# US 9,437,394 B1 · Method of operating a charged particle microscope and charged particle microscope operating according to such method

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- Number: US 9,437,394 B1
- Document ID: US9437394B1
- Status: Granted
- Kind: B1
- Application number: 14/228,391
- Filed: 2014-03-28
- Granted: 2016-09-06
- Earliest claimed: 2014-03-28
- Assignees: Carl Zeiss Microscopy GmbH
- Applicants: Carl Zeiss Microscopy GmbH; Carl Zeiss Microscopy Ltd.
- Inventors: Simon Diemer; Janina Schulz; Emanuel Heindl; David Roddom
- Web page: https://patentgrep.com/patent/US9437394B1

## CPC

- H01J 37/21: Means for adjusting the focus
- H01J 37/222: {Image processing arrangements associated with the tube}
- H01J 37/263: {Contrast, resolution or power of penetration}
- H01J 2237/153: Correcting image defects, e.g. stigmators
- H01J 2237/21: Focus adjustment
- H01J 2237/216: Automatic focusing methods
- H01J: ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS (spark-gaps H01T; electric arc lamps H05B31/00; particle accelerators H05H)

## Abstract

A method of operating a charged particle microscope comprises: providing settings of a focus, an x-stigmator and an y-stigmator of the charged particle microscope; and then repeatedly performing adjusting the charged particle microscope to the settings, recording an image of an object using the settings, determining a sharpness measure from the recorded image, and changing at least one of the settings of the focus, the x-stigmator and the y-stigmator based on the sharpness measure until a stop criterion is fulfilled. Herein, the determining of the sharpness measure comprises: determining an orientation of an intensity gradient at each of a plurality of locations within one of the recorded image and a processed image generated by processing the recorded image, and determining the sharpness measure based on the plurality of determined orientations.

## Claims

### Claim 1

A method of operating a charged particle microscope, the method comprising:
providing settings of a focus, an x-stigmator and an y-stigmator of the charged particle microscope; and then
repeatedly performing
adjusting the charged particle microscope to the settings,
recording an image of an object using the settings,
determining a sharpness measure of the image as a whole from the recorded image, the sharpness measure indicating an overall sharpness of the recorded image, and
changing at least one of the settings of the focus, the x-stigmator and the y stigmator based on the sharpness measure
until a stop criterion is fulfilled;
wherein the determining of the sharpness measure comprises:
determining an orientation of an intensity gradient at each of a plurality of locations within one of the recorded image and a processed image generated by processing the recorded image, and
determining the sharpness measure based on the plurality of determined orientations.

### Claim 2 (depends on 1)

The method of claim 1, wherein the determining of the sharpness measure comprises determining a first orientation such that a function depending on the orientation as an argument has a minimum at the first orientation, wherein the function depends on a number of locations of the plurality of locations at which the determined orientation of the intensity gradient is within an orientation interval about the orientation argument.

### Claim 3 (depends on 1)

The method of claim 1, wherein the determining of the sharpness measure comprises determining a first orientation such that a function depending on the orientation as an argument has a minimum at the first orientation, wherein the function depends on a sum of amounts of the intensity gradients at the locations at which the determined orientation of the intensity gradient is within an orientation interval about the orientation argument.

### Claim 4 (depends on 2)

The method of claim 2, wherein the function depends on an average of amounts of the intensity gradients at the locations at which the determined orientation of the intensity gradient is within the orientation interval about the orientation argument.

### Claim 5 (depends on 2)

The method of claim 2, wherein the sharpness measure depends on a value of the function at the minimum.

### Claim 6 (depends on 5)

The method of claim 5, wherein the sharpness measure is proportional to the value of the function at the minimum.

### Claim 7 (depends on 5)

The method of claim 5, wherein the determining of the sharpness measure comprises determining a second orientation such that the function depending on the orientation as an argument has a maximum at the second orientation, and wherein the sharpness measure depends on a value of the function at the maximum.

### Claim 8 (depends on 7)

The method of claim 7, wherein the sharpness measure is inversely proportional to the value of the function at the maximum.

### Claim 9 (depends on 7)

The method of claim 7, wherein the stop criterion is fulfilled when a difference between the value of the function at the maximum and the value of the function at the minimum is below a threshold.

### Claim 10 (depends on 1)

The method of claim 1, wherein the processing of the recorded image comprises generating a Fourier transform.

### Claim 11 (depends on 1)

The method of claim 1, wherein the processing of the recorded image comprises a smoothing step.

