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

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!