Image based metrology of surface deformations

1. A method for detecting areas of localized tilt on a sample using an imaging reflectometer, the method comprising:

illuminating a measurement area on the sample using an input beam;

performing a first imaging reflectometry measurement, wherein performing the first imaging reflectometry measurement comprises:

receiving, at an imaging sensor, a first imaging beam reflected from the sample, wherein an iris positioned at an aperture plane of the imaging reflectometer is configured so that the first imaging beam passes through a pupil of the iris and the iris does not block any portion of a first beam reflected from the sample; and

obtaining a first image of the measurement area using the first imaging beam reflected from the sample and received at the imaging sensor, the first image comprising a first plurality of pixels;

performing a second imaging reflectometry measurement, wherein performing the second imaging reflectometry measurement comprises:

receiving, at the imaging sensor, a second imaging beam reflected from the sample, wherein the iris positioned at the aperture plane of the imaging reflectometer is configured so that the second imaging beam passes through the pupil of the iris and the iris blocks a portion of a second beam reflected from the sample; and

obtaining a second image of the measurement area using the second imaging beam reflected from the sample and received at the imaging sensor, the second image comprising a second plurality of pixels;

detecting the areas of localized tilt within the measurement area, wherein detecting the areas of localized tilt comprises comparing first reflectance intensity values of pixels in the first image with second reflectance intensity values of corresponding pixels in the second image.

2. The method of claim 1 wherein detecting the areas of localized tilt further comprises identifying pixels in the first image where the first reflectance intensity values are different from the second reflectance intensity values of corresponding pixels in the second image by more than a predetermined amount.

3. The method of claim 1 wherein a difference in height between an area of localized tilt and surrounding flat areas on the sample is determined based on a difference in reflectance intensity values between pixels in the first image and corresponding pixels in the second image.

4. The method of claim 1 wherein a diameter of the pupil of the iris is smaller during the second imaging reflectometry measurement than during the first imaging reflectometry measurement.

5. The method of claim 1 wherein a size of the pupil of the iris is variable.

6. The method of claim 1 wherein comparing the first reflectance intensity values of the pixels in the first image with the second reflectance intensity values of the corresponding pixels in the second image comprises dividing the second reflectance intensity values of the pixels in the second image by the first reflectance intensity values of the corresponding pixels in the first image.

7. The method of claim 1 wherein detecting the areas of localized tilt comprises comparing, for each of the first plurality of pixels and each of the second plurality of pixels, a first reflectance intensity value of a pixel in the first image with a second reflectance intensity value of a corresponding pixel in the second image.

8. The method of claim 1 further comprising determining a direction of tilt associated with the areas of localized tilt, wherein determining the direction of tilt comprises:

arranging a knife edge to block a first portion of the first imaging beam reflected from the sample and a second portion of the second imaging beam reflected from the sample; and

identifying regions within the areas of localized tilt where the direction of tilt is orthogonal to the knife edge based on a change in reflectance intensity values between the first image and the second image.