Packaging of devices for topside cooling by replacing air-bridges with Su-8 polymer bridges

This paper takes an in-depth look at the qualifications of Su-8 photoresist as a low-K dielectric in the fabrication of integrated circuits (IC's). Su-8 is a unique photo definable material that has been traditionally used in MEMS and waveguide fabrication. With its very large aspect ratios (50:1) (1) and very vertical sidewalls it has been used in the fabrication of some very unique MEMS structures. Although these two qualities are eye catching, it is its permanent qualities and resistance to wet chemistry that intrigues a packaging engineer. At this point, there is no common wet chemistry found in a clean room that can selectively remove Su-8 once it has been fully processed. This poses some very interesting possibilities for its use as a permanent dielectric for on-wafer processing in IC's as well as for electronic packaging that can withstand harsh environments. Specifically this paper discusses replacing air bridges on MESFET devices with polymer bridges fabricated from Su-8. This polymer bridge structure has several impacts for on-wafer processing as well as topside cooling packaging techniques for microwave devices. For on-wafer processing, replacing airbridges with polymer bridges eliminates the need for wafer thinning, through-wafer vias, and provides a dielectric interface material for Cu interconnects. Eliminating processing steps as well as the fragile topside air-bridge structures improves both the yield and the reliability. From a packaging perspective, fabricating polymer bridges allows further topside processing to take place in order to build packaging structures that can both remove the FET generated heat from the topside of the device as well as have access to the electrical I/O's.