EE 143 Microfabrication Technology Lecture 14m Etching I CTN 3 4 10 Wet Etching Wet etching dip wafer into liquid solution to etch the desired film Generally isotropic thus inadequate for defining features 3 m wide Wet Etching Film to be etched C Nguyen Reactant 2 14 10 9 PR Si LecM 4 1 Diffusion of the reactant to the film surface 2 Reaction adsorption reaction desorption 3 Diffusion of reaction products from the surface C Nguyen 2 14 10 10 1 Isotropic Limited to 3 m features But this is also an advantage of wet etching e g if used for undercutting for MEMS 2 Higher cost of etchants DI water compared w dry etch gas expenses in general but not true vs deep etchers 3 Safety Chemical handling is a hazard 4 Exhaust fumes and potential for explosion Need to perform wet etches under hood 5 Resist adhesion problems Need HMDS but this isn t so bad Could be as simple as dissolution of the film into the solvent solution Usually it involves one or more chemical reactions Oxidation reduction redox is very common a Form layer of oxide b Dissolve react away the oxide Advantages 1 High throughput process can etch many wafers in a single bath 2 Usually fast etch rates compared to many dry etch processes 3 Usually excellent selectivity to the film of interest C Nguyen PR Wet Etching Limitations There are many processes by which wet etching can occur LecM 4 PR EE 143 Microfabrication Technology Wet Etching cont EE 143 Microfabrication Technology PR Si o Reaction o products LecM 4 Solvent bath General Mechanism o EE 143 Microfabrication Technology wafer etch 2 14 10 11 EE 143 Microfabrication Technology Copyright 2010 Regents of the University of California at Berkeley LecM 4 C Nguyen 2 14 10 12 EE 143 Microfabrication Technology Lecture 14m Etching I CTN Wet Etch Limitations cont Wet Etch Limitations cont 7 Bubble formation as a reaction by product If bubbles cling to the surface get nonuniform etching 6 Incomplete wetting of the surface Pockets where wetting hasn t occurred yet eventually it will occur Solvent bath Bubble gaseous by product PR Wetted surface PR Non uniform etching Film to be etched But this will lead to nonuniform etching across the wafer wafer 3 4 10 Solution Agitate wafers during reaction Si wafer For some etches e g oxide etch using HF the solution is to dip in DI water first then into HF solution the DI water wets the surface better LecM 4 C Nguyen 2 14 10 13 EE 143 Microfabrication Technology Some Common Wet Etch Chemistries nitric acid 1 forms a layer of SiO2 100 plane H2SiF6 HNO2 H2 H2O hydrofluoric acid x 100 coordinate 1 0 0 100 plane to this vector C Nguyen 2 14 10 110 plane to this vector structure Two merged FCC cells offset by a 4 in x y and z axes From right available bonds cm2 111 available bonds cm2 110 available bonds cm2 100 15 EE 143 Microfabrication Technology Copyright 2010 Regents of the University of California at Berkeley e a y Silicon has the basic diamond 2 etches away the SiO2 LecM 4 14 z 110 plane a y Different mixture combinations yield different etch rates EE 143 Microfabrication Technology 2 14 10 z a Wet Etching Silicon isotropic C Nguyen Silicon Crystal Orientation z Common Si HNO3 6HF LecM 4 LecM 4 111 y 110 x coordinate 1 1 0 defines vector 111 plane x coordinate 1 1 1 111 plane to resulting vector Increasing EE 143 Microfabrication Technology C Nguyen 2 14 10 16 EE 143 Microfabrication Technology Lecture 14m Etching I CTN Anisotropic Wet Etching Anisotropic Wet Etching cont Can get the following Anisotropic etches also available for single crystal Si 111 Orientation dependent etching 111 plane more densely packed than 100 plane Faster E R 3 4 10 100 SiO2 Slower E R on a 100 wafer Si 54 7 in some solvents 110 One such solvent KOH isopropyl alcohol SiO2 111 e g 23 4 wt KOH 13 3 wt isopropyl alcohol 63 wt H2O on a 110 wafer E R 100 100 x E R 111 EE 143 Microfabrication Technology LecM 4 C Nguyen Si 2 14 10 17 Quite anisotropic EE 143 Microfabrication Technology Wet Etching SiO2 HF nt 300nm native oxide can get this just by exposing Si to air 1 2nm thick LecM 4 C Nguyen 18 Use hot phosphoric acid 85 phosphoric acid 180oC Etch rate 10 nm min quite slow Problem PR lifted during such etching Solution use SiO2 as an etch mask E R 2 5 nm min A hassle dry etch processes more common than wet Wet etchining aluminum Typical etch solution composition 80 phoshporic acid 5 nitric acid 5 acetic acid 10 water H2PO4 Solution add a surfactant e g Triton X to the BHF before the contact clear etch 1 Improves the ability of HF to wet the surface hence get into the contact 2 Suppresses the formation of etch by products which otherwise can block further reaction if by products get caught in the contact EE 143 Microfabrication Technology 2 14 10 Wet etching silicon nitride Problem Contact hole is so thin that surface tensions don t allow the HF to get into the contact Generally the case for VLSI circuits oxide C Nguyen More Wet Etch Chemistries SiO2 6HF H2 SiF6 2H2O Generally used to clear out residual oxides from contacts bubble LecM 4 2 14 10 19 HNO3 CH3COOH H2O 1 Forms Al2O3 aluminum oxide 2 Dissolves the Al2O3 Problem H2 gas bubbles adhere firmlly to the surface delay the etch need a 10 50 overetch time Solution mechanical agitation periodic removal of wafers from etching solution EE 143 Microfabrication Technology Copyright 2010 Regents of the University of California at Berkeley LecM 4 C Nguyen 2 14 10 20 EE 143 Microfabrication Technology Lecture 14m Etching I CTN Wet Etch Rates f K Williams 3 4 10 Film Etch Chemistries For some popular films EE 143 Microfabrication Technology LecM 4 C Nguyen 2 14 10 21 EE 143 Microfabrication Technology Copyright 2010 Regents of the University of California at Berkeley LecM 4 C Nguyen 2 14 10 22
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