Automatic focusing method of scanning electron microscope

Abstract

The present invention relates to an autofocusing technology for a scanning electron microscope using interlaced scanning. The autofocusing method for a scanning electron microscope is as follows: while the scanning position of an electron beam is shifted by a predetermined number of pixels in a direction perpendicular to the scanning direction, the sample is repeatedly scanned with an electron beam, thereby generating an interlaced image of a pattern (160) formed on the surface of the sample, while the focus position and irradiation position of the electron beam are changed, the process of generating the interlaced image of the pattern (160) is repeatedly performed, thereby generating multiple interlaced images of the pattern (160), calculating multiple sharpness of each of the multiple interlaced images, and determining the best focus position according to the multiple sharpness.

Description

Technical Field

[0001] The invention relates to an automatic focusing technology of a scanning electron microscope, in particular to an automatic focusing technology utilizing interlaced scanning. Background Art

[0002] Scanning electron microscopes are used for wafer inspections such as dimensional measurement of wiring patterns formed on the wafer surface and defect detection of the wiring patterns. Scanning electron microscopes generally have an autofocus function that automatically focuses an electron beam on the wafer surface. Fig.11 is a schematic diagram for explaining the conventional autofocus technology. Fig.11 As shown in FIG. 1 , the scanning electron microscope generates a plurality of images while shifting the focus position little by little, and calculates the clarity of each image. More specifically, the scanning electron microscope performs a differential process on each image and calculates the clarity of the pattern edge on the image. Then, the scanning electron microscope determines the focus position corresponding to the image with the highest clarity.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Laid-Open No. 2007-109408 Summary of the invention

[0006] Problem that the invention aims to solve

[0007] However, if Fig.11 As shown in the figure, the conventional autofocus technology takes a considerable amount of time to generate multiple images. As a result, the throughput of wafer inspection is reduced. In addition, since the electron beam is repeatedly irradiated on the wafer to generate multiple images, the film (such as the protective film) formed on the wafer surface is charged. As a result, the shape of the pattern appearing in the image is deformed and the brightness becomes uneven.

[0008] Therefore, an object of the present invention is to provide a method that can complete focus adjustment in a short time and prevent a sample from being charged.

[0009] Technical means of solving problems

[0010] In one embodiment, an autofocus method for a scanning electron microscope is provided, comprising: repeatedly scanning the sample with the electron beam while shifting the scanning position of the electron beam by a predetermined number of pixels in a direction perpendicular to the scanning direction, thereby generating an interval-removed image of a pattern formed on the surface of the sample; performing the step of generating the interval-removed image of the pattern multiple times while changing the focal position and irradiation position of the electron beam, thereby generating multiple interval-removed images of the pattern; calculating multiple sharpnesses of each of the multiple interval-removed images, and determining the optimal focus position based on the multiple sharpnesses.

[0011] In one approach, the focal position of the electron beam is changed by changing a voltage applied to a deflector of the scanning electron microscope.

[0012] In one embodiment, the scanning range of the electron beam when generating each thinned image covers the entire field of view of the scanning electron microscope.

[0013] Effects of the Invention

[0014] Since each thinned-out image is composed of fewer pixels than a normal image, a scanning electron microscope can generate multiple thinned-out images in a shorter time. Therefore, focus adjustment can be completed in a shorter time than with previous autofocus technologies. Furthermore, since the multiple thinned-out images are images of parts with different patterns, the same part of the sample is not repeatedly irradiated with the electron beam when generating multiple thinned-out images. In other words, the scan lines of the electron beam do not overlap. Therefore, it is possible to prevent the sample from being charged and generate an accurate image of the sample. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a schematic diagram showing one embodiment of an image generation system including a scanning electron microscope.

[0016] Figure 2 It is a diagram for explaining the process of generating the first thinned-out image of the pattern.

[0017] Figure 3 Yes means through Figure 2 Figure 2. Thinned-out image of a pattern generated by the process shown.

[0018] Figure 4 It is a diagram for explaining the process of generating the second thinned-out image of the pattern.

[0019] Figure 5 Yes means through Figure 4 Figure 2. Thinned-out image of a pattern generated by the process shown.

[0020] Figure 6It is a diagram for explaining the process of generating the third thinned-out image of the pattern.

[0021] Figure 7 Yes means through Figure 6 Figure 2. Thinned-out image of a pattern generated by the process shown.

[0022] Figure 8 A diagram showing a plurality of thinned images.

[0023] Fig. 9 A diagram illustrating a process of determining an optimal focus position based on a plurality of sharpnesses.

[0024] Fig.10 Yes means Figure 1 A schematic diagram of one embodiment of a computer is shown.

