A low-power, micromachined, double spiral hotplate for MEMS gas sensors

This paper presents the design, fabrication and reliability testing of a double spiral platinum-based MEMS hotplate for gas sensing applications. The structure of MEMS hotplate consists of a 0.7 mu m-thick thermally grown SiO2 membrane of size 120 mu m x 120 mu m over which a double spiral platinum resistor is laid out. The hotplate membrane is supported by its four arms connected to the Si-substrate. The design and simulation of the hotplate structure was carried out using MEMS-CAD Tool COVENTORWARE. Based on the design, a double spiral platinum resistor of 103 Omega is fabricated on SiO2 membrane using lift-off technique. The platinum deposition is carried out using DC sputtering technique. Bulk micromachining of Si is done from front side of the structure to create the suspended SiO2 membrane. The temperature coefficient of resistance (TCR) of platinum is measured and found to be 2.19 x 10(-3)/ degrees C. The TCR value is used for temperature estimation of the hotplate. The test results show that the microhotplate consumes only 20 mW power when heated up to 500 degrees C. For reliability testing of fabricated structure, the hotplate is continuously operated at 300 degrees C for 1.8 h. Also, it can sustain at least 61 cycles pulse-mode operation at 530 degrees C with ultra-low resistance and temperature drifts. The structure can sustain a maximum temperature and current of 611 degrees C and 11.55 mA respectively without any damage.