Enabling Transfer of Ultra-Thin Layers of GaN for Demonstration of a Heterogenous Stack on Copper Heat Spreader

This letter demonstrates heterogeneous stacking through postfabrication transfer of a thin layer of AlGaN/GaN high-electron-mobility transistors (HEMTs) and Si metal–oxide–semiconductor field-effect transistors (MOSFETs) onto a copper heat spreader. Both devices were fabricated on silicon substrate using separate process flows. Transfer of ~3- $\mu \text{m}$ GaN layer grown on silicon, however, leads to cracks due to the high-stress gradients arising from large lattice and thermal mismatch. Electroplated copper was used to improve mechanical strength, allowing transfer of the GaN layer using Cu–In bonding. Use of electroplated copper ensures high thermal conductivity to the bonded heat spreader. Next, an ultrathin silicon layer ( $\sim 1.5~\mu \text{m}$ ) with functional NMOS transistors was stacked above the GaN devices using a cost-effective epoxy bonding approach. The Cu–In bonding not only improved thermal dissipation but also led to enhanced performance of the GaN device due to the compressive stress induced by the bonding process. This compressive stress will also provide reliability against coefficient of thermal expansion (CTE) mismatch at higher operating temperatures.