0.5keV Xe + ion beam nano smoothing of ULE® substrate after processing with 3.0-10.0keV Xe + ion beam

Ion beam figuring (IBF) is a suitable technology for the final shape correction of substrates used in the projection optics of EUVL tools. Generally the ULE(R) substrate with surface roughness of 0.060-0.080nm rms can be produced with mechanical machining methods. However, it is very difficult to obtain the shape accuracy of approximately 0.120nm rms using mechanical machining methods. Therefore, ion beam figuring (IBF) may be adapted for the final shape correction of the substrates. In this paper, we investigated a method in which a low energy 0.5keV Xe^+ ion beam is used for smoothing of ULE(R) substrates which were first figured using high energy (3.0, 5.0, 7.0 and 10.0keV) Xe-ions beams (1mm diameter). The result shows that the ULE(R) substrate is roughened by 3.0-10.0keV Xe^+ ion beam (first stage processing) can be smoothened by 0.5keV Xe^+ ion beam (second stage processing) and reached to 0.120nm rms. Therefore, our proposed smoothing method is one of the best methods for the figure error correction of the ULE(R) substrate for EUVL projection optics.