Low Loss Multi-Layer Wiring for Superconducting Microwave Devices

TL;DR

This paper introduces a fabrication technique for low-loss multi-layer wiring in superconducting microwave circuits using removable SiO2 scaffolding, enabling high-performance quantum devices with minimal added loss.

Contribution

It presents a novel method to create low-loss multi-layer wiring with removable dielectrics, compatible with standard foundry processes, and demonstrates its effectiveness in superconducting quantum circuits.

Findings

Achieved a resonator loss of ~3.9 x 10^-8 per bridge, 100 times lower than dielectric-supported bridges.

Successfully integrated freestanding airbridges in quantum circuits, maintaining qubit lifetimes over 30 μs.

Demonstrated the technique's versatility for various circuit components like crossovers and control line jumpers.

Abstract

Complex integrated circuits require multiple wiring layers. In complementary metal-oxide-semiconductor (CMOS) processing, these layers are robustly separated by amorphous dielectrics. These dielectrics would dominate energy loss in superconducting integrated circuits. Here we demonstrate a procedure that capitalizes on the structural benefits of inter-layer dielectrics during fabrication and mitigates the added loss. We separate and support multiple wiring layers throughout fabrication using SiO$_2$ scaffolding, then remove it post-fabrication. This technique is compatible with foundry level processing and the can be generalized to make many different forms of low-loss multi-layer wiring. We use this technique to create freestanding aluminum vacuum gap crossovers (airbridges). We characterize the added capacitive loss of these airbridges by connecting ground planes over microwave frequency $\lambda/4$ coplanar waveguide resonators and measuring resonator loss. We measure a low power resonator loss of $\sim 3.9 \times 10^{-8}$ per bridge, which is 100 times lower than dielectric supported bridges. We further characterize these airbridges as crossovers, control line jumpers, and as part of a coupling network in gmon and fuxmon qubits. We measure qubit characteristic lifetimes ($T_1$'s) in excess of 30 $\mu$s in gmon devices.