JP2019124492A - electronic microscope - Google Patents

Abstract

To make the penetration length of an electron beam and the traverse length of an X-ray as small in change as possible even if the incident angle of the electron beam is changed in a two-dimensional direction.SOLUTION: The electron microscope according to the present invention measures samples which have the same shape such that a first distance through which an electron beam passes in the samples when the electron beam is irradiated from the first direction is equal to a second distance through which the electron beam passes in the samples when the electron beam is irradiated from the second direction. An X-ray detector detects X-rays emitted from the samples when the electron beam is irradiated in each of the first and second directions.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an electron microscope.

  Trace elements may be added to materials for the purpose of functional expression of materials and improvement of high temperature characteristics. For example, in the case of a magnetic material, the main phase is a crystal structure that determines the characteristics of the magnetic material, and the characteristics of the magnetic material are developed by the cyclic structure of the crystal structure being arranged periodically. By replacing any part of the crystal structure with an additional element, the magnetic material can exhibit new functionality. In order to understand the cause of functional expression, it is measured whether or not the additional element is substituted by the main phase, and further, which part of the crystal structure of the main phase is substituted by the additional element (that is, substitution site) There is a need to.

  Conventionally, measurement of substitution sites has been performed by X-ray absorption fine structure (XAFS) radiation or neutron diffraction. In synchrotron radiation and neutron diffraction, while the beam size is several tens of μm to several mm, the crystal structure of the main phase is micron size, making measurement difficult. Moreover, in the radiation facility, it is difficult to measure a replacement site with short TAT (Turn Around Time) due to the limitation of machine time.

  In the case of using an electron microscope, since the electron beam can be narrowed to a nanometer, substitution sites can be measured in micron-sized particle units. Furthermore, since it is a laboratory level measurement, it can measure by short TAT. The ALCHEMI method (Atom Location by Channeling-Enhanced MIcroanalysis) described in Non-Patent Document 1 below is a method of measuring a substitution site using an electron beam and characteristic X-rays excited by the electron beam, and has already been useful experimentally. Sex is shown.

  The ALCHEMI method is a method of analyzing the atomic structure of a sample by analyzing the spectrum of X-rays emitted from the sample while changing the incident direction of the electron beam irradiated to the sample. In order to improve the analysis accuracy of the ALCHEMI method, it is important that the number of atomic rows of the sample at the incident site does not change significantly even if the incident direction of the electron beam is changed. In the process of X-rays being emitted from the sample, the sample itself absorbs X-rays to some extent, so if the distance through which the X-rays pass differs greatly from direction to direction, the detection accuracy of X-rays varies depending on the direction. Absent. Therefore, it is also important that the sample thickness does not change significantly even if the incident direction of the electron beam changes.

  Patent Document 1 below discloses a method of creating a cylindrical sample and observing the sample while rotating the sample around its central axis. The electron beam is incident on the sample from a direction orthogonal to the rotation axis. Therefore, even if the incident angle of the electron beam is changed on the plane orthogonal to the rotation axis, the penetration length of the electron beam does not change.

Patent No. 3767895

J. Taftφ and J. C. H. Spence, Ultramicroscopy, 9, 243 (1982)

  When a cylindrical sample as described in Patent Document 1 is used, the penetration length of the electron beam does not change even if the incident angle of the electron beam is changed on a plane orthogonal to the rotation axis. However, in the ALCHEMI method, the incident angle of the electron beam is changed in two dimensions. Therefore, in the case of the cylindrical sample described in Patent Document 1, the incident angle of the electron beam may be changed on a plane parallel to the rotation axis. Then, the penetration accuracy of the electron beam and the traverse length of the X-ray differ depending on the incident angle of the electron beam, so that the measurement accuracy is lowered.

