Measurements of device parameters on large arrays of Josephson interferometers

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 x 6.35-mm Si chip. Threshold currents I/sub m/(0), subgap resistances R/sub j/, and V/sub m/ : the product of R/sub j/ and I/sub m/(0) : were investigated. Statistical analysis indicated that the standard deviation sigma of I/sub m/(0) and R/sub j/ ranged between 6 and 18% and that of V/sub m/ ranged between 5 and 7%. A systematic spatial dependence of I/sub m/(0) and R/sub j/ 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 I/sub m/(0) (and R/sub j/) 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