Charged particle beam device

Abstract

The purpose of the present invention is to provide a charged particle beam device capable of obtaining an observation image in which the influence of periodically generated noise is suppressed. A charged particle beam device according to the present invention generates images of a sample on the basis of detection signals of secondary charged particles, and generates an integrated image by integrating two or more of the images. Further, the duration for which the charged particle beam is scanned from a start position to an end position in order to acquire one image is made different for at least any two of the images that are integrated, whereby the noise included in the images that are integrated is reduced in the integrated image (see fig. 10).

Description

Charged particle beam device

[0006]

[0001] The present invention relates to a charged particle beam device. Background art

[0002] A charged particle beam device (e.g., scanning transmission electron microscope: STEM) can generate an observation image (SEM image) of a sample by detecting secondary charged particles generated by irradiating the sample with a charged particle beam. In order to reduce the noise in the observation image, one observation image may be generated by integrating a plurality of observation images. For example, as a method for reducing image noise due to sudden disturbances, a method of image integration in which scanning images acquired at a short frame rate of about 1 second are superimposed is used.

[0003] Patent Document 1 discloses that when integrating SEM images, a normalized integration operation is performed to output an integration image in which the luminance value of the integration image is always in the state of "1" during the integration process, thereby displaying a frame integration image without a sense of incongruity (no dark display) during the integration process.

[0004] Patent Document 2 describes image integration (a method for removing sudden noise). When the obtained scan image contains noise components, a clearer image with less noise can be obtained by integrating scan images of the same structure. In image integration, the first image is acquired and stored in the image memory. Next, the second image of the same structure is acquired and stored in the image memory. Although the first image and the second image are images acquired continuously, even when the same structure (field of view) is photographed, there may be a shift in the X direction and the Y direction due to the influence of disturbances to the device. In that case, the shift is corrected by image processing technology to generate an integrated image. Sudden noise may be mixed into the photographed image. In such a case, if the photographed image is continuously integrated with the integrated image, the noise components are averaged, and a clear integrated image can be photographed.

[0005] WO2017 / 090204 JP-A-2012-049049

[0006] Conventional image integration methods, such as those described in Patent Documents 1 and 2, attempt to reduce noise by integrating multiple observation images. However, there is a problem in that, for disturbances that fluctuate sinusoidally over time, image noise is not reduced or is difficult to reduce even when using image integration methods. This is because, when noise appears periodically, each image being integrated will contain similar noise, and therefore the noise remains even after image integration.

[0007] This invention has been made in view of the above-mentioned problems, and aims to provide a charged particle beam apparatus that can obtain observation images with the effects of periodically occurring noise suppressed.

[0008] The charged particle beam apparatus according to the present invention generates an image of a sample based on the detection signal of secondary charged particles and generates an integrated image by integrating two or more of the above images. Furthermore, for at least two of the above integrated images, the time lengths for scanning the charged particle beam from the start position to the end position to acquire one image are made different from each other, thereby reducing the noise contained in the above integrated images on the integrated image.

[0009] The charged particle beam apparatus according to the present invention makes it possible to obtain observation images that suppress the effects of periodically occurring noise. Other problems, configurations, and advantages of the present invention will become clear from the following description of embodiments.

[0010] This is a diagram illustrating the configuration of a charged particle beam apparatus according to Embodiment 1. This diagram illustrates the procedure for acquiring STEM images. This is a schematic diagram illustrating the general procedure for image integration. This diagram schematically shows the state when a periodic structure is modulated by noise. This shows the mathematical expression of conventional image integration. This diagram shows conventional image integration in spectral form. This is an example of a user interface for setting parameters when the charged particle beam apparatus according to the present invention acquires an integrated image. This is a schematic diagram showing the procedure for acquiring an integrated image by the charged particle beam apparatus according to the present invention. This is the mathematical expression of image integration when the frame time is changed. This diagram shows image integration in spectral form when the frame time is changed. This is a schematic diagram showing the process by which the charged particle beam apparatus according to Embodiment 2 performs image integration.

