Design and Fabrication of Argonne/KICP Detectors for CMB Polarization.

We present the design, microfabrication and assembly of dual-polarization absorber-coupled Transition Edge Sensor (TES) bolometer detectors for cosmic microwave background B-mode polarization studies. The device consists of two separate dies incorporating suspended silicon nitride membranes within silicon frames, carefully aligned perpendicularly and fixtured face-to-face. Polarization sensitivity around 95 GHz is provided by a single dipole-like absorber element; we briefly analyze this absorber-in-waveguide configuration in closed form using the EMF method. Proximity effect Mo/Au bilayers provide control of the TES critical temperature between 400 mK and 600 mK, with a normal resistance Rn∼1Ω. DC magnetron sputtering, wet etching, and liftoff were employed for TES fabrication. Optimization of the superconducting Mo thin film utilized independent RF bias applied to the substrate during deposition in a confocal geometry. This technique allows outstanding thin film uniformity and stress to be achieved simultaneously over a 4” wafer surface, leading to excellent superconducting properties. Device thermal conductance in the range from 25–250pW/K was achieved using suspended silicon nitride membranes 1 micron thick. The value of the thermal conductance depends very strongly upon the surface roughness, and therefore in turn upon the treatment of the silicon nitride material during fabrication. [ABSTRACT FROM AUTHOR]