Method and system for enhancing image quality
This invention relates to methods and systems for enhance the signal-to-noise ratio of an image scanned by a charged particle beam. In an embodiment, a sequence of grayscales of a pixel is recorded first, extreme values of the sequence of grayscales are then identified and removed, and the remained grayscales are used to determine a nominated grayscale of the pixel.
1. A method for enhancing image quality, comprising:
repeatedly scanning a pixel by a charged particle beam;
recording a sequence of grayscales of the pixel scanned by the charged particle beam;
removing extreme values from the sequence of grayscales; and
determining a nominated grey level of the pixel from remained grayscales in order to enhance the image quality,
wherein the extreme grayscales of each pixel include less than a lower limit (LL) and more than a upper limit (UL) of grayscales of each pixel, in which the lower limit is μ minus 0.67σ, the upper limit is μ plus 0.676σ, and μ and σ respectively stand for the mean and the standard deviation of the grayscales of each pixel.
2. The method of claim 1 , wherein the nominated grey level is the average or the median of the remained grayscales.
3. The method of claim 1 , wherein the extreme values include the highest 25% and the lowest 25% of the sequence of grayscales.
4. The method of claim 1 , wherein the extreme values include the highest 10% and the lowest 10% of the sequence grayscales.
5. The method of claim 1 , wherein the sequence is a time mode.
6. The method of claim 5 , wherein the charged particle beam is an electron beam.
7. The method of claim 6 , wherein the pixel is imaged from a specimen.
8. The method of claim 7 , wherein the specimen is an extreme ultraviolet (EUV) mask.
9. The method of claim 7 , wherein the specimen is a silicon wafer.
10. A system for enhancing image quality, comprising:
a charged particle beam repeatedly scanning a pixel;
means for recording a sequence of grayscales of the pixel scanned by the charged particle beam;
means for removing extreme values from the sequence of grayscales; and
means for calculating an average or a median of remained grayscales to obtain an nominated grayscale of the pixel in order to enhance the image quality,
wherein the extreme grayscales of each pixel include less than a lower limit (LL) and more than a upper limit (UL) of grayscales of each pixel, in which the lower limit is μ minus 0.67σ, the upper limit is μ plus 0.67σ, and μ and σ respectively stand for the mean and the standard deviation of the grayscales of each pixel.
11. The system of claim 10 , wherein the charged particle beam is an electron beam.
12. The system of claim 11 , wherein the pixel is imaged from a specimen.
13. The system of claim 12 , wherein the specimen is an extreme ultraviolet (EUV) mask.
14. The system of claim 12 , wherein the specimen is a silicon wafer.
15. A method for enhancing image quality, comprising:
scanning a specimen to obtain a plurality of frames by using a charged particle beam;
removing extreme grayscales of each pixel of the plurality of frames;
averaging remained grayscales of each pixel of the plurality of frames to obtain a scanned image of the specimen in order to enhance the image quality,
wherein the extreme grayscales of each pixel include less than a lower limit (LL) and more than a upper limit (UL) of grayscales of each pixel, in which the lower limit is μ minus 0.67σ, the upper limit is μ plus 0.67σ, and μ and σ respectively stand for the mean and the standard deviation of the grayscales of each pixel.
16. The method of claim 15 , wherein the plurality of frames have a time sequence in the scanning step.
17. The method of claim 16 , wherein the charged particle beam is an electron beam.
18. The method of claim 17 , wherein the specimen is an extreme ultraviolet (EUV) mask.
19. The method of claim 17 , wherein the specimen is a silicon wafer.
20. The method of claim 18 , wherein at least two frames from the plurality of frames with most the extreme grayscales in a statistical result are obtained.
21. A non-transitory computer readable medium encoded with a computer program for enhancing image quality of an inspection system, comprising the steps of:
recording a sequence of grayscales of a pixel scanned by a charged particle beam;
removing extreme values from the sequence of grayscales; and
determining a nominated grey level of the pixel from remained grayscales in order to enhance the image quality,
wherein the extreme grayscales of each pixel include less than a lower limit (LL) and more than a upper limit (UL) of grayscales of each pixel, in which the lower limit is μ minus 0.67σ, the upper limit is μ plus 0.67σ, and μ and σ respectively stand for the mean and the standard deviation of the grayscales of each pixel.
22. The computer readable medium of claim 21 , wherein the nominated grey level is the average or the median of the remained grayscales.
23. The computer readable medium of claim 21 , wherein the extreme values include the highest 25% and the lowest 25% of the sequence of grayscales.
24. The computer readable medium of claim 21 , wherein the extreme values include the highest 10% and the lowest 10% of the sequence grayscales.
25. The computer readable medium of claim 21 , wherein the sequence is a time mode.
26. The computer readable medium of claim 21 , wherein the charged particle beam is an electron beam.