Optical scanning apparatus which detects scanning spot focus error

Claim 1

An apparatus for optically scanning a surface of an object, comprising: a radiation source for supplying a scanning beam of radiation; an objective lens system for focusing said scanning beam to form scanning spot on said surface which produces a reflected beam therefrom, such reflected beam having a divergence which varies in accordance with deviations from correct focus of said radiation spot on said surface, such divergence having a nominal value corresponding to the correct focus; and a focus error detection system for detecting such deviations in focus of said radiation spot on said surface; characterized in that said focus error detection system comprises: two planar diffraction gratings arranged in succession in the path of the reflected beam, the grating period of the second grating being equal to that of the first grating multiplied by a factor determined by said nominal divergence value of the reflected beam, the grating strips of the second grating being parallel to those of the first grating but offset with respect thereto transversely to the grating strips; said first grating projecting an image thereof on the second grating having a grating period which varies with variations in the divergence of the reflected beam, and said second grating projecting an interference pattern of dark and light bands having a period and intensity distribution which varies in accordance with changes in the grating period of the projected image of said first grating; and at least two radiation-sensitive detectors on which the interference pattern projected by said second grating is formed, said detectors being separated by a bounding line there-between which extends parallel to said grating strips, said detectors producing signals corresponding to the intensity distribution of the interference pattern formed thereon.

Claim 2

An apparatus for optically scanning a surface of an object, comprising: a radiation source for supplying a scanning beam of radiation; an objective lens system for focusing said scanning beam to form scanning spot on said surface which produces a reflected beam therefrom, such reflected beam having a divergence which varies in accordance with deviations from correct focus of said radiation spot on said surface, such divergence having a nominal value corresponding to the correct focus; and a focus error detection system for detecting such deviations in focus of said radiation spot on said surface; characterized in that said focus error detection system comprises: two planar diffraction gratings arranged in succession in the path of the reflected beam, the grating period of the second grating being equal to that of the first grating multiplied by a factor determined by said nominal divergence value of the reflected beam, the grating strips of the second grating being parallel to those of the first grating but offset with respect thereto transversely to the grating strips; said first grating projecting an image thereof on the second grating having a grating period which varies with variations in the divergence of the reflected beam, and said second grating projecting an interference pattern of dark and light bands having a period and intensity distribution which varies in accordance with changes in the grating period of the projected image of said first grating; and at least two radiation-sensitive detectors on which the interference pattern projected by said second grating is formed, said detectors being separated by a bounding line there-between which extends parallel to said grating strips, said detectors producing signals corresponding to the intensity distribution of the interference pattern formed thereon.