| 摘要 |
An apparatus for die to die inspection of masks having transparent phase shifting elements and a method of die to die inspection of masks having transparent phase shifting elements. Light from a light source is directed through a transparent mask substrate and a phase shifting mask element to a first objective lens, and through the transparent mask substrate and another phase shifting mask element to a second objective lens. Light from the first objective lens is then given a 180 DEG phase shift by a phase adjustment unit. Light from the phase adjustment unit and the second objective lens is combined at a split mirror and directed to a detector. The method makes use of the fact that the intensity of the light at the detector is proportional to the square of the cosine of one half of the phase angle between the light from the phase adjusting unit and light from the second objective lens. If the intensity of light reaching the detector is not zero, or very small, the mask has a defect.
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