Download - Plasma-Enhanced Chemical Vapor Deposition (PECVD) Epitaxial Thin Film Growth Emil Blix Wisborg
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 example1. Gas-phase
decomposition2. Diffusion to surface3. Physical adsorption4. Diffusion along surface5. Decomposition6. 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 reactionsReaction 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
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
I.H.Hutchinson: Introduction to Plasma Physics, http://silas.psfc.mit.edu/introplasma/chap1.html
1
4
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
I.H.Hutchinson: Introduction to Plasma Physics, http://silas.psfc.mit.edu/introplasma/chap1.html
E
• Precursor gas and carrier gas mixed in reaction chamber• Ionization to plasma by RF electric field
Process steps
A. Barron, ‘Chemical Vapor Deposition’ , Connexions Web site, Mar 12, 2014. http://cnx.org/content/m25495/1.2/
• 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
Reactors
• S. A. Campbell, Fabrication Engineering at the Micro- and Nanoscale, 4th Ed., Oxford University Press (2013)
Hot wall 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-and-growth/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.html• 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!