Enhanced electron-beam lithography to reduce the frequency scatter of 200–400 GHz superconducting microstrip resonators for on-chip filterbank spectrometers

Integrated superconducting spectrometers provide the instantaneous bandwidth, sensitivity, and scalable architecture required for large-scale spectroscopic surveys in submillimeter-wave astronomy and cosmology. However, the accuracy with which the resonant frequencies of superconducting microstrip bandpass filters can be spaced has limited the spectral resolution of these spectrometers with continuous spectral coverage to F/ΔF < 500. The origin of this frequency scatter has been largely unknown. In this work, we demonstrate a fourfold improvement in the frequency spacing of superconducting microstrip resonators by optimizing electron-beam lithography. We find that reducing the electron-beam step size also reduces the random frequency scatter and that avoiding main-field stitching across filter patterns can eliminate a systematic frequency shift between groups of resonators, indicating the different origins of these two modes of frequency deviation. These findings demonstrate that nanometer-scale control of lithographic processes is imperative for the realization of next-generation integrated superconducting spectrometers with higher spectral resolution.