Development of Lumped Element Kinetic Inductance Detectors for the W-Band

Abstract

We are developing a lumped element kinetic inductance detector (LEKID) array which can operate in the W-band (75$-$110 GHz) in order to perform ground-based cosmic microwave background (CMB) and mm-wave astronomical observations. The W-band is close to optimal in terms of contamination of the CMB from Galactic synchrotron, free-free, and thermal interstellar dust. In this band, the atmosphere has very good transparency, allowing interesting ground-based observations with large ($>$30 m) telescopes, achieving high angular resolution ($<$0.4 arcmin). In this work we describe the startup measurements devoted to the optimization of a W-band camera/spectrometer prototype for large aperture telescopes like the 64-m Sardinia Radio Telescope. In the process of selecting the best superconducting film for the LEKID, we characterized a 40-nm-thick aluminum 2-pixel array. We measured the minimum frequency which can break CPs (i.e., $h\nu =2\Delta \left( T_\mathrm{c}\right) =3.5\,k_\mathrm{B}T_\mathrm{c}$) obtaining $\nu =95.5$ GHz, which corresponds to a critical temperature of 1.31 K. This is not suitable to cover the entire W-band. For an 80-nm layer the minimum frequency decreases to 93.2 GHz, which corresponds to a critical temperature of 1.28 K; this value is still suboptimal for W-band operation. Further increase of the Al film thickness results in bad performance of the detector. We have thus considered a Titanium–Aluminum bi-layer [10-nm-thick Ti $+$ 25-nm-thick Al, already tested in other laboratories (Catalano et al. in Astron Astrophys 580:A15, 2015)], for which we measured a critical temperature of 820 mK and a cut-on frequency of 65 GHz, so this solution allows operation in the entire W-band.