JP4995542B2 - Autofocus method of charged particle beam apparatus, computer program, and recording medium - Google Patents

Description

  The present invention relates to an autofocus method, a computer program, and a recording medium for a charged particle beam apparatus that irradiates an observation target region on a sample with a charged particle beam and forms a sample image with secondary charged particles generated from the sample.

  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.

  In such an autofocus device, as a typical method of focus value detection, there is a method of acquiring a plurality of images while changing the focus and determining an optimum focus value by referring to the plurality of images.

  FIG. 6 is a diagram for explaining an autofocus method of a conventional charged particle beam apparatus. In this example, a plurality of images (FIGS. 6A to 6E) are acquired while changing the focal position by Δf from the reference focus position F, and the sharpness of the image in each image is evaluated. Is used to calculate an optimum focus value. In the example shown in FIG. 5, an image of F + Δf × 2 (FIG. 6C) is detected as having an optimum focus value.

  A similar method is described in Patent Document 1. In Patent Document 1, in a charged particle beam apparatus that focuses an electron beam on a sample and scans the beam in a predetermined area on the sample, the focus state of the beam on the sample is changed stepwise. At each focus stage, a specific area on the sample is scanned with a beam, the detection signal obtained based on the beam scan is appropriately processed to determine the peak accuracy, and the determined peak accuracy is compared. In such a case, the peak position is selected, and the focus state of the beam is set to the focus state of the electron beam corresponding to the selected peak position. These are methods for obtaining an appropriate focus position by changing the focus for each frame in which a sample image is taken.

As another method of detecting the focal position, there is a method of determining an optimum focus value by narrowing a scan area at the time of detecting the focal position and changing the focus. FIG. 7 is a diagram showing another autofocus method of the conventional charged particle beam apparatus. In this example, a charged particle beam is scanned on a sample a plurality of times in the same narrow area, and the focus is changed for each scan. In the example shown in FIG. 7, the scan region is limited to the center (FIG. 7A), the focus is changed, and images at respective focus values are arranged in order to obtain an image (FIG. 7B). That is, in the image of FIG. 7B, an image at the focus value: F is displayed at the uppermost position, an image of Focus + Δf is displayed on the next line, and Δf is sequentially added downward. An image is displayed. In this example, the sharpness of the image in each row is obtained from the obtained image of FIG. 6B, and an optimum focus value is calculated from this sharpness.
JP 2002-334678 A

  By the way, in the former method of acquiring a plurality of images while changing the focus, since it is necessary to acquire a plurality of images, there is a problem that the execution time becomes long. In other words, it is necessary to obtain a plurality of images for calculating the optimum focus value, and this problem becomes more prominent as the observation field of view becomes higher in definition (the number of pixels increases).

  In addition, the latter method in which the scan area is narrowed can obtain an optimum focus value at high speed. However, the charged particle beam is irradiated many times on a narrow area of the sample, so that the damage of the portion becomes large. is there. In addition, there is a drawback that autofocus cannot be performed when there is no pattern in the narrowed area.

  Accordingly, the present invention solves the above-described problems, detects the focal position at high speed and with high efficiency, and prevents the charging (charging), damage, etc. of the sample, and the autofocus method and computer for the charged particle beam apparatus An object is to provide a program and a recording medium.

According to a first aspect of the present invention, in the autofocus method of the charged particle beam apparatus, the charged particle beam comprises: a converging unit that converges the charged particle beam at a focal position; and a scanning unit that scans the charged particle beam two-dimensionally. In an autofocus method of a charged particle beam apparatus that acquires an image signal based on secondary charged particles from a sample irradiated with a beam and forms a sample image, when scanning a sample region for one frame composed of a plurality of scanning lines The focusing means is driven to move the focal position of the charged particle beam over a predetermined range, and when the number of edges of the sample image appearing in the image signal is equal to or less than the predetermined number, the image signal is ignored and a score value is obtained. An image information acquisition step and an image signal representing the sample image of one frame obtained during the focal point movement are analyzed to obtain an appropriate focal point of the charged particle beam. A focus position detection step of acquiring location autofocusing method of the charged particle beam device and a focus position setting step of setting the converging means in a proper focus position said.

