Sol1 - MIT OpenCourseWare http/ocw.mit.edu 2.830J 6.780J ESD.63J Control of Manufacturing Processes(SMA 6303 Spring 2008 For information about

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MIT OpenCourseWare http://ocw.mit.edu 2.830J / 6.780J / ESD.63J Control of Manufacturing Processes (SMA 6303) Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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MIT 2.830J/6.780J Assignment 1 (2008) — Solutions Problem 1 — exemplary solution provided courtesy of Eehern Wong. Choose Cantilever-beam MEMS accelerometer a) Outputs: Output geometry - Width, length, thickness, sidewall angles Output constitutive properties - Young’s modulus, density, resonant frequency (function of stiffness and mass, which are functions of above geometry and constitutive properties) b) Fabrication: Photolithography – Use a chrome mask, pattern wafer with photoresist by applying UV light to harden resist. Use resist mask to pattern oxide mask. Use oxide mask to wet etch silicon (next step). Lithographic processes determine the top planer dimension of the part (width, length). Wet chemical etching – Etch with KOH for anisotropic etch of silicon. Determine endpoint when cantilever released, overetching is okay since anisotropic. The wet etch contributes primarily to the z-direction geometry of the part since sidewall angles are determined by the etchant used (KOH is anisotropic) and the crystallographic orientation of the silicon substrate. Young’s modulus and density are functions of the bulk silicon, and are process independent. However, since stiffness and density are also functions of volume, and therefore geometry, they are affected by both lithography resolution and etch time/chemical. c) Process parameters: Photolithography • Equipment state - UV on/off, power density of tool, time of exposure • Equipment properties - UV power density dependence on temperature/time • Material states - Prepolymer, exposed polymer, developed/cured polymer • Material properties - Viscosity, catalytic density Wet chemical etching • Equipment state - Concentration, temperature, time of KOH • Equipment properties - KOH etch rate as a function of conc, temp, time • Material states - Bulk silicon, silicon and KOH product • Material properties - Crystallographic orientation, silicon type, mask material d) Control variables: Photolithography – Exposure time, softbake time, softbake temperature, develop time, develop spin speed Wet chemical etching – Chemical etchant, etchant concentration, chemical bath temperature, etch time
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e) Output sensitivity to control variables: Photolithography Exposure time, softbake time, softbake temperature, develop time, develop spin speed – all affect the width and length of the oxide mask used to etch the cantilever. Alignment of the cantilever to crystallographic orientations, also affect width and length because of etch plane selectivity. Wet chemical etching
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This note was uploaded on 09/24/2010 for the course MECHE 2.830J taught by Professor Davidhardt during the Spring '08 term at MIT.

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Sol1 - MIT OpenCourseWare http/ocw.mit.edu 2.830J 6.780J ESD.63J Control of Manufacturing Processes(SMA 6303 Spring 2008 For information about

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