Scalable 3D carbon nanotube structures for bolometric infrared sensor arrays
The infrared detector market has experienced steady growth over the last 10 years and is expected to reach hundreds of million dollars by 2025. The demand for more accurate, lower cost, and lower power consumption IR sensors is driven by emerging applications such as autonomous vehicles. Uncooled infrared sensors work on a bolometer principle, where the infrared energy is absorbed, increasing the temperature of the device. This temperature change can be measured electrically using a thermistor.
Carbon nanotubes (CNTs) have high mechanical strength, excellent IR absorption, low thermal mass, and are electrically conductive. These properties make CNT structures especially suitable for IR sensing, where the IR absorber and detector in a device can be combined in one element. A common structure for a microbolometer is a thin suspended bridge connected to the substrate via small legs, which is advantageous as the thermal mass is minimized, and there is a small thermal link to the substrate. Previous CNT bolometer literature studies focused on optimising the device electrical properties, and so far, no good fabrication methods for suspended CNT structures have been developed. The work described in this thesis demonstrates a novel method for fabricating suspended CNT-based bolometers, which is significantly better from a scalability point of view than literature CNT infrared sensors, and also requires fewer processing steps than conventional MEMS processes.
CNTs can be grown in 3D structures, such as suspended bridges, using a stress engineering process that involves two lithography steps, followed by CNT synthesis via chemical vapour deposition. The device performance was enhanced via two different methods: laser-assisted silicon etching and dry VO2 functionalization. The laser processing method allows for the creation of thin SiN membranes on which CNTs can be grown. This improves the heat retention of the 3D CNT structures by reducing their thermal link to the substrate. The CNT-selective VO2 functionalization increases the temperature coefficient of resistance of the devices by a factor of 3 in the linear region and up to 30 times in the transition region.
The resulting devices are some of the smallest CNT bolometers presented in literature and have a high responsivity (1723 V/W) at fast response times (<15 ms). With some improvements to the device electrical noise e.g. by thermal annealing and alumina encapsulation, these devices could approach the performance of low-end uncooled thermal cameras. Considering these results alongside the scalable fabrication methods developed in this work, these 3D CNT structures are a promising platform for the development of low-cost IR sensor arrays.