Radiation damage to multilayer mirrors has been intently studied in the view of the EUV lithography (EUVL) application in recent years.
To investigate the radiation damage, a reflectance measurement system for EUVL mirrors was developed at beam line 9 at the NewSUBARU SR facility. This system can irradiate the mirror using EUV radiation from a long undulator (10.8 m) and simultaneously measure changes in reflectance caused by radiation damage. The actual measurement of the power density of the EUV radiation at the sample mirror was about 500 mW/mm2, which is sufficiently intense for quickly investigating radiation damage. The EUV wavelength, 13.5 nm, was selected from the undulator radiations by using a planar multilayer mirror with a maximum reflectance of 13.5 nm. The θ and 2 θ stages were adopted for reflectance measurements, making the system more valuable and flexible. Because the system is equipped with a removable pinhole to restrict the incident beam size and x-z automatic stages, it can also be used to measure the spatial distribution of the reflectance and photoemission current.
The ultimate vacuum was in the order of 10-5 Pa even though the automatic stages were moving. Some aspects, which depend on the atmospheres, capping layers on mirrors, and flux density of the irradiation beam, were measured. The photoemission current was also measured. These measurements provide important information about the extent of the radiation damage and whether or not it is proportional to the flux density.