Computer-aided method for determining the elemental fraction, in particular the Li fraction, of an identified element with a low atomic number, and corresponding device for processing the data

Abstract

The present invention relates to a computer-aided method for determining the elemental fraction of an identified element, in particular of low atomic number, in particular lithium, of an examination area of ​​a sample bombarded with primary electrons, wherein a backscattered electron signal, preferably a backscattered electron image, captured using a backscattered electron detector and a spectroscopic elemental composition of the examination area determined using an X-ray spectroscopic detector such as an EDX detector are obtained. According to the invention, in order to determine the fraction of the identified element, a measured gray value S determined from the backscattered electron signal is used. M When combined with the elemental fraction of the spectroscopic elemental composition, a viable quantitative identification can be achieved. The invention further relates to a device for processing data and a computer product for carrying out the method.

Description

[Technical Field]

[0001] The present invention relates to a computer-aided method for determining the elemental fraction of an identified element, particularly one with a low atomic number, in particular lithium, of an examination area of ​​a sample bombarded with primary electrons, wherein a backscattered electron signal, preferably a backscattered electron image, captured using a backscattered electron detector and a spectroscopic elemental composition of the examination area determined using an X-ray spectroscopy detector, such as an EDX detector, is obtained.

[0002] The invention also relates to a device for processing data.

[0003] The invention further relates to a computer program product.

[0004] It is known from the prior art that both the topography and also the elemental composition of an examination area of ​​a sample can be determined by electron microscopy. For this purpose, primary electrons are typically emitted into the examination area using an electron gun, then inelastically scattered electrons are detected using a secondary electron detector, elastically scattered electrons are detected using a BSE detector (backscattered electron detector), and / or emitted X-ray radiation is detected using an X-ray spectroscopy detector, typically an EDX detector (energy-dispersive X-ray spectroscopy detector) or a WDX detector (wavelength-dispersive X-ray spectroscopy detector). In position-dependent rendering, based on the detected secondary electrons, a gray value image of the sample surface suitable for displaying the topography of the sample surface in the examination area can be generated, based on the detected backscattered electrons (BSE), a gray value image of the sample surface suitable for depicting the spatial distribution of elements on the sample surface can be generated, and based on the detected X-ray radiation, the elemental composition of the sample surface can be determined.

[0005] The depiction of spatial elemental distributions by backscattered electron detectors is usually based on the dependence of the backscattered electron yield on the mean atomic number of the examination area: the higher the mean atomic number, the higher the backscattered electron yield and the brighter the corresponding grey values ​​in the grey value image.

[0006] Identification of elemental composition by EDX is usually based on the energy-dependent depiction of detected X-ray radiation and the element-specific assignment of characteristic X-ray radiation peaks. However, difficulties usually result: elements with low atomic numbers, typically elements with atomic numbers lower than 5, i.e., elements with atomic numbers lower than boron, can no longer be detected. Quantitative analysis is often only feasible for high atomic numbers, often starting from atomic number 7. This can be explained by the fact that the characteristic X-ray radiation of such elements with low atomic numbers is very low energy and is often absorbed both by the sample itself and by the window of the X-ray radiation detector. In particular, through the use of windowless X-ray radiation detectors, the detection limit can be extended to low atomic numbers; achievable detection limits are usually higher than 20 wt% and up to 30 wt%. However, the detection and, in particular, quantitative identification of elements with low atomic numbers proves to be inherently uncertain. This is particularly true for elements such as lithium or beryllium.

[0007] This is what the present invention addresses. The aim of the present invention is to specify a method of the type mentioned first when it allows in a feasible manner the quantitative identification of identified elements, especially those with low atomic numbers.

[0008] Besides, the objective is to specify the type of device that is first mentioned when it allows in a feasible manner the quantitative identification of identified elements, especially those with low atomic numbers.

[0009] Furthermore, the objective is to specify a computer program product of the type first mentioned when it allows for the quantitative identification of identified elements, particularly those with low atomic numbers, in a feasible manner.

