Uncooled infrared detection based on aluminium nitride piezoelectric resonator

• Main Author: Ang, Wan Chia
• Other Authors: Tan Chuan Seng
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: http://hdl.handle.net/10356/65601
• Institution: Nanyang Technological University

Infrared (IR) radiation is an electromagnetic radiation with wavelength ranging from 0.75 µm to 1,000 µm and photon energy from 1.24 meV to 1.7 eV. IR sensors can be generally categorized into either photon detectors or thermal detectors. Traditionally, IR detection was mainly employed in the fields of astronomy, military and surveillance using photon detectors. Thermal detectors (also called uncooled detectors) were comparatively less explored due to their unsatisfactory performance. Since the emergence of micromachining technologies in the early 1990s, the sensing performance of micro-electro-mechanical systems based thermal IR detectors have been boosted up, which is sufficient for some low-end applications in the field of civilian, medical, spectroscopy, etc. Among all available thermal IR sensing technologies, resonant detectors appeared to be the promising candidates that are competitive with photon detectors because of their low noise characteristic and highly accurate frequency readout. In this thesis, thermal IR detectors based on aluminium nitride (AlN) piezoelectric resonators have been successfully fabricated, characterized and evaluated. A brief introduction to IR radiation and currently available IR detection technologies are first given, thereby providing a motivation to evaluate the thermal AlN resonant detectors in this project. An overview of AlN piezoelectric resonators is provided, followed by the working principle and theory of IR detection. Detailed design considerations of AlN resonant detectors are described. A fully CMOS compatible fabrication process is developed to enable the integration between the detectors and CMOS readout circuits. The detectors in up-side-down design with optimized thermal isolation structure are successfully fabricated. Resonant and thermal behaviour of the fabricated AlN resonant detectors are characterized using network analyzer in a vacuum prober equipped with temperature chuck and radio frequency feedthroughs. IR sensing characterization of the detectors is accomplished using blackbody source and optical filter in the same vacuum prober. The measurement data is then fitted with modified Butterworth van Dyke equivalent circuit model. The measured performance of the detectors is presented along with figure-of-merits including responsivity, noise performance, and noise-equivalent-temperature-difference. The sensing response as a function of IR radiant power, operating temperatures, and structural design parameters is also illustrated. It is experimentally proven that the AlN resonant detectors are responsive to the IR range of 0.5 – 20 µm. The anchors dimension does not have significant effect on resonant characteristic of the IR detectors due to high acoustic impedance of molybdenum (Mo). However, the longer and narrower anchors give longer response time with improved responsivity. The optimum interdigitated transducer electrode metallization ratio is chosen based on the resonant characteristic and IR absorption. It is worth noting that the devices with less number of electrode fingers give better sensitivity due to improved temperature coefficient of frequency. The best performing devices operating at resonant frequency of 388 MHz has anchor dimensions of 12.5 μm × 31.5μm × 200 nm, interdigitated transducer electrode metallization ratio of 0.75 μm with 12 electrode fingers, IR sensing area of 150 μm × 166 μm, and edge reflector distance of quarter-wavelength of the operating resonant frequency. The peak absorption of at least 90 % (responsivity of 6.0 /W) is achievable with resonant quality factor of > 1000, anti-resonant quality factor of > 1200 and effective coupling coefficient of 0.8 %. With the low noise characteristic, a noise-equivalent-temperature difference of about 20 mK is obtained at room temperature operation. It is also demonstrated that the AlN resonant detectors are working well at elevated temperatures up to 300 C. Although the device quality factor and sensitivity degrade with temperature due to material softening and increasing thermal noise, the noise-equivalent-temperature-difference of about 45 mK is achievable with response time of 3.20 ms.