Large Scale Integration Issues in Superconducting Circuits
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Superconducting digital logic technology, Rapid Single Flux Quantum (RSFQ), is established as the fastest technology in microelectronics based on Josephson junction devices. Technological challenges in large scale integration of superconductor circuits (RSFQ) namely: flux trapping, low power dissipation and multi-chip modules are addressed here. One of the main limitations of the integration of the RSFQ circuits is flux trapping, which arises due to the presence of residual magnetic field during transition of the circuit into superconducting state. Flux trapping is an unusual problem specific in only superconducting circuits. The effect of flux trapping in RSFQ circuits is studied, with a controlled 3-D magnetic field setup, quantitatively. Layout configuration with different moat patterns making RSFQ circuits tolerant to external magnetic field up-to 20mG was also developed. Low power issue was addressed by developing a new logic family. Power independent logic, an improved form of RSFQ logic, enables the circuits to operate only when needed and circuits can be switched off the remaining time; thereby eliminating static power consumption by retaining the logic state of the circuit. Flux trapping has also beeninvestigated in power independent cells. Current recycling is a technique to reduce the bias current applied for the operation of the RSFQ circuits. In this technique the cells are biased by serial connections. The reduction in supply current is proportional to number of blocks connected in series The effect of magnetic field on the operating margins of the current recycling circuits has also been studied. Operation of Multi-chip modules, one of the most viable solutions to develop large scale circuits, was demonstrated. Multi-chip modules with lithographically designed bumps is presented where data transmission rate between two superconducting chips exceed 100GHz. Finally, the development of the multi-modulator ADC for low noisefront end under-development is discussed.