EE 143 Microfabrication Technology Lecture Module 3 Film Deposition CTN EE 143 Microfabrication Technology Spring 2010 Prof Clark T C Nguyen Dept of Electrical Engineering Computer Sciences University of California at Berkeley Berkeley CA 94720 Lecture Module 3 Film Deposition EE 143 Microfabrication Technology LecM 3 C Nguyen 2 14 10 1 2 14 10 2 Thin Film Deposition EE 143 Microfabrication Technology LecM 3 C Nguyen Copyright 2010 Regents of the University of California at Berkeley 2 11 10 EE 143 Microfabrication Technology Lecture Module 3 Film Deposition CTN Thin Film Deposition Methods for film deposition Evaporation Sputter deposition Chemical vapor deposition CVD Plasma enhanced chemical vapor deposition PECVD Epitaxy Electroplating Atomic layer deposition ALD Evaporation Heat a metal Al Au to the point of vaporization Evaporate to form a thin film covering the surface of the Si wafer Done under vacuum for better control of film composition EE 143 Microfabrication Technology LecM 3 C Nguyen 2 14 10 3 Evaporation Filament Evaporation System W filament Al staples 1 Pump down to vacuum reduces film contamination and allows better thickness control 2 Heat W filament melt Al wet filament 3 Raise temperature evaporate Al mean free path Vacuum Pump wafer EE 143 Microfabrication Technology kT 2 Pd 2 k Boltzmann Constant T temperature P pressure d diameter of gas molecule LecM 3 C Nguyen 2 14 10 4 Copyright 2010 Regents of the University of California at Berkeley 2 11 10 EE 143 Microfabrication Technology Lecture Module 3 Film Deposition CTN Evaporation cont can be 60m for a 4 particle at 10 4 Pa 0 75 Torr thus at 0 75 Torr get straight line path from Al staple filament to wafer Problem Shadowing Step Coverage Source Get an open Problem line of sight deposition Solns i Rotate water during evaporation ii Etch more gradual sidewalls Source Better Solution forget evaporation sputter deposit the film EE 143 Microfabrication Technology LecM 3 C Nguyen 2 14 10 5 Sputter Deposition Use an energetic plasma to dislodge atoms from a material target allowing the atoms to settle on the wafer surface Not as low a vacuum as evaporation 100 Pa 750 mTorr Vacuum Pump EE 143 Microfabrication Technology Ar Ar Target Al SiO2 Si2N4 ZnO Ti plasma wafer LecM 3 C Nguyen 2 14 10 6 Copyright 2010 Regents of the University of California at Berkeley 2 11 10 EE 143 Microfabrication Technology Lecture Module 3 Film Deposition CTN Sputter Deposition Process Step by step procedure 1 Pump down to vacuum 760 Torr 100 Pa 1 Pa 9 8 10 6 atm 0 0075012 Torr atm 750 mTorr 2 Flow gas e g Ar 3 Fire up plasma create Ar ions apply dc bias or RF for non conductive targets 4 Ar ions bombard target dislodge atoms 5 Atoms make their way to the wafer in a more random fashion since at this higher pressure 60 m for a 4 particle plus the target is much bigger Result better step coverage EE 143 Microfabrication Technology LecM 3 C Nguyen 2 14 10 7 Problems With Sputtering 1 Get some Ar in the film 2 Substrate can heat up up to 350oC causing nonuniformity across the wafer but it still is more uniform than evaporation 3 Stress can be controlled by changing parameters e g flow rate plasma power from pass to pass but repeatability is an issue Solution use Chemical Vapor Deposition CVD EE 143 Microfabrication Technology LecM 3 C Nguyen 2 14 10 8 Copyright 2010 Regents of the University of California at Berkeley 2 11 10 EE 143 Microfabrication Technology Lecture Module 3 Film Deposition CTN Chemical Vapor Deposition CVD Even better conformity than sputtering Form thin films on the surface of the substrate by thermal decomposition and or reaction of gaseous compounds Desired material is deposited directly from the gas phase onto the surface of the substrate Can be performed at pressures for which i e the mean free path for gas molecules is small This combined with relatively high temperature leads to Excellent Conformal Step Coverage Types of films polysilicon SiO2 silicon nitride SiGe Tungsten W Molybdenum M Tantalum Ta Titanium Ti EE 143 Microfabrication Technology LecM 3 C Nguyen 2 14 10 9 The CVD Process Reactant gas inert diluting gases are introduced into the reaction chamber a Gas species move to the substrate Gas Flow Gas Stream Reactants adsorb Atoms migrate and react chemically to form films This determines the ultimate conformality of the film i e determines step coverage b onto the substrate e e d c d Reaction by products desorbed from surface Wafer Energy required to drive reactions supplied by several methods Thermal i e heat photons electrons i e plasma EE 143 Microfabrication Technology LecM 3 C Nguyen 2 14 10 10 Copyright 2010 Regents of the University of California at Berkeley 2 11 10 EE 143 Microfabrication Technology Lecture Module 3 Film Deposition CTN The CVD Process cont Gas phase processes Step by Step CVD Sequence a Reactant gases inert diluting gases are introduced into reaction chamber b Gas species move to the substrate c Reactants adsorbed onto the substrate d Atoms migrate and react chemically to form films This determines to a large extent whether or not a film is conformal i e better step coverage Surface processes Not Conformal low T not enough adatom migration Conformal High T Plenty of adatom migration e Reaction by products desorbed and removed from reaction chamber EE 143 Microfabrication Technology LecM 3 C Nguyen 2 14 10 11 CVD Modeling Simplified Schematic Ng Jg Js Ns surface Governing Equations Ng conc of reactant molecules in the gas stream Ns conc of reactant molecules at the surface Js flux of gas molecules at the surface Jg flux of molecules diffusing in from the gas stream Effective diffusion const for the gas molecule J s k s N s ks surface reaction rate const Dg N g N s hg N g N s J g EE 143 Microfabrication Technology LecM 3 Vapor phase masstransfer coefficient C Nguyen 2 14 10 12 Copyright 2010 Regents of the University of California at Berkeley 2 11 10 EE 143 Microfabrication Technology Lecture Module 3 Film Deposition CTN CVD Modeling cont J s J J g J Ns s ks Otherwise reactants will build up somewhere hg J 1 ks k s hg hg N g J N g k s hg N g k h s g growth rate hg J J J hg N g hg N g ks ks J flux molecules incorporated unit volume N k s hg N g Ng J k s hg growth rate N k s hg N N EE 143 Microfabrication Technology LecM 3 C Nguyen 2 14 10 13 CVD Modeling cont Case ks hg surface reaction rate mass transfer rate growth rate h g Ng N mass transfer limited Case hg ks mass transfer rate
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