Microfabrication technology for large LEKID arrays : from NIKA2 to future applications

TL;DR

This paper reviews the development and microfabrication advancements of LEKID arrays from NIKA2 to future large-scale applications, highlighting improvements in sensitivity, frequency coverage, and fabrication techniques for CMB research.

Contribution

It presents new microfabrication methods enabling large LEKID arrays with maintained sensitivity for future CMB satellite missions.

Findings

LEKID arrays achieved near photon-noise limited performance in 120-300 GHz band.

Demonstrated LEKID arrays working in 80-120 GHz range with improved sensitivity.

Scaled up NIKA2 to thousands of pixels without sensitivity loss.

Abstract

The Lumped Element Kinetic Inductance Detectors (LEKID)demonstrated full maturity in the NIKA (New IRAM KID Arrays)instrument. These results allow directly comparing LEKID performance with other competing technologies (TES, doped silicon) in the mm and sub-mm range. A continuing effort is ongoing to improve the microfabrication technologies and concepts in order to satisfy the requirements of new instruments. More precisely, future satellites dedicated to CMB (Cosmic Microwave Background) studies will require the same focal plane technology to cover, at least, the frequency range of 60 to 600 GHz. Aluminium LEKID developed for NIKA have so far demonstrated, under real telescope conditions, performance approaching photon-noise limitation in the band 120-300 GHz. By implementing superconducting bi-layers we recently demonstrated LEKID arrays working in the range 80-120 GHz and with sensitivities approaching the goals for CMB missions. NIKA itself (350 pixels) is followed by a more ambitious project requiring several thousands (3000-5000) pixels. NIKA2 has been installed in October 2015 at the IRAM 30-m telescope. We will describe in detail the technological improvements that allowed a relatively harmless 10-fold up-scaling in pixels count without degrading the initial sensitivity. In particular we will briefly describe a solution to simplify the difficult fabrication step linked to the slot-line propagation mode in coplanar waveguide.