Surface Optimization of Aluminum Resonators for Robust Quantum Device Fabrication

TL;DR

This study explores surface treatment techniques for aluminum resonators to enhance their performance and stability in quantum devices, focusing on post-processing methods suitable for industry-scale fabrication.

Contribution

It introduces novel surface modification strategies, including plasma treatments and selective etching, to reduce dielectric losses in aluminum resonators after extended ambient exposure.

Findings

Remote oxygen plasma reduces dielectric loss.

Fluorine-based plasma increases dielectric loss.

Sequential HF vapor and phosphoric acid etching achieves low dielectric loss.

Abstract

Aluminum remains the central material for superconducting qubits, and considerable effort has been devoted to optimizing its deposition and patterning for quantum devices. However, while post-processing of Nb- and Ta-based resonators has been widely explored, primarily focusing on oxide removal using buffered oxide etch (BOE), post-treatment strategies for Al resonators remain underdeveloped. This challenge becomes particularly relevant for industry-scale fabrication with multichip bonding, where delays between sample preparation and cooldown require surface treatments that preserve low dielectric loss during extended exposure to ambient conditions. In this work, we investigate surface modification approaches for Al resonators subjected to a 24-hour delay prior to cryogenic measurement. Passivation using self-limiting oxygen and fluorine chemistries was evaluated utilizing different plasma processes. Remote oxygen plasma treatment reduced dielectric losses, in contrast to direct plasma, likely due to additional ashing of residual resist despite the formation of a thicker oxide layer on both Si and Al surfaces. A fluorine-based plasma process was developed that passivated the Al surface with fluorine for subsequent BOE treatment. However, increasing fluorine incorporation in the aluminum oxide correlated with higher loss, identifying fluorine as an unsuitable passivation material for Al resonators. Finally, selective oxide removal using HF vapor and phosphoric acid was assessed for surface preparation. HF vapor selectively etched SiO2 while preserving Al2O3, whereas phosphoric acid exhibited the opposite selectivity. Sequential application of both etches yielded dielectric losses as low as $\delta_\mathrm{LP} = 5.2 \times 10^{-7}$ ($Q\mathrm{i} \approx 1.9\,\mathrm{M}$) in the single photon regime, demonstrating a promising pathway for robust Al-based resonator fabrication.