According to a second aspect of the present invention, in the autofocus method for a charged particle beam apparatus, the charged particle beam comprises: a converging unit for converging the charged particle beam at a focal position; and a scanning unit for two-dimensionally scanning the charged particle beam. In an autofocus method of a charged particle beam apparatus that acquires an image signal based on secondary charged particles from a sample irradiated with a beam and forms a sample image, when scanning a sample region for one frame composed of a plurality of scanning lines An image information acquisition step of driving the focusing means to move the focal position of the charged particle beam over a predetermined range, and analyzing an image signal representing the sample image of one frame obtained during the focal movement Gets the focal position of the charged particle beam, obtains a score value of the image signal representing the sharpness of the sample image, the number of edges EN the score value S (y) a focus position detection step of removing the influence of the score value by amount of the pattern by dividing y), autofocus method of the charged particle beam device and a focus position setting step of setting a proper focus position the said converging means It is.

  According to a third aspect of the present invention, in the autofocus method of the charged particle beam apparatus according to the first or second aspect, in the image information acquisition step, the focal position is moved based on a sawtooth wave having a plurality of periods. Features.

According to a fourth aspect of the present invention, in the autofocus method for a charged particle beam apparatus according to any one of the first to third aspects, in the focal position detection step, a score value of an image signal representing the sharpness of the sample image And obtaining the appropriate focus position based on the score value.

According to a fifth aspect of the present invention, in the autofocus method for a charged particle beam apparatus according to any one of the first to fourth aspects, in the focal position detecting step, a score obtained from a plurality of image signals obtained by scanning. The values are averaged to obtain an appropriate focal position.

According to a sixth aspect of the present invention, in the autofocus method for a charged particle beam apparatus according to any one of the first to fifth aspects, in the image information acquisition step, the charged particle beam is scanned in the intersecting direction to obtain one frame. A plurality of images are acquired, and in the focal position detection step, an appropriate focal position is obtained from the plurality of images.

A seventh aspect of the present invention is the charged particle beam device autofocus method according to any one of the first to sixth aspects, wherein the image information acquisition step obtains a plurality of frames of images, Scanning the charged particle beam by changing at least one of the scanning direction, the deflection amount, the position, and the number of periods of the focal position of the charged particle beam;
In the focal position detection step, an appropriate focal position is obtained based on the obtained images of a plurality of frames.

The invention of claim 8 is a computer program characterized in that the autofocus method for a charged particle beam apparatus according to any one of claims 1 to 7 is executed by a computer.

A ninth aspect of the present invention is a recording medium in which the program according to the eighth aspect is stored.

  According to the present invention, in the autofocus operation of the charged particle beam apparatus, an appropriate focal position can be acquired while scanning one frame of the sample at a minimum. Focus operation can be performed. Further, noise can be reduced by integrating the sample images over a plurality of times. Furthermore, since the charged particle beam is not irradiated on the sample locally, the sample can be prevented from being charged or damaged.

  A scanning electron microscope which is an example of a charged particle beam apparatus to which the present invention is applied will be described below. FIG. 1 is a block diagram showing a configuration of a scanning electron microscope according to the embodiment.

  As shown in FIG. 1, the scanning electron microscope 10 corrects the aberration of the electron beam 13 generated from the electron beam source 12 in the upper part of the lens barrel 11 by an alignment coil 14 (scanning means) and a stig coil 15 (scanning means). Then, scanning is performed, the focus is adjusted by the objective lens coil 16 (convergence means), and the material 21 such as a wafer is scanned. Then, the charged particles 17 such as secondary electrons and reflected electrons generated from the sample 21 in the sample chamber 20 are detected by the detector 18, the signal from the detector 18 is processed by the microscope control device 30, and the scanning means The sample image is displayed on the image display device 35 so as to be observable in synchronization with the scanning. In addition, an autofocus device 40 is connected to the scanning electron microscope 10 according to this example.

  Next, the autofocus device 40 will be described. FIG. 2 is a block diagram showing the configuration of the autofocus device of the scanning electron microscope according to the embodiment. As shown in FIG. 2, the autofocus device 40 includes a focus position changing device 41, a focus position detecting device 42, and a focus position setting device 43.

