Charged particle microscope and measurement image correction method thereof

A charged particle microscope corrects distortion in an image caused by effects of drift in the sampling stage by measuring the correction reference image in a shorter time than the observation image, making corrections by comparing the shape of the observation image with the shape of the correction reference image, and reducing distortion in the observation images. The reference image for distortion correction is measured at the same position and magnification as when acquiring images for observation. In order to reduce effects from drift, the reference image is at this time measured within a shorter time than the essential observation image. The shape of the observation image is corrected by comparing the shapes of the reference image and observation image, and correcting the shape of the observation image to match the reference image.

TECHNICAL FIELD

The present invention relates to a charged particle microscope and a method for correcting measurement images by utilizing the charged particle microscope.

BACKGROUND ART

The charged particle microscope is widely used for observing the structure of substances at a high magnification. However, drift can sometimes occur due to the characteristics of the specimen and the equipment stage. Charged particle microscopes generally shift the imaging field of view by moving a stage carrying a specimen placed on a sampling stage. However, due to problems with mechanical precision, the stage does not suddenly stop even if stop operation was initiated and still continues to moves even though only a small distance. Drift is caused mainly by the slight movement of the sampling stage after stop operation.

In the charged particle microscope, the capture of the image for observation requires a long time ranging from a few seconds to several dozen seconds and moreover is imaging that is enlarged to a high magnification so that even just a slight amount of drift causes distortion to appear in the image. However, finding what extent of image displacement has occurred due to drift, or finding in what direction the displacement amount occurred, just in the image where distortion occurred was impossible (in the related art) so preventing distortion from entering the image at the time of measurement or some type of method for correcting distortion in the image is needed.

One way to prevent distortion from entering an image during measurement, is to start the observation after waiting for the sampling stage to come to a complete stop after operation to stop the sampling stage movement however the image capture efficiency in that case is extremely poor. In order to resolve the problem, a variety of methods to correct image distortion due to drift were contrived.

The patent literature 1 for example discloses a drift correction method to correct slow-scan images by utilizing results from finding the drift amount (drift speed) per unit of time in the X direction and the Y direction from two fast-scan images (television scanning image) in order to find the displacement amount, due to drift in the image captured by slow-scan.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

The method that starts measurement after waiting for movement of the sampling stage to stop requires waiting for the sampling stage to come to a complete stop whenever the sampling stage moves and so there is a drastic drop in operability during observation. The drift correction method disclosed in patent literature 1 on the other hand, corrects one image from among at least three images, however the number of captured images relates to the direct device throughput and damage to the specimen and so a drift correction method is needed that achieves the drift correction from as few images as possible.

The present invention has the object of providing drift correction from as few captured images as possible.

Solution to Problem

In order to resolve the aforementioned problems, a reference image is measured in order to correct distortion at a position and magnification identical to the image acquired for observation. The reference image is at this time measured within a shorter time than the actual observation image in order to lessen the effects of drift. The reference image performs the measurement in a short time compared to the observation image so the signal volume decreases and the reference image cannot be used for making observations. However the reference image has little distortion compared to the observation image and so correctly reflects the shape of the specimen. The shape of the observation image is corrected by comparing the shapes of the reference image and observation image, and then correcting the shape of the observation image to match the shape of the reference image.

Advantageous Effects of the Invention

The present invention does not require waiting until the sampling stage has fully stopped before starting observation. Moreover, the correction of the related art required the acquisition of three images however the present invention can perform correction with two images.

DESCRIPTION OF EMBODIMENTS

First Embodiment

The embodiments of the present invention are described next while referring to the drawings.

FIG. 1is a drawing showing the structure of the device for achieving the present invention. A charged particle microscope101is a microscope utilizing charged particles and that obtains image at a high magnification by irradiating charged particles onto the specimen. Generally known microscopes that use an electron beam are the scanning electron microscope and transmission electron microscope. Images acquired by the charged particle microscope101are obtained by the image input device102. Images acquired by the image input device102can be stored in the storage device105. The images measured by using the charged particle microscope101and the image input device102are corrected on the arithmetic logic unit (ALU)104based on correction conditions input by the correction condition input device103, and the images are output from the image output device106.

FIG. 2is images showing the content implemented by the present invention. An observation image201is an image measured for observation purposes using the charged particle microscope101. The measurement is made in a period ranging from a few to several dozen seconds in order to obtain high image quality and so contains much distortion. The correction reference image202like the observation image201is an image measured by using the charged particle microscope101. The correction reference image202measures faster than the observation image201in order to alleviate the effects of drift in the sampling stage. The observation image201for example measures at 40 seconds and so when the correction reference image202measures at 40 milliseconds the effect of drift in the sampling stage is 1/1000th. A corrected observation image203is acquired by comparing the shape of the observation image201with the correction reference image202and making corrections. The sampling stage drift movement is for here the case where moving in the lateral direction and where moving in the vertical direction, and moreover in the diagonal direction utilizing the lateral and vertical components.

