A CMOS-MEMS Fluorescence Quenching Gas Sensor Encapsulated with Silicon-Based LED Reflector

This study demonstrates a simple approach to enhance the sensitivity and reduce the power consumption of a fluorescence quenching based gas sensor using a silicon-based encapsulation as the optical reflector (Fig. 1). The design consists of a CMOS-MEMS gas sensor, a blue LED, and a silicon optical reflector integrated using the bonding technologies (Fig. 1a). Moreover, the gas sensor containing the vertically integrated photo sensor, resistive temperature detector (RTD), thermal isolation trenches, and cavities for gas sensing materials is implemented using the TSMC $0.35\mu\mathrm{m}$ CMOS process (Fig. 1b). The gas concentration is detected on the basis of the fluorescence intensity measured by the photo sensor on the CMOS platform [1] [2]. This design has three merits: (1) simple approach to integrate the optical reflector to sensing chip to enhance the performances of gas sensor, and wafer level packaging can be achieved for future applications, (2) the reflector could reflect and redirect the high intensity emitting light from the top-side of LED to enhance the sensitivity of gas sensor (Fig. 1b), and (3) the power consumption of the LED as well as the gas sensor can be reduced. In applications, the concentration of O2 is measured. Measurement results show the sensitivity of gas concentration for the proposed design with reflector and the reference design without reflector are $0.26\mu\mathrm{A}/\%$ (O2/N2) and $0.019\mu\mathrm{A}/\%$ (O2/N2) respectively. Moreover, the LED driving current reduced from 100 mA to 20 mA by adding the reflector (reducing power consumption for blue-LED).