Low-Temperature (260 °C) Solderless Cu-Cu Bonding for Fine-Pitch 3-D Packaging and Heterogeneous Integration

Low-temperature solderless Cu-Cu bonding is an important technology for advanced packaging, such as fine-pitch 3-D packaging and heterogeneous integration. In this study, we prepared an oxidation-free Cu surface using an optimized N2 plasma process in a two-step Ar/N2 plasma treatment based on the design of the experiment method, to improve direct Cu-Cu bonding quality at 260 °C. The N2 plasma treatment process was optimized using the Minitab optimizer with chemical state results obtained by X-ray photoelectron spectroscopy (XPS) analysis. The Cu surface treated under the two-step Ar/N2 plasma with optimized N2 plasma conditions not only formed Cu nitride well, but the surface remained without further oxidation for at least one week at room temperature. The Cu-Cu bonding was performed at the low bonding temperature of 260 °C and low bonding pressure of 0.9 MPa for 1 hour, and the bonded interface was evaluated using scanning acoustic tomography (SAT) and field emission scanning electron microscope (FE-SEM) images. We measured shear strength to estimate the bonding interface quality of the oxidation-free Cu-Cu bonding specimen. A maximum shear strength of 62.6 MPa was obtained. Our bonding results demonstrated remarkably improved Cu-Cu bonding quality compared with other previous Cu-Cu joint studies that used Sn, Cu/Ag nanoparticles, or Cu composites.