Optical system with compensation lens

1. An optical system used in a charged particle beam inspection system, comprising:

one or more optical lenses; and

a compensation lens configured to compensate a drift of a focal length of a combination of the one or more optical lenses from a first medium to a second medium.

2. The optical system of claim 1 , wherein the optical system is a position detection system comprising:

a projection module including a projection lens and configured to project a first light beam to a sample;

a receiving module including a receiving lens and configured to receive a second light beam reflected from the sample; and

a detection module configured to detect a position of the sample based on the second light beam.

3. The optical system of claim 2 , wherein the one or more optical lenses include the projection lens included in the projection module and the receiving lens included in the receiving module.

4. The optical system of claim 2 , wherein the detection module is disposed in the first medium, and the projection module and the receiving module are disposed in the second medium.

5. The optical system of claim 1 , wherein the optical system is an imaging system comprising:

an illumination module configured to project a first light beam to a sample;

an objective lens configured to conjugate a second light beam reflected from the sample; and

a detection module configured to detect an image of the sample based on the second light beam.

6. The optical system of claim 5 , wherein the one or more optical lenses include the objective lenses.

7. The optical system of claim 5 , wherein the detection module is disposed in the first medium, and the objective lens is disposed in the second medium.

8. The optical system of claim 1 , wherein the optical system is an illumination system comprising:

an illumination module configured to project a first light beam to a portion of a sample under a charged particle beam;

an objective lens configured to conjugate a second light beam reflected from the sample; and

a detection module configured to detect an image of the portion of the sample based on the second light beam.

9. The optical system of claim 8 , wherein the one or more optical lenses include the objective lenses.

10. The optical system of claim 8 , wherein the detection module is disposed in the first medium, and the objective lens is disposed in the second medium.

11. The optical system of claim 1 , wherein the first medium is air, and the second medium is vacuum.

12. The optical system of claim 1 , wherein the charged particle beam inspection system is an electron beam inspection system.

13. The optical system of claim 1 , wherein a focal length of the compensation lens is configured based on a focal length of each one of the one or more lenses, a position of each one of the one or more lenses, and refractive indices of the first medium and the second medium.

14. A method for operating an optical system used in a charged particle beam inspection system, the optical system including one or more optical lenses, the method comprising:

compensating, via a compensation lens in the optical system, a drift of a focal length of a combination of the one or more lenses in a first medium, comprising:

emulating, by the compensation lens, an effect when the one or more lenses of the optical system performs under a second medium different from the first medium; and

exposing the one or more lens in the optical system in the second medium to operate without the compensation lens.

15. The method of claim 14 , wherein the optical system is a position detection system, and the method further includes:

projecting, by a projection module including a projection lens, a first light beam to a sample;

receiving, by a receiving module including a receiving lens, a second light beam reflected from the sample; and

detecting, by a detection module, a position of the sample based on the second light beam.