Investigations on low-temperature thermocompression bonding of passivated aluminum for enhanced wafer-level packaging and heterogeneous integration

Aluminum-to-Aluminum (Al/Al) bonding is critical for wafer-level packaging and heterogeneous integration in modern CMOS compatible semiconductor technologies. Due to electronic devices become smaller and more complex, Al/Al bonding offers a scalable solution with improved thermal management with good electrical performance. Recent advances in low-temperature thermal compression bonding highlight the potential for reliable, high-throughput integration, establish Al/Al bonding a key enabler of semiconductor technology. Due to the property of forming a strong oxide, high temperatures and pressures are typically required to create successful Al-Al bonding at wafer level. In this study, a low-temperature thermocompression bonding process for aluminum limited to a maximum temperature of 350 degrees C is investigated. In contrast to conventional processes, no wet chemical treatment is used to remove surface oxides on the aluminum. Instead, an in-situ PVD deposition technique is used in which an ultra-thin layer of precious metal is applied to prevent oxidation of the aluminum. Titanium is chosen next to palladium specifically for its role in passivating the aluminum surface to ensure improved stability and prevent unwanted oxidation effects during the bonding process. In this study, an additional layer under the aluminum is also being investigated to reduce the surface roughness of the aluminum. This layer is strategically introduced to improve the overall roughness and quality of the aluminum surface to address important aspects of the bonding process. Thermocompression bonding was performed on both unpatterned and patterned wafers, with the patterned wafers using 60 mu m and 80 mu m wide frames for bonding. The subsequent investigations included the evaluation of the interface quality by atomic force microscopy (AFM), scanning electron microscopy (SEM) and the evaluation of the reliability by shear tests.