تجزیه و تحلیل حساسیت از اثر بسته بندی سنسور فشار piezoresistive مبتنی بر سیلیکون

The silicon-based pressure sensor is one of the major applications in the MEMS device. Nowadays, the silicon piezoresistive pressure sensor is a mature technology in the industry, but its requirement in terms of sensing accuracy and stability is more rigorous than that of many advanced applications. The major factor affecting the sensing stability of the piezoresistive pressure sensor is its thermal and packing effects. For a packaged pressure sensor, silicone gel is usually used to protect the die surface, so the thermal and packaging effects caused by the silicone gel should be taken into consideration to obtain better sensing sensitivity and stability. For fast design and optimization purpose, a finite element method (FEM) is adopted for sensor performance evaluation, packaging-induced signal variation, and thermal/packaging effects will be examined in this research. Several experiments are also performed to validate the finite element model. After the simulation is validated, an optimization analysis is carried out under different packaged pressure sensor design parameters. The simulation results show that the different geometry of the protection gel will influence pressure sensitivity significantly; base on analysis results, this research will conclude a design guideline for pressure sensor packages with concave and convex type of protection gel.

Since the piezoresistive effect was discovered, the applications of piezoresistive sensors have been widely employed in mechanical signal sensing. The fundamental concept of the piezoresistive effect is the change in resistivity of a material resulting from an applied loading. This effect in silicon material was first discovered by Smith [1] in the 1950s and has been applied extensively in mechanical signal measurement for years. Smith proposed the change in conductivity under stress in bulk n-type material and designed an experiment to measure the longitudinal as well as transverse piezoresistance coefficients. Pfann et al. [2] presented the shear piezoresistance effect; they designed several types of semiconductor stress gauges to measure longitudinal, transverse, and shear stresses and torque. In addition, a Wheastone bridge type gauge in mechanical signal measurement is employed. On the other hand, piezoresistance coefficient is a function of impurity concentration and temperature; hence the thermal effect will influence the measurement result of a piezoresistive sensor. Piezoresistive pressure sensor design was widely studied in the 1990s in the MEMS and electronic packaging fields. Jaeger et al. [3] and [4] employed piezoresistive sensors made on silicon chips to measure the stresses within electronic packaging devices. Kanda [5] applied the MEMS process to fabricate piezoresistive pressure sensors on {1 0 0} and {1 1 0} wafers for optimum design considerations. Recently, the FEM has been widely adopted for stress prediction, thermal effect reduction, packaging design and reliability enhancement of piezoresistive sensors. Pancewicz et al. [6] used FEM to obtain the output voltage of the pressure sensor, and compared the simulation data with experimental results. Schilling et al. [7] also applied FEM analysis for sensor performance simulation and discussed the packaging effects on the silicon piezoresistive pressure sensors. Moreover, Peng et al. [8] and [9] used FEM demonstrated a promising result for the prediction of sensor performance. For the application of FEM in the optimum design of the pressure sensor, Krondorfer et al. [10] used FEM software to predict thermal effect resulting from the fabrication processes of pressure sensor packaging. To operate the piezoresistive pressure sensor in a harsh environment, silicone gel is usually required to protect the die surface. However, some design parameters such as silicone gel that will affect sensor sensitivity and stability were not discussed in previous research. For this reason, the material and geometry of silicone gel are considered in this study to investigate thermal and packaging effects. Furthermore, to achieve better sensor performance, FEM parametric and factorial design analysis is performed. The design parameters include silicon chip length and thickness, membrane length, silicone gel thickness and material, and so on.

In this study, the pressure sensor with silicone gel to protect die surface is investigated. Base on the validated FEM results, the analysis procedures for pressure sensor proposed in this research is found to be feasible. In addition, the procedure is a reliable tool for the design of sensor performance. The parametric study shows that a larger Young's modulus of silicone gel can enhance sensor stability. However, if the peripheral boundary of silicone gel is constrained, excessive values of Young's modulus will give an extreme pressure on the silicon membrane, thus inducing a deformation of silicon membrane. The analytic results of gel thickness effect under different temperature situation show an unobvious variation in output voltage when the silicone gel is not constrained. On the other hand, when the peripheral condition of silicone gel is constrained, the thicker silicone gel makes a better stability of sensor performance. However, the excessive thickness of gel will influence the sensitivity and stability of packaged pressure sensor. Compared to these two models, model2 shows a batter stability performance than model1. In the abovementioned case, either thinner or larger membrane can upgrade sensor sensitivity. In addition, a thicker or a shorter silicon chip can enhance sensor stability. Moreover, using the factorial analysis, the results show that the main effect factors of package effect include die thickness and die length when the boundary condition of silicone gel is not constrained. However, if the peripheral condition of the silicone gel is constrained, numerous factors that mainly influence the packaging effect of pressure sensor, such as the membrane length, gel thickness, and Young's modulus of silicone gel, give a large contribution to the output voltage. Based on the above findings, it can be concluded that the proper selection of sensor structure in terms of geometry and the composition of silicone gel material not only enhance sensor sensitivity but also reduce the thermal effects and the packaging influence.