  The present invention has been made in view of the above problems. Even if the incident angle of the electron beam is changed in a two-dimensional direction, the penetration length of the electron beam and the traverse length of the X-ray do not change as much as possible. The purpose is to

  In the electron microscope according to the present invention, when the electron beam is irradiated from the first direction, the electron beam passes the inside of the sample at a first distance, and when the electron beam is irradiated from the second direction, the inside of the sample is the electron beam Measure a sample having a shape that is equal to the second distance that L passes through. The X-ray detector detects X-rays emitted from the sample when the electron beam is irradiated in each of the first and second directions.

  According to the electron microscope of the present invention, even if the incident angle of the electron beam is changed in the two-dimensional direction, the penetration length of the electron beam and the traverse length of the X-ray can be made uniform. Therefore, the analysis accuracy of the ALCHEMI method can be improved.

It is a figure explaining the principle of the ALCHEMI method. It is a figure which shows a mode that a sample is processed. FIG. 1 is a block diagram of an electron microscope 100 according to a first embodiment. It is the figure which expanded the periphery of the sample 24. FIG. It is the figure which expanded the periphery of the sample 24 further. It is a figure explaining the procedure which produces the sample 24. FIG. It is a figure explaining the process of processing the sample 24. FIG. It is a figure which defines fluctuation [Delta] r of a hemispherical shape. It is a figure which shows the relationship between X-ray intensity fluctuation (I / I ') and allowable film thickness fluctuation ((DELTA) r).

<Principle and problem of ALCHEMI method>
In the following, in order to facilitate understanding of the principle of the present invention, first, the principle of the ALCHEMI method described in Non-Patent Document 1 and the subject regarding the penetration length of the electron beam will be described. After that, the configuration of the present invention will be described.

  FIG. 1 is a diagram for explaining the principle of the ALCHEMI method. When the electron beam 26a is incident on the sample 24 at the angle shown in FIG. 1A, the electron beam 26a transmits an atomic row in the sample 24, and a transmitted wave and a diffracted wave are generated to interfere with each other. Under this condition, it is assumed that a standing wave is generated in the atomic row of atom A. In this case, in the X-ray spectrum detected by the X-ray detector, the X-ray intensity of the atom A is high and the X-ray intensity of the atom B is low (FIG. 1 (c)). When the electron beam 26 b is incident at the angle shown in FIG. 1 (b), it is assumed that a standing wave is generated in the atomic row of the atom B. In the X-ray spectrum in this case, the X-ray intensity of the atom B is high, and the X-ray intensity of the atom A is low (FIG. 1 (d)). Thus, the X-ray spectral intensity depends on the incident angle of the electron beam. Since the X-ray intensity of the atom C in the case of FIG. 1B is higher than the X-ray intensity of the atom C in FIG. 1A, it is understood that the atom C substitutes the atom B.

  In FIG. 1, although the irradiation angle of the electron beam 26a is inclined by θx in the x direction, since the atoms are arranged three-dimensionally, in the ALCHEMI method, the irradiation angle is further inclined by θy in the y direction and measurement is performed. . In order to increase the analysis accuracy of the three-dimensional structure, it is necessary to change θx and θy as finely as possible. One X-ray spectrum is acquired for each given angular condition (θx1, θy1). For example, when the angle is set to 0 ° ≦ θx ≦ 180 °, 0 ° ≦ θy ≦ 180 °, and measurement is performed while changing θx and θy by 1 °, 32761 (= 181 × 181) X-ray spectra are obtained It will be done. By analyzing this data by the ALCHEMI method described in Non-Patent Document 1, it is possible to know the three-dimensional atomic arrangement structure and substitution structure and the ratio thereof.

  As described above, in the ALCHEMI method, the number of atomic strings at the place where the electron beam is irradiated is important information. On the other hand, the AlCHEMI method is a measurement method that widely changes the incident direction of the electron beam with respect to the sample. Therefore, in order to improve the analysis accuracy, the number of atomic rows does not change significantly even if the incident direction of the electron beam changes. This is very important. Further, when analyzing light elements, the energy of the characteristic X-ray excited by the electron beam is low, so the effect of the sample itself absorbing X-rays between the sample and the X-ray detector can not be ignored. Therefore, it is necessary to devise that the sample thickness does not change for each observation condition (incident angle of the electron beam).

