An argon ion beam milling process for native AlOx layers enabling coherent superconducting contacts

Abstract

We present an argon ion beam milling process to remove the native oxide layer forming on aluminum thin films due to their exposure to atmosphere in between lithographic steps. Our cleaning process is readily integrable with conventional fabrication of Josephson junction quantum circuits. From measurements of the internal quality factors of superconducting microwave resonators with and without contacts, we place an upper bound on the residual resistance of an ion beam milled contact of 50 mΩ • µm 2 at a frequency of 4.5 GHz. Resonators for which only 6% of the total foot-print was exposed to the ion beam milling, in areas of low electric and high magnetic field, showed quality factors above 10 6 in the single photon regime, and no degradation compared to single layer samples. We believe these results will enable the development of increasingly complex superconducting circuits for quantum information processing.

Loss Mechanisms and Quasiparticle Dynamics in Superconducting Microwave Resonators Made of Thin-Film Granular Aluminum

“…Surprisingly, when the ground plane is fabricated from aluminum, we observe an increase of Q i by a factor of 2 for an increase of the p MS by a factor of 10, presumably due to phonon trapping in the lower gap aluminum ground plane [68]. This result directly suggests QPs are a dominant dissipation source, which is confirmed by measurements on aluminum resonators [53], with 10 times smaller α compared to resonators A-C, and similar p MS , showing approximately a factor of ten increase in Q i [cf. grey rhombus in Fig.…”

“…The waveguide sample holders are successively surrounded by a series of shields and absorbing materials, to minimize stray radiation and magnetic fields (cf. [53] and Supplemental Material [54]).…”

Implementation of a Transmon Qubit Using Superconducting Granular Aluminum

“…The copper waveguide sample holder equipped with the 2D vector magnet (highlighted in color) is mounted at the dilution stage of a tabletop Sionludi dilution refrigerator[57] (gray scale), with a base temperature T base ≈ 20-30 mK. The flat copper cylinder visible in the lower part of the image is the lid of the outer shield (not shown), which consists of successive copper (Cu) and aluminum (Al) cylinders, similar to Ref [58]…”

“…The presence of contact junctions is expected because the Al grains in grAl are uniformly covered by an amorphous AlO x oxide. Taking into account the typically measured 15% kinetic inductance fraction in Al thin films of comparable geometry[58], we estimate the intrinsic kinetic contribution of the Al film to be 70 pH. The remaining 130 pH are associated with the contact junctions, from which we calculate a critical current I c ≈ 5 μA for each of them, with corresponding critical current density j c;JJ ≈ 0.13 mA=μm 2 , comparable to the critical current…”

Inductively shunted transmon qubit with tunable transverse and longitudinal coupling

“…III B [Eq. (42)], the equivalent in this case is a critical (minimum) number of junctions k crit that ensures a positive curvature of the potential. From here on, our strategy is the one described in Sec.…”