EE 143: Microfabrication TechnologyLecture 14m: Etching ICTN 3/4/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 9Wet EtchingEE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 10Wet Etching• Wet etching: dip wafer into liquid solution to etch the desired filmª Generally isotropic, thus, inadequate for defining features < 3μm-wide• General Mechanism -1. Diffusion of the reactant to the film surface2. Reaction: adsorption, reaction, desorption3. Diffusion of reaction products from the surfacewaferetchSolvent bathPR PRSiPR PRSiooFilm to be etchedo ReactantReactionproductsEE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 11Wet Etching (cont.)• There are many processes by which wet etching can occurª Could be as simple as dissolution of the film into the solvent solutionª 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 bath2. Usually fast etch rates (compared to many dry etch processes)3. Usually excellent selectivity to the film of interestEE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 12Wet Etching Limitations1. Isotropicª Limited to <3μm featuresª But this is also an advantage of wet etching, e.g., if used for undercutting for MEMS2. Higher cost of etchants & DI water compared w/ dry etch gas expenses (in general, but not true vs. deep etchers)3. Safetyª Chemical handling is a hazard4. Exhaust fumes and potential for explosionª Need to perform wet etches under hood5. Resist adhesion problemsª Need HMDS (but this isn’t so bad)EE 143: Microfabrication TechnologyLecture 14m: Etching ICTN 3/4/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 13Wet Etch Limitations (cont.)6. Incomplete wetting of the surface:ª 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 betterwaferBut this will lead to nonuniformetching across the wafer.Pockets where wetting hasn’t occurred, yet (eventually, it will occur).Wetted surfaceSolvent bathEE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 14Wet Etch Limitations (cont.)7. Bubble formation (as a reaction by-product)ª If bubbles cling to the surface → get nonuniform etchingNon-uniform etchingPRPRSi waferBubble (gaseous by-product)Film to be etchedSolution: Agitate wafers during reaction.EE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 15Some Common Wet Etch ChemistriesWet Etching Silicon:Common: Si + HNO3+ 6HF H2SiF6+ HNO2+ H2+ H2O(isotropic)(nitric acid)(hydrofluoric acid)(1) forms a layer of SiO2(2) etches away the SiO2Different mixture combinations yield different etch rates.EE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 16Silicon Crystal Orientation• Silicon has the basic diamond structureª Two merged FCC cells offset by (a/4) in x, y, and z axesª From right:# available bonds/cm2<111># available bonds/cm2<110># available bonds/cm2<100>xyza[100](100) planeIncreasing@ coordinate (1,0,0)<100> plane ⊥to this vectorxyza][110](110) planexyza][111]e(111) planecoordinate (1,1,0) defines vector<110> plane ⊥to this vector@ coordinate (1,1,1), <111> plane ⊥ to resulting vectorEE 143: Microfabrication TechnologyLecture 14m: Etching ICTN 3/4/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 17Anisotropic Wet EtchingAnisotropic etches also available for single crystal Si:ª Orientation-dependent etching: <111>-plane more densely packed than <100>-planeSlower E.R.Faster E.R.…in some solventsOne such solvent: KOH + isopropyl alcohol(e.g., 23.4 wt% KOH, 13.3 wt% isopropyl alcohol, 63 wt% H2O)E.R.<100>= 100 x E.R.<111>EE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 18Anisotropic Wet Etching (cont.)Can get the following:(on a <100> - wafer)Si54.7°<111> <100>SiO2(on a <110> - wafer)Quite anisotropic!Si<110> <111>SiO2EE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 19Wet Etching SiO2SiO2+ 6HF H2+ SiF6+ 2H2OGenerally used to clear out residual oxides from contactsnative oxidecan get this just by exposing Si to air 1-2nm-thickProblem: Contact hole is so thin that surface tensions don’t allow the HF to get into the contactGenerally the case for VLSI circuitsoxideHFbubblent300nmSolution: add a surfactant (e.g., Triton X) to the BHF before the contact clear etch1. 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 contactEE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 20More Wet Etch Chemistries• Wet etching silicon nitrideª Use hot phosphoric acid: 85% phosphoric acid @ 180oCª Etch rate ~ 10 nm/min (quite slow)ª Problem: PR lifted during such etchingª Solution: use SiO2as an etch mask (E.R. ~2.5 nm/min)( A hassle → dry etch processes more common than wet• Wet etchining aluminumª Typical etch solution composition:ª Problem: H2gas bubbles adhere firmlly to the surface →delay the etch → need a 10-50% overetch timeª Solution: mechanical agitation, periodic removal of wafers from etching solution80% phoshporic acid, 5% nitric acid, 5% acetic acid, 10% water(HNO3)(CH3COOH) (H2O)(H2PO4)(1) Forms Al2O3(aluminum oxide)(2) Dissolves the Al2O3EE 143: Microfabrication TechnologyLecture 14m: Etching ICTN 3/4/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 21Wet Etch Rates (f/ K. Williams)EE 143: Microfabrication Technology LecM 4 C. Nguyen 2/14/10 22Film Etch Chemistries• For some popular
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