JP5499282B2 - Aberration correction method and aberration correction apparatus in scanning transmission electron microscope - Google Patents

Description

  The present invention relates to an aberration correction method and an aberration correction apparatus in a scanning transmission electron microscope.

  A scanning transmission electron microscope (STEM) scans a sample with a converged electron beam and maps the intensity of detection signals from transmitted or scattered electrons from the sample in synchronization with this scanning. It is an electron microscope which obtains an image, a STEM image. In recent years, the scanning transmission electron microscope has attracted attention as an electron microscope capable of obtaining a very high spatial resolution at the atomic level. Since this spatial resolution depends on the beam diameter of the electron beam incident on the sample, it is very important to reduce aberrations in order to achieve high resolution. As a direct means for reducing aberration, an aberration corrector such as a quadrupole or hexapole is used.

  Further, in order to obtain a high spatial resolution in a short time during observation, it is effective to calculate aberrations occurring at that time and grasp them in advance. As a calculation method, Patent Document 1 discloses that an incident electron beam is shifted by a known distance in two directions orthogonal to each other in a sample plane, and aberrations are corrected by using Ronchigrams acquired before and after that. A method is disclosed. Specifically, each Ronchigram is divided into a plurality of small regions, and the amount of movement (that is, the magnification) of the incident electron beam before and after each shift is calculated using a cross-correlation function for a specific structure in each region. Since two movement amounts along the x and y directions on the sample surface are obtained from the movement amount for each shift, four movement amount components are calculated for each small region. These movement amounts correspond to the second partial differentiation of the aberration function χ with respect to the convergence angle and azimuth angle of the electron beam that forms the image of each small region. Therefore, the partial differential is fitted twice using the least square method for the obtained plurality of movement amounts, and each aberration coefficient is obtained therefrom. Each aberration is reduced by controlling the aberration corrector using this aberration coefficient.

  As another aberration calculation method, Patent Document 1 divides a single Ronchigram into small regions and obtains a plurality of diffractograms by executing Fourier transform for each region. Discloses a method of calculating an aberration coefficient from the above. The positions of the light and dark rings appearing in each diffractogram correspond to the double partial differentiation of the aberration function χ with respect to the convergence angle and azimuth angle of the electron beam, so by performing the same fitting as above, each aberration coefficient can be calculated from these. Seeking.

  As yet another aberration calculation method, Patent Document 1 shows an aberration calculation method using minute detectors arranged at different positions. In this case, the movement amount of each image is calculated with respect to the bright field image obtained from each detector. When the position of each detector is associated with the convergence angle and azimuth angle of the incident electron beam, the amount of movement of each image corresponds to one-time differentiation of the aberration function χ with respect to the convergence angle or azimuth angle. Accordingly, the above-described one-time differentiation is fitted to the obtained plurality of movement amounts using the least square method, and each aberration coefficient is obtained therefrom.

US Pat. No. 6,552,340

  The aberration correction method using the Ronchigram of Patent Document 1 requires an electro-optical conversion element such as a scintillator for observing the Ronchigram and an imaging device such as a CCD camera. However, in a scanning transmission electron microscope, a detector for detecting electrons that form a bright field image and a dark field image is basically fixed in a lens barrel. Therefore, in order to observe the Ronchigram, the electron-light conversion element must be installed in a retractable manner in front of the detector. In addition, since the bright-field image and the dark-field image are newly formed by the subsequent detector after the electro-optical conversion element is retracted, a time delay occurs after aberration correction. In high-resolution observation at the atomic level, aberrations may change due to changes over time such as electron beam drift, so the above time delay cannot be ignored. Further, the imaging device is very expensive, and the manufacturing cost increases.

  Further, since the Ronchigram is observed, aberration calculation cannot be executed during high-resolution observation. Also from this point, a time delay occurs after aberration correction.

  An object of the present invention is to provide an aberration correction method and an aberration correction apparatus that minimizes a time delay accompanied by a change in aberration and has high accuracy in calculating an aberration coefficient in high-resolution observation.

  A first aspect of the present invention is an aberration correction method for a scanning transmission electron microscope having an aberration corrector, the step of causing an electron beam to enter a detector having a plurality of detection surfaces, and a dark field by the electron beam. Simultaneously capturing a bright field image including angle information of the electron beam for each image and the detection surface; calculating an aberration coefficient from the plurality of bright field images using the dark field image as a position reference of an observation image; And a step of controlling the aberration corrector so as to reduce aberration based on the calculated aberration coefficient.

