SNS and SIS Josephson junctions with dimensions down to the submicron region prepared by a unified technology

Abstract

Several applications of Josephson junctions (JJs) in low-temperature superconducting circuits such as programmable voltage standards and SFQ logical circuits or SQUID sensors require JJs with sub-µm dimensions. A method for preparing superconductor-isolator-superconductor (SIS) JJs as well as superconductor-normal metal-superconductor (SNS) JJs was developed which allows the preparation of both types of junctions by nearly the same technological process. Titanium is used as the normal metal for the SNS JJ. Ti can be anodically oxidized and a technology similar to the well-tried Nb-Al technology for SIS JJs can be applied to the SNS system, too. By electron-beam exposure of a negative deep UV photoresist (ma-N 2400) a stable resist mask is generated which has to withstand the following three self-aligned process steps: etching of the Nb counterelectrode by RIE, isolation by anodization and lifting of a SiO planarization layer. By this process SNS (Nb-Ti-Nb) and SIS (Nb-Al/AlOx-Nb) Josephson junctions with smallest junction areas of 0.49 µm2 were prepared and characterized by dc measurements and under the influence of microwave irradiation. The Nb-Ti-Nb junctions have IcRn products of 87 µV. They can be used for programmable Josephson voltage standard circuits at frequencies of 70 GHz.

Transport and Noise Properties of sub-100-nm Planar Nb Josephson Junctions with Metallic Hf - Ti Barriers for nano-SQUID Applications

“…The latter feature is in contrast to other conventional overdamped junctions, like Dayem bridges (see, e.g., Ref. [19]), small superconductor-insulator-superconductor (SIS) [20] or intrinsically shunted SNIS JJs [21], which typically have V c of the order of 100 μV. However, SNS junctions combine a number of crucial advantages, making them suitable for the realization of nano-SQUIDs.…”

Nanomembrane-Based Mesoscopic Superconducting Hybrid Junctions

“…This value is substantially larger than other devices fabricated using niobium-normal metal-niobium junctions, usually having I c R n products in the range of hundreds of microvolts up to a few millivolts. 43,44 This can be partially explained by two distinct factors present in our system. First, the short normal metal channel length of 50 nm leads to an inherently high I c (I c ∝ L -1 ).…”

Aluminum hard mask technique for the fabrication of high quality submicron Nb/Al–AlOx/Nb Josephson junctions

“…While a precise and reproducible definition of the JJ size is routinely performed with the help of electron-beam lithography, the application of anodic oxidation in such processes remains a challenge. This is due to the fact that the Nb 2 O 5 layer formed during the anodic oxidation creeps under the resist mask protecting the top electrode of the JJ (this is known as 'encroachment' [1,2]). Even for a plasma hardened resist and an anodization voltage of 20 V, a significant encroachment was still observed [1].…”

“…Since we found that a certain Nb 2 O 5 thickness is needed to ensure low subgap leakage currents, this was not an option for us. Other groups have replaced the resist mask with an insulating hard mask such as SiO or SiO 2 [2,[7][8][9][10], which entirely eliminates the encroachment. These hard masks have to be removed by an additional process step, either by reactive ion etching (RIE) through the insulator right above the JJ or by chemical mechanical polishing (CMP).…”