Patent Number: 
Section: claims

1. The scanning electron microscope comprising: a stage for mounting an object substrate;  an electron gun for emitting electron beam;  a converging lens for converging the electron beam emitted from the electron gun;  scanning unit for two-dimensionally scanning a surface of the object substrate with the electron beam converged by the converging lens;  an objective lens for focusing the electron beam converged by the converging lens in a spot-like shape on the surface of the object substrate;  a detector for detecting an intensity of at least one of secondary electron or reflected electron or absorbed electron generated from the object substrate by scanning with the electron beam by the scanning unit and outputting an analog image signal;  an A/D conversion unit for sampling the analog image signal detected by and outputted from the detector and converting the analog image signal into a digital image signal;  a switching control unit for controlling to switch at least the scanning unit so that a digital image signal having a low magnification based on a wide image taking field of view and a digital image signal having a high magnification based on a narrow image taking field of view are provided to be switched from the A/D conversion unit; and  a beam spot diameter control unit for controlling to switch a spot diameter of the electron beam at the surface of the object substrate in accordance with the magnification when the scanning unit is controlled so as to switch by the switching control unit. 2. The scanning electron microscope according to  claim 1 , wherein the beam spot diameter control unit controls a spot diameter of the electron beam based on information concerning a surface texture at a portion of the object substrate for taking an image when the detector takes the image with the wide image taking field of view by controlling so as to switch the scanning unit by the switch control unit. claim 1 3. The scanning electron microscope according to  claim 1  or  2 : claim 1 2 wherein the beam spot diameter control unit is constituted to control to move the stage in a direction of irradiating with the electron beam. 4. The scanning electron microscope according to  claim 1  or  2 : claim 1 2 wherein the beam spot diameter control unit is constituted by controlling the objective lens. 5. The scanning electron microscope according to  claim 1  or  2 : claim 1 2 wherein the beam spot diameter control unit is constituted by controlling current of the electron beam emitted from the electron gun. 6. The scanning electron microscope according to  claim 1 , further comprising: claim 1 a control unit for controlling to restrain pseudo noise components generated from a portion of the object substrate for taking an image when the detector takes the image with the wide image taking field of view by controlling so as to switch the scanning means by the switching control unit. 7. The scanning electron microscope according to  claim 1 , further comprising: claim 1 a signal processing unit for reducing pseudo noise components at high frequencies by carrying out a signal processing in accordance with a surface texture at a portion of the object substrate for an analog image signal outputted from the detector when the detector takes an image with the wide image taking field of view by controlling so as to switch the scanning unit by the switching control unit. 8. The scanning electron microscope according to  claim 1 , further comprising: claim 1 a signal processing unit for reducing pseudo noise components at high frequencies by carrying out a signal processing in accordance with a surface texture of a portion of the object substrate for a digital image signal provided by the A/D conversion unit when the detector takes an image with the wide image taking field of view by controlling so as to switch the scanning unit by the switching control unit. 9. The scanning electron microscope according to  claim 7  or  8 : claim 7 8 wherein the signal processing unit is constituted to carry out a filtering signal processing.