  Usually, when observing a sample using an electron microscope, the sample is sliced only at a target portion so that the incident electron beam does not scatter and spread within the sample. For example, in a general-purpose electron microscope with an acceleration voltage of about 100 kV to 300 kV, it is necessary to thin the sample to a thickness of several nm to several hundred nm depending on the acceleration voltage and the atomic number of the sample. At the time of thinning the sample, electrolytic polishing or ion beam processing is used. In particular, in order to extract a sample with high precision from a target view and thin it out, FIB (Focused Ion Beam) processing is often used. This makes it possible to process flat exfoliated samples.

  FIG. 2 is a view showing how a sample is processed. In the ALCHEMI method, the incident direction of the electron beam to the sample is changed in a two-dimensional direction, so when using a flat plate-like thinned sample, it is oblique to the electron beam incident perpendicularly to the sample surface. Comparing the time of incidence, the penetration length of the electron beam and the distance (crossing length) traversed by the X-ray in the sample are different from each other (FIG. 2 (a)).

  In a three-dimensional electron microscope that observes a fine three-dimensional structure by CT (Computed Tomography), a cylindrical sample as shown in FIG. 2B is used because it is desirable to make the penetration lengths of electron beams as uniform as possible. In this case, even if the incident direction of the electron beam is changed on a plane orthogonal to the central axis of the cylinder, the penetration length of the electron beam can be easily aligned by rotating the sample about the central axis. (Figure 2 (b) lower left). However, in the ALCHEMI method, since it is necessary to change the incident direction of the electron beam in a two-dimensional direction, for example, the incident direction of the electron beam is changed on a plane parallel to the central axis of the cylinder. In this case, the penetration length of the electron beam can not be made uniform (FIG. 2 (b) lower right).

Embodiment 1
FIG. 3 is a block diagram of the electron microscope 100 according to the first embodiment of the present invention. The electron microscope 100 is configured to acquire a signal used to analyze the atomic structure of the sample 24 using the ALCHEMI method.

  The electron beam 26 emitted from the electron gun 11 (controlled by the electron gun control circuit 11 ', hereinafter similarly described) is a condenser lens 12 (condenser lens control circuit 12'), a condenser diaphragm 13 (condenser diaphragm control circuit 13 '). ), Deflector 14 for axis deviation correction (deflector control circuit 14 for axis deviation correction), aberration corrector 15 (aberration corrector control circuit 15 '), deflector 16 for image shift, deflector control circuit for image shift 16 '), objective lens 18 (objective lens control circuit 18'), and projection lens 22 (projection lens control circuit 22 '), the target analysis place on the sample holder 20 (sample holder control circuit 20') is hemispherical thin film The focused sample 24 is focused and irradiated. The electron beam 26 is two-dimensionally scanned on the sample surface by the scanning deflector 17. The intensity of the transmission electron beam 27 is detected by the electron detector 21 (electron detector control circuit 21 '), and synchronized with the scanning position, and displayed on the computer 23 to display the structure / composition / electronic state of the sample. A transmission electron beam image having a corresponding contrast can be observed.

  FIG. 4 is an enlarged view of the periphery of the sample 24. As shown in FIG. For convenience of the description, only components necessary for explanation are shown. The electron beam 26 is collimated by the condenser lens 12 and then enters the off-axis error correcting deflector 14. The electron beam 26 is incident on the sample 24 obliquely at an angle Θx by the off-axis deflection deflector 14. The electron beam 26 has a certain width, but the angle of incidence on the sample 24 is always Θ x (ie parallel beams). From the portion of the sample 24 irradiated with the electron beam 26, characteristic X-rays 28 corresponding to the elemental species of the portion are generated. The X-ray detector 19 (X-ray detector control circuit 19 ′) detects the characteristic X-ray 28.

