1EE 245: Introduction to MEMSLecture 12: Mechanics of MaterialsCTN 10/5/09Copyright © 2009 Regents of the University of CaliforniaEE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 34Wafer BondingEE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 35Fusion Bonding• Two ultra-smooth (<1 nm roughness) wafers are bonded without adhesives or applied external forces• Procedure:ª Prepare surfaces: must be smooth and particle-free( Clean & hydrate: O2plasma, hydration, or HF dipª When wafers are brought in contact at room temperature, get hydrogen bonding and/or van der Waals forces to hold them togetherª Anneal at 600-1200oC to bring the bond to full strength• Result: a bond as strong as the silicon itself!Works for Si-to-Si bonding and Si-to-SiO2bondingWorks for Si-to-Si bonding and Si-to-SiO2bondingHydrate surfacesContact and annealLap down the top waferEE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 36Fusion Bonding Example• Below: capacitive pressure sensor w/ fusion-bonded features[Univ. of Southampton]EE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 37Anodic Bonding• Bonds an electron conducting material (e.g., Si) to an ion conducting material (e.g., sodium glass = Pyrex)• Procedure/Mechanism:ª Press Si and glass togetherª Elevate temperature: 180-500oCª Apply (+) voltage to Si: 200-1500V( (+) voltage repels Na+ions from the glass surface( Get net (-) charge at glass surface( Attractive force between (+) Si and (-) glass → intimiatecontact allows fusing at elevated temp.ª Current drops to zero when bonding is completeTemperaturePressureVoltageCurrent2EE 245: Introduction to MEMSLecture 12: Mechanics of MaterialsCTN 10/5/09Copyright © 2009 Regents of the University of CaliforniaEE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 38Anodic Bonding (cont.)• Advantage: high pressure of electrostatic attraction smoothes out defects• Below: 100 mm wafers, Pyrex glass 500 μm-thick, 430oC, 800V, N2@ 1000 mbarEE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 39Metal Layer Bonding• Pattern seal rings and bond pads photolithographically••Eutectic bondingEutectic bondingª Uses eutectic point in metal-Si phase diagrams to form silicidesª Au and Si have eutectic point at 363oCª Low temperature processª Can bond slightly rough surfacesª Issue: Au contamination of CMOS••Solder bondingSolder bondingª PbSn (183oC), AuSn (280oC)ª Lower-T processª Can bond very rough surfacesª Issue: outgassing (not good for encapsulation)••ThermocompressionThermocompressionª Commonly done with electroplated Au or other soft metalsª Room temperature to 300oCª Lowest-T processª Can bond rough surfaces with topographyEE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 40Thermocompression Bonding• Below: Transfer of hexsil actuator onto CMOS wafer[Singh, et al, Transducers’97]EE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 41Hexsil MEMS• Achieves high aspect ratio structures using conformal thin films in mold trenches• Parts are demolded (and transferred to another wafer)• Mold can be reused• Design with honeycomb structure for strength3EE 245: Introduction to MEMSLecture 12: Mechanics of MaterialsCTN 10/5/09Copyright © 2009 Regents of the University of CaliforniaEE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 42Hexsil MEMS Actuator• Below: Transfer of hexsil actuator onto CMOS wafer[Singh, et al, Transducers’97]EE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 43Silicon-on-Insulator (SOI) MEMS• No bonding required• Si mechanical structures anchored by oxide pedestals• Rest of the silicon can be used for transistors (i.e., CMOS compatible)Cross Section Top ViewNitrideSiliconSiliconSiO2NitrideEE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 44SOI MEMS Examples[Brosnihan]Micromirror[Analog Devices]EE C245: Introduction to MEMS Design LecM 6 C. Nguyen 9/28/07 45The SCREAM Process• SCREAM: SSingle CCrystal RReactive EEtching and MMetallization
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