Fabrication of Antenna-Coupled KID Array for Cosmic Microwave Background Detection

Kinetic inductance detectors (KIDs) have become an attractive alternative to traditional bolometers in the sub-mm and mm observing community due to their innate frequency multiplexing capabilities and simple lithographic processes. These advantages make KIDs a viable option for the O(500,000) detectors needed for the upcoming Cosmic Microwave Background-Stage 4 experiment. We have fabricated an antenna-coupled MKID array in the 150 GHz band optimized for CMB detection. Our design uses a twin-slot antenna coupled to an inverted microstrip made from a superconducting Nb/Al bilayer as the strip, a Nb ground plane and a SiN$$_x$$x dielectric layer in between, which is then coupled to an Al KID grown on high-resistivity Si. We present the fabrication process and measurements of SiN$$_x$$x microstrip resonators.