Effects of annealing conditions on temperature coefficient of resistance of Pt/AlOx thin-film thermistors

Uncooled microbolometer arrays incorporating vertically aligned carbon nanotube absorbers are increasingly being adopted in satellite instrumentation for monitoring the Earth's radiation budget. A key requirement for such microbolometers is a thermistor having high-temperature coefficient of resistance (TCR), low 1/f noise while surviving high-temperature carbon nanotube growth at 800 degrees C. In the present work, Pt thin-film thermistors are fabricated on SiNx/SiO2/Si substrates using DC magnetron sputtering. To achieve enhanced adhesion of the Pt thin film, an interlayer of AlOx is deposited on the substrate via reactive high-power impulse magnetron sputtering. To maximize the positive TCR, the Pt/AlOx is subjected to different annealing conditions by varying temperature, time, and gaseous environment. With an increase in the annealing temperature and duration, Pt/AlOx thin film exhibits an improvement in TCR. Microstructural and morphological investigations suggest that improvement in TCR is related to the recrystallization of Pt and the resulting increase in grain size. A relatively high TCR of 0.308%/degrees C (TCR of bulk Pt = 0.359%/degrees C) was obtained at an operational range of 20-50 degrees C when Pt was annealed at 800 degrees C for 1 h and 3 h in air and Ar, respectively. Deposition of Pt without an AlOx interlayer resulted in thin film blistering and delamination when annealed at 800 degrees C for 1 h in air. A Pt/AlOx thermistor with a TCR of 0.308%/degrees C, annealed at 800 degrees C for 3 h in Ar has the potential for use in microbolometers that must undergo high-temperature growth of vertically aligned carbon nanotube absorbers.