Electron microscope and method of aberration measurement

1. An electron microscope comprising:

an electron beam source for producing an electron beam;

an illumination lens system for focusing the electron beam onto a sample;

a scanner for scanning the focused electron beam over the sample;

an aperture stop having a plurality of detection angle-limiting holes for extracting rays of the electron beam having mutually different detection angles from the electron beam transmitted through the sample and one reference hole for extracting rays of the electron beam passed through the reference hole having a reference detection angle;

a detector for detecting the rays of the electron beams passed through the aperture stop, said detector having a first detector segment and a second detector segment, said first detector segment being operative to detect rays of the electron beam passed through said plurality of detection angle-limiting holes, and said second detector segment being operative to detect rays of the electron beam passed through the reference hole,

an image generator for generating a first STEM (scanning transmission electron microscope) image based on the output from said first detector segment and a second STEM image based on the output from the second detector segment; and

an aberration calculator for finding an aberration in said illumination lens system based on the first STEM image and the second STEM image.

2. The electron microscope as set forth in claim 1 , wherein said aberration calculator finds the aberration in said illumination lens system by calculating a correlation function between said first STEM image and said second STEM image.

3. The electron microscope as set forth in claim 1 ,

wherein said first detector segment has an annular shape and said second detector segment being positioned in an area surrounded by the first detector segment.

4. The electron microscope as set forth in claim 3 , wherein said first detector segment is split into a plurality of subsegments capable of independently detecting rays of the electron beam.

5. A method of measuring an aberration in an illumination lens system of an electron microscope having an electron beam source for producing an electron beam, said illumination lens system for focusing the electron beam onto a sample, a scanner for scanning the focused electron beam over the sample, and a detector for detecting the electron beam transmitted through the sample; said method comprising the steps of:

(a) placing a first aperture stop having a plurality of detection angle-limiting holes for extracting rays of the electron beam having mutually different detection angles from the electron beam transmitted through the sample at a position between the sample and the detector, detecting the rays of the electron beams and obtaining a first STEM (scanning transmission electron microscope) image;

(b) placing a second aperture stop having a reference detection angle-limiting hole for extracting rays of the electron beam having a reference detection angle from the electron beam transmitted through the sample at a position between the sample and the detector, detecting the rays of the electron beam and obtaining a second STEM image; and

(c) calculating the aberration in said illumination lens system based on the first STEM image and the second STEM image.

6. The method of measuring an aberration in an illumination lens system of an electron microscope as set forth in claim 5 ,

wherein said steps of (a) and (b) are executed by using the single aperture stop having both a plurality of detection angle-limiting holes and a reference detection angle-limiting hole.

7. The method of measuring an aberration in an illumination lens system of an electron microscope as set forth in claim 5 ,

wherein the aberration is found based on calculation of a correlation function between said first STEM image and said second STEM image in step (c).

8. An electron microscope comprising:

an electron beam source for producing an electron beam;

an illumination lens system for focusing the electron beam onto a sample;

a scanner for scanning the focused electron beam over the sample;

a first aperture stop having a plurality of detection angle-limiting holes for extracting rays of the electron beam having mutually different detection angles from the electron beam transmitted through the sample;

a second aperture stop having a reference detection angle-limiting hole for extracting rays of the electron beam having specific detection angle from the electron beam transmitted through the sample, wherein the first and second aperture stops may be arranged in two stages along the path of the electron beam transmitted through the sample or selectively placed in a path of the electron beam transmitted through the sample;

a detector for detecting the rays of the electron beams passed through the aperture stops;

an image generator for generating a first STEM (scanning transmission electron microscope) image based on results of detections performed by said detector by detecting the rays of the electron beams passed through the first aperture stop and for generating a second STEM image based on results of detections performed by said detector by detecting the rays of the electron beam passed through the second aperture stop; and

an aberration calculator for finding an aberration in said illumination lens system based on the first STEM image and the second STEM image.

9. The electron microscope as set forth in claim 8 , wherein said aberration calculator finds the aberration in said illumination lens system by calculating a correlation function between said first STEM image and said second STEM image.