Uncooled Infrared Detectors With Adaptive Response

Woo-Bin Song and Joseph J. Talghader

Uncooled infrared detectors are almost universally designed to maximize sensitivity. Commercial focal plane arrays designed for the 8-12?m wavelength range can easily detect temperature variations in a scene less than 100mK. However, this sensitivity comes at a price. If a scene contains "hot" features along with room temperature features, the "hot" parts of the scene will appear white or overexposed because of the extremely high signal. We have developed a microbolometer device where the responsivity can be actively controlled. The concept is demonstrated in Figure 1, where a small portion of the support beams of a microbolometer are actuated to the substrate. This increases the thermal conductance of the device, which reduces the responsivity.

Figure 2 shows a SEM image of an individual device and Figure 3 shows a small array. We have demonstrated devices with two orders of magnitude of tuning in responsivity as shown in Figure 4 and over an order of magnitude in detectivity as shown in Figure 5. These devices could be used in several applications such as seeing detail in fire emergencies where hot areas cannot be imaged by other means. Military applications to see detail in plumes or conflict sites are also likely.

Figure 1 - Conceptual diagram of a tunable detectivity microbolometer. In diagram (a), the device stands in a state of low thermal conductance and high detectivity. In diagram (b), the device supports have been actuated into contact with the substrate, creating a high thermal conductivity and low detectivity state. In practice only the edge farthest from the ground support contacts the substrate to prevent stiction.

Figure 2 - SEM image of an adaptive responsivity uncooled infrared detector.

Figure 3 - Optical microscope image of a small array of adaptive microbolometers.

Figure 4 - Responsivity versus frequency of an adaptive microbolometer. The responsivity is controlled by the voltage applied between the bolometer plate and substrate.

Figure 5 - Detectivity vs. frequency for an adaptive detectivity microbolometer. The response varies by over an order of magnitude when the lowest device support is snapped down at 17V.