Control method and charged particle beam device

Abstract

The purpose of the present disclosure is to provide technology whereby aberration can be properly evaluated in a charged particle beam device equipped with an aberration corrector. In a control method according to the present disclosure, a charged particle beam is scanned, and charged particles that are reflected from an optical element or pass through the optical element are detected after further passing through a beam-limiting aperture, and the aberration of the charged particle beam is evaluated on the basis of a feature amount of a projected image of an opening of the beam-limiting aperture (see fig. 8).

Description

Control method, charged particle beam device 【0001】 This disclosure relates to a technique for evaluating aberrations of optical elements in a charged particle beam apparatus. 【0002】 A charged particle beam apparatus is a device that performs processing such as obtaining an observation image of a sample by irradiating the sample with a charged particle beam. The performance of a charged particle beam apparatus is affected by the aberrations of the optical elements. 【0003】 Patent Document 1 discloses a method for measuring aberrations in a scanning transmission electron microscope (STEM). In this document, adjustments are made using aberration correctors and deflection coils so that features caused by aberrations appearing in the scanning transmission electron microscope image disappear. 【0004】 Patent document 2 discloses a charged particle beam apparatus equipped with a double mirror type aberration corrector (DMC). 【0005】 Japanese Patent Publication No. 2007-180013 (corresponding U.S. Patent USP 7,619,220) WO2018 / 016961 (corresponding U.S. Published Patent US10679,819) 【0006】 According to the aberration corrector disclosed in Patent Document 2, electron beam aberrations can be corrected by using an electrostatic mirror. However, it is not possible to determine an appropriate correction amount without knowing the degree of aberration. Patent Document 1 discloses a method for identifying aberrations based on the movement of shapes appearing on a STEM image. However, the method disclosed in Patent Document 1 requires acquiring numerous images with different beam irradiation conditions. Furthermore, it requires a thin sample through which electrons can pass, which is not usually effective with scanning electron microscopes or ion microscopes. 【0007】 This disclosure has been made in view of the above-mentioned problems, and aims to provide a technology that enables appropriate evaluation of aberrations in a charged particle beam apparatus equipped with an aberration corrector without requiring a transparent sample. 【0008】The control method according to this disclosure involves scanning a charged particle beam to detect charged particles that have been reflected from or passed through the optical element, and then passed through a beam limiting aperture. The method then evaluates the aberration of the charged particle beam based on the characteristic quantities of the projected image of the aperture of the beam limiting aperture. 【0009】 According to the control method described herein, aberrations can be appropriately evaluated in a charged particle beam apparatus equipped with an aberration corrector. Other issues, configurations, advantages, etc. will become clear from the following description of the embodiments. 【0010】 This is a schematic diagram illustrating an optical system including optical elements to be measured for aberration. This is a diagram showing a schematic configuration example of a scanning electron microscope 200 equipped with an aberration corrector. This is a diagram showing a specific configuration example of an aberration corrector 223. This is a diagram showing the configuration of the aberration corrector 223 illustrated in Figure 3 and a measuring device 401 that measures the aberration of an electrostatic mirror, which is part of the aberration corrector 223. This is a schematic diagram showing how the angle of the beam position on the beam shielding unit (electrode 402) changes with the scan angle. This shows the change in the SEM image (Pupil Scan; image when pupil scanning is performed) when the focus conditions of the charged particle beam are changed. This is a diagram explaining the trajectory of backscattered electrons, etc., emitted from the first electrostatic mirror 302 when beam scanning is performed on the first electrostatic mirror 302 using a Wien filter. This graph shows the relationship between the scanning angle when scanning the beam on the first electrostatic mirror 302 (optical element 102) using a Wien filter (scanning deflection unit 101) and the beam position (angle-converted value) when the beam reaches the aperture (conversion electrode 403) that restricts the passage of the beam. This flowchart illustrates an example of an operation sequence for automatically outputting a spherical aberration index value to a charged particle beam apparatus. This flowchart shows one example of a procedure for determining chromatic aberration. This graph shows the change in radius when the beam energy is changed. 【0011】In scanning probe systems such as scanning electron microscopes, the beam is focused using an electron lens. Such lenses have aberrations, which limit the reduction of the probe diameter at the focal point. In other words, aberrations are a factor that reduces image resolution. 【0012】 To suppress the occurrence of such aberrations and generate high-resolution images, it is desirable to measure the aberrations of optical elements (such as mirrors and lenses). More preferably, in order to selectively correct the aberrations of aberration correctors, it is desirable to accurately measure the aberrations caused by optical elements and perform aberration correction according to the measurement results. 【0013】 The following describes methods for measuring aberrations in a charged particle beam apparatus equipped with an electrostatic mirror aberration corrector, primarily for correcting chromatic aberration CC and spherical aberration C3. However, this disclosure is not limited to this, and the aberration measurement methods described later can also be applied to systems including other aberration correctors or systems that correct aberrations of optical elements other than lenses. 【0014】 In the case of mirror-type aberration correctors, the electron beam needs to be aligned with the ideal optical axis of the mirror. For example, if positive lens aberration is present, this aberration can be canceled out by adjusting the voltage applied to the electrodes that make up the mirror to generate negative mirror aberration (a transfer optical system can also be used as an option). 【0015】 The following describes a new method for qualitatively and quantitatively evaluating chromatic and spherical aberrations of mirrors, primarily through alignment and aberration measurements, even if the electron microscope column is not perfectly aligned. 