In situ spectroscopic ellipsometry as a surface-sensitive tool to probe thin film growth

Abstract

Sputtered thin film and multilayer X-ray mirrors are made routinely at the Advanced Photon Source (APS) for APS users. Precise film growth control and characterization are critical in fabricating high-quality X-ray mirrors. Film thickness calibrations were carried out using in situ and ex situ spectroscopic ellipsometry, interferometry, and X-ray scattering. To better understand the growth and optical properties of different thin film systems, we have carried out a systematic study of sputtered thin films of Au, Rh, Pt, Pd, Cu, and Cr grown on Si substrates, using in situ ellipsometry. Multiple data sets were obtained in situ for each film material with incremental thicknesses and were analyzed using both flat-film and rough-film models. We found that the initial growth of these metal films on Si is correlated with their chemical bond strength with oxygen in the native oxide of Si. Metals with higher bond strength (such as Cr) grow smoother at the early stage of growth and have a better adhesion to the Si wafer. The Cr film becomes rougher as its thickness increases. Au and Rh films grow smoothly on a thin (6-nm) Cr-covered Si wafer. But on a thick (100-nm) Cr/Si film, Rh films were initially as rough as the thick Cr film. The roughness decreases as the Rh film thickness increases. When Cr is used as a ‘glue’ layer in metal on oxide systems, it is better to use a thin (<10 nm) Cr film in order to obtain a smooth metal film. We conclude that in situ spectroscopic ellipsometry as a surface-sensitive tool can also be used to probe the growth and morphology of the thin film system.