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 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)

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 → AlN → 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 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 1 4 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 Cold wall S. A. Campbell, Fabrication Engineering at the Micro- and Nanoscale, 4th Ed., Oxford University Press (2013)

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 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!