(Invited) Cu Surface Passivation with Self-Assembled Monolayer (SAM) and Its Application for Wafer Bonding at Moderately Low Temperature

Introduction 3D IC is identify as a technology platform to augment present CMOS scaling to ensure continuous performance enhancement (“More Moore”) and functional diversification (“More than Moore”). 3D IC is simply the process of stacking and interconnecting several chips vertically. While TSV pitch <10 μm has been demonstrated [1], scaling of micro-bump to this pitch has not kept up with the pace due to process issues. Cu/Sn micro-bump [2] also has poorer electrical, mechanical and thermal properties due to the formation of inter-metallic compounds. Cu-Cu bonding provides an alternative for future scaling to10 μm pitch with improved physical properties. Besides the technical merits, a number of process challenges must be overcome for Cu-Cu bonding to be a viable solution including low temperature bonding (< 300 C) at high throughput. One major barrier to low temperature bonding is the deterioration of Cu surface during exposure in ambient. Several methods for Cu-Cu bonding have been reported [3-5] with promising results, but these processes are usually UHV-based, corrosive or require ultra-smooth surface. In this paper, low temperature (< 300 C) thermo-compression bonding of bump-less Cu-Cu in the context of 3D IC application is discussed. The Cu surface degradation is overcome by the application of a self-assembled monolayer (SAM) of alkane-thiol as a temporary passivation layer. This method is potentially attractive as it is non-UHV and non-corrosive.