【0016】 Figure 1 is a schematic diagram illustrating an optical system that includes optical elements to be measured for aberration. Figure 1 shows an example configuration in which multiple optical elements are arranged in the order through which a beam (or charged particle) for aberration measurement passes. 【0017】The optical system illustrated in Figure 1 comprises: an optical element 102; a scanning deflection unit 101 that scans the beam on or within the optical element 102; a beam shielding unit 103 having a beam limiting aperture 105 that partially restricts the passage of the beam that has passed through the optical element 102, or secondary electrons (SE) and backscattered electrons (BSE) emitted from the optical element 102; and a detection unit 104 that detects the beam or charged particles that have passed through the beam shielding unit 103. In the case of reflection by the optical element 102 (such as mirror electrons), the beam shielding unit 103 and the detection unit 104 may be positioned on the opposite side of the optical element 102, or on the scanning deflection unit 101. 【0018】 The scanning deflection unit 101 is, for example, a magnetic field deflector, an electric field deflector, or an electric field / magnetic field superposition deflector, and is configured to scan the beam over the pupil of the optical element 102. The optical element 102 is, for example, an electrostatic mirror type aberration corrector, an electrostatic lens, a magnetic field lens, a multipole type deflector, or a composite optical element combining these optical elements. 【0019】 The beam shielding unit 103 includes, for example, a beam limiting aperture 105 of known size and shape located at a predetermined distance from the optical element 102. The detection unit 104 is configured to detect electrons that have passed through the beam shielding unit 103. The detection unit 104 may include, for example, a scintillator and a photodetector that converts the light emitted by the scintillator into an electrical signal. In the optical system illustrated in Figure 1, the scanning plane relative to the optical element 102 is configured to image onto the beam limiting aperture 105. 【0020】Although not shown in the diagram, the system illustrated in Figure 1 includes a controller (one or more computer systems) that controls small changes (such as ±10%) in the beam acceleration voltage without significantly altering other beam control settings in order to evaluate chromatic aberration and the like. This one or more computer systems is configured to supply scanning signals to the scanning deflection unit 101, supply control signals to the optical element 102, and function as a controller or image processing device that processes detection signals output from the detection unit 104. 【0021】 The computer system, acting as an image processing device, is configured to image the detection signal in synchronization with the beam scanning of the aperture of the optical element 102. 【0022】 One or more computer systems include a storage medium that stores applications (at least one of which, such as calculation formulas (mathematical models), tables, and learning models) for determining the aberrations of optical elements based on detection signals, and one or more processors that calculate the aberrations according to these models, etc. The computer system functions as a data processing unit that determines the aberrations based on the setting conditions of the optical elements and deflectors, and the output data of the detector. 【0023】 The data processing unit and the like measure the intensity profile T(r,φ) of the so-called "pupil scan" of charged particles that have passed through the beam shielding unit 103 and reached the detection unit 104, as a function of the deflection signal and deflection width of the scanning deflection unit 101. The data processing unit and the like are also configured to determine the aberration index value and / or the beam aperture angle of the optical element based on the combination of the optical distance between the scanning surface of the optical element 102 and the beam limiting aperture 105, the aperture size of the beam limiting aperture 105, and the measured intensity profile T(r,φ). 【0024】<Example of Scanning Electron Microscope Configuration> Figure 2 shows a schematic example of the configuration of a scanning electron microscope (SEM) 200 equipped with an aberration corrector. An electron beam is extracted from the tip 201 by the extraction electrode 202 and accelerated by an accelerating electrode and accelerating tube (not shown). The accelerated electron beam passes through the ideal optical axis 203, is focused by a condenser lens 204, which is a form of focusing lens, and then deflected by a scanning deflector 205. As a result, the electron beam is scanned on the sample 209 in one or two dimensions. 【0025】 The electron beam incident on the sample 209 is decelerated by a negative voltage applied to electrodes built into the sample stage 208, and is focused by the lens action of the objective lens 206 before being irradiated onto the surface of the sample 209. The sample stage 208 is located inside the vacuum sample chamber 207. 【0026】 Electrons 210 (secondary electrons, backscattered electrons, etc.) are emitted from the irradiated area on the sample 209. The emitted electrons 210 are accelerated toward the tip (electron source) 201 by an acceleration effect based on a negative voltage applied to electrodes built into the sample stage 208. 【0027】 The accelerated electrons 210 collide with the conversion electrode 212, generating secondary electrons 211. The secondary electrons 211 emitted from the conversion electrode 212 are captured by the detector 213, and the output I of the detector 213 changes depending on the amount of secondary electrons captured. The brightness of the display device changes in response to this change in output I. For example, when forming a two-dimensional image, the deflection signal to the scanning deflector 205 and the output I of the detector 213 are synchronized to form an image of the scanning region. Note that the SEM illustrated in Figure 1 shows an example in which electrons 210 emitted from the sample 209 are converted into secondary electrons 211 at the conversion electrode 212 before detection, but the configuration is not limited to this. For example, a configuration in which the electron doubling tube and the detection surface of the detector are placed on the trajectory of the accelerated electrons may be adopted. 【0028】 The control device 214 supplies the necessary control signals to each optical element of the SEM according to an operating program called an imaging recipe for controlling the SEM. 