Novel E-beam lithography technique for in-situ junction fabrication: the controlled undercut

TL;DR

This paper introduces a new shadow evaporation method using asymmetric undercuts for in-situ fabrication of high-quality Josephson junctions and capacitors, improving robustness and size control.

Contribution

The novel technique enables in-situ junction fabrication without suspended bridges, increasing mechanical robustness and junction size range, with demonstrated improved qubit coherence times.

Findings

Successful fabrication of Al/AlOx/Al Josephson junctions and phase qubits

Enhanced qubit coherence times compared to previous methods

Effective control of junction size from 0.01 to over 10,000 microns squared

Abstract

We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by E-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in-situ fabrication of junctions and capacitors without the use of the well-known suspended bridge[1]. The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 0.01 to more than 10000 micron square. We have fabricated Al/AlOx/Al Josephson junctions, phase qubit and capacitors using a 100kV E- beam writer. Although this high voltage enables a precise control of the undercut, implementation using a conventional 20kV E-beam is also discussed. The phase qubit coherence times, extracted from spectroscopy resonance width, Rabi and Ramsey oscillations decay and energy relaxation measurements, are longer than the ones obtained in our previous samples realized by standard techniques. These results demonstrate the high quality of the junction obtained by this controlled undercut technique.