[0025] Fig.11 This is a schematic diagram for explaining the conventional autofocus technology. DETAILED DESCRIPTION

[0026] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0027] Figure 1 is a schematic diagram showing one embodiment of an image generation system having a scanning electron microscope. Figure 1 As shown, the image generation system includes: a scanning electron microscope 100 that generates an image by scanning a sample in the X direction and the Y direction perpendicular to each other using an electron beam; and a computer 150 that controls the operation of the scanning electron microscope. The scanning electron microscope 100 includes: an electron gun 111 that emits an electron beam composed of primary electrons (charged particles), a focusing lens 112 that focuses the electron beam emitted from the electron gun 111, an X deflector 113 that deflects the electron beam in the X direction, a Y deflector 114 that deflects the electron beam in the Y direction, and an objective lens 115 that focuses the electron beam on a wafer 124 as an example of a sample.

[0028] The X deflector 113 and the Y deflector 114 are disposed below the objective lens 115. In one embodiment, the X deflector 113 and the Y deflector 114 may also be disposed above the objective lens 115. The X direction in which the X deflector 113 deflects the electron beam and the Y direction in which the Y deflector 114 deflects the electron beam are perpendicular to each other.

[0029] The focusing lens 112 and the objective lens 115 are connected to a lens control device 116, and the operations of the focusing lens 112 and the objective lens 115 are controlled by the lens control device 116. The lens control device 116 is connected to a computer 150. The X deflector 113 and the Y deflector 114 are connected to a deflection control device 117, and the deflection operations of the X deflector 113 and the Y deflector 114 are controlled by the deflection control device 117. The deflection control device 117 is also connected to the computer 150. The secondary electron detector 130 and the reflected electron detector 131 are connected to an image acquisition device 118. The image acquisition device 118 is configured to convert the output signals of the secondary electron detector 130 and the reflected electron detector 131 into an image. The image acquisition device 118 is also connected to the computer 150.

[0030] The XY stage 121 disposed in the sample chamber 120 is connected to a stage control device 122, and the position of the XY stage 121 is controlled by the stage control device 122. The stage control device 122 is connected to a computer 150. The sample transport device 140 for placing the wafer 124 on the XY stage 121 in the sample chamber 120 is also connected to the computer 150.

[0031] The electron beam emitted from the electron gun 111 is focused by the focusing lens 112, and then focused by the objective lens 115, and deflected by the X deflector 113 and the Y deflector 114 to irradiate the surface of the wafer 124. When the primary electrons of the electron beam irradiate the wafer 124, secondary electrons and reflected electrons are emitted from the wafer 124. The secondary electrons are detected by the secondary electron detector 130, and the reflected electrons are detected by the reflected electron detector 131. The detection signals of the secondary electrons and the detection signals of the reflected electrons are input to the image acquisition device 118 and converted into images. The images are sent to the computer 150.

[0032] The image generation system configured as described above has an autofocus function that causes the electron beam to automatically focus on the pattern formed on the surface of the wafer 124. The autofocus function of the present embodiment utilizes interlaced scanning. That is, the image generation system generates a plurality of thinned images of different parts of the pattern formed on the wafer 124, and determines the optimal focus position based on the clarity of the plurality of thinned images obtained. The autofocus function of the present embodiment is described below.

[0033] The electron beam emitted from the electron gun 111 is focused by the focusing lens 112, and then focused by the objective lens 115 and deflected by the X deflector 113 and the Y deflector 114 to irradiate the surface of the wafer 124. Figure 2As shown, the X deflector 113 and the Y deflector 114 shift the scanning position of the electron beam by a predetermined number of pixels in the Y direction perpendicular to the X direction as the scanning direction, and at the same time repeatedly scan the pattern 160 on the wafer 124 in the X direction using the electron beam, thereby generating a pattern 160 as shown in FIG. Figure 3 A thinned-out image of pattern 160 is shown. Figure 2 In the example shown, the predetermined plurality of pixels is n pixels (n is a natural number). Since the thinned image is obtained by scanning the wafer 124 at predetermined intervals within the field of view 165 of the scanning electron microscope 100, only a portion of the pattern 160 appears in the thinned image.

[0034] The scanning range of the electron beam when generating the thinned image of the pattern 160 covers the entire field of view 165 of the scanning electron microscope 100. That is, in order to generate one thinned image, Figure 2 As shown, the electron beam scans from one end to the other end of the pre-set field of view 165. Therefore, the scanning lines of the electron beam when generating one interpolated image are evenly distributed in the entire field of view 165. Therefore, since the scanning range of the electron beam covers the entire field of view 165, even if the pattern 160 is not at the center of the field of view 165, an interpolated image of the pattern 160 can be obtained. In the case where the pattern 160 is too small compared to the size of the field of view 165, the interpolated image of the pattern 160 can be reliably generated by reducing the number of the above-mentioned plurality of pixels (i.e., n) representing the moving distance (scanning interval) of the scanning position in the Y direction.

