title: A novel monolithic focal plane array for mid-IR imaging
Creators: name: Xie, Chengzhi
Publisher: name: University of Glasgow
Year: 2017
Subjects: subject: Mid-IR Imaging, Focal Plane Array (FPA), InSb, GaAs, Sb-based semiconductor material, photodetector, MESFET.

description: The use of Mid-infrared (mid-IR) imagers has great potential for a number of applications in gas sensing and medical diagnostics, but so far for many of those non-defence fields it has been significantly limited by their high price tag. One of the reasons behind the great cost of mid-IR imagers is that most of them need to operate at cryogenic temperatures. Thanks to more than half a century of research, state-of-the-art mid-IR photodetectors have finally achieved premium detection performance without the need for cryogenic cooling. Some of them have even demonstrated very promising results, suggesting room temperature operation is on the horizon. As a result, the cost associated with cooling equipment has been significantly suppressed. However, most mid-IR imagers are still based on hybrid technologies needing a great number of die-level process steps and being prone to connection failure during thermal cycles. The high manufacturing cost this entails is also preventing a wider diffusion of mid-IR imagers. Currently, there is still a lack of an effective monolithic approach able to achieve low-cost mass production of mid-IR imagers in the same way as monolithic integration has been widely used for imagers working at visible wavelengths. This thesis presents a novel monolithic approach for making mid-IR imagers based on co-integration of mid-IR photodetectors with GaAs-based MESFETs on the same chip. The initial focus of the project was the development of the fabrication steps for delivery of prototype devices. In order to achieve monolithic fabrication of pixel devices made in either indium antimonide (InSb) or indium arsenide antimonide (InAsSb) on a gallium arsenide (GaAs) substrate, various highly controllable etch processes, both wet and dry etch based, were established for distinct material layers. Moreover, low temperature annealed Ohmic contacts to both antimonide-based materials and GaAs were used. The processing temperatures used never exceeded 180˚C, preventing degradation of photodetector performance after fabrication of transistors, thus avoiding well-known thermal issues of InSb fabrication. Furthermore, an intermediate step based on polyimide was developed to provide a smoothing section between the lower MESFET and upper photodetector regions of the pixel device. The polyimide planarisation enabled metal interconnects between the fabricated devices regardless of the considerable etch step (&gt; 6 µm) created after multiple mesa etches. Detailed electrical and optical measurements demonstrated that the devices were sensitive to mid-IR radiation in the 3 to 5 µm range at room temperature, and that each pixel could be isolated from its contacts by switching off the co-integrated MESFET. Following the newly developed fabrication flow, InSb-based mid-IR imaging arrays (in two sizes, 4×4 and 8×8) are presented here for the first time, with pixel addressing achieved by monolithically integrated GaAs MESFETs. By demonstrating real-time imaging results obtained from these array devices at room temperature, implementation of a new type of monolithic focal plane array for mid-IR imaging has been confirmed. The device is suitable for further scaling (up to 64×64 pixel and beyond) and potential commercialisation. More importantly, the monolithic approach developed in this work is very flexible, as a number of III-V materials with mid-IR detecting capabilities can be grown on GaAs substrates, meaning alternative semiconductor layer structures could also be investigated in the near future.