Improved packaging techniques for LWIR microbolometers

Abstract

A new generation of Infrared (IR) camera sensors, based on MicroBolometer Arrays (MBA) has been developed. This technology is lower cost and has no need for active cooling, unlike the previous generation. It, therefore, has the potential to bring thermal cameras to new application avenues, such as thermography for quality assurance and control, automated driver assistance systems in cars and unmanned aerial aircraft systems, as well as other surveillance, security, and safety applications. Packaging of these MBAs, like most Micro-Electro-Mechanical Systems (MEMS), is challenging because these devices are sensitive and have strict requirements when it comes to processing techniques, processing temperature, and vacuum operating level. Developing a realistic and scalable packaging process for these MBAs using existing techniques and improving them, is the main goal of this thesis. A packaging process based on CMOS fabrication techniques and Solid Liquid InterDiffusion (SLID) bonding has been developed and is presented in detail. Detailed descriptions of the processes, including photolithography for patterning and electroplating for deposition of seal frames, DRIE for micromachining the cap wafer, and a justified bonding temperature profile are presented along with common characterization techniques. The articles included in this thesis present techniques developed to improve this packaging through specific seal frame patterns and improved anti-reflective techniques. In “Squeeze-out and bond strength of patterned CuSn SLID seal-frames” from 2022, I demonstrate that patterning of seal frames may increase their shear strength by as much as 400%, and another pattern shows a 46% reduction in squeeze-out. The strongest of the seal frames, which contained 2x50 μm wide openings inside the bond, did not appear to reduce squeeze-out, which goes against the earlier findings in the article “Investigation of seal frame geometry on Sn squeeze-out in Cu-Sn SLID bonds” from 2021. The difference in findings can be explained by the improved analysis technique; For the first article, point measurements were made, while in the second the entire seal frame area was compared. For these reasons, the findings of the 2022 article are thought to be more reliable meaning that seal frames with openings inside the bond have an insignificant impact on squeeze-out. In “Simulated effects of wet-etched induced surface roughness on IR transmission and reflection” I use COMSOL Multiphysics to estimate that roughness in silicon as a result of wet-etching actually improves transmission for LWIR. I later apply this phenomenon of sub-wavelength structures that improve transmission, also referred to as moth-eye structures, to produce specific structures in “Moth-eye anti-reflection structures in silicon for Long Wave IR applications”. In this article, an oxide mask and an alternating isotropic-anisotropic recipe is used to produce structures that increase transmission by 5%. Simulations show that with a few alterations to the recipe, the structures could be improved to further increase the transmission by a total of ∼10%. I came to the idea of implementing moth-eye structures and defined their parameters through literature review and simulation myself, and the recipe was developed with the assistance of the research group at Ginzton Laboratories, Stanford, led by Prof. Solgaard. These moth-eye structures are intended to be patterned inside the cavity on the cap wafer. This is necessary because ∼30% of LWIR light is reflected at a single air-silicon interface and the anti-reflection measures on the inside of the package must be able to survive the bonding temperature of SLID (270°C), and traditional Anti-Reflective Coatings (ARC) are generally sensitive to temperature changes due to their multiple layers. In the article “Temperature Resistant Anti-Reflective Coating for LWIR imaging on Si-wafer”, however, we present an ARC capable of maintaining its anti-reflective properties after being heat-cycled to 300°C. The ARC lowers reflection by 15% and was developed using a simple 2-layer structure composed of ZnS and YF3 and deposition at 100°C. Through the work described above, this thesis contributes to the state-of-the-art knowledge of electronic packaging by: 1) Further developing the technique of SLID bonding. 2) Investigating the impact of silicon processing on IR transmission. 3) Investigating and describing the fabrication of novel moth-eye structures. 3) Developing an ARC with hitherto unseen application areas.