Wirebond crosstalk and cavity modes in large chip mounts for superconducting qubits

TL;DR

This paper models and experimentally verifies how wirebond crosstalk and cavity modes affect large superconducting qubit chip mounts, leading to improved design with enhanced isolation and mode identification.

Contribution

It introduces a simple impedance ladder model for wirebond crosstalk, validated by simulations and measurements, guiding the design of mounts with better isolation for larger qubit chips.

Findings

Crosstalk decreases exponentially with distance at low frequencies

Resonance frequency increases as chip-ground capacitance decreases

Achieved -65 dB isolation at 6 GHz in the new mount design

Abstract

We analyze the performance of a microwave chip mount that uses wirebonds to connect the chip and mount grounds. A simple impedance ladder model predicts that transmission crosstalk between two feedlines falls off exponentially with distance at low frequencies, but rises to near unity above a resonance frequency set by the chip to ground capacitance. Using SPICE simulations and experimental measurements of a scale model, the basic predictions of the ladder model were verified. In particular, by decreasing the capacitance between the chip and box grounds, the resonance frequency increased and transmission decreased. This model then influenced the design of a new mount that improved the isolation to -65 dB at 6 GHz, even though the chip dimensions were increased to 1 cm by 1 cm, 3 times as large as our previous devices. We measured a coplanar resonator in this mount as preparation for larger qubit chips, and were able to identify cavity, slotline, and resonator modes.