【0029】 The signal detected by the detector 213 is converted into a digital signal by the A / D converter 217 and sent to the image processing unit 218. The image processing unit 218 generates an integrated image by integrating the signals obtained by a plurality of scans in terms of frames. 【0030】 The image obtained by one scan of the scanning area is called an image of one frame. For example, when integrating images of 8 frames, an integrated image is generated by adding and averaging the signals obtained by 8 two-dimensional scans in terms of pixels. It is also possible to scan the same scanning area multiple times, generate and store a plurality of images of one frame for each scan. 【0031】 Further, the image processing unit 218 includes an image memory 220 which is an image storage medium for temporarily storing digital images, and a CPU (Central Processing Unit) 219 which calculates feature amounts (dimension values such as the widths of lines and holes, roughness index values, index values indicating pattern shapes, area values of patterns, pixel positions serving as edge positions, etc.) from the images stored in the image memory 220. 【0032】 The scanning electron microscope 200 further includes a storage medium 221 for storing measurement values of each pattern, luminance values of each pixel, etc. Regarding the overall control of the scanning electron microscope 200, on the workstation 222, operations of necessary devices, confirmation of detection results, etc. can be realized by a graphical user interface. 【0033】 The image memory 220 is configured to store the output signal of the detector (a signal proportional to the amount of electrons emitted from the sample) at the corresponding address (x, y) on the memory in synchronization with the scanning signal supplied to the scanning deflector 205. Further, the image processing unit 218 also functions as an arithmetic processing device that generates a line profile from the luminance values stored in the image memory 220, specifies edge positions using a threshold method or the like, and measures the dimensions between the edges. 【0034】An SEM that measures dimensions based on such line profile acquisition is called a CD-SEM (Critical Dimension SEM) and is used to measure various feature amounts in addition to measuring the line width of semiconductor circuits. For example, irregularities called line edge roughness (LER) exist at the edges of circuit patterns and are factors that change circuit performance. The CD-SEM can be used to measure LER. 【0035】 In the present embodiment, a scanning electron microscope, which is a type of charged particle beam device, is taken as an example. However, the aberration corrector described below can also be applied to other charged particle beam devices such as ion microscopes that irradiate liquid metals such as hydrogen ions, helium ions, or gallium, or focused ion beam (FIB: Focused Ion Beam) devices, other than scanning electron microscopes. 【0036】 <Configuration Example and Operation Outline of Aberration Corrector> An aberration corrector 223 for suppressing aberrations generated by optical elements such as lenses is provided in the scanning electron microscope 200. The aberration corrector 223 includes the following: a magnetic field deflector 224 for deflecting (off-axis) an electron beam from the ideal optical axis 203 of the scanning electron microscope 200; an aberration correction unit 226 for correcting the aberrations of the off-axis electron beam; a magnetic field deflector 225 for deflecting the trajectory of the electron beam emitted from the aberration correction unit 226 so as to coincide with the ideal optical axis 203 of the electron beam; and a measurement device 401 provided for measuring aberrations. 【0037】 FIG. 3 is a diagram showing a specific configuration example of the aberration corrector 223. The aberration correction unit 226 includes a double mirror aberration corrector. In the double mirror aberration corrector, a first electrostatic mirror 302 and a second electrostatic mirror 303 are arranged with a predetermined intermediate space therebetween. 【0038】The electron beam incident on the magnetic field deflector 224 located within the aberration corrector 223 is deflected in direction a by the magnetic field B1 of the magnetic field deflector 224 and incident on the orthogonal electromagnetic field generation unit 301. The electron beam incident on the orthogonal electromagnetic field generation unit 301 is deflected in direction b by the deflecting electromagnetic field generated by the orthogonal electromagnetic field generation unit 301 and heads towards the first electrostatic mirror 302. 【0039】 The first electrostatic mirror 302 is biased to a potential equivalent to the incoming energy of the electron beam. Therefore, electrons are decelerated by the first electrostatic mirror 302 and reflected by an equipotential surface where the electrons' kinetic energy reaches zero. Thus, the equipotential surface where the kinetic energy of individual electrons reaches zero provides a reflection surface for these electrons. By curving this equipotential surface, negative spherical aberration and chromatic aberration can be introduced into the backscattered electron beam, which is arranged to at least partially compensate for the aberrations of the electron microscope lens. 【0040】 The reflected electron beam, reflected in direction c by the first electrostatic mirror 302, passes through the orthogonal electromagnetic field generation unit 301 and heads toward the second electrostatic mirror 303. The orthogonal electromagnetic field generation unit 301 is configured to have its deflection conditions adjusted so as not to deflect the trajectory of the reflected electron beam reflected by the first electrostatic mirror 302. Specifically, in the orthogonal electromagnetic field generation unit 301 illustrated in Figure 3, the deflection conditions of each deflector are adjusted so that the deflection effect of the upper and lower electrostatic deflectors 307 and 308 cancels out the deflection effect of the magnetic field deflector 309 placed between the electrostatic deflectors 307 and 308. Therefore, directions c and d are the same. 【0041】 The electron beam that has passed through the orthogonal electromagnetic field generation unit 301 is reflected in direction e by the second electrostatic mirror 303 and re-enters the orthogonal electromagnetic field generation unit 301. The aberration correction unit 226 functions as an aberration corrector by generating an aberration that compensates for the aberration of the electron beam by having at least one of the first electrostatic mirror 302 and the second electrostatic mirror 303 generate an aberration. 