Preferably, the aberration coefficient calculating step includes a step of calculating an image movement amount between the plurality of bright field images and a step of calculating a geometric aberration amount in each bright field image from each of the image movement amounts.
The image movement amount is preferably calculated using a cross-correlation function.

  The plurality of detection surfaces of the detector are preferably arranged at equal intervals in the angular direction and the radial direction.

  The detector is installed on the outside of the bright field image detector, a bright field image detector having a plurality of detection surfaces for acquiring the plurality of bright field images as the plurality of detection surfaces, and the dark field image detector. And a dark field image detector having one detection surface for acquiring a field image.

  Preferably, the imaging step further includes a step of simultaneously capturing a dark field image and a plurality of bright field images by rotating an electron beam from the sample on the detector.

  It is preferable that the imaging step further includes a step of rotating the plurality of detection surfaces to simultaneously capture a dark field image and a plurality of bright field images.

  The aberration correction method further includes defocusing as a pre-process of the imaging process, simultaneously acquiring a dark-field image and a bright-field image including angle information of the electron beam for each detection surface; Adjusting the relative position of the transmitted electron beam from the sample and the detector so that the composition of each positional deviation vector of the bright field image at the time of defocusing with respect to the dark field image at the time of focusing is minimized. It is preferable to provide.

  The aberration correction method preferably further includes a step of enlarging or reducing the electron beam from the sample.

  A second aspect of the present invention is an aberration correction apparatus for a scanning transmission electron microscope having an aberration corrector, the detector having a plurality of detection surfaces on which an electron beam is incident, and (a) a dark field image by the electron beam. And simultaneously taking a bright field image including angle information of the electron beam for each detection surface, and (b) calculating an aberration coefficient from the plurality of bright field images using the dark field image as a position reference of an observation image, and c) a control device that controls the aberration corrector so as to reduce aberration based on the calculated aberration coefficient.

  When calculating the aberration coefficient, the control device further calculates an image movement amount between the plurality of bright field images, and calculates a geometric aberration amount in each bright field image from each of the image movement amounts. preferable.

  In the control device, the image movement amount is preferably calculated using a cross-correlation function.

  The plurality of detection surfaces of the detector are preferably arranged at equal intervals in the angular direction and the radial direction.

  The detector is installed on the outside of the bright field image detector, a bright field image detector having a plurality of detection surfaces for acquiring the plurality of bright field images as the plurality of detection surfaces, and the dark field image detector. And a dark field image detector having one detection surface for acquiring a field image.

  The aberration correction device includes a rotating lens installed between a sample and the detector, and the control device further controls the electron lens to rotate the electron beam on the detector, It is preferable to capture a plurality of bright field images simultaneously.

  The detector includes an electron-light conversion element that converts an electron beam into light, and an optical transmission path that divides the electron-light conversion element as the plurality of detection surfaces and transmits light from each detection surface; It is preferable that a plurality of photodetectors convert light transmitted from the optical transmission path into electrical signals for the plurality of detection surfaces.

  It is preferable that the optical transmission path includes a rotation mechanism that rotates the plurality of detection surfaces within an electron beam incident surface of the electron-optical conversion element by changing a light transmission path.

  The control device defocuses before capturing the dark field image and the plurality of bright field images, and simultaneously acquires a dark field image and a bright field image including angle information of the electron beam for each detection surface. It is preferable to adjust the relative position between the electron beam and the detector so that the composition of each positional deviation vector of the bright field image at the time of defocusing with respect to the dark field image at the time of defocusing is minimized.

  In the aberration correction method and the aberration correction apparatus, a bright field image is acquired from each of a plurality of detection surfaces having different positions, and a dark field image is acquired at the same time. In calculating each aberration coefficient using these simultaneously acquired observation images, a dark field image with little deviation is used as a position reference, so the aberration coefficient calculation accuracy can be improved, and aberration correction based on this result enables high resolution. Can be achieved.

  Further, since the aberration coefficient is calculated using the observation image, it is not necessary to switch the observation mode. That is, since the aberration correction can be executed immediately after the scanning transmission image is observed, the time delay between the aberration correction and the actual observation can be kept to a minimum. This is very effective for high-resolution observation at the atomic level where it is desired to minimize changes in the set values of the electron optical system.

