Fabrication and Test of Large Area Spider-Web Bolometers for CMB Measurements

Detecting the primordial ’B-mode’ polarization of the cosmic microwave background is one of the major challenges of modern observational cosmology. Microwave telescopes need sensitive cryogenic bolometers with an overall equivalent noise temperature in the nK range. In this paper, we present the development status of large area (about 1 cm2) spider-web bolometer, which imply additional fabrication challenges. The spider-web is a suspended Si3N41 μ m-thick and 8-mm diameter with mesh size of 250 μ m. The thermal sensitive element is a superconducting transition edge sensor (TES) at the center of the bolometer. The first prototype is a Ti–Au TES with transition temperature tuned around 350 mK, new devices will be a Mo–Au bilayer tuned to have a transition temperature of 500 mK. We present the fabrication process with micro-machining techniques from silicon wafer covered with SiO2- Si3N4CVD films, 0.3 and 1 μ m- thick, respectively, and preliminary tests.