壓阻式十字樑結構微型壓力感測器之研製

Abstract

本論文旨在設計一高穩定度壓力感測器，以微機電製程之體型加工技術，在p型的SOI矽晶圓上製作壓阻式微型壓力感測器，在感測壓力的方型隔膜上另貼附十字型橫樑薄膜，以增進感測器的線性度；隔膜背面為壓力作用凹槽空間，以深離子反應蝕刻機進行深度蝕刻。此微型壓感器的製程容易掌控，敏感度高，訊號處理簡潔，可達到良好的線性度。 本文探討的重點為隔膜的面積與厚度、壓阻器的大小與位置、矽晶圓的摻雜濃度等參數對靈敏度的影響。經初步探研後，設計三種隔膜配合五種壓阻器尺寸，總共五種構型。首先利用電腦模擬軟體計算施加壓力時，隔膜的應力響應、壓阻器的應變。由數值模擬的結果顯示：薄膜面積2300  2300 m2、厚度為20 m時，會有最大的應力響應，其最大應力為178.05 MPa，靈敏度為8.75 mV/V/bar；驗證壓力計所承受的應力在材料的安全強度300 MPa內，並可輸出達到量測準確度的電壓。 SOI矽晶圓的組件層表面經過熱蒸鍍鋁薄膜，製成金屬導線；接著以深離子反應蝕刻，定義壓阻器與隔膜。晶圓製作完成後，經切割打線，封裝於螺絲頭後，即進行壓力測試與校準。經過多次的實驗量測與數據分析，驗證這些壓力感測器可在0~1.5 bar的範圍內正確運作，且歷次敏感度差值不超過0.4 mV/V/bar，具有重複性；實驗結果顯示壓力計的敏感度在4.93 ~ 9.98 mV/V/bar之間，最高達9.98 mV/V/bar，最佳非線性度為0.33% FS；量測敏感度可藉由調整感測電路的放大倍率，達到方便換算的數值。 本文所研發的壓力感測器經測試後，可應用於一般環境的壓力量測。由於感壓計是以半導體製程研製，可以批次作業量產，單位成本遠低於以傳統機械加工方式製作的產品。再者，由於壓力感測的體積小，質量輕，因此可縮減構裝尺寸，且更耐衝擊，整合於控制電路模組將更具方便性。

Highly stable pressure sensors were developed in this study. The sensors were fabricated via bulk micromachining technology for micro-electromechanical systems. The sensors are designed with piezoresistors situated at the edges of the square diaphragm on a p-type device layer of a silicon-on-insulator wafer. To improve linearity, cross beam membrane are attached on the deformable diaphragm. The backside cavity of the diaphragm was etched by a deep reactive ion etcher up to the designated thickness. This configuration is easy to fabricate, and would possess high sensitivity, simple signal processing, and can achieve good linearity. This research focused on the sensing diaphragm area and thickness, sizes of piezoresistors and their locations, and effect of the doping concentration of the device layer on the measuring sensitivity. After preliminary explorations, five discrete models involving three areas in diaphragm and five sizes in piezoresistor were setup for numerical simulation coupling mechanical and electrical fields. The computational results provide sufficient information to verify that the respondent stresses are within the allowable strength of material and the piezoresistor bridge delivers significant voltage output for accurate measurement. The simulation results also evidence that the diaphragm size of 2300  2300 m2 and thickness of 20 m2 would have a most significant response stress of 178.05 MPa, a sensitivity of 8.75 mV/V/bar. It is obviously that the maximum respondent stress is well within the allowable strength of the material of 300 MPa. With validation of the designs by numerical simulation, five models of pressure sensor were fabricated by semiconductor process technology on a SOI wafer. The processing consisted of E-beam aluminum deposition and lift-off for metal wires, deep etching to define piezoresistor, central bosses, and pressure sensing cavity on the backside. Upon finishing the manufacturing, cutting, and packaging processes, detail calibration were carried out to characterize sensitivity and linearity of the pressure sensor. The correlated data demonstrate that these sensors can stably and accurately measure pressure in the range of 0 to 1.5 bar with a sensitivity of 4.93 ~ 9.98 mV/V/bar. The deviations of sensitivity between test runs were within 0.4 mV/V/bar which shows good repeatability. These sensors also exhibit a very good nonlinearity of 0.33% per full scale output. The output voltage can be magnified by tuning a resistor of the input instrumentation amplifier to a convenient reading corresponding to actual applied pressure. The developed modular pressure sensor and actuator can be integrated into modern instrumentations as well as for intelligent appliances. Since the MEMS processing is compatible with the CMOS manufacturing, the present micro pressure module can be integrated with the other electric control circuits.