JP5592085B2 - Autofocus image determination method in charged particle beam apparatus, autofocus image determination apparatus in charged particle beam apparatus, charged particle beam apparatus, computer program, and recording medium - Google Patents

Description

The present invention relates to a method for determining an autofocus image in a charged particle beam apparatus and a charged particle beam apparatus for determining an image captured using an autofocus apparatus in a charged particle beam apparatus such as a scanning electron microscope, a drawing apparatus, and a semiconductor inspection apparatus. The present invention relates to an autofocus image determination device, a charged particle beam device, a computer program, and a recording medium.

  In recent years, a charged particle beam apparatus such as a scanning electron microscope is required to have a high-accuracy and wide-area observation field of an electron beam applied to a sample, and an autofocus apparatus is also required to have high focus accuracy at high speed. Further, it is necessary to prevent damage to the sample at the time of observation as much as possible, and a reduction in damage is required.

  Examples of the charged particle beam apparatus provided with such an autofocus apparatus include the following. In Patent Document 1, scanning is performed by deflecting an electron beam by a deflecting means, detecting charged particles generated on a sample having an arbitrary shape, and obtaining a sample image. This scanning is performed by changing the excitation intensity of the objective lens. The step of obtaining the sharpness of the edge component in a predetermined direction of the sample shape having an arbitrary shape from the sample image obtained while performing the change, and the change of the excitation intensity of the objective lens and the sharpness of each obtained predetermined direction are obtained. Based on the step of calculating the peak position corresponding to the excitation intensity of the objective lens in each predetermined direction, it is determined whether or not the corresponding astigmatism correction coil is to be corrected based on the distance between the peak positions in the predetermined direction. A method for automatically adjusting an electron beam apparatus comprising steps is described.

  Further, in Patent Document 2, in a semiconductor pattern shape evaluation apparatus using a length measurement SEM, in order to collectively manage various data stored in a database, a coordinate system between various data is associated with each other. An imaging recipe for observing a semiconductor pattern in a length measurement SEM is generated by selecting a part or all of the data, and using the selected data, and managing the various data in time series, A semiconductor pattern shape evaluation apparatus for correcting an imaging recipe when there is a change is described.

  Furthermore, Patent Document 3 discloses a step of acquiring a line profile from a sample when the current or voltage of the objective lens is sequentially changed by a predetermined width, and differentiating each acquired line profile to obtain the sum of absolute values thereof. An autofocus method is described which includes a step of calculating and a step of adjusting the current or voltage of the objective lens when the calculated sum is maximum or adjacent sums are substantially equal.

See Japanese Patent Application Laid-Open No. 2007-109408 See JP 2007-147366 A, paragraph 0037. See Japanese Patent Application Laid-Open No. 2007-178864, paragraph numbers 0018 to 0049.

  Here, in the above-described conventional apparatus, when an image is picked up by the charged particle beam apparatus, the image is picked up as it is after focusing by autofocus setting by various methods. For this reason, even when the focus setting by the autofocus setting is properly performed, only a defocus image that is not set to an appropriate focus may be obtained due to a subsequent change in the state of the apparatus or the sample.

  In general, it is known that the focus position shifts when the sample is charged up, and defocusing occurs in this way when the sample is charged up for some reason after the focus position is determined by autofocus.

  When such defocusing occurs, when the operator manually performs imaging, it can be dealt with by performing imaging again when the operator sequentially observes the captured images and finds defocusing. However, in an apparatus that picks up a large number of samples by automatic operation, it is very complicated to inspect the picked-up image after the imaging of the large number of samples is completed and to find out the defocus, and to pick up the image again.

  Accordingly, the present invention provides an autofocus image determination method for a charged particle beam device, an autofocus image determination device for a charged particle beam device, and a charge device that can automatically determine whether or not an autofocus image is satisfactory. An object is to provide a particle beam device, a computer program, and a recording medium.