[0011] <Embodiment 1> <Overview of Charged Particle Beam Apparatus> Figure 1 is a diagram of the configuration of a charged particle beam apparatus according to Embodiment 1 of the present invention. Here, a scanning transmission electron microscope (STEM) is described as an example of a charged particle beam apparatus, but the present invention can also be applied to other charged particle beam apparatuses such as scanning electron microscopes (SEM).

[0012] The electron beam 002 generated by the electron gun 050 is focused by the electron lens 054 and irradiated onto the sample 011 on the sample holder 010 mounted on the sample stage 051. Electrons that pass through the sample 011 are detected by the detector 055. The main control device 057 acquires the detection signal output by the detector 055 and creates an image. The controller 053 controls the sample stage 051.

[0013] A scan image can be obtained by applying a current having a scan waveform to a scan coil 060 installed above the sample 011, and synchronizing the scan waveform with the transmitted electrons detected by the detector 055 in the main control device 057.

[0014] <STEM Image Acquisition Method> Figure 2 is a diagram illustrating the procedure for acquiring a scanning transmission image (STEM image). When acquiring a STEM image, the electron beam 002 is focused to a size smaller than the object to be observed and irradiated onto the sample 011. The electron beam 103 that has passed through the sample 011 is detected by a detector 055 installed below or above the sample 011.

[0015] When the horizontal direction of the sample is defined as X and the vertical direction as Y, the electron beam is scanned by applying a current amplified from the X-direction scan waveform 101 to a coil that deflects the electron beam in the X direction. After scanning one line in the X direction, the line is shifted one line in the Y direction and scanned again in the X direction. The Y-direction scan waveform 102 also has a sawtooth waveform, similar to the X-direction scan waveform 101.

[0016] The main control unit 057 digitizes the brightness signal 104 based on the electron beam 103 detected by the detector 055 and stores it in the image memory 120. At this time, the main control unit 057 generates a two-dimensional coordinate address (pixel unit) for storing image data in the image memory 120 based on the scan signal. The brightness signal 104 is stored in the image memory 120 according to this address. The main control unit 057 outputs the data generated in the image memory 120 as a scanned image 111 on the display 110.

[0017] Figure 3 is a schematic diagram illustrating the general procedure for image integration. When integrating images 201 and 205, the positions of each image must be aligned. Therefore, by correcting the X-direction and Y-direction displacement of image 201, image 203 is obtained (image 202 shows the displacement in each direction). Image displacement can be caused, for example, by sample drift. Images 203 and 205 are integrated. No noise exists at position 204 on image 203. On the other hand, noise exists at position 204 in image 205 obtained from the same sample. By integrating images 203 and 205, the noise at position 204 is reduced. Furthermore, another image 206 obtained from the same sample has no noise at position 204. By further integrating image 206, the noise at position 204 can be further reduced.

[0018] <Image Integration When Periodic Noise is Introduced> Figure 4 schematically shows what happens when a periodic structure is modulated by noise. Consider the case where periodic noise is introduced into a scan image when observing a periodic structure. When periodic noise 251 is introduced into a periodic arrangement of atoms 250, the periodic arrangement of atoms 250 undergoes amplitude modulation and is shifted from its original position in the scan image, as shown in arrangement 252.

[0019] Figure 5 shows the mathematical representation of conventional image integration. If the periodic structure of atoms is represented by equation 254 and the periodic noise by equation 253, the periodic structure whose amplitude has been modulated by the noise can be represented as shown in equation 255. Similarly, the periodic structure in the second and subsequent images can be represented as shown in equations 256 and 257.

[0020] Figure 6 shows a representation of conventional image integration in spectral form. The first term of Equation 255 corresponds to fc, the second term to fc-fm, and the third term to fc+fm, which are the corresponding spectra. Mathematically, image integration means averaging multiple qs (q1 to qn) shown in Figure 5. In this case, the periodic noise component expressed by Equation 253 does not decrease during averaging.

