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 AlO X Nb junctions to new material systems. Two major limiting factors of higher frequency operation are the increase in subgap leakage that occurs in AlO X barriers as current densities approach and exceed 10 kA cm 2 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/AlN/NbTiN junctions. The AlN 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 m 2 . 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.Index Terms-Aluminum nitride, inductively couple plasma, niobium titanium nitride, SIS.

Investigation of Material Growth and Fabrication Processes for THz Frequency SIS Mixers

“…[34] to nitridation of Al films via parallel-plate plasma [35], ion gun [36], and ICP [24]. Previous attempts at trilayer growth utilizing ICP grown AlN barriers at UVML produced fairly low-leakage high-quality SIS junctions, however it was shown that the repeatability of the normal state resistance junction area product, ( ), of successive wafers drifted over time by up to an order of magnitude [38]. The ability to predict and control junction impedance is paramount to the realization of superconducting RF circuits, such a drift in is a limiting factor.…”

“…Upon the start of my PhD research, our research group had demonstrated the ability to produce fairly low-leakage high-quality Nb/Al-AlN/Nb SIS junctions utilizing ICP grown AlN barriers, however, it was shown that the repeatability of , drifted over time by up to an order of magnitude [38]. Such a drift in made it impossible to realize SIS based mixers of proper design impedance.…”

Effect of metamaterial engineering on the superconductive properties of ultrathin layers of NbTiN

“…Moreover, ultrathin superconducting films are known to exhibit extremely high values of the critical magnetic field H c . 8 The ability to maintain (or even increase) the T c of a superconducting coating while keeping its critical magnetic field at the same level as for the ultrathin films can lead to many advances in technological applications of superconductors such as THz SIS (superconductor/insulator/superconductor) receivers, 9,10 superconducting single photon detectors (SNSPDs), 11 qubits, and quantum electronics. [12][13][14] Multilayered SIS metamaterial coatings with tailored thicknesses for superconductor and insulator materials are foreseen to shield vortex penetration and enhance the maximum sustainable magnetic field for niobium cavities used in particle accelerators.…”