The invention of claim 1 comprises a converging means for converging the charged particle beam at a focal position and a scanning means for two-dimensionally scanning the charged particle beam, and 2 from the sample irradiated with the charged particle beam. in autofocus image determination method in the charged particle beam device for acquiring an image of the sample based on the following the charged particle signal, the scanning of a specific region selected by driving the converging means and said scanning means from within an imaging region of the sample The line is sequentially scanned with a charged particle beam while changing the focal position, and preliminary imaging is performed to acquire a preliminary image signal of the specific region from the obtained secondary charged particle signal, and the maximum sharpness in the preliminary image signal is obtained. comprising determining the scanning lines, a determination of the image obtained by the main imaging of an imaging area of the sample a state of the converging means in the scan line as the in-focus state in the row Umono The acquired image signal profile scan line reaches the maximum sharpness position in the preliminary imaging to acquire the image signal profile of scan lines corresponding to the scan line reaches the maximum sharpness in the preliminary imaging in the present imaging The similarity between the two image signal profiles is obtained from the two image signal profiles, the similarity is compared with a first threshold value, and the amount of deviation from the center position of the image at which the similarity is minimum is calculated as a second value. This is an autofocus image determination method in a charged particle beam apparatus, characterized in that an image obtained by performing main imaging is determined in comparison with the threshold value.

The invention of claim 2 comprises a converging means for converging the charged particle beam at a focal position and a scanning means for two-dimensionally scanning the charged particle beam, and the secondary from the sample irradiated with the charged particle beam. In an autofocus image determination apparatus in a charged particle beam apparatus that acquires an image of a sample based on a charged particle signal, a scanning line of a specific area selected from an imaging area of the sample is driven by driving the scanning means and the focusing means. Scanning with a charged particle beam while sequentially changing the focal position, performing preliminary imaging to obtain a preliminary image signal of the specific area from the obtained secondary charged particle signal, and scanning with the maximum sharpness in the preliminary image signal determine lines, there is performed the determination of the image obtained by performing the imaging of the imaging area of the sample state of the converging means in the scan line as the focus state Obtaining means for obtaining an image signal profile scan line reaches the maximum sharpness position, the image signal profile of scan lines corresponding to the scan line reaches the maximum sharpness in the preliminary imaging in the present imaging in the pre-shot A similarity between the two image signal profiles is obtained from the two image signal profiles, the similarity is compared with a first threshold value, and a deviation amount from the image center position at which the similarity is minimized Is an autofocus image determination apparatus in a charged particle beam apparatus, characterized by comprising means for comparing an image with a second threshold and determining an image obtained by performing main imaging.

  According to a third aspect of the present invention, there is provided a charged particle beam apparatus comprising the image determination apparatus in the charged particle beam apparatus according to the second aspect.

The invention according to claim 4 includes a converging unit that converges the charged particle beam at a focal position, and a scanning unit that scans the charged particle beam in a two-dimensional manner, from a sample irradiated with the charged particle beam. In a charged particle beam apparatus that acquires an image of a sample based on a secondary charged particle signal, a scanning position of a specific area selected from an imaging area of the sample is driven by sequentially driving the scanning unit and the converging unit. Scanning with a charged particle beam while changing, performing preliminary imaging to acquire a preliminary image signal of the specific area from the obtained secondary charged particle signal, and determining a scanning line having the maximum sharpness in the preliminary image signal A computer program for performing autofocus image determination of an image obtained by performing main imaging of the imaging area of the sample with the state of the converging means in the scanning line as a focused state. An image signal profile of a scanning line that has a maximum sharpness position in the preliminary imaging, and a scanning line corresponding to the scanning line that has the maximum sharpness in the preliminary imaging in the main imaging. Obtaining an image signal profile; obtaining a similarity between the two image signal profiles from the two image signal profiles; comparing the similarity with a first threshold; and further determining a position at which the similarity is minimized. Comparing the amount of deviation from the center position of the image with a second threshold and determining the image obtained by performing the main imaging .

  The invention according to claim 5 is a recording medium in which the computer program according to claim 4 is stored.

  According to the present invention, an image signal profile of a scanning line at the maximum sharpness position in preliminary imaging and an image signal profile of a scanning line corresponding to the specific scanning line that performed the preliminary imaging in the main imaging are acquired, Since the suitability of autofocus is determined by comparing both image signal profiles, it can be determined appropriately and quickly whether or not the captured image has been obtained in the intended focus state.

