Shapiro and Shapiro
Apparatus with four light detectors for checking surface of mask with pellicle
 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. 
Nikon Corporation
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G01N G01N G03F G03F
additional inventional inventional inventional
Kazunori Sunao Kinya Fuminori
Imamura Murata Kato Hayano