| 摘要 |
PURPOSE:To measure accurately an amount of displacement of a specimen surface even if it is in a tilted position, by directing parallel beams of light into a position outside the lens axis and installing a position sensor in a light path of the reflected beam returning through a lens. CONSTITUTION:A light flux from a source 1 is directed into a position outside the axis of a lens 2 for irradiating a specimen surface 3. Further, after these beams of reflected light are collected by the lens 2, they are directed onto a position sensor 9 through mirror 7, lens 8, etc. The lens 2 is moved by an automatic focusing mechanism and a distance to the specimen surface 3 is adjusted arbitrainly. By this arrangement, if the specimen surface 3 displaces to a position 3', the path of the reflected beam shifts as broke lines and image developed positions of the reflected beams P1-P4 are deviated from a point P0 on the sensor 9. This deviation signal of the sensor 9 is issued out as a feedback signal to the automatic focusing mechanism and the light flux from the source 1 is focused always on the surface of the specimen 3. At this moment, an amount of displacement of the surface 3 can automatically be measured from the amount of displacement of the lens 2. |
