We are developing a Lumped Element Kinetic Inductance Detector ( LEKID ) array able to 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 SRT ( 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 able to break CPs ( i.e . h \nu = 2 \Delta \left ( T _ { c } \right ) = 3.5 k _ { B } T _ { c } ) obtaining \nu = 95.5 GHz , that 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 [ 1 ] ) , 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 .