Fabrication of Al/AlO _x /Al junctions using pre-exposure technique at 30-keV e-beam voltage

Abstract

We fabricate high-quality Al/AlO x /Al junctions using improved bridge and bridge-free techniques at 30-keV e-beam voltage, in which the length of undercut and the size of junction can be well controlled by the pre-exposure technique. The dose window is 5 times as large as that used in the usual Dolan bridge technique, making this technique much more robust. Similar results, comparable with those achieved using a 100-keV e-beam writer, are obtained, which indicate that the 30-keV e-beam writer could be an economic choice for the superconducting qubit fabrication.

Optimizing Josephson Junction Reproducibility in 30 kV E-Beam Lithography: An Analysis of Backscattered Electron Distribution

“…Various groups have developed distinct strategies to enhance the reproducibility of Josephson junctions using low-energy 30 kV electron beam lithography. These improvements primarily focus on variations in junction geometry and the employment of different resist stacks [18,19], which have facilitated the fabrication of superconducting qubits with relaxation and coherence times ranging from hundreds of nanoseconds to tens of microseconds [20][21][22] for diverse applications. Despite these advancements, the underlying mechanisms contributing to the enhanced reproducibility of the junctions remain ambiguous.…”

“…Some groups claim that robustness is compromised due to stress that occurs to the PMMA layer depending on the geometry used [23,24]. Others deal with this by pre-exposing the bottom resistance [18]. Our study addresses this issue by focusing on enhancing fabrication reproducibility through the strategic selection of optimal geometries that correctly engineer the doses of backscattered electrons on the bridge area.…”