EE C245 – ME C218Introduction to MEMS DesignFall 2007Fall 2007Prof Clark TC NguyenProf. Clark T.-C. NguyenDept of Electrical Engineering & Computer SciencesDept. of Electrical Engineering & Computer SciencesUniversity of California at BerkeleyBerkeley, CA 94720yLt 8 Sf Mi hii IEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 1Lecture 8: Surface Micromachining ILecture Outline• Reading: Senturia Chpt. 3, Jaeger Chpt. 11, Handout: gpgp“Surface Micromachining for Microelectromechanical Systems”•Lecture Topics:•Lecture Topics:ª Polysilicon surface micromachiningª Stictionª Residual stressª Topography issuesªNickel metal surface micromachiningªNickel metal surface micromachiningª 3D “pop-up” MEMSª Foundry MEMS: the “MUMPS” processª The Sandia SUMMIT processEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 2Polysilicon Surface-Micromachining• Uses IC fabrication instrumentation exclusivelyNitrideInterconnectPl iliSacrificialOxideStructuralPolysilconIsolationOidinstrumentation exclusively•Variations: sacrificial layer thickness, fine- vs. large-PolysiliconPolysilconOxide,ggrained polysilicon, in situvs. POCL3-dopingSilicon SubstrateFree-St diHydrofluoricAcidRlWaferStandingPolysiliconBeamReleaseEtchant300 kHz FoldedBeam EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 3Silicon Substrate300 kHz Folded-Beam Micromechanical ResonatorPolysiliconPolysiliconEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 4Why Polysilicon?• Compatible with IC fabrication processesª Process parameters for gate polysilicon well knownªOnly slight alterations needed to control stress for ªOnly slight alterations needed to control stress for MEMS applications• Stronger than stainless steel: fracture strength of polySi ~ 23 GP l 0 2GP1GP 2-3 GPa, steel ~ 0.2GPa-1GPa • Young’s Modulus ~ 140-190 GPa•Extremely flexible: maximum strain before fracture ~ 0 5%Extremely flexible: maximum strain before fracture ~ 0.5%• Does not fatigue readily• Several variations of polysilicon used for MEMSª LPCVD polysilicon deposited undoped, then doped via ion implantation PSG source POClor B-source dopingimplantation, PSG source, POCl3,or B-source dopingª In situ-doped LPCVD polysiliconª Attempts made to use PECVD silicon, but quality not very good (yet) →etches too fast in HF so release is EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 5good (yet) →etches too fast in HF, so release is difficultPolysilicon Surface-Micromachining P FlProcess FlowEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 6Layout and Masking Layers• At Left: Layout for a folded-beam capacitive combcapacitive comb-driven micromechanical resonator• Masking Layers:1stPolysilicon: POLY1( f)POLY1(cf)Anchor Opening: ANCHOR(df)2ndPolysilicon: POLY2(cf)()Cii bdi AA′Capacitive comb-drive for linear actuationFoldedbeam support EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 7Folded-beam support structure for stress reliefSurface-Micromachining Process Flow• Deposit isolation LTO (or PSG):ª Target = 2μmª 1 hr. 40 min. LPCVD @450oCNitrideInterconnectIsolationCross-sections through A-A′• Densify the LTO (or PSG)ª Anneal @950oC for 30 min.• Deposit nitride:ªT 100NitridePolysiliconIsolationOxideªTarget = 100nmª 22 min. LPCVD @800oC• Deposit interconnect polySi:ªTarget = 300nmSilicon SubstrateªTarget = 300nmª In-situ Phosphorous-dopedª 1 hr. 30 min. LPCVD @650oCPhotoresist• Lithography to define poly1 interconnects using the POLY1(cf) Silicon Substrateinterconnects using the POLY1(cf) mask• RIE polysilicon interconnects:ª CCl4/He/O2@300W,280mTorrEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 842,• Remove photoresist in PRS2000Silicon SubstrateSurface-Micromachining Process Flow• Deposit sacrificial PSG:ª Target = 2μmª 1 hr. 40 min. LPCVD @450oCSacrificialOxidePhotoresist• Densify the PSGª Anneal @950oC for 30 min.• Lithography to define anchors using the ANCHOR(df) maskª Align to the poly1 layer•Etch nch sSilicon Substrate•Etch anchorsª RIE using CHF3/CF4/He @350W,2.8TorrªRemove PR in PRS2000ªRemove PR in PRS2000ª Quick wet dip in 10:1 HF to remove native oxideSilicon SubstrateStructural Polysilcon• Deposit structural polySiª Target = 2μmEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 9ª In-situ Phosphorous-dopedª 11 hrs. LPCVD @650oCSilicon SubstrateSurface-Micromachining Process Flow• Deposit oxide hard maskª Target = 500nmª 25 min. LPCVD @450oC•St A lOxide Hard Mask•Stress Annealª 1 hr. @ 1050oCª Or RTA for 1 min. @ 1100oC in 50 sccm N2Silicon Substrate• Lithography to define poly2 structure (e.g., shuttle, springs, drive & sense electrodes) using drive & sense electrodes) using the POLY2(cf) maskª Align to the anchor layerª Hard bake the PR longer to make it strongermake it stronger• Etch oxide mask firstª RIE using CHF3/CF4/He @350W,2.8Torr E h l l iliSilicon Substrate•Etch structural polysiliconª RIE using CCl4/He/O2@300W,280mTorrª Use 1 min. etch/1 min. rest EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 10increments to prevent excessive temperatureSilicon SubstrateSurface-Micromachining Process Flow• Remove PR (more difficult)ªAsh in OplasmaªAsh in O2plasmaª Soak in PRS2000Silicon Substrate• Release the structuresª Wet etch in HF for a ll d h HydrofluoricAcidWafercalculated time that insures complete undercutting ª If 5:1 BHF, then ~ 30 min.ªIf 48 8 t % HF 1 iFSt diReleaseEtchantªIf 48.8 wt. % HF, ~ 1 min.• Keep structures submerged in DI water after the etchFree-StandingPolysilicon Beam• Transfer structures to methanol•Supercritical CO2dry releaseEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 11Supercritical CO2dry releaseSilicon SubstratePolysilicon Surface-Micromachined Examples• Below: All surface-micromachined in polysilicon using variants of the described process flowFree-Free Beam ResonatorFoldedFolded--Beam CombBeam Comb--Driven ResonatorDriven ResonatorThreeThree--Resonator Micromechanical FilterResonator Micromechanical FilterEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/23/08 12Structural/Sacrifical Material CombinationsStructural Material Sacrificial Material EtchantPolySiSiO PSG LTOHF BHFPoly-SiSiO2, PSG, LTOHF, BHFAl
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