  FIG. 5 is an enlarged view of the area around the sample 24. As shown in FIG. The sliced sample 24 is placed on the sample holder 20. The sample 24 has a hemispherical portion centered on the analysis point 29 of particular interest, and a plate-shaped support portion that holds the hemispherical portion. An electron beam 26 is incident on the sample 24 at various angles. Instead of or in combination with deflecting the electron beam 26, a device such as a two-axis tilting sample stage tilts the sample holder 20 to change the relative angle between the electron beam 26 and the sample 24. You may

  FIG. 6 is a view for explaining the procedure for producing the sample 24. As shown in FIG. The periphery of the target view of the mother sample is grooved, and the microprobe 30 is brought into contact with the target view. Next, an organic tungsten gas is flowed in the FIB apparatus, and the tip of the microprobe 30 is irradiated with an ion beam to solidify the organic tungsten gas to fix the microprobe 30 and the sample 24 (FIG. 6A). Next, the sample 24 is extracted by the microprobe 30, and the sample 24 is moved to the sample stage 31 (FIG. 6 (b)). Next, an organic tungsten gas is again flowed to the contact portion between the sample stage 31 and the sample 24 to irradiate the ion beam, thereby temporarily fixing the sample stage 31 and the sample 24 (FIG. 6 (c)). Subsequently, after an unnecessary sample portion on the side of the microprobe 30 is cut off by FIB, the fixation between the sample stage 31 and the sample 24 is strengthened to make this permanent fixation (FIG. 6 (d)). Next, the sample stage 31 and the sample 24 are rotated by 90 ° in the FIB apparatus, and the orientation relationship between the ion beam irradiation direction and the sample 24 is made the relationship of FIG. Since the ion beam cuts the sample 24 to a depth approximately proportional to the irradiation time, first, the FIB irradiation time is controlled by FIB # 1 so that the FIB irradiation time is controlled so that the sample 24 remains in the shape of a dotted line. Next, the lower surface is processed flat with FIB # 2. By the above operation, the hemispherical-shaped sample shown in FIG. 5 can be produced. By mechanically fixing the sample stage 31 on the sample holder 20, the sample 24 is subjected to observation using the electron microscope 100.

  The X-ray detector 19 detects characteristic X-rays 28 at each incident angle of the electron beam 26, and outputs a signal representing the detection result. The computer 23 can identify the substituted elemental species by analyzing the characteristics represented by the detection signal according to the ALCHEMI method illustrated in FIG. Furthermore, it can be specified which site in the crystal structure of the sample 24 has been replaced. Since the specific method of the ALCHEMI method is known, the detailed description is omitted here.

<Embodiment 1: Summary>
The electron microscope 100 according to the first embodiment irradiates the electron beam 26 to the sample 24 having a hemispherical shape from a plurality of incident angles. The penetration length of the electron beam 26 incident on the hemispherical portion is equal regardless of which direction it is incident. Therefore, even if the incident angle of the electron beam 26 is changed in the two-dimensional direction, the spectra of the characteristic X-rays 28 are considered to be equivalent. Thereby, the analysis accuracy of the ALCHEMI method using the characteristic X-ray 28 can be improved.

Second Embodiment
In Embodiment 2 of the present invention, the procedure for preparing a hemispherical sample will be described in more detail. The FIB apparatus can process a three-dimensional structure of any shape by scanning an ion beam focused to a few nm diameter on a two-dimensional surface. Since the ion beam cuts the sample to a depth generally proportional to the irradiation time, the height of the sample 24 along the beam irradiation direction can be controlled by controlling the FIB irradiation time.

FIG. 7 is a view for explaining the process of processing the sample 24. As shown in FIG. FIG. 7A is a top view showing the planar shape of the sample. In the following, it is assumed that a hemispherical sample of radius r is produced with the analysis point 29 as the origin (0, 0) of the coordinates. FIG. 7 (b) is a side view of FIG. 7 (a). The FIB processing is started from the sample surface before processing, and the apex height of the hemisphere, that is, the hemisphere radius is r. Therefore, from the unprocessed sample surface up to a height where there is a vertex of the hemisphere is required to scrape off a thickness of uniform t _0. The plate-like portion of the sample 24 is formed by further shaving the thickness (t ₀ + r).