It is a schematic diagram which shows an example of the scanning electron microscope which concerns on an Example. It is a block diagram which shows the control system of a scanning electron microscope. It is a flowchart which shows the imaging procedure of a scanning electron microscope. It is explanatory drawing for demonstrating the procedure which performs autofocus. It is a flowchart which shows the procedure which determines an autofocus image. The preliminary imaging is shown, (a) is an acquired image, (b) is a graph showing the profile at the sharpest position. This figure shows main imaging, where (a) is an acquired image and (b) is a graph showing a profile at a focus setting position. FIG. 7 shows a comparison state of profiles, (a) is a graph showing a profile Fa (t), and (b) is a graph showing a profile Fb (t). It is for demonstrating calculation of similarity S (t), (a) is a schematic diagram which shows the calculation start state, (b) respectively shows the end state of calculation.

  The charged particle beam apparatus as a mode for carrying out the present invention is a scanning electron microscope. A scanning electron microscope scans an electron beam two-dimensionally, focuses and irradiates a sample, detects a secondary electron beam or a reflected electron beam from a sample with a secondary electron detector, and forms a scanned image. To do. For this reason, the scanning electron microscope includes a converging means for converging the electron beam at the focal position and a scanning means for two-dimensionally scanning the electron beam, and is based on a secondary electron signal from the sample irradiated with the electron beam. When imaging the sample, the scanning means is driven to perform secondary charging by scanning with the charged particle beam sequentially having different focal positions in the specific area selected from the imaging area of the sample and scanning at different focal positions. Preliminary imaging for obtaining a preliminary image of the specific area from a particle signal is performed, and a state in which a scanning line with the highest sharpness of the image signal is obtained at different focal positions in the preliminary image is defined as an in-focus state of the convergence means. And an autofocus device that performs the actual imaging of the sample with the focusing means in the in-focus state.

  The scanning electron microscope also includes an autofocus image determination device that determines whether or not the captured image is captured in the intended state of the autofocus image captured in the autofocus state set in the preliminary imaging. The autofocus image determination device includes a means for acquiring an image signal profile of a scanning line at a maximum sharpness position in preliminary imaging, and an image signal profile of a scanning line corresponding to a specific scanning line that has performed the preliminary imaging in the main imaging. And a means for determining the quality of the autofocus image by comparing both image signal profiles.

  The comparison of the image signal profiles is performed by comparing at least one of the similarity between the two image signal profiles and the shift amount of the position where the similarity is maximized with a threshold value.

  A scanning electron microscope as a charged particle beam apparatus according to an embodiment of the present invention will be described below. FIG. 1 is a schematic diagram illustrating an example of a scanning electron microscope according to the embodiment, FIG. 2 is a block diagram illustrating a control system of the scanning electron microscope, and FIG. 3 is a flowchart illustrating an imaging procedure of the scanning electron microscope.

  The basic structure of the scanning electron microscope is substantially the same as that of the charged particle beam apparatus (scanning electron microscope) described in Patent Document 1 or the like, and a commonly used one can be adopted. As shown in FIG. 1, the scanning electron microscope 10 according to the embodiment uses an electron beam 13 generated from an electron beam source 12 in the upper part of a lens barrel 11 as an alignment coil 14 (scanning means) and a stig coil 15 (second scanning). Correction is performed by the correction means), the focus is adjusted by the objective lens coil 16 (convergence means), and the sample 18 disposed in the sample chamber 17 is scanned. Charged particles 19 such as secondary electrons and reflected electrons generated from the sample 18 are detected by a detector 20, and a sample image is displayed by an image display means 23 (see FIG. 2) such as a monitor.

  In the present invention, as shown in FIG. 2, the scanning electron microscope 10 determines whether or not the autofocus device 21 that causes the scanning electron microscope 10 to perform autofocus imaging and the autofocused image are in an intended state. An autofocus image determination device 22 for determination, an image display means 23, and input means 24 such as a keyboard and a mouse are provided. Here, the autofocus device 21 and the autofocus image determination device 22 can be configured as a computer including a CPU, HDD, ROM, RAM, I / O device, and the like, and the computer program according to the present invention is stored in this computer. The recording medium such as a CD-ROM is stored and executed by a predetermined method to control the respective devices 12, 14, 15, 16 of the scanning electron microscope 10 and image processing in the autofocus device 21 and the autofocus image determination device 22. Functions and judgment processing functions are realized.