【0042】The electron beam, reflected by the second electrostatic mirror 303 and incident on the orthogonal electromagnetic field generation unit 301, is deflected in direction f by the orthogonal electromagnetic field generation unit 301 and directed toward the ideal optical axis 203 of the scanning electron microscope. A magnetic field deflector 225 is positioned at the intersection of the electron beam trajectory and the ideal optical axis 203. The magnetic field B3 generated by the magnetic field deflector 225 has a predetermined energy and is adjusted to deflect the electron beam, which is directed toward direction f, toward the ideal optical axis 203. The magnetic field deflector 225 functions to return the electron beam, whose aberrations have been corrected by the aberration correction unit 226, back toward the ideal optical axis 203 of the scanning electron microscope. 【0043】 Lenses 304 and 305 refract the incident beams b and d to reach the correct beam diameter at the first electrostatic mirrors 302 and 303, and adjust the amount of negative aberration. Next, the mirror voltage is adjusted to refocus the beam to the crossover 321. The aberrations of the beam incident on each electrostatic mirror are corrected by reflection by an electric field with a potential distribution that cancels out the aberrations on the electrostatic mirror. 【0044】 The orthogonal electromagnetic field generation unit 301 includes an aperture 310 through which the electron beam passes when aberration correction is performed using the aberration correction unit 226, and an aperture 311 through which the electron beam passes when the aberration correction unit 226 is not used (no aberration correction is performed). The control device 214 switches between (a) turning on the aberration correction function by supplying a predetermined voltage or current to the magnetic field deflectors 224 and 225 and the aberration correction unit 226, and (b) turning off the aberration correction function by not supplying current to the magnetic field deflectors 224 and 225, thereby using the two apertures. In Figure 3, an example is shown in which the beam is deflected off-axis using the two magnetic field deflectors 224 and 225, and the beam incident from off-axis is deflected so that it passes through the optical axis. However, the beam may also be deflected using an electrostatic deflector or a Wien filter. 【0045】The electrostatic deflectors 307 and 308 include electrodes, and the magnetic field deflector 309 includes a magnetic pole. The orthogonal electromagnetic field generation unit 301 illustrated in Figure 3 is an E-B-E unit in which an electrode (E), a magnetic pole (B), and an electrode (E) are arranged in that order from the electron source (chip 201) side. A spacer capable of maintaining a predetermined distance between the electrode and the magnetic pole may be placed between them. 【0046】 The electrodes constituting the electrostatic deflectors 307 and 308 include multiple pairs of electrodes for generating electric fields E1 and E2. The magnetic poles constituting the magnetic field deflector 309 are configured to generate a magnetic field B2. The E-B-E unit illustrated in Figure 3 enables the aforementioned deflection (deflection from orbit a to b and deflection from orbit e to f) without deflecting the electron orbits (orbits c to d) traveling from the first electrostatic mirror 302 to the second electrostatic mirror 303, by generating electric fields E1 and E2 and a magnetic field B2. 【0047】 <Configuration of Aberration Corrector and Aberration Measurement Device> Figure 4A shows the configuration of the aberration corrector 223 illustrated in Figure 3 and the measurement device 401 which measures the aberration of the electrostatic mirror, which is part of the aberration corrector 223. In Figure 4A, the same components as in Figure 3 are given the same part numbers. The electrostatic deflectors 307, 308 and the magnetic field deflector 309 each include at least one pair of electrodes and one pair of magnetic poles, and these electrodes and magnetic poles constitute a Wien filter. The Wien filter corresponds to the scanning deflection unit 101 in Figure 1. That is, the Wien filter is configured to scan the beam on the first electrostatic mirror 302 while satisfying the Wien condition, and to deflect the trajectory of the reflected electrons from the first electrostatic mirror in the direction of h in the figure. The first electrostatic mirror 302 corresponds to the optical element 102 in Figure 1. 【0048】Charged particles deflected in the direction h in Figure 4A by the Wien filter (electrostatic deflector 307, electrostatic deflector 308, magnetic field deflector 309) are further deflected by the magnetic field deflector 224 toward the optical axis of the electron beam (dotted line in the figure), and the trajectory of the deflected charged particles becomes i in Figure 4A. Electrode 402 is an aperture plate in the beam shielding unit 103 and corresponds to the beam limiting aperture 105 in Figure 1. Reflected electrons deflected toward trajectory i collide with the conversion electrode 403, and the conversion electrode 403 generates secondary electrons j. The generated secondary electrons j are captured by the detector 404, converted into an electrical signal, and then transmitted to the image processing unit 218, which consists of one or more computers. Detector 404 corresponds to the detection unit 104 in Figure 1. 【0049】 In the example shown in Figure 4A, a method was described in which reflected electrons and accelerated secondary electrons are first converted into secondary electrons j by the conversion electrode 403 and then detected. However, the method is not limited to this, and a detector that directly detects electrons may be placed at the position of the conversion electrode 403. 【0050】 <Spherical Aberration Measurement Method> As illustrated in Figure 1, when the optical element 102 (e.g., a lens) accurately deflects the beam, it passes through the aperture regardless of the beam's deflection direction (trajectories 107, 108, and 109 in the example of Figure 1), and the detection unit 104 detects the beam that has reached it, so the 2D image becomes brighter at all pixels. 【0051】 If a deflection occurs depending on the position of the optical element due to some kind of aberration (such as astigmatism, coma aberration, or spherical aberration), the detection unit 104 detects a signal only if the deflection due to the aberration of the optical element is corrected by the lens deflection caused by the lens misfocus. For example, spherical aberration causes a deflection proportional to the cube of the distance to the axis, and misfocus causes a deflection proportional to the distance to the axis, so there is only one distance to the axis at which both deflections are opposite. 