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HW4%20Solutions

# HW4%20Solutions - EEL5225 Principles of MEMS Transducers...

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Principles of MEMS Transducers Page 1 of 4 Prepared by D. Arnold September 30, 2010 EEL5225 Principles of MEMS Transducers HW4 Fall 2010 Semester Assigned: Monday, 9/27 Due: Friday, 10/1 1. A 4:1:1 HNO 3 : HF : HC 2 H 3 O 2 solution is used to etch silicon. How long would it take to etch a hole all the way through a 500 μm-thick wafer? 2. You are asked to figure out how to fabricate a 5 μm thick, 500 μm x 500 μm square silicon membrane for a pressure sensor from a 500 μm thick silicon wafer. Because the device must maintain CMOS- compatibility, you decide to use a backside TMAH etch. Also, to maintain precise membrane thickness you decide to use an electrochemical etch stop, with the pn junction achieved via ion implantation and drive in. a. Find the (100) etch rate of 20% TMAH at 95°C. b. Assuming a (100)/(111) etch plane selectivity of 25:1, determine the (111) etch rate. c. How large should the backside mask square opening be? d. How long will the etch take? e. What material would you use for a backside etch mask? How thick does it need to be? For 4:1:1 HNO 3 : HF : HC 2 H 3 O 2 solution: there is 66.66 % HNO 3 , 16.67% HF and 16.67 % HC 2 H 3 O 2 From the chart: To etch silicon, Hence, to etch 500 micron thick wafer, time: a. (100) etch rate of 20% TMAH at 95

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