Optics: measuring and testing – By light interference
Patent
1999-02-08
2000-10-17
Font, Frank G.
Optics: measuring and testing
By light interference
3562392, 356345, 356352, 356519, G01B 902
Patent
active
061340146
ABSTRACT:
A method and apparatus for inspecting photomasks having phase shifting elements which shift the phase of light but are otherwise transparent. A coherent light source is directed through a mask to be inspected, through an objective lens, through an 180.degree. phase shifting unit, and to an image divider. The coherent light source is also directed through a transparent reference substrate to the image divider. The mask to be inspected is formed on a transparent mask substrate having the same thickness and formed from the same material as the transparent reference substrate. The intensity of the light exiting the image divider is proportional to the square of the cosine of 1800 plus the phase angle between the light exiting the reference substrate and the light exiting the mask under test. The light exiting the image divider is directed to a CCD image sensor. An image computer compares the output of the CCD image sensor with an image formed from the image database and identifies defects in the mask under test.
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Lin Shy-Jay
Tzu San-De
Ackerman Stephen B.
Font Frank G.
Natividad Phil
Prescott Larry J.
Saile George O.
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