Method for manufacturing sample for tomography and method for acquiring tomography data

Abstract

To provide a method for manufacturing a sample for tomography and a method for acquiring tomography data that enable easy and highly accurate alignment of tomography data groups while solving the problem of inaccurate alignment.SOLUTION: The method for manufacturing a sample for tomography using a transmission electron microscope includes a metal oxide powder adhesion step of adhering a metal oxide powder having an atomic number of 14 or less and dispersed in a liquid to an observation surface of an observation target as an observation sample surface. The method for acquiring tomography data includes: placing the above-described sample in a sample chamber of the transmission electron microscope; performing alignment using the metal oxide powder having an atomic number of 14 or less existing on the sample surface as markers; and obtaining tomography data of the sample.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for preparing a sample for tomography and a method for acquiring tomography data. [Background technology]

[0002] TEM (Transmission Electron Microscope) and STEM (Scanning Transmission Electron Microscope) are instruments used to obtain enlarged projection images of samples, and are transmission electron microscopes (hereinafter, TEM and STEM are collectively referred to as transmission electron microscopes) that are also suitable for observing ultrafine structures on the nanometer scale. Tomography is known as one of the observation techniques for transmission electron microscopes. Tomography is a useful measurement method that uses computed tomography to observe structural factors within materials in three dimensions. Tomography involves taking multiple transmission images (sometimes referred to as tomography data sets) while tilting the sample relative to the electron beam, and then reconstructing the series of tilt transmission images (sometimes referred to as a tomography data set) to obtain a 3D image. To obtain this 3D image, the tilt axis must be aligned with the central axis of the sample during the reconstruction calculation. Therefore, to obtain accurate 3D images, the series of tilt transmission images must be aligned to this ideal state. The accuracy of this alignment is known to have a significant impact on the results of 3D image reconstruction, and precise correction is required by calculating the pair correlation for each tilt image (tilted transmission image) and tracking the trajectory of a marker applied to the sample at each angle.

[0003] Various studies have been conducted to address the above-mentioned issues. For example, Japanese Patent Application Laid-Open No. 2017-26612 (Patent Document 1) describes a method for aligning a tomography data set, which uses a charged particle beam to create reference holes and then improves sample positioning, acquisition, alignment, reconstruction, and visualization of the tomography data set. Furthermore, Japanese Patent Application Laid-Open Publication No. 2012-2552 (Patent Document 2) describes a method for aligning a group of tomography data, which is characterized in that when a sample is processed into a needle-shaped sample using a focused ion beam, a mask is formed from the sample, thereby separating areas where gold nanoparticles are to be attached from areas where they are not to be attached within a vapor deposition device. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-026612 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-002552 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent documents 1 and 2 are useful inventions that improve the accuracy of aligning tomography data sets. However, when a reference hole is formed in a sample as in the method described in Patent document 1, damage from sample processing may remain inside the sample. In addition, the edge effect tends to make it difficult to obtain accurate transmission images near the reference hole. Furthermore, the method disclosed in Patent Document 2 uses gold microparticles as markers, but the gold microparticles are reflected when a transmission image is acquired using a transmission electron microscope, and in the three-dimensional image created by combining tilted transmission images, part of the image is lost due to the gold microparticles, so an accurate observation image cannot be obtained. Therefore, an object of the present invention is to provide a method for manufacturing a tomography sample and a method for acquiring tomography data that solve the problem of inaccurate alignment and facilitates high-precision alignment of a group of tomography data. [Means for solving the problem]

[0006] The present invention has been made in view of the above-mentioned problems. That is, one aspect of the present invention is a method for manufacturing a sample for tomography of a transmission electron microscope, which includes a metal oxide powder deposition step of depositing metal oxide powder having an atomic number of 14 or less dispersed in a liquid onto the surface of the sample to be observed. Another aspect of the present invention is a method for acquiring tomography data, which comprises placing the above-mentioned sample in the sample chamber of a transmission electron microscope, aligning the sample using metal oxide powder with atomic number 14 or less present on the sample surface as a marker, and obtaining tomography data of the sample. [Effects of the Invention]

[0007] According to the present invention, it is possible to improve the accuracy of tomography data and acquire tomography data that can be aligned with high precision, thereby facilitating the reconstruction of the acquired tomography data group. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 shows a secondary electron image (SE image), a dark-field image (STEM DF image), a bright-field image (STEM BF image), and an element map of a metal material. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, a method for producing a tomography sample according to an embodiment of the present invention will be described in detail with reference to examples. The method will be described assuming that tomography data of a metal material is obtained. Prepare a sample for tomography. Sample preparation up to the coating process can be done using the usual method. For example, it can be done as follows. (1) Sample preparation: A predetermined portion of the target metal material is machined to obtain a small piece. (2) Polishing: The small pieces are polished to a smooth surface to obtain small pieces for thin sections. (3) Thin section preparation: The small pieces for thin sections are thinned to a thickness of several tens to several hundreds of nanometers by ion beam milling or electrolytic polishing to obtain thin sections. (4) Cleaning: The flakes are cleaned in an ultrasonic cleaner to remove surface dirt and residues.

