UC Riverside

Superconducting digital logic electronics based on Josephson junctions is established as the ultra-fast speed and ultra-low power consumption microelectronics device. Significant progress has been made in the development of superconducting digital circuits based on low Tc superconductors. Rapid single flux quantum (RSFQ) logic has been experimentally demonstrated to operate up to 770 GHz clock frequency with a bit-error rate close to the measurement limit. The power consumption is extremely low in such high-frequency operation, about three orders of magnitude less than that of CMOS logic. The energy dissipation of Quantum flux parametron (QFP) has been reduced to the order of zJ per transition. However, low Tc digital circuit requires more complicated cryogenic systems to operate below liquid helium temperature (sub-kelvin temperatures). An effort has been made to develop high Tc digital circuits for operation above nitrogen temperatures. Further development is needed to solve a number of problems. In order to enable circuit functionalities and complexities required for computing, a very large-scale integration of superconducting circuit is needed. Compared to the typical CMOS circuit integration scale, due to the physical limitations on the size of superconducting logic cells made by the traditional fabrication process, the present superconducting integrated circuit density is several orders of magnitude lower.

The realization of using a focused helium ion beam (FHIB) to write an over-damped Josephson junction on the material of yttrium barium copper oxide (YBCO) will further extend the digital logic technology to high Tc superconductors and allow the circuit to be tested at liquid helium temperature, which will also make the integration with nitrogen-cooled circuits easier. Furthermore, a multilayer process is presently used to make Josephson junction, which is complicated and limits the scale of the circuit. The ion irradiation technology makes the circuits simpler. It is possible to fabricate nanoscale circuits, allowing to design of more complex circuits and solving the challenge of large-scale integration. In this dissertation, we show the simulations and measurements of some basic RSFQ cells made by high Tc materials, which can monitor the propagation of signals and prove the logic function of the chip fabricated by FHIB technology. In the future, we will continue the prospects for a further increase of the operation temperature and implementation of a complex digital circuit.