Recent advances in high temperature superconductor multichip modules

Two different techniques for fabricating high temperature superconducting (HTS) MCM-D substrates have been developed and tested. The first unit consists of two digital gallium arsenide bare die connected by YBa₂Cu₃O_7-δ (YBCO) HTS interconnects to form two ring oscillators on a 2.25 cm² MCM-D substrate. The interconnections consist of two wiring layers of YBCO separated by a 4-5 μm silicon dioxide interlevel dielectric. The signal lines are routed between power and ground lines which form an interconnected mesh power system (IMPS) and, thereby, the module avoids the necessity of having two additional layers for power and ground planes. Connection between the two YBCO layers is accomplished with low contact resistance 40 μm gold vias through the interlevel dielectric layer. The signal interconnects have 50 μm linewidths and 75 μm spacings. Electrical connections between the die and the MCM substrate and between the substrate and the PC board were made using ultrasonic Al wire bonds to low contact resistance gold/YBCO bondpads on the MCM substrate. The second module, known as the Flip-Mesh superconducting MCM; provides an alternative to the multilayer MCM-D substrate described above. It involves using flip chip bonding techniques to connect multiple single-layer substrates, thereby reducing the processing complexity of fabricating multiple layers. X-plane and Y-plane interconnects are fabricated on separate substrates and interconnected using solder bumps. The IMPS topology is also utilized in this structure so that power, ground, and signals can be fabricated on two planes. The initial Flip-Mesh design incorporates 100 μm (4 mil) solder bump vias with similar spacings, which results in a low packing density for MCM-D technology, but a high density for I/O technology