Design And Fabrication Of The CMOS Compatible Micro-hotplate Gas Sensor

Due to the merits of small size, low power consumption, rapid thermal response and easy integration, micro-hotplates (MHPs) are widely applied to vacuum sensor, microcalorimeter, micro gas sensor and other microdevices. Hence, the micro-hotplate gas sensor has become an important research direction among microdevices. In this thesis, the CMOS compatible micro-hotplate gas sensor has been studied in terms of the manufacturing process design, device fabrication and performance testing.Firstly, the basic structure and technological process of the CMOS-compatible micro-hotplate gas sensor are designed from the view of machinability. In consideration of the compatibility with CMOS and the stability of gas sensor, the tungsten plug between the first metal and the second metal in the CMOS processes is served as the heating resistor and the thermistor. Moreover, noble metal electrodes are prepared on top of aluminum electrodes, because aluminum electrodes tend to form an oxide at high temperature for a long time, leading to a poor electrical measurement. Taking into account the adiabatic design of micro-hotplate and the preparation process of sensing films, the front side micro-machining process is introduced to achieve the releasing of micro-hotplate. According to our laboratory conditions, two sets of feasible Post-CMOS processes are proposed.Secondly, based on the above design scheme, the CMOS-compatible micro-hotplate gas sensor is achieved by CMOS foundry and a key research and optimization of the Post-CMOS processes are performed. The fabrication of device can be divided into CMOS processing and Post-CMOS processing. The device is accomplished by the process of CMOS foundry followed by the Post-CMOS process including the releasing of free-standing membrane, the deposition of noble metal on the electrode and the processing of sensitive film. The issues for two different post-CMOS processes are analyzed and discussed and then an optimized whole process is proposed, improving the processing efficiency and yield. These works provide the basis for further study on the single-chip integration of gas sensors with integrated circuits.Finally, the performance test of the manufactured CMOS compatible micro-hotplate gas sensor is carried out, including tungsten resistor properties, thermal characteristics, and gas sensitivity. Measurements indicate that the tungsten resistor has a satisfying process consistency with the majority of the resistance among210Ω～230Ω, and a large temperature coefficient of resistance (1.9‰/℃), which means that the tungsten can be served as a heater and thermometer. The tungsten resistor exhibits excellent high temperature stability, when operating at300℃under constant heating mode for336h, the variation of the resistance is smaller than0.3%. The gas sensor has the characteristic of low power consumption about only19mW at300℃and rapid thermal response of only8ms for heating up to300℃. After the deposition of sensing films, a good response is achieved for50ppm alcohol. The above measurements entirely meet the design aims of CMOS-compatible micro-hotplate gas sensor, and verify the rationality and feasibility of the structure design and process scheme.