JP4744592B2 - Electron beam apparatus and method for measuring stray magnetic field in electron beam apparatus - Google Patents

Description

  The present invention relates to an electron beam apparatus having an electron lens and a method for measuring a stray magnetic field inside the apparatus.

  In an electron beam apparatus having an electron optical system such as an electron microscope, a magnetic field type lens is usually used as an electron lens. The magnetic lens uses the image forming action by a local strong magnetic field parallel to the traveling direction of the electron beam (optical axis of the electron optical system), and is the most commonly used electron lens. The strength of the magnetic field of the electron lens corresponds to the change of the focal length, and is used for focus adjustment in the objective lens and for changing the image magnification in the imaging lens system. On the other hand, the magnetic field in the direction perpendicular to the traveling direction of the electron beam directly changes the traveling direction of the electron beam as a Lorentz force. Therefore, it is generally used as a deflector for electron beams in a relatively weak magnetic field.

  In the case of an electron beam, unlike an electron beam, the electron beam passage path must be evacuated, so that the electron optical system is generally constructed in a metal sealed container. At this time, by using a high permeability material (for example, pure iron, permalloy, etc.) for the vacuum container, it is possible to shield the magnetic field outside the container from entering the electron beam path, and it is relatively easy to take measures against the magnetic field. Can be taken. However, unlike shielding against an electric field, shielding against a magnetic field only lowers the magnetic field strength of the space or part that needs relatively countermeasures by preferentially transmitting the magnetic field to the high permeability material side. In principle, it cannot be completely shielded.

  For this reason, when high-resolution image observation, especially high-resolution image observation methods using aberration correction devices, and electron beam holography must be controlled with high accuracy, the electron beam path must be Careful measures are required for the remaining stray magnetic field, especially the time-varying magnetic field. In particular, as described above, the magnetic field component perpendicular to the optical axis has a great influence because it directly deflects the electron beam, and its measurement and countermeasures are important.

  In general, as disclosed in Non-Patent Document 1, for example, the magnitude and position of the stray magnetic field are estimated using the fact that the stray magnetic field deflects the electron beam temporally. FIG. 13 schematically shows the principle. FIG. 14 (FIGS. 14A and 14B) shows measurement conditions and measurement results. When there is a magnetic field 26 (magnetic flux density ΔB) perpendicular to the optical axis 2 that changes with time in the traveling direction of the electron beam in the direction of travel of the electron beam, the electrons with velocity v incident in the magnetic field are as shown in FIG. A constant-velocity circular motion with a radius of rotation r around a certain point 25 is performed. As a result, the electron beam draws an orbit 27 that exits the magnetic field region 26 with a deflection angle α. This deflection angle α is based on the balance between Lorentz force and centrifugal force that electrons receive from the magnetic field.

(Where λ = h / mv was used). Here, e is the charge amount of electrons, h is the Planck constant, and λ is the wavelength of the electron beam.

  The deflection angle α to the electron beam is observed at a distance L on the downstream side in the traveling direction of the electron beam in the region where the stray magnetic field exists (see FIG. 14A). From ΔSource = 2αL, the shape of the electron beam spot 10 is distorted. Alternatively, it is measured as splitting (see FIG. 14B), and the stray magnetic field ΔB is obtained.

  As is clear from FIG. 14, in this conventional stray magnetic field measurement method, it is desirable that the region where the stray magnetic field exists and the observation position of the electron beam spot 10 are separated (L → large) in order to measure with high sensitivity. In addition, it is desirable that the spot 10 is small and at the same time the observation magnification of the spot position is high. However, optical systems that satisfy these conditions often deviate greatly from the sample image observation conditions. Therefore, even if the stray magnetic field is measured, a different optical system must be separately constructed when observing the sample image, and the measurement result may not directly relate to the quality of the observed image.

  As another technique related to the electron beam apparatus, in Patent Document 1, in addition to a basic electron microscope main body, means for inputting a spatial resolution required in holographic observation, input values, and apparatus-specific parameters A device for calculating the electron biprism position and the sample position for realizing the spatial resolution required from the above, and a mechanism for moving these two positions for realizing the obtained calculation results are provided. It is disclosed. As a sample position moving means, a rotary mechanism using a screw pitch is disclosed. Non-Patent Document 2 describes a dark field imaging experiment as a method for recording a lattice image with high contrast without being affected by the aberration of the electron lens. Non-Patent Document 3 describes the phase of the electron wave, the phase difference between the two waves, and the Aharanov-Bohm effect.

JP 2007-335083 A Kenichi Goto and Shuichi Yamazaki Co-edition: Detailed explanation Electromagnetics Seminar (1983) (Kyoritsu Shuppan) p400 Tetsuya Akashi, Akiara Sugawara, Takaho Yoshida, Tsuyoshi Matsuda, Ken Harada and Akira Tonomura: Proceedings of 16th International Microscopy Congress (IMC16), (Sapporo, Japan, September 3-8, 2006) Vol. 2, pp. 585. A. Tonomura: Electron Holography, 2nd ed. (Springer, Heidelberg. Germany, 1999), Captor 6, page 52.

  As described above, a stray magnetic field, especially a magnetic field that has a component perpendicular to the optical axis and whose intensity and direction change over time, is when the fluctuations are applied during image acquisition (within the exposure time) in an electron microscope. Will be observed by superimposing different images, which directly causes deterioration of the electron microscope image. Therefore, not only the entire device but also the electron beam path is intensively covered with a high permeability material, or the entire device is actively applied with a magnetic field in the opposite direction and the same strength as the problematic floating magnetic field. Countermeasures such as offsetting are taken. In any case, measuring a stray magnetic field that directly affects the electron microscope image is important for determining the countermeasure policy and for evaluating the quality of the countermeasure.

  However, in an electron microscope optical system that is sealed in a vacuum and composed of a large number of electron lenses, it is practically difficult to detect the stray magnetic field ΔB using a sensor. The beam was made into a spot shape, and measurement or qualitative observation was only performed through nonuniformity (extension in one direction) of the spot, splitting or vibration of the spot, and the like. This method using electron beam spots has different detection sensitivities depending on the spot size. In a normal transmission electron microscope, the optical system for observing the electron beam spot is an optical system for imaging and observing the sample image. Therefore, in high-resolution observation and electron beam holography, even if the degree of the stray magnetic field is evaluated by an optical system that magnifies and projects the electron beam spot, the stray magnetic field that affects image observation can be evaluated. However, after all, the magnetic field measures were taken by trial and error, and the actual effect was observed through experiments to know the extent of the effect.

