Fabrication of Diffusion-Cooled Hot-Electron Bolometers Using Electron-Beam Lithography

Astronomical interest in observing phenomena within the terahertz frequency spectrum has driven demand for extremely sensitive heterodyne receivers. Traditionally, niobium-based SIS tunnel junctions are used by the radio astronomy community in such receivers for observations below 700GHz. Research on novel devices for heterodyne mixing aims to extend experimental capabilities of sensitive receivers into the THz frequency domain. For mixing applications much beyond 1THz, superconducting hot-electron bolometers (HEB) may offer the best compromise solution for heterodyne receiving. The physical layout of a diffusion-cooled bolometer, the smallest in the family of HEBs, consists of a small absorber material, typically a 10 to 12nm thick niobium microbridge, contacting large cooling pads on opposite ends. These cooling pads are typically thick gold structures, and are much wider than the microbridge. In order to ensure that electron-electron interactions are the dominate cooling mechanism in the bolometer, the pad-to-pad spacing must be very small; electrons must diffuse out of the niobium absorber area in a time period less than the electron-phonon interaction time. The length of the microbridge is therefore on order of the electron diffusion length. Microbridge dimensions are too small to be fabricated with conventional photolithographic techniques. Typical devices are around 200nm wide and 300nm long. A method of bolometer fabrication employing electron beam lithography is presented, along with DC I-V characteristics and superconducting transition temperature results.