  The microscope control device 30 and the autofocus device 40 realize the functions by causing a computer to execute a program. This program can be stored in a recording medium such as a CD and a DVD, and the computer realizes the above functions by being installed in the computer.

  The focal position changing device 41 drives the converging means to move the focal position of the charged particle beam over a predetermined range when scanning a sample region for one frame composed of a plurality of scanning lines. In other words, in this example, the focus position changing device 41 drives the objective lens coil 16 via the microscope control device 30 and adds a specified amplitude ΔF of the charged particle beam from the reference focus position F during one frame. Irradiation is performed by changing to a certain position (F + ΔF) a plurality of times (for example, 4 to 64 times).

  FIG. 3 is a schematic diagram showing the operation of the focal position changing apparatus according to the embodiment. As shown in FIG. 3A, the focal position changing device 41 changes the focal position to F to F + ΔF over four periods when photographing one area of the sample 21 (FIG. 3B). This change is made by a sawtooth wave. This change may be caused by another waveform such as a triangular wave. Further, the repetition period is not limited to 4, and may be 1, 2, 8, 16,.

The detector 18 detects the charged particles 17 from the sample 21, and the microscope control device 30 obtains an image signal of one frame of the sample. In this operation, the scanning of the charged particle beam is performed at least once on the entire surface of the observation region of the sample, so that the sample is not damaged. Note that scanning can integrate the images multiple times to reduce noise in the sample image.

  When scanning is performed while the focal position of the charged particle beam is changed in this way, an image signal shown in FIG. 3B is obtained. In this example, the image obtained from the image signal can be divided into four for each period of the sawtooth wave. In a place where each image (divided image) is in focus for each cycle, the pattern (pattern) of the sample appears clearly, and before and after that, the pattern is gradually blurred without being focused.

  Next, the focal position detection device 42 will be described. The focal position detection device 42 extracts a pattern line (edge) from the image acquired by scanning by the focal position changing device 41, analyzes the image information of the edge, and obtains a score value of the image signal representing the sharpness. The proper focal position is obtained based on the score value.

  In addition, the focal position detection device 42 performs correction to prevent the influence of the influence on the score value when there are many patterns of the sample due to the image acquisition cycle (position) or when there is no partial pattern. . This correction is based on obtaining score values of image signals representing the sharpness of the sample image in each cycle based on the obtained four-cycle images and averaging these score values.

  Further, the focal position detection device 42 performs correction to eliminate the influence of the number of sample patterns in the sample image. This correction is performed by removing the focus position detection target when the number of edges in the image signal in one cycle is smaller than a predetermined number.

  Hereinafter, an operation algorithm of the focal position detection device 42 will be described. FIG. 4 is a flowchart showing the operation of the focal position detection device, and FIG. 5 is a schematic diagram showing the operation of the focal position detection device. In the description, the y direction indicates the direction in which the focus is changed, the x direction indicates the direction in which the focus is not changed, and f indicates the focus change direction (on the electron beam optical axis).

  The focal position detection device 42 calculates and evaluates the sharpness of each divided image according to the following procedure. First, the focal position detection device 42 sets the number of repetitions (ST1). The initial value is 0, and “1” is added every time the process is repeated to calculate the number of repetitions.

  Next, the focus width is determined (ST2). When the focus fluctuation width (ΔF) is larger than the threshold value 1 (an arbitrary predetermined constant), it is determined that an error has occurred. The focus amplitude is increased or decreased by subsequent processing. This is because if the swing width is too large, the focus accuracy becomes poor.

  Next, the number of repetitions is determined (ST3). If the number of repetitions is greater than threshold value 2 (arbitrary constant), an error is determined. If an appropriate focus value cannot be obtained even after repeating a certain number of times, it is determined that an appropriate focus value cannot be obtained.

  Next, image acquisition is performed (ST4). The focus position changing device 41 obtains an image (FIG. 5A) in which the focus is changed according to the focus swing specified within one frame. Further, the period data (period, gain, start) at which the focus is changed at this time is acquired. Also store position offsets).

