具應力隔絕結構之三軸壓阻式加速度感測器設計與實現

Design and Implementation of Three-Axis Piezoresistive Accelerometer with Stress Isolation Structure

謝協伸

指導教授 ： 方維倫

國立清華大學/工學院/動力機械工程學系/博士(2012年)

摘要

以微機電技術為基礎之三軸加速度感測器在偵測三軸加速度量值時需要與外界環境有物理上的實質連接；而三軸加速度感測器本質上是一個力量的感測器。故為了增進感測器的精準度，對於隔絕影響感測器輸出的環境外力或是溫度造成的應力干擾將是一個重要的課題。因此，本文將設計一具有應力隔絕結構的感測器，以實現隔絕環境外力或是溫度造成的應力干擾之效果。
本文設計一應力隔絕結構來改善習知三軸加速度感測器的性能。因此，三軸加速度感測器因環境干擾(如外力傳遞或是溫度變化)的性能改變可以明顯的降低。為了驗證其應力隔絕的效果及製程的可行性，本文選擇三軸壓阻式加速度感測器為研究對象，並成功的利用體微細加工技術對於SOI晶片製作出具應力隔絕結構之三軸壓阻式加速度感測器。實驗結果顯示在具有懸浮彈簧及質量塊的感測器被加熱到環境溫度150 ℃時，其出平面變形由0.72 um降為0.1 um。感測器的0g偏差溫度飄移及靈敏度溫度飄移成功的被降低；而對於環境外力或是變形的傳遞所造成的干擾訊號輸出也成功的降低一個數量級。
除此之外，本研究進一步分析及探討以環氧樹脂封裝具應力隔絕結構之三軸加速度感測器的特性。研究結果顯示具有應力隔絕結構之三軸壓阻式加速度感測器可成功的降低因環氧樹脂塑膠封裝製程殘留應力所造成的輸出訊號飄移(約降低一個數量級)。此外，此三軸加速度感測器依然維持習知感測器的性能，靈敏度為0.12 ~ 0.17 mV/V/g， 而非線性小於 1.02 %。
最後，本文進一步分析及探討具應力隔絕結構之三軸加速度感測器的應力隔絕結構幾何尺寸及應力隔絕性能之最佳化。由分析結果可知，具最佳化設計參數組合之三軸壓阻式加速度感測器，其應力隔絕結構的尺寸變成98 um (守護環結構的長/寬及連接橋的長的和，先前為138 um)；並再進一步降低三軸壓阻式加速度感測器因為外框變形所產生的訊號飄移一個數量級。

關鍵字

三軸加速度感測器 ； 壓阻式加速度感測器 ； 加速度感測器結構設計 ； 環境應力隔絕 ； 封裝應力隔絕 ； 結構最佳化 ； 有限元素分析

並列摘要

This study designs and implements a stress isolation guard-ring structure to improve the performances of the existing accelerometers. Thus, the environment disturbances, such as temperature variation and force/deflection transmittance, for a packaged accelerometer are significantly reduced. In application, the 3-axis piezoresistive accelerometer has been fabricated using bulk micromachining process on the SOI wafer. Experiment results show the out of plane deformation of the suspended spring-mass on packaged accelerometer is reduced from 0.72 um to 0.10 um at a 150 ℃ temperature elevation. The temperature coefficient of zero-g offset for the presented sensor is reduced, and the temperature-induced sensitivity variation is minimized as well. Measurements also demonstrate the guard-ring design successfully reduces the false signals induced by the force and displacement transmittance disturbances for one order of magnitude.
The accelerometers with guard-ring structure is further capped with glass substrate to form the glass/Si/glass sandwich and then encapsulated in plastic package. The testing results on these packaged accelerometers have shown that the guard ring structure successfully suppresses the performance shift caused by plastic packaging process for one order of magnitude. Thus, the inexpensive plastic encapsulated package for accelerometers can be implemented on the real products. Moreover, the 3-axis acceleration sensing for the presented accelerometer with guard-ring has also been demonstrated with sensitivities of 0.12 ~ 0.17 mV/V/g and non-linearity < 1.02 %.
Finally, this study further reports an optimum design to shrink the size of guard-ring, yet maintain the performance of accelerometer. Under the assistant of FEM and Taguchi method, the performance of the accelerometer is improved both in offset shift and sensitivity shift for one order of magnitude, and the size of the stress isolation structure (the sum of guard-ring length/width and connection bridge length) has been shrunk for 29 % (from 138 um to 98 um). Moreover, the unwanted higher vibration modes are far away from the first three modes. The proposed accelerometer design keeps the advantages of the original 3-axis accelerometer design.

參考文獻

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