[0035] The scanning electron microscope 100 further generates a plurality of thinned images of the pattern 160 by repeatedly performing the process of generating thinned images of the pattern 160 while changing the focal position and irradiation position of the electron beam. More specifically, the scanning electron microscope 100 generates a plurality of thinned images of the pattern 160 by repeatedly performing the process of generating thinned images of the pattern 160 while shifting the initial scanning position of the electron beam by 1 pixel in the Y direction and changing the focal position of the electron beam.

[0036] Figure 4 1 is a diagram for explaining the process of generating the second thinned-out image of the pattern 160. Figure 4 As shown in Figure 2, the initial scanning position of the electron beam moves 1 pixel in the Y direction. Figure 2 Similarly to the process shown in FIG. 1 , the pattern 160 on the wafer 124 is generated by repeatedly scanning the pattern 160 on the wafer 124 with the electron beam in the X direction while shifting the scanning position of the electron beam by n pixels in the Y direction. Figure 5 The thinned-out image of pattern 160 is shown. Figure 2 and Figure 4As can be seen from the comparison, in the process of generating the first thinned-out image and the process of generating the second thinned-out image, the electron beam scans different parts of the pattern 160. Therefore, Figure 5 The location of the pattern 160 that appears in the thinned-out image shown is different from Figure 3 The location of the pattern 160 appearing in the illustrated thinned-out images is different.

[0037] Figure 6 1 is a diagram for explaining the process of generating the third thinning image of the pattern 160. Figure 6 As shown in FIG. 1 , the initial scanning position of the electron beam is further moved by 1 pixel in the Y direction. Figure 2 and Figure 4 Similarly to the process shown in FIG. 1 , the pattern 160 on the wafer 124 is generated by repeatedly scanning the pattern 160 on the wafer 124 with the electron beam in the X direction while shifting the scanning position of the electron beam by n pixels in the Y direction. Figure 7 A thinned-out image of pattern 160 is shown. Figure 7 The location of the pattern 160 that appears in the thinned-out image shown is different from Figure 3 and Figure 5 The location of the pattern 160 that appears in the illustrated thinned-out image is different.

[0038] By repeating the same process while changing the focal position of the electron beam, the following Figure 8 When these thinned-out images are combined, a single image in which the pattern 160 appears repeatedly can be formed. In this way, a plurality of thinned-out images of different parts of the pattern 160 are generated and a technology of combining these thinned-out images to obtain a single image is called interlaced scanning.

[0039] In the present embodiment, when the thinning image is repeatedly generated, the focal position of the electron beam is changed by changing the voltage applied to the X deflector 113 and the Y deflector 114. More specifically, each time the thinning image is repeatedly generated, the computer 150 issues a command to the deflection control device 117 to change the voltage applied to the X deflector 113 and the Y deflector 114.

[0040] The X deflector 113 and the Y deflector 114 are generally used for the purpose of deflecting the electron beam, but the X deflector 113 and the Y deflector 114 also have the function of changing the focal position of the electron beam. That is, in the present embodiment, when repeatedly generating the thinned-out image, the X deflector 113 and the Y deflector 114 change the focal position of the electron beam while deflecting the electron beam. The reason why the X deflector 113 and the Y deflector 114 are used to change the focal position of the electron beam instead of the objective lens 115 is that the X deflector 113 and the Y deflector 114 can quickly respond to the change of the applied voltage, and can change the focal position of the electron beam more quickly than the objective lens 115.

[0041] Each time the thinned-out image of pattern 160 is repeatedly generated, the focal position of the electron beam changes, so Figure 8 As shown, the clarity of each thinned image is different. The clarity is an indicator indicating the clarity of the edge of the pattern 160 appearing in the image. That is, the higher the clarity, the closer the focus position of the electron beam is to the pattern surface. The computer 150 is configured to calculate a plurality of clarity of each of the plurality of thinned images, and determine the optimal focus position based on the plurality of clarity.

[0042] Fig. 9 is a diagram for explaining an embodiment of a process for determining an optimal focus position based on a plurality of sharpnesses. Fig. 9 The coordinate system shown. The vertical axis of the coordinate system represents the clarity, and the horizontal axis represents the focus position. The computer 150 calculates the clarity of the edge of the pattern 160 in each thinned-out image by calculating the brightness difference between the pixels in each thinned-out image and adding the obtained differences. The computer 150 plots a plurality of points determined by the plurality of sharpnesses of the plurality of thinned-out images and the corresponding plurality of focus positions on the coordinate system, creates an approximate curve 170 of the plurality of points, and determines the focus position corresponding to the peak point of the approximate curve 170. The determined focus position is the optimal focus position. The computer 150 issues an instruction to the lens control device 116, causing the lens control device 116 to operate the objective lens 115 to achieve the focus position determined above.

