Patent Number: 
Section: claims

1. A scanning electron microscope comprising:an electron source;a magnetic field type objective lens for focusing a primary electron beam emitted from the electron source on a specimen and having a lens gap opening toward the specimen;a first deflector for deflecting the primary electron beam off-axis of the objective lens; anda second deflector, comprising an electrostatic deflector, for deflecting the primary electron beam, so that a deflecting force of the objective lens against the primary electron beam is canceled when the primary electron beam is deflected off-axis of the objective lens,wherein the second deflector creates an electric field that suppresses off-axis deviation of the primary electron beam being caused by a magnetic field created by the objective lens. 2. The scanning electron microscope according to claim 1, wherein the first deflector serves also as a scanning deflector for scanning the primary electron beam. 3. The scanning electron microscope according to claim 1, wherein the second deflector is an octupole deflector. 4. The scanning electron microscope according to claim 3, wherein:the octupole deflector has an insulating base plate provided with a primary electron beam passing aperture and insulating slits formed so as to extend radially from the electron beam passing aperture, andopposite surfaces of a part of the base plate around the electron beam passing aperture and side surface of the electron beam passing aperture and the insulating slits are coated with conductive films. 5. The scanning electron microscope according to claim 4, wherein:the insulating base pate has a conductive, cylindrical part formed around the primary electron beam passing aperture, andthe conductive, cylindrical part of the insulating base plate is inserted in a primary electron beam passing aperture of the objective lens. 6. The scanning electron microscope according to claim 3, wherein:the octupole deflector has a part inserted in a primary electron beam passing aperture of the objective lens, anda shielding electrode is disposed so as to screen partly a deflecting electric field created by the octupole deflector. 7. A scanning electron microscope comprising:an electron source;a magnetic field type objective lens for focusing a primary electron beam emitted from the electron source on a specimen and having a lens gap opening toward the specimen;a first deflector for deflecting the primary electron beam off-axis of the objective lens; anda second deflector, comprising an electrostatic deflector, for deflecting the primary electron beam, so that an aberration created by a deflecting force of the objective lens against the primary electron beam is cancelled when the primary electron beam is deflected off-axis of the objective lens,wherein the second deflector creates an electric field that suppresses off-axis deviation of the primary electron beam being caused by a magnetic field created by the objective lens. 8. The scanning electron microscope according to claim 7, wherein the first deflector serves also as a scanning deflector for scanning the primary electron beam. 9. The scanning electron microscope according to claim 7, wherein the second deflector is an octupole deflector. 10. The scanning electron microscope according to claim 9, wherein:the octupole deflector has an insulating base plate provided with a primary electron beam passing aperture and insulating slits formed so as to extend radially from the electron beam passing aperture, andopposite surfaces of a part of the base plate around the electron beam passing aperture and side surface of the electron beam passing aperture and the insulating slits are coated with conductive films. 11. The scanning electron microscope according to claim 10, wherein:the insulating base plate has a conductive, cylindrical part formed around the primary electron beam passing aperture, andthe conductive, cylindrical part of the insulating base plate is inserted in a primary electron beam passing aperture of the objective lens. 12. The scanning electron microscope according to claim 9, wherein:the octupole deflector has a part inserted in a primary electron beam passing aperture of the objective lens, anda shielding electrode is disposed so as to screen partly a deflecting electric field created by the octupole deflector.