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Berkeley ELENG C245 - Lecture 12: Mechanics of Materials

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EE C245 ME C218 Introduction to MEMS Design Fall 2007 Prof Clark T C Nguyen Dept of Electrical Engineering Computer Sciences University of California at Berkeley Berkeley CA 94720 Lecture 12 Mechanics of Materials EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 1 Lecture Outline Reading Senturia Chpt 3 8 Jaeger Chpt 11 Handouts Bulk Micromachining of Silicon Wafer to Wafer Bonding for Microstructure Formation Lecture Topics Vapor Phase Etching of Silicon Laser Assisted Silicon Etching Wafer Bonding Mechanics of Materials Stress Strain Poisson Ratio Material Properties EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 2 1 Deep Reactive Ion Etching DRIE The Bosch process Inductively coupled plasma Etch Rate 1 5 4 m min Two main cycles in the etch Etch cycle 5 15 s SF6 SFx etches Si Deposition cycle 5 15 s C4F8 deposits fluorocarbon protective polymer CF2 n Etch mask selectivity SiO2 200 1 Photoresist 100 1 Issue finite sidewall roughness scalloping 50 nm Sidewall angle 90o 2o EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 3 DRIE Issues Etch Rate Variance Etch rate is diffusion limited and drops for narrow trenches Adjust mask layout to eliminate large disparities Adjust process parameters slow down the etch rate to that governed by the slowest feature EE C245 Introduction to MEMS Design Lecture 12 Etch Etch rate rate decreases decreases with with trench trench width width C Nguyen 10 4 07 4 2 DRIE Issues Footing Etch depth precision Etch stop buried layer of SiO2 Due to 200 1 selectivity the vertical etch practically just stops when it reaches SiO2 Problem Lateral undercut at Si SiO2 interface footing Caused by charge accumulation at the insulator Charging induced potential perturbs the E field Poor charge relaxation and lack of neutralization of e s at insulator Distorts the ion trajectory Result strong and localized damage to the structure at Si SiO2 interface footing Ion flux into substrate builds up potential EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 5 Metal Interlayer to Prevent Footing a Photolithography 1 f Silicon Thinning sacrificial b Preparatory trenches g Photolithography 2 c Metal interlayer deposition d Lift off remove PR Pre defined metal interlayer grounded to substrate supplies e s to neutralize charge and prevent charge accumulation at the Si insulator interface EE C245 Introduction to MEMS Design Lecture 12 e Anodic Bonding C Nguyen h DRIE i Remove metal interlayer j Metallize 10 4 07 6 3 Footing Prevention cont Below DRIE footing over an oxide stop layer Right efficacy of the metal interlayer footing prevention approach No No metal metal interlayer interlayer Kim Stanford Footing No footing With With metal metal interlayer interlayer Kim Seoul Nat Univ EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 7 DRIE Examples High aspectratio gear Tunable Capacitor Yao Rockwell Microgripper Keller MEMS Precision Instruments EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 8 4 Vapor Phase Etching of Silicon Vapor phase Xenon Difluoride XeF2 2XeF2 g Si s 2Xe g SiF4 g Set up Xactix XeF2 Xe sublimes at room T Etcher Closed chamber 1 4 Torr Pulsed to control exothermic heat of reaction Etch rate 1 3 m min isotropic Etch masks photoresist SiO2 Si3N4 Al other metals Issues Etched surfaces have granular structure 10 m roughness Hazard XeF2 reacts with H2O in air to form Xe and HF Inductor w no substrate Pister EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 9 Laser Assisted Chemical Etching Laser creates Cl radicals from Cl2 reaction forms SiCl2 Etch rate 100 000 m3 s Takes 3 min to etch 500 500 125 m3 trench Surface roughness 30 nm rms Serial process patterned directly from CAD file At right Laser assisted etching of a 500x500 m2 terraced silicon well Each step is 6 m deep EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 10 5 Wafer Bonding EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 11 Fusion Bonding Two ultra smooth 1 nm roughness wafers are bonded without adhesives or applied Hydrate surfaces external forces Procedure Prepare surfaces must be smooth and particle free Contact and anneal Clean hydrate O2 plasma hydration or HF dip When wafers are brought in contact at room temperature get hydrogen bonding and or Lap down the top wafer van der Waals forces to hold them together Anneal at 600 1200oC to bring the bond to full strength Result a bond as strong as the Works Works for for Si to Si Si to Si bonding bonding and Si to SiO silicon itself bonding and Si to SiO22 bonding EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 12 6 Fusion Bonding Example Below capacitive pressure sensor w fusion bonded features Univ of Southampton EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 13 10 4 07 14 Anodic Bonding Bonds an electron conducting material e g Si to an ion conducting material e g sodium glass Pyrex Procedure Mechanism Press Si and glass together Elevate temperature 180 500oC Temperature Apply voltage to Si 2001500V voltage repels Na ions Pressure from the glass surface Get net charge at glass surface Attractive force between Voltage Si and glass intimiate contact allows fusing at Current elevated temp Current drops to zero when bonding is complete EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 7 Anodic Bonding cont Advantage high pressure of electrostatic attraction smoothes out defects Below 100 mm wafers Pyrex glass 500 m thick 430oC 800V N2 1000 mbar EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 15 Metal Layer Bonding Pattern seal rings and bond pads photolithographically Eutectic bonding Uses eutectic point in metal Si phase diagrams to form silicides Au and Si have eutectic point at 363oC Low temperature process Can bond slightly rough surfaces Issue Au contamination of CMOS Solder bonding PbSn 183oC AuSn 280oC Lower T process Can bond very rough surfaces Issue outgassing not good for encapsulation Thermocompression Commonly done with electroplated Au or other soft metals Room temperature to 300oC Lowest T process Can bond rough surfaces with topography EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 16 8 Thermocompression Bonding Below Transfer of hexsil actuator onto CMOS wafer Singh et al Transducers 97 EE C245 Introduction to MEMS Design Lecture 12 C Nguyen 10 4 07 17 10 4 07 18 Hexsil MEMS Achieves high aspect ratio structures using conformal thin


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Berkeley ELENG C245 - Lecture 12: Mechanics of Materials

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