Cooler-free operation (up to 120^oC) of microbolometers based on amorphous VO_x thermistor deposited at different growth temperatures

The recent growth of the autonomous driving market increased demands for IR image sensors due to the detection capability in the absence of visible light, e.g., night, rainy, or foggy conditions. Since the cost-effectiveness is essential for commercialization of autonomous vehicles, cooler-free operations of IR detectors at high ambient temperature for autonomous vehicles is in high demand. Microbolometers are the most widely used thermal detector in commercial thermal cameras because of its advantages like portability and low cost. However, the upper limit of conventional microbolometers is approximately 70^oC since the temperature coefficient of resistance (TCR) is lower than 1 %/K of TCR value that is required for ROICs in microbolometers. Thus, high temperature coefficient of resistance (TCR) value and low resistance is required.<br/><br/>In this work, we report on the amorphous vanadium oxide thin films as thermistor materials for microbolometers. Vanadium oxide thin films were deposited using pulsed DC sputtering processes under different growth temperatures from 160^oC to 230^oC. We also studied their electrical properties and effects on the performances of fabricated microbolometers. These processes were carried out under 350^oC which is suitable for ROIC integration without any doping or annealing process. XRD analysis was carried out to identify the structural properties of thin films and the results show that thin films grown at different temperatures were amorphous phases. The sheet resistances of thin films decrease from 330 to 70 kΩ/sq with increasing growth temperature. XPS analysis confirms that it is due to the increase of oxygen vacancies at higher growth temperature. Amorphous VO_x thin films show non-hysteresis behavior up to 120^oC, which in turn shows clear activation energies. The TCR values at room temperature and activation energies of the films increased from 2.44 to 2.8 (-%/K) and from 0.145 eV to 0.190 eV by decreasing growth temperature. These thin films also showed higher than 1 %/K of TCR value even at 120^oC. Amorphous VO_x based microbolometers have been fabricated using electron-beam lithography and lift-off techniques. Characterizations of fabricated microbolometers were carried out under various ambient temperatures from room temperature to 120^oC. Responsivities of fabricated microbolometers based on the amorphous VO_x thermistors change from 1.14 × 10³ V/W to 3.04 × 10³ V/W at room temperature and from 3.02 × 10² V/W to 9.54 × 10² V/W at 120^oC. The fabricated microbolometers showed high frequency operation properties as well. Thermal time constants were from 1.38 to 0.95 ms at room temperature and from 1.77 to 1.22 ms at 120^oC which show that the microbolometers are suitable for high-speed camera applications such as autonomous vehicles.<br/><br/>This work reveals the correlations between growth temperatures of vanadium oxide thin films and their electrical properties. Amorphous VO_x based microbolometers showed good performance not only at room temperature but also at high ambient temperature up to 120^oC without any cooling device. The results of this work will be highly encouraging for the development of IR detector for autonomous vehicle.