具有可變電感的CMOS MEMS三軸加速度計

Abstract

本論文使用CMOS-MEMS製程設計出具有可變電感的三軸加速度計，並將三軸的感測製作於單一質量塊上。不同於容易受殘餘應力影響的電容式加速度計，我們直接使用彈簧來當作可變電感並搭配固定的MIM電容組成一個LC振盪器，當晶片受到一加速度時，彈簧會因為質量塊的牽引而發生彈簧線圈面積的改變並產生電感值的變化，同時LC振盪器的輸出頻率也會跟著改變，而振盪器的頻率變化會跟加速度成正比，故可以利用輸出頻率的變化來偵測加速度大小。 本篇論文設計的晶片有兩顆，Sample I與Sample II皆使用1P6M 0.18 um製程。Sample I的機械共振頻率在出平面方向為8.28 kHz(Q值3.45)，水平面方向為19.2 kHz；而Sample II的X、Y與Z共振頻率分別為6.49 kHz(Q值216)、6.56 kHz(Q值218)與6.418 kHz(Q值210)。結合電性量測後得到Sample I的Z軸靈敏度為25.66 KHz/grms @1 grms - 3.5 grms；Sample II的X、Y與Z軸靈敏度分為37.1 KHz/grms、61.2 KHz/grms @0 grms - 5 grms與3.32 KHz/grms @1 grms - 4 grms。

This thesis proposes two tri-axis CMOS-MEMS accelerometers with variable inductors and combine them into a single proof-mass for each. Different from the capacitive accelerometer sensitive to thin-film residual stress, using spring as a variable inductor can prevent from it. The proposed accelerometers use variable inductors and MIM capacitors to form LC oscillators as the capacitors are independent from acceleration. Due to the force and displacement of the proof-mass caused by external acceleration, the coil area of the spring inductors are changed as well as the inductances, and frequencies of LC oscillators are also changed. The frequency deviations in LC oscillators are proportional to the external acceleration. As a result, the sensing method is feasible for detecting the acceleration. Two devices including Sample I and Sample II which has been designed were both fabricated using TSMC 1P6M 0.18 um process in this thesis. The measured mechanical resonant frequencies of Sample I is 8.28 kHz for out-of-plane axis (Q is 3.45) and 19.2 kHz for in-plane axis; another one, Sample II is 6.49 kHz for the X-axis(Q is 216)、6.56 kHz for the Y-axis(Q is 218) and 6.418 kHz for the Z-axis(Q is 210). The measurement results combining with electrical characteristics show that Sample I can achieve 25.66 KHz/grms @1 grms - 3.5 grms for Z-axis；Sample II can achieve 37.1 KHz/grms、61.2 KHz/grms @0 grms - 5 grms and 3.32 KHz/grms @1 grms - 4 grms for X、Y and Z-axis respectively.