Ex-situ Tunnel Junction Process Technique Characterized by Coulomb Blockade Thermometry

TL;DR

This study presents a wafer-scale fabrication method for tunnel junctions, characterized by Coulomb blockade thermometry, demonstrating low resistance deviation and potential for precise quantum device applications.

Contribution

It introduces an ex-situ tunnel junction fabrication process with detailed resistance uniformity analysis using Coulomb blockade thermometry.

Findings

Resistance deviation below 7.5% across 150 mm wafer

Array resistance deviation within ~5% of theoretical limits

Successful operation of Coulomb blockade thermometry at sub-1 K

Abstract

We investigate a wafer scale tunnel junction fabrication method, where a plasma etched via through a dielectric layer covering bottom Al electrode defines the tunnel junction area. The ex-situ tunnel barrier is formed by oxidation of the bottom electrode in the junction area. Room temperature resistance mapping over a 150 mm wafer give local deviation values of the tunnel junction resistance that fall below 7.5 % with an average of 1.3 %. The deviation is further investigated by sub-1 K measurements of a device, which has one tunnel junction connected to four arrays consisting of N junctions (N = 41, junction diameter 700 nm). The differential conductance is measured in single-junction and array Coulomb blockade thermometer operation modes. By fitting the experimental data to the theoretical models we found an upper limit for the local tunnel junction resistance deviation of ~5 % for the array of 2N+1 junctions. This value is of the same order as the minimum detectable deviation defined by the accuracy of our experimental setup.