  An object of the present invention is to provide a measurement method of a stray magnetic field that exists on an electron optical system in an electron beam apparatus and that can detect a stray magnetic field that affects imaging with high sensitivity and quantitatively evaluate the stray magnetic field. .

  An example of a representative one of the present invention is as follows. That is, the present invention includes an electron beam light source, an irradiation optical system for irradiating the sample with an electron beam emitted from the light source, a sample holding device for holding the sample irradiated with the electron beam, An electron beam apparatus having an imaging lens system for forming an image of a sample and an image observation / recording device for observing or recording the sample image, the irradiation optical system, the sample holding device, An information processing device for controlling the imaging lens system and the image observation / recording device is provided, and the sample is moved to a plurality of positions on the optical axis of the electron beam, and images of the sample obtained at the respective positions A stray magnetic field existing on a passage path of an electron beam from the sample to the sample image observation surface in the apparatus is measured from a change in contrast or the image contrast.

  According to the present invention, it becomes possible to measure the influence of a stray magnetic field quantitatively and with high accuracy under the same imaging optical system and optical conditions as those for sample observation, and the stray magnetic field remaining in the optical system can be observed. It is possible to measure and evaluate the degree of the effect on the magnetic field and the degree of effectiveness of the countermeasure against the stray magnetic field.

  The present invention uses a crystal sample and measures a stray magnetic field in the optical system from the contrast of the sample image observed at a magnification of the lattice image observation or a change in contrast in the high resolution image (lattice image) observation optical system. A method and apparatus enabling it are provided. The principle of this method is that a phase difference (a phase difference caused by the Aharanov-Bohm effect (Non-Patent Document 3) generated by a temporally fluctuating magnetic field existing in a space surrounded by a path of two waves when two waves are subjected to interference imaging. (Refer to the above)) as a deterioration of contrast of interference fringes, which is in principle different from the method for detecting deflection of the electron beam spot, which measures the degree to which the electron beam is deflected by the Lorentz force received from the stray magnetic field. .

  In the present invention, the intensity and direction of the stray magnetic field as electromagnetic noise constantly fluctuate in time, but when the degree of fluctuation does not change greatly within the time required for a series of interference image observations, the degree of fluctuation is reduced. In order to measure and evaluate, it is possible to move the sample to different points on the optical axis or place multiple samples, observe the interference image at each position, and form the interference image from the degree of contrast change Measure the degree of fluctuation of the stray magnetic field existing in the two-wave electron beam path. As the two-wave path becomes farther away, the space enclosed by the path becomes larger, and the amount of the stray field taken into the closed space increases, so that the stray field can be detected with high sensitivity. In addition, when the sample has a small lattice interval and a large diffraction angle, such as a crystal, and the low acceleration voltage electron beam has a large diffraction angle, the stray magnetic field can be detected with high sensitivity.

Hereinafter, the principle of the stray magnetic field measurement of the present invention will be described first.
<1. Two-wave separation and lattice image (two-wave interference imaging)>
The method of the present invention uses an interference phenomenon that occurs between a plurality of electron beams starting from the same light source and passing through different orbits. A crystal is used to divide into multiple waves. This corresponds to an amplitude division type interferometer, and an optical system similar to the lattice image observation method is required for imaging. Therefore, an evaluation experiment under the same optical conditions as in the lattice image observation method is examined. For simplicity, a model is given for a two-wave imaging lattice image experiment. This is a method of recording a lattice image (interference fringes) by two waves diffracted by a crystal lattice.

  As shown in FIG. 1, the explanation is made using two diffracted waves symmetric with respect to the optical axis (g, -g), but it is generally applicable to imaging systems using essentially any two waves such as transmitted waves and scattered waves. It holds true. In FIG. 1, 2 is an optical axis, 3 is a sample, 5 is an objective lens (main surface of the objective lens), 8 is an interference fringe, 11 is an image (crossover) of an electron source imaged by the objective lens, 22 is An electron beam wavefront, 27 is an electron beam trajectory, 36 is a rear focal plane of the objective lens, 71 is a sample image plane by the objective lens, and 93 is a beam stopper.

  First, various parameters of the optical system are determined as shown in FIG. 1, and when the magnification of the optical system is M, the following equations are obtained from Bragg diffraction conditions (equation (3)).

  Here, a is the distance between the objective lens 5 (main surface of the objective lens) and the sample 3, b is the distance between the objective lens 5 (objective lens main surface) and the image plane 71 of the sample, θ is the Bragg angle, and d is The crystal lattice spacing, f is the focal length of the objective lens, and q is the distance between the optical axis 2 and the image 11 (diffraction point) of the electron source by the objective lens.

  Considering the diffraction point 11 on the rear focal plane 36 of the lens as a light source, the interference fringe spacing s is

And an interference fringe (lattice image) having a spatial frequency twice the spatial frequency of the crystal lattice spacing d. That is, the diffraction angle θ depends on the wavelength λ, but the obtained grating image depends only on the magnification and not on the wavelength.

  In FIG. 1, for the sake of simplicity, the rotation of the electron beam about the optical axis by the magnetic lens and the rotation of the image accompanying the rotation are not drawn. However, the omission of the generality of the electron optical system discussed in the present invention is lost. It is not a thing. Also, the lens itself omits the principal surface represented by a single line. The same abbreviations are applied to the diagrams representing the optical system in FIG.

Further, the interference fringes 8 composed of the two diffracted waves (g, -g) depicted in FIG. 1 are called dark-field grating images, and are not affected by the aberration of the electron lens (strictly, the influence of the aberration). This is known as a method of recording a lattice image with high contrast (see Non-Patent Document 2). In order to effectively use the coherence of the electron beam, it is desirable that the defocus amount Δf be based on the equation (7). Here, C _s is the spherical aberration coefficient, hkl is the order of the diffracted wave g _hkl , and q _hkl is the spatial frequency of the diffracted wave g _hkl .

<2. Electron orbit and phase difference>
The method of the present invention utilizes an interference phenomenon that occurs between electron beams starting from the same light source and passing through different orbits. FIG. 2 is a schematic diagram depicting the relationship between the electron trajectory 27 and the wavefront 22 (equal phase plane). In the case of the relationship between the electron source 1 and the observation point (electron source image 10) as shown in FIG. 2, the phase difference of the electron beam when the electron beam from the light source 1 reaches the observation point 10 through each orbit 27. Δφ is expressed by equation (8).

