Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Fabricating NIS tunnel junctions on single crystal superconducting substrate by a lithographic process



Anna Clark, Marcel L. van den Berg, Joel Ullom


We present a novel design for a solid-state microrefrigerator using Normal-Insulator-Superconductor (NIS) tunnel junctions. Cooling of the normal electrode is achieved by electrons tunneling into the superconducting electrode, providing a means of continuous refrigeration at temperatures below 300 mK. The slow diffusion of quasiparticles away from the tunnel barriers limits the cooling power of thing film NIS microrefrigerators for two reasons. First, the efficiency of energy removal from the normal electrode is reduced as quasiparticles build up in the superconducting electrode. Second, phonons produced by quasiparticle recombination near the junction can enter and heat the normal electrode. Using single crystals as both the substrate and the superconducting electrode can prevent the accumulation of quasiparticles near the tunnel barrier. A large volume, high purity electrode will allow quasiparticles to rapidly move away from the junction, thereby eliminating the self-heating. We are developing a photolithographic process to fabricate NIS devices on Al single crystal substrates. Presented here are results from test devices with a 50 υm x 50 υm Al-Al2O3-Cu tunnel junction deposited on SiN substrates.
IEEE Transactions on Applied Superconductivity


NIS junctions, tunneling, Al single crystal, microrefrigerators


Clark, A. , van den Berg, M. and Ullom, J. (2003), Fabricating NIS tunnel junctions on single crystal superconducting substrate by a lithographic process, IEEE Transactions on Applied Superconductivity (Accessed April 14, 2024)
Created May 31, 2003, Updated October 12, 2021