Bi-layer kinetic inductance detectors for space observations between 80–120 GHz

¹ Laboratoire de Physique Subatomique et de Cosmologie, LPSC, Université Grenoble-Alpes, CNRS/IN2P3, 53 rue des Martyrs, 38026 Grenoble Cedex, France
e-mail: catalano@lpsc.in2p3.fr
² Institut Néel 38042, CNRS, Université Joseph Fourier Grenoble I 38042, 25 rue des Martyrs, 38026 Grenoble, France
³ Centre de Sciences Nucléaires et de Sciences de la Matière (CSNSM), CNRS/IN2P3, Bât. 104–108, 91405 Orsay, France
⁴ Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), CNRS and Université de Grenoble, 38026 Grenoble, France
⁵ LNGS – Laboratori Nazionali del Gran Sasso, via G. Acitelli 22, 67100 Assergi L’ Aquila, Italy

Received: 28 March 2015
Accepted: 28 May 2015

Abstract

We have developed lumped element kinetic inductance detectors (LEKIDs) that are sensitive in the frequency band from 80 to 120 GHz. In this work, we take advantage of the so-called proximity effect to reduce the superconducting gap of aluminium (Al), otherwise strongly suppressing the LEKID response for frequencies smaller than 100 GHz. We designed, produced, and optically tested various fully multiplexed arrays based on multi-layer combinations of Al and titanium (Ti). Their sensitivities were measured using a dedicated closed-circle 100 mK dilution cryostat and a sky simulator, which allowed us to reproduce realistic observation conditions. The spectral response was characterised with a Martin-Puplett interferometer up to THz frequencies and had a resolution of 3 GHz. We demonstrate that Ti–Al LEKID can reach an optical sensitivity of about 1.4 × 10^-17 W/Hz^0.5 (best pixel), or 2.2 × 10^-17 W/Hz^0.5 when averaged over the whole array. The optical background was set to roughly 0.4 pW per pixel, which is typical for future space observatories in this particular band. The performance is close to a sensitivity of twice the CMB photon noise limit at 100 GHz, which drove the design of the Planck HFI instrument. This figure remains the baseline for the next generation of millimetre-wave space satellites.