The first term Δφ _{1 on} the right side of equation (8) is the geometric optical path difference (wavenumber path integral), and the second term Δφ ₂ is the contribution of the electric field and corresponds to the refractive index in the case of light rays. The third term Δφ ₃ is a contribution of the magnetic field and is characterized by being independent of the acceleration voltage (electron beam wavelength). In the present invention, the first term Δφ ₁ of the geometric optical phase difference and the third term Δφ ₃ of the magnetic field contribution are examined in order to consider the influence of the stray magnetic field on the electron microscope image. (See Non-Patent Document 3)
<3. Effect of stray magnetic field on geometric optical path>
In the previous section, the sample was a crystal and the incident electron beam was subjected to Bragg diffraction by the sample. Further, as shown in FIG. 3, a stray magnetic field 24 (magnetic flux) is included in the path of two diffracted waves (g, -g). Consider the case where density ΔB) exists. The objective lens that has the greatest influence on the image quality, such as the resolution of the electron microscope image, has a structure in which electron optical components such as the sample holder and the objective aperture are arranged in the vicinity of the lens main surface. The actual situation is weaker than the part. Therefore, it is assumed that the stray magnetic field that fluctuates with time exists locally in the range of the width l on the downstream side of the flow of the electron beam on the rear focal plane 36 of the objective lens. Because of the above circumstances, generality is not lost even with this assumption.

FIG. 3 shows an optical system in the case where a floating magnetic field region is included. Considering a certain moment, each electron orbit 27 diffracted by the crystalline sample 3 is deflected by the Lorentz force in the stray magnetic field ΔB, and the lattice image 8 moves laterally (in FIG. 3, the right side). direction). Since the image observation point is only in the vicinity of the optical axis 2 because of the high magnification, it is observed that the fringe position of the lattice image is moved sideways in the observation image. That is, the phase of the observed interference fringe changes. When the change amount Δφ ₁ of the phase of the lattice image at the observation point is obtained from the optical path length from the imaginary light source 15 caused by the floating magnetic field ΔB, the displacement amount of the position of the imaginary light source 15 is in a first order approximation range (Equation 19). in because it is L [alpha, each optical path length L _R of the right diffraction point g (virtual source), the optical path length L _L of the left diffraction point -g (virtual source)

It becomes. As described above, the optical path difference Δl and the phase difference Δφ ₁ are obtained.

From the equation (12), the phase difference Δφ ₁ due to the geometric optical path difference ΔL of the lattice image due to the stray magnetic field does not depend on the wavelength λ, and the intensity of the stray magnetic field (magnetic flux density ΔB) and the existence region 1 of the magnetic field It turns out that it depends. That is, when the stray magnetic field ΔB that changes with time exists in the region l, the interference fringes 8 that differ by the phase difference Δφ ₁ are superimposed and observed, and the contrast of the interference fringes 8 deteriorates. That is, there is a certain relationship between the observed fringe contrast and the stray magnetic field, and this relationship can be regarded as constant within the time of a series of experiments.
<4. Phase difference due to magnetic field>
As in the third term of equation (8), the magnetic field contribution to the phase difference Δφ ₃ does not depend on the acceleration voltage. According to Stokes' theorem, the change is determined by the magnetic flux density B existing between the two orbits (Equation (13)). This is the Aharanov-Bohm effect (AB effect) (see Non-Patent Document 3).

Therefore, if the area of the floating magnetic field existing region S surrounded by the two-wave electron beams is obtained, the phase difference Δφ ₃ is obtained. As in the previous section, in the case of the parameters of the optical system shown in FIG. 3 and the region where the stray magnetic field exists, the area S surrounded by the orbits 27 of the two diffracted electron beams is expressed as in Expression (14).

Accordingly, the phase difference Δφ ₃ is expressed as shown in Expression (15).

That is, similarly to the equation (12), when the stray magnetic field ΔB that changes with time exists in the region l, the interference fringes 8 that are different by the phase difference Δφ ₃ are observed to be superimposed and observed. The contrast of the image deteriorates. However, unlike Equation (12), Equation (15) includes parameters (electron beam wavelength λ, crystal lattice spacing d, sample position a) that can be changed by the experimenter.
<5. Evaluation method of stray magnetic field>
Since the stray magnetic field ΔB fluctuates in the positive and negative directions, the amount of fringe fringes that appears as a result of temporal integration is handled as the sum of the absolute values of the respective phase differences. That is, from the equations (8), (12), and (15), the phase difference between the two waves is Δφ,

It is represented by

  That is, there is a fixed relationship between the observed fringe contrast and the stray magnetic field based on Equation (16), but the contribution from the stray magnetic field (phase difference based on Equation (15)) is determined by the experimenter. There remains room for parameter changes as follows.

  (1) Change the wavelength λ by changing the acceleration voltage of the electron beam.

  (2) Change the lattice spacing d by changing the orientation of the target crystal or crystal.

  (3) Change the sample position a.

  By changing these parameters, it is possible to obtain information about stray magnetic fields. Among the parameters (λ, d, and a) that can be changed by the experimenter, the sample position (distance between the sample and the lens main surface) a is changed as shown in FIG. It is the basis of the present invention to provide a method for evaluating the stray magnetic field ΔB and a device that makes it possible.

For example, the lattice image is observed while changing the sample position (increasing the distance a between the sample and the lens main surface), and when the contrast of the lattice image is lost, the time-varying phase difference is exactly 2π (Δφ = 2π), the distance a _lim between the sample 3 and the lens main surface 5 at that time is obtained, and the relationship of the equation (17) is obtained with respect to the strength of the floating magnetic field ΔB from the equation (16).

  In the equation (17), the stray magnetic field existence region l is undecided. However, since it is normally considered to be about the focal length f of the target electron lens (here, the objective lens), it is assumed that l = f and the equation (18) Get.

  From this equation (18), it is possible to estimate the stray magnetic field. Note that the disappearance of the contrast of the lattice image is considered, for example, when the contrast of the lattice image falls below the noise level in comparison with the contrast of the amorphous image and the lattice image 8 cannot be effectively discriminated.

