Development of high- Q superconducting resonators for use as kinetic inductance detectors

One of the greatest challenges in the development of future space based instruments for sub-mm astronomy is the fabrication of very sensitive and large detector arrays. Within this context we have started the development of Microwave Kinetic Inductance Detectors (MKID’s). The heart of each detector consists of a high- Q superconducting quarter wavelength microwave resonator. As a result it is easy to multiplex the readout by frequency division multiplexing. The predicted fundamental sensitivity limit of the MKID is due to quasiparticle creation-recombination noise, leading to a NEP ∼ 1 × 10 - 20 W / Hz , low enough for any envisionable application in the sub-mm, optical and X-ray wavelength ranges. We describe experiments with these resonators, made of 150 nm Ta films with a 5 nm Nb seed layer on high purity Si substrates with a resonance frequency around 3 GHz. We measure the Q factors, responsivity, noise and noise equivalent power of several resonators. We find Q factors in excess of 1 × 10 5, high enough for the multiplexing of more than 10 4 pixels. The quasiparticle lifetime in our film is measured to be 25 μs. which gives, together with the measured phase noise, a NEP of ∼ 4 × 10 - 16 W / Hz at 1 kHz. At lower frequencies the noise increases.