Investigation of Cu-to-Cu and Oxide-to-Oxide Bonding

While device scaling of planar devices faces physical limitations, vertically stacked devices are becoming more important due to package miniaturization, cost reduction and enhanced functionality. 3D packaging technology improves performance and electrical signal and power integrities, and becomes a key packaging technology for the next generation heterogeneous integration. We demonstrated both Cu-Cu bonding and SiO2-SiO2 bonding at 300°C under the 700 kPa using the two step Ar-N2 plasma treatment process prior to bonding. Currently, it is difficult to fabricate Cu/SiO2 hybrid bonded specimens in our laboratory, so Cu bonding and oxide bonding were investigated separately in this experiment. The effect of two-step Ar-N
2
plasma treatment process on both Cu and SiO
2
surface was studied using XPS (X-ray photoelectron spectroscopy). A 3–4 nm copper nitride (Cu4N) layer was formed on the copper surface to prevent additional Cu oxidation, and some Si-O bonds were broken on the oxide surface to form NSi
3
and NSiO
2
. After the plasma treatments, Cu-Cu bonding and oxide-to-oxide bonding were carried out separately. According to the bonded interface measurements by SAT (scanning acoustic tomography), the Cu-Cu bonded interface was excellent, while the SiO
2
-SiO
2
bonded interface showed very fine voids even though the RMS roughness was 0.092 nm. In addition, the particle removal process in a wafer dicing process was evaluated with a positive photoresist for die-to-wafer bonding. In addition, for die-to-wafer bonding, it is necessary to remove particles generated in the wafer dicing process, and for this purpose, a positive photoresist coating process was evaluated.