Measurements of device parameters on large arrays of Josephson interferometers

Abstract

Measurements on large arrays of Josephson interferometers were carried out to obtain individual device parameters and their spread. Each array contained 784 identical interferometers with junction diameters of 5 and 7 μm and was deposited on a 6.35×6.35-mm Si chip. Threshold currents Im(0), subgap resistances Rj, and Vm - the product of R j and Im(0) - were investigated. Statistical analysis indicated that the standard deviation σ of Im(0) and R j ranged between 6 and 18% and that of Vm ranged between 5 and 7%. A systematic spatial dependence of Im(0) and Rj on the chips was obtained which is attributed as being due to nonuniformities in the rf oxygen discharge used to form the tunnel barrier. With improvements in the process, namely, eliminating these nonuniformities, the standard deviation in Im(0) (and Rj) can be reduced to about 5%. From the observed variations in the junction areas it is estimated that the random variation in the Josephson current density is about 4%. Large scale applications of these interferometers in the design of computer memory and logic thus seem feasible.