| 摘要 |
PURPOSE:To enhance the measuring accuracy of a film thickness, by a method wherein two glass plates are fixed to a rotatable shaft so as to provide a minute offset angle therebetween and inserted in two beam paths close to each other to compensate the difference between the beam paths. CONSTITUTION:A beam path difference compensator 6 is constituted of phase compensation plates 6A, 6B each made of a thin glass plate. The plates 6A, 6B are fixed to a rotary shaft so as to provide a minute offset angle therebetween and made revolvable from the outside. If said plates 6A, 6B are inserted in the beam paths of a double dual beam path laser interference membrane measuring apparatus, the differences between the beam paths change corresponding to the angle formed by the plates 6A, 6B and the beam paths and a film thickness can be calculated from the angle of rotation of the plate 6A when the differences between the beam paths are adjusted. As the offset angle of the plates 6A, 6B becomes smaller, a measuring range becomes narrower but accuracy is improved. Therefore measuring accuracy can be enhanced by the appropriate offset angle.
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