Improving wafer-scale Josephson junction Jc variation by new oxidation process of Oxide Barrier

Hsiao-Wen Chang^1*, Yen-Pin Chang¹, Chuang-Ping Chiu¹, Tse-Jun Chen¹, Ming-Jye Wang¹

¹Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan

* Presenter:Hsiao-Wen Chang, email:hwchang@asiaa.sinica.edu.tw

A superconductor-insulator-superconductor (SIS) tunnel junction has increased greatly in recent decades because it is a strong candidate for the implementation of quantum bits (qubits). How to fabricate this tunnel barrier with ultrathin thickness (~1 nm), uniformity, leakage-free, and run-to-run reproducibility? These questions have recently taken on a new urgency with the rise of superconducting circuits in quantum computing, circuit quantum electrodynamics, and metrology.

We have developed a novel 2-step Al oxidation process, instead of 1-step oxidation exposure used in the regular oxidation process [1], for fabricating Nb/AlOx-Al/Nb junctions in the Jc ranging from 11.88 kA/cm2 to 2.04 kA/cm2 (as shown in Fig. a). Our 2-step oxidation process consists of an initial Al oxidation with a low oxygen partial pressure (~0.1mTorr) and a follow-up oxidation process with a higher oxygen partial pressure (~12 mTorr). Jc decays exponentially following with higher oxygen partial pressure (~12 mTorr) oxide-time from 8.67 mins to 300 mins. 2 inch-wafer-scale Jc variation for Jc is 5.06 kA/cm2 and 2.04 kA/cm2 are in the range of -10.8% to 8.58% and -11.93% to 14.74 %, respectively (as shown in Fig. b and c). Comparing to the 1-step oxidation process (Jc is 6.38 +/- 0.749 kA/cm2, and oxygen partial pressure ~12 mTorr), the best wafer-scale statistics (σJc)⁄〈 Jc 〉 =5.44 % instead of 11.9 %.

Our 2-step oxidation process was shown to further improve the wafer-scale uniformity which is a useful capability as tolerances become tighter for quantum processors, photon detectors and mixers growing in complexity, size, and qubit number.

[1] S. K. Tolpygo et al., (2017) Properties of Unshunted and Resistively Shunted Nb/AlOx-Al/Nb Josephson Junctions With Critical Current Densities from 0.1 mA/μm2 to 1 mA/μm2, IEEE Trans. Appl. Supercond., 27(4), 26-30.

Keywords: Superconductivity, Josephson junction, Critical current density, Aluminum oxide barrier, Oxidation process