Soft X-ray reflectometry for the inspection of interlayer roughness in stacked thin film structures

Abstract

A key element of semiconductor fabrication is the precise deposition of thin films. Amongst other aspects, the quality of interfaces between different materials plays a crucial role for the success of further process steps. We here present soft X-ray reflectometry measurements on stacked thin film samples of silicon and silicon-germanium in various concentrations as they are produced for complementary field-effect transistor (CFET) applications. Synchrotron-based, angle- and energy-resolved broadband reflectance data sets can be modeled using a matrix-method approach that describes reflection, absorption, and diffuse scattering off the interfaces. This method is often used to determine the optical constants of materials in the EUV spectral region as parameters of the fitting procedure. We here show that the method is equally well suited to investigate roughness and layer intermixing between different deposited materials. These roughness parameters alongside the actual thicknesses of the individual layers also result from the physical modelling of the measured data. The method is inherently non-destructive and very sensitive, down to approximately 50nm depth and as such gives valuable information. To further qualify our findings, we compare the data to scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (STEM-EDX) to give insight into the atomic structure at the interfaces.