[0021] <Settings for Creating Integrated Images (User Setting of Dwell Time Change Range)> Figure 7 shows an example of a user interface for setting parameters when the charged particle beam apparatus according to the present invention acquires integrated images. It is possible to set Dwell time t (301), which is the electron beam residence time per image pixel, Dwell time change range wt (302), which is changed for each image acquisition, and the number of images to acquire n (303), which determines how many images to acquire.

[0022] The initial value Ti for one frame time is determined from the following: (a) the number of pixels in the image Np, (b) the Dwell time, and (c) the total waiting time per frame tw (Ti = Np × t + tw), which is determined from the waiting time between the X-direction scan of one line and the X-direction scan of the next line. Therefore, the charged particle beam apparatus can automatically calculate and input this on the screen. The final value Tf for one frame time is automatically input according to Np × (t + wt × n) + tw.

[0023] Depending on the nature of the periodic noise, it may be better to integrate multiple images when generating a single integrated image, or it may not be. Therefore, in Figure 7, the number of acquired images n (303) can be set so that the user can specify the number of integrated images according to the nature of the periodic noise.

[0024] <Image Acquisition Method> Figure 8 is a schematic diagram showing the procedure for acquiring an integrated image using the charged particle beam apparatus according to the present invention. The first image 350 is captured within the frame time Ti set in Figure 7. Next, image 351 is captured at frame time T1. Frame time T1 is the time interval obtained by changing frame time Ti by the Dwell time change wt. Similarly, frame time is sequentially changed by the Dwell time change wt. By integrating image 351 with image 350, an integrated image 352 is obtained. The same image integration is repeated until frame time reaches Tf, and the final integrated image 353 is obtained. The alignment described in Figure 3 is also performed as appropriate for each image.

[0025] <Mathematical representation of integrated images when Dwell time is changed> Figure 9 shows the mathematical representation of integrated images when the frame time is changed. Equations 253 to 255 are the same as in Figure 5. By changing Dwell time, the mathematical representation of the second image changes as shown in equation 256', where fm changes to fm1 (fm ≠ fm1). Similarly, fm changes sequentially thereafter. Through integrated images (averaging of q1 to qn), the first term of the equation does not change, but the second and third terms are averaged out and reduced, thereby reducing noise.

[0026] Figure 10 shows the image integration in spectral form when the frame time is varied. The equation in Figure 9 is expressed spectrally as shown in Figure 10. While fc remains constant through averaging, fc-fm2, fc-fm1, fc+fm1, and fc+fm2 decrease with averaging. In the spectral representation, it can be seen that the noise component decreases as Dwell time is varied for each captured image.

[0027] More specifically, let's assume that the first image is composed of frequency components fc and fc±fm1 (middle of Figure 10), and the second image is composed of frequency components fc and fc±fm2. In the first image, fc±fm1 exists as a frequency component other than fc. Furthermore, when the second image is integrated, fc±fm1 does not exist in the second image, so this component is reduced in the integrated image by averaging. Similarly, fc±fm2 is also reduced by averaging. Through this repetition, frequency components other than fc gradually decrease with integration. In other words, frequency components fm1, fm2, etc., caused by periodic noise are gradually reduced by averaging.

[0028] <Embodiment 2> <Settings for Integrated Image Creation (Pre-setting of Dwell Time Change Range)> Figure 11 is a schematic diagram showing the process by which a charged particle beam apparatus according to Embodiment 2 of the present invention performs image integration. In Embodiment 1, an example was described in which the user arbitrarily sets the change range wt of Dwell Time. On the other hand, the change range wt or the one-frame time of each captured image may be set in advance, and the setting may be read out each time an image is captured.

