Fabrication of sub-μm Nb/Al-AlO$_x$/Nb Josephson Junctions with Electron Beam Lithography for Qubit Applications

Lucia Capogna, Giovanni Costabile, Paola Barbara +2 more

Gustavo Moreto, Diego Molina, Everton Arrighi +1 more

Haifeng Yu, Cao Wen-Hui, Zhu Xiao-Bo +6 more

Nb/Al–AlO x /Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al–AlO x /Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with V m = 100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm 2 , which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.

Vijay Patel, W. Chen, Shawn Pottorf +1 more

The authors describe a process for fabrication of Nb/AlO/sub x//Nb Josephson junction circuits for quantum computation. The process involves only one etch step and incorporates electron beam lithography and a self-aligned lift-off of the dielectric resulting in a short turn-around time of 4-5 days. Deep submicron junctions of size down to 0.15 /spl times/ 0.15 /spl mu/m/sup 2/ have been fabricated and tested. Results of junction quality measurements will be presented. In particular, a subgap resistance of /spl sim/1 G/spl Omega/ measured at /spl sim/0.4 K indicates that the junction's subgap leakage should not be a limitation for quantum computation.

Paola Barbara, G. Costabile

W. Chen, Alexander V. Rylyakov, Vijay Patel +2 more

Rapid Single Flux Quantum (RSFQ) T-flip flops (TFFs) operating up to 770 GHz have been demonstrated at 4.2 K. The devices, consisting of either resistively shunted or unshunted Josephson junctions, are fabricated using a planarized Nb/AlO/sub x//Nb trilayer process. Electron beam lithography is used to pattern all levels with a minimum junction area less than 0.1 /spl mu/m/sup 2/. Critical current densities of 0.5 mA//spl mu/m/sup 2/ and 2.5 mA//spl mu/m/sup 2/ are used for the shunted (tested at 1.8 K) and unshunted devices (tested at 4.2 K) respectively. The input and output frequencies of the TFFs are obtained from the input and output voltages by the Josephson relation. The output voltage is exactly half of the input voltage when the divide-by-two operation is correct.