CMOS Compatible Midinfrared Wavelength-Selective Thermopile for High Temperature Applications

In this paper, the design, fabrication, and testing of a CMOS compatible mid-infrared (mid-IR) thermopile sensor comprising a stacked double layer thermopile and an interferometric absorber is reported. The devices are all fabricated in CMOS compatible process. The length and width of the thermocouple are 600 and 12 μm, respectively. One thermopile consists of 96 thermocouple pairs. The thickness of the poly-Si strips is 300 nm and the thickness of the SiO₂ electrical isolation layer is 150 nm. The interferometric absorber is formed by a stacked three layer of 7-nm TiN, 250-nm amorphous silicon, and 300-nm Al. The measurement using blackbody with temperature of 470 °C was conducted at ambient temperature from -50 °C to 300 °C. The results show that the responsivity and the detectivity of this thermopile mid-IR sensor vary from 130 to 463 V/W and 3.8 · 106 cm · Hz^1/2 · W^-1 to 1.3 · 107 cm · Hz^1/2 · W^-1, respectively. And the time constant of the mid-IR sensor is 33 ms. The maximum output voltage is measured at 190 °C, where the responsivity and the detectivity are 425.7 V/W and 1.25 · 107 cm · Hz^1/2 · W^-1, respectively.