charged particle beam equipment

Abstract

In order that, according to a defocused state at the start of an AF operation, a just focus search range is adjusted and focus adjustment is completed quickly, precisely, stably, and automatically, the present invention provides a charged particle beam device and a sample observation method, wherein: a defocus amount is calculated by using a first image blur amount, which is calculated from the change amount of sharpness obtained by performing defocus processing on one photographed image, and a second image blur amount, which is calculated in accordance with photographing conditions for an observation subject by changing the magnification for the observation subject in accordance with the value of the first image blur amount; the defocus amount is used to determine a first excitation current value; and the first excitation current value is used to start an AF operation for the observation subject.

Description

[Technical Field]

[0001] The present invention relates to a charged particle beam device and a sample observation method for irradiating a sample with a charged particle beam to perform observation. [Background technology]

[0002] In recent years, R&D efforts in fields such as semiconductors, materials, and biotechnology have become increasingly sophisticated. This demands faster development cycles, necessitating a deeper understanding of performance-related phenomena. Scanning electron microscopes (SEMs) allow for easy observation of various samples at nanometer-order resolution, making them an essential tool in R&D settings across a wide range of fields. SEM observation procedures can be broadly divided into sample preparation, field of view search, and optical axis adjustment. Optical axis adjustment is essential for obtaining clear images, and focus adjustment, in particular, is performed frequently during field of view search. Therefore, the time required for focus adjustment significantly impacts the overall time required for SEM observation. To address this issue, many SEMs are equipped with an automatic focus adjustment function (autofocus (AF)). AF essentially calculates the image sharpness by adjusting the focal position, and the position with the highest image sharpness is set as just focus. However, the focus search range in AF is often tied to observation conditions such as magnification, and often does not take into account the focal position at which AF begins. Therefore, when the search range is narrow, AF is completed in a short time, but exact focus often does not exist within the search range, reducing the AF success rate. On the other hand, when the search range is wide, the AF success rate increases, but because the number of SEM images that can be used to evaluate sharpness per second is limited, the focus adjustment accuracy decreases or the required time increases. In other words, there is a trade-off between AF adjustment accuracy, adjustment speed, and success rate, making it difficult to increase speed while maintaining AF adjustment accuracy.

[0003] In response to this, for example, Patent Document 1 discloses a technique for increasing the speed of AF while maintaining the accuracy of AF adjustment. This publication provides a technique for estimating the defocus amount at the start of AF and narrowing the search range to increase the speed while maintaining the accuracy of AF adjustment. In Patent Document 1, the amount of blur of an image at the start of AF is calculated using images with two different blur states, and the amount of defocus at the start of AF is estimated by referring to a correlation table between the amount of blur and the true defocus amount. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-218206 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the method of Patent Document 1 is applied to a charged particle beam device such as an SEM, the following two problems arise. The first problem is a response delay that occurs when adjusting the focus. In a charged particle beam device such as an SEM, the focus position is adjusted by controlling the excitation current flowing through an electromagnetic lens, so after changing the focus position, it is necessary to wait until the image stabilizes. As a result, it takes a long time to obtain two images with different blur conditions, as in Patent Document 1, and it is not possible to speed up AF. The second issue is the high observation magnification of the charged particle beam device. Patent Document 1 assumes use in an optical system such as a camera, and the observation magnification has little effect on the calculation of the amount of blur in an image when the zoom magnification is around 10x. On the other hand, the zoom magnification of a charged particle beam device easily exceeds 1000x, so the amount of blur in an image may not be calculated appropriately when the observation magnification is high. That is, at high observation magnifications, the focal position at the start of AF often falls within the first defocus range described in Patent Document 1, resulting in a problem in which the amount of blur is not calculated appropriately.