The hemispherical parts vary in height depending on the location. Assuming that the coordinates in the xy plane with the analysis point 29 as the origin are (x, y) and the processing thickness at the coordinates (x, y) is t (x, y), the ion beam irradiation time is T (x, y) It becomes = A · t (x, y). A is a sputtering rate (unit: sec / nm, for example) which varies depending on the material to be scraped / ion beam species / ion beam energy. The processed thickness t (x, y) varies depending on the place. When viewed from the z direction, in the inside of the hemisphere, that is, in the region of (x ² + y ² ) ≦ r ² , t (x, y) = r− (r ² − (x ² + y ² )) ^1/2 . Outside the hemisphere and on the sample surface, that is, in the region of (x ² + y ² ) ≧ r ² and −a ≦ x ≦ a, −b ≦ y ≦ b, t (x, y) = r.

  There are various methods of beam scanning as shown in FIG. 7C to FIG. 7E, but once the processing position coordinates are determined, the ion beam irradiation time can be uniquely determined. Therefore, the procedure of FIG. 7 (b) can be carried out with any scanning shape.

  In the present invention, the sample has a hemispherical shape, but it is difficult to produce an ideal hemisphere as long as it is processed by an ion beam. How much the sample shape of the profile deviated from the hemisphere is permitted depends on the pursuit of the analysis accuracy. Therefore, in the following, we take the idea of shape fluctuation and show the basis for defining it.

FIG. 8 is a diagram defining the hemispherical fluctuation .DELTA.r. The outer peripheral surface of an ideal hemisphere of radius r is defined by the spherical equation x ² + y ² + z ² = r ² . If the allowable shape fluctuation in the radial direction is defined as Δr, then (r−Δr) ² ≦ x ² + y ² + z ² ≦ (r + Δr) ² holds. When the sample shape fluctuates, the length of the electron beam 26 passing through the sample also fluctuates, and the X-ray dose to be excited also fluctuates. Furthermore, the excited characteristic X-rays 28 are emitted out of the sample while being absorbed in the sample. In particular, in the case of X-rays having an energy released from light elements of less than 1 keV, the influence of absorption can not be ignored.

  Assuming that the intensity of the characteristic X-ray 28 emitted at each of the hemispherical radius r and (r + Δr) is I and I ′, (I−I ′) / I = Io · [exp (−r / λ) It becomes -exp (-(r + delta r) / lambda) / Io * exp (-r / lambda) = 1 -exp (-delta r / lambda). From this, the relationship of Δr = λ · ln (I / I ′) is derived. That is, since the originally obtained X-ray dose is I 'where I should be, the difference between these becomes the measurement error.

  FIG. 9 is a view showing the relationship between the X-ray intensity fluctuation (I / I ') and the allowable film thickness fluctuation Δr. The allowable measurement error differs depending on the evaluation content. Here, as an example, the relationship between the X-ray intensity fluctuation (I / I ′) at 180 nm, which is a typical mean free path (for example, boron in silicon), and the allowable film thickness fluctuation Δr is shown. Since I / I ′ = 1 (= 100%) ideally, the allowable film thickness fluctuation Δr in this case is zero. When I / I '= 0.8 (= 80%) or so is sufficient, the allowable film thickness fluctuation Δr is approximately 40 nm. In the analysis of trace elements, etc., I / I '= 0.99 (= 99%) or so is required, and in this case, the allowable film thickness fluctuation Δr is about 1.8 nm. Thus, the fluctuation of the sample shape can be defined in relation to the required analysis accuracy.

<About the modification of the present invention>
The present invention is not limited to the embodiments described above, but includes various modifications. The above-described embodiments are described in detail in order to explain the present invention in an easy-to-understand manner, and are not necessarily limited to those having all the described configurations. Further, part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Moreover, it is possible to add, delete, and replace other configurations for part of the configurations of the respective embodiments.