【0052】 For simplicity, we will explain the aberrations caused by mirrors by considering only spherical aberration and focus shift. The aberration r is given by r = C １ α + C ３ α ３It can be expressed as (α > 0). C １ is the focus deviation, C ３ represents the spherical aberration coefficient, and α is the beam angle. A method for determining whether negative spherical aberration (C ３ < 0) has occurred will be described. In the case of under-focus (C １ < 0), r becomes negative C ３ has both positive and negative solutions depending on the value of C １ . For just focus (C １ = 0), r becomes negative only when C ３ is negative. In the case of over-focus (C １ > 0), r can be either positive or negative depending on the magnitude of C ３ . By over-focusing, the change in aberration r can be observed. Here, it should be noted that the aberration that cancels the aberration of the objective lens is called negative spherical aberration. 【0053】 Fig. 4B is a schematic diagram showing how the angular conversion value of the beam position on the beam limiting aperture 105 (electrode 402) changes with the scan angle. The angular conversion value of the beam position can be expressed as r* = r / D using the aberration r. In the cases of under-focus and just focus, the beam passing position changes monotonically, but in the case of over-focus, there appears a range where the beam cannot pass through the aperture of the upper Einzel lens (TEL) depending on the scan angle. By observing this range in the scan image, spherical aberration can be observed. 【0054】 Fig. 5 is a diagram showing an example of an image generated based on the charged particles detected by the detection unit 104 (in the example of Fig. 5, since an electron microscope is used, it is an electron microscope image (SEM image)). The detection unit 104 is configured to detect charged particles that have passed through the aperture of the beam limiting aperture 105. Since the charged particle beam 106 irradiated onto the beam limiting aperture 105 is two-dimensionally scanned by the scanning deflection unit 101, the image generated based on the output of the detection unit 104 is a projection image of the beam limiting aperture 105. 【0055】Figure 5 shows the changes in the SEM image when the focusing conditions of the charged particle beam are changed. Focusing conditions include, for example, the acceleration voltage of the electron beam (Vacc) and the voltage applied to the optical element (V). ＭＦ This is done by adjusting the following. To change the focus conditions, etc., the control parameters of other optical elements may also be changed. 【0056】 If negative spherical aberration occurs in the optical element 102 (e.g., a mirror), a ring-shaped projection image can be observed on the overfocus side. Based on this phenomenon, the aberration may be determined from the images obtained when the focus conditions are varied. For example, one or more computer systems may be configured to extract features of the projection image from the image obtained based on the output of the detection unit 104, and to evaluate the aberration and its degree from the extracted features. 【0057】 One or more computer systems (such as an image processing device 406) may be configured to read an image evaluation program from a predetermined storage medium for evaluating an image obtained when the beam condition is shifted to overfocus, for example, and to evaluate the acquired image. For example, it may be configured to detect a ring-shaped pattern by image recognition and identify the occurrence of spherical aberration based on the detection. 【0058】 Figure 6 illustrates the trajectories of backscattered electrons and the like emitted from the first electrostatic mirror 302 when beam scanning is performed with a Wien filter on the first electrostatic mirror 302. The left side of Figure 6 shows the electron trajectories cs, hs, and is emitted from the first electrostatic mirror 302 when there is no aberration, and the right side of Figure 6 shows the electron trajectories cl and hl emitted from the first electrostatic mirror 302 when negative spherical aberration exists in the first electrostatic mirror 302. 【0059】As illustrated in the left side of Figure 6, if the optical element (electrostatic mirror) has no aberrations, regardless of the beam irradiation position 601, 602 (scanning position), the electrons emitted from the mirror follow almost the same trajectory as the electrons incident on the mirror and return in the direction of incidence. On the other hand, if the optical element has, for example, negative spherical aberration, the electrons emitted from the mirror are deflected due to the effect of that aberration. The right side of Figure 6 shows a state in which the electrons emitted from the mirror are deflected by an angle δα compared to the case without aberrations. The relationship between such an angle δα and spherical aberration Cs can be defined as shown in Equation 1. 【0060】 【0061】 Similar equations can be derived for other geometric aberrations (e.g., astigmatism and coma aberration) in relation to the angle δα and each geometric aberration. Furthermore, combined aberrations can be represented using pupil scan images. By using through-focus, a series of pupil scan images can be acquired to improve the accuracy of each geometric aberration. 【0062】 Z in Equation 1 ｃｃｐ This is the position of the common crossover, which coincides with the magnetic field deflector 309 in Figure 3. First, the crossover will be explained with reference to Figure 3. The mirror corrector is located in a chamber surrounded by the upper electrode (electrode 402) and the lower Einzel lens (BEL, not shown). The upper Einzel lens creates a crossover 322 on the optical axis 203. The crossover is the point where the electron beam is narrowest. The lower Einzel lens shifts this crossover to the objective lens surface. An electron microscope equipped with a mirror corrector has two optical axes: the optical axis 203 of the electron microscope and the optical axis 320 of the mirror corrector. If the focal length of the mirror corrector is 2f, the height of the electron microscope's crossover 322 and the height of the mirror's crossover 321 are made to match by placing the first electrostatic mirror 302 at a position 2f above the same height as the crossover 322 and the second electrostatic mirror 303 at a position 2f below. Since these are common crossovers between optical axes 203 and 320, this is called a common crossover. Ｍ δα is the focal position of the mirror (a position that changes according to the mirror voltage) and is a device-specific constant. δα in Equation 1 can be obtained from Equation 2. 【0063】 【0064】 α in Equation 2 ＴＥＬ is the angle of view of the upper Einzel lens, and α ｃｃｐ This is the beam scanning angle at the common crossover position. 【0065】 α in Equation 1 (= α ｐ ) is the radius value of the high-brightness region when in focus, as illustrated in Figure 5, and one or more computers, based on image processing and mathematical models, etc., α ｐ It is configured to derive the following. 【0066】 One or more computers installed in or configured to receive the output of an electron microscope derive the spherical aberration Cs using mathematical models, such as those exemplified by Equations 1 and 2, which are stored in a predetermined storage medium. 