Design and testing of high-speed interconnects for superconducting multi-chip modules

Superconducting single flux quantum (SFQ) circuits can process information at extremely high speeds, in the range of hundreds of GHz. SFQ circuits are based on Josephson junction cells for switching logic and ballistic transmission for transferring SFQ pulses. Multi-chip modules (MCMs) are often used to implement larger complex designs, which cannot be fit onto a single chip. We have optimized the design of wideband interconnects for transferring signals and SFQ pulses between chips in flip-chip MCMs and evaluated the importance of several design parameters such as the geometry of bump pads on chips, length of passive microstrip lines (MSLs) and number of corners in MSLs as well as flux trapping and fabrication effects on the operating margins of the MCMs. Several test circuits have been designed to evaluate the above mentioned features and fabricated in the framework of a 4.5 kA cm ^-2 HYPRES process. The MCM bumps for electrical connections have been deposited using a wafer-level electroplating process. We have found that, at the optimized configuration, the maximum operating frequency of the MCM test circuit, a ring oscillator with chip-to-chip connections, approaches 100GHz and is not noticeably affected by the presence of MCM interconnects, decreasing by only about 3% with respect to the same circuit with no inter-chip connections.