Uncooled thermal MWIR imagers for high-temperature imaging applications

Microbolometers are well-established sensing elements for uncooled thermal imaging applications. Benefits in both costs and power consumption allow microbolometers to be a competitive alternative as compared to cooled infrared detectors in most common infrared imaging scenarios. Until now, microbolometers are designed and optimized for the long wavelength infrared (LWIR) regime ranging from 8 μm to 14 μm. However, the mid wavelength infrared (MWIR) regime ranging from 3 μm to 5 μm is also of great interest for a wide range of applications that can benefit from the advantages of a technological concept relying on microbolometers. For this reason, Fraunhofer IMS developed an uncooled thermal imager based on microbolometers targeting the wavelength spectrum of the MWIR for high temperature imaging applications. A novel imager technology based on Fraunhofer IMS's microbolometer process for lateral leg bolometers providing QVGA resolution (320 x 240) in case of a pixel pitch of 17 μm but transferred to the MWIR regime will be presented here. In order to increase the sensitivity in the MWIR, the transmission characteristics of the vacuum package have been adopted to meet the requirements of this wavelength region. The resulting spectral sensitivity of our MWIR imagers was verified by means of an electro-optical test setup making use of a high temperature black body radiator. In addition, the actual design of the microbolometer membrane has been optimized to reduce the overall thermal capacitance, resulting in thermal time constants up to 30 % lower than those of our standard LWIR imager.