Study of extreme low temperature and load solid-phase Sn-Ag system bonding mechanism for 3D ICs

The objective of this study is i) to optimize a new nano solid phase, Ag-Sn thin-film bonding system for wafer-level 3D-stacking for 3D ICs, and ii) to clarify its bonding mechanism. As reported in our previous study, we achieved bonding at a much lower temperature (180°C), with lower load (0.4MPa) and much shorter time (5 min) compared to Cu-Cu direct bonding. Moreover, the bonded interface had high heat resistance (> 480°C) when we deposit the main bonding material of Sn (low melting point metal) and Ag (formation compound with Sn) as a multi-layer film. By performing detailed analysis using TEM, we find that formation of Sn-Ag type intermetallic compound occurs and that the bonding volume contracts, i.e., Sn+Ag ⇒ Sn-Ag IMC: about 6%. This contraction results in extremely high pressure being applied to an un-bonding process caused by micron roughness and an impurity layer, e.g., oxide, at the interface. A bonding process we call “self-compression bonding” then sequentially proceeds at unbonded interface regions. We hypothesize good bonding is achieved over the entire interface in a short time period due to self-compression bonding.