It is a figure for demonstrating the principle of this invention. It is a figure for demonstrating the principle of this invention. It is a figure which shows the structure of the aberration correction apparatus which concerns on 1st Embodiment of this invention. It is a figure which shows the detection surface of the detector which concerns on 1st Embodiment of this invention. It is a functional block diagram of the aberration corrector control device according to the first and second embodiments of the present invention. It is a figure which shows the procedure which rotates a detection surface in the optical transmission line which concerns on 1st and 2nd embodiment of this invention. It is a figure which shows the structure of the aberration correction apparatus which concerns on 2nd Embodiment of this invention. It is a figure which shows the detection surface of the detector which concerns on 2nd Embodiment of this invention. It is a figure which shows an example of a structure of the scanning transmission electron microscope carrying the aberration correction apparatus which concerns on each embodiment of this invention.

(principle)
The principle of aberration correction according to the present invention will be described with reference to the drawings. FIG. 1 schematically shows an example of the trajectory of the electron beam 1 from the vicinity of the sample 41 introduced into the STEM to the detector 11 in the acquisition of a bright field image by a scanning transmission electron microscope (STEM) (see FIG. 9). FIG. The sample 41 is installed at a position away from the front focal plane 57a of the objective lens 57 by a focal length, and the detector 11 is installed at a position away from the sample 41 by the camera length. This figure shows a state in which the electron beam 1 passes through the front focal plane 57a of the objective lens 57 and converges toward the sample 41 by the convergence action of the objective lens.

  Α is the convergence angle (incident angle) of the electron beam 1 with respect to the sample 41, and θ is the azimuth angle of the electron beam 1 on the sample 41. When there is no aberration, each electron beam should be concentrated at one point of the sample 41 as shown by the dotted line regardless of the convergence angle α and the azimuth angle θ. However, since there is actually aberration (geometric aberration), the electron beam 1 crosses the optical axis 2 in front of the sample 41 as the convergence angle α to the sample 41 increases, and the irradiation position is the convergence of the electron beam. The larger the angle α, the farther away from the assumed irradiation position. If this aberration is due to the spherical aberration of the objective lens 57, as is well known, this deviation is proportional to the cube of the convergence angle α.

  Under the influence of such aberration, when the electron beam 1a that has passed on the optical axis 2 forms the bright field image shown in FIG. 2A, electrons having a convergence angle α (α ≠ 0) with respect to the sample 41. As shown in FIG. 2B, the bright field image formed by the line 1b is accompanied by a positional shift with respect to the bright field image of FIG. This is because the electron beam 1b needs to be further shifted in order to form an image of the atom A (in other words, to irradiate the atom A) due to the deviation of the irradiation position due to the aberration.

  That is, a plurality of bright field images formed by electron beams with different convergence angles α are necessarily accompanied by positional deviations due to aberration. That is, assuming that one observation image of the sample 41 is used as a reference and the amount of positional deviation of these bright field images is represented by positional deviation vectors Fα and θ, the vector in the opposite direction indicates the aberration appearing in each bright field image. This corresponds to the geometric aberration vectors Gα and θ.

  On the other hand, a front focal plane (also referred to as an aperture plane) 57 a of the objective lens 57 is an angular space plane of the electron beam 1. That is, as conceptually shown in the upper part of FIG. 1, when each position of the electron beam on the front focal plane 57a is expressed in polar coordinates, the radial component and the angular component are expressed by the convergence angle α and the azimuth angle θ, respectively. It can be expressed uniquely. The aberration function χ at the front focal plane 57a is expressed as the sum of the following wavefront aberrations that are functions of the convergence angle α and the azimuth angle θ. In consideration of handling only the axial aberration in the high-resolution observation at the atomic level, the aberration function χ (α, θ) is expressed by the following equation (1).

χ (α, θ) = defocus (defocus) + 2 times astigmatism
+ On-axis frame + 3 times astigmatism
+ Spherical aberration + star aberration +4 astigmatism
+ 4th order coma + Threelobe aberration +5 times astigmatism
+ 5th order spherical aberration + 6th astigmatism ...
That is,

It is.

The components Gα and Gθ in the convergence angle direction and the azimuth direction of the geometric aberration vectors Gα _and θ are obtained by partial differentiation of the aberration function χ with respect to the convergence angle α and the azimuth angle θ, respectively.

That is, by obtaining bright field images in each of a plurality of sets of the convergence angle α and the azimuth angle θ, geometrical aberration vectors Gα _and θ are obtained for the number of the sets. Each aberration coefficient can be calculated by performing the target process.