  Needless to say, in the expression (17), the stray magnetic field existence region 1 is not limited to the focal length f of the electron lens. For example, when it can be considered that a stray magnetic field exists between the pole piece gaps of the electron lens, the strength of the stray magnetic field ΔB can be obtained from equation (17) by replacing the gap length G with the stray magnetic field existence region l. good. In addition, an arithmetic expression equivalent to Expression (18) may be obtained by using an arbitrary parameter that can roughly specify the existence area of the stray magnetic field in the electron beam apparatus instead of the existence area l of Expression (17).

The procedure for measuring the stray magnetic field according to the present invention is shown as a flowchart in FIG. Each will be explained. It is assumed that necessary information is previously input by the observer through a computer monitor screen and recorded in the storage means.
(Step 1): A lattice image observation condition, for example, the wavelength of the electron beam, the lattice interval, and the observation condition (the amount of defocus, the magnification at the time of recording, etc.) are determined.
(Step 2): The observer operates the apparatus as necessary to adjust the optical system of the apparatus to the observation conditions defined in Step 1.
(Step 3): An image of the amorphous part in the sample is observed under the same conditions as the lattice image observation, and the image is recorded on a recording medium or a recording apparatus.
(Step 4): The noise level of the image is determined from the obtained image of the amorphous part. For this, for example, the apparatus may automatically determine using Fourier transform of an image.
(Step 5): The optical system is adjusted to the observation conditions defined in Step 1.
(Step 6): A lattice image is observed and the image is recorded on a recording medium or a recording apparatus.
(Step 7): The contrast of the lattice image is evaluated.
(Step 8): Compare with the noise level of the image obtained in Step 4.
(1) When the contrast of the lattice image is larger than the noise level (Yes) → A movement amount for further moving the sample from the lens main surface is determined (Step 9) → The sample is moved (Step 10) → Step 5 Return to.
(2) When the contrast of the lattice image is not larger than the noise level (No) → Proceed to Step 11.
(Step 11): The distance a _lim from the lens main surface to the sample is calculated based on the sample position or the integrated value of the movement amount that has moved the sample so far, and the result is recorded in the recording device.
(Step 12): Based on the equation (18), the stray magnetic field amount ΔB is obtained and recorded in the recording device.
(Step 13): The stray magnetic field amount ΔB obtained in Step 10 is displayed on the monitor.
(Step 14): A series of measurements is terminated.

The flowchart of FIG. 5 is realized when the experimenter inputs necessary information to the apparatus while watching the monitor screen, such as the condition setting in step 1 and the determination in step 8 and the like. It is also possible to construct a system that automatically measures the stray magnetic field ΔB based on the flowchart of FIG. That is, the apparatus may automatically perform a series of measurements.
<6. Evaluation accuracy of stray magnetic field>
Let us evaluate how sensitive the method of the present invention is to a stray magnetic field. The coefficient h / 2e in the equation (17) or the equation (18) corresponds to the amount of magnetic flux (Φ ₀ = 2 × 10 ⁻¹⁵ Wb (Weber)) held by just one superconducting magnetic flux quantum. The ratio λ / d between the wavelength λ of the electron beam and the lattice spacing d is about 1/50 to 1/100. In other words, the stray magnetic field strength ΔB that can be evaluated is an area where one magnetic flux quantum defines a floating magnetic field existence region l (for example, if the floating magnetic field existence region l is about the focal length f of the objective lens, f ^{2 to} 3 mm ² ). This corresponds to a very weak magnetic field (for example, geomagnetism 3 × 10 ^-5 Wb / m ² (= 0.3G (Gauss)) corresponds to 15,000 / mm ² when converted to the surface density of magnetic flux quantum). Therefore, it can be seen that this method can be evaluated for a weak magnetic field of 1/10 ⁵ or less of the geomagnetism.
<Electron beam device>
FIG. 6 schematically shows the entire electron beam apparatus as a system relating to the present invention. FIG. 6 is a schematic diagram assuming the use of a general-purpose transmission electron microscope, but the present invention is not limited to the form described in this schematic diagram.

  The electron source 1 is positioned at the most upstream part in the direction in which the electron beam flows, and the electron beam emitted from the electron source 1 is converted into an electron beam having a predetermined speed by the acceleration tube 40, and then the irradiation optical system 4 (first optical system 4). The sample 3 is irradiated through an irradiation (condenser) lens 41 and a second irradiation (condenser) lens 42. The sample 3 is installed in a movable sample holding mechanism 38, and the optical axis is controlled by a sample control unit 39. The position of the sample in the direction, that is, the distance a between the sample and the main lens surface is adjusted, and the electron beam transmitted through the sample 3 is subjected to processing such as selection of a diffracted wave by the objective aperture 92, and the traveling direction of the electron beam The image is formed by the objective lens 5 on the downstream side of the sample 3. This imaging action is performed by a plurality of imaging lens systems (the first imaging lens 61, the second imaging lens system on the downstream side of the objective lens 5). Imaging lens 62, third imaging lens 63, fourth imaging lens 6 4), interference fringes 8 are finally formed on the observation / recording surface 89 of the electron beam apparatus, and an image observation / recording medium 79 is provided corresponding to the observation / recording surface 89 of the electron beam apparatus. As will be described in detail later, the control system computer 51 functions as an information processing apparatus for controlling the entire apparatus and observing a sample image or measuring a stray magnetic field. By changing the sample position (distance between the sample and the lens main surface) a, the image formed on the observation / recording surface 89, that is, the lattice image or the interference fringes 8 is recorded in the image observation / recording device. That is, the lattice image or interference fringe 8 is recorded in the image recording device 77 through an image observation / recording medium 79 such as an electron microscope film or a CCD camera controlled by the control unit 78.

  The movable sample holding mechanism 38 may be, for example, a rotary mechanism using a screw pitch (see Patent Document 1) or a mechanism using a piezoelectric element. In the case of the rotation mechanism, the rotation angle gives the sample position a. In the case of the piezoelectric element mechanism, the sample position a is known from the applied voltage.

  The voltage applied to the electron source 1, the accelerating tube 40, the position of the sample 3, the excitation state of each electron lens, and the like of the control system computer 51, which is an information processing apparatus for controlling the entire apparatus, and each unit connected thereto A control system, that is, an electron source control unit 19, an acceleration tube control unit 49, a first irradiation lens control unit 48, a second irradiation lens control unit 47, a sample control unit 39, and an objective lens control unit 59 , Objective aperture control unit 97, first imaging lens control unit 69, second imaging lens control unit 68, third imaging lens control unit 67, fourth imaging lens control unit 66, controlled by the image observation / recording medium control unit 78. 52 is a monitor of the control computer, and 76 is a monitor of the image observation / recording device 77.

