Light-anodization process for high-J/sub c/ micron and submicron superconducting junction and integrated circuit fabrication

Abstract

We have developed a new approach for high critical current density ( ) small junction fabrication. The key step is light anodization that forms a thin double-layer of Al 2 O 3 Nb 2 O 5 oxides around the junction area and on the sidewalls of the junction. This anodization ring is a good dry-etch stop, so the via for the junction contact can be larger than the junction area. The anodization ring can also protect the junction from plasma damage during dry etching and sputtering steps; therefore, it can reduce the junction leakage current and critical-current spread. The new technique is very simple and cost effective compared with the CMP approach. It needs only one additional mask and process step. We have used the technique to fabricate high-submicron Nb/Al-AlO /Nb tunnel junctions with very low critical-current spreads. Using this technique, we have also fabricated Nb SQUID's and various Nb digital IC's.

Characterization of the reliability and uniformity of an anodization-free fabrication process for high-quality Nb/Al–AlOx/Nb Josephson junctions

“…In contrast to conventional SNEP [9], our fabrication process completely avoids anodization techniques for the lateral junction definition. Typically, anodization is used to define the junction area and to insulate the base electrode as well as the sidewalls of the counter-electrode reducing the probability for shorts between the base electrode and additional wiring layers and protecting the tunnel barrier in succeeding fabrication steps (see, for example, [21,22]). However, standard anodization techniques require electrical connections to the structures that are to be anodized which is contrary to the fact that a rf-SQUID is by design electrically floating.…”

Plasma process-induced damage to Josephson tunnel junctions in superconducting integrated circuits

“…It is known that these processing steps can potentially degrade the quality of Josephson junctions, especially when the thin tunnel barrier around the perimeter of the junction is exposed to processing chemicals. This problem is usually solved by anodizing the Al/AlO x layer around the junctions and Nb sidewalls of the junction CE in a process often referred to as light anodization [4]. This is essentially a very light modification of the original selective Nb anodization (SNAP) and selective Nb etch (SNEP) processes employed for Josephson junction fabrication many years ago [5,1] but performed at a lower voltage.…”

Optimization of Nb/Al-AlO x /Nb Josephson junctions through wafer-scale anodic oxidation: a systematic characterization and performance analysis

“…The electrolyte used for anodic oxidation was a mixture of ammonium pentaborate (390 g), ethylene glycol (2.56 l), and deionized water (1.76 l) [23,28,29]. Ammonium pentaborate, which provides H + , promotes the reaction [30], whereas ethylene glycol improves the ion mobility and surface flatness of the films [31,32].…”

“…The process involves placing a wafer with SIS junctions at the anode and a metal cathode plate opposite, with a constant current applied between them. Various methods of SIS junction anodization have been explored, including selective Nb anodization [20], selective Nb etching process [21], self-aligned contact fabrication [22], light anodization process [23], and selective capacitive anodization process [24]. Apart from SIS junction protection, anodic oxidation is also used to decrease the SIS junction scale, particularly in fabricating junctions with a scale <1 µm by sidewall oxidation; anodization spectroscopy is used to identify the SIS junction layer structure before the device microfabrication process [25,26], and Nb 2 O 5 films with high dielectric constants are used to create large capacitors in superconducting electronics [13,27].…”