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 for emitting an electron beam, an objective lens for controlling the electron beam, a table for setting a specimen, and a detector for detecting secondary charged particles emanated from the specimen by the irradiation of the electron beam emitted from the electron beam source and controlled by the objective lens; anda height detection system for detecting height of the specimen set on the table;wherein the height detection system includes:an illumination system configured to direct a first and a second beam of light through a mask with a multi-slit pattern to a surface of the specimen at substantially opposite azimuth angles and at substantially equal angles of incidence;first and second detectors, wherein the first detector is configured to detect a first multi-slit image by the first beam of light reflected from the surface of the specimen and to generate an output signal responsive to the detected first multi-slit image, and wherein the second detector is configured to detect a second multi-slit image by the second beam of light reflected from the surface of the specimen and to generate an output signal responsive to the detected second multi-slit image; anda device configured to receive the output signals from the first and second detectors and to generate a comparison signal from the output signals, the comparison signal being responsive to the height of the specimen;wherein the objective lens is controlled so as to focus the electron beam on the specimen by adjusting a focus control current or a focus control voltage of the objective lens in accordance with the comparison signal. 2. An electron beam apparatus according to claim 1, further comprising an image processing unit which obtains an image from the detected secondary charged particles and which processes the obtained image to detect a defect on the surface of the specimen. 3. An electron beam apparatus according to claim 1, further comprising a processing device, wherein the processing device is configured to receive the comparison signal and to alter a focus setting of an optical column by controlling the objective lens via the focus control current or the focus control voltage in accordance with the comparison signal. 4. An electron beam apparatus according to claim 1, wherein the output signal generated by the first detector is responsive to a height detected value obtained from the first multi-slit image, and wherein the output signal generated by the second detector is responsive to a height detected value obtained from the second multi-slit image. 5. An electron beam apparatus according to the claim 1, wherein the comparison signal is substantially independent of patterned features on the specimen. 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;wherein the step of optically assessing a height of the specimen includes the steps of:directing a first and a second beam of light through a mask with a multi-slit pattern 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;detecting first and second multi-slit images by the first and the second beams of light reflected from the surface of the specimen;generating a first output signal responsive to the detected first multi-slit image and a second output signal responsive to the detected second multi-slit image; andgenerating a comparison signal from the first and second output signals, the comparison signal being responsive to the height of the specimen,wherein the step of irradiating and scanning an electron beam is performed while controlling an objective lens so as to focus the electron beam on the specimen by adjusting a focus control current or a focus control voltage of the objective lens in accordance with the comparison signal. 7. A method according to the claim 6, further comprising the step of processing an image from the detected secondary charged particles to detect a defect on the surface of the specimen. 8. A method according to claim 6, wherein the first output signal responsive to the detected first multi-slit image is responsive to a height detected value obtained from the first multi-slit image, and wherein the second output signal responsive to the detected second multi-slit image is responsive to a height detected value obtained from the second multi-slit image. 9. A method according to claim 6, wherein the comparison signal is substantially independent of patterned features on the specimen. 10. 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 for emitting an electron beam, an objective lens for controlling the electron beam, a table for setting a specimen, and a detector for detecting secondary charged particles emanated from the specimen by the irradiation of the electron beam emitted from the electron beam source and controlled by the objective lens; anda height detection system for detecting height of the specimen set on the table;wherein the height detection system includes:an illumination system configured to direct a first and a second beam of light through a mask with a multi-slit pattern to a surface of the specimen at substantially opposite azimuth angles and at substantially equal angles of incidence;first and second detectors, wherein the first detector is configured to detect a first multi-slit image by the first beam of light reflected from the surface of the specimen and to generate an output signal responsive to the detected first multi-slit image, and wherein the second detector is configured to detect a second multi-slit image by the second beam of light reflected from the surface of the specimen and to generate an output signal responsive to the detected second multi-slit image; anda device configured to receive the output signals from the first and second detectors and to generate a comparison signal from the output signals, the comparison signal being responsive to the height of the specimen;wherein the objective lens is controlled in accordance with the comparison signal so as to maintain a substantially constant working distance between an optical column of the electron beam.