A monolithic CMOS microhotplate-based conductance type gas sensor system is described. A bulk micromachining technique is used to create suspended microhotplate structures. The thermal properties of the microhotplates include a one-millisecond thermal time constant and a 10 :C/mW thermal efficiency. The polysilicon used for the microhotplate heater exhibits a temperature coefficient of resistance (TCR) of 1.067x10-3/:C. Tin(IV) oxide and titanium(IV) oxide (SnO2, TiO2) sensing films are grown over post-patterned gold sensing electrodes using low-pressure chemical vapor deposition (LPCVD). Isothermal responses are measured from these films during exposure to different gas molecules and concentrations. Interface circuit design and implementation are also presented. Bipolar transistors are found to be a good choice for the heater driver, and MOSFET switches are suitable for addressing sensing films. An on-chip operational amplifier produces a robust output signal. Examples of gas sensing results are demonstrated for concentrations down to 100 parts per billion (ppb).
Citation: IEEE Sensors Journal
Pub Type: Journals