A model for spectroscopic ellipsometry analysis of plasma-activated Si surfaces for direct wafer bonding

In this work, the impact of plasma treatment on Si wafers with native oxide on top was systematically investigated using spectroscopic ellipsometry. A general applicable three-layer optical model structure for ellipsometry data fitting was developed and employed on samples treated with the N2, O2, and N2/O2 mixture plasma. Oxide-growth, amorphization of crystalline Si, and the formation of a transition layer between the SiO2 and the amorphous Si were detected. The estimated thicknesses of produced layers were confirmed by complementary methods, which allow precise ultra-thin layers thicknesses detection, namely, angle-resolved x-ray photoelectron spectroscopy and transmission electron microscopy. The depth-resolved chemical composition and the direct thickness measurements of the produced amorphous structure revealed pronounced elemental gradients and the absence of sharp interfaces. Nitrogen gas used in the plasma process was found to be implanted mainly at the interface of the SiO2/transition layer in the form of Si3N4. However, it was verified that it is feasible to employ one general ellipsometry model consisting of SiO2/transition layer/amorphous Si on crystalline Si for all plasma species due to comparably low Si3N4 concentrations as well as its refractive index n and absorption k similarity to SiO2. Spectroscopic ellipsometry is nondestructive and can be efficiently applied to analyze whole wafers without any sample preparation. It can be concluded that the present approach to ellipsometry model development and verification is well suited for plasma-activated direct wafer bonding processes.