Simulation study of cleaning induced extreme ultraviolet reflectivity loss mechanisms on mask blanks

摘要

It is widely recognized in the semiconductor industry that getting to defect-free extreme ultraviolet (EUV) mask blanks is critical in achieving high volume chip manufacturing yield beyond the 22 nm half-pitch node. Finished mask blanks are normally subjected to a cleaning process to get rid of the loosely adhered particles on the top. It is important that this cleaning process does not degrade the properties of the multilayer blank or introduce additional particles or pits during the process. However, standard cleaning processes used to clean multilayer blanks can result in EUV reflectivity loss, loss of uniformity in reflectivity, increased roughness, and add pits and particles on mask blanks. The standard cleaning process consists of multiple steps, each of which may cause the oxidation of the ruthenium capping layer, as well as the underlying bilayers, etching of the multilayer stack, and increased roughness of the bilayers, thus leading to a loss in EUV reflectivity. It is a challenging task to experimentally correlate the processing steps to the resulting damage and to quantify the reflectivity loss. Further, due to the high cost of materials we have not been able to perform extensive experiments to determine the root cause of problems. In this work, we have combined mask blank cleaning using standard processes, x-ray photoelectron spectroscopy, transmission electron microscope cross section, and atomic force microscope studies with simulations to quantify the impact of the multilayer oxidation, etching, and roughness on the EUV reflectivity loss and mask blank degradation.