[0010] According to the invention, the object is achieved in a method of the type mentioned at the beginning when the backscattered electron signal, in particular the measured gray value determined from the backscattered electron image, is combined with the elemental fraction of the spectroscopic elemental composition in order to determine the fraction of the identified element.

[0011] The present invention is based on the idea of ​​using the dependence of the backscattered electron yield or backscatter coefficient on atomic number to compensate for the lack of detection sensitivity of X-ray spectroscopy. The gray value of a backscattered electron image or of one or more of its pixels, also referred to as the measured gray value, is a function of the average atomic number of the examination area of ​​the sample. Elements with low atomic numbers, such as lithium or beryllium, reduce the gray value of the backscattered electron image compared to the gray value image in which the element is not present. The spectroscopic elemental composition of the examination area, i.e., the elemental composition of the examination area determined by X-ray spectroscopy, combined with the gray value allows the identification of elements or fractions thereof that are not or only weakly represented by the spectroscopic elemental composition. In particular, identified elements with low atomic numbers, especially those with atomic numbers less than 5, preferably lithium or beryllium, can be determined quantitatively or proportionally in this way. This allows for a simple and feasible identification of fractions of one or more identified elements that would otherwise be determined entirely or only by expensive methods.

[0012] It should be understood that, according to the convention in the art and according to this specification, gray value and measured gray value are typically used as synonyms for brightness value or intensity value, which indicate the brightness or intensity of a measured signal or backscattered electron signal of one or more pixels of an image. The term gray value, in particular, should be viewed independently of any specific coloring and, in principle, can be combined with any desired color. A gray value may thereby be an average or representative gray value of several other gray values. It is usually provided that the backscattered electron image is generated from the backscattered electron signal captured by the backscattered electron detector, and therefore the intensity of the backscattered electron signal is typically referred to as the gray value or measured gray value. However, generation of the backscattered electron image from or from the backscattered electron signal is not required at all. For example, the backscattered electron signal or its measured gray value may be used alternatively or cumulatively for measuring individual points or lines in the inspection area.

[0013] It is feasible if the combination between the measured gray value and the elemental fraction of the spectroscopic elemental composition occurs using the backscattering coefficient of the elemental fraction. According to the convention in the art, the backscattering coefficient typically refers to the ratio of backscattered electrons emitted between a surface area or an inspection area and the primary electrons that strike the surface. The backscattering coefficient can be taken from the literature and / or from simulation results and / or determinations using calibration measurements. The gray value S of the backscattered electron signal or backscattered electron image can be calculated using the backscattering coefficient η, in particular the average or representative backscattering coefficient

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[0014] It is feasible if the reference gray value is calculated using the elemental fraction of the spectroscopic elemental composition, and the fraction of the identified element is determined using a comparison or combination, in particular the difference, of the measured and reference gray values. Since the measured gray value is calculated from the backscattered electron signal, on the one hand, and the reference gray value is calculated using the elemental fraction determined by X-ray spectroscopy, on the other hand, from their comparison, the fraction of the identified element that is not present in the spectroscopic elemental composition or its elemental fraction, or that is present only to a barely noticeable extent or is not represented thereby, can be easily determined.

[0015] For high feasibility it has proven effective if the comparison of the measured and reference grey values ​​is realized by means of their difference, however, cumulatively or alternatively they may also be placed in relation by means of the formation of a proportion or ratio or by means of another mathematical comparison or combination method customary in the art.

[0016] It is advantageous if the reference grey value is determined using the respective backscattering coefficients of the elemental fractions of the spectroscopic elemental composition, which allows a particularly feasible calculation of the comparative grey value.

[0017] Typically, the measured gray value is an average or representative measured gray value and the reference gray value is possibly an average or representative reference gray value, whereby averaging is usually performed over the examination area and / or the detected elements.

