Heat-transfer microstructures for integrated circuits

TLDR: In this paper, a liquid partially fills an array of micron-wide repentant capillaries in the heat sink substrate, so that surface tension holds the polished back of an IC in intimate thermal contact with the sink.

Abstract: : The design of high-speed integrated circuits and systems is often constrained by thermal considerations. As late as 1981 it was authoritatively predicted that the maximum achievable power flux for liquid-cooled, densely-packed integrated circuits (ICs) would be about 20 W/sq cm. Convective heat-transfer theory indicates that well over 1000 W/sq cm can be compactly removed from ICs at normal operating temperatures, provided microscopic (e.g., 50-microns wide) extended-surface structures are used. The difficulty of constructing high-conductance, low-stress thermal interfaces between ICs and heat sinks suggests the use of an integral heat sink. Accordingly, IC microfabrication techniques were employed to design, fabricate, and test novel, ultracompact water-cooled, laminar-flow, optimized plate-fin and pin-fin heat sinks directly within standard-thickness silicon substrates. Worst-case thermal resistances as low as 0.083 deg C/W were measured from 1-sq cm thin-film resistors (e.g., a 108 deg C temperature rise at 1309 W), in good agreement with predictions. Further increases in heat transfer are achievable. The use of integral liquid-cooled heat sinks in multichip systems presents potential yield, reliability, cost and packaging problems. Attachment of unmodified ICs to micro-heat sinks seems a more attractive approach. A novel die-attachment technique has been developed which avoids the problems of conventional attachments. In this technique, a liquid partially fills an array of micron-wide repentant capillaries in the heat sink substrate, so that surface tension holds the polished back of an IC in intimate thermal contact with the heat sink. The bond is void-free, virtually stress-free, long-lived, and allows repeated detachment and replacement of ICs without damaging the heat sink substrate. The repentant grooves were fabricated by a novel process using electroless plating of nickel onto vertical silicon microgrooves.