[0006] The present invention has been made in consideration of the above-mentioned problems, and an object of the present application is to provide a charged particle beam device that performs AF quickly, accurately, and stably, regardless of the observation magnification and blur state at which AF is performed. [Means for solving the problem]

[0007] In order to achieve the above object, the present invention provides a charged particle beam device configured to perform a defocus process on one captured image, calculate a defocus amount using a first image blur amount calculated from the amount of change in sharpness and a second image blur amount calculated based on the imaging conditions for the object to be observed, determine a first excitation current value using the defocus amount, and start focus adjustment for the object to be observed using the first excitation current value. In addition, to achieve the above object, there is provided a sample observation method comprising the steps of: performing a defocus process on one captured image; calculating a first amount of image blurring from the amount of change in sharpness; changing the magnification of the object to be observed in accordance with the value of the first amount of image blurring; calculating a defocus amount using a second amount of image blurring calculated based on imaging conditions for the object to be observed; and determining a first excitation current value using the defocus amount and starting AF on the object to be observed using the first excitation current value. [Effects of the Invention]

[0008] According to the present invention, even in the case of a high observation magnification such as in a charged particle beam device, it is possible to complete AF quickly, accurately, and stably, regardless of the focal position at the start of AF. Specifically, compared to conventional high-precision AF that searches a narrow range, the adjustment speed is approximately doubled while maintaining the same adjustment accuracy, and the AF success rate is improved. Also, compared to conventional AF that searches a wide range, the adjustment accuracy is approximately doubled and the adjustment speed is approximately doubled while maintaining the same AF success rate. This has led to improved throughput in focus adjustment and observation with charged particle beam equipment. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a configuration of a charged particle beam device according to the present invention. [Figure 2] FIG. 4 is a diagram illustrating an example of a GUI of an input display unit. [Figure 3] FIG. 10 is a diagram showing an example of a flowchart of the entire AF. [Figure 4] FIG. 10 is a diagram illustrating an example of a flowchart of a method for calculating the amount of image blur. [Figure 5] FIG. 10 is a diagram illustrating an example of a flowchart for estimating a defocus amount. [Figure 6] FIG. 10 is a diagram illustrating an example of a flowchart for determining a defocus direction. [Figure 7] 10A and 10B are diagrams showing the relationship between image sharpness in the vertical and horizontal directions and the defocus state before and after astigmatism (vertical and horizontal directions) is introduced. [Figure 8] FIG. 10 is a diagram illustrating an example of a flowchart of focus adjustment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of a charged particle beam device and a sample observation method according to the present invention will be described with reference to the drawings. [Example]

[0011] First, one configuration of a charged particle beam apparatus (hereinafter referred to as the present apparatus) for carrying out a sample observation method will be described with reference to FIG.

[0012] As shown in the figure, this apparatus comprises an electron gun 101 that irradiates an electron beam, a focusing lens 102, an aperture 103, a deflection coil 104, a stigma coil 105, an objective lens 106, a sample stage 108, a secondary electron detector 109, and an image forming unit 112. The electron gun 101 irradiates an electron beam 110, the focusing lens 102 and the objective lens 106 focus the electron beam, the aperture 103 adjusts the aperture angle of the electron beam, and the deflection coil 104 scans the electron beam 110 and deflects its irradiation direction. The secondary electron detector 109 detects secondary electrons 111 that are generated when the electron beam 110 irradiates an observation sample 107. The image forming unit 112 forms a charged particle beam image based on a signal from the secondary electron detector 109 and can transmit the image to a calculation processing unit 113. The electron beam 109 may be a charged particle beam such as ions, the secondary electrons 110 may be reflected electrons or transmitted electrons, and the secondary electron detector 108 may be a reflected electron detector or transmitted electron detector.

[0013] The device also includes a calculation processing unit 113 that processes the image transmitted from the image forming unit 112 and calculates the amount of blur and defocus. The calculation processing unit 113 calculates the amount of blur and defocus based on the image transmitted from the image forming unit 112 and the parameters transmitted from the control unit 114, and then calculates the value of the excitation current, which is a parameter of the objective lens corresponding to the focal position at which the search for just focus begins, and transmits the value to the control unit 114.

[0014] This apparatus is equipped with a control unit 114 that controls parameters related to the electron optical systems 101 to 106 and the sample stage 108. In particular, parameters related to the polarization coil 104 can be adjusted to change the observation magnification, the stigma coil 105 can be adjusted to perform astigmatism correction, and parameters related to the objective lens 106 can be adjusted to change the focal position. The control unit 114 can acquire and hold parameters related to the electron optical systems 101 to 106, and can transmit these parameters to the calculation processing unit 113.

[0015] The device also includes an input display unit 115 that allows the operator to input parameters necessary for AF execution and displays a button for the operator to start AF execution. Fig. 2 shows an example of a Graphical User Interface (GUI) 116 of the input display unit 115, which has an AF start button 117, an input field 118 for a magnification step that specifies the amount of change in the observation magnification used when adjusting the observation magnification to an appropriate observation magnification for calculating the amount of blur, an input field 119 for an upper limit of the amount of blur that determines whether the amount of blur has been calculated appropriately, and an input field 120 for a lower limit of the amount of blur.

