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.
The reduction of the pixel pitch of uncooled thermal imagers is still ongoing and by the end of 2021, reached a size of only 8µm within commercially available products. In case of those small pixel sizes, design and manufacturing requirements become more challenging by facing the main technological objectives of maximizing the temperature rise within the microbolometer membrane and simultaneously, being sensitive towards the latter. Tackling these challenges, Fraunhofer IMS provides a manufacturing process for uncooled thermal imagers optimized for the LWIR regime based on a scalable microbolometer technology incorporating vertical nanotubes. Based on this technological concept, we already demonstrated microbolometers with full electro-optical functionality for pixel sizes ranging from 17µm down to 6µm. A comprehensive study of our measurements regarding the scalable microbolometer technology will be presented here. This includes a discussion about design requirements in correlation to the achieved electro-optical results covering electrical noise, NETD, thermal time constant and spectral absorption characteristics. Those key parameters will be summarized and evaluated with respect to the reduction of the active microbolometer area in case of a shrinked pixel size.
Mid-wave and long-wave infrared (IR) are two bands of interest for uncooled infrared imaging cameras. While longwave infrared detectors are sensitive to human body temperature, mid-wave infrared detectors are useful to detect “hot sources”. In addition, various gases have absorption bands in the mid-wave IR range, so that environmental monitoring or gas detection should be mentioned as further applications. To realize multispectral uncooled thermal imaging detectors, Fraunhofer IMS investigated the absorption properties of plasmonic metamaterial absorbers made of metal-insulator-metal (MIM) structures. High and multispectral absorption is particularly desirable for various microengineering applications, including microbolometers. The MIM absorbers are developed to be adaptable to Fraunhofer IMS nanotube microbolometer technology.
We report here the first results of simulation and experimental characterization of MIM test structures for multispectral absorption. The test structures consist of upper periodic metal structures, a middle dielectric layer and a lower metal reflector layer to produce surface plasmon resonance at desired absorption wavelengths. For a CMOS-compatible MIM absorber, various materials and thicknesses are being studied to realize selective absorption. We demonstrate the optical characterization of the test structures by Fourier transform infrared (FTIR) measurements and the influence of size, thickness and materials of MIM structures to achieve high selective absorption in a narrow wavelength range.
The scalability of Fraunhofer IMS’s nanotube microbolometer technology for uncooled thermal imaging is demonstrated. Thermal insulation of the microbolometers is realized by means of vertically oriented nanotubes arranged underneath the sensor membrane. Thus, thermal insulation is decoupled from the latter, allowing for a variable scaling of the microbolometer, independently of its thermal insulation. Key parameters as electro-optical responsivity, noise, NETD and thermal time constant are presented for pixel pitches varying between 17 μm, 12 μm, 8.5 μm and 6 μm with the latter value being close to the fundamental optical limit in the LWIR, but full electro-optical functionality being maintained.
Besides nowadays challenges in contactless measurement of body temperature, the market for uncooled thermal imager continuously increased in the last years. The size of the camera core is a parameter, that needs to follow the miniaturization of the whole camera body. State-of-the-art value for pixel sizes of microbolometers in uncooled thermal imagers is 10 μm. Pushing the microbolometer size to the optical limit, Fraunhofer IMS provides a manufacturing process for FIR-imagers (uncooled thermal imagers) based on a scalable microbolometer technology. Taking this scalable technology as a basis, we are introducing a fully implemented uncooled thermal imager with 6 μm pixel size. The 6 μm microbolometers are made by Fraunhofer IMS’s manufacturing technology for a thermal MEMS isolation realized by vertical nanotubes. Performance of the 6 μm microbolometers is estimated by a 17 μm digital readout integrated circuit in QVGA resolution. Responsivity and number of electrical defect pixels as well as NETD are determined by an electro-optical characterization based on a test setup with a black body at two different temperatures. NETD of the 6 µm microbolometers is estimated to be at 611 mK. Supporting the quantitative measurements, FIR test images will be presented to demonstrate the microbolometer’s functionality in a fully implemented uncooled thermal imager. In summary, a fully implemented uncooled thermal imager with QVGA resolution based on a 6 μm nanotube-microbolometer detector is presented here. Compared with commercially available uncooled thermal imagers, the highly limited absorption area of our microbolometers with structure sizes below the target wavelength causes an accordingly higher NETD. Nevertheless, it can be stated that 6 μm pixel size still shows the capability of absorbing infrared radiation at wavelengths of approximately 10 μm in an uncooled thermal imager.
Uncooled FIR-imagers decreased in pixel pitch from latest state-of-the-art value of 17 µm to 10 µm. Following this trend of a reduction of pixel size, Fraunhofer IMS provides a manufacturing process for FIR-imagers (IRFPAs) based on a scalable microbolometer technology. Beside conventional approaches of a thermal isolation of microbolometer membranes realized by lateral legs, Fraunhofer IMS developed a manufacturing process for a thermal isolation realized by nanotubes. To demonstrate the scalability of the nanotube-microbolometers the nanotube contact is applied to microbolometer membranes with 12, 10, 8 and 6 µm pixel size on top of a 17 µm digital readout integrated circuit (ROIC). The arrays are sealed by a chip-scale vacuum package to evaluate the microbolometers’ performance by means of a complete IRFPA. Quantitative measurement results for the responsivity as well as qualitative test pictures of the 12, 10 and 8 µm nanotube-microbolometers will be presented. A direct visual comparison in a test scene demonstrates no obvious decrease in sensitivity between 12 and 8 µm. Only at 6 µm pixel size a reduced sensitivity is observed. In summary, a fully working uncooled IRFPA with QQVGA resolution based on a 6 µm nanotube-microbolometer technology is presented here. The scalability of the nanotube-microbolometer technology from state-of-the-art pixel sizes down to 6 µm is demonstrated.
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