  Next, an edge threshold value is calculated (ST5). Necessary processing such as smoothing processing is performed on the above-mentioned image, and y1% and y2% from the darker than the histogram obtained by integrating the brightness and frequency of the image as axes (y1% is about 2% and y2 respectively). The edge threshold value is calculated based on the difference (y2−y1).

  Next, a score value is calculated (ST6). For the acquired image, the score value S (y) and the number of edges EN (y) are calculated for each line of the image. A schematic diagram of the score value S (y) obtained is shown in FIG. Here, the difference D (x, y) is calculated by assuming that the image is I (x, y), m, n are arbitrary constants, the edge threshold is ET, and the number of edges is EN (y). The difference D (x, y) is obtained by Expression (1).

  However, if D (x, y) ≦ 0, D (x, y) = 0. The difference D (x, y) can also be obtained by the following equation (2) (m, n, o, p are arbitrary constants).

  The score value S (y) and the number of valid data E (y) for each row are obtained as follows. Here, IX is an image size in a direction in which the focus is not changed, and EN (y) is a number where D (x, y)> 0. Further, the threshold value ENi is calculated from EN (y). ENi may be an arbitrary constant, or may be a value obtained by dispersion of EN (y) or the like.

  Y satisfying EN (y) <ENi is obtained by executing the following calculation except that S (y) = 0 and E (y) = 0.

Here, by dividing S (y) by EN (y), it becomes difficult to be influenced by the number of patterns.
Further, it is possible to exclude from the calculation places where it is determined that the pattern is small by comparison with ENi.

Next, the average of the score values is obtained (ST7). When the number of focus periods in the image is i, the focus values for the position in the Y direction of the image in each period are F1 (f), F2 (f)... Fi (f), and the Y direction of the image in each period S1 (f), S2 (f)... Si (f), and the number of effective data E (f) at each focus position is calculated from E (y). ... (5)
Here, the sharpness profile of each period is added.

  Next, a temporary peak is calculated (ST8), and an effective period is determined (ST9). A peak position P is obtained for the obtained result S (f). Average values A1 (f), A2 (f)... Ai (f) around the peak position of each cycle are obtained as follows. W is an arbitrary constant.

Here, in order to obtain the effective sawtooth wave period i, the following processing is performed.
The maximum value Amax of Ai is obtained, and only the period i where Amax / n <Ai is valid. (N is an arbitrary constant)

  Further, for example, a method of obtaining the variance Aσ of Ai and enabling only the period i where Amax / n <Ai can be employed (n is an arbitrary constant). By performing this process, it is possible to exclude from the calculation a cycle in which it is determined that there are few patterns.

Next, the focus value is calculated (ST10). Using only the effective period i thus obtained, the operation of (5) and the expression (6) are re-executed, and the peak position is calculated for the obtained S (f). However, S (f) where S (f) = 0 is excluded and approximated. As a result, even if there is an area without a pattern, it is excluded at the time of approximation and does not affect the calculation of the optimum focus value. When calculating the peak position, a quadratic approximation centered on the peak position can be used, and the intersection of linear approximations within a certain range (20% to 80%, etc.) on the left and right sides of the peak position can be used. can do. Then, from the obtained peak position, the optimum focus value, to calculate the coefficient of determination R ² indicating the degree of coincidence between the actual data and the approximate value.

Further, determination coefficients are compared (ST11), and a focus value is set (ST12). The coefficient of determination R ² obtained compared threshold 3 (predetermined constant), when the predetermined threshold is smaller than 3, to expand the range of the focus range (the [Delta] F) (ST14), setting the number of repetitions again The processing from (ST1) is repeated. Further, when the coefficient of determination R ² is larger than the threshold value 3 is set to the focus position detecting device 42 the calculated focus value as the set focus value.

  Next, focus range determination 2 is performed (ST13). The threshold value 4 that can ensure a predetermined accuracy is determined by using the focus range value. If ΔF is equal to or smaller than the threshold value 4, the process ends normally. If not, the focus range is reduced and the number of repetitions is set again. Repeat from (ST1).

