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

In this paper we present the design and characterization measurements performed on Kinetic Inductance Detectors produced on sapphire and policrystalline diamond substrates. Designed to be part of a nuclear fusion polarimetric diagnostic instrument, 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$\%$, in accordance with the guidelines of the upcoming ITER reactor \cite{zeeland2013} \cite{Donne2007}. 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 relatively high $T_C$ of bulk NbN ($\sim$16~K) enables operation at liquid helium temperatures, simplifying the setup of the final detection system when compared to the sub-kelvin temperatures employed in astrophysical KID arrays. The DC characterization measurements performed on test strips patterned on the two substrate materials highlighted large differences in the thin films quality, with the superconductor deposited on diamond showing reduced critical temperature, lower \textcolor{red}{critical} current density and increased values of the kinetic inductance compared to the strips on sapphire. ... mehrThis difference is believed to be mainly due to the higher lattice constant and thermal expansion coefficient mismatch between film and substrate in the case of diamond, \textcolor{red}{with the} different surface finish quality of the crystalline samples at our disposal \textcolor{red}{also likely playing a role.} The response to microwave read-out tones of the prototypes obtained from the same films follows a similar behavior, with the devices produced on sapphire generally outperforming those on diamond. Nevertheless, diamond is a promising candidate for this kind of application, \textcolor{red}{especially considering} the advantages given by its radiation hardness. The devices on both substrates showed a response to THz radiation, bolometric in nature, that fulfills the requirement guidelines and represent a good starting point to optimize the design for the application at hand.