Titanium Nitride Film on Sapphire Substrate with Low Dielectric Loss for Superconducting Qubit

Coherence times of superconducting qubits are often limited by dielectric loss of the materials. Among multiple candidates of material systems, the combination of titanium nitride (TiN) film and sapphire substrate shows its potential because of the crystallinity, chemical stability against oxidization, and high quality at interfaces. In this work, we report TiN films deposited by magneto sputtering on sapphire substrate achieving low dielectric loss on the substrate-metal (SM) interface. We systematically characterize a series of 2D transmons fabricated with identical batches of TiN base layer, but with different geometries of qubit shunting capacitors covering various participation ratios of SM interface. We then quantitatively extract a loss tangent at the SM interface smaller than 7.5 × 10⁻⁴ in 1 nm disordered layer. With such low level of dielectric loss, the optimal relaxation time (T₁) and decoherence time (T₂, spin echo) of 2D transmon reach up to 302.2 us and 165.4 us respectively. This work suggests that TiN films on sapphire substrate are an ideal material system for long-coherence superconducting qubits.