[0018] For high accuracy, it is beneficial if one or more calibration samples are measured using a backscattered electron detector to assign specific gray values, especially gray value contributions, to specific elements, element fractions, or elemental compositions. The relationship between the measured gray values ​​and the spectroscopic elemental compositions, and therefore the combination, can thus be more precisely demonstrated. In particular, the reference gray values ​​and / or backscattering coefficients can thus be determined in a feasible manner. Advantageously, the measurement of system-dependent effects can be taken into account in this way. It should be understood that the gray values ​​assigned to elements or elemental compositions are usually average or representative gray values. Samples with known compositions, especially single-element calibration samples, usually with the highest possible purity, are typically used as calibration samples, also referred to as calibration standards. Correlations between specific elements or elemental compositions and reference gray values ​​or their contributions can thus be easily generated.

[0019] Similarly, it is correspondingly advantageous if at least one calibration sample made of or formed from the identified element is measured or is measured using a backscattered electron detector in order to assign the gray value, in particular the gray value contribution, to the identified element or a fraction of the identified element. The fraction of the identified element can thus be determined in a particularly precise manner. This allows for a precise combination of the measured gray value and the spectroscopic element composition. In particular, the associated backscattering coefficient can thus be determined feasibly, in particular taking into account the measurement of system-dependent effects. It should be understood that this is usually the average gray value or gray value contribution. Alternatively or additionally, it may be advantageous if the assignment of the gray value or gray value contribution to the fraction of the identified element is performed by simulation, in particular Monte Carlo simulation.

[0020] In the measurement of the aforementioned calibration samples, the gray value S is usually determined as a function of the atomic number Z, i.e., S / Z, or a corresponding calibration curve is formed. Here, non-linear relationships may appear, which makes the calibration difficult. In this type of calibration or in the measurement of the calibration samples, the ratio of the gray value S and the backscattering coefficient η is determined as a function of the atomic number Z, i.e., (

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[0021] It has proven effective if the backscattered electron detector is operated during the measurement of one of the aforementioned calibration samples in such a way that the contrast and / or brightness of the determined grey value is in the range of 20% to 80%, in particular 30% to 70%, preferably 40% to 60% of the maximum contrast or brightness value of the relevant grey value spectrum, which can usually be set using the operating parameters of the backscattered electron detector.

[0022] Typically, an examination area of ​​the sample is bombarded with a primary electron beam, and electrons backscattered from the examination area are detected using a backscattered electron detector to capture a backscattered electron signal or a backscattered electron image, and X-rays emitted from the examination area are detected using an X-ray spectroscopy detector, such as an EDX detector and / or a WDX detector, to determine the spectroscopic elemental composition. This allows measurements to be carried out in particular by standard methods, such as electron microscopy, in particular scanning electron microscopy or transmission electron microscopy. The detection using the backscattered electron detector and the detection using the X-ray spectroscopy detector can be performed in parallel or sequential manner in time.

[0023] It is advantageous if the detection using the backscattered electron detector and the detection using the X-ray spectrometric detector are carried out in an essentially parallel manner in time, which allows low error measurements and, correspondingly, precise identification of the fraction of the identified element.

[0024] In principle, detection using a backscattered electron detector and detection using an X-ray spectrometer detector can be performed at the same or different primary electron beam voltages for accelerating the primary electrons. Backscattered electrons and X-rays usually have different interaction volumes, i.e., they are emitted from different sample volume regions. The interaction volume of X-ray radiation is therefore usually larger than that of backscattered electrons. To make them equal, it is advantageous if detection using a backscattered electron detector and detection using an X-ray spectrometer detector are performed at different primary electron beam voltages for accelerating the primary electrons. Detection using a backscattered electron detector is usually performed at a higher primary electron beam voltage than detection using an X-ray spectrometer detector. It has proven effective if the respective primary electron beam voltages are selected so that the interaction volume depths of the interaction volumes in the detection using the backscattered electron detector and the detection using the X-ray spectrometer detector are essentially the same size at at least the 70th percentile, particularly at least the 80th percentile, preferably at least the 90th percentile, and particularly preferably at least the 95th percentile of the respective interaction volume depths. Although measurements cannot usually be performed in parallel in time, adjusting the interaction volume advantageously allows for particularly high accuracy when identifying the fraction of the identified element. It can be beneficial if the respective primary electron beam voltages are determined by simulation, in particular Monte Carlo simulation. In this way, the interaction volumes can be particularly well adjusted to each other. In certain cases where the material structure to be inspected is larger than the interaction volume in detection using an X-ray spectroscopic detector, approximately equal primary electron beam voltages can usually be used.