### Claim 12 (depends on 1)

The method of claim 1, wherein the changing the settings based on the sharpness measure is performed using an optimization method for non-linear functions having at least one parameter, wherein at least one value of the at least one setting of the focus, the x-stigmator and the y-stigmator is used as the at least one parameter of the non-linear function.

### Claim 13 (depends on 12)

The method of claim 12, wherein the optimization method is a downhill simplex method.

### Claim 14 (depends on 1)

The method of claim 1, wherein the stop criterion is fulfilled when an absolute value of a difference between the sharpness measure of a current repetition and the sharpness measure of a previous repetition is below a threshold.

### Claim 15 (depends on 1)

The method of claim 1, further comprising, when the stop criterion is fulfilled, recording at least one image of the object using the settings.

### Claim 16

A charged particle microscope, comprising:
a charged particle source generating a charged particle beam;
at least one focusing lens traversed by the charged particle beam, the at least one focusing lens including an objective lens focusing the charged particle beam at a working distance from the objective lens;
an x-stigmator traversed by the particle beam;
an y-stigmator traversed by the particle beam;
an object mount configured to hold the object at a distance from the objective lens; and
a controller configured to provide settings of a focus, an x-stigmator and an y-stigmator of the charged particle microscope, and
to repeatedly perform:
adjusting the charged particle microscope to the settings,
recording an image of an object using the settings,
determining a sharpness measure of the image as a whole from the recorded image, the sharpness measure indicating an overall sharpness of the recorded image, and
changing at least one of the settings based on the sharpness measure, until a stop criterion is fulfilled;
wherein the determining of the sharpness measure comprises:
determining an orientation of an intensity gradient at each of a plurality of locations within one of the recorded image and a processed image generated by processing the recorded image, and
determining the sharpness measure based on the plurality of determined orientations; and
wherein the adjusting of the charged particle microscope to the settings includes at least one of
adjusting at least one of an excitation of the at least one focusing lens and a distance of the object mount from the objective lens according to the setting of the focus,
adjusting an excitation of the x-stigmator according to the setting of the x-stigmator, and
adjusting an excitation of the y-stigmator according to the setting of the y-stigmator.

### Claim 17 (depends on 16)

The charged particle microscope of claim 16, wherein the determining of the sharpness measure comprises determining a first orientation such that a function depending on the orientation as an argument has a minimum at the first orientation, wherein the function depends on a sum of amounts of the intensity gradients at the locations at which the determined orientation of the intensity gradient is within an orientation interval about the orientation argument.

### Claim 18 (depends on 17)

The charged particle microscope of claim 17, wherein the function depends on an average of amounts of the intensity gradients at the locations at which the determined orientation of the intensity gradient is within the orientation interval about the orientation argument.

### Claim 19 (depends on 17)

The charged particle microscope of claim 17, wherein the sharpness measure depends on a value of the function at the minimum.

### Claim 20 (depends on 19)

The charged particle microscope of claim 19, wherein the sharpness measure is proportional to the value of the function at the minimum.

### Claim 21 (depends on 19)

The charged particle microscope of claim 19, wherein the determining of the sharpness measure comprises determining a second orientation such that the function depending on the orientation as an argument has a maximum at the second orientation, and wherein the sharpness measure depends on a value of the function at the maximum.

### Claim 22 (depends on 21)

The charged particle microscope of claim 21, wherein the sharpness measure is inversely proportional to the value of the function at the maximum.

### Claim 23 (depends on 21)

The charged particle microscope of claim 21, wherein the stop criterion is fulfilled when a difference between the value of the function at the maximum and the value of the function at the minimum is below a threshold.

### Claim 24 (depends on 16)

The charged particle microscope of claim 16, wherein the processing of the recorded image comprises generating a Fourier transform.

### Claim 25 (depends on 16)

The charged particle microscope of claim 16, wherein the processing of the recorded image comprises a smoothing step.

### Claim 26 (depends on 16)

The charged particle microscope of claim 16, wherein the changing the settings based on the sharpness measure is performed using an optimization method for non-linear functions having at least one parameter, wherein at least one value of the at least one setting of the focus, the x-stigmator and the y-stigmator is used as the at least one parameter of the non-linear function.

### Claim 27 (depends on 26)

The charged particle microscope of claim 26, wherein the optimization method is a downhill simplex method.

### Claim 28 (depends on 16)

The charged particle microscope of claim 16, wherein the stop criterion is fulfilled when an absolute value of a difference between the sharpness measure of a current repetition and the sharpness measure of a previous repetition is below a threshold.

### Claim 29 (depends on 16)

The charged particle microscope of claim 16, wherein when the stop criterion is fulfilled, at least one image of the object is recorded using the settings.