【0067】 Furthermore, instead of using a mathematical model that includes calculation formulas, a database model or a learning model labeled with aberration index values ​​for image information may be used. In addition, although this embodiment describes an example of determining spherical aberration Cs based on the radius of the high-luminance region when in focus, it is not limited to this, and the radius value when in focus may be estimated by extrapolation from the radius values ​​under two different focus conditions, and spherical aberration Cs may be determined based on this estimated value. 【0068】 Furthermore, instead of determining the spherical aberration Cs, a model may be prepared that defines the relationship between the control signal given to the aberration corrector and the radius value, and the control signal may be determined from this model. Alternatively, a model may be used to determine the degree of aberration (e.g., large, medium, small). If the information obtained by the detection unit 104 is related to aberration, a model may be prepared that defines the relationship between that information and the aberration index value (including the aberration itself, the relationship between the magnitudes of aberrations, and the control signal that changes depending on the degree of aberration). 【0069】Figure 7 is a graph showing the relationship between the scanning angle when scanning a beam on the first electrostatic mirror 302 (optical element 102) using a Wien filter (scanning deflection unit 101) and the beam position (angle-converted value) when the beam reaches the aperture (conversion electrode 403) that restricts the passage of the beam. The curves 701, 702, and 703 illustrated in Figure 7 show the relationship between the scanning angle and the beam's arrival position when overfocused, just focused, and underfocused, respectively. 【0070】 Figure 7 shows the relationship between two parameters when the optical element 102 has negative spherical aberration. When the optical element 102 has negative spherical aberration (curve 701), the beam passes through the aperture in each of the scanning angle ranges 704, 705, and 706 and is detected by the detection unit 104. That is, a signal is detected when the beam is deflected to the negative side from the scanning center (scanning angle zero) (scanning angle range 704), at the scanning center (scanning angle range 705), and when the beam is deflected to the positive side from the scanning center (scanning angle range 706). As a result, when there is negative spherical aberration, an image 503 with a high-brightness region at the ring 501 and center point 502 is formed based on the output signal from the detection unit 104. 【0071】 As described above, by utilizing the phenomenon in which the pupil scanning image formed based on the output of the detection unit 104 changes depending on the degree and type of aberration occurring in the optical element, it becomes possible to evaluate the type and degree of aberration. In other words, by scanning a beam over an optical element using a scanning deflector, detecting charged particles that pass through or are reflected by the optical element after passing them through a beam-passage limiting aperture, and generating an image based on this detection, it becomes possible to evaluate the type and degree of aberration. 【0072】Figure 8 is a flowchart illustrating an operational sequence for automatically outputting spherical aberration index values ​​from a charged particle beam apparatus. The flowchart in Figure 8 shows a focus adjustment sequence for acquiring an in-focus image based on image evaluation in order to derive the index value using a mathematical model as illustrated in Equation 1. Unlike focusing to bring the image into focus, it is necessary to evaluate the shape of the projected image to determine whether or not it is in focus, so in the image evaluation step, for example, the dimensions and shape of the high-brightness region are evaluated. 【0073】 One or more computer systems (e.g., an image processing unit 218 and a workstation 222) provided in a scanning electron microscope 200 as illustrated in Figure 2 generate an image based on the output of the measurement device 401 (S801). At this time, the first electrostatic mirror 302 has initial conditions set, and under these conditions, it generates an image based on beam scanning of the first electrostatic mirror 302 using electrostatic deflectors 307, 308 and a magnetic field deflector 309. Next, one or more computer systems perform an evaluation of the acquired image (S802). In S802, for example, the dimensions of the high-brightness portion of the projected image of the aperture of the electrode 402 are measured. 【0074】 As illustrated in Figure 5, the pattern that appears on the projected image changes depending on the focus conditions and the resulting aberrations. Therefore, by identifying the shape and size (feature quantities) of the pattern using image recognition or other methods, and inputting information such as shape and size into a database or other model that defines the relationship between the identified result and the focus state, the focus state can be determined. 【0075】If the focus state is determined to be overfocus or underfocus, focus adjustment (S803) is performed. In the flowchart illustrated in Figure 8, if the result of the image evaluation is in focus, a spherical aberration index value is output based on the input of information such as the shape and size of the pattern into the model, etc. (S804, S805). Alternatively, in S803, the focus may be deliberately changed to the overfocus or underfocus side, and the type of aberration may be determined from the image information at that time. As mentioned above, if the optical element has negative spherical aberration, a ring-shaped pattern is formed, so such a pattern image may be stored in advance as reference information, and the type of aberration may be identified by calculating the degree of agreement using pattern matching and threshold determination. 【0076】 With a charged particle beam apparatus equipped with an operating program as illustrated in Figure 8, it becomes possible to automatically determine the index value of spherical aberration. 【0077】 <Method for Measuring Chromatic Aberration> Next, we will explain the method for measuring chromatic aberration. The same apparatus as exemplified in Figure 4 can be used for measuring chromatic aberration. Chromatic aberration Cc can be approximated using the defocus amount df as shown in Equation 3. 【0078】 【0079】 Vacc is the energy of the beam passing through the optical element, and ΔVacc is the beam energy fluctuation caused by changing the applied voltage to the optical element, etc. k is a device-specific constant. The defocus amount df is the focus fluctuation amount that occurs with changes in beam energy, etc. 