  The focal position detection device 42 determines the focus value by the above procedure. The focus position setting device 43 can adjust the objective lens coil 16 and the like based on the focus value, and can execute an autofocus operation of the scanning electron microscope 10.

  In the above example, the processing by the focus position changing device 41 and the focus position detecting device 42 has been described to be executed every time. However, the autofocus device sharpens the focus (image) by an external switching operation such as an operator. You may make it perform evaluation of a degree. Further, the set focus value of the focus position detection device 42 is compared with the current focus value, and a warning is displayed when the difference exceeds a certain range, or the focus position setting device 43 is automatically adjusted. be able to.

  In the embodiment, a wafer is used as a sample. However, the sample is not limited to this, and may be an arbitrary sample such as a mask on which a pattern detectable by an electron beam is formed. The charged particle beam apparatus is not limited to the scanning electron microscope, and may be an electron beam irradiation apparatus such as a semiconductor manufacturing / inspection apparatus. Furthermore, the charged particles used in the charged particle beam apparatus are not limited to electrons but may be other charged particles such as ions.

  Further, in the above example, the focal position changing device has the same scanning direction during scanning of one frame, but it can be performed in two directions intersecting the scanning direction.

In the above example, the case where the focal position is obtained by acquiring an image of one frame with the scanning direction, the deflection amount, the position, and the number of periods of the focal position of the fixed charged particle beam has been described. The apparatus shall obtain images of multiple frames, and in each frame, the charged particle beam shall be scanned with at least one of the scanning direction, deflection amount, position, and period number of the focal position of the charged particle beam being different. The focal position detection device may obtain an appropriate focal position based on the obtained images of a plurality of frames. In this way, the autofocus operation can be performed with high accuracy even when the pattern is only locally or when the edge of the pattern is only in a specific direction.

Further, in the above claims, the present invention has been described as a method, but when the present invention is specified as an apparatus invention, the following means can be cited.

  In this case, the first means includes 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 an autofocus device of a charged particle beam apparatus that obtains an image signal based on the next charged particle and forms a sample image, the converging means is driven to scan the sample region for one frame composed of a plurality of scanning lines. A focal position changing device that moves the focal position of the charged particle beam over a predetermined range, and an image signal representing the sample image of one frame obtained during the focal change is analyzed to obtain an appropriate focal position of the charged particle beam. And a focus position setting device that sets the convergence means so as to irradiate the obtained focus position with a charged particle beam. An autofocus device of a charged particle beam device according to symptoms.

  Further, the second means is the focal position changing device of the charged particle beam autofocus device described in the first means, wherein the focal position is moved a plurality of times in the predetermined range in one frame region of the sample, An image signal at each moving time is acquired, and the focal position detection device acquires an appropriate focal position of the charged particle beam based on the image signal at each moving time.

  Further, the third means is the charged particle beam autofocus apparatus according to the first or second means, wherein the focal position changing apparatus performs movement of the focal position based on a sawtooth wave. It is what.

  Further, the fourth means obtains a score value of an image signal representing the sharpness of the sample image in the charged particle beam autofocus device focal position detection device according to any one of the first to third means, The proper focus position is acquired based on the score value.

  The fifth means is the charged particle beam autofocus device according to any one of the first to fourth means, wherein the focus position detection device is a plurality of image signals obtained by scanning the focus position detection device. The score values obtained from the above are averaged to obtain an appropriate focal position.

  The sixth means is the charged particle beam autofocus device according to any one of the first to fifth means, wherein the focal position detection device has a predetermined number of edges of the sample image appearing in the image signal. When the number is less than or equal to the number, the score value is obtained by ignoring the image signal.

  The seventh means is the charged particle beam autofocus device according to any one of the first to sixth means, wherein the focal position changing device scans the charged particle beam in the intersecting direction to perform one frame. And obtaining an appropriate focal position from the plurality of images.

The means of claim 8 is the charged particle beam autofocus device according to any one of the first to seventh means, wherein the focal position changing device obtains an image of a plurality of frames. The charged particle beam is scanned by changing at least one of the scanning direction, the deflection amount, the position, and the number of periods of the focal position of the particle beam, and the focal position detection device is based on the obtained images of a plurality of frames. It is characterized by obtaining an appropriate focal position.