For the electron beam convergence angle α and azimuth angle θ, for example, the position of the electron beam detection surface may be specified. For example, using a multi-segment detector having a plurality of detection surfaces with different detection positions associated with the convergence angle α and the azimuth angle θ, a bright-field image is detected from the electron beam incident on each detection surface. A plurality of information (ie, angle information (convergence angle α and azimuth angle θ) of the electron beam 1 incident on the detection surface) are simultaneously acquired, and a plurality of geometric aberration vectors Gα _and θ for each bright-field image are calculated from these. . Each aberration is a function having the convergence angle α and the phase angle θ (radial component and angular component in the front focal plane) as variables, and therefore the detection surface must be divided into at least two for both variables. Preferably,

Further, as the number of geometric aberration vectors Gα _and θ necessary for calculating each aberration coefficient increases (that is, as the number of divided detection surfaces increases), aberrations with lower orders can be calculated with higher accuracy.

Although one of a plurality of bright field images can be used as a reference image that defines the starting point of the geometric aberration vectors Gα _and θ for each bright field image, in the present invention, the above-described plurality of bright field images are used as the reference image. A dark field image acquired simultaneously with the field image is used. As is well known, a bright-field image is an image formed by a coherent transmission electron beam (0th-order diffracted electron beam), and contrast inversion occurs due to the sample thickness or focus. ) May be difficult to recognize. Further, since the bright field image is easily displaced by a change in the incident angle of the electron beam due to geometric aberration or the like, it is not suitable as an image for setting the reference position of the geometric aberration vectors Gα _and θ. On the other hand, since the dark field image is formed by scattered electrons that are incoherent, the image is not reversed and the specific structure can be easily recognized. In addition, under the conditions of image observation by STEM, the angle dependence of the scattering intensity with respect to the incident electrons is gentle, so that the dark field image hardly shifts with respect to the change in the incident angle of the electron beam due to geometric aberration or the like. Further, since the dark field image is acquired simultaneously with the bright field image, the drift of the nano-order image which is concerned in the high resolution measurement at the atomic level due to the temporal change of the magnetic field or electric field generated by the electron optical system or the like is eliminated. Therefore, such a dark field image is acquired simultaneously with the bright field image obtained from each detection surface, and the dark field image is used as a position reference regarding the deviation of each bright field image, thereby calculating the geometric aberration vectors Gα _and θ. Accuracy can be improved.

  Each calculated aberration coefficient is used, for example, in an aberration corrector installed on the STEM irradiation system side to suppress aberration. In this case, since each aberration coefficient can be obtained immediately from the bright field image and dark field image acquired at the same time, the aberration corrector can be controlled in a short time, and high before the above-mentioned drift occurs. A resolution observation is possible.

The basic principle of the present invention is as described above. However, when a plurality of bright-field images are actually acquired using a multi-division detector, all detectors can detect image misalignment based on aberrations. Further, it is desirable to align the center of the multi-segment detector with the central axis of the transmission electron beam from the sample 41. For this alignment, the electron beam 1 is defocused using the objective lens 57 to simultaneously acquire a dark field image and a plurality of bright field images, and using the same method as the calculation of the geometric aberration vectors Gα _and θ described above. Then, the positional deviation vectors Fα and θ of the bright field images with respect to the dark field image at the time of defocusing are calculated, and the relative positions of the transmission electron beam and the multi-divided detector are adjusted so that the combination is minimized.

  Specifically, in the observation of the bright field image using the multi-segment detector, when the incident electron beam to the sample 41 is defocused, the movement direction of each bright field image is radially centered on a certain point. Take advantage of the nature of moving. That is, when the optical axis of the incident electron beam coincides with the center of the multi-divided detector, the positional deviation vectors Fα and θ of the bright field images with the dark field image as the reference position are substantially equal in size and radial. However, when the center of the multi-segment detector is off the optical axis of the incident electron beam (in an extreme example, the optical axis of the optical axis of the incident electron beam is outside the multi-segment detector) This makes it difficult to obtain a bright-field image for calculating each aberration coefficient and to calculate a positional deviation vector thereof. Therefore, each positional deviation vector F′α, θ at the time of defocusing is calculated, and the transmission electron beam is deflected so that the combination is minimized, or the multi-divided detector is placed on a plane perpendicular to the optical axis. Move mechanically. Thereby, the center axis | shaft of a transmission electron beam and the center of a multi-division detector can be made to correspond.

