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

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

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]