Patent Number: 
Section: claims

1. A method for adjusting an electron beam for controlling focus of said electron beam with which a sample is irradiated, by adjusting a negative voltage applied to a semiconductor wafer, the method comprising:creating a distribution function approximating potentials concentrically distributed on said semiconductor wafer; andadjusting the negative voltage applied to said semiconductor wafer using said distribution function and coordinates on said semiconductor wafer where said electron beam is irradiated. 2. The method according to claim 1, further comprising measuring the potentials at a plurality of points on said semiconductor wafer and creating said distribution function based on the resultant measurement data. 3. The method according to claim 2, further comprising measuring the potentials on said semiconductor wafer and re-creating said distribution function based on the resultant measurement data concerning the potentials. 4. The method according to claim 2, further comprising calculating a difference between said resultant measurement data and an approximation value of a potential obtained by said distribution function, and, if the difference is larger than a predetermined threshold, said resultant measurement data is excluded from the creation of said distribution function. 5. The method according to claim 1, wherein said distribution function is an even function. 6. A charged-particle optical system controlling apparatus for transmitting a negative-voltage application control signal to a scanning electron microscope in which focus of an electron beam is controlled by application of a negative voltage to a semiconductor wafer, whereinthe negative voltage applied to said semiconductor wafer is calculated based on a distribution function approximating potentials concentrically distributed on said semiconductor wafer and on coordinates on said semiconductor wafer where said electron beam is irradiated. 7. The apparatus according to claim 6, wherein said distribution function is created based on measurement data concerning the potentials measured on said semiconductor wafer. 8. The apparatus according to claim 6, wherein said distribution function is re-created based on measurement data concerning the potentials measured on said semiconductor wafer. 9. The apparatus according to claim 7, wherein a difference between said measurement data and an approximation value of a potential obtained by said distribution function is calculated, and, if the difference is larger than a predetermined threshold, said measurement data is excluded from the creation of said distribution function. 10. The apparatus according to claim 6, wherein said distribution function is an even function. 11. A scanning electron microscope comprising:an electron source; anda control unit for adjusting focus of an electron beam emitted by said electron source by varying a negative voltage applied to a semiconductor wafer, whereinsaid control unit calculates the negative voltage applied to said semiconductor wafer based on a distribution function approximating potentials concentrically distributed on said semiconductor wafer and on coordinates on said semiconductor wafer where said electron beam is irradiated. 12. The scanning electron microscope according to claim 11, wherein said distribution function is created based on measurement data concerning the potentials measured on said semiconductor wafer. 13. The scanning electron microscope according to claim 11, wherein said distribution function is re-created based on measurement data concerning the potentials measured on said semiconductor wafer. 14. The scanning electron microscope according to claim 12, wherein a difference between said measurement data and an approximation value of a potential obtained by said distribution function is calculated, and, if the difference is larger than a predetermined threshold, said measurement data is excluded from the creation of said distribution function. 15. The scanning electron microscope according to claim 11, wherein said distribution function is an even function. 16. A method of adjusting an electron beam for controlling focus of an electron beam with which a sample is irradiated, by adjusting a negative voltage applied to a semiconductor wafer, the method comprising:creating a distribution function approximating a potential distribution that develops on said semiconductor wafer such that the potential is highest at the center of said semiconductor wafer and gradually decreases towards an edge of said wafer; andadjusting the negative voltage applied to said semiconductor wafer using said distribution function and coordinates on said semiconductor wafer where said electron beam is irradiated. 17. A charged-particle optical system control apparatus, comprising:a system creating a distribution function approximating a potential distribution that develops on a semiconductor wafer such that the potential is highest at the center of said semiconductor wafer and gradually decreases towards an edge of said wafer; and adjusting a negative voltage applied to said semiconductor wafer using said distribution function and the coordinates on said semiconductor wafer where an electron beam is irradiated. 18. A scanning electron microscope, comprising:a system creating a distribution function approximating a potential distribution that develops on a semiconductor wafer such that the potential is highest at the center of said semiconductor wafer and gradually decreases towards an edge of said wafer; and adjusting a negative voltage applied to said semiconductor wafer using said distribution function and coordinates on said semiconductor wafer where an electron beam is irradiated. 19. A method for operating a scanning electron microscope, comprising:creating a distribution function approximating a potential distribution that develops on a semiconductor wafer such that the potential is highest at the center of said semiconductor wafer and gradually decreases towards an edge of said wafer; andadjusting a negative voltage applied to said semiconductor wafer using said distribution function and coordinates on said semiconductor wafer where an electron beam is irradiated.