Defect detection method

1. A defect detection method comprising:

irradiating light from a light source in an optical system and obtaining a plurality of optical images of a sample having a repeated pattern having a size smaller than a resolution of the optical system, while changing the conditions of the optical system;

performing correction processing for the plurality of optical images with the use of at least one of noise filtering and convolution filtering;

shifting a position of the other optical images based on any of the plurality of optical images, obtaining a relationship between shift amounts of the other optical images and a change of correlation of a gray scale value using base pattern noise between the plurality of optical images, and

performing positional alignment of the plurality of optical images based on the shift amount obtained when the correlation is highest; and performing defect detection of the sample with the use of the plurality of optical images after the positional alignment.

2. The defect detection method according to claim 1 , wherein the step of performing the defect detection is a step of plotting each pixel of the plurality of optical images in a gray scale value space, and separating a pixel having a defect and a pixel having no defect.

3. The defect detection method according to claim 2 , wherein the step of separating the pixel having a defect and the pixel having no defect is performed using at least one of: clustering of the each pixel in the gray scale value space; a distance from a correlation straight line of the plurality of optical images to each pixel; and a difference image of the plurality of optical images.

4. The defect detection method according to claim 1 , wherein the correlation is evaluated using at least one of: a covariance between the plurality of optical images; a correlation coefficient between the plurality of optical images; a sum of square of a difference between the plurality of optical images; and a sum of absolute values of a difference between the plurality of optical images.

5. The defect detection method according to claim 1 , wherein the plurality of optical images include an optical image imaged by transmission of the irradiated light through the sample and an optical image imaged by reflection of the irradiated light by the sample.

6. The defect detection method according to claim 1 , wherein the plurality of optical images are captured by changing a focal position between the optical system and the sample.

7. The defect detection method according to claim 1 , wherein the plurality of optical images include an optical image imaged when the optical system is regarded as a bright field and an optical image imaged when the optical system is regarded as a dark field.

8. The defect detection method according to claim 1 , wherein the plurality of optical images are captured by changing a polarization state of the light from the light source.

9. The defect detection method according to claim 1 , further comprising performing correction in which dynamic ranges between the optical images are matched.

10. The defect detection method according to claim 1 , further comprising performing correction in which a tone of the gray scale value of each of the plurality of optical images is inverted.

11. The defect detection method according to claim 1 , further comprising correcting an image distortion of the plurality of optical images.

12. The defect detection method according to claim 1 , wherein the light from the light source is DUV (deep ultraviolet radiation) light.