Electromagnetic lens for electron beam exposure apparatus

1. An electromagnetic lens comprising:

an electromagnetic coil wound to be rotationally symmetrical about an optical axis of an electron beam; and

a pole piece including an inner wall covering an inner circumference side of the electromagnetic coil, an upper end wall covering the electromagnetic coil at an entering side of the electron beam, a lower end wall covering the electromagnetic coil at an emitting side of the electron beam, an outer wall covering an outer circumference side of the electromagnetic coil, and a gap formed by cutting out at least a part of the inner wall, the lower end wall, and the upper end wall to be rotationally symmetrical about the optical axis,

wherein a thickness of the inner wall is thinnest at a portion close to the gap, and gradually becomes thicker as a distance from the gap increases, and a width of the electromagnetic coil in a radial direction is more increased as being closer to the gap.

2. The electromagnetic lens according to claim 1 , wherein the thickness of the thickest portion of the inner wall is thicker than a thickness of the outer wall.

3. The electromagnetic lens according to claim 1 , wherein

the gap is formed in at least a part of the inner wall, the lower end wall, and the upper end wall, and

a surface of the inner wall on an optical axis side is in parallel with the optical axis, and a surface of the inner wall on an electromagnetic coil side slants to be closer to the optical axis as extending toward the gap.

4. The electromagnetic lens according to claim 3 , wherein the electromagnetic coil has a trapezoidal cross section taken along a plane in parallel with the optical axis.

5. The electromagnetic lens according to claim 3 , wherein at least the part of the inner wall and the lower end wall with the gap formed therein is disposed to be opposed to a surface of a sample where an image of the electron beam is to be formed.

6. The electromagnetic lens according to claim 1 , wherein the gap is formed at a center of the inner wall.

7. The electromagnetic lens according to claim 1 , wherein a clearance between the pole piece and the electromagnetic coil is a flow path for a coolant.

8. The electromagnetic lens according to claim 7 , wherein the gap in the pole piece is sealed with a sealing member made of a nonmagnetic material.

9. An electron beam exposure apparatus comprising an electromagnetic lens,

wherein the electromagnetic lens includes:

an electromagnetic coil wound to be rotationally symmetrical about an optical axis of an electron beam; and

a pole piece including an inner wall covering an inner circumference side of the electromagnetic coil, an upper end wall covering the electromagnetic coil at an entering side of the electron beam, a lower end wall covering the electromagnetic coil at an emitting side of the electron beam, an outer wall covering an outer circumference side of the electromagnetic coil, and a gap formed by cutting out at least a part of the inner wall, the lower end wall, and the upper end wall to be rotationally symmetrical about the optical axis,

wherein a thickness of the inner wall is thinnest at a portion close to the gap, and gradually becomes thicker as a distance from the gap increases, and a width of the electromagnetic coil in a radial direction is more increased as being closer to the gap.

10. The electron beam exposure apparatus according to claim 9 , further comprising:

a plurality of electron guns configured to emit electron beams, and

a plurality of the electromagnetic lenses on optical axes of the respective electron beams.