High Density and Heterogeneous Packaging of MCMs by Epitaxial Liftoff Technology

Epitaxial Liftoff (ELO) Packaging is the packaging of extremely thin crystalline semiconductor films of only a few microns thick. This is done by removing the semiconductor substrate after fabrication of the devices on the wafer. The results are increased efficiency of the device and subsequent package with respect to thermal, electrical, and density issues. One major concern in high density MCM's is the thermal density of such a package. Through the use of ELO, the device layer itself usually separated from the heat sink by the distance of the semiconductor substrate, may be immediately coupled to a heat sink improving thermal efficiency of the MCM Furthermore, by using circuits with the substrate removed, both device and packaging parasitics may be eliminated or reduced, reducing power consumption of the MCM This paper presents an Epitaxial Liftoff process flow for packaging of high density circuits and bonding of the thin film devices and chips to virtually any substrate. The process uses standard flip chip equipment and technology, but process modifications have been introduced to accommodate the extremely thin device structures. The techniques to bond the circuits to heat sinks, transparent substrates, or flexible substrates - potentially achieving flexibility of the substrate and the chip itself- are presented.