Josephson circuit designs often place tight tolerance requirements on the junction critical current values. In manufacturing Josephson junctions to pre-specified critical and photolithographic size is important. We present a language to describe critical current reproducibility and reproducibility using a database of a thousand individual Josephson junctions in addition to junction arrays, manufactured over many months in the TRw superconducting electronics process line. We measure less than 2 percent (lo) spread critical currents for 10 micrometer junctions. Fig. 1. Similarly, junction current density may deviate from designed values. The equations current, accurate control over both wafer critical current density Jc = Jc, ("JA (2) uniformity. We have statistically analyzed junction d = ddesign + d~ (3)define a total fraCtiOna1 Critical CLUrent density departure frOm design, JA> and a total dimensional departure frOm design, dA.

We have designed and constructed a primary thermometry system for operation from 4.2 K to temperatures well below 0.65 K, the bottom of the International Temperature Scale (ITS-90). The system is composed of a thin-film resistive-SQUID (R-SQUID) and a dc SQUID readout amplifier fully integrated on a single chip. We have fabricated our SQUIDs using Ti/Pd/Au shunt resistors, which remain normal conductive down to mK temperatures. Thermal and electrical constraints demand that the readout amplifier to the R-SQUID have a low system noise. We used a dc SQUID series array in a direct readout flux-locked loop scheme to achieve a system bandwidth in excess of 1 MHz with a white noise level less than 1 pad&.

Niobium carbonitride (NbCN) Josephson circuits can operate over a wider temperature range than either niobium or niobium nitride circuits. Higher operating temperature places NbCN technology more comfortably within the range of closed cycle refrigerators, a key factor in aerospace applications. We have fabricated tunnel junctions from NbCN films with transition temperatures up to 18 kelvin. High quality NbCN tunnel junction fabrication generally requires low stress films with roughness less than the barrier thickness (=20 A). We have developed scanning tunneling microscopy as a tool for measuring and optimizing film smoothness. Junctions formed in situ with AlN tunneling barriers show reproducible I-V characteristics. Unlike NbN, NbCN oxidizes readily, enabling junction definition by a modified SNAP process. This SNAP lithography technique successfully produced NbCN/AlN/NbCN junctions with Vm values (at 3 mV) up to 27 mV. These results demonstrate a fabrication technology compatible with realistic closed cycle refrigerator operating temperatures.

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