Development of Highly Reproducible and Robust Absorber Coatings for Bolometric Detector Arrays

We will develop fabrication processes to realize controlled impedance absorber coatings for bolometric sensor applications.  Although there are a variety of different coatings currently employed with existing detector assemblies, they are either difficult to integrate or do not meet the science requirements over envisioned mission lifecycles.   We will demonstrate absorbers with highly reproducible and lasting properties applicable for general use in the microwave through the infrared.

Our objective is to develop controlled impedance absorbers operating in the mid-to-far infrared spectral band (20 microns and long ward).  These absorbers are critical components in a large number of planned terrestrial (e.g., GISMO 2), balloon borne (e.g., BETTII and PIPER), airborne (e.g., HAWC+ and HIRMES), and future space missions.  Unfortunately, existing coatings are either difficult to reproduce, incompatible with other sensor processing steps, or susceptible to aging.  These attributes result in schedule delays and/or a reduction of science because of a transient optical efficiency over the instrument lifetime.

Our near term goal is to develop an absorber material for HIRMES, which requires delivery of flight focal plane arrays prior to April 2017 in order for the instrument build to be completed on schedule.   A NbTiN absorber has been baselined for HIRMES, because Fourier Transform Spectrometer measurements of absorber prototypes have shown that this material is resistive at relevant frequency (~15 THz), has a low residual stress (which is required in order to achieve optimal optical coupling), and has a superconducting transition temperature > 10K.  This last attribute is important for a background-limited instrument like HIRMES, because the NbTiN acts like a high pass filter.  At frequency < 750GHz, the absorber is invisible, and, consequently, there is significantly lower radiation loading on the detectors.   

We will coat the NbTiN films with ultrathin dielectric coatings in order to passivate them against aging in ambient conditions, and if these films are acceptable, we will have, for the first time, developed broadband coatings that are robust and immune to aging after annealing encountered during detector fabrication and over the course of an instrument development and deployment lifecycle (~years at ambient temperature).