  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.