  In addition, in the actual apparatus, there are a deflection system for changing the traveling direction of the electron beam, a diaphragm mechanism other than the objective diaphragm 92 for limiting the region through which the electron beam passes, and the like. The apparatus is also controlled by a control system connected to the computer 51.

  The control system computer 51 inputs information necessary for search of observation conditions or operation control of an input device 53, a calculation means, and an electron microscope for inputting control parameter values and the like necessary for controlling the sample position through a GUI or the like. Storage means for storing, for example, a memory, various storage devices, and an output device including a display monitor are provided. The information stored in the storage means includes information necessary for the apparatus to determine conditions necessary for the lattice image observation, such as information on the material of the sample and observation conditions. Furthermore, various kinds of software for executing the above information search or operation control are stored, and these software are executed by the arithmetic means.

  The software controls the irradiation optical system, sample holding device, imaging lens system, and image observation / recording device, executes a series of arithmetic processing based on the flowchart of FIG. A program that functions as an information processing apparatus for measuring a magnetic field is also included.

  When measuring the stray magnetic field, the information processing apparatus moves the sample to a plurality of positions on the optical axis of the electron beam, and determines the image contrast of the sample obtained at each position or the change in the image contrast. It functions to measure stray magnetic fields existing on the electron beam passage path from the sample to the sample image observation surface.

In other words, when performing stray magnetic field measurement, the information processing apparatus controls the lattice image observation optical system and controls the sample holding mechanism to move the sample to a plurality of positions on the optical axis of the electron beam. The image contrast of the sample at each position obtained by the observation / recording apparatus or the change in the image contrast is obtained, the minimum distance a _lim is measured, and the arithmetic means calculates the expression in the apparatus by the equation (17) or the equation (18). A function of calculating a floating magnetic field existing on the passage of the electron beam to the sample and the sample image observation surface and displaying it on the monitor 52 is executed. In this case, the stray magnetic field existence region 1 in the equation (17) for calculating the stray magnetic field is L _B in FIG. 6 such as the focal length f of the objective lens 5 and the pole piece gap length as described above. The range in which the displayed magnetic field remains may be specified, set as appropriate, and employed as an alternative to equation (18).

  The setting of the noise level of the image and the observation / evaluation of the contrast of the lattice image may be performed, for example, while the experimenter looks at the image displayed on the monitor 76.

  The experimenter takes necessary magnetic field shielding measures for the electron beam apparatus based on the calculation result of the stray magnetic field displayed on the monitor, and prepares for the subsequent original measurement of the electron beam apparatus.

  It should be noted that the diaphragm system other than the objective diaphragm 92 that limits the region through which the electron beam passes is not directly related to the present invention, and is omitted in this figure. As shown in this schematic diagram, the electro-optical device is assembled in the vacuum vessel 18 and continuously evacuated by a vacuum pump. However, the vacuum system is not directly related to the present invention, and is therefore omitted. .

As a first embodiment of the present invention, a specific configuration example of a dark field lattice image observation optical system in an electron beam apparatus and a stray magnetic field measurement method using the same will be described with reference to FIG.
FIG. 7 shows a schematic diagram of a dark field lattice image observation optical system when the present invention is implemented. For the sake of simplicity, a state is shown in which an objective aperture 92 and a beam stopper 93 in which only two waves symmetrical to the optical axis 2 contribute to image formation are inserted in the rear focal plane 36 of the objective lens 5. In FIG. 7A, for example, a crystalline sample 3 is set at a normal sample position, multiwaves subjected to Bragg diffraction are taken into the objective lens 5, and two waves selected by the objective aperture 92 and the beam stopper 93 are connected. It is a schematic diagram showing that the interference fringes 8 (lattice image) generated on the image plane 71 of the objective lens are further magnified by the imaging lens 61 on the downstream side in the traveling direction of the electron beam. Reference numeral 72 denotes an image surface of the sample formed by the first imaging lens 61. The imaging lens 61 corrects the sample magnification associated with the change in the distance between the crystalline sample 3 and the objective lens 5 with the movement of the crystalline sample 3 accompanying the stray magnetic field measurement.

  7B is a schematic diagram when the position of the sample is moved along the optical axis 2 toward the light source side of the electron beam in the optical system of FIG. 7A. Since the Bragg angle (θ = λ / d) is determined only by the wavelength λ of the electron beam and the lattice spacing d of the crystal, the diffracted wave in the vicinity of the objective lens spreads spatially compared to the case of A in FIG. Yes. That is, the magnitude of the influence of the stray magnetic field ΔB on the lattice image 8 depends on the size of the closed space surrounded by the diffracted wave (area S in the equation (15)), which is uniquely determined geometrically. .

  The magnitude of the stray field ΔB remaining in the stray field region 24 in the optical system can be directly observed and measured as the contrast degradation of the lattice image 8.

  The information processing apparatus moves the sample to a plurality of positions on the optical axis of the electron beam, and based on the image contrast of the sample at each position or a change in the image contrast, the sample and the sample in the apparatus are calculated by a calculation unit. The stray magnetic field ΔB existing on the electron beam passing path to the image observation plane is calculated and displayed on the monitor.

  In the example of FIG. 7, the sample position is moved to the light source side. However, if only the parameter is changed, the sample position may be moved to the lens side as long as an image can be formed by the objective lens. In general, at the time of high-resolution observation, the sample position is arranged on the light source side only slightly from the front focal position of the objective lens. Therefore, the parameter can be greatly changed by moving the sample position to the light source side as shown in FIG.

  In addition, the decrease in the magnification of the objective lens due to the movement of the sample position to the light source side is compensated by increasing the magnification of the imaging lens system downstream of the objective lens in the traveling direction of the electron beam, and finally recorded. The magnification of the lattice image to be made is made constant. This is a measure for reducing the influence of the performance of the recording system (for example, MTF (Modulation Transfer Function)). In FIG. 7B, in order to show constant magnification, the position of the imaging lens 61 is changed and drawn. However, in an actual electron microscope, the same effect can be obtained by changing the focal length of the imaging lens 61. It has gained.

