Microelectromechanical Systems Project 1,2,3

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EE-446 MEMS, Spring 2018, Project #3: COMSOL Design and Simulation of Surface-micromachined MEMS Comb Accelerometer Note: This is an optional project. You do NOT have to do it, but it is a good opportunity for you to earn bonus points to improve your final total grade. If you choose to do it, please submit your report to me on or before 04/24/2018 (Tuesday) in class, and you can get up to 50 bonus points added to your project #2 grade. You can do it either independently or two students can work jointly as a group and submit a joint report. If you choose to do it, even if you cannot complete the project by 04/24/2018, you can still submit your partial results/report to me, and you will get partial bonus points based on the results you obtained. A poly-silicon MEMS comb accelerometer device is shown in Figure 1 and 2. The device prototype comes from the ADXL50 accelerometer developed by Analog Devices Inc. It has been successfully used for automobile air-bag deployment. As a MEMS designer, you are asked to design such a comb accelerometer. Actual MEMS accelerometer contains 46 differential capacitance groups. For simplification, here we only design 24 finger groups. The device structure is shown in Figure 1 and 2. The design parameters are listed in Table 1. Given the density of poly-Si: ρ=2.33×103kg/m3, Young’s modulus of poly-Si: E=1.70×1011Pa.

Table 1. Design parameters of MEMS comb accelerometer Design parameters Dimensions Device thickness t 2µm Beam width Wb 2µm Beam length Lb 210µm Mass width Wm (70+2× the sum of last two

digits of your student ID) µm Mass length Lm 372µm Movable finger width Wf 4µm Movable finger length Lf 160µm Fixed finger width Wff 4µm Fixed finger length Lff 194µm Total number of sensing fingers Ns 24 Capacitance gap d0 2µm Gap d1 between neighboring sensing

finger groups 4µm

Outmost device area (width × length) 460μm×630μm Anchor size (width × length) 30μm×30μm

In the above table, the mass width Wm is decided as (70+2×last two digits of your student ID) µm. For example, if the last two digits of your student ID are “34”, then your mass width Wm=70+2×(3+4)=84µm. (1). The cross-sectional view and top view of the MEMS comb accelerometer are shown

in Figure 1 and 2 separately. Briefly explain the working principle of this MEMS accelerometer device.

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(2). Hand calculations of the device: i). Calculate the total sensitive mass Ms of the accelerometer, which consists of the movable mass, plus the mass of 24 movable fingers. ii). Calculate the total spring constant ktot of the device. iii). Calculate the displacement sensitivity of the whole device. That is, the lateral displacement Sdx in response to unit gravity acceleration input (1g) in X direction. iv). Calculate the resonant frequency of the accelerometer. (3). The top-view of the accelerometer is shown in Figures 2 and 3. Try to decide the coordinates of the key points of the device. Based on these coordinates, please develop the 3D COMSOL model for the comb accelerometer. Capture the screen-shot of your 3D model in COMOL, print it out and attach it with your report. Also capture the screenshot of the meshed model of your accelerometer device, print it out and attach it with your report. (4). Use COMSOL to simulate the device displacement sensitivity Sd along horizontal X direction for unit gravity acceleration (1g=9.8m/s2). Capture the screenshot showing the displacement contour plot of your MEMS accelerometer in COMSOL, print it out and attach it with your report. (5). Compare the displacement sensitivity you obtained in COMSOL simulation with the previous hand-calculation result in previous step (2), what is the relative percentage error for hand-calculation? Error=|Sd(COMSOL) – Sd(hand_calculation)/(Sd(COMSOL)|×100%

Figure 1. Cross-section view of the comb accelerometer

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Figure 2. Top view of poly-Si surface-micromachined comb accelerometer

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Figure 3. Top view of MEMS accelerometer to decide coordinates of keypoints

Due day: 04/24/2018, Tuesday in class.