Wafer Level Capping Technology for Vacuum Packaging of Microbolometers

This paper presents novel technology developments for vacuum wafer level packaging of microbolometer arrays for thermal infrared sensors targeting applications in automotive, safety, and security/surveillance. The concept is based on fabrication of large cap structures on temporary carrier wafers and their subsequent transfer bonding to device wafers. The objective of the presented work was to develop and test wafer level vacuum packaging for MEMS microbolometer arrays (MBA) fabricated on read out integrated circuit (ROIC) wafers. For that, related MBA layouts integrating diverse Pirani vacuum test structures were fabricated on 200-mm silicon wafers. With intent of hermeticity, all wafer bonding steps were done by AuSn soldering using seal rings, which were deposited by electroplating. The relevant process flows with alternative process options as well as the obtained results of the capping approaches are presented and discussed extensively in this article. For characterization of the sealing results, Pirani test structures were utilized. First, their resistance vs. pressure behavior was determined under controlled reference vacuum. The measured resistance values of identical structures after capping were then compared with the reference data to estimate the residual vacuum inside the cavity of the bonded cap structures.