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!