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Unformatted text preview: EE C245 / ME C218 INTRODUCTION TO MEMS DESIGN FALL 2009 PROBLEM SET #3 Issued: Tuesday, Sept. 29, 2009 Due (at 7 p.m.): Tuesday, Oct. 13, 2009 , in the EE C245 HW box in 240 Cory. 1. The following pages comprise a surface micromachining process flow for a clamped-clamped micromechanical beam with layout shown below. No details are spared in this flow; even equipment names are given, as are diagnostic steps used to verify select process steps. Furnace program names (for equipment in the UC Berkeley Microlab) are also given. These details are included to present a more realistic situation. In doing this problem, you must sift through the extraneous information and concentrate on the recipe information (i.e., temperatures, times, doses, etc.). For etch steps, if the etch uses a plasma or RIE process, assume perfect anisotropy. Also, as- sume that any etch time is determined by first calculating the time needed to etch through the nominal film thickness based on the nominal etch rate, then adding a 30% overetch to remove any small remaining spots of material. Assume that after you develop your photoresist, it has a sidewall angle of 90 . Also assume that the photoresist will have the given thickness in the field regions and have a perfectly flat upper surface. SP1 (cf) SNC (df) SG1 (df) SP2 (cf) SG2 (df) SN1 (df) SA1 (df) 50 50 8 40 40 60 8 70 40 60 80 40 30 20 24 20 50 20 16 A A B B C C 5 Figure 1: Full layout view EE C245 / ME C218 INTRODUCTION TO MEMS DESIGN FALL 2009 The black color is the background color of the layout editor. (This is field for all masks.) When considering etches in this problem, assume the following selectivities (estimated from Kirt Williams, Etch Rates for Micromachining Processing). As a reminder, the definition of selectivity is S A/B = ER A /ER B . Etchant Layer A Layer B Selectivity S A/B SF 6 + He Nitride Photoresist 1:1 Oxide 2:1 Silicon 1:3 CF 4 + CHF 3 + He Oxide Photoresist 3:1 Nitride 4:1 Silicon 4:1 Cl 2 + HBr Silicon Photoresist 1:1 Oxide 100:1 Nitride 1:2 CH 3 COOH+HNO 3 +H 2 SO 4 Nickel Photoresist 5:1 Oxide 300:1 Nitride 300:1 Gold 500:1 HF (release) Oxide Stoichiometric Nitride 250:1 (a) Draw the cross-section of the structures along the A-A, B-B, and C-C lines in the layout: (i) after step 14.2 of the process; and (ii) at the end of the process. Here, you should get the thickness dimensions correct (to within 100 nm or 20%, whichever is finer). Draw the length (horizontal) dimensions using a compressed scale. If any structures completely detach from the wafer, please show this clearly in the final sketch. (b) If the wafer is immersed in HF too long, something very bad happens. What is this? What is the longest time that the wafer can be immersed in HF before this happens? Is this enough to completely release the structure?...
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- Fall '09