Combined Surface-Activated Bonding Technique for Low-Temperature Cu/SiO2 Hybrid Bonding

Low-temperature wafer bonding is one of key enabling technologies for manufacturing silicon-on-insulator (SOI), micro-electromechanical systems (MEMS), and the emerging three-dimensional (3D) integration. Among many other bonding approaches, low-temperature SiO₂-SiO₂ bonding and Cu-Cu bonding have been extensively studied because of their high bonding quality and CMOS compatibility. As an evolution of SiO₂-SiO₂ bonding and Cu-Cu bonding, Cu/SiO₂ hybrid bonding provides both ultrafine metal interconnections and enhanced bonding stability during the single bonding process. However, this bonding approach remains difficult because of the challenging simultaneous surface activation of the hybrid surface. For instance, plasma treatment is effective for SiO₂ surface activation, but it may induce formation of Cu₂O/CuO and increase of surface roughness on Cu surface, which prevents successful hybrid bonding at low temperatures. Therefore, novel bonding techniques with compatibility with hybrid surface are highly desired for low-temperature Cu/SiO₂h hybrid bonding. 

This work develops a combined surface-activated bonding (SAB) technique for low-temperature Cu/SiO₂ hybrid bonding at below 200 °C. The technique involves combinations of Ar beam bombardment, Si deposition, and water vapor exposure for prebonding surface activation prior to bonding in vacuum. Homogeneous (i.e., SiO₂-SiO₂ and Cu-Cu) and heterogeneous (i.e., Cu-SiO₂) wafer bonding were carried out with the same bonding technique. Surface properties of the surface-activated wafers have been investigated using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). Bonding strength has been measured by crack-opening method and tensile test. Bonding interfaces have been inspected by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). Mechanisms of prebonding surface activation and bonding interface evolution during low-temperature postbonding annealing are discussed based on our results.