Thermoelectric infrared microsensor using suspended membranes made by silicon micromachining
Résumé
The main activity of our laboratory is based on the design and the fabrication of thermal microsensors whose infrared radiometers are a significant part. These microsensors work by converting infrared radiation into spatial periodical temperature gradients patterned on substrate. An array of plated bimetallic microthermocouples is designed to produce the output voltage from the maximum generated temperature differences. Until now, planar sensors featuring a metering area from 3 to 100 mm2 have been realized on glass or kapton (submitted for SPIE 2001) substrates. To take advantages of silicon technology, it is necessary to adapt the structure of the sensors to such a material. Indeed, the sensitivity of this kind of sensor is approximately proportional to the thermal resistance between hot and cold junctions. Thus, taking into account the low thermal resistivity of silicon, membranes must be recessed what makes it possible to strongly increase the thermal resistance of the zone located under the junctions of the thermocouples. However, to preserve the advantage of immunity against convection, these membranes must present quite particular properties of symmetry and periodicity compared to the thermocouples. In this case, the microthermopile is constituted of alternatively doped N and P polysilicon layers. Using a simple model, operation principle is analyzed and the microradiometer sensitivity can be computed. The microsensors are manufactured within the framework of a national project (CNRS Interlab) and the successive steps of the technological process are described. Lastly, the first experimental results are presented.