Rapid Single Flux Quantum Random Access Memory

A new design concept for the Josephson-junction random access memory (RAM) has been developed. In contrast to previous RAMs based on single flux quantization (SFQ), in our system READ and WRITE operations employ ballistic transfer of SFQ pulses along bit lines (either Josephson transmission lines, or passive superconducting microstrip lines, or their combination). The basic memory cells are the single-junction SQUIDs, connected serially by the bit lines and inductively coupled to word lines. READ and WRITE operations are performed by sending SFQ pulses in appropriate directions along bit lines, and dc currents of appropriate polarity into word lines. This approach allows design of very dense memories with n<2 Josephson junctions per bit, memory cell area smaller than 80λ2 (where λ is the minimum feature size), and the critical parameter margin well above ±20%. In this paper we present the general structure of the RSFQ RAM, as well as design and results of testing of the basic memory cell and decoder circuitry using Hypres’ 3.5-µm, l-kA/cm2 Nb-trilayer technology.