YBCO Microbolometers for Infrared Detection: Enhancement of Performances with Regionally Thinned Microbridges

Abstract : Silicon nitride micro bridges (50x50 micrometers squared, 0.6 micrometers 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 micrometers. Micro bolometers were fabricated by sputtering a YBaCuO thin film over the bridges. The experimental results showed that the regionally thinned micro bridges have lower thermal time constants wavelength (electronics and physics) (about 1.6 ms) than those of the standard pixel configurations (2.6 ms). On the other hand, the fact that these regionally thinned micro bolometers having detectivity D* values comparable to or even six times superior to those of the standard pixel showed that the decrease in response time does not result in lowered detection performance. The simulation results also show that, as the amount of material removed is increased, the thermal time constant drops significantly while the (wavelength / 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 micro bridge 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) provides significant mechanical support to the micro structure. This confirms the validity of the regionally thinned micro bridges approach.