Asymmetric line edge roughness of multilayer grating reference materials

Line edge roughness (LER) in a one dimensional Si/SiO2 multilayer grating reference material with 20 nm nominal pitch size was investigated. It was shown for the first time that the LER of Si on SiO2 edges was about three times larger than that of SiO2 on Si edges. The asymmetric LERs led to asymmetric uncertainties and it must be considered when using the multilayer grating reference material. Moreover, the origin of asymmetric LER was investigated based on distinguishing contributions of interfacial roughness and interfacial diffusion. The interfacial roughness was determined to be symmetric because the surface roughness of the substrate, the Si single layer, the Si/SiO2 bilayer and the Si/SiO2 multilayer were almost the same with the value about 0.1 nm. Whereas, the interfacial diffusion layer of Si on SiO2 interface was about two times larger than that of SiO2 on Si interface through X-ray reflectivity (XRR) reverse fitting and transmission electron microscopy (TEM) image analysis. The asymmetric interfacial diffusion layer was proposed to be the main reason of the observed asymmetric LERs of the Si/SiO2 multilayer grating reference material.