  Further, the number of detection surfaces used for acquiring the bright field image and the dark field image can be adjusted as appropriate by enlarging or reducing the electron beam from the sample 41 with an electron lens such as a projection lens in front of the sample 41, for example. . If the electron beam is enlarged, the number of detection surfaces for acquiring a bright field image can be increased, and the calculation accuracy of each aberration coefficient can be improved. On the other hand, if the electron beam is reduced, the detection area of electrons necessary for forming the dark field image can be increased and the detection scattering angle is enlarged, so that the dark field image having a good S / N ratio as the position reference. Can be obtained.

(First embodiment)
The aberration correction apparatus according to the first embodiment of the present invention will be described with reference to FIGS.

  Hereinafter, the aberration correction apparatus 10 according to the present embodiment includes a detector 11 as a multi-segment detector mounted on the STEM 50 (see FIG. 9), and a deflector 31 installed between the sample 41 and the detector 11. And an aberration corrector control device 20 that processes the detection signal from the detector 11 and controls the aberration corrector 55 and the deflector 56 (see FIG. 9) based on the processing result. These details will be described below.

  First, the detector 11 will be described. As shown in FIG. 3, the detector 11 divides the electron-light conversion element 12 that converts an electron beam into light and the electron-light conversion element 12 as a plurality of detection surfaces, and transmits light from each detection surface. And a plurality of photodetectors 14 for converting the light for each of the plurality of detection surfaces transmitted from the optical transmission path 13 into an electrical signal.

  The electron-light conversion element 12 is, for example, a single scintillator or a fluorescent plate, and converts incident electrons into light having an intensity that can be detected by the subsequent photodetector 14.

  The optical transmission line 13 is, for example, a so-called bundle optical fiber in which a large number of optical fibers are bundled, and an end portion on the side of the electron-light conversion element 12 is integrally formed so as to receive light from the entire surface of the electron-light conversion element 12. The other side is branched so that the received light is transmitted to each photodetector according to the light receiving position. In other words, the light transmission path 13 has a light emitting surface of the electro-optical conversion element 12 as a plurality of detection surfaces D1 to D16 having different positions and corresponding to the convergence angle α and the azimuth angle θ in a plurality of light detection regions. Configured to split.

  The photodetector 14 is, for example, a combined device of a photomultiplier tube (photomultiplier) and a preamplifier, and converts the light emitted from the branched optical transmission path 13 into an electric signal and amplifies it. This signal is input to an image processing unit 22 (described later) of the aberration corrector control device 20 as a detection signal of an electron beam incident on each light emitting surface.

As described in the principle section, each aberration is a function having a convergence angle α and a phase angle θ (radial component and angular component on the front focal plane) as variables, so the detection surface is divided into at least two parts for both variables. There is a need to. On the other hand, the greater the number of geometric aberration vectors Gα _and θ necessary for calculating each aberration coefficient, the higher the accuracy of aberrations with lower orders. Therefore, the light transmission path 13 is divided at equal intervals in the phase angle direction (angular direction) and the radial direction of the light emitting surface of the electro-optical conversion element 12. FIG. 4 shows an example of this division, which is divided into four in the phase angle direction and four in the convergence angle direction, and the light-emitting surface of the electron-light conversion element 12 is divided into 16 as detection surfaces D1 to D16. In this case, the number of branches of the optical transmission line 13 is 16, and the number of photodetectors 14 is also 16.

  In this embodiment, out of these detection surfaces D1 to D16, the detection surfaces D13 to D16 located outside are used as the detection surfaces of the dark field image detector, and the detection surfaces D1 to D12 located inside thereof are used as the bright field. Used as the detection surface of the image detector. However, the number of detection surfaces used as the dark field image detector and the bright field image detector is not limited to the above, and can be set as appropriate according to the conditions of the sample to be observed, the intensity of the electron beam, the acceleration voltage, etc. It is.

  Next, the aberration corrector control device 20 of the present embodiment will be described. FIG. 5 is a functional block diagram of the aberration corrector controller 20. As shown in this figure, the aberration corrector control device 20 includes a calculation unit 21, an image processing unit 22, and a power supply control unit 23, and passes through a STEM control device 62 that controls the overall STEM, a communication line 26, and the like. Connected. The aberration corrector control device 20 may include a display device 24 that displays the calculation result of the aberration by the calculation unit 21 and an input device 25 that receives an operation from the operator. The aberration corrector control device 20 may be mounted in the STEM control device 62 without using the communication line 26.

