Plasma-Enhanced Chemical Vapor
Deposition (PECVD)
Epitaxial Thin Film Growth
Emil Blix Wisborg
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 400-1200°C
CVD process example
1. Gas-phase
decomposition
2. Diffusion to surface
3. Physical adsorption
4. Diffusion along surface
5. Decomposition
6. Desorption of reaction
by-products
S. A. Campbell, Fabrication Engineering at the Micro- and Nanoscale,
4th Ed.,
Oxford University Press (2013)
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
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
Epitaxy
GaN →
AlGaN →
AlGaN →
AlN
→
GaN →
GaN →
GaN →
GaN →
Dr. Alan Doolittle, Georgia Tech, ECE6450: CVD and Epitaxy
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
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
PECVD theory – plasma reactions
Reaction
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
Adsorption
Rg + S→RS
CH2 + S→(CH2)S
Sputtering
A+ + BS → A + B
Ar+ + AlS → Ar + Al
Reactions with surfaces
Secondary electron emission A+ + S → S + e
O+ + S → S + e
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:
–
–
–
–
1
Particle-surface collision rate: 4 n v
v ~ √{T/m}
velectron > vion
Drain of electrons from plasma
I.H.Hutchinson: Introduction to Plasma Physics,
http://silas.psfc.mit.edu/introplasma/chap1.html
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,
http://silas.psfc.mit.edu/introplasma/chap1.html
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, 2014.
http://cnx.org/content/m25495/1.2/
Reactors
Hot wall
•
S. A. Campbell, Fabrication Engineering at the Micro- and
Nanoscale, 4th Ed., Oxford University Press (2013)
Cold wall
PECVD trends (SiH4 based processes)
Oxford Instruments, Plasma Technology.
http://www.ndl.org.tw/cht/doc/3-1-1-0/T19/T19_B1.pdf
Advantages of using PECVD
• Low operating temperature
• Uniform coating of different shapes
Conformal step coverage of PECVD SixNy
Royal Philips Electronics,
http://www.hitech-projects.com/dts/docs/pecvd.htm
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
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)
PECVD at UiO
• Advanced Vacuum
Vision 310 MKII
• Located in the
cleanroom
• SiO2
• Si3N4
• SiON
• a-Si
• Up to 12” wafer size
• No polymers or organic
materials
References
•
•
•
•
•
•
•
•
Wikipedia: ‘Plasma-enhanced chemical vapor deposition’.
http://en.wikipedia.org/wiki/Plasma-enhanced_chemical_vapor_deposition
http://www.oxford-instruments.com/products/etching-deposition-andgrowth/plasma-etch-deposition/pecvd
S. A. Campbell, Fabrication Engineering at the Micro- and Nanoscale, 4th Ed.,
Oxford University Press (2013)
A. Barron, ‘Chemical Vapor Deposition’ , Connexions Web site.
http://cnx.org/content/m25495/1.2/
T. Finstad, FYS4310: Materials Science of Semiconductors
TimeDomain CVD Inc., ‘Capacitive Plasmas’
http://timedomaincvd.com/CVD_Fundamentals/plasmas/capacitive_plasma.ht
ml
Wikipedia: ‘Thin film’. http://en.wikipedia.org/wiki/Thin_film
Jung-Hyun Park: Deposition of Coatings by PECVD.
http://www.docstoc.com/docs/59194062/Deposition-of-Coatings-by-PECVD
All websites accessed latest at March 12, 2014
Questions?
Thank you for your attention!