Patent ID: 7970199

Claim:
An apparatus for inspecting a surface of a specimen, the apparatus comprising: a laser light source which emits a laser beam having a certain spectrum width; a modulation system including a first branch optical element which branches an optical path of the laser beam emitted from the laser light source into two, each of frequency modulation elements which modulates each of two laser beams branched by the first branch optical element with mutually different frequencies, each of optical path length adjustment elements which adjusts respective optical path lengths of the each of the two laser beams branched by the first branch optical element, and a first combination optical element which combines laser beams which are modulated with the mutually different frequencies by the each of the frequency modulation elements and whose the each of the optical path lengths is adjusted by the each of the optical path length adjustment elements; a laser irradiation optical system including a second branch optical element which branches a laser beam that has been combined and output by the modulation system, an specimen light irradiation optics which irradiates a specimen with one laser beam branched by the second branch optical element, a reflecting plate, and a reference light irradiation optics which irradiates the reflecting plate with other laser beam branched by the second branch optical element; a laser detection optical system including a sensor element which receives a specimen laser beam reflected from the specimen irradiated with the one laser beam by the laser irradiation optical system and a reference laser beam reflected from the reflecting plate irradiated with the other laser beam by the laser irradiation optical system which are combined into a beam by passing through the second branch optical element; and a signal processor which processes a signal obtained by detecting the combined laser beam in the sensor element of the laser detection optical system so as to obtain information on a surface condition of the specimen, wherein each of the optical path length adjustment elements includes an optical path length adjustment plate configured to adjust the respective optical path length of the two branched laser beams emitted from the first branch optical element so as to maximize the interference intensity of at least one of the output from the first combination optical element and the laser beams detected by the sensor element.