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Berkeley ELENG C245 - Lecture Notes

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EE C245 – ME C218Introduction to MEMS DesignFall 2008Fall 2008Prof Clark TC NguyenProf. Clark T.-C. NguyenDept of Electrical Engineering & Computer SciencesDept. of Electrical Engineering & Computer SciencesUniversity of California at BerkeleyBerkeley, CA 94720yLt 7 P M d l IV Et hi I l t ti EE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 1Lecture 7: Process Modules IV: Etching, Implantation, DiffusionLecture Outline• Reading: Senturia, Chpt. 3; Jaeger, Chpt. 2, 3, 5, 6gpgpª Etchingª Ion implantationªDiffusionªDiffusionEE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 2Dry EtchingAll based upon plasma processes• Physical sputtering• Plasma etchingplasma processes.RF ( l ld b )(+) ions generated by inelastic collisions with g• Reactive ion etching~RF (also, could be μwave)Develop () bias)energetic e-1’sGet avalanche effect because more e-1’s h i i +++++++++Develop (-) bias)Plasma (partially ionized gas composed of ions, come out as each ion is generated.+++waferE-fieldpy g pe-’s, and highly reactive neutral species)Develops (+) charge to ∴(+) ions will be accelerated EE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 3Develops (+) charge to compensate for∴(+) ions will be accelerated to the waferPhysical Sputtering (Ion Milling)• Bombard substrate w/ energetic ions → etching via physical momentum transfer• Give ions energy and directionality using E-fields• Highly directional → very anisotropicionsPR PRplasmafilmSiEE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 4Steep vertical wallProblems With Ion MillingPR etched down to herePRPRPRfilmOnce through the film the Sithe film, the etch will start barreling through the Si1. PR or other masking material etched at almost th t th fil t b t h d through the Sithe same rate as the film to be etched → very poor selectivity!2. Ejected species not inherently volatile →get jp ygredeposition → non-uniform etch → grass!•Because of these problems ion milling is not used often EE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 5Because of these problems, ion milling is not used often (very rare)Plasma Etching• Plasma (gas glow discharge) creates reactive species that chemically react w/ the film in questionRl h b l i i b i i h•Result: much better selectivity, but get an isotropic etch1plasmaPlasma Etching Mechanism:256plasma1. Reactive species generated in a plasma.2Reactive species diffuse to the PR PR3452.Reactive species diffuse to the surface of material to be etched.3. Species adsorbed on the surface.4Chemical reactionFilm to be etchedSi4.Chemical reaction.5. By-product desorbed from surface.6. Desorbed species diffuse into the MOST IMPORTANT STEP! (determines EE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 6bulk of the gasSTEP! (determines whether plasma etching is possible or not.)Ex: Polysilicon Etching w/ CF4and O2CF4CF4++ CF3++ CF2++ CF++ F++ F0+ CF2++ …N t l di l plasmaNeutral radical (highly reactive!)e-+ CF4→CF3+ F + e-SipSiCF6, SiF4both volatile dry etching is possible.∴e+ CF4→CF3+ F + e• F° is the dominant reactant → but it can’t be given a direction → thus, get isotropic etch!PRF0F0SiF4isotropiccomponentpolySiEE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 7Ex: Polysilicon Etching w/ CF4and O2F0F0isotropicPRF0F0SiF4pcomponentpolySi• Problems:1Isotropic etching1.Isotropic etching2. Formation of polymer because of C in CF4ª Solution: add O2to remove the polymer (but note that this reduces the selectivity, Spoly/PR)• Solution:ªUse Reactive Ion Etching (RIE)EE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 8ªUse Reactive Ion Etching (RIE)Reactive Ion Etching (RIE)• Use ion bombardment to aid and enhance reactive etching in a particular directionªRl di i l i i hi !ªResult: directional, anisotropic etching!• RIE is somewhat of a misnomerªIt’s not ions that react rather it’s still the neutral ªIt s not ions that react … rather, it s still the neutral species that dominate reactionª Ions just enhance reaction of these neutral radicals in a specific direction• Two principle postulated mechanisms behind RIE1 Surface damage mechanism1. Surface damage mechanism2. Surface inhibitor mechanismEE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 9RIE: Surface Damage Mechanism• Relatively high energy impinging ions (>50 eV) produce lattice damage plasma++reactiveproduce lattice damage at surface• Reaction at these PRPR++radicaldamaged sites is enhanced compared to reactions at undamaged filmSireactions at undamaged areasEnhanced reaction overResult: E.R. at surface >> E.R. on sidewallsEE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 10Result E.R. at surface E.R. on sidewallsRIE: Surface Inhibitor Mechanism• Non-volatile polymer layers are a product of plasma++reactiveypfreaction• They are removed by hi h di i l PRPR+++reactiveradicalhigh energy directional ions on the horizontal surface, but not filmSino reactionsurface, but not removed from sidewalls(+) ions breakup no reactionget () n raup the polymer layerget reactionRltER @ f ER id llEE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 11Result:E.R. @ surface >> E.R. on sidewallsDeep Reactive-Ion Etching (DRIE)The Bosch process:• Inductively-coupled plasmaypp• Etch Rate: 1.5-4 μm/min• Two main cycles in the etch:ªEh l(515 ) F(F) ªEtch cycle(5-15 s): SF6(SFx+) etches SiªDeposition cycle: (5-15 s): C4F8ªDeposition cycle: (515 s): C4F8deposits fluorocarbon protective polymer (CF2-)n• Etch mask selectivity:ª SiO2~ 200:1ªPhotoresist ~ 100:1ªPhotoresist 100:1• Issue: finite sidewall roughness ª scalloping < 50 nmEE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 12pg• Sidewall angle: 90o±2oDRIE Issues: Etch Rate Variance• Etch rate is diffusion-limited and drops for narrow trenchestrenchesª Adjust mask layout to eliminate large disparitiesEtch rate decreasesª Adjust process parameters (slow down the etch rate to that governed by the Etch rate decreases with trench widthEE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 13to that governed by the slowest feature)Semiconductor DopingSemiconductor DopingEE C245: Introduction to MEMS Design Lecture 7 C. Nguyen 9/18/08 14Doping of Semiconductors• Semiconductors are not


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Berkeley ELENG C245 - Lecture Notes

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