[0043] Each culled image is composed of Fig.11 The ordinary image shown is composed of fewer pixels, so the scanning electron microscope 100 can generate multiple spaced-out images in a shorter time. Therefore, the focus adjustment can be completed in a shorter time than the previous autofocus technology. Further, since the multiple spaced-out images are images of different parts of the pattern 160 on the chip 124, the same position of the chip 124 will not be repeatedly irradiated by the electron beam when generating multiple spaced-out images. In other words, the scanning lines of the electron beam do not overlap. Therefore, the chip 124 can be prevented from being charged, and an accurate image of the sample can be generated.

[0044] Fig.10 1 is a schematic diagram showing the structure of a computer 150. The computer 150 has a storage device 1162 for storing programs and data, a processing device 1120 such as a CPU (central processing unit) for performing calculations according to the programs stored in the storage device 1162, an input device 1163 for inputting data, programs, and various information into the storage device 1162, an output device 1140 for outputting processing results and processed data, and a communication device 1150 for connecting to a network such as the Internet.

[0045] The storage device 1162 includes a main storage device 1111 that can access the processing device 1120 and an auxiliary storage device 1112 that stores data and programs. The main storage device 1111 is, for example, a random access memory (RAM), and the auxiliary storage device 1112 is a storage device such as a hard disk drive (HDD) or a solid state drive (SSD).

[0046] The input device 1163 includes: a keyboard, a mouse, and further includes: a recording medium reading device 1132 for reading data from a recording medium and a recording medium port 1134 for connecting the recording medium. The recording medium is a computer-readable recording medium that is a non-temporary tangible object, such as an optical disk (such as a CD-ROM, a DVD-ROM) or a semiconductor memory (such as a USB flash drive, a memory card). Examples of the recording medium reading device 1132 include optical drives such as CD drives and DVD drives, and card readers. Examples of the recording medium port 1134 include a USB terminal. The program and / or data electrically stored in the recording medium are imported into the computer 150 through the input device 1163 and stored in the auxiliary storage device 1112 of the storage device 1162. The output device 1140 includes: a display device 1164 and a printing device 1142.

[0047] The above-mentioned embodiments are recorded for the purpose of enabling persons with general knowledge in the technical field to which the present invention belongs to implement the present invention. Various modifications of the above-mentioned embodiments are self-evident to those skilled in the art, and the technical concept of the present invention may also be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is interpreted as the broadest scope of the technical concept defined by the scope of the claims.

[0048] Industrial availability

[0049] The present invention can be used for the automatic focusing technology of a scanning electron microscope using interlaced scanning.

[0050] Explanation of symbols

[0051] 100 Scanning Electron Microscope

[0052] 111 Electron Gun

[0053] 112 Focusing lens

[0054] 113 X Deflector

[0055] 114 Y deflector

[0056] 115 Objective

[0057] 116 Lens control device

[0058] 117 Deflection Control Device

[0059] 118 Image acquisition device

[0060] 120 Sample Room

[0061] 121 XY stage

[0062] 122 Stage control device

[0063] 124 chips

[0064] 130 Secondary Electron Detector

[0065] 131 Reflected electron detector

[0066] 140 Sample transport device

[0067] 150 Computer

[0068] 160 Patterns

[0069] 165 Field of View

[0070] 170 Approximate curve.

Claims

1. An automatic focusing method for a scanning electron microscope, characterized in that: The sample is repeatedly scanned with the electron beam while shifting the scanning position of the electron beam by a predetermined number of pixels in a direction perpendicular to the scanning direction, thereby generating a thinned image of a pattern formed on the surface of the sample, The step of generating a thinned image of the pattern is performed a plurality of times while changing the focal position and the irradiation position of the electron beam, thereby generating a plurality of thinned images that are images of different parts of the pattern, calculating a plurality of resolutions of each of the plurality of thinned images having different resolutions, determining an optimal focus position according to the plurality of sharpnesses, Changing the irradiation position of the electron beam means that the initial scanning position of the electron beam each time the process of generating the interval-removed image of the pattern is performed is moved to the same side in a direction perpendicular to the scanning direction compared with the initial scanning position of the electron beam the last time the process of generating the interval-removed image of the pattern was performed.

2. The automatic focusing method according to claim 1, characterized in that: The focal position of the electron beam is changed by changing the voltage applied to the deflector of the scanning electron microscope.

3. The automatic focusing method according to claim 1, characterized in that: The scanning range of the electron beam when generating each thinned-out image covers the entire field of view of the scanning electron microscope.

Images