【0080】 If optical elements such as the first electrostatic mirror 302 or lenses, as illustrated in Figure 4, have chromatic aberration Cc, the focus shifts due to changes in beam energy, as can be understood from Equation 3. Therefore, if the amount of focus fluctuation that occurs in response to energy fluctuations can be measured, the chromatic aberration Cc can be derived. The focus shift may be normalized as a function of the voltage applied to the electrostatic mirror (or the difference before and after the change in applied voltage) and then expressed as the amount of defocus. 【0081】 Equation 4 is given by defocus amount df and radius α. ｐ , and the change in radius (angle) due to defocusing dα １ This shows the relationship. a is a constant specific to the device. Based on equations 3 and 4, and parameters obtained by measurement, the chromatic aberration Cc can be determined. 【0082】 【0083】 Figure 9 is a flowchart showing an example of a procedure for determining chromatic aberration. Each step is performed by one or more computer systems (for example, an image processing unit 218 and a workstation 222). 【0084】 One or more computer systems first set the optical element 102 to its initial state (for example, a state in which the defocus amount of the electrostatic mirror is adjusted to zero), and then use the scanning deflection unit 101 to scan the beam on or within the optical element and acquire a projection image of the beam limiting aperture 105 (S901). Next, the high-brightness portion of the projection image is measured, and the radius α at that time is measured. ｐ (=α ｐ１ ) is derived (S902). 【0085】 Next, the applied voltage to the electrostatic mirror is set, for example, to obtain a beam energy that is at least one different from the initial state (S903). At this time, other parameters may be changed to change the focus by a predetermined or arbitrary amount of defocus. After changing the beam energy, the radius α is set in that state. ｐ２ Measure (S904). 【0086】 By following the procedure described above, radius data for different beam energies can be obtained, and from this radius data, the defocus amount can be calculated as a function of beam energy. From the radius before and after the change in beam energy, dα¹ (= |α ｐ２ -α ｐ１ The value of | can be determined (S905). From the obtained dα1 and equations 3 and 4, the chromatic aberration Cc can be determined (S906). 【0087】Figure 10 is a graph showing the change in radius when the beam energy is changed. (Breath energy change rate, defocus amount df, radius α) ｐ The value changes as shown in Figure 10 according to equations 3 and 4. Based on this change, the chromatic aberration Cc can be determined using the flowchart in Figure 9. 【0088】 The flowchart in Figure 9 illustrates a process for deriving chromatic aberration Cc based on the control of an electron beam apparatus by one or more computer systems and the acquired images. However, instead of calculating the chromatic aberration value, the presence of chromatic aberration may simply be displayed on a display device provided with the electron beam apparatus. Furthermore, one or more computers may derive control signals for aberration correctors in conjunction with, or instead of, information regarding chromatic aberration. By pre-storing mathematical models, tables, learning models, etc., containing the control signals for aberration correctors or related parameters in a predetermined storage medium, such control can be performed automatically. In addition to the flowchart for calculating spherical aberration, the flowchart in Figure 9 can also be used to further calculate chromatic aberration by changing the initial energy of the charged particle beam. 【0089】 <Regarding Variations of the Disclosure> This disclosure is not limited to the embodiments described above and includes various variations. For example, the embodiments described above are described in detail for the purpose of explaining this disclosure clearly and do not necessarily have all the configurations described. Also, parts of one embodiment can be replaced with the configuration of another embodiment. Also, configurations of other embodiments can be added to the configuration of one embodiment. Also, parts of the configuration of each embodiment can be added, deleted, or replaced with parts of the configuration of other embodiments. 【0090】In the embodiments described above, one or more computer systems have been shown to calculate the spherical aberration or chromatic aberration of the optical element 102 (aberration corrector 223). However, instead of or in addition to this, the angle δα may be calculated, or a control signal for controlling the optical element 102 may be calculated. For example, instead of calculating the aberration itself, a control signal for controlling the optical element 102 to suppress the aberration may be calculated. In any case, these calculations are performed while scanning the irradiation position of the charged particle beam or controlling the beam energy, so one or more computer systems perform these calculations in the process of controlling the scanning electron microscope 200. 【0091】 In the embodiments described above, the control device 214 and the image processing unit 218 can be configured by hardware such as circuit devices that implement their functions, or by software that implements their functions being executed by a computing device such as a CPU (Central Processing Unit). 【0092】 In the embodiments described above, a scanning electron microscope 200 was described as an example of a charged particle beam apparatus, but the disclosure is not limited thereto and can also be applied to other charged particle beams, such as ion beam apparatuses. 【0093】 200: Scanning electron microscope 218: Image processing unit 222: Workstation 223: Aberration corrector

Claims

