First Demonstration of VGA Format Microbolometer FPAs using Semi-Conducting SWCNT Networks for Uncooled LWIR Image Sensor

Uncooled infrared sensors of bolometer type have a wide range of applications such as security, military, food inspection, health care, and automotive night vision system. Currently, the highly sensitive device development is the key issue for further expansion of demand. The bolometer is an infrared detector of long wave infrared (LWIR) region for radiant heat by means of an infrared absorber having a temperature sensitive electrical resistance material. Infrared radiation strikes the absorber material heating it and thus changing resistor material resistance. Therefore, bolometer’s performance is strongly limited by temperature coefficient of resistance (TCR) of the resistor. The conventional resistor is generally based on vanadium oxide (VO_x) with TCR of about -2%/K [1] and an outstanding resistor is essential to achieve highly sensitive infrared detectors. Recently, single-walled carbon nanotubes (SWCNTs) have been expected as promising materials with high TCR and high chemical stability. We have reported that semi-conducting SWCNT networks extracted by the “Electric-field inducing Layer Formation (ELF)” method [2] show high TCR which is close to -6%/K [3]. The pristine SWCNTs are fundamentally a mixture of both semi-conducting and metallic SWCNTs. The ELF method is the remarkable promising technique to extract semi-conducting SWCNTs with high purity, which show stable device performance and excellent electrical transportation property.<br/><br/>In this study, we fabricated the Video Graphics Array (VGA) format focal plane allays (FPAs) with semiconducting SWCNTs for the first time and characterized them. Previously, we have developed a VO_x microbolometer with a suspended structure for thermally separation between the infrared absorber and the substrate by micro electro mechanical system (MEMS) process [4]. By utilizing this technology and constructing a new MEMS structure suitable for the CNT networks, we have successfully developed a SWCNT microbolometer FPAs. Device chip which was mounted on the ceramic carrier was installed in a vacuum dewar. For responsivity measurement, infrared radiation from cavity type blackbody was irradiated onto the microbolometer through the ZnSe optical window. This radiation was chopped by an optical chopper and the response signal of microbolometer was measured by lock-in amplifier.<br/><br/>The responsivity of LWIR region was estimated to be around 10⁵ V/W in our FPAs with semi-conducting SWCNTs. This is about three times higher than that in FPAs with VO_x. TCR of our semi-conducting SWCNT networks was above -6 %/K, which is approximately three times higher than that of VO_x. Therefore, it means high TCR of semi-conducting SWCNT network was responsible for the increased sensitivity.<br/><br/>In conclusion, we showed the high-purity semi-conducting SWCNT networks extracted by ELF method effectively work for higher responsivity of microbolometer FPAs. In the future, we will evaluate the performance in more detail and obtain infrared images by combining a microbolometer and a readout circuit.<br/><br/>Acknowledgments: Part of this study was supported by Innovative Science and Technology Initiative for Security Grant No. JPJ004596, ATLA, Japan.<br/><br/>[1] C. Chen, et. al., Sen. Act. A. Phys. 90, 2001, 212.<br/>[2] K. Ihara, et. al., J. Phys. Chem. C, 115, 2011, 22827.<br/>[3] T. Tanaka. et. al., 2022 MRS Fall Meeting, NM02.09.08.<br/>[4] N. Oda. et. al., Proc. SPIE 6940, Infrared Technology and Applications XXXIV, 69402Y (2008).