CMOS monolithic metal-oxide sensor system comprising a microhotplate and associated circuitry

A gas sensor system fabricated in industrial CMOS technology is presented, which includes, for the first time, a microhotplate and the necessary driving and control circuitry on a single chip. Post-complementary-metal-oxide-semiconductor (CMOS) fabrication steps, such as micromachining of the membrane structure, the deposition of noble metal on the electrodes, and the processing of the sensitive metal-oxide layer, have been developed to be fully compatible with the industrial CMOS process. Temperatures up to 350°C were reached on the hotplates using a low-voltage power supply (5 V). A symmetric hotplate design with a temperature homogeneity of better than 2% in the heated area was realized. The integrated temperature controller regulates the membrane temperature with a resolution of ±0.3°C in the tracking mode. The temperature increase on the bulk chip owing to heat transfer through the membrane is less than 2% of the respective membrane operation temperature (6°C at 350°C membrane temperature). The gas sensing performance of the sensor was assessed by test measurements with carbon monoxide (CO). The gas tests evidenced a limit of detection of less than 5 ppm CO.