  Next, procedures for autofocus imaging and autofocus image determination will be described. FIG. 3 is a flowchart showing an imaging procedure of the scanning electron microscope, and FIG. 4 is an explanatory diagram for explaining a procedure for performing autofocus.

First, the procedure from autofocus imaging to autofocus image will be described. First, in order to execute autofocus imaging by the autofocus device 21, preliminary imaging is performed (step S1), and then the autofocus value, that is, the state of the convergence means in the main imaging is set from the preliminary image obtained by the preliminary imaging. Then (step S2), the main imaging is performed to acquire the main image (step S3). Then, the autofocus image determination device 22 determines the autofocus image (step S4). If the intended image is obtained as a result of the determination, the imaging is terminated. If the intended image is not obtained, error processing is performed (step S5). The error processing performs necessary processing such as displaying an image to that effect or adding a flag and calling an operator.

Next, autofocus imaging will be described. The preliminary imaging is performed by imaging, for example, a central area of the sample 18. At this time, imaging is performed by sequentially changing the focus for each line. For example, when an area as shown in FIG. 4A (the scanning line is N rows) is imaged, an image shown in FIG. 4B is obtained. In the example shown in FIG. 4 , the focus value in the scanning line corresponding to the uppermost stage in FIG. 1... N−1) to obtain a preliminary image ( FIG. 4B) .

  Therefore, the in-focus state of each scanning line is analyzed to obtain a focused scanning line, and the focus value at this time is set as the focus value in actual imaging. This analysis processing is based on determining the sharpness of the image in each scanning line and determining the scanning line having the maximum sharpness. In this example, when n = (A−1), that is, F = F + Δf × A maximum sharpness is obtained. The sharpness is acquired by a known method. At this time, when the sample 18 is irradiated with the electron beam 13 and a sample image is formed by electrons generated from the sample 18, it is preferable to change the focus while scanning one frame. This makes it possible to execute autofocus at high speed and with little damage to the sample.

  Next, the autofocus image determination device 22 will be described. FIG. 5 is a flowchart showing a procedure for determining an autofocus image, FIG. 6 shows preliminary imaging, (a) is an acquired image, (b) is a graph showing a profile at the sharpest position, and FIG. 7 is main imaging. (A) is an acquired image, (b) is a graph showing a profile at a focus setting position, FIG. 8 shows a profile comparison state, and (a) shows a profile Fa (t). Graph (b) is a graph showing the profile Fb (t).

  In this process, first, the profile Fa (t) (see FIG. 6) in the scanning line in which the maximum sharpness is obtained in the preliminary image, and the profile Fb (t in the imaging line corresponding to the determination line of the image in the main imaging are further obtained. (See FIG. 7) (step ST2). Here, Fa (t) and Fb (t) may be those of a single scanning line, or an average value of a plurality of adjacent scanning lines may be used to reduce noise.

  The obtained two profiles Fa (t) and Fb (t) are the same or similar if the main imaging is performed in the focused state determined by the preliminary imaging. However, if the in-focus state is not maintained for reasons such as charging of the sample after preliminary imaging, they are not similar. In this autofocus image determination process, the autofocus image is determined in a similar state between the two profile profiles Fa (t) and Fb (t). When the image is captured without the influence of charge-up and the autofocus value is set normally, the profile Fa (t) and the profile Fb (t) have substantially the same shape. In the example shown in FIG. 8, the two peaks Pa1 and Pa2 of the profile Fa (t) are shifted from the two peaks Pb1 and Pb2 of the profile Fb (t). In such a case, the two profiles are not similar, For some reason, it is considered that the focusing condition is lost during the main shooting, and the captured image is in a defocused state different from the intended image.