  The image processing unit 22 creates a dark field image based on the detection signal output from the photodetector 14 corresponding to the detection surfaces D13 to D16. At the same time, a bright field image is created based on detection signals output from the photodetectors 14 corresponding to the detection surfaces D1 to D12. The image data of the dark field image is stored in a storage device (not shown) such as a memory in the aberration corrector controller 20. The image data of each bright field image is also stored in a storage device (not shown) together with the convergence angle α and the azimuth angle θ associated with each detection surface.

The computing unit 21 reads out a dark field image from a storage device (not shown), uses this image as a position reference, and calculates geometric aberration vectors Gα _and θ of each bright field image by mathematical processing such as a cross-correlation function. As a result, the components Gα and Gθ of the geometric aberration vectors Gα _and θ at the convergence angles α and the azimuth angles θ are obtained. The calculation unit 21 selects a predetermined number of sets from a plurality of sets based on the convergence angle α and the azimuth angle θ and the obtained geometric aberration vectors Gα _and θ, and uses a mathematical process such as a least square method. Then, each aberration coefficient of the geometric aberration is calculated.

  The power supply control unit 23 controls the excitation current and the like of the aberration corrector 55 based on the aberration coefficient calculated by the calculation unit 21 to reduce the aberration. Specifically, for example, based on the aberration coefficient calculated by the calculation unit 21, a setting value such as an excitation current for reducing aberration is determined, and a control signal based on this setting value is transmitted via the communication line 26 to the STEM control device. 62. In this case, based on this set value, the STEM control device 62 sets the excitation current of the aberration corrector 55 and the like, and the aberration is reduced.

When the defocus is used to match the optical axis of the incident electron beam with the center of the detector 11, the power supply control unit 23 transmits a setting signal to the objective lens 57 (see FIG. 9) to the STEM control device 62, The STEM control device 62 changes the focal length of the objective lens 57 based on this signal. Thereafter, similarly to the calculation of the geometric aberration vectors Gα _and θ, the image processing unit 22 simultaneously acquires the dark field image and each bright field image, and the calculation unit 21 calculates the positional deviation vectors Fα and θ of each bright field image. The setting value of the deflector 31 is determined so as to calculate and minimize the combination. Based on this set value, the power supply controller 23 applies a current or voltage to the deflector 31 to deflect the electron beam from the sample 41 so that the optical axis of the incident electron beam coincides with the center of the detector 11.

  The optical transmission path 13 may include a rotation mechanism 17 that rotates the plurality of detection surfaces D1 to D16 within the electron beam incident surface of the electron-light conversion element 12 by changing the light transmission path. . In this case, the optical transmission line 13 is configured to be rotatable between an optical transmission line 13a arranged on the vacuum side and an optical transmission line 13b arranged on the atmosphere side. The rotation mechanism 17 rotates the optical transmission path 13b around the central axis while maintaining the central axis of the entire optical transmission path 13, and closely contacts the end surface 13c of the optical transmission path 13a with the end surface 13d of the optical transmission path 13b. . For example, in FIG. 6, assuming that the end faces 13c and 13d of the optical transmission lines 13a and 13b are separated by boundary lines as indicated by solid lines for each detection surface, the optical transmission line 13b is rotated around its axial direction. Then, the boundary line of the end face 13d of the optical transmission path 13b is aligned with the position indicated by the dotted line of the end face 13c of the optical transmission path 13a, and they are attached to each other. Since the whole detection surface is rotated by this rotation, a set of convergence angle α and azimuth angle θ different from the set of convergence angle α and azimuth angle θ before rotation can be defined. For example, when the optical transmission line 13b is rotated by 45 ° as shown in FIG. 6, the newly obtained convergence angle is (α + 22.5) °. Thus, by changing the position of the detection surface before and after rotation, the number of bright-field images used for calculating the aberration coefficient can be doubled, and the calculation accuracy of the aberration coefficient can be improved. Further, since the optical transmission path on the atmosphere side is rotated, the operation is simple and requires a short time. Therefore, the change with time of aberration is minimized.

  The detection surface may be rotated by providing a rotating lens (not shown) for rotating the electron beam from the sample 41 between the sample 41 and the electron-light conversion element 12. The rotating lens is an axisymmetric lens, and is disposed, for example, on the focal plane between the intermediate lens 59 and the projection lens 60.

(Second Embodiment)
An aberration correction apparatus according to the second embodiment of the present invention will be described with reference to FIGS. Since the aberration correction apparatus 10 ′ according to the present embodiment and the aberration correction apparatus 10 according to the first embodiment are different only in the configuration of the detector 11, the description of the same parts as those in the first embodiment is omitted. To do.

