Patent Number: 
Section: claims

1. An electron beam apparatus equipped with a height detection system configured to assess a height of a specimen during use, comprising:an electron beam unit having an electron beam source, electro optical lenses, a table for setting a specimen and controllable in height direction, and a detector for detecting charged particles emanated from the specimen by the irradiation of an electron beam emitted from the electron beam source and converged and deflected by the electro optical lenses; anda height detection system for detecting height of the specimen set on the table while the specimen is irradiated by the electron beam,wherein the height detection system further comprises:an illumination system configured to direct a first and a second beam of light to a surface of the specimen at substantially opposite azimuth angles and at substantially equal angles of incidence while the specimen is irradiated by the electron beam;a collection system configured to collect the first beam of light specularly reflected from the surface of the specimen and to collect the second beam of light specularly reflected from the surface of the specimen while the specimen is irradiated by the electron beam;first and second detectors, wherein the first detector is configured to generate an output signal responsive to the collected first beam of light, and wherein the second detector is configured to generate an output signal responsive to the collected second beam of light while the specimen is irradiated by the electron beam; anda device configured to receive the output signals from the first and second detectors while the specimen is irradiated by the electron beam and to generate a comparison signal from the output signals while the specimen is irradiated by the electron beam, wherein the comparison signal is responsive to the height of the specimen. 2. An electron beam apparatus according to the claim 1, wherein the output signal generated by the first detector is further responsive to a position of the collected first beam of light on the first detector, and wherein the output signal generated by the second detector is further responsive to a position of the collected second beam of light on the second detector. 3. An electron beam apparatus according to the claim 1, wherein the comparison signal is substantially independent of patterned features on the specimen. 4. An electron beam apparatus according to the claim 1, further comprising a processing device coupled to the system and configured to alter a height of the specimen in response to the comparison signal by altering a position of a stage configured to support the specimen. 5. An electron beam apparatus according to claim 1, further comprising a processing device, wherein the processing device is configured to receive the comparison signal while the specimen is irradiated by the electron beam, and wherein the processing device is further configured to alter a focus setting of an optical column in response to the comparison signal. 6. A method for assessing a height of a specimen in an electron beam apparatus, comprising the steps of:irradiating and scanning an electron beam on a specimen set on a table;detecting secondary charged particles emanated from the specimen by the irradiation of the electron beam; andoptically assessing a height of the specimen set on the table during the irradiation of the electron beam;wherein the step of optically assessing a height of the specimen further comprises the steps of:directing a first and a second beam of light to a surface of the specimen at substantially opposite azimuth angles and at substantially equal angles of incidence such that the first and second beams of light illuminate the surface of the specimen while the specimen is irradiated by the electron beam;collecting the first and second beams of light specularly reflected from the surface of the specimen;generating an output signal responsive to the collected first beam of light and an output signal responsive to the collected second beam of light; andgenerating a comparison signal from the output signals, wherein the comparison signal is responsive to the height of the specimen. 7. A method according to the claim 6, wherein the step of generating the output signals comprises directing the collected first beam of light to a first detector and the collected second beam of light to a second detector. 8. The method of claim 6, wherein the output signal responsive to the collected first beam of light is further responsive to a position of the collected first beam of light on a first detector and wherein the output signal responsive to the collected second beam of light is further responsive to a position of the collected second beam of light on a second detector. 9. The method of claim 6, wherein the comparison signal is substantially independent of patterned features on the specimen. 10. The method of claim 6, further comprising the steps of altering the height of the specimen in response to the comparison signal to maintain a substantially constant working distance between an optical column of the electron beam apparatus. 11. The method of claim 6, further comprising the steps of altering the height of the specimen by altering a position of the table on which the specimen is set. 12. The method of claim 6, further comprising the steps of altering a focus setting of an optical column of the electron beam apparatus in response to the comparison signal. 13. An optical inspection apparatus equipped with a height detection system configured to assess a height of a specimen during use, comprising:an optical inspection apparatus having a light source, an illumination optical unit, a table for setting a specimen and controllable in height direction, and a detection optical system for detecting light reflected from the specimen by the irradiation of light emitted from the light source and passed through the illumination optical unit; anda height detection system for detecting height of the specimen set on the table while the specimen is illuminated by the light;wherein the height detection system further comprises:an illumination system configured to direct a first and a second beam of light to a surface of the specimen at substantially opposite azimuth angles and at substantially equal angles of incidence while the specimen is illuminated by the light;a collection system configured to collect the first beam of light specularly reflected from the surface of the specimen and to collect the second beam of light specularly reflected from the surface of the specimen while the specimen is illuminated by the light;first and second detectors, wherein the first detector is configured to generate an output signal responsive to the collected first beam of light, and wherein the second detector is configured to generate an output signal responsive to the collected second beam of light while the specimen is illuminated by the light; anda device configured to receive the output signals from the first and second detectors while the specimen illuminated by the light and to generate a comparison signal from the output signals while the specimen is illuminated by the light, wherein the comparison signal is responsive to the height of the specimen. 14. An optical inspection apparatus according to claim 13, wherein the output signal generated by the first detector is further responsive to a position of the collected first beam of light of the first detector, and wherein the output signal generated by the second detector is further responsive to a position of the collected second beam of light of the second detector. 15. An optical inspection apparatus according to claim 13, wherein the comparison signal is substantially independent of patterned features on the specimen.