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Windows for Defining the Action Area of Nb Tunnel Junctions

IP.com Disclosure Number: IPCOM000052276D
Original Publication Date: 1981-May-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 50K

Publishing Venue

IBM

Related People

Broom, RF: AUTHOR [+3]

Abstract

Many applications of Josephson tunnel junctions require the area of the junction to be defined by a "window" in a relatively thick insulator placed over the base electrode, as shown in Fig. 1. An essential step in obtaining good tunneling characteristics with Nb base electrodeg is sputter etching of the surface in a high energy, low pressure Ar plasma, before growth of the tunneling oxide. However, in a clean system and with extended cleaning time, a conducting film of back-sputtered Nb is deposited on the side walls of the window. Being nearly vertical at the lower edge, this film oxidizes much more slowly than the base electrode, forming effectively a shunt resistance across the tunnel barrier of the completed junction which contributes a high leakage current.

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Windows for Defining the Action Area of Nb Tunnel Junctions

Many applications of Josephson tunnel junctions require the area of the junction to be defined by a "window" in a relatively thick insulator placed over the base electrode, as shown in Fig. 1. An essential step in obtaining good tunneling characteristics with Nb base electrodeg is sputter etching of the surface in a high energy, low pressure Ar plasma, before growth of the tunneling oxide. However, in a clean system and with extended cleaning time, a conducting film of back- sputtered Nb is deposited on the side walls of the window. Being nearly vertical at the lower edge, this film oxidizes much more slowly than the base electrode, forming effectively a shunt resistance across the tunnel barrier of the completed junction which contributes a high leakage current. The effect can be eliminated by changing the profile at the border of the window. Suitable examples are shown in Fig. 2 and 3.

In Fig. 2, tapered side walls are used which can be obtained by plasma etching. Due to the oblique angle of incidence of the sputtered Nb, the film deposited on the edge is very thin, and it oxidizes more rapidly than in the case of a near-vertical film. The combination of these two factors results in a greatly reduced leakage conductance. Fig. 3 shows an undercut window edge which can be formed by,depositing the insulator on a thin dielectric or metal film (e.g., 100 Angstroms thickness), which is then selectively etche...