Multichip modules of high packing density and with multi-gigahertz bandwidth are mandatory to transfer the on-chip circuit speed to the system level. Since the circuits are low impedance, the VLSI chip and the multichip system call for high currents. Because the power lines can be made of zero resistance buses, the voltage drop and cryogenic power supply waste can be made very small. On the MCM, the signals themselves can once again be transferred by superconductive transmission lines. It always will be necessary to have multiple levels of wiring for a multichip system, and this is achieved by alternate layers of ground and connection lines which are spaced to form strip-line or microstrip transmission lines. These low impedance lines have exceptionally low crosstalk, enhanced by the magnetic field expulsion inherent in superconductivity. Such MCMs have already been built at three levels of metallization and are now being fabricated at five levels for a 20-chip superconductive crossbar.
An alternative approach has been demonstrated by KYOCERA in conjunction with MITI's Electro-Technical Laboratory. They have built a multilevel ceramic MCM with nine levels of tungsten connectors for powering and three levels of superconductor for signal transport. The ``breakthroughs'' here were the ability to make a ceramic that matched the thermal expansion of the superconductive chips and the fabrication of superconductive lines on this medium. The MCM is designed to mount ETL's 4-chip prototype supercomputer and drive it at 1.25 GHz, 3.75 GHz, and finally, at 10 GHz clock rates.