矽質微型壓阻式壓力感測器之熱遲滯現象研究

Investigation of thermal hysteresis phenomenon of Si-based piezo-resistive pressure sensor

沈傑(Jie Shen)

指導教授 ： 江國寧

國立清華大學/工學院/動力機械工程學系/碩士(2007年)

摘要

近幾年來，矽質壓力感測器的發展日趨成熟，並應用於許多先進系統中。然而，在現今應用端強調微小化和高精密度的需求下，其各項重要性質的研究更是刻不容緩。因此，為了更精確的量測，和避免感測器對溫度極為敏感而產生的熱遲滯現象，本研究將使用模擬與實驗的方式，觀測由溫度負載所造成熱遲滯電壓的因素，並進行分析與討論。
矽質壓阻式壓力感測器主要是利用微機電製程技術製作而成。其工作原理為藉由施加壓力導致薄膜產生形變；再由壓阻效應原理使得壓阻元件的形變產生電阻值變化，並搭配惠司同電橋電路轉換成電壓變化，以電壓變化反推求得所施加之壓力。然而，壓阻式壓力感測器對外界溫度的敏感度極高，這份特性可能使其準確度降低。當壓阻式壓力感測器受溫度循環負載於導線層產生殘餘應力時，將產生同一溫度於不同受力週期點所量測之輸出電壓漂移，此即稱為熱遲滯現象，而電壓漂移之差值則稱為熱遲滯電壓。為了能減少熱遲滯現象對於壓力感測器的誤差影響，分析鋁導線殘餘應力與熱遲滯電壓的關係成了本研究之主要目標之一。
熱遲滯現象主要是由於在溫度負載下，材料之熱膨脹係數的不匹配和鋁導線的非線性性質交互作用所產生的殘餘應力導致。故本研究將由溫度循環負載中，導入鋁導線的材料非線性性質參數；並以壓力感測器之製程為基礎，以有限單元法模擬壓阻式壓力感測器的熱遲滯現象，並與實驗比對以驗證模擬的可靠度。進而針對修改製程模擬的方式導入蝕刻過程，探討感測器產生熱遲滯電壓的變化情形。此外，本文亦針對潛變效應對壓力感測器熱遲滯電壓的影響，進行一系列的模擬分析與實驗探討。
最後，利用以上熱遲滯現象於壓阻式壓力感測器之影響進行實驗及模擬的分析討論，期望能由研究中所得之結論對日後設計人員有所助益。

關鍵字

矽質壓力感測器 ； 熱遲滯現象 ； 殘餘應力 ； 材料非線性

並列摘要

Silicon piezoresistive pressure sensor technology has recently been gaining ground in its use in many advanced applications. In order to meet the needs of mechanical signal sensing in the industry, the different characteristics of pressure sensors need to be crucially taken into account. Therefore, our research employs simulations and experiments to determine the factors which produce the thermal hysteresis voltage in thermal cycle loadings. The purpose of this is to achieve measurement accuracy and avoid the thermal hysteresis phenomenon.
The silicon piezoresistive pressure sensor is fabricated by the MEMS process, and utilizes the ion implant technique to form the piezoresistors on the silicon substrate. The main principle for operation is that the external pressure loading causes the deflection on the silicon membrane. Then a piezoresistive effect results in a resistance change in the piezoresistor on the silicon membrane. Using the Wheatstone Bridge transforms the mechanical signal to an output voltage in order to obtain the unknown pressure loading. However, the sensitivity of the piezoresistive pressure sensor is relatively high for the environmental temperature, thereby reducing accuracy. Therefore, the drifts of the output voltage in the same temperature result in a residual stress on the aluminum trace under thermal cycle loading. This situation is called the thermal hysteretic phenomenon, and the variation in output voltage is called the thermal hysteresis voltage. Given these, one goal of the current research is to analyze the relation between the residue stress of the trace and the thermal hysteresis voltage in order to reduce the measurement error in the thermal hysteresis phenomenon.
The thermal hysteresis phenomenon is produced by the thermal expansion coefficients’ mismatch with the nonlinear properties of the aluminum trace in the thermal cycle loadings. Furthermore, this research will input the nonlinear properties of aluminum trace in ANSYS® and will base on the process of the pressure sensor to obtain the thermal hysteresis voltage. After several numerical analyses of the thermal hysteresis voltage, experiments will be performed to validate the simulation results. We will revise the process simulation for the etching effect in order to analyze the thermal hysteresis voltage’s differences. The research will also do a series of simulations and experiments on the creep effect that usually affects the thermal hysteresis voltage in the pressure sensor.
To sum up, both the simulations and experiments systematically discuss the hysteresis phenomenon of the pressure sensor. The study’s conclusions will hopefully provide designers with relevant guidelines in the relative field.

參考文獻

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