Image based metrology of surface deformations
Methods for detecting areas of localized tilt on a sample using imaging reflectometry measurements include obtaining a first image without blocking any light reflected from the sample and obtaining a second image while blocking some light reflected from the sample at the aperture plane. The areas of localized tilt are detected by comparing first reflectance intensity values of pixels in the first image with second reflectance intensity values of corresponding pixels in the second image.
1. A method for detecting areas of localized tilt on a sample using an imaging reflectometer configured for dark field measurements, the method comprising:
illuminating a measurement area on the sample using an input beam, a portion of the input beam passing through an illumination pupil;
receiving, at an imaging sensor, portions of an imaging beam reflected from the areas of localized tilt on the sample, the portions of the imaging beam reflected from the areas of localized tilt passing through an imaging pupil, wherein the imaging pupil is configured to block portions of an imaging beam reflected from flat areas on the sample;
obtaining a first image of the measurement area using the portions of the imaging beam reflected from the areas of localized tilt on the sample and received at the imaging sensor; and
detecting the areas of localized tilt within the measurement area based on locations of bright spots within the first image.
2. The method of claim 1 wherein the illumination pupil has a first numerical aperture (NA), and the imaging pupil has a second NA larger than the first NA.
3. The method of claim 1 wherein a one-dimensional pattern is provided at the illumination pupil and a one-dimensional pattern is provided at the imaging pupil, the one-dimensional pattern at the illumination pupil being an inverse of the one-dimensional pattern at the imaging pupil, and an orientation of the one-dimensional pattern at the illumination pupil being aligned with the orientation of the one-dimensional pattern at the imaging pupil, the method further comprising determining a direction of tilt associated with the areas of localized tilt, wherein determining the direction of tilt comprises:
rotating the one-dimensional pattern at the illumination pupil and the one-dimensional pattern at the imaging pupil by about 90°;
obtaining a second image of the measurement area using the portions of the imaging beam reflected from the areas of localized tilt on the sample and received at the imaging sensor; and
determining the direction of tilt associated with the areas of localized tilt based on a change in reflectance intensity values between the first image and the second image.
4. The method of claim 3 wherein the one-dimensional pattern at the illumination pupil and/or the one-dimensional pattern at the imaging pupil is generated by electronic control, and rotation of the one-dimensional pattern at the illumination pupil and/or the one-dimensional pattern at the imaging pupil is provided by electronically generating a new one-dimensional pattern rather than by rotating a mechanical pattern generator.