Thin films of vanadium oxide were prepared and studied for the electro-optical properties of semiconductor-metal transition. Vanadium oxide films with thickness of ~ 0.15μm were deposited on SiO2/Si substrates by reactive RF magnetron sputtering using a pure vanadium target under various ratios of argon and oxygen gases. The oxygen content in the mixed atmosphere has a significant influence on the semiconductor-metal transition characteristics. Both thermochromic and electrochromic modes were studied. In thermochromic mode, the oxide film deposited in an O2/Ar ratio of 1.2% exhibits 90% optical transmission in semiconducting state at room temperatures, while very low transmission at 5% in metallic state at 65°C, in the wavelength region of 8 to 12μm. In the near infrared region of 1 to 2μm, the transmission is about 60% in the semiconducting state and a few percents in the metal state. A corresponding three-order variation of resistivity was observed over the transition. The refractive indices (n and k) of the vanadium oxide films were measured using an ellipsometer in the near infrared region between 1 and 2 μm in both states. The index n decreases in metal state while k increases. The electrochromic phase transition of vanadium oxide was investigated by applying a pulsed voltage to minimize the heating effect. The required charge density for the phase transition is consistence with the Mott metal-insulator model. Longwave IR switching and modulation were demonstrated by electrically induced semiconductor-metal transition.
Silicon nitride microbridges (50x50 mm2, 0.6 mm thick), suspended over a silicon substrate, were patterned and thinned. These patterns consist of 2 to 12 windows that were thinned to approximately 0.3 mm. Microbolometers were fabricated by sputtering a YBaCuO thin film over the bridges. The experimental results showed that the regionally thinned microbridges have a lower thermal time constants t (about 1.6 ms) than that of the standard pixel configuration (2.6 ms). On the other hand, the fact that the regionally thinned microbolometers having detectivity D* values comparable to or even six times superior than that of the standard pixel showed that the decrease in response time is not penalized by loss of detection performance. The simulation results also show that as the amount of material removed is increased, the thermal time constant drops significantly while the (τ/G)1/2 ratio (where G is the thermal conductance of the pixel) only decreases slightly, suggesting that the reduced response time will not cause a significant drop in detectivity D*. The simulation results of mechanical integrity show that a specific regionally thinned microbridge design has 22 % higher stiffness than that of a standard pixel design with similar thermal properties. The fact that thick regions remained on the regionally thinned pixels (like the edges of the pixels) provide significant mechanical support to the microstructures. This confirms the validity of the regionally thinned microbridges approach.