Low Loss Multi-Layer Wiring for Superconducting Qubits*

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. I will discuss 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 silicon oxide scaffolding, then remove it post-fabrication. 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 coplanar waveguide resonators and measuring resonator loss. We measure a low power resonator loss of 3.9E-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 networks in nine and fifteen gmon linear chains, as well as fluxmon qubits. We measure gmon characteristic lifetimes (T1's) in excess of 30 microseconds.