A novel CMOS-compatible polysilicon/titanium thermopile

In this paper, we present a 0.8-μm 1P2M CMOS process compatible polysilicon/titanium thermopile with a gold black absorption layer. Instead of an aluminum layer of 0.6 μm thickness in the first metallization process, a titanium layer of 0.1 μm thickness was introduced to the fabrication of a CMOS compatible thermopile in order to enhance sensitivity by lowering the thermal conductance of the sensor. After the CMOS process, a 1.2-mm-square floating thermopile structure was formed by using a front-side anisotropic etching process in dual-doped tetra-methyl ammonium hydroxide (TMAH) solution, in which the etch rates of aluminum thin film are relatively low. The experimental data reveal that the etch rate of a thermally evaporated aluminum film could be smaller than 10 Å/min in 5 wt.% TMAH solution with the addition of 4 gm/l ammonium peroxodisulfate (AP) and 24 gm/l silicic acid. Eventually, a 0.8-mm-square porous gold black layer was thermally evaporated upon the sensor surface to serve as an infrared absorption layer. The FTIR spectrum shows that the absorber has near perfect absorptance in the wavelength range of 5 to 15 μm. The completed thermopile sensor consists of 96 poly-Si/Ti thermocouples pairs and has a resistance of 43.5 kilo-ohms. For reducing environment noises, the sensor chip was encapsulated in a TO5 metal-can with an 8-14 μm filter. The measurement result indicates the sensor has a thermal time constant of 103 ms in atmosphere and 173 ms in a pressure of 16 mTorr respectively. In addition to an increasing time constant, the output voltage also raises by a factor of 2.3 in vacuum environment.