Interface sharpness in stacked thin film structures: a comparison of soft X-ray reflectometry and transmission electron microscopy

BackgroundA key element of semiconductor fabrication is the precise deposition of thin films. Among other aspects, the quality of interfaces between different materials plays a crucial role for the success of further processing steps.AimWe here present a combined quantitative study of soft X-ray reflectometry measurements compared to scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM-EDX) on stacked thin film samples of silicon and silicon-germanium (SiGe).ApproachThe thin film structures feature two distinct germanium concentrations in the SiGe layers and are produced for complementary field-effect transistor applications. We use synchrotron-based, angle-, and energy-resolved broadband reflectance to investigate the sharpness of the layer interfaces, which is accessible through rigorous modeling of the acquired data. Complementary, the samples are investigated using STEM-EDX on thin lamellas across the interfaces, which give a direct representation of the interface sharpness through the varying germanium content.ResultsLayer thicknesses and interface properties are studied with the two methods. As a side-product of the measurement, the optical constants of the different SiGe compounds are determined and reported.ConclusionsWe find a very high correlation of the retrieved values between both methods and discuss their comparability and limits.