[0025] It is advantageous if the method is implemented to determine the fraction of lithium or beryllium, or if said elements are envisaged as identifying elements. Through the combination or comparison of the measured grey values ​​and the spectroscopic elemental composition, or in particular the reference grey values, these elements or their fractions can be determined in a feasible and low-cost manner, in particular within the scope of electron microscopy. This is particularly true for lithium, which can only be quantitatively determined using methods conventional in the art and at particularly great expense.

[0026] The fraction x of an identified element is the measured gray value S M and the reference gray value S V The difference between the gray values ​​is expressed as a ratio to the reference gray value, i.e.,

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[0027] It is advantageous if the distribution of the identified element is shown or depicted along the surface of the sample or examination area. In particular, it is thus possible to adapt the examination area or its extensions, in particular iteratively, in order to increase the accuracy of the measurement or identification of the identified element. This can preferably be done within the scope of elemental mapping.

[0028] According to the present invention, the object is achieved by a device for processing data of the type mentioned at the beginning, which comprises means such as at least one processor adapted to carry out one of the methods described above, in particular to carry out a combination between the measured gray values ​​determined from the backscattered electrons and the spectroscopic elemental composition in a defined manner in order to determine the fraction of the identified element, or in particular to calculate the measured gray values ​​from the backscattered electron signal, in particular the backscattered electron image, on the one hand, and to calculate the reference gray values ​​using the elemental fraction of the spectroscopic elemental composition on the other hand, and then to determine the fraction of the identified element by comparing the measured gray values ​​and the reference gray values. A feasible identification of the fraction of the identified element can be carried out in this way, as well as the aforementioned advantages and effects. Typically, the device comprises one or more processors or is embodied in or as a computer.

[0029] It is to be understood that the device according to the invention may be embodied in accordance with or in a similar manner to the features, advantages and effects described in particular within the scope of the method for determining elemental fractions, in particular elemental fractions, described above, and the same also applies to the device according to the invention relating to this method.

[0030] It has proven advantageous if the device is embodied as part of an electron microscope or is coupled to the electron microscope for data transmission. This allows a particularly viable identification of the identified element or its fraction. The electron microscope can thereby be embodied, for example, as a scanning electron microscope or a transmission electron microscope. It is understood that the method for determining the element fraction is thereby typically realized or performed within the scope of electron microscopy measurements. The electron microscope advantageously comprises a backscattered electron detector for capturing backscattered electron signals, in particular backscattered electron images, and an X-ray spectroscopic detector for determining the spectroscopic elemental composition of the examined area of ​​the sample.

[0031] In the case of a transmission electron microscope (TEM), due to its structural principle, atomic number-dependent electron scattering usually occurs essentially in the direction of the primary electron's emission or in the direction of the primary electron passing through the sample. Accordingly, in the specific case of a transmission electron microscope, electrons penetrating the sample that are suitable for affecting atomic number contrast correspond to backscattered electrons for purposes of this specification and should be specifically considered to be comprised or included by the term backscattered electrons. Accordingly, in this case, an electron detector that detects this type of electron suitable for generating atomic number contrast should be specifically considered to be comprised or included by the term backscattered electron detector for purposes of this specification. In the case of a transmission electron microscope, a high-angle annular darkfield detector (HAADF detector) has proven particularly useful as a backscattered electron detector, in which electrons are detected, typically concentrically around the optical axis of the transmission electron microscope, on the side of the sample facing away from the primary electron source of the transmission electron microscope. Atomic number contrast is thereby typically scaled as the square of the atomic number.

