Patent Number: 
Section: claims

1. A focused ion beam apparatus comprising:a scanning electron microscope having an objective lens that generates a magnetic field on a sample;an ion source;a focused ion beam optics to focus the ion beam emitted from the ion source on the sample;a canceling magnetic field generator to generate a canceling magnetic field on the optical axis of the ion beam; anda canceling magnetic field control unit for controlling the canceling magnetic field generated from the canceling magnetic field generator with the exciting current of the objective lens of the scanning electron microscope as an input signal thereto,wherein the deflection of the ion beam under the effect of a magnetic field external to the focused ion beam optics is canceled. 2. A focused ion beam apparatus according to claim 1, wherein the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other at a substantially single point on the sample. 3. A focused ion beam apparatus according to claim 2, wherein the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other substantially at 90 degrees to each other, andwherein an electron detector is arranged on the optical axis of the scanning electron microscope on the side of the crossing point far from the objective lens of the scanning electron microscope. 4. A focused ion beam apparatus according to claim 1, further comprising a magnetic shield for covering the focused ion beam optics. 5. A focused ion beam apparatus according to claim 4, wherein the magnetic shield covers the part of the focused ion beam optics far from the sample, andwherein the distance L from the end surface of the magnetic shield nearer to the sample to the sample is given as            ∫      L      0        ⁢                            ⅆ                      z            1                                                              V              acc                        -                          ϕ              ⁡                              (                                  z                  1                                )                                                        ⁢                        ∫          L                      z            1                          ⁢                                            B              y                        ⁡                          (              z              )                                ⁢                                          ⁢                      ⅆ            z                                ≅  0where Vacc is the ion beam accelerating voltage, Φ (z1) the electric potential at the coordinate z1 on the optical axis of the ion beam, By(z) a magnetic field component perpendicular to the optical axis of the ion beam at the coordinate z on the optical axis, and the origin of the coordinate is located at the ion beam spot position on the sample in the absence of a magnetic field. 6. A focused ion beam apparatus according to claim 1, wherein the ion beam includes a plurality of ion beams having a plurality of different mass-to-charge ratios. 7. A focused ion beam apparatus according to claim 6, wherein the ion beams having the plurality of different mass-to-charge ratios are a plurality of types of isotope ion beams. 8. A focused ion beam apparatus according to claim 1, wherein the ion source is a Ga liquid metal ion source. 9. A focused ion beam apparatus according to claim 1,wherein the canceling magnetic field generator is arranged on the optical axis of the ion beam and has a beam path allowing the entire the ion beam to pass therethrough. 10. A focused ion beam apparatus according to claim 9,wherein the canceling magnetic field generator is covered by a magnetic shield. 11. A focused ion beam apparats comprising:a scanning electron microscope having an objective lens that generates a magnetic field on a sample;an ion source;a focused ion beam optics to focus the ion beam emitted form the ion source on a sample; anda canceling magnetic field generator to generate a canceling magnetic field on the optical axis of the ion beam;at least one magnetic field sensor in the neighborhood of the focused ion beam optics; anda canceling magnetic field control unit for controlling the canceling magnetic field generated from the canceling magnetic field generator with the output of the magnetic field sensor as an input signal thereto,wherein the deflection of the ion beam under the effect of a magnetic field external to the focused ion beam optics is canceled. 12. A focused ion beam apparatus according to claim 11, wherein the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other at a substantially single point on the sample. 13. A focused ion beam apparatus according to claim 12, wherein the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other substantially at 90 degrees to each other, andwherein an electron detector is arranged on the optical axis of the scanning electron microscope on the side of the crossing point far from the objective lens of the scanning electron microscope. 14. A focused ion beam apparatus according to claim 11, further comprising a magnetic shield for covering the focused ion beam optics. 15. A focused ion beam apparatus according to claim 14, wherein the magnetic shield covers the part of the focused ion beam optics far from the sample, andwherein the distance L from the end surface of the magnetic shield nearer to the sample to the sample is given as            ∫      L      0        ⁢                            ⅆ                      z            1                                                              V              acc                        -                          ϕ              ⁡                              (                                  z                  1                                )                                                        ⁢                        ∫          L                      z            1                          ⁢                                            B              y                        ⁡                          (              z              )                                ⁢                                          ⁢                      ⅆ            z                                ≅  0where Vacc is the ion beam accelerating voltage, Φ(z1) the electric potential at the coordinate z1 on the optical axis of the ion beam, By(z) a magnetic field component perpendicular to the optical axis of the ion beam at the coordinate z on the optical axis, and the origin of the coordinate is located at the ion beam spot position on the sample in the absence of a magnetic field. 16. A focused ion beam apparatus according to claim 11, wherein the ion beam includes a plurality of ion beams having a plurality of different mass-to-charge ratios. 17. A focused ion beam apparatus according to claim 16, wherein the ion beams having the plurality of different mass-to-charge ratios are a plurality of types of isotope ion beams. 18. A focused ion beam apparatus according to claim 11, wherein the ion source is a Ga liquid metal ion source. 19. A focused ion beam apparatus according to claim 11,wherein the canceling magnetic field generator is arranged on the optical axis of the ion beam and has a beam path allowing the entire the ion beam to pass therethrough. 20. A focused ion beam apparatus according to claim 19,wherein the canceling magnetic field generator is covered by a magnetic shield. 21. A focused ion beam apparatus comprising:a scanning electron microscope having an objective lens that generates a magnetic field on a sample;an ion source;a focused ion beam optics to focus the ion beam emitted from the ion source on a sample; anda canceling magnetic field generator to generate a canceling magnetic field on the optical axis of the ion beam; wherein:the deflection of the ion beam under the effect of a magnetic field external to the focused ion beam optics is canceled,the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other at a substantially single point on the sample,the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other substantially at 90 degrees to each other, andthe apparatus further comprises an electron detector arranged on the optical axis of the scanning electron microscope on the side of the crossing point far from the objective lens of the scanning electron microscope. 22. A focused ion beam apparatus according to claim 21, further comprising a magnetic shield for covering the focused ion beam optics. 23. A focused ion beam apparatus according to claim 22, wherein the magnetic shield covers the part of the focused ion beam optics far from the sample, and wherein the distance L from the end surface of the magnetic shield nearer to the sample to the sample is given as            ∫      L      0        ⁢                            ⅆ                      z            1                                                              V              acc                        -                          ϕ              ⁡                              (                                  z                  1                                )                                                        ⁢                        ∫          L                      z            1                          ⁢                                            B              y                        ⁡                          (              z              )                                ⁢                                          ⁢                      ⅆ            z                                ≅  0where Vacc is the ion beam accelerating voltage, Φ(z1) the electric potential at the coordinate z1 on the optical axis of the ion beam, By(z) a magnetic field component perpendicular to the optical axis of the ion beam at the coordinate z on the optical axis, and the origin of the coordinate is located at the ion beam spot position on the sample in the absence of a magnetic field. 24. A focused ion beam apparatus according to claim 21, wherein the ion beam includes a plurality of ion beams having a plurality of different mass-to-charge ratios. 25. A focused ion beam apparatus according to claim 24, wherein the ion beams having the plurality of different mass-to-charge ratios are a plurality of types of isotope ion beams. 26. A focused ion beam apparatus according to claim 21, wherein the ion source is a Ga liquid metal ion source. 27. A focused ion beam apparatus according to claim 21,wherein the canceling magnetic field generator is arranged on the optical axis of the ion beam and has a beam path allowing the entire the ion beam to pass therethrough. 28. A focused ion beam apparatus according to claim 27,wherein the canceling magnetic field generator is covered by a magnetic shield.