A Device-Level Vacuum-Packaging Scheme for Microbolometers on Rigid and Flexible Substrates

This paper reports on the design, fabrication, and characterization of device-level vacuum-packaged microbolometers on rigid Si wafers and flexible polyimide substrates. Semiconducting yttrium barium copper oxide (commonly referred to as YBCO) serves as the bolometric material. Operating micromachined bolometers in vacuum reduces the thermal conductance G_th from the detector to the substrate. If flexibility of the substrate is not to be sacrificed, then the vacuum packaging needs to be done at the device level. Here, the microbolometers are fabricated on a silicon nitride support membrane, isolated from the substrate using surface micromachining. Suitable materials as well as various dimensions in the vacuum cavity are determined using finite-element method (FEM)-based CoventorWARE. A vacuum cavity made of Al₂O₃ has been designed. The thermal conductance G_th of bolometers with the geometry implemented in this work is the same for devices on rigid and flexible substrates. The theoretical value of G_th was calculated to be 4.0 x 10^-6 W/K for devices operating in vacuum and 1.4 x 10^-4 W/K for devices operating at atmospheric pressure. Device-level vacuum-packaged microbolometers on both rigid Si and flexible polyimide substrates have been fabricated and characterized for optical and electrical properties. A low thermal conductance of 1.1 X 10^-6 W/K has been measured six months after fabrication, which implies an intact vacuum cavity.