Targeting Superconducting Resonator Frequency on Different Substrate Materials

Accurate frequency targeting of superconducting readout resonators is required for optimal performance of superconducting quantum computers. Fabrication processes result in larger distributions and offsets from designed parameters than might be assumed from the accuracy of the lithography. For superconducting devices, this can lead to issues with frequency crowding and shifts the expected coupling to the qubit. We used a 4 K RF wafer prober to form a statistical data set for the center frequency of niobium superconducting readout resonators on both silicon and sapphire substrates. The frequency distributions are much tighter on the sapphire substrate compared with the silicon which we attribute to the different amounts of etching of the substrate during metal lithography. The cryogenic RF wafer prober allows us to accurately measure the resonant frequency of resonators before dicing, and improve targeting by compensating for fabrication variation across a wafer. Changing the substrate from silicon to sapphire allows us to further improve the targeting and yield.