  In a grating image observation optical system, a plurality of waves having different propagation angles with respect to the optical axis, such as transmitted waves and diffracted waves, are involved in interference. Therefore, the influence of spherical aberration and chromatic aberration of the objective lens and the amount of defocus are interference fringes Image). However, in the first embodiment (FIG. 7), only two waves symmetric with respect to the optical axis 2 are selected by the objective aperture 92 and the beam stopper 93 and used for image formation. Offset. Therefore, when compared with FIG. 8, a measurement result with less error can be obtained. However, the out-of-focus amount is a value determined by the equation (8).

  According to the present embodiment, it becomes possible to measure the influence of the stray magnetic field quantitatively and with high accuracy under the same imaging optical system and optical conditions as in the sample observation, and the stray magnetic field remaining in the optical system can be observed. When measures are taken for the effect on the image and countermeasures against the stray magnetic field, it is possible to measure and evaluate the effectiveness of the measures and implement appropriate magnetic shielding.

  As a second embodiment of the present invention, a specific configuration example of the lattice image observation optical system in the electron beam apparatus will be described with reference to FIG.

FIG. 8 shows a configuration example of an optical system when the present invention is implemented. The beam stopper 93 is not used for the example of FIG. Other relationships are exactly the same as in FIG.
8A and 8B, for example, the crystalline sample 3 is set at a normal sample position, and a multiwave (three waves) subjected to Bragg diffraction is imaged by the objective lens 5 and is generated on the image plane 71 of the objective lens. It is a schematic diagram showing that the interference fringe 8 (lattice image) is further enlarged by the imaging lens 61 on the downstream side in the traveling direction of the electron beam. The magnitude of the stray field ΔB remaining in the stray field region 24 in the optical system can be directly observed and measured as the contrast degradation of the lattice image 8.

  When performing stray magnetic field measurement, the information processing apparatus uses the image contrast of the sample at each position obtained by the image observing / recording apparatus, or changes in the image contrast, in the calculation means in accordance with the equation (18) or the like. The floating magnetic field ΔB existing on the electron beam passing path from the sample to the sample image observation surface is calculated and displayed on the monitor.

  In the example of FIG. 8, since the beam stopper 93 is not used, there is an advantage that the floating magnetic field can be measured under exactly the same observation conditions as the high-resolution image observation at the time of normal sample image observation. On the other hand, the effects of spherical aberration and chromatic aberration of the objective lens, and the effect of defocusing are superimposed on the interference fringes (lattice image). It is. That is, since interference of a plurality of waves is involved, in multiwave imaging, an error tends to increase in the evaluation according to Expression (18). However, relative observations regarding the magnitude of the stray magnetic field ΔB will not be wrong if the observation conditions are aligned.

FIG. 9 (FIG. 9A, FIG. 9B) shows an example of the experimental result of FIG. FIG. 9A shows a stripe interval of 72 pm when the sample position is moved to the 0.6 mm light source side (Δa = 0.6 mm) from the normal high-resolution observation position (a ₀ = 2.4 mm font change only) as shown in FIG. 8A. FIG. 9B is a lattice image with a stripe interval of 72 pm when the sample position is moved to the 3.6 mm light source side from the normal high-resolution observation position (Δa = 3.6 mm). Furthermore, when the sample position was moved to the 2 mm light source side (Δa = 5.6 mm), a lattice image with a stripe interval of 72 pm was not observed. _{Assuming that} a _lim = 8 mm and the stray magnetic field existence region 1 is about the focal length of the objective lens (f = 1.6 mm), the stray magnetic field is magnetic flux density in the direction perpendicular to the two waves forming the lattice image. It can be evaluated that about 1 × 10 ^-9 Wb / m ² (= 1 × 10 ^-5 G) remains. However, in this evaluation, as described above, the influence of spherical aberration and the like is ignored. The experiment was conducted using a gold single crystal thin film as the sample and an electron beam acceleration voltage of 400 kV.

FIG. 10 shows an example of the experimental results similar to those of FIG. 9 after the experiment in FIG. In the experiment using the same gold crystal thin film and the same acceleration voltage (400 kV), but the sample position was moved to the 5.6 mm light source side from the normal high resolution observation position (a ₀ = 2.4 mm font change only) (Δa = 5.6 mm), that is, a lattice image with a stripe interval of 72 pm was observed even when a = 8 mm. From this result, it can be seen that the influence of the stray magnetic field is reduced. This is the manifestation of the effect of magnetic field shielding measures.

  According to the present embodiment, it becomes possible to measure the influence of the stray magnetic field quantitatively and with high accuracy under the same imaging optical system and optical conditions as in the sample observation, and the stray magnetic field remaining in the optical system can be observed. It is possible to measure and evaluate the degree of the influence on the image and the effectiveness of the countermeasure against the stray magnetic field, and implement appropriate magnetic field shielding.

As a third embodiment of the present invention, a specific configuration example of the measuring method of the azimuth angle dependence of the stray magnetic field by the rotation of the beam stopper and a stray magnetic field measuring method using the same will be described with reference to FIG.
FIG. 11 is a schematic diagram when the beam stopper 93 inserted into the rear focal plane 36 of the objective lens 5 is rotated about the optical axis 2 as a central axis. The change of the sample position and the imaging lens system at the lower stage of the objective lens are omitted.

  Although the Bragg diffraction wave is generated in all directions in the crystal body, since the diffraction wave exists only discretely, the orientation dependence of the floating magnetic field ΔB cannot be measured finely. Still, the direction of the stray magnetic field ΔB measured approximately corresponds to the long direction of the flat beam stopper 93. In this figure, the electron lens is drawn like an optical glass lens, but this is a schematic diagram, and the image rotation accompanying the rotation of the electron beam around the optical axis 2 by the electron lens is also omitted. . This omission on drawing is the same for other drawings.

When performing stray magnetic field measurement, the information processing device measures the minimum distance a _lim based on the image contrast of the sample at each position obtained by the image observation / recording device, or a change in the image contrast, The stray magnetic field ΔB existing on the electron beam passing path from the sample to the sample image observation surface in the apparatus is calculated and displayed on the monitor.

  According to the present embodiment, it becomes possible to measure the influence of the stray magnetic field quantitatively and with high accuracy under the same imaging optical system and optical conditions as in the sample observation, and the stray magnetic field remaining in the optical system can be observed. It is possible to measure and evaluate the degree of the influence on the image and the effectiveness of the countermeasure against the stray magnetic field, and implement appropriate magnetic field shielding.

