Effects of the film thickness of a ground plane in the SFQ circuits with a dc-power layer

We have investigated the effect of a dc-power (DCP) layer on the performance of single flux quantum (SFQ) circuits by changing the film thickness of a ground plane (GP). The DCP layer was placed below the GP and was used as bias current feeds. Two measurements were conducted. One was direct detection of the magnetic flux above the GP by using SQUIDs. The results indicated that the GPs with film thicknesses of more than 400 nm were effective to completely suppress the flux under the condition that the return current was extracted from the ground plane. The other was evaluation of the operating margins of the 408 bit shift register circuits (SRs) with 8032 Josephson junctions and a total designed bias current of 1.05 A. The results showed that a 500 nm thick GP had an advantage over a 300 nm thick GP in the dependence of the upper margin on the bias current port of the SR