Properties of InGaAs/InP thermoelectric and surface bulk micromachined infrared sensors
Applied Physics Letters
We present a concept for the realization of InGaAs/InP micromachined thermoelectric sensors. The advantages of InGaAs lattice matched to InP combine perfectly for this application. The high selectivity of wet chemical etching of InP against InGaAs is ideally suited for surface bulk micromachining. Thermoelectric InGaAs sensors profit from the high thermal resistivity combined with high electrical conductivity and Seebeck effect. Thanks to the material parameters a responsivity of 257 V/W and
... y of 257 V/W and relative detectivity of 6.4ϫ10 8 cm Hz Ϫ1/2 /W are expected for infrared sensors. Long-wave, infrared thermal radiation provides useful information regarding the surface temperature of an object and allows its characterization by contactless techniques. Silicon-and polysilicon-based technology serves the purpose using bulk micromachining 1,2 and offers CMOS IC compatibility. Another approach that has recently been explored is the use of AlGaAs/GaAs micromachined sensors that are compatible with MESFET technology. 3 We report here a novel possibility for realizing high performance sensors using InP-based III-V technology. This approach provides the possibility of high performance operation while combining the advantages of simple fabrication procedures due to the highly selective etching properties of InP/InGaAs. The principle of operation of thermal infrared sensors is based on the conversion of infrared radiation into heat. To measure this heat quantity it is necessary to monitor the temperature increase of the radiation absorber. Hence, thermal isolation is required for enhanced temperature differences and the resulting changes with respect to the ambient can be measured with the thermoelectric effect. Thermal isolation can be achieved by the fabrication of thin floating membranes that carry the absorber. The supporting arms of the floating membrane itself can be the thermocouples-similar to the free-standing thermopile concept realized in AlGaAs. 4 This system is operational at room temperature and in principle at any other temperature. One needs, however, to ensure that the temperature of the object to be measured is well above the sensor temperature so that the resulting signal can be detected. The absorber is ideally a blackbody providing the advantage of broadband response. In terms of wavelength such a sensor can detect wavelengths only below 10 m when its temperature is 300 K ͑Wien's law͒. The material parameter and design requirements can be derived by considering the measurement principle. 3 The key material parameters are the thermal resistivity W of the supporting bridges, the Seebeck coefficient S, and electrical re-sistivity of the thermopile assuming that the thermopile and supporting bridge are one and the same.