  In the detector 11 according to the first embodiment, a part of the electron-light conversion element 12 is used as a dark field image detector, whereas in the detector 11 according to the second embodiment, the electron-light conversion is performed. The entire surface of the element 12 is used as a bright-field image detector, and an annular dark-field detection surface 12b as a dark-field image detector is installed outside the element 12. The dark field detection surface 12b is a multi-channel plate or the like that detects electrons with high sensitivity. Therefore, in this embodiment, in addition to the photodetector 14, a preamplifier 15 for amplifying the detection signal from the dark field detection surface 12b is provided. The output signal of the preamplifier 15 is input to the image processing unit 22, where image data of a dark field image is created.

  The configuration of the optical transmission line 13 in the present embodiment is the same as that described in the first embodiment. The light-emitting surface of the electro-optical conversion element 12 is divided as a plurality of bright-field image detection surfaces that are associated with the convergence angle α and the azimuth angle θ and have different positions.

As described in the principle section, since the dark field image is used as a position reference for calculating the geometric aberration vectors Gα _and θ of each bright field image, a dark field image is individually formed using a plurality of detection surfaces. Less important. Therefore, according to this embodiment, the number of photodetectors can be reduced, and the configuration of the aberration correction apparatus is simplified.

  A scanning transmission electron microscope (STEM) in which the aberration correction apparatus according to each of the above embodiments is mounted will be described. A scanning transmission electron microscope having a known configuration can be applied except for the aberration corrector controller 20. FIG. 9 shows an example, and the scanning transmission electron microscope 50 is equipped with an aberration correction apparatus 10 ′ according to the second embodiment.

  The scanning transmission electron microscope 50 includes an electron gun 51 that generates an electron beam 1, a converging lens 52 of at least one stage, an aberration corrector 55, a deflector 56 for scanning and axial alignment, an objective lens 57, and a sample. A sample stage 58 for introducing 41 into the irradiation region of the electron beam 1, an intermediate lens 59, a projection lens 60, a detector 11, and a STEM control device 62 for controlling the electron optical system are provided. A deflector (not shown) for adjusting the axis of the electron beam is appropriately provided between the lenses.

  The STEM control device 62 includes a power source 63 that applies a voltage or current to the electron gun 51 and the above-described electron optical system, a power source control unit 64 that controls the output voltage or output current of the power source, and detection from the detector 11. An image processing unit 65 that creates an observation image such as a bright-field image and a dark-field image using the signal, a display device 66 that displays the observation image and an operation screen, and input from an operator (for example, an observation magnification and an observation region) And an input device 67 for receiving the input value. These are connected to each other by a bus or the like, and are controlled by arithmetic processing of the arithmetic unit 68.

  The electron beam 1 generated by the electron gun 51 applied with a high voltage is accelerated toward the converging lens 52. The electron beam 1 is converged by the converging lens 52 and passes through the aberration corrector 55 and the deflector 56. Thereafter, the light is converged to a thinner beam by the objective lens 57 and irradiated onto the sample 41. At this time, the aberration corrector 55 corrects each aberration such as spherical aberration, and the deflector 56 deflects the electron beam 1 in two directions perpendicular to the optical axis 2 in order to obtain a scanning transmission image. Then, the electron beam 1 is scanned. As described in each embodiment, the aberration corrector 55 is also controlled from the aberration corrector 10 '. The electron beam 1 transmitted through the sample 41 or scattered from the sample 41 once converges on the back focal plane (not shown) of the objective lens 57, then passes through the intermediate lens 59 and the projection lens 60 and enters the detector 11. To do.

  A detection signal is output to the detector 11 based on the incident electron beam 1. This detection signal is input to the image processing unit 65 of the STEM control device 62. When aberration correction is performed, this detection signal is also input to the image processing unit 22 of the aberration correction apparatus 10 ′. The image processing unit 65 of the STEM control device 62 creates an observation image (image data) such as a bright field image and a dark field image based on this detection signal. This observation image is appropriately recorded in a storage device (not shown) such as a memory or a hard disk and displayed on the display device 24.

