Scalable and controllable qubit manufacturing via advanced Nb-based trilayer fabrication techniques

Superconducting qubits are a promising candidate for quantum computing due to their relatively high coherence times and fast, high-fidelity gates. However, further improvements in scalability and performance are needed. Transitioning from aluminium (Al) to niobium (Nb)-based qubits offers advantages, such as fewer sources of quasiparticle-induced decoherence and operation at higher temperatures due to Nb's higher energy gap and critical temperature (T_c). Previous studies have shown that Nb trilayer qubits maintain quality factors up to 1.1K [1], significantly higher than Al qubits before quasiparticles become active. Earlier approaches used CVD to create spacer layers, which prevented shorting but introduced high-temperature processing issues that altered the AlOx tunnel barrier and left dielectric residues, contributing to decoherence. Here, we present a novel fabrication method that mitigates these problems. The dielectric spacer is deposited at lower temperatures, reducing interface contamination and enabling precise thermal control over tunnel resistance. We use ebeam lithography to accurately align and reduce junction areas, achieving a 90+% yield of functional devices at the chip level, with the yield continuing to improve as the process is refined. We are now scaling this process to 4-inch wafers, containing multiple devices and test structures, which we characterise at both room and cryogenic temperatures.

[1]Anferov, Alexander, et al. PRA 21.2 (2024): 024047.