Anal. Chem. 2004, 76, 4437-4445
Metal Oxide-Based Monolithic Complementary Metal Oxide Semiconductor Gas Sensor Microsystem Markus Graf,*.† Diego Barrettino,† Stefano Taschini,† Christoph Hagleitner,‡ Andreas Hierlemann,† and Henry Baltes†
Physical Electronics Laboratory, Swiss Federal Institute of Technology, ETH Zurich, HPT H8, CH-8093 Zurich, Switzerland
A fully integrated gas sensor microsystem is presented, which comprises for the first time a micro hot plate as well as advanced analog and digital circuitry on a single chip. The micro hot plate is coated with a nanocrystalline SnO2 thick film. The sensor chip is produced in an industrial 0.8-µm CMOS process with subsequent micromachining steps. A novel circular micro hot plate, which is 500 × 500 µm2 in size, features an excellent temperature homogeneity of (2% over the heated area (300-µm diameter) and a high thermal efficiency of 6.0 °C/mW. A robust prototype package was developed, which relies on standard microelectronic packaging methods. Apart from a microcontroller board for managing chip communication and providing power supply and reference signals, no additional measurement equipment is needed. The onchip digital temperature controller can accurately adjust the membrane temperature between 170 and 300 °C with an error of (2 °C. The on-chip logarithmic converter covers a wide measurement range between 1 kΩ and 10 MΩ. CO concentrations in the sub-parts-per-million range are detectable, and a resolution of (0.1 ppm CO was achieved, which renders the sensor capable of measuring CO concentrations at threshold levels. In recent years, there have been increasing interest and development efforts in miniaturizing gas sensors and systems. Particular endeavors have been made to monitor environmentally relevant gases, such as carbon monoxide (CO), methane (CH4), and ozone (O3). Commonly used chemically sensitive materials for these target gases are wide-band-gap semiconducting oxides, such as tin oxide, tungsten oxide, or indium oxides, which are operated at elevated temperatures of 200-400 °C.1-3 At those high temperatures, the oxides show considerable resistance changes upon exposure to a multitude of inorganic gases and volatile organics. The most prominent example is tin dioxide (SnO2), which shows large resistance changes upon exposure to the above* E-mail:
[email protected]. † Swiss Federal Institute of Technology. ‡ Now at IBM Research, CH-8803 Ru ¨ schlikon, Switzerland. (1) Madou, M. J.; Morrison S. R. Chemical Sensing with Solid State Devices; Academic Press: New York, 1989. (2) Go ¨pel, W.; Reinhardt, G. Sensors Update; Baltes, H., Go¨pel, W., Hesse, J., Eds.; Verlagsgesellschaft mbH: Weinheim, Germany, 1991, Vol.1. (3) Heiland, G.; Kohl, D. In Chemical Sensor Technology; Seiyama, T., Ed.; Elsevier: Amsterdam, 1988. 10.1021/ac035432h CCC: $27.50 Published on Web 06/24/2004
© 2004 American Chemical Society
mentioned gases at operating temperatures between 250 and 350 °C and has been engineered and provides long-term stability over months.4-6 The miniaturization efforts in the context of metal oxide-based gas sensors follow several major trends: (a) development of micromachined sensor platforms,7-10 (b) micro- and nanotechnological fabrication of the sensing materials,11,12 and (c) design of application-specific integrated circuits with smart features.8,13-15 During the past several years, so-called “micro hot plates” have been developed in order to shrink the overall dimensions and to reduce the thermal mass of metal oxide gas sensors.7,9,16 They consist of a thermally isolated area with a heater structure, a temperature sensor, and contact electrodes for the sensitive layer. By using such microstructures, high operation temperatures can be reached at comparably low power consumption (
