCl + 2In InCl + H2 2HCl + 2Ga GaCl + H2 Deposition reactions at InP substrate 2GaCl + As2 + H GaAs + 2HCl 2GACl + P2 + H GaP + 2HCl 2InCl + P2 + H InP + 2HCl 2InCl + As2 + H InAs + 2HCl
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Generally in CVD ① a A(g) + bB(g) cC(s)+ dD(g) ② Reversible ; thermodynamics is applicable to reactions Chemical vapor Deposition : reactant gases enter the reactor from the outside of the system Chemical vapor transport reactions : solid or liquid sources are contained within closed or open reactors need carrier gases to transport source materials But, type of chemical reaction is same same CVD
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2.2 Thermodynamics of CVD Reaction feasibility where are chemical reactions going? chemical thermodynamics at equilibrium : feasibility How fast are they getting there? chemical kinetics ; growth rates, speed of reaction For chemical reaction aA + bB cC G = G + RTlnK, If the system is in equilibrium, G = 0 and -G = RTlnK For many practical cases, G G since the ai differ little from the standard-state activities, which are taken to be unity. G = G + RTlnK G from Ellingham diagram Standard free energy change G 0 for large critical sited nuclei. (1 atm. & T) If G<< 0, polycrystal formation is promoted.
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(Example) At 1000K, G = Kcal/mole logK=13
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2. 2. 2 Conditions of Equilibrium
Evaluate the partial pressures of the involved species within the reactor In situ mass spectroscopic analysis revealed the presence of unexpected species. (eq) SiCl4 (g) + 2H2(g) Si(s) + 4HCl(g) (1400℃) For Si-Cl-H system : 8 species were detected during the reaction of chlorosilanes SiCl4, SiCl3H, SiCl2H2, SiClH3, SiH4, SiCl2, HCl, H2
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SiCl4 (g) + 2H2(g) Si(s) + 4HCl(g) (1400℃)
For Si-Cl-H system : chemical reactions and partial pressures of 8 species SiCl4, SiCl3H, SiCl2H2, SiClH3, SiH4, SiCl2, HCl, H2
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We need two more equations :
① PSiCl4 + PSiCl3H + PSiCl2H2 + PSiClH3 + PSiH4 + PSiCl2 + PHCl + PH2 = Pt(atm) ② molar ratio is fixed (known) : (eq) the mass of Cl in SiCl4 : mCl = 4MCl (mSiCl4/MSiCl4) from ideal gas law : number of moles of Cl : in SiCl4
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Evaluate Ki from G vs. T
Ellingham-diagram
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For 1400℃ is recommended for epitaxial deposition of Si since Si in the gas phase is minimized. [see Si/Cl] For [Cl/H] = 0.1 [Si/Cl] is higher than obtained for epitaxial deposition at the same temperature high Si concentration in the gas phase polycrystalline Si
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Deposition of SiC using CH4 and SiCl4
Gibbs phase rule : f = n f = number of degrees of freedom or variance in the system n = numbr of components or different atomic species = number of phases n=4, =2, and f=4 f=temperature, pressure, [H/Cl], [Si/C]
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