Development of Nb/Al-AlN/NbTiN SIS Junctions With ICP Nitridation

Abstract

Increasing the operating frequency of SIS receivers requires a shift from Nb/Al-AlOX/Nb junctions to new material systems. Two major limiting factors of higher frequency operation are the increase in subgap leakage that occurs in AlOX barriers as current densities approach and exceed 10 kA/cm2 and the increased loss in Nb electrodes above ~ 700 GHz. A promising alternative structure is the hybrid Nb/Al-AlN/NbTiN junction. Realization of these devices has been difficult due to the challenge of fabricating devices with repeatable current densities and electrical characteristics. We present on the fabrication and dc testing of Nb/AIN/NbTiN junctions. The A1N barrier is formed using our inductively coupled plasma (ICP) technique which allows for independent control of both ion energy and current density. This improved control enables the repeatable synthesis of high quality and high Jc barriers. Nb and NbTiN electrodes are deposited by unbalanced dc magnetron. A new fabrication process was developed to enable fabrication of junctions with area as small as 0.28 mum2. The relationship between barrier thickness and plasma conditions is determined by in-situ discrete ellipsometry. Ellipsometry results were verified by comparison with measured current-voltage characteristics. I(V) curves for a range of junction sizes are presented. Plans for in-situ Faraday monitoring of the energy and current density of the ICP nitridation plasma will also be discussed.