FIG. 3is illustrations for describing the correction of lateral drift. A profile position301is set along the line for verifying the shape of the image at a desired location laterally on the observation image201. A profile position302is also set on the correction reference image202at the same position as the profile position301set on the observation image. The profile303is an image profile for the profile position301that was set on the observation image201. The profile304is an image profile for the profile position302that was set on the correction reference image202. The amount of movement in the lateral direction can be found by comparing the shape of these two profiles. The drift amount for the entire image can be obtained by sequentially detecting the drift amount on each line while shifting the position where the profile was set from the top edge to the bottom edge of the image.

FIG. 4is a drawing showing the drift amount in the lateral direction along the entire image. The actual measured drift amount402contains an error due to effects of noise and fluctuations in the shape during measurement, etc. However, the drift amount continuously fluctuates when the cause of the drift is established as the drift from the time after the stop of sampling stage stop operation until, the sampling stage stops. Variations due to the effects of noise and so on can be alleviated by calculating the approximate curve from the measured drift amount. The method for calculating the approximate curve is the least squares approximation.

FIG. 5is illustrations showing the method for correcting the drift in the lateral direction. In the observation image201, a lateral drift-corrected image501can be obtained by shifting laterally one line at a time in the lateral direction according to the approximation curve of the drift amount found fromFIG. 4.

FIG. 6is a drawing for describing the method for correcting the drift amount in the vertical direction. The lateral drift amount is featured in being measured as the image displacement however vertical drift appears as extensions and contractions in the image. Methods that simply compare profiles on a line therefore cannot detect the vertical drift amount. The vertical drift amount is therefore calculated by measuring what section in the correction reference image202matches a sectional area within the observation image. In this method, a reference region601is first of all set at an optional position in the lateral drift-corrected image501. A correction position search region602is next set at a position identical to the reference region601that was set in the lateral drift-corrected image501on the correction reference image202. The vertical drift amount can next be calculated by detecting a position having the same shape as the reference region601from the correction position search region602. The correction position search region602is however set to a region wider than the reference region601in order to detect a section having the same shape as the reference region601within the correction search region602. A method to search for an identical shape is widely known as template matching using a normalized correlation. The drift amount for the entire image can be calculated by calculating the drift amount while shifting the reference region601and correction position search region602up and down. In the vertical drift amount, effects from noise and so on may sometimes cause variations in the drift amount the same as with the lateral drift amount so an approximate curve is also found for the vertical drift amount.

FIG. 7shows the method for correcting the vertical drift amount. In the lateral drift-corrected image501, the vertical drift amount can be performed by shifting the image vertically according to the approximate curve for the drift amount found inFIG. 6. However, drift in the vertical direction cannot be corrected just by merely shifting and copying the vertical line. Such correction is not possible because the drift amount in the vertical direction appears on the image as extensions and contractions in the shape. A corrected observation image203can be obtained by calculating which position in the lateral drift-corrected image501matches which of the respective pixels in the corrected observation image203according to the approximate curve for the drift amount found inFIG. 6; and calculating the value of each pixel in the corrected observation image203from performing interpolation calculation on the lateral drift-corrected image501.

The areas (profile position301,302, reference region601, correction position search region602) for detecting the drift amount and method for finding the correction curve from the drift amount must be set in order to calculate the lateral and vertical drift amounts. One way to set these areas and method is to establish fixed areas and a method beforehand. Moreover, a method to find the area for detecting the drift amount and find the correction curve from the detected drift amount can be input by the correction condition input device103.

FIG. 8shows the processing flow of the present invention. An acquire reference image801obtains the correction reference image202. An acquire observation image802obtains the observation image201. The lateral drift amount is then calculated by the calculate lateral drift amount803using the correction reference image202obtained by the acquire reference image801and the observation image201obtained by the acquire observation image802. A find approximate lateral drift amount804sets an approximate lateral drift amount calculated by calculate lateral drift amount803and calculates an approximate curve for the lateral drift amount. In the correct the lateral drift correction805, the lateral drift amount is corrected by using the approximate curve for the lateral drift amount and the observation image201, and acquires the lateral drift-corrected image501.

A calculate the vertical drift amount806calculates the vertical drift amount by using the lateral drift-corrected image501corrected in the lateral drift correction805and the correction reference image202obtained in acquire reference image801. The find approximate vertical drift amount807approximates the vertical drift amount calculated by vertical drift amount806, and calculates the approximate curve of the vertical drift amount. The correct the vertical drift808corrects the vertical drift amount by utilizing the lateral drift-corrected image501and the approximate curve for the vertical drift amount, and obtains the corrected observation image203.

Second Embodiment

The second embodiment of the present invention is described next usingFIG. 9. In addition to the methods described so far, user support is further provided by way of the operating screen. A warning screen901contains a warning character string904, a first button902, and a second button903. If drift was detected in the image then the warning screen901is displayed to the user to show that the captured image contains drift. The user at this same time selects on the warning screen901whether or not to execute drift correction of the captured image. To make the selection, the user clicks a first button902or a second button903to permit or abort drift correction.

LIST OF REFERENCE SIGNS