[0032] According to the present invention, another object is achieved with a computer program product of the type mentioned at the beginning, which comprises commands that, when executed by a computer or the aforementioned device for processing data, cause said computer or device to carry out a method for determining elemental fractions, in particular to carry out a combination between measured gray values ​​determined from backscattered electrons and spectroscopic elemental compositions in a defined manner in order to determine the fraction of an identified element, or in particular to calculate measured gray values ​​from a backscattered electron signal, in particular a backscattered electron image, on the one hand, and to calculate reference gray values ​​using the elemental fraction of the spectroscopic elemental composition, on the other hand, and then to determine the fraction of an identified element using a comparison of the measured and reference gray values. Similar to the aforementioned advantages and effects, an executable identification of the fraction of an identified element can be carried out in this way. Advantageously, the computer program product is embodied as part of a device for processing data or is intentionally installed in said device.

[0033] It is to be understood that the computer program product according to the invention may be embodied in accordance with or in a similar manner to the features, advantages and effects described in particular within the scope of the method for determining elemental fractions, in particular elemental fractions, described above.

[0034] Advantageously, a computer-readable storage medium is provided having the computer program product stored thereon, which enables the executable execution of the computer program product or the execution of the method for determining elemental fractions. The storage medium may be embodied as a volatile or non-volatile storage medium. The storage medium may conveniently be embodied as part of the aforementioned device for processing data. [Brief explanation of the drawings]

[0035] Additional features, benefits and advantages will become apparent from the exemplary embodiments described below, with reference to the drawings.

[0036] [Figure 1] FIG. 1 is a schematic diagram showing the conceptual sequence of a method for determining the fraction of lithium as an identifying element. [Figure 2] FIG. 1 shows a backscattered electron image of a LAX410 alloy sample. [Figure 3] FIG. 1 shows a backscattered electron image of an LSX2021 alloy sample. [Figure 4] 1 is a calibration curve determined using calibration standards. [Figure 5] 1 is a calibration curve determined using calibration standards. DETAILED DESCRIPTION OF THE INVENTION

[0037] The measured gray value S of the sample surface M The gray value of the backscattered electron image, referred to as the reference gray value S, is a function of the average atomic number of the surface of the sample. Elements with low atomic numbers, such as lithium, reduce the gray value. Based on the elemental composition of the surface determined by X-ray spectroscopy, where lithium is not displayed due to the insufficient sensitivity of X-ray spectroscopy for lithium, a reference gray value S is used. V The measured gray value S of the backscattered electron image is calculated by M Refer to the gray value S V By combining this with the measured gray value S, the Li content at the surface of the sample can be determined. M and the reference gray value S V are thereby designated as representative values, respectively. Backscattered electron images, referred to as BSE images according to convention in the art, are typically captured using a backscattered electron detector or BSE detector. The elemental composition of the surface is commonly determined by energy dispersive X-ray spectroscopy, referred to as EDX.

[0038] Figure 1 shows the conceptual basis for the example of a Mg-Li-Al ternary alloy. The backscattering coefficient of Li, η Li , and the backscattering coefficient η of a mixture of Mg and Al at the ratio y rest with the gray value S M , where S M =f(ηLi ,η rest ,x Li ) can be assigned to a backscattered electron image (BSE image) of the examined area of ​​the specimen. In this case, x Li means the ratio of Li to the mixture of Mg and Al. Furthermore, the backscattering coefficients of Al and Mg, η Al and η Mg with η rest =f(η Al ,η Mg ,y) also holds. The backscattering coefficient η Al , η Mg and η Li can be determined from the literature or by simulation. y can be determined by X-ray spectroscopy, and S M can be determined from the backscattered electron image. The reference gray value S from the Al and Mg fractions determined by EDX V By identifying the reference gray value S, the functional relationship can be solved mathematically. V Preferably, this can be done using backscattered electron images of calibration standards to accurately determine .

[0039] As a result, x Li can be calculated and the fraction of Li on the surface of the examined sample can thus be determined.

