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2. Chemical Vapor Deposition (CVD)

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Presentation on theme: "2. Chemical Vapor Deposition (CVD)"— Presentation transcript:

1 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

2 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

3 MOSFET Dielectric SiN Al(Cu, Si) Poly-Si SiO2 Gate oxide N+ p
Source Drain Barrier metal

4 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

5 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”

6 For optoelectronic devices by hydride VPE
Binary : GaAs, InP, GaP, InAs Ternary : (Ga, In)As, Ga(As, P) Quaternary : (Ga, In, As, P)

7 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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9 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

10 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.

11 (Example) At 1000K, G = Kcal/mole  logK=13

12 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

13 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

14 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

15 Evaluate Ki from G vs. T
Ellingham-diagram

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17 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

18 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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