  In order to make a similar determination, a similarity S (t) for the obtained Fa (t) and Fb (t) is calculated (step ST3). This similarity S (t) is calculated based on Equation 1. This calculation process is performed by executing a program on a computer constituting the autofocus image determination device 22. Here, Expression 1 is obtained by normalizing the difference between the profiles Fb (t) while shifting the profile Fa (t) within a predetermined range. The gain fluctuations of the profiles Fa (t) and Fb (t) are as follows: Canceled.

  FIG. 9 is a diagram for explaining the calculation procedure of the similarity S (t), where (a) is a schematic diagram showing a calculation start state, and (b) is a schematic diagram showing a calculation end state. Profile Fa (t) and profile Fb (t) are distributed in the range of 0 to l (w−1). When Expression 1 is applied in this state, the portion of t = c to (w−c−1) (shaded portion) of the profile Fb (t) is sequentially shifted over the width c and is calculated as Fa (t). Here, the width c is an invalid area for calculation. That is, at the start of comparison, as shown in FIG. 9A, Fa (0), Fb (c),..., Fa (w-2c-1) and Fb (w-c-1) The calculation is continued and the calculation is continued by shifting Fb (t) by one, and finally Fb (c) and Fa (2c), Fb (w−c−1) and Fa (w−1) are calculated. Each operation value is added. In this calculation, if the profile Fa (t) and the profile Fb (t) are equal, the value of the similarity S (t) is “1”. The larger the difference between the two, the larger the value of the similarity S (t). Becomes smaller.

In this example, in order to determine the similarity, the maximum value of the obtained similarity S (t) is compared with a threshold T1 that is a predetermined first threshold (step S4). Note that the threshold value T1 may be determined in advance by an imaging experiment or the like. When the maximum value of the similarity S (t) is smaller than the threshold value T1 (yes in step ST4), the two profiles Fa (t) and the profile Fb (t) have no similarity, and the actually captured image is desired. It is subjected to error processing as not being an autofocus image (step ST8).

When the maximum value of the similarity S (t) is equal to or greater than the threshold T1 (no in step ST5), a position T at which the similarity S (t) is the minimum is obtained (step ST5). and in close position to the image central portion (w-1/2) as is, the difference ΔT between the position T and the central position location is reduced.

In this example, this value ΔT is compared with a predetermined second threshold value T2 (step ST6). If the difference is larger than the threshold value T2, the positions of the profile Fa (t) and the profile Fb (t) are compared. The deviation is large, there is no similarity between the two profiles, and the actual captured image is not subjected to error processing (step ST8). Note that the threshold value T2 may be determined in advance by an imaging experiment or the like.

  When the value ΔT is equal to or smaller than the threshold value T2, the next image is taken assuming that the actually captured image is a desired autofocus image (step ST7).

  As described above, according to this embodiment, it is possible to automatically determine the quality of the obtained autofocus image. For this reason, when a sample is photographed in automatic operation, error processing can be performed even if defocusing occurs, and automatic sample photographing can be efficiently performed.

  In this embodiment, the quality of the autofocus image is determined based on the maximum value of the similarity S (t) and the value ΔT. The determination of the quality is based on the similarity S (t) and the vicinity thereof. Other data such as S (t−k) to S (t + k) may be used, and the determination may be made based on the correlation coefficient after the second order.

  In the above-described embodiment, the case where the autofocus value is acquired in the central region of the imaging region at the time of autofocus imaging has been described as an example. However, in order to obtain the autofocus value, the sample region for one frame including a plurality of scanning lines is scanned. In this case, the focusing means is driven to move the focal position of the charged particle beam over a predetermined range, and an image signal is analyzed with respect to the obtained sample image whose focal point is changed within one frame to obtain an appropriate charged particle beam. The present invention can be applied to a charged particle beam apparatus that obtains a focal position and sets a converging means so that the obtained focal position is irradiated with a charged particle beam.

  Further, although the above embodiment has been described for the case where it is applied to a scanning electron microscope, it can be used to determine the success or failure of autofocus of other charged particle beam apparatuses such as a drawing apparatus and a semiconductor inspection apparatus.

