Patent Number: 
Section: claims

1. A specimen observing method based on an electron microscope, comprising:a step of picking up enlarged specimen images by changing focal position of an electron beam in relation to a specimen; a step of image-calculating image sharpness coefficients of the enlarged specimen images; and a step of deciding the number of peaks on a curve indicative of the relation between focal position of the electron beam and image sharpness coefficient, wherein when two peaks are determined to exist in said decision, an astigmatism correction process proceeds, said astigmatism correction process including: a step of picking up enlarged specimen images by changing astigmatism correction current of a stigmator in X direction; a step of image-calculating image sharpness coefficients of the enlarged specimen images; a step of determining a minimum position sandwiched by two peaks on a curve indicative of the relation between the astigmatism correction current of said stigmator in X direction and the image sharpness coefficient; a step of setting the astigmatism correction current of said stigmator in X direction to a current value corresponding to said minimum position; a step of picking up enlarged specimen images by changing astigmatism correction current of a stigmator in Y direction; a step of image-calculating image sharpness coefficients of the enlarged specimen images; a step of determining a minimum position sandwiched by two peaks on a curve indicative of the relation between astigmatism correction current of said stigmator in Y direction and image sharpness coefficient; and a step of setting the astigmatism correction current of said stigmator in Y direction to a current value corresponding to said minimum position.  2. A specimen observing method based on an electron microscope according to claim 1, wherein when two peaks are determined to exist in said decision, said minimum position sandwiched by the two peaks is determined before said astigmatism correction process proceeds and the focal position of the electron beam is set to a position corresponding to said minimum position. 3. A specimen observing method based on an electron microscope, comprising:a step of picking up enlarged specimen images by changing focal position of an electron beam in relation to a specimen; a step of image-calculating an angular direction component of image sharpness coefficient of the enlarged specimen image; a step of deciding an astigmatism correction direction from a result of the calculation of the angular direction component of image sharpness coefficient of said enlarged specimen image; a step of picking up enlarged specimen images by changing the astigmatism correction current of stigmator in said determined direction; a step of calculating image sharpness coefficients of said enlarged specimen images; a step of determining a minimum position sandwiched by two peaks on a curve indicative of the relation between astigmatism correction current of said stigmator and image sharpness coefficient; and a step of setting the astigmatism correction current of said stigmator to a current value corresponding to said minimum position.  4. A specimen observing method based on an electron microscope according to claim 1, wherein a focus correction process proceeds after the astigmatism correction has been completed, said focus correction process including:a step of picking up enlarged specimen images by changing focal position of an electron beam in relation to a specimen; a step of calculating image sharpness coefficients of said enlarged specimen images; a step of determining a peak position on a curve indicative of the relation between focal position of the electron beam and image sharpness coefficient; and a step of setting the focal position of the electron beam to a position corresponding to said peak position.  5. A specimen observing method based on an electron microscope according to claim 1, wherein the image sharpness coefficient is image-calculated in respect of an edge enhanced image of the enlarged specimen image. 6. A specimen observing method based on an electron microscope for performing a process of making the focus of the electron beam coincident with a specimen after a process of correcting astigmatism of the electron beam, wherein a process of making focal position of the electron beam substantially coincident with the specimen before the process of correcting astigmatism of the electron beam, said process of making focal position of the electron beam substantially coincident with the specimen including:a step of picking up enlarged specimen images by changing the focal position of the electron beam in relation to the specimen; a step of calculating pixel mean values of said enlarged specimen images; a step of determining a minimum position on a curve indicative of the relation between the focal position of said electron beam and the pixel mean value; and a step of setting the focal position of said electron beam to a position corresponding to said minimum position, and said process of correcting astigmatism of the electron beam including the individual steps as recited in any one of claims 1 to 3.  7. A specimen observing method based on an electron microscope for performing a process of focusing an electron beam on a specimen after a process of correcting astigmatism of the electron beam, wherein a process of making focal position of the electron beam substantially coincident with the specimen is carried out before the process of correcting astigmatism of the electron beam, said process of making focal position of the electron beam substantially coincident with the specimen including:a step of picking up enlarged specimen images by changing focal position of the electron beam in relation to the specimen; a step of calculating image sharpness coefficients of said enlarged specimen images; a step of calculating pixel mean values of said enlarged specimen images; a step of determining a ratio between the image sharpness coefficient and the pixel mean value at each focal position; a step of determining a maximum position on a curve indicative of the relation between the focal position of the electron beam and the ratio; and a step of setting the focal position of the electron beam to a position corresponding to said maximum position, and said process of correcting astigmatism of the electron beam including the individual steps as recited in any one of claims 1 to 3.  