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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.

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Many mechanical and thermal characteristics, for example the air damping, of suspended micromachined structures are sensitive to the ambient pressure. Thus, micromachined devices such as the gyroscope and accelerometer have different ambient pressure requirements. Commercially available process platforms could be used to fabricate and integrate devices of various functions to reduce the chip size. However, it remains a challenge to offer different ambient pressures for micromachined devices after sealing them by wafer level capping (WLC). This study exploits the outgassing characteristics of the CMOS chip to fabricate chambers of various pressures after the WLC of the Si-above-CMOS (TSMC 0.18 µm 1P5M CMOS process) MEMS process platform. The pressure of the sealed chamber can be modulated by the chamber volume after the outgassing. In other words, the pressure of hermetic sealed chambers can be easily and properly defined by the etching depth of the cavity on an Si capping wafer. In applications, devices sealed with different cavity depths are implemented using the Si-above-CMOS (TSMC 0.18 µm 1P5M CMOS process) MEMS process platform to demonstrate the present approach. Measurements show the feasibility of this simple chamber pressure modulation approach on eight-inch wafers.