Combining simulation and optical microscopy to determine inspection mode
A best optical inspection mode to detect defects can be determined when no defect examples or only a limited number of defect examples are available. A signal for a defect of interest at the plurality of sites and for the plurality of modes can be determined using electromagnetic simulation. A ratio of the signal for the defect of interest to the noise at each combination of the plurality of sites and the plurality of modes can be determined. A mode with optimized signal-to-noise characteristics can be determined based on the ratios.
1. A system comprising:
a light source configured to direct a beam of light at a wafer;
a detector that collects the beam of light reflected from the wafer; and
a processor in electronic communication with the detector, wherein the processor is configured to:
receive test images from the detector and reference images of the wafer for a plurality of modes;
determine difference images from the test images and the reference images;
determine noise based on grey levels in the difference images at a plurality of sites;
determine a signal intensity for a defect of interest at the plurality of sites and for the plurality of modes using electromagnetic simulation, wherein the electromagnetic simulation, for the plurality of modes, determines predicted grey level values for one of the test images and one of the reference images and subtracts the predicted grey level values for the one of the test images and the one of the reference images whereby a difference grey level of the defect of interest is determined;
determine a ratio of the signal intensity for the defect of interest to the noise at each combination of the plurality of sites and the plurality of modes, wherein the signal intensity for the defect of interest and the noise in each of the ratios corresponds to a same site and mode in the plurality of sites and the plurality of modes; and
determine a mode with optimized signal-to-noise characteristics based on the ratios.
2. The system of claim 1 , wherein the mode with optimized signal-to-noise characteristics is determined by examining outliers in a distribution.
3. The system of claim 1 , wherein the mode with optimized signal-to-noise characteristics is determined by finding a maximum of the ratios.
4. The system of claim 1 , wherein the mode with optimized signal-to-noise characteristics is determined by finding a maximum mean of the ratios.
5. The system of claim 1 , wherein the mode with optimized signal-to-noise characteristics is determined by finding a maximum median of the ratios.
6. A method comprising:
receiving, at a processor, test images and reference images of the wafer for a plurality of modes;
determining difference images from the test images and the reference images using the processor;
determining noise based on grey levels in the difference images at a plurality of sites using the processor;
determining, using the processor, a signal intensity for a defect of interest at the plurality of sites and for the plurality of modes using electromagnetic simulation, wherein the electromagnetic simulation, for the plurality of modes, determines predicted grey level values for one of the test images and one of the reference images and subtracts the predicted grey level values for the one of the test images and the one of the reference images whereby a difference grey level of the defect of interest is determined;
determining, using the processor, a ratio of the signal intensity for the defect of interest to the noise at each combination of the plurality of sites and the plurality of modes, wherein the signal intensity for the defect of interest and the noise in each of the ratios corresponds to a same site and mode in the plurality of sites and the plurality of modes; and
determining, using the processor, a mode with optimized signal-to-noise characteristics based on the ratios.
7. The method of claim 6 , wherein the mode with optimized signal-to-noise characteristics is determined by examining outliers in a distribution.
8. The method of claim 6 , wherein the mode with optimized signal-to-noise characteristics is determined by finding a maximum of the ratios.
9. The method of claim 6 , wherein the mode with optimized signal-to-noise characteristics is determined by finding a maximum mean of the ratios.
10. The method of claim 6 , wherein the mode with optimized signal-to-noise characteristics is determined by finding a maximum median of the ratios.
11. The method of claim 6 , further comprising adjusting a system to use the mode with optimized signal-to-noise characteristics.
12. The method of claim 11 , wherein the system is an optical inspection system for semiconductor wafers.
13. The method of claim 6 , wherein the test images are within a care area.
14. A non-transitory computer-readable storage medium, comprising one or more programs for executing the following steps on one or more computing devices:
determine difference images from test images and reference images for a plurality of modes;
determine noise based on grey levels in the difference images at a plurality of sites;
determine a signal intensity for a defect of interest at the plurality of sites and for the plurality of modes using electromagnetic simulation, wherein the electromagnetic simulation, for the plurality of modes, determines predicted grey level values for one of the test images and one of the reference images and subtracts the predicted grey level values for the one of the test images and the one of the reference images whereby a difference grey level of the defect of interest is determined;
determine a ratio of the signal intensity for the defect of interest to the noise at each combination of the plurality of sites and the plurality of modes, wherein the signal intensity for the defect of interest and the noise in each of the ratios corresponds to a same site and mode in the plurality of sites and the plurality of modes; and
determine a mode with optimized signal-to-noise characteristics based on the ratios.
15. The non-transitory computer-readable storage medium of claim 14 , wherein the mode with optimized signal-to-noise characteristics is determined by examining outliers in a distribution.
16. The non-transitory computer-readable storage medium of claim 14 , wherein the mode with optimized signal-to-noise characteristics is determined by finding a maximum of the ratios.
17. The non-transitory computer-readable storage medium of claim 14 , wherein the mode with optimized signal-to-noise characteristics is determined by finding a maximum mean of the ratios.
18. The non-transitory computer-readable storage medium of claim 14 , wherein the mode with optimized signal-to-noise characteristics is determined by finding a maximum median of the ratios.