[0010] <Metal oxide powder deposition process> Next, in the present invention, the above-mentioned thin flake is used as an observation sample to be observed, and a metal oxide powder having an atomic number of 14 or less dispersed in a liquid is attached to the surface of this observation sample. Note that, in the present invention, "metal oxide powder having an atomic number of 14 or less" refers to a metal oxide whose metal element has an atomic number greater than 14. In the present invention, if the metal in the metal oxide has an atomic number greater than 14, there is a possibility that the transmission of the electron beam may be hindered in the observation step described below. Of the metal oxide powders with atomic numbers equal to or less than 14, it is preferable to use SiO2 powder, which is easily available and easily dispersible in organic solvents. For example, Figure 1 shows a secondary electron image (SE image), dark-field image (STEM DF image), bright-field image (STEM BF image), and elemental map of a metal material observed at 250,000 magnification. The measurement conditions were an accelerating voltage of 200 kV, a probe size of 0.5 nm, a camera length of 25 m, and an elemental map size of 256 × 256 pixels. The arrow in the secondary electron image indicates metal oxide particle 1. Metal oxide particle 1 is SiO2 powder with an average primary particle size of 40 nm, and Si is a metal element with atomic number 14. From Figure 1, we can see that SiO2 metal oxide particles 1 can be seen in the secondary electron image and elemental map, but cannot be seen in the bright-field image, which is the best suited for tomography data. For this reason, in this invention, we use metal oxide powders with atomic numbers of 14 or less.

[0011] The particle size of the metal oxide powder used in the present invention can be selected depending on the magnification of observation and is not particularly limited. For example, when observing at a magnification of 1,000,000 times, a metal oxide powder with a particle size of 150 nm or less should be used. The preferred particle size is 100 nm or less, and the more preferred particle size is 50 nm or less. The solution used to disperse the metal oxide powder is not particularly limited as long as it does not adversely affect the properties of the observation sample. For example, if the observation sample is made of steel, organic solvents such as ethanol or acetone, which dry quickly and are unlikely to cause rust on the sample, can be used. Furthermore, because static electricity can cause the SiO2 powder to aggregate, reducing its dispersibility in the solution, it is preferable to vibrate the SiO2 powder with an ultrasonic vibrator for at least 3 minutes before application to remove any aggregates. The amount of solution applied should be such that the observation surface is wetted with the solution containing the dispersed metal oxide. Furthermore, since residual moisture in the sample after application can lead to oxidation, it is preferable to dry the coated sample after the metal oxide powder application process. The observation image shown in Figure 1 above was obtained by ultrasonically vibrating SiO2 metal oxide particles for more than three minutes, dispersing the metal oxide particles in a liquid, and then dripping the metal oxide powder onto the surface of the observation sample. Besides the aforementioned dripping method, other typical methods for adhering metal oxide powder to the surface of the observation sample include coating and immersion.

[0012] <Observation process> The observation sample to which the metal oxide powder has been attached by the metal oxide powder attachment step described above is placed in the sample chamber of a transmission electron microscope, and tomography data (transmission image) is obtained with the transmission electron microscope. As mentioned above, metal oxide powders with atomic numbers below 14 have small mass numbers and are easily transmitted by electron beams, so they have almost no effect on transmission images. On the other hand, metal oxide powder 1, which serves as a marker, can be recognized in elemental maps. This effect enables highly accurate alignment when acquiring tomography data using a transmission electron microscope. Therefore, as shown in Figure 1, when bright-field images (STEM BF images) are acquired while acquiring elemental maps, and multiple transmission images are taken while tilting the observation sample relative to the electron beam to obtain a series of tilted transmission images (tomography data sets), the metal oxide particles function as markers for alignment.

[0013] According to the method of the present invention, it is not necessary to provide a reference hole as in the conventional method, and there is no influence of the edge effect.Furthermore, it is clear that the marking object, such as a gold particle, does not appear in the image. As described above, the present invention makes it possible to improve the accuracy of tomography data and acquire tomography data that can be aligned with high precision, thereby facilitating the reconstruction of acquired tomography data sets. [Explanation of symbols]

[0014] 1. Metal oxide powder

Claims

1. A method for producing a sample for tomography of a transmission electron microscope, comprising: A method for manufacturing a tomography sample, comprising a metal oxide powder deposition step of depositing a metal oxide powder having an atomic number of 14 or less dispersed in a liquid onto the surface of an observation sample to be observed.

2. A method for acquiring tomography data, comprising placing the sample according to claim 1 in a sample chamber of a transmission electron microscope, aligning the sample using metal oxide powder with an atomic number of 14 or less present on the sample surface as a marker, and obtaining tomography data of the sample.

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