# US 9,436,985 B1 · Method and system for enhancing image quality

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- Number: US 9,436,985 B1
- Document ID: US9436985B1
- Status: Granted
- Kind: B1
- Application number: 14/595,582
- Filed: 2015-01-13
- Granted: 2016-09-06
- Earliest claimed: 2013-02-26
- Assignees: Hermes Microvision, Inc.
- Applicants: Hermes Microvision Inc.
- Inventors: Chiyan Kuan; Joe Wang; Van-Duc Nguyen
- Web page: https://patentgrep.com/patent/US9436985B1

## CPC

- G03F 7/7065: {Defects, e.g. optical inspection of patterned layer for defects}
- G01N 23/2251: using incident electron beams, e.g. scanning electron microscopy [SEM]
- G03F 1/86: by charged particle beam [CPB]
- G06T 5/40: using histogram techniques
- G06T 5/50: using two or more images, e.g. averaging or subtraction
- G06T 5/77: Retouching; Inpainting; Scratch removal
- H01J 37/222: {Image processing arrangements associated with the tube}
- H01J 37/28: with scanning beams {(H01J37/268, H01J37/292, H01J37/2955 take precedence)}
- G01N 21/9501: {Semiconductor wafers (manufacturing processes per se of semiconductor devices implementing a measuring step H10P74/20)}
- G01N 2223/42: image digitised, -enhanced in an image processor
- G01N 2223/6116: semiconductor wafer
- G06T 5/80: Geometric correction
- G06T 2207/20172: Image enhancement details
- G06T 2207/20216: Image averaging
- G06T 2207/30148: Semiconductor; IC; Wafer
- G06V 10/30: Noise filtering
- G03F: PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR; (phototypographic composing devices B41B; photosensitive materials or processes for photographic purposes G03C; electrophotography, sensitive layers or processes therefor G03G)
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)
- G06T: IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
- H01J: ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS (spark-gaps H01T; electric arc lamps H05B31/00; particle accelerators H05H)
- G06V: IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING

## Abstract

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.

## Claims

### Claim 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.

### Claim 2 (depends on 1)

The method of claim 1, wherein the nominated grey level is the average or the median of the remained grayscales.

### Claim 3 (depends on 1)

The method of claim 1, wherein the extreme values include the highest 25% and the lowest 25% of the sequence of grayscales.

### Claim 4 (depends on 1)

The method of claim 1, wherein the extreme values include the highest 10% and the lowest 10% of the sequence grayscales.

### Claim 5 (depends on 1)

The method of claim 1, wherein the sequence is a time mode.

### Claim 6 (depends on 5)

The method of claim 5, wherein the charged particle beam is an electron beam.

### Claim 7 (depends on 6)

The method of claim 6, wherein the pixel is imaged from a specimen.

### Claim 8 (depends on 7)

The method of claim 7, wherein the specimen is an extreme ultraviolet (EUV) mask.

### Claim 9 (depends on 7)

The method of claim 7, wherein the specimen is a silicon wafer.

### Claim 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.

### Claim 11 (depends on 10)

The system of claim 10, wherein the charged particle beam is an electron beam.

### Claim 12 (depends on 11)

The system of claim 11, wherein the pixel is imaged from a specimen.

### Claim 13 (depends on 12)

The system of claim 12, wherein the specimen is an extreme ultraviolet (EUV) mask.

### Claim 14 (depends on 12)

The system of claim 12, wherein the specimen is a silicon wafer.

### Claim 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.

### Claim 16 (depends on 15)

The method of claim 15, wherein the plurality of frames have a time sequence in the scanning step.

### Claim 17 (depends on 16)

The method of claim 16, wherein the charged particle beam is an electron beam.

### Claim 18 (depends on 17)

The method of claim 17, wherein the specimen is an extreme ultraviolet (EUV) mask.

### Claim 19 (depends on 17)

The method of claim 17, wherein the specimen is a silicon wafer.

### Claim 20 (depends on 18)

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.

### Claim 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.

### Claim 22 (depends on 21)

The computer readable medium of claim 21, wherein the nominated grey level is the average or the median of the remained grayscales.

### Claim 23 (depends on 21)

The computer readable medium of claim 21, wherein the extreme values include the highest 25% and the lowest 25% of the sequence of grayscales.

### Claim 24 (depends on 21)

The computer readable medium of claim 21, wherein the extreme values include the highest 10% and the lowest 10% of the sequence grayscales.

### Claim 25 (depends on 21)

The computer readable medium of claim 21, wherein the sequence is a time mode.

### Claim 26 (depends on 21)

The computer readable medium of claim 21, wherein the charged particle beam is an electron beam.