Substrate temperatures of liquid nitrogen cooled multichip modules utilizing wirebonded die

Substrate temperatures for liquid nitrogen cooled MCM assemblies utilizing wire-bonded chips were investigated to determine the thermal limits for using superconducting chip-to-chip interconnects. A boiling curve was obtained for this configuration from Sandia´s ATC-03 assembly test chips using integral surface resistance heaters and diode thermometers which indicated that 23 W/cm² could be dissipated by nucleate boiling before the onset of film boiling at 90 K, 13°C above nitrogen´s 77 K boiling point at atmospheric pressure. This measured boiling curve was incorporated into a finite-element model of an array of face-up 1 cm² silicon chips on an MgO substrate and used to predict the temperature on the surface of the interconnecting substrate as a function of chip power dissipation, distribution of the powered area on the chip, and chip separation. ICs on such an MCM should not be operated in the film boiling regime because temperature fluctuations peaking as high as 118 K could propagate from the upper surface of the chips, through the silicon and die attach material and down to the superconducting interconnects. Therefore the onset of film boiling anywhere on the assembly represents an upper limit of power dissipation for the system. 1 cm² chips separated by 2 mm could dissipate 30 W each before film boiling began in the middle of the top of the chips resulting in a maximum substrate temperature of 89.8 K directly under that spot. Since film boiling in this closest-packed configuration begins at a temperature in the vicinity of Tc for YBCO systems, decreasing critical current density may limit the operating temperature of these interconnects before the onset of film boiling on the chips. Superconductors based on thallium or mercury have sufficient headroom to avoid this further limitation