[0016] Input to the input display unit 115 does not necessarily have to be in the form of the GUI 116, but may be in the form of a text file that is read and reflected.

[0017] Next, a sample observation method using this apparatus will be described. Fig. 3 is a diagram showing an example of a flowchart of the entire AF according to the first embodiment.

[0018] First, a first image blur amount B1 of an initial SEM image, which is necessary for estimating the defocus amount at the start of AF, is calculated (121). Figure 4 shows an example of a flowchart of a method for calculating the first image blur amount B1. In the figure, first, an initial SEM image immediately after the start of AF is acquired from the image forming unit 112 (128) and sent to the calculation processing unit 113.

[0019] Next, the calculation processing unit 113 performs defocus processing 129 on the acquired SEM. The defocus processing is basically performed using Gaussian blur. The value of standard deviation b, which indicates the amount of blur in Gaussian blur, may be any value equal to or greater than 0.

[0020] A calculation processing unit 113 uses two images, the original initial SEM image and the initial SEM image after defocus processing, to calculate a first image blur amount B1 of the original initial SEM image (131). The first image blur amount B1 is calculated by calculating the sharpness of each of the original initial SEM image and the initial SEM image after defocus processing (129) (130) and using the sharpness ratio (131).

[0021] In AF used in optical systems such as cameras, the amount of blur is calculated using images acquired at multiple different focal positions, as in Patent Document 1, for example. However, charged particle beam devices such as SEMs often use objective lenses that use a magnetic field, and acquiring images at multiple focal positions requires waiting several seconds for the magnetic field to stabilize. In this embodiment, this problem is solved by calculating the amount of blur using two images: an initial SEM image and an initial SEM image that has been defocused by the calculation processing unit 113.

[0022] The method for calculating the first image blur amount B1 (131) will be described in detail. The boundaries between regions in an image with no blur are described by a step function U. The brightness F(x) near the boundary in an SEM image can be expressed as F(x,y) = CU(x) + D, where x is the pixel coordinate and C and D are constants. Furthermore, U(x) is 1 when x ≥ 0 and 0 when x < 0. Assuming that the blur of an image formed by a charged particle beam device such as an SEM is modeled using a point spread function, an SEM image I0 with a blur amount B can be expressed as I0(x,y) = F(x,y) × G(B), where × represents the convolution product and G represents the kernel used for Gaussian blur.

[0023] Next, we will explain how to find B using this formula. First, consider the defocusing process for I0(x,y). If the image after defocusing is I1(x,y), then I1(x,y) = I0(x,y) × G(b). Here, b represents the amount of blur used in this defocusing process. Next, consider the differentiation of I1(x,y). From ∇I1(x,y) = ∇((CU(x) + D) × G(B) × G(b)), ∇I1(x,y) = C(2π(b 2 +B 2 )) -0.5 exp(-x 2 / 2(b 2 +B 2 )) The derivative of I0(x,y) is calculated in the same way, and the ratio of the absolute values of the two derivatives, ∇I0 / ∇I1, is ((b 2 +B 2 ) / B 2 ) -0.5 exp(-x 2 / 2B 2 +x 2 / 2(b 2 +B 2 )) where ∇I0 / ∇I1 is at its maximum value at x=0, and the ratio R at this maximum is ((b 2 +B 2 ) / B 2 ) -0.5 Therefore, it is clear that the value does not depend on the pixel coordinate x. In other words, B=b(R 2 -1) -0.5 If we know the amount of blur b of the defocus processing performed on I0(x,y) and R, we can calculate the amount of blur B that the original image had.

[0024] Therefore, the calculation processing unit 113 calculates the ratio of the differential values of the original initial SEM image and the defocused initial SEM image for each pixel, and obtains the ratio that is maximum near the boundary, thereby obtaining R. For example, a method can be used in which the average value obtained by maximum pooling a pixel size of 10 × 10 is defined as R. Furthermore, if the image has a large amount of noise and it is expected that the ratio of differential values near the boundary will not be maximum, it is sufficient to obtain only the differential values derived from the edges by combining edge enhancement processing or the like.

