Patent Number: 
Section: claims

1. A focusing method for a charged particle beam comprising:setting a lens value in an imaging lens;setting first coil values in first and second alignment coils, wherein a charged particle beam is temporarily swung outside and then swung back to an opening center of a first aperture plate member by deflection of the first and second alignment coils in which the first coil values have been set;measuring a first position of the charged particle beam focused by the imaging lens in which the lens value has been set, on a second aperture plate member, wherein the charged particle beam having passed through the opening center of the first aperture plate member by deflection of the first and second alignment coils in which the first coil values have been set is focused on a position which is different from a position on the second aperture plate member, by the imaging lens in which the lens value has been set;setting second coil values in the first and second alignment coils, wherein the charged particle beam is temporarily swung outside and then swung back to the opening center of the first aperture plate member by deflection of the first and second alignment coils in which the second coil values have been set and an amount of deflection of the first and second alignment coils in which the second coil values have been set is different from an amount of deflection of the first and second alignment coils in which the first coil values have been set;measuring a second position of the charged particle beam focused by the imaging lens in which the lens value has been set, on the second aperture plate member, wherein the charged particle beam having passed through the opening center of the first aperture plate member by deflection of the first and second alignment coils in which the second coil values have been set is focused on a position which is different from a position on the second aperture plate member, by the imaging lens in which the lens value has been set; andcalculating a difference between the first and second positions of the charged particle beam on the second aperture plate member;wherein the lens value is variably changed, and a plurality of the differences are calculated depending on the lens values variably changed; andfurther comprising: setting a lens value which corresponds to a minimum difference of the plurality of the differences, in the imaging lens. 2. The method according to claim 1, wherein the first and second coils are arranged above the first aperture plate member. 3. The method according to claim 1, wherein the imaging lens is arranged between the first and second aperture plate members. 4. The method according to claim 1, further comprising:detecting a current of the charged particle beam having passed through the second aperture plate member when amounts of deflection of the charged particle beam obtained by the first and second alignment coils are changed for each of focal positions of the charged particle beam controlled by the lens. 5. The method according to claim 4, wherein the difference between the first and second positions is calculated by using a current value of the charged particle beam detected and an amount of deflection of the charged particle beam obtained by a deflector. 6. The method according to claim 1, wherein a ratio of coil values is used when the first coil values are set in the first and second alignment coils. 7. The method according to claim 6, wherein a coil value setting in one of the first and second alignment coils is calculated by multiplying the ratio by a current value of another of the first and second alignment coils. 8. The method according to claim 1, wherein when the lens value is variably changed, a position focused by the imaging lens is adjusted to a focal position where the difference between the first and second positions is made smaller. 9. The method according to claim 1, wherein the difference between the first and second positions is calculated in directions of two-dimensional directions on the second aperture plate member. 10. A focusing method for a charged particle beam comprising:setting first coil values in first and second alignment coils, wherein a charged particle beam is temporarily swung outside and then swung back to an opening center of a first aperture plate member by deflection of the first and second alignment coils in which the first coil values have been set;setting a lens value in an imaging lens; andmeasuring a first position of the charged particle beam focused by the imaging lens in which the lens value has been set, on a second aperture plate member, wherein the charged particle beam having passed through the opening center of the first aperture plate member by deflection of the first and second alignment coils in which the first coil values have been set is focused on a position which is different from a position on the second aperture plate member, by the imaging lens in which the lens value has been set;wherein the lens value is variably changed, and a plurality of the first positions are calculated depending on the lens values variably changed;further comprising:setting second coil values in the first and second alignment coils, wherein the charged particle beam is temporarily swung outside and then swung back to the opening center of the first aperture plate member by deflection of the first and second alignment coils in which the second coil values have been set and an amount of deflection of the first and second alignment coils in which the second coil values have been set is different from an amount of deflection of the first and second alignment coils in which the first coil values have been set;setting the lens value in the imaging lens; andmeasuring a second position of the charged particle beam focused by the imaging lens in which the lens value has been set, on the second aperture plate member, wherein the charged particle beam having passed through the opening center of the first aperture plate member by deflection of the first and second alignment coils in which the second coil values have been set is focused on a position which is different from a position on the second aperture plate member, by the imaging lens in which the lens value has been set;wherein the lens value is variably changed, and a plurality of the second positions are calculated depending on the lens values variably changed;further comprising:calculating a plurality of differences, each of the plurality of differences being a difference between corresponding positions of the first and second positions on the second aperture plate member; andsetting a lens value which corresponds to a minimum difference of the plurality of the differences, in the imaging lens.