Optical compensation in high numerical aperture photomask inspection systems for inspecting photomasks through thick pellicles

摘要

An objective lens system having reconfigurable optical components that enable the inspection of inspection surfaces in the absence of a pellicle or through a thin membrane pellicle, and using the same system, also enabling the inspection of inspection surfaces through a thick pellicle. An objective lens system includes a first group and a second group of optical elements. The first group of optical elements enables high numerical aperture and beam contraction. The second group of optical elements is capable of two mode operation enabling, in one mode, inspection through a thin membrane pellicle or in the absence of a pellicle and in another mode, enabling inspection through a thick pellicle. The system can also be enhanced through the use of an interposable aberration corrector plate that is used to correct optical aberrations caused by the presence, absence, or thickness of pellicles.