[0025] Furthermore, the differential value of the image does not necessarily have to be a value calculated by a differentiation operation. The amount of blur can be calculated in the same way as in the case of the differential value, using a sharpness ratio, which is a value proportional to the differential value of the image. For example, sharpness defined using a frequency analysis method such as Fourier transform or wavelet transform can be used.

[0026] This determines whether the calculated first image blur amount B1 is appropriate. In charged particle beam devices such as SEMs that perform observations at high magnifications, the blur amount often exceeds the image size. In this case, the differential value or sharpness may not be calculated appropriately, and the first image blur amount may also be calculated incorrectly. Therefore, if the first image blur amount is too large compared to the image size, the observation magnification is reduced until the first image blur amount B1 is smaller than the image size. This reduces the blur amount on the SEM image, allowing a more appropriate blur amount to be calculated than when the magnification is high. Conversely, if the observation magnification is reduced too much and the calculated first image blur amount B1 is extremely small, it is recommended to increase the observation magnification to increase the calculated blur amount. The appropriateness of the first image blur amount B1 is determined using the blur amount upper limit value 119 and blur amount lower limit value 120 input into the GUI 116 of the input display unit 115.

[0027] Furthermore, if the initial SEM image has depth when the magnification is reduced, it is advisable to estimate the first amount of image blur at the periphery of the field of view where the initial image was acquired when AF was started.

[0028] Next, a method of estimating the defocus amount D1 using the first blurred image amount B1 after the first image blur amount B1 has reached an appropriate value for the image size will be described.

[0029] FIG. 5 is a diagram showing an example of a flowchart for estimating the defocus amount.

[0030] First, the control unit 114 acquires observation conditions such as parameters related to the aperture angle α of the electron beam 110 (133). These parameters are sent to the calculation processing unit 113, which then calculates the correlation between the excitation current value and the second image blur amount B2 on the SEM image based on these parameters (134). For example, if the distance between the objective lens and the object plane is d1, the distance between the objective lens and the image plane is d2, the aperture angle of the electron beam at the object plane is α0, the observation magnification is m, and the defocus amount from the sample is D, the second image blur amount B2 is calculated as mDα0d1 / d2(i), where i represents the excitation current value.

[0031] Finally, the calculation processing unit 113 can calculate the defocus amount D1 as an excitation current value from the first image blur amount B1 by referring to the calculated second image blur amount B2 (135).

[0032] FIG. 6 is a diagram showing an example of a flowchart for determining the defocus direction.

[0033] Next, to determine the position where focus adjustment starts, it is necessary to determine the defocus direction. Because the calculated defocus amount D1 is an absolute value, it is not possible to determine whether the focal position is above (overfocus) or below (underfocus) the sample when AF starts. Therefore, it is necessary to determine the defocus direction, which corresponds to the sign of the defocus amount.

[0034] In order to determine the defocus direction, astigmatism is intentionally introduced. Normally, it is desirable to properly correct astigmatism during focus adjustment; if astigmatism remains, two just-focus positions will appear. These two just-focus positions are in focus in directions that are approximately perpendicular to each other. For example, one just-focus position is in focus vertically, while the other just-focus position is in focus horizontally. The midpoint between these two just-focus positions is the true just-focus position, and the focus positions change in directions that are approximately perpendicular to each other before and after the true just-focus position. In other words, the defocus direction can be estimated by determining the direction in which the focus is better when astigmatism is introduced.

[0035] Figure 7 shows the relationship between image sharpness in the vertical and horizontal directions and the defocus state before and after introducing astigmatism (vertical and horizontal directions). Figure 7 (a) shows the case without astigmatism, and (b) shows the case with astigmatism.

[0036] Sharpness is an index that indicates how clear an image is, and in this embodiment, it is defined so that sharpness is greatest at the position where the focus is best. When astigmatism is properly corrected, vertical and horizontal sharpness is greatest at the true just-focus position. On the other hand, when astigmatism is introduced, two focal points appear before and after the true just-focus position, and the midpoint between these two focal points is the true just-focus position.

[0037] It can also be seen that the increase and decrease in sharpness are reversed on the overfocus side and underfocus side. On the overfocus side, vertical sharpness increases and horizontal sharpness decreases, whereas on the underfocus side, horizontal sharpness increases and vertical sharpness decreases. The defocus direction can be determined by using this relationship. In this embodiment, astigmatism is introduced using the stigma coil 105 that corrects astigmatism, and if the increase in vertical sharpness before and after the introduction is greater than that in the horizontal direction, it is considered to be overfocus. If the increase in sharpness in the horizontal direction is greater than that in the vertical direction, it is determined that the image is underfocused, and the defocus direction is determined accordingly.

