1. Plasma-Enhanced Chemical Vapor Deposition (PECVD)
Epitaxial Thin Film Growth
Emil Blix Wisborg

2. What is CVD? Chemical Vapor Deposition
Deposition of a solid phase from a gaseous phase
Volatile precursor gases react or decompose on a heated substrate
Operating temperatures °C

3. CVD process example Gas-phase decomposition Diffusion to surface
Physical adsorption
Diffusion along surface
Decomposition
Desorption of reaction by-products
S. A. Campbell, Fabrication Engineering at the Micro- and Nanoscale, 4th Ed.,
Oxford University Press (2013)

4. Thin films
A layer of material ranging from a few Ångstrøms to several microns
Electronic semiconductor devices
Solar cells
Batteries
Optical coatings
Mirrors
Antireflection coating

5. Epitaxy
Deposition of a crystalline overlayer on a crystalline substrate
Continous crystal structure
Homoepitaxy
Film and substrate same material
High purity layers and doping control
Heteroepitaxy
Film and substrate different material
Bandgap engineering

6. Epitaxy GaN → AlGaN → AlN → GaN →
Dr. Alan Doolittle, Georgia Tech, ECE6450: CVD and Epitaxy

7. What is PECVD? Plasma-enhanced CVD
Energy required for reaction comes from plasma rather than from temperature
Wafers can be kept at low temperature
The plasma is created by RF electromagnetic waves

8. PECVD theory – plasma Fractionally ionized gas
High free electron content
Two main types:
Hot (thermal) plasma
kT > Eionization
Thermal equilibrium, Te≈Tgas
Cold plasma
Created by electric fields or radiation
Non-thermal equilibrium, Te >>Tgas

9. PECVD theory – plasma reactions
General equation
Example
Reactions with electrons
Ionization
e + A → A+ + 2e
e + N2 →N2+ + 2e
Excitation
e + A → A* + e
e + O2 → O2* + e
Dissociation
e + AB → e + A + B
e + SiH4 → e + SiH3 + H
Dissociative ionization
e + AB → 2e + A+ + B
e + TiCl4 → 2e + TiCl3+ + Cl
Dissociative attachment
e + AB → A− + B
e + SiCl4 → Cl− + SiCl3
Reactions with surfaces
Adsorption
Rg + S→RS
CH2 + S→(CH2)S
Sputtering
A+ + BS → A + B
Ar+ + AlS → Ar + Al
Secondary electron emission
A+ + S → S + e
O+ + S → S + e

10. PECVD theory – sheath
The plasma forms a thin potential drop at all surfaces - sheath
Causes an electric field from the plasma to the surface
If E = 0:
Particle-surface collision rate: n v
v ~ √{T/m}
velectron > vion
Drain of electrons from plasma 1 4
I.H.Hutchinson: Introduction to Plasma Physics,

11. PECVD theory – sheath
The plasma forms a thin potential drop at all surfaces - sheath
Causes an electric field from the plasma to the surface
Plasma becomes positively charged
Positively charged particles are accelerated toward the surface E
I.H.Hutchinson: Introduction to Plasma Physics,

12. Process steps Precursor gas and carrier gas mixed in reaction chamber
Ionization to plasma by RF electric field
Energetic electrons dissociate precursor molecules to free radicals
Particles move to substrate
Radicals adsorbed onto substrate (and reactor walls)
Layer formation
Density increased by ion bombardment
A. Barron, ‘Chemical Vapor Deposition’ , Connexions Web site, Mar 12,

13. Reactors Hot wall Cold wall
S. A. Campbell, Fabrication Engineering at the Micro- and Nanoscale, 4th Ed., Oxford University Press (2013)

14. PECVD trends (SiH4 based processes)
Oxford Instruments, Plasma Technology.

15. Advantages of using PECVD
Low operating temperature
Uniform coating of different shapes
Conformal step coverage of PECVD SixNy
Royal Philips Electronics,

16. Advantages of using PECVD
Low operating temperature
Uniform coating of different shapes
Good step coverage
High packing density – hard and environmentally stable
Continuous variation of film characteristics as a function of depth
Stress reduction

17. Drawbacks Toxic precursors and byproducts High equipment cost
Limited capacity
Contamination from precursor and carrier gas molecules
Silane (SiH4) often used as Si source
Hard to obtain stoichiometry
Silicon nitride (SixNy) and silicon oxide (SiOx)

18. PECVD at UiO Advanced Vacuum Vision 310 MKII Located in the cleanroom
Si3N4
SiON
a-Si
Up to 12” wafer size
No polymers or organic materials

19. References
Wikipedia: ‘Plasma-enhanced chemical vapor deposition’.
S. A. Campbell, Fabrication Engineering at the Micro- and Nanoscale, 4th Ed., Oxford University Press (2013)
A. Barron, ‘Chemical Vapor Deposition’ , Connexions Web site.
T. Finstad, FYS4310: Materials Science of Semiconductors
TimeDomain CVD Inc., ‘Capacitive Plasmas’
Wikipedia: ‘Thin film’.
Jung-Hyun Park: Deposition of Coatings by PECVD.
All websites accessed latest at March 12, 2014

20. Questions?
Thank you for your attention!