[0029] The frame time Ti for acquiring the first image 601 is set in advance. Similarly, the frame time for the second image is set in advance. When capturing each image, the corresponding frame time (600) is read out, and the image is captured according to that frame time. Image integration (602) is performed each time an image is acquired to obtain the final integrated image 603. The final integrated image 603 is a clear image with reduced sudden and periodic noise.

[0030] <Regarding Variations of the Invention> The present invention is not limited to the embodiments described above, but includes various variations. For example, the embodiments described above are described in detail to make the present invention easier to understand, and it is not necessary to have all of the described configurations. Also, a part of one embodiment can be replaced with the configuration of another embodiment. Also, a configuration of another embodiment can be added to the configuration of one embodiment. Furthermore, a part of the configuration of each embodiment can be added, deleted, or replaced with a part of the configuration of another embodiment.

[0031] In the embodiments described above, one frame time (duration) refers to the duration of scanning the charged particle beam from the starting position to the ending position in order to acquire one image. For example, in Figure 2, the duration of scanning the electron beam 002 for all x and y coordinates of one image can be considered as one frame time.

[0032] In the above embodiments, the main control device 057 may switch between a variable frame mode, which implements the integration procedure according to the present invention (the procedure described in Figures 8 to 10, etc.) in order to reduce periodic noise, and other modes. That is, if it is known that periodic noise will have an effect, the variable frame mode may be implemented, and if no effect will occur, other modes (for example, a mode that implements conventional image integration) may be implemented.

[0033] 002: Electron beam 050: Electron gun 051: Sample stage 053: Controller 054: Electron lens 055: Detector 057: Main control unit

Claims

1. A charged particle beam apparatus for irradiating a sample with a charged particle beam, comprising: a scanner for scanning the irradiation position of the charged particle beam on the sample; a detector for detecting secondary charged particles that pass through the sample, are reflected from the sample, or are generated from the sample by irradiating the sample with the charged particle beam, and outputting a detection signal representing the result; and a main control device for generating an image of the sample based on the detection signal, wherein the main control device generates an integrated image by integrating two or more of the images, and the main control device reduces noise contained in the integrated images on the integrated image by making the time lengths for scanning the charged particle beam from a start position to an end position to acquire one image different for at least two of the integrated images.

2. The main control device is configured to switch whether or not to implement a variable frame mode such that the time lengths of at least two of the images to be integrated are different from each other, and the main control device reduces the noise on the integrated image caused by the periodic noise by implementing the variable frame mode when periodic noise is mixed into the sample, as described in claim 1.

3. The charged particle beam apparatus according to claim 1, characterized in that the main control device integrates the signal component represented by the first frequency of periodic noise mixed into the sample as the integrated image for the first image among the images to be integrated, and the main control device integrates the signal component represented by the first frequency as the integrated image for the second image among the images to be integrated, which is different from the first image among the images to be integrated, and which is represented by a second frequency different from the first frequency of the periodic noise and does not have the first frequency, thereby reducing the signal component represented by the first frequency in the integrated image.

4. The charged particle beam apparatus according to claim 1, characterized in that the main control device receives a designation input specifying the time length for at least two of the images to be integrated, and the main control device controls the scanner to scan the charged particle beam according to the time length specified by the designation input.

5. The charged particle beam apparatus according to claim 4, wherein the main control device provides a user interface for the user to input the specified input, and the main control device controls the scanner to scan the charged particle beam according to the specified input input on the user interface.

6. The charged particle beam apparatus according to claim 1, wherein the main control device provides a user interface for the user to input the number of images to be integrated in order to generate one integrated image, and the main control device generates the integrated image by integrating the images according to the number of images input on the user interface.

7. The charged particle beam apparatus according to claim 1, characterized in that the main control device generates the integrated image by integrating the positions of the images to be integrated relative to each other.

8. The charged particle beam apparatus according to claim 1, wherein the main control device stores the time length in advance for at least two of the images to be integrated, and the main control device controls the scanner to scan the charged particle beam according to the stored time length.

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