  As a fourth embodiment of the present invention, a specific configuration example of the measuring method of the azimuth dependency of the stray magnetic field by the rotation of the sample and the beam stopper and a measuring method of the stray magnetic field using the same will be described with reference to FIG. .

  FIG. 12 is a schematic diagram when the sample 3 and the beam stopper 93 inserted into the rear focal plane 36 of the objective lens 5 are both rotated about the optical axis 2 as a central axis.

  Thus, by rotating both the sample and the beam stopper at the same time, the azimuth angle dependence of the free magnetic field ΔB can be measured in detail. In the third embodiment, since the diffracted wave exists only in the space in a discrete manner, it has been described that the direction of the floating magnetic field ΔB can be measured only in a discrete manner, but this is a solution to the drawback.

  When stray magnetic field measurement is performed, the information processing device uses the image contrast of the sample at each position obtained by the image observation / recording device, or changes in the image contrast, and the calculation means observes the sample and the sample image in the device. The stray magnetic field ΔB existing on the electron beam passing path to the surface is calculated and displayed on the monitor.

  According to the present embodiment, it becomes possible to measure the influence of the stray magnetic field quantitatively and with high accuracy under the same imaging optical system and optical conditions as in the sample observation, and the stray magnetic field remaining in the optical system can be observed. It is possible to measure and evaluate the degree of the influence on the image and the effectiveness of the countermeasure against the stray magnetic field, and implement appropriate magnetic field shielding.

It is a schematic diagram showing an imaging optical system of a dark field lattice image regarding the principle of the present invention. It is a schematic diagram which shows the relationship between an electron orbit and a wave front regarding the principle of this invention. FIG. 3 is a schematic diagram showing changes in the orbit of a diffracted electron beam due to a stray magnetic field in relation to the principle of the present invention. FIG. 4 is a schematic diagram showing a change in the trajectory of a diffracted electron beam due to a change in a sample position, regarding the principle of the present invention. It is a flowchart which shows the procedure of the floating magnetic field measurement which becomes one Embodiment of this invention. It is a schematic diagram which shows the outline | summary of an example of the electron beam apparatus with which this invention is applied. It is a schematic diagram which shows the method at the time of using a dark field lattice image observation optical system as Example 1 of this invention. FIG. 7 is a schematic diagram showing a method when a lattice image observation optical system is used in the apparatus of FIG. 6 as Example 2 of the present invention. In Example 2, it is a figure which shows the experimental result when it moves to the 0.6mm light source side from the normal high-resolution observation position using the method of this invention ((DELTA) a = 0.6mm). In Example 2, it is a figure which shows the experimental result when it moves to the 3.6mm light source side from the normal high-resolution observation position using the method of this invention ((DELTA) a = 3.6mm). In Example 2, it is a figure which shows an experimental result when it moves to the 5.6mm light source side from the normal high-resolution observation position using the lattice image observation optical system after a magnetic field shielding countermeasure ((DELTA) a = 5.6mm). It is a schematic diagram which shows the method when rotating a beam stopper centering | focusing on an optical axis as Example 3 of this invention. It is a schematic diagram which shows the method when rotating a sample and a beam stopper centering | focusing on an optical axis as Example 4 of this invention. It is a schematic diagram which shows the principle of the deflection | deviation of the electron beam by the magnetic field (Lorentz force) known conventionally. It is a schematic diagram which shows the conventional measuring method (measurement condition) of the deflection | deviation of the electron beam by a magnetic field. It is a schematic diagram which shows the conventional method (measurement result) of the deflection | deviation of the electron beam by a magnetic field.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electron source, 10 ... Image of electron source, 11 ... Image of electron source imaged by objective lens (crossover), 12 ... Image of crossover imaged by 1st imaging lens, 15 ... Crossover virtual image formed by objective lens, 18 ... vacuum container, 19 ... electron source control unit, 2 ... optical axis, 22 ... electron wavefront, 24 ... floating magnetic field region, 25 ... circular arc by magnetic field The center of rotation of the electron beam, 26 ... the magnetic field region, 27 ... the trajectory of the electron beam, 3 ... the sample, 36 ... the rear focal plane of the objective lens, 38 ... the sample holding mechanism, 39 ... the sample control unit, 4 ... Irradiation optical system, 40 ... acceleration tube, 41 ... first irradiation (condenser) lens, 42 ... second irradiation (condenser) lens, 47 ... second irradiation lens control unit, 48 ... first irradiation lens Control unit, 49 ... Accelerating tube Control unit, 5 ... objective lens, 51 ... control system computer, 52 ... control system computer monitor, 53 ... control system computer interface, 59 ... objective lens control unit, 61 ... first imaging lens, 62 ... first 2 imaging lens, 63 ... third imaging lens, 64 ... fourth imaging lens, 66 ... fourth imaging lens control unit, 67 ... third imaging lens control unit, 68 ... Control unit for second imaging lens, 69... Control unit for first imaging lens, 71... Image plane of sample by objective lens, 72... Image plane of sample by first imaging lens, 76. Monitor of recording device, 77: Image recording device, 78: Image observation / recording medium control unit, 79: Image observation / recording medium, 8: Lattice image or interference fringe, 89 ... Observation / recording surface, 92: Objective Ri, 93 ... beam stopper, 97 ... objective aperture of the control unit.

Claims (20)

An electron beam light source, an irradiation optical system for irradiating the sample with an electron beam emitted from the light source, a sample holding device for holding the sample irradiated with the electron beam, and an image of the sample are formed An electron beam apparatus having an imaging lens system for performing an image observation and recording apparatus for observing or recording the sample image,
An information processing device that controls the irradiation optical system, the sample holding device, the imaging lens system, and the image observation / recording device;
The sample is moved to a plurality of positions on the optical axis of the electron beam, and the image contrast of the sample obtained at each position, or the change of the image contrast, the sample and the sample image observation surface in the apparatus An electron beam apparatus characterized by measuring a stray magnetic field existing on a path through which an electron beam passes. In claim 1,
2. The electron beam apparatus according to claim 1, wherein the image of the sample is a crystal lattice image, and the change in the image contrast is a state in which the contrast of the lattice image is smaller than a predetermined level. In claim 2,
An electron beam apparatus characterized in that a predetermined level at which the lattice image contrast is compared and determined is a contrast of an image formed by an amorphous portion of the sample, and is a noise for the lattice image. In claim 3,
The information processing apparatus includes:
Let the stray magnetic field inside the device be ΔB,
Let e be the charge of an electron
Let the Planck constant be h,
The wavelength of the electron beam is λ,
The observed lattice spacing of the crystal used as the sample is d,
Let f be the focal length of the electron lens to be measured,
When the distance between the main surface of the electron lens to be measured and the sample can be changed and the minimum distance at which the lattice image is not observed is a _lim ,
An electron beam apparatus, wherein the stray magnetic field is evaluated by the following formula (18).