  In the above embodiment, the calculator 23 can obtain Δr by obtaining the difference between the X-ray intensities in the respective incident directions of the electron beam 26 using the detection result by the X-ray detector 19. If the calculated Δr does not fall within the above-described allowable range, the calculator 23 may output an alert to that effect. Thus, the user can take measures such as replacing the sample 24.

  In the above embodiment, it has been described that the portion (replacement site) of the crystal structure of the sample 24 replaced by the additive element is analyzed by the ALCHEMI method. It is assumed that the functionality is expressed by replacing the part of the crystal structure with the additive element because the crystal structure has periodicity. Therefore, the possibility of applying the present invention to other molecular structures having similar periodicity is considered.

11: Electron gun 11 ': Electron gun control circuit 12: Condenser lens 12': Condenser lens control circuit 13: Condenser aperture 13 ': Condenser aperture control circuit 14: Axis shift correction deflector 14': Axis shift correction deflector Control circuit 15: aberration corrector 15 ': aberration corrector control circuit 16: image shift deflector 16': image shift deflector control circuit 17: scan deflector 17 ': scan deflector control circuit 18: objective Lens 18 ′: objective lens control circuit 19: X-ray detector 19 ′: X-ray detector control circuit 20: sample holder 20 ′: sample holder control circuit 21: electron detector 21 ′: electron detector control circuit 22: projection Lens 22 ': Projection lens control circuit 23: Computer 24: Sample 26: Electron beam 27: Transmission electron beam 28: Characteristic X-ray 29: Analysis point 30: Micro probe 31: Sample stand

Claims (6)

An electron microscope that irradiates a sample with an electron beam,
An electron beam source for emitting the electron beam,
A deflector for irradiating the electron beam from a plurality of angles with respect to the sample by changing an angle at which the electron beam is incident on the sample;
An X-ray detector that detects X-rays generated from the sample when the sample is irradiated with the electron beam;
A stage on which the sample is placed;
Equipped with
The sample has a convex portion protruding from the stage toward the electron beam source,
The convex portion has a first distance at which the electron beam passes through the sample when the sample is irradiated with the electron beam from the first direction, and the convex portion is different from the first direction with respect to the sample. When irradiated with the electron beam from two directions, it has a shape in which the second distance through which the electron beam passes through the sample becomes equal to each other.
The X-ray detector irradiates the sample with the X-ray emitted from the sample when the sample is irradiated with the electron beam from the first direction, and the sample emits the electron beam from the second direction with the sample. An electron microscope characterized in that when the X-ray emitted from the sample is detected, a signal representing a characteristic in a region of the sample irradiated with the electron beam is acquired. When the X-ray detector irradiates the sample with the electron beam from a plurality of angles, the X-ray detector detects the X-ray generated from the sample, thereby the area of the sample irradiated with the electron beam. The electron microscope according to claim 1, wherein a signal used to identify an element added to a sample is acquired. The X-ray detector is replaced by the element in the crystal structure of the sample by detecting the X-rays generated from the sample when the sample is irradiated with the electron beam from a plurality of angles. The electron microscope according to claim 2, wherein a signal used to identify the location of the substitution site is acquired. At least a part of the convex portion has a spherical shape,
Assuming that the radius of the spherical shape is r and the allowable error as an error of the radius is Δr, all the portions of the spherical shape have a radius within the range from r + Δr to r−Δr. The electron microscope according to claim 1, characterized in that: The intensity of the X-ray passing through the portion having the radius r in the spherical shape is I, and the intensity of the X-ray passing through the portion having the radius r + Δr in the spherical shape is I '. Assuming that the mean free path in the sample of the electron beam incident on the sample or the mean free path of the X-ray emitted from the sample is λ, the inequality: Δr ≦ λ · ln (I / I ′) holds. The electron microscope of Claim 4 characterized by the above-mentioned. The electron microscope further includes an arithmetic unit that calculates the radius of the convex portion based on the intensity of the X-ray detected by the X-ray detector.
The electron microscope according to claim 5, wherein the computing device outputs an alert to that effect when the radius of the convex portion calculated does not satisfy the inequality.

Images