Very low resistance Al/Cu joints for use at cryogenic temperatures

TL;DR

This paper introduces two effective methods for creating ultra-low resistance copper-aluminum contacts at cryogenic temperatures, with potential applications in advanced cryogenic systems and heat switches.

Contribution

It presents novel gold plating and indium insertion techniques for low-resistance Al/Cu joints, achieving the lowest reported resistances for such contacts.

Findings

Gold plated contacts achieve resistances as low as 3 nΩ.

Both methods produce stable, low-resistance joints over time.

Surface preparation and compression are key to success.

Abstract

We present two different techniques for achieving low resistance ($<$20 n$\rm \Omega$) contacts between copper and aluminium at cryogenic temperatures. The best method is based on gold plating of the surfaces in an e-beam evaporator immediately after Ar plasma etching in the same apparatus, yielding resistances as low as 3 n$\rm \Omega$ that are stable over time. The second approach involves inserting indium in the Al/Cu joint. For both methods, we believe key elements are surface polishing, total removal of the aluminum oxide surface layer, and temporary application of large (typ. 11 kN) compression forces. We believe the values for gold plated contacts are the lowest ever reported for a Cu/Al joint of a few $\rm cm^{2}$. This technology could simplify the construction of thermal links for advanced cryogenics applications, in particular that of extremely low resistance heat switches for nuclear demagnetization refrigerators.