Image capturing device and inspection apparatus and inspection method

1. An image capturing device comprising:

a light source configured to emit light having a predetermined wavelength;

a polarization beamsplitter configured to reflect the light from the light source;

a Faraday rotator having an optical material configured to rotate a polarization plane of the light reflected from the polarization beamsplitter by changing intensity of a magnetic field or changing the thickness of the optical material;

an objective lens configured to illuminate an inspection target with the light transmitted through the Faraday rotator;

a sensor configured to capture an optical image of the inspection target by causing the light reflected by the inspection target to be incident through the objective lens, the Faraday rotator, and the polarization beamsplitter;

an image processor configured to obtain a gradation value in each pixel with respect to the optical image and acquire (a) a rotation angle of the polarization plane of the light rotated by the Faraday rotator for minimizing a standard deviation of the gradation value, or (b) a rotation angle for minimizing a value which is obtained by dividing the standard deviation of the gradation values of a plurality of optical images obtained by changing the rotation angle, by a square root of an average gradation value obtained from the gradation value, wherein a rotation angle of the Faraday rotator is adjusted to the rotation angle acquired by the image processor; and

a defect detector configured to detect a defect of the inspection target based on an optical image which is captured while the magnetic field is applied to the Faraday rotator.

2. The image capturing device according to claim 1 , wherein the Faraday rotator rotates a polarization plane of the light before the transmission through the Faraday rotator such that the polarization plane rotates 90 degrees by transmitting the light back and forth through the Faraday rotator, by applying the magnetic field.

3. The image capturing device according to claim 2 , wherein the Faraday rotator includes an optical material that transmits the light, and

the magnetic field is applied to the optical material by one selected from a group consisting of an electromagnet, a permanent magnet, and a combination of the electromagnet and permanent magnet.

4. The image capturing device according to claim 1 , further comprising: a half-wavelength plate configured between the polarization beamsplitter and the inspection target,

wherein the half-wavelength plate changes a polarization direction of the light with which the inspection target is illuminated.

5. The image capturing device according to claim 4 , wherein the half-wavelength plate is configured between the polarization beamsplitter and the Faraday rotator.

6. The image capturing device according to claim 4 , wherein the half-wavelength plate is configured between the Faraday rotator and the inspection target.

7. The image capturing device according to claim 4 , wherein the half-wavelength plate includes a rotation mechanism, and

the rotation mechanism changes the polarization direction of the light by changing an angle of the half-wavelength plate.

8. The image capturing device according to claim 1 , wherein the predetermined wavelength of the light from the light source and a numerical aperture of an objective lens defines a resolution limit, wherein the resolution limit is a value at which a repetitive pattern formed in the inspection target is not resolved.

9. The image capturing device according to claim 1 ,

wherein a plurality of permanent magnets having different intensities of the magnetic field are prepared for the Faraday rotator, and

wherein the rotation angle of the Faraday rotator is adjusted by selecting appropriate one of the plurality of permanent magnets so as to obtain the rotation angle acquired by the image processor.

10. The image capturing device according to claim 1 ,

wherein a plurality of optical materials having different thicknesses are prepared for the Faraday rotator,

wherein the rotation angle of the Faraday rotator is adjusted by selecting appropriate one of the plurality of optical materials so as to obtain the rotation angle acquired by the image processor.

11. The image capturing device according to claim 1 ,

wherein a coil through which an electric current passes to generate the magnetic field is provided in the Faraday rotator,

wherein the rotation angle of the Faraday rotator is adjusted by controlling the current so as to obtain the rotation angle acquired by the image processor.