CCAT: Characterization of the first science-grade MKID array for the Prime-Cam 850 GHz module

The Fred Young Submillimeter Telescope (FYST) is a 6-meter crossed-Dragone telescope developed by the CCAT collaboration. Sited at 5600 m on Cerro Chajnantor in the Atacama Plateau of Chile, FYST aims to provide superior atmospheric transmission and a wide field of view for submillimeter observations. Prime-Cam is a first-generation instrument for FYST designed to house up to seven separate instrument modules. Among these, the 850 GHz module represents the highest-frequency band and is optimized for ultra-sensitive broadband polarimetry and imaging. This module is designed to deploy ~38,000 polarization-sensitive lumped-element titanium-nitride (TiN) microwave kinetic inductance detectors (MKIDs) across three arrays. Thus the 850 GHz module will have the most submillimeter MKIDs in a single instrument module to date. These detectors have adopted a novel two-octave design to maximize the multiplexing achieved using a RFSoC readout system. We review the design parameters and fabrication process for the first 850 GHz science-grade array. The preliminary measurements of the array show a fabrication yield of 99%, highlighting the successful application of the design and fabrication process. We further present the cryogenic characterization of the first full MKID array developed for the Prime-Cam 850 GHz module. Multiple tests were performed on the devices including resonator frequency mapping, quality factor measurements, optical load sweeps and noise performance. From these measurements, we discuss the measured optical efficiency, sensitivity, and uniformity across the array and the expected on-sky performance of the module. The 850 GHz module will be deployed for observing in 2027.