  A ninth means is a charged particle beam apparatus comprising the autofocus apparatus according to any one of the first to eighth aspects.

It is a block diagram which shows the structure of the scanning electron microscope which concerns on embodiment. It is a block diagram which shows the structure of the autofocus apparatus of the scanning electron microscope which concerns on embodiment. It is a schematic diagram which shows operation | movement of the focus position change apparatus which concerns on embodiment. It is a flowchart which shows operation | movement of the focus position detection apparatus which concerns on embodiment. It is a schematic diagram which shows operation | movement of the focus position detection apparatus which concerns on embodiment. It is a figure explaining the autofocus method of the conventional charged particle beam apparatus. It is a figure which shows the conventional charged particle beam apparatus other autofocus methods.

Explanation of symbols

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 Charged particle 18 Detector 20 Sample chamber 21 Sample 30 Microscope control device 35 Image display device 40 Autofocus device 41 Focus position Changing device 42 Focus position detecting device 43 Focus position setting device

Claims (9)

Converging means for converging the charged particle beam at a focal position and scanning means for two-dimensionally scanning the charged particle beam, and image signals are obtained based on secondary charged particles from the sample irradiated with the charged particle beam. In an autofocus method of a charged particle beam device that acquires and forms a sample image,
When the sample area for one frame consisting of a plurality of scanning lines is scanned, the focusing means is driven to move the focal position of the charged particle beam over a predetermined range, and the number of edges of the sample image appearing in the image signal is predetermined. When the number is less than or equal to a number, an image information acquisition step of obtaining a score value ignoring the image signal
A focus position detecting step of analyzing an image signal representing the sample image of one frame obtained during the focus movement and acquiring a focus position of an appropriate charged particle beam;
A focus position setting step of setting the convergence means to the proper focus position; and an autofocus method for a charged particle beam apparatus. Converging means for converging the charged particle beam at a focal position and scanning means for two-dimensionally scanning the charged particle beam, and image signals are obtained based on secondary charged particles from the sample irradiated with the charged particle beam. In an autofocus method of a charged particle beam device that acquires and forms a sample image,
An image information acquisition step of driving the converging means to move the focal position of the charged particle beam over a predetermined range when scanning a sample region of one frame composed of a plurality of scanning lines;
Analyzing the image signal representing the sample image of one frame obtained during the focal point movement to obtain an appropriate focal position of the charged particle beam, obtaining a score value of the image signal representing the sharpness of the sample image, A focus position detecting step of removing the influence of the score value due to the variation of the pattern by dividing the score value S (y) by the number of edges EN (y);
A focus position setting step of setting the convergence means to the proper focus position; and an autofocus method for a charged particle beam apparatus.   3. The charged particle beam apparatus autofocus method according to claim 1, wherein in the image information acquisition step, the focal position is moved based on a sawtooth wave having a plurality of periods. 4. The focus position detection step includes obtaining a score value of an image signal representing the sharpness of the sample image, and acquiring the appropriate focus position based on the score value. 5. 2. An autofocus method for a charged particle beam apparatus according to item 1 . Charged according to the in focus position detection step, any one of claims 1 to 4 a plurality of times the score values obtained from the image signal obtained by the scanning by averaging and wherein the obtaining a proper focus position Autofocus method for particle beam apparatus. In the image information acquisition step, the charged particle beam is scanned in the intersecting direction to acquire a plurality of images of one frame,
Wherein the focal position detection step, autofocus method of the charged particle beam device according to any one of claims 1 to 5, characterized in that to obtain a proper focus position by the plurality of images. In the image information acquisition step, an image of a plurality of frames is obtained. In each frame, at least one of the scanning direction, the deflection amount, the position, and the period number of the focal position of the charged particle beam is different, and the charged particle beam is changed. Scan
Wherein the focal position detection step, autofocus method of the charged particle beam device according to any one of claims 1 to 6, characterized in that to obtain a proper focus position based on the image of the plurality of frames obtained. A computer program for executing the autofocus method for a charged particle beam apparatus according to any one of claims 1 to 7 by a computer.   A recording medium storing the program according to claim 8.