[0038] Furthermore, with regard to the settling time that was an issue when the excitation current value of the objective lens 106 was changed, the stigma coil that corrects astigmatism generally has fewer coil turns than the objective lens, resulting in a shorter settling time, so introducing astigmatism does not pose a settling time problem.

[0039] The defocus direction may also be determined by shifting the objective lens or the sample stage 108 toward the focal position and checking the increase or decrease in sharpness.

[0040] Based on the estimated defocus amount D1 and defocus direction, a calculation processing unit 113 calculates an excitation current value corresponding to a focal position that results in a just-in-focus position, and transmits the calculated value to a control unit 114. The control unit 114 changes the excitation current value to the transmitted value.

[0041] Next, focus adjustment is performed by searching for the just-in-focus position while finely changing the focus position. Fig. 8 is a diagram showing an example of a flowchart for focus adjustment.

[0042] First, the control unit 114 returns the observation magnification to the value at the start of AF. After returning the magnification, it estimates the defocus amount D2 and defocus direction using the same procedure as above. Based on these estimated results, it starts searching for just focus. For example, it repeatedly acquires SEM images and calculates the sharpness of the SEM images while continuously changing the focal position or excitation current value, and searches for the focal position at the excitation current value that maximizes sharpness as the just focus position. After searching, it changes the excitation current value to the one that results in the just focus position, and AF is completed.

[0043] According to the present invention described above in detail, the just focus search range can be adjusted according to the defocus amount at the start of AF, so that automatic focus adjustment can be performed quickly, accurately, and stably.

[0044] The present invention is not limited to the above-described embodiments and includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, some or all of the above-described configurations, calculation processing units, control units, etc. may be realized in hardware, for example, by designing them as integrated circuits. [Explanation of symbols]

[0045] 101...electron gun, 102...focusing lens, 103...diaphragm, 104...deflection coil, 105...stigma coil, 106...objective lens, 107...sample, 108...sample stage, 109...secondary electron detector, 110...electron beam, 111...secondary electrons, 112...image forming unit, 113...calculation processing unit, 114...control unit, 115...input display unit, 116...GUI of input display unit, 117...AF execution start button, 118...magnification step input field, 119...blur amount upper limit input field, 120...blur amount lower limit input field, 139...correlation between sharpness and focus position when there is no astigmatism, 140...correlation between sharpness and focus position when there is astigmatism

Claims

1. A charged particle beam device, a first image blur amount calculated from a change in sharpness by performing a defocus process on one captured image; and a second image blur amount calculated based on an imaging condition for the observation object; and determining a first excitation current value using the defocus amount, and starting focus adjustment for the observation object using the first excitation current value; A charged particle beam device characterized by:

2. 2. The charged particle beam device according to claim 1, changing a magnification of the object to be observed according to the value of the first amount of image blur, and repeating the calculation of the first amount of image blur; A charged particle beam device characterized by:

3. 2. The charged particle beam device according to claim 1, In addition to the defocus amount, a defocus direction is estimated by introducing astigmatism or shifting a focus position, the defocus amount and the defocus direction are used to determine the first excitation current value, and focus adjustment for the observation object is started using the first excitation current value. A charged particle beam device characterized by:

4. 2. The charged particle beam device according to claim 1, The electron gun includes an electron gun for irradiating an electron beam, a focusing lens for focusing the electron beam, an aperture, a deflection coil, a stigma coil, an objective lens, a sample stage, and a secondary electron detector. A charged particle beam device characterized by:

5. 5. The charged particle beam device according to claim 4, an image forming unit that forms an image based on an output of the secondary electron detector; a calculation processing unit that processes the image; an input display unit that displays outputs of the image forming unit and the calculation processing unit; and a control unit. A charged particle beam device characterized by:

6. 6. The charged particle beam device according to claim 5, the control unit controls the focusing lens based on the first excitation current value. A charged particle beam device characterized by:

7. 6. The charged particle beam device according to claim 5, The control unit changing a magnification of the object to be observed according to the value of the first amount of image blur, and performing control so as to repeatedly calculate the first amount of image blur; A charged particle beam device characterized by:

Images