Superconductor ICs: the 100-GHz second generation
Fabricating the novel Josephson junctions
Today's Josephson junction circuits are fabricated with a process technology that is reliable, reproducible, and rugged. The superconductor is made of niobium (Nb), or sometimes niobium nitride (NbN). A tunnel barrier of aluminum oxide creates sandwich-type Josephson junctions.
Two advances were key. In 1981, at the Sperry Research Center, in Sudbury, Mass., Harry Kroger, Larry Smith, and Don Jillie introduced what is now known as the trilayer process. Instead of depositing and then patterning each layer of the Josephson junction separately, as before, the whole wafer was completely covered with three layers of films: a first layer of superconductor Nb, then the barrier of amorphous silicon, and a final Nb superconducting layer. Next, the trilayer was patterned, using standard semiconductor processes, into discrete junctions that were later interconnected with superconducting lines. In principle, this trilayer process can be scaled into the deep submicron regime.
The second advance was the use of aluminum oxide rather than silicon oxide to form the junction barrier. In 1983, at AT&T Bell Laboratories in Murray Hill, N.J., Jochen Geerk, Michael Gurvitch, John Washington, and Maurice Washington found that Josephson junctions were much more reliable if an aluminum layer, which was briefly oxidized to form the necessary 1-2 nm of insulator, was sandwiched between the two niobium superconducting layers. (This process has been widely adopted in another recent technology, magnetotunneling devices for memories and sensors.) Such junctions show no degradation with multiple cooling cycles to 5 K; in addition, they can be stored indefinitely at room temperature.
The combined Nb/Al trilayer process is now used universally to make niobium superconductor digital ICs in R&D laboratories [see figure above]. It also appears in commercial systems. One example is primary voltage standards for the Système Internationale definition of the volt. Another is sensors that measure magnetic fields from the heart and brain.
But while the United States had used the trilayers to make just a small number of Josephson junctions, Japan took the final step of expanding that number. In a project funded by the Ministry of International Trade and Industry, researchers at Fujitsu, Hitachi, NEC, and Japan's Electrotechnical Laboratory (ETL) showed how the Nb/Al trilayers could be developed into a complete fabrication process for which the junction count is, in principle, limited only by the available lithography and wafer size.
Further, in an effort to raise chip operating temperature to 9-10 K, ETL, Hypres, and TRW have replaced the Nb/Al trilayer process with one that uses NbN as the superconductor and magnesium oxide as the barrier.
--D.K.B., E.K.T., & J.M.R