8. A specimen observing method based on an electron microscope according to claim 6, wherein said process of making focal position of the electron beam substantially coincident with said specimen is performed at a lower magnification than a specimen observation magnification. 9. A specimen observing method based on an electron microscope according to claim 1, wherein when setting the astigmatism correction current to a current value corresponding to said minimum position, hysteresis of astigmatism correction current of said stigmator and astigmatism correction amount is taken into account. 10. A specimen observing method based on an electron microscope according to claim 1, wherein the focal position of the electron beam in relation to said specimen is changed by changing excitation current of the objective lens and when setting the focal position of said electron beam to a position corresponding to said minimum position, hysteresis of the excitation current of objective lens and the focal position is taken into consideration. 11. A specimen observing method based on an electron microscope according to claim 1 further comprising a step of, when the number of peaks is determined to be two in said decision, determining an astigmatic difference amount from said two peaks. 12. A specimen observing microscope based on an electron microscope according to claim 11, wherein a range within which said astigmatism correction current is changed is determined on the basis of said astigmatic difference amount. 13. A specimen observing method based on an electron microscope according to claim 11, wherein when the astigmatic difference amount is larger than a predetermined threshold value, said astigmatism correction process proceeds. 14. A specimen observing method based on an electron microscope according to claim 13, wherein measurement of said astigmatic difference amount and said astigmatism correction process are executed repetitively until said astigmatic difference amount becomes smaller than said threshold value. 15. An electron beam apparatus for observing a specimen, the apparatus comprising:an electron microscope for irradiating the specimen with an electron beam and generating enlarged images of the specimen from the electron beam irradiation of the specimen, the electron microscope including a stigmator; and a controller for controlling operation of the electron microscope, configured to cause the electron microscope to implement steps comprising: picking up enlarged specimen images by changing focal position of the electron beam in relation to a specimen; image-calculating image sharpness coefficients of the enlarged specimen images; and deciding the number of peaks on a curve indicative of the relation between focal position of the electron beam and image sharpness coefficient, wherein when two peaks are determined to exist in said decision, an astigmatism correction process proceeds, said astigmatism correction process including: picking up enlarged specimen images by changing astigmatism correction current of the stigmator in X direction; image-calculating image sharpness coefficients of the enlarged specimen images; determining a minimum position sandwiched by two peaks on a curve indicative of the relation between the astigmatism correction current of the stiginator in the X direction and the image sharpness coefficient; setting the astigmatism correction current of the stigmator in the X direction to a current value corresponding to said minimum position; picking up enlarged specimen images by changing astigmatism correction current of the stigmator in Y direction; image-calculating image sharpness coefficients of the enlarged specimen images; determining a minimum position sandwiched by two peaks on a curve indicative of the relation between astigmatism correction current of the stigmator in the Y direction and image sharpness coefficient; and setting the astigmatism correction current of the stigmator in the Y direction to a current value corresponding to said minimum position.  16. An electron beam apparatus for observing a specimen, the apparatus comprising:an electron microscope for irradiating the specimen with an electron beam and generating enlarged images of the specimen from the electron beam irradiation of the specimen, the electron microscope including a stigmator; and a controller for controlling operation of the electron microscope, configured to cause the electron microscope to implement steps comprising: picking up enlarged specimen images by changing focal position of the electron beam in relation to a specimen; image-calculating an angular direction component of image sharpness coefficient of the enlarged specimen images; deciding an astigmatism correction direction from a result of the calculation of the angular direction component of image sharpness coefficient of said enlarged specimen images; picking up enlarged specimen images by changing the astigmatism correction current of the stigmator in said determined direction; calculating image sharpness coefficients of said enlarged specimen images; determining a minimum position sandwiched by two peaks on a curve indicative of the relation between astigmatism correction current of the stigmator and image sharpness coefficient; and setting the astigmatism correction current of the stigmator to a current value corresponding to said minimum position.