Design and Comparison of Diamond- and Sapphire-Based NbN KIDs for Fusion Plasma Polarimetric Diagnostics

Design and characterization measurements performed on kinetic inductance detectors (KIDs) produced on sapphire and polycrystalline diamond substrates are presented. Designed as part of a nuclear fusion polarimetric diagnostic, the foreseen plasma-probing frequency of the final devices is 1.3 THz with a maximum response time under 10 ms and cross-polarization target accuracy of 1%. These detectors are based on superconducting micro-resonators that undergo de-tuning upon absorption of radiation. The main characteristics of the devices include polarization sensitiveness and lumped-elements multi-pixel configuration produced from photo-lithographed niobium nitride (NbN) thin films. The direct current (DC) and microwave characterization measurements highlight large differences in the thin film quality, with the superconductor deposited on diamond showing reduced critical temperature, lower critical current density, and increased values of the kinetic inductance. This difference is likely due to the higher lattice constant and thermal expansion coefficient mismatch between film and substrate in the case of diamond, with the different surface finish quality of the crystalline samples available also playing a role. The devices on both substrates show a bolometric response to THz radiation that fulfills the requirement guidelines and represent a good starting point to optimize the design for the application at hand.