Junction fabrication by shadow evaporation without a suspended bridge

Abstract

We present a novel shadow evaporation technique for the realization of junctionsand capacitors. The design by e-beam lithography of strongly asymmetricundercuts on a bilayer resist enables in situ fabrication of junctions and capacitorswithout the use of the well-known suspended bridge (Dolan 1977 Appl. Phys.Lett. 31 337–9). The absence of bridges increases the mechanical robustnessof the resist mask as well as the accessible range of the junction size, from10 − 2 µm2 to more than104 µm2. We havefabricated Al/AlOx/Al Josephson junctions, phase qubit and capacitors using a 100 kV e-beam writer. Althoughthis high voltage enables a precise control of the undercut, implementation using aconventional 20 kV e-beam is also discussed. The phase qubit coherence times, extractedfrom spectroscopy resonance width, Rabi and Ramsey oscillation decays and energyrelaxation measurements, are longer than the ones obtained in our previous samplesrealized by standard techniques. These results demonstrate the high quality of the junctionobtained by this bridge-free technique.