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several kinds of micro-electro-mechanical systems are sensitive to pressure. Some need to interface to ambient condition in order to aim intended function, but others claim hermetic packages to keep the constant internal pressure over MEMS devices operation time [1][2]. This study presents the novel method to control the pressure level of different chambers fabricated using the same wafer-level-packaging (WLP) process. As indicated in Fig. 1, due to the out-gassing of CMOS and MEMS chips both, the chamber pressure of a hermetic sealed MEMS device can be modulated by varying the cavity depth of the cap Si substrate. Thus, pressure of hermetic sealed chambers can be easily specified by the etching depth of capped Si. In applications, the Si-above-CMOS (TSMC 0.18μm 1P5M CMOS process) MEMS process platform has been employed to demonstrate the present approach. The fabrication results demonstrate that the chamber pressure is modulated by the cavity depth of Si cap.

The implementation of differential Pirani gauge for accurately measuring wafer-level package pressures was demonstrated. We proposed a multiple-sensor-solution, where two Pirani gauges were constructed under different pressures; one in sealed micro-cavity for measuring pressures and the other one in opened micro-cavity as a reference. Ambient pressure, structural dimension variations, and resistivity differences among wafers/lots, were captured through the differential scheme for error compensations, allowing accurate pressure determinations. Presented Pirani utilized small gaps (~2μm) between heater and dual heat sinks to obtain wide operation range (0.05~100 Torr) and high sensitivity (~10000 ppm/Torr). With 5X error reductions and high stabilities, the proposed device was successfully used in examining reliabilities and monitoring processes of wafer-level packages.

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This article presents a side electromechanical instability of slender comb-fingers in MEMS electrostatic devices. Two models are developed for predicting such side pull-in of comb-fingers; one is based in the lumped modeling technique and another is developed by FEM approach. In the slender finger array, the deflection of every finger and pull-in voltage can be calculated by the models. Results indicate every comb-finger bends with different amplitudes. In the finger array, few sided fingers are strongly deflected, the extreme sided fingers in particular, while other inner fingers show negligible deformations. We suggest these extreme sided fingers can dominate the side pull-in of the comb-fingers and should be considered with careful device design by employing the proposed models in this paper.