DESCRIPTION OF SYMBOLS 1 Electron beam 2 Optical axis 10, 10 'Aberration correction apparatus 11 Detector 12 Electron-light conversion element 13 Optical transmission path 20 Aberration corrector control apparatus 31 Deflector D1-D16 Detection surface 50 Scanning transmission electron microscope

Claims (18)

An aberration correction method in a scanning transmission electron microscope having an aberration corrector,
A step of causing an electron beam from a sample to enter a detector having a plurality of detection surfaces;
Simultaneously capturing a dark field image by the electron beam and a bright field image including angle information of the electron beam for each detection surface;
Calculating an aberration coefficient from the plurality of bright field images using the dark field image as a position reference of an observation image;
And a step of controlling the aberration corrector so as to reduce aberration based on the calculated aberration coefficient. A method of correcting aberrations in a scanning transmission electron microscope. The aberration coefficient calculating step includes:
Calculating an image movement amount between the plurality of bright-field images;
The aberration correction method according to claim 1, further comprising: calculating a geometric aberration amount in each bright-field image from each of the image movement amounts. The aberration correction method according to claim 2, wherein the image movement amount is calculated using a cross-correlation function. The aberration correction method according to claim 3, wherein the plurality of detection surfaces of the detector are arranged at equal intervals in the angular direction and the radial direction. The detector is
A bright field image detector having a plurality of detection surfaces for acquiring the plurality of bright field images as the plurality of detection surfaces;
The aberration correction method according to claim 3, further comprising: a dark field image detector that is installed outside the bright field image detector and has a single detection surface for acquiring the dark field image. The aberration according to claim 1, wherein the imaging step further includes a step of simultaneously imaging a dark field image and a plurality of bright field images by rotating an electron beam from the sample on the detector. Correction method. The aberration correction method according to claim 1, wherein the imaging step further includes a step of rotating the plurality of detection surfaces to simultaneously capture a dark field image and a plurality of bright field images. Furthermore, as a pre-process of the imaging process,
Defocusing and simultaneously obtaining a dark field image and a bright field image including angle information of the electron beam for each detection surface;
Adjusting the relative position between the transmitted electron beam from the sample and the detector so that the composition of the positional deviation vector of each bright field image at the time of defocusing with respect to the dark field image at the time of defocusing is minimized. The aberration correction method according to claim 1, further comprising: The aberration correction method according to claim 1, further comprising a step of enlarging or reducing the electron beam from the sample. An aberration correction apparatus for a scanning transmission electron microscope having an aberration corrector,
A detector having a plurality of detection surfaces on which an electron beam is incident;
(A) simultaneously capturing a dark field image by the electron beam and a bright field image including angle information of the electron beam for each detection surface;
(B) calculating an aberration coefficient from the plurality of bright field images using the dark field image as a position reference of an observation image;
(C) An aberration correction apparatus for a scanning transmission electron microscope, comprising: a control device that controls the aberration corrector so as to reduce aberration based on the calculated aberration coefficient. When calculating the aberration coefficient, the control device further calculates an image movement amount between the plurality of bright field images, and calculates a geometric aberration amount in each bright field image from each of the image movement amounts. The aberration correction device according to claim 10, wherein The aberration correction apparatus according to claim 11, wherein the image movement amount is calculated using a cross-correlation function. The aberration correction apparatus according to claim 12, wherein the plurality of detection surfaces of the detector are arranged at equal intervals in an angular direction and a radial direction. The detector is
A bright field image detector having a plurality of detection surfaces for acquiring the plurality of bright field images as the plurality of detection surfaces;
The aberration correction apparatus according to claim 12, further comprising: a dark field image detector that is installed outside the bright field image detector and has one detection surface for acquiring the dark field image. In addition, a rotating lens installed between the sample and the detector,
The aberration according to claim 10, wherein the control device further controls a rotating lens to rotate an electron beam on the detector and simultaneously captures a dark field image and a plurality of bright field images. Correction device. The detector is
An electron-light conversion element that converts an electron beam into light;
Dividing the electro-optical conversion element as the plurality of detection surfaces, and an optical transmission path for transmitting light from each of the detection surfaces;
A plurality of photodetectors for converting the light transmitted from the optical transmission path into electrical signals for each of the plurality of detection surfaces;
The aberration correction device according to claim 13 or 14, characterized by comprising: 17. The optical transmission path includes a rotation mechanism that rotates the plurality of detection surfaces within an electron beam incident surface of the electron-optical conversion element by changing a light transmission path. Aberration correction device. The control device, before capturing the dark field image and the plurality of bright field images,
While defocusing, simultaneously obtaining a dark field image and a bright field image including angle information of the electron beam for each detection surface,
The relative position between the electron beam and the detector is adjusted so that the composition of each displacement vector of the bright field image at the time of defocusing with respect to the dark field image at the time of the defocusing is minimized. Item 11. The aberration correction device according to Item 10.