Parallel-Plate Capacitor Titanium Nitride Kinetic Inductance Detectors for Infrared Astronomy

The Balloon Experiment for Galactic INfrared Science (BEGINS) is a concept for a sub-orbital observatory that will operate from \(\lambda\) = 25 to 250 \(\upmu\)m to characterize dust in the vicinity of high-mass stars. The mission’s sensitivity requirements will be met by utilizing arrays of 1840 lens-coupled, lumped-element kinetic inductance detectors (KIDs) operating at 300 mK. Each KID will consist of a titanium nitride (TiN) parallel strip absorbing inductive section and parallel plate capacitor deposited on a Silicon (Si) substrate. The parallel plate capacitor geometry allows for reduction of the pixel spacing. At the BEGINS focal plane, the detectors require optical NEPs from \(2\times 10^{-16}\) to \(6\times 10^{-17}\) W/\(\sqrt{\text {Hz}}\) from 25 to 250 \(\upmu\)m for optical loads ranging from 4 to 10 pW. We present the design, optical performance and quasiparticle lifetime measurements of a prototype BEGINS KID array at 25 \(\upmu\)m when coupled to Fresnel zone plate lenses. For our optical set up and the absorption efficiency of the KIDs, the electrical NEP requirement at 25 \(\upmu\)m is \(7.6\times 10^{-17}\) W/\(\sqrt{\text {Hz}}\) for an absorbed optical power of 0.36 pW. We find that over an average of five resonators the the detectors are photon noise limited down to about 200 fW, with a limiting NEP of about \(7.4\times 10^{-17}\) W/\(\sqrt{\text {Hz}}\). Future arrays will be coupled to microlens arrays and have higher optical efficiencies.