1. 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)

2. Chemical Vapor Deposition (CVD)
Thin Film Deposition
PVD
CVD
CVD : Film species are supplied in the form of a precursor gas

3. Chemical Vapor Deposition (CVD)
Horizontal:
Barrel:
Pancake:
From Ohring, Fig. 4-13, p. 178

4. Heterogeneous and homogeneous reactions
CVD Chemistry
Heterogeneous and homogeneous reactions
From Herman, Fig. 8.3, p. 173
From Sze, Fig. 19, p. 323

5. 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

6. 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

7. 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

8. CVD Systems
CVD
LP-CVD
AP-CVD
PE-CVD
VPE

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

10. 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

11. LP-CVD Systems
~ 10 mT - 1 T
Uses low pressures to enhance diffusion and mean free path of gas molecules toward the substrates

12. 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

13. 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

14. Poly-Si CVD
LPCVD at °C :
SiH4(g) → Si(s) + 2H2(g)

15. SiO2 CVD
°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

16. SiN CVD
APCVD at 700 – 900°C :
3SiH4(g) + 4NH3(g) → Si3N4(s) + 12H2(g)
LPCVD at °C :
3SiCl2H2(g) + 4NH3(g) → Si3N4(s) + 6HCl(g) + 6H2(g)

17. W CVD
°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

18. Vapor Phase Epitaxy (VPE)
Epitaxy: a single crystal substrate acts as a template for a film of identical or related crystal structure

19. 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)

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

21. 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)

22. 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

23. MOCVD
From Herman, Fig & 8.15, p. 180

24. Metalorganics
From Herman, Fig. 8.21, p. 187

25. MOCVD
From Hitchman, Appendix 6.3, p. 383

26. MOCVD
From Ohring, Table 4-5, p. 187

27. MOCVD
From Hitchman, Appendix 6.2, p. 382

28. MOCVD TMG AsH3(g) + Ga(CH3)3(g) → GaAs(s) + 3CH4(g)
From Herman, p. 192

29. MOCVD
From Herman, Fig. 8.23, p. 190

30. MOCVD
From Herman, Fig. 8.17, p. 182

31. MOCVD
From Ohring, Table 4-6, p. 189

32. CVD Films and Coatings
From Ohring, Table 4-1, p. 154

33. 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

34. 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

35. 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

36. 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

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

38. 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

39. CVD Kinetics
At steady-state,
Js = Jg
Growth rate,
R = Js/N = [ kshg / (ks + hg) ] (Ng/N)
N = film atomic density

40. 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 

41. 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

42. 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

43. PE-CVD PECVD (N plasma) : 2SiH4 + N2(g) → 2SiNH(s) + 3H2(g)
PECVD (Ar plasma) :
SiH4(g) + NH3(g) → SiNH(s) + 3H2(g)

44. 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

45. 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

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

47. PE-CVD
from Hitchman, Fig. 7.2, p. 392

48. 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

49. 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

50. 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)

51. Afterglow/Remote Plasma
metastable
neutrals
plasma
decaying
plasma
substrate
additional
gas input

52. 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