In the field of microbolometer technology, there are two concepts for tuning the absorption to the desired spectral range. Absorbers based on a λ/4 resonator, which is the standard design for commercial uncooled IR imagers, and Plasmonic Metamaterial Absorbers (PMAs). This paper provides an overview of the optical results of both Fraunhofer IMS’s concepts and a comparison between the two technological approaches in terms of their targeted spectral ranges and bandwidths. We show that the quarter-wavelength resonator-based microbolometers provide a broadband absorption spectrum and the PMA-based microbolometers can have a narrow-band absorption spectrum with a bandwidth of about 0.5 μm. In addition, we show how λ/4 absorbers are primarily suited to thermography, while the PMAs can provide a promising platform for gas cameras.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.