Optical and structural characterization of CeO2/B4C multilayers near Boron K-edge energy

A search for novel materials for making multilayers of high reflectivity has been driven by the vigorous demand towards miniaturizing photonics. A typical consumer of high performance multilayers (MLs) is the extreme ultraviolet lithography (EUVL) based on the 13.5 nm laser produced plasma (LPP) source. To sustain “Moore’s law” and print fine features below 10 nm on integrated circuits (IC), source of radiation for the EUVL has to shift towards even shorter wavelengths where 6.x nm wavelength seems to be immediate successor. However, the 6.x nm EUV lithography needs MLs of reflectivity performance above 70 % to support high volume manufacturing (HVM). It is clear that more work is required particularly on the development of MLs with high reflectance, stable to thermal heat and sufficient lifetime. In this work new MLs of B4C/CeO2 are deposited, analyzed and characterized for the first time. Combinations of X-ray reflectometry (XRR) and EUV reflectance measurements near resonance edge of boron are analyzed to derive structural and optical parameters of MLs. ML coatings of B4C/CeO2 MLs have shown similar reflectance performance with the leading candidate MLs around 6.x nm wavelength. Analysis shows that interlayer diffusion is a major reason for low reflectivity performance. Cross-sectional scanning electron microscopy (SEM) images of the MLs have proved formation of interlayer diffusion.