Improvement of Design and Fabrication for Equally Spaced Resonant Frequencies in Niobium Lumped Element Kinetic Inductance Detectors

We have developed a lumped element kinetic inductance detector (LEKID) process to suppress non-uniformity in a distribution of resonant frequencies. Non-uniformity of the frequency interval between neighboring resonant frequencies can be caused by the crosstalk among the resonators in the frequency domain. We find that the non-uniformity is attributed not only to electromagnetic coupling among the resonators, but also to the non-uniform depth of over-etching on the silicon substrate surface produced during the reactive ion etching of resonator electrodes. The former is mitigated by forming a ground shield around the resonators, and the latter can be removed by forming a thin AlN layer on the substrate that acts as an etch-stopper. With the improvements, we achieve a uniform distribution of resonant frequencies that meets our requirements.