CMOS thermoelectric infrared sensors

Infrared sensors manufactured with the fabrication technology for integrated cir¬ cuits fulfill the demand for inexpensive, reliable devices compatible with on-chip signal processing circuitry. We report thermoelectric infrared sensors fabricated using a standard industrial CMOS process, combined with a subsequent micromachining step for thermal isolation of the sensing structures. We cointegrate analog and digital circuitry to amplify the weak sensor signal, and process the signal with application specific circuitry to obtain a "smart sensor" system. Infrared sensors are important elements in a number of industrial applications. One such product is a passive intrusion alarm detector. The thermal radiation emitted by a human intruder and collected by a mirror system is detected from which an alarm signal is generated. The goal of this work is to develop a thermo¬ electric infrared sensor for this purpose. We require a very sensitive sensor, as temperature differences of 3 K at a distance of 15 m have to be resolved. This implies that on the sensor itself, a temperature difference of a few mK between the radiation absorbing structure and the environment must be measured. To detect this small temperature difference we use thermocouples fabricated with the mate¬ rials provided by industrial CMOS technology, such as aluminum and nand p-doped polysilicon. The thermocouples provide an output voltage of more than 300 uV/K. To obtain a larger output signal, several thermocouples are connected in series to form a thermopile. To attain good thermal isolation of the sensing structure, the thermopile is placed on a 3 to 4 (jm thin cantilever beam or mem¬ brane, which consists only of the CMOS dielectric layers possessing a low thermal conductivity of about 2 to 3 }iW/mK. We demonstrate that the materials used for the passivation of CMOS circuitry may be utilized to absorb infrared radiation, as they show strong absorption bands in a wavelength range of 8 to 14 |jm. About 30% to 40% of the incoming radiation can thus be absorbed and converted into heat, i.e. a temperature increase, without using a non-CMOS material. We investigated a number of different thermoelectric sensor types aimed at high responsivity, high yield and low fabrication cost. The experimental investigations