A method for controlling a charged particle beam apparatus equipped with optical elements, A step of scanning the irradiation position of the charged particle beam onto the optical element, A step of detecting charged particles after they have been reflected from or passed through the optical element by scanning the charged particle beam, and have subsequently passed through a beam limiting aperture. A step of obtaining a projection image of the opening of the beam limiting aperture based on the detected charged particles, A step of calculating at least one of the following based on the feature quantities of the projection image: a control signal to control the optical element to compensate for the aberration of the charged particle beam, a value that characterizes or quantifies the aberration of the charged particle beam, or the difference between the incident angle and the reflection angle of the charged particles reflected from the optical element. A control method characterized by having the following features. 　 The optical element comprises a first mirror and a second mirror arranged to face each other along a second optical axis parallel to the first optical axis of the charged particle beam, The control method further includes the step of deflecting the charged particle beam to irradiate the first mirror, In the step of scanning the charged particle beam, the irradiation position of the charged particle beam with respect to the first mirror is scanned, and the charged particle beam reflected from the first mirror is deflected to irradiate the beam limiting aperture. The control method according to claim 1, characterized by the features described above. 　 In the calculation step described above, the focus shift of the charged particle beam is determined based on at least one of the size or shape of the projected image, and the focus of the charged particle beam is adjusted to suppress the focus shift. The control method according to claim 1, characterized by the features described above. 　 The optical element is configured to operate as an aberration corrector by generating an aberration that compensates for the aberration of the charged particle beam in at least one of the first mirror and the second mirror, which are arranged to face each other along a second optical axis parallel to the first optical axis of the charged particle beam. In the calculation step described above, the spherical aberration and / or geometric aberration of the optical element are calculated based on the diameter of the high-luminance region that appears in the projected image. The control method according to claim 1, characterized by the features described above. 　 In the calculation step described above, the focus misalignment of the charged particle beam is determined based on at least one of the size of the projected image or the shape of the projected image, and the focus of the charged particle beam is adjusted so that the focus misalignment is less than a threshold. In the calculation step, the spherical aberration is calculated based on the diameter and / or the shape of the high-luminance region when the focus shift is less than the threshold. The control method according to claim 4, characterized by the features described above. 　 The optical element is configured such that the position in the height direction along the first optical axis of the crossover point where the charged particle beam is most narrowly focused on the first optical axis, and the position in the height direction along the second optical axis of the crossover point where the charged particle beam is most narrowly focused on the second optical axis, coincide with each other at the common crossover position. In the calculation step described above, the spherical aberration is calculated based on the common crossover position, the focal positions of the first and second mirrors, and the deflection angle at which the charged particle beam reflected by the first mirror is deflected by the spherical aberration. The control method according to claim 4, characterized by the features described above. 　 In the calculation step described above, the spherical aberration is calculated by inputting the measurement result of the diameter into a model that defines the relationship between the diameter and the spherical aberration. The control method according to claim 4, characterized by the features described above. 　 In the calculation step described above, the focus shift of the charged particle beam is determined based on at least one of the size of the projected image or the shape of the projected image. In the calculation step described above, the type and degree of the aberration are evaluated based on the shape that appears in the projected image when the charged particle beam is overfocused. The control method according to claim 1, characterized by the features described above. 　 In the calculation step described above, it is determined whether or not negative spherical aberration occurs in the optical element based on the shape of the projected image. In the calculation step described above, if negative spherical aberration occurs, the negative spherical aberration is calculated based on the diameter of the high-luminance region that appears in the projected image when the charged particle beam is overfocused. The control method according to claim 8, characterized in that it is a control method. 　 In the calculation step described above, the amount of defocusing of the optical element is changed, and the amount of change in the radius of the projected image before and after the change is measured. In the calculation step described above, the chromatic aberration of the optical element is calculated using the defocus amount, the change amount, and the rate of change of the beam energy of the charged particle beam. The control method according to claim 1, characterized by the features described above. 　 The optical element is composed of at least one of an electrostatic mirror, a lens, or a multipole. The control method according to claim 1, characterized by the features described above. 　 In the calculation step described above, a mathematical model is used to calculate at least one of the following: the focus shift, the astigmatism, the coma aberration, and the spherical aberration, using a mathematical model that describes the relationship between the combined deflection field generated by a combination of two or more deflectors that deflect the charged particle beam and at least one of the following: the focus shift, the astigmatism, the coma aberration, and the spherical aberration, which occur in the charged particle beam deflected by the combined deflection field. The control method according to claim 1, characterized by the features described above. 　 A charged particle beam apparatus equipped with optical elements, A scanning deflection unit that scans the irradiation position of the charged particle beam onto the optical element, A detection unit that detects charged particles after they have been reflected from or passed through the optical element by scanning the charged particle beam, and have subsequently passed through a beam limiting aperture. A computer system that controls the optical element, Equipped with, The aforementioned computer system, A step of obtaining a projection image of the opening of the beam limiting aperture based on the detected charged particles, A step of calculating at least one of the following based on the feature quantities of the projection image: a control signal to control the optical element to compensate for the aberration of the charged particle beam, or the aberration of the charged particle beam, or the difference between the incident angle and the reflection angle of the charged particles reflected from the optical element. Implement A charged particle beam apparatus characterized by the following features.

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