DESCRIPTION OF SYMBOLS 10 Scanning electron microscope 11 Lens tube 12 Electron beam source 13 Electron beam 14 Alignment coil 15 Stig coil 16 Objective lens coil 17 Sample chamber 18 Sample 19 Charged particle 20 Detector 21 Autofocus device 22 Autofocus image determination device 23 Image display means 24 Input means

Claims (5)

A converging means for converging the charged particle beam at a focal position; and a scanning means for two-dimensionally scanning the charged particle beam, based on a secondary charged particle signal from a sample irradiated with the charged particle beam. in autofocus image determination method in the charged particle beam device for acquiring an image of the sample, sequentially changing the focal position of the scanning lines of the specific region selected from among an imaging region of the sample by driving the converging means and said scanning means However, scanning with a charged particle beam, performing preliminary imaging to acquire a preliminary image signal of the specific region from the obtained secondary charged particle signal, determining a scanning line that has the maximum sharpness in the preliminary image signal, the determination of an image obtained by performing the main scan of the imaging region of the sample the state of the converging means in the scan line as the focus state a line Umono,
Obtain an image signal profile of the scanning line that has reached the maximum sharpness position in the preliminary imaging,
Obtaining an image signal profile of a scanning line corresponding to the scanning line having the maximum sharpness in the preliminary imaging in the main imaging;
The similarity between the two image signal profiles is obtained from the two image signal profiles, the similarity is compared with a first threshold value, and the amount of deviation from the image center position at the position where the similarity is minimized is calculated as a second value. An autofocus image determination method in a charged particle beam apparatus, characterized in that an image obtained by performing main imaging is compared with a threshold value. A focusing means for converging the charged particle beam at a focal position and a scanning means for two-dimensionally scanning the charged particle beam, and a sample based on a secondary charged particle signal from the sample irradiated with the charged particle beam In an autofocus image determination apparatus in a charged particle beam apparatus that acquires an image of
The scanning means and the converging means are driven to scan a scanning line in a specific area selected from the imaging area of the sample with a charged particle beam while sequentially changing the focal position, and from the obtained secondary charged particle signal, Preliminary imaging for obtaining a preliminary image signal of a specific area is performed, a scanning line having the maximum sharpness is determined in the preliminary image signal, and the state of the convergence means in the scanning line is set as a focused state in the imaging area of the sample. The image obtained by performing the main imaging is determined ,
Means for acquiring an image signal profile of a scanning line that has reached a maximum sharpness position in the preliminary imaging;
Means for acquiring an image signal profile of a scanning line corresponding to the scanning line having the maximum sharpness in the preliminary imaging in the main imaging;
The similarity between the two image signal profiles is obtained from the two image signal profiles, the similarity is compared with a first threshold value, and the amount of deviation from the image center position at the position where the similarity is minimized is calculated as a second value. An autofocus image determination apparatus in a charged particle beam apparatus, comprising: means for determining an image obtained by performing main imaging in comparison with a threshold value. A charged particle beam apparatus comprising the image determination apparatus in the charged particle beam apparatus according to claim 2. A focusing means for converging the charged particle beam at a focal position and a scanning means for two-dimensionally scanning the charged particle beam, and a sample based on a secondary charged particle signal from the sample irradiated with the charged particle beam In the charged particle beam apparatus that acquires the image of the sample, the scanning unit and the converging unit are driven to scan the scanning line of the specific region selected from the imaging region of the sample with the charged particle beam while sequentially changing the focal position. Then, preliminary imaging is performed to acquire a preliminary image signal of the specific area from the obtained secondary charged particle signal, a scanning line having the maximum sharpness is determined in the preliminary image signal, and the convergence means in the scanning line is determined. A computer program for determining an autofocus image of an image obtained by performing main imaging of an imaging region of a sample with a state as a focused state,
Obtaining an image signal profile of a scanning line that has reached a maximum sharpness position in the preliminary imaging;
Obtaining an image signal profile of a scanning line corresponding to the scanning line having the maximum sharpness in the preliminary imaging in the main imaging;
Obtaining a similarity between the two image signal profiles from the two image signal profiles;
This similarity is compared with the first threshold, and the amount of deviation from the center position of the image at which the similarity is minimum is compared with the second threshold to determine the image obtained by performing the main imaging. The steps of
Computer program, characterized in that it comprises a.   A recording medium storing the computer program according to claim 4.