Integrating Atomic Layer Deposition and Ultra-High Vacuum Physical Vapor Deposition for In Situ Fabrication of Tunnel Junctions

TL;DR

This paper presents an integrated ALD-UHV sputtering system enabling in situ fabrication of high-quality, ultrathin tunnel barriers for Josephson junctions, addressing surface nucleation challenges on metals.

Contribution

It introduces a novel integrated system combining ALD and UHV sputtering with in situ transfer, enabling improved control over ultrathin dielectric tunnel barrier fabrication.

Findings

Successful fabrication of leak-free Nb/Al/Al2O3/Nb Josephson tunnel junctions.

Demonstrated control over tunnel barrier thickness from sub-nm to 1 nm.

Validated the use of an Al wetting layer for ALD initiation on metal surfaces.

Abstract

Atomic Layer Deposition (ALD) is a promising technique for growing ultrathin, pristine dielectrics on metal substrates, which is essential to many electronic devices. Tunnel junctions are an excellent example which require a leak-free, ultrathin dielectric tunnel barrier of typical thickness around 1 nm between two metal electrodes. A challenge in the development of ultrathin dielectric tunnel barrier using ALD is controlling the nucleation of dielectrics on metals with minimal formation of native oxides at the metal surface for high-quality interfaces between the tunnel barrier and metal electrodes. This poses a critical need for integrating ALD with ultra-high vacuum (UHV) physical vapor deposition. In order to address these challenges, a viscous-flow ALD chamber was designed and interfaced to an UHV magnetron sputtering chamber via a load lock. A sample transportation system was implemented for in situ sample transfer between the ALD, load lock, and sputtering chambers. Using this integrated ALD-UHV sputtering system, superconductor-insulator-superconductor (SIS) Nb/Al/Al2O3/Nb Josephson tunnel junctions were fabricated with tunnel barriers of thickness varied from sub-nm to ~ 1 nm. The suitability of using an Al wetting layer for initiation of the ALD Al2O3 tunnel barrier was investigated with ellipsometry, atomic force microscopy, and electrical transport measurements. With optimized processing conditions, leak-free SIS tunnel junctions were obtained, demonstrating the viability of this integrated ALD-UHV sputtering system for the fabrication of tunnel junctions and devices comprised of metal-dielectric-metal multilayers.