In claim 1,
The information processing apparatus includes:
A function to set the lattice image observation conditions of the sample;
A function for adjusting the optical system of the apparatus to predetermined observation conditions;
A function of observing the image of the amorphous part in the sample under the same conditions as the lattice image observation and recording those images,
From the obtained image of the amorphous part, the function of determining the noise level of the image,
A function of observing the lattice image and recording the image;
A function of evaluating the contrast of the lattice image;
Compared with the obtained noise level of the image, when the contrast of the lattice image is larger than the noise level, a moving amount for further moving the sample from the lens main surface is determined, and the sample holding mechanism is controlled. The sample is moved to repeat observation, recording, evaluation and comparison of the image, and when the contrast of the lattice image is not larger than the noise level, from the lens main surface of the imaging lens system to the sample. A function to calculate and record the distance of
An electron beam apparatus having a function of evaluating the stray magnetic field based on the calculated distance. In claim 5,
An electron beam apparatus, wherein the lattice image and the evaluation result of the floating magnetic field are displayed on a monitor. In claim 5,
The imaging lens system includes an objective lens and one or a plurality of imaging lenses for enlarging the lattice image generated on the image plane of the objective lens on the downstream side in the traveling direction of the electron beam. Electron beam equipment. In claim 1,
An electron beam apparatus, wherein a transmitted wave or a scattered wave is selected using a diaphragm when an image of the sample is formed. In claim 8,
An electron beam apparatus characterized in that the diaphragm is rotated around the optical axis in a plane perpendicular to the optical axis. In claim 9,
An electron beam apparatus, wherein the sample is rotated around the optical axis in a plane perpendicular to the optical axis. An electron beam source;
An irradiation optical system for irradiating the sample with an electron beam emitted from the light source;
A sample holding mechanism for holding a sample irradiated by the electron beam;
An imaging lens system for forming an image of the sample;
An image observation / recording device for observing or recording the sample image;
An information processing device that controls the irradiation optical system, the sample holding device, the imaging lens system, and the image observation / recording device;
The information processing apparatus includes:
The sample is used as a diffraction grating for amplitude-dividing an electron beam starting from the same light source into at least two waves and passing through different trajectories,
Fluctuation in phase difference caused by a temporally varying floating magnetic field existing in a space surrounded by the path of each wave when the at least two waves are subjected to interference imaging is evaluated as deterioration in contrast of interference fringes. An electron beam device. In claim 11,
Changing the distance between the sample and the principal surface of the lens of the imaging lens system that is the first downstream in the direction of travel of the electron beam from the sample;
An interference fringe composed of the at least two waves is observed at each position of the sample, and exists in the path of the at least two electron beams leading to the formation of the interference fringe due to the change in contrast. An electron beam apparatus for measuring the degree of fluctuation of the stray magnetic field. In claim 12,
Let the stray magnetic field inside the device be ΔB,
Let e be the charge of the electron beam,
Let the Planck constant be h,
Let λ be the wavelength of the electron beam,
The observed lattice spacing of the crystal used as the sample is d,
The region where the stray magnetic field exists is l,
The interference fringe is observed while changing the distance between the sample and the lens main surface, and the distance a _lim between the sample and the lens main surface when the contrast of the interference fringe is lost is obtained. ) An electron beam apparatus characterized by obtaining the strength of the floating magnetic field ΔB.

In claim 13,
An electron beam apparatus characterized in that the intensity of the floating magnetic field ΔB is obtained by replacing the existence region 1 in the equation (17) with the focal length f of the electron lens of the imaging lens system to be measured. In claim 13,
The presence region 1 in the equation (17) is replaced with the pole piece gap length G of the electron lens of the imaging lens system to be measured, and the intensity of the floating magnetic field ΔB is obtained. Wire device. In claim 11,
An electron beam apparatus comprising: an objective aperture and a beam stopper that are inserted into a rear focal plane of the electron lens of the imaging lens system and that contribute only two-wave electron beams to imaging. A method for measuring stray magnetic fields in an electron beam device,
The electron beam apparatus includes an electron beam light source, an irradiation optical system for irradiating the sample with an electron beam emitted from the light source, a sample holding mechanism for holding the sample irradiated with the electron beam, An imaging lens system for forming an image of a sample, an image observation / recording device for observing or recording the sample image, the irradiation optical system, the sample holding device, the imaging lens system, and the image It has an information processing device that controls the observation and recording device,
Moving the sample to a plurality of positions on the optical axis of the electron beam;
From the image contrast of the sample obtained at each of the positions, or a change in the image contrast, the stray magnetic field existing on the electron beam passing path from the sample to the sample image observation surface is measured,
A method for measuring a stray magnetic field in an electron beam apparatus, comprising: displaying a measurement result of the stray magnetic field on a monitor. In claim 17,
The image of the sample is a crystal lattice image, and the change in the image contrast is a state in which the contrast of the lattice image is smaller than a predetermined level. . In claim 17,
Let the stray magnetic field inside the device be ΔB,
Let e be the charge of an electron
Let the Planck constant be h,
Let λ be the wavelength of the electron beam,
The observed lattice spacing of the crystal used as the sample is d,
Let f be the focal length of the electron lens to be measured,
When the distance between the main surface of the electron lens to be measured and the sample can be changed and the minimum distance at which the lattice image is not observed is a _lim ,
A method for measuring a stray magnetic field in an electron beam apparatus, wherein the stray magnetic field is evaluated by the following formula (18).

In claim 17,
A beam stopper inserted in the rear focal plane of the lens of the imaging lens system that is the first downstream in the traveling direction of the electron beam from the sample is rotated about the optical axis, or the sample and the beam stopper And measuring the stray magnetic field by rotating the optical axis about the optical axis as a central axis.

Images