Apparatus with four light detectors for checking surface of mask

Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system

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250572, 356237, G01N 2188, G01N 2132

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active

048899984

ABSTRACT:
An apparatus for detecting foreign particles on a transparent pellicle comprises a first illuminating device having a first optical axis crossing a surface of the pellicle at a predetermined angle for supplying a light beam onto said transparent pellicle along said first optical axis and a second illuminating device having a second optical axis crossing the surface of said pellicle at an angle different from said predetermined angle for supplying a light beam to said transparent pellicle along said second optical axis. The apparatus further includes a detector for comparing quantity of light of a light beam scattered by a foreign particle and directed to a predetermined direction from said first illuminating device with quantity of light of a light beam scattered by the foreign particle and directed to a predetermined direction from said second illuminating device and for determining a surface of the transparent pellicle on which said foreign particle exists.

REFERENCES:
patent: 4172666 (1979-10-01), Clarke
patent: 4468120 (1984-08-01), Tanimoto et al.
patent: 4541715 (1985-09-01), Akiyama et al.
patent: 4610541 (1986-09-01), Tanimoto et al.
patent: 4669875 (1987-06-01), Shiba et al.
patent: 4731855 (1988-03-01), Suda et al.
patent: 4776693 (1988-10-01), Imamura et al.
Akikazu Tanimoto et al, "Proceedings of SPIE", pub. 1984, vol. 470, pp. 242-249.

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