| Humidity sensors are now increasingly used in both commodity and industry applications. With the rapid development of silicon-based frabrication technology, especially the CMOS process and MEMS technology, it's now possible to realize a complete integrated humidiy-sensoring system with both the MEMS sensors and the CMOS signal-handling circuitry fabricated on a single chip. This integration techonology, called the CMOS compatible integration process, has now become an important subject of interest in recent studies.Knowledge background and development history of both MEMS humidity sensors and MEMS packaging technologies are presented at the beginning of this paper. The packaging techniques of humidity sensors are then introduced as the major topic of this paper. Recent works worldwide on these subjects are summurized and compared, and their future developments are predicted.In this paper, a new packaging technique of CMOS compatible humidity sensor is proposed. The technique employs a modified SOP package with a profoudly recessed ceramic substrate and a plastic cap with via-holes as the humidty interface. A special PVDF membrane are attached on the innner surface of the cap as a filter against particles and other contaminations in the air. The humidity sensor are attached, wire-bonded and sealed to finish the package.The packged humidity sensors are carefully measured and tested to obtain their major characteristics, such as range, linearity, sensitivity, delay, temperature coefficient, and so on . The sensors can present near-linear responses in their allowable 15% - 95%RH enviorment. The temperature coefficient of the sensor capacitance is 0.092%/℃. Their maximum response delay at 85%RH is 10%.Several viable signal-handling circuitry are presented at the end of this paper, and each is tested to select the best one for the sensor system. Results show that the output frequency ratio can reach 9.68Hz/%RH, which is an reasonable value for most applications. |
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