[0040] The identification of the lithium fraction as an identifying element is described below as an example using two Mg-Li-based alloys, LAX410 and LSX2021. For this purpose, a backscattered electron image of each of the examined regions, referred to as ROIs (regions of interest) according to the convention in the art, of the alloy samples of each alloy is determined, and the spectroscopic elemental composition of each examined region is determined by energy dispersive X-ray spectroscopy (EDX). Table 1 shows the nominal compositions of LAX410 and LSX2021 in wt%. The alloy samples were produced using an induction furnace under an argon protective gas atmosphere. A field-emission scanning electron microscope was used to identify the alloy samples, a silicon drift detector was used for EDX, and a four-quadrant semiconductor detector was used to capture the backscattered electron images.

[0041] Table 1: Nominal composition in wt% of LAX410 and LSX2021 [Table 1] FIG. 2 shows a backscattered electron image of an LAX410 alloy sample, in which three examination regions, designated ROI 1, ROI 2, and ROI 3, which are examined in more detail in terms of their elemental composition, are marked. In a corresponding manner, FIG. 3 shows a backscattered electron image of an LSX2021 alloy sample, in which two examination regions, ROI 1 and ROI 2, are marked. These backscattered electron images are embodied in this case as gray value images with a gray value spectrum of 0 to 255.

[0042] Based on the elemental composition determined by EDX, also referred to as the spectroscopic elemental composition, of each test area, a respective average reference gray value

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[0043] formula

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[0044] Table 2: For LAX410: Mg, Al, and Ca fractions in wt% determined by X-ray spectroscopy; average measured gray values ​​of backscattered electron images

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[0045] It has thus been shown that the method according to the invention allows for the quantitative identification of elements with low atomic numbers, in particular lithium, with high accuracy, in that the backscattered electron signals, in particular the backscattered electron image, and the results of X-ray spectroscopy measurements are synergistically combined. Since the backscatter or backscatter coefficient of the backscattered electrons is typically dominated by the nuclei of the respective elements, the bonding state of the identified elements thereby advantageously plays a secondary role.

Claims

1. 1. A computer-assisted method for determining an elemental fraction of an identified element, including at least lithium or beryllium, of an examination area of ​​a specimen bombarded with primary electrons, comprising: A backscattered electron signal or a backscattered electron image is acquired using a backscattered electron detector, and a spectroscopic elemental composition of the examination area is determined using an X-ray spectroscopic detector, and a measured gray value S is determined from the backscattered electron signal to determine the fraction of the identified element. Ｍ is combined with an elemental fraction of the spectroscopic elemental composition, A method wherein a reference grey value S V is calculated using the elemental fraction of the spectroscopic elemental composition, and the fraction of the identified element is determined using the difference between the measured grey value S M and the reference grey value S V .

2. The method of claim 1, wherein the reference gray value S V is determined using the respective backscattering coefficients of the elemental fractions of the spectroscopic elemental composition.

3. 3. The method of claim 1, wherein one or more calibration samples are measured with the backscattered electron detector to assign specific grey values ​​to specific elements or elemental compositions.

4. 4. The method according to claim 1, wherein a calibration sample comprising or formed from the identified element is measured using the backscattered electron detector to assign a grey value to the identified element.

5. 5. The method of any one of claims 1 to 4, wherein the examination area of ​​the specimen is bombarded with a primary electron beam, electrons backscattered from the examination area are detected using a backscattered electron detector to capture the backscattered electron signal, and X-rays emitted from the examination area are detected using an X-ray spectroscopy detector such as an EDX detector to determine the spectroscopic elemental composition.

6. A device for processing data, comprising means such as at least one processor adapted to carry out the method according to any one of claims 1 to 5.

7. 7. The device of claim 6, embodied as part of an electron microscope or coupled to an electron microscope for data transmission.

8. A computer program comprising instructions which, when executed by a computer, cause said computer to carry out the method of any one of claims 1 to 5.

9. A computer-readable storage medium storing the computer program of claim 8.

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