Computer-assisted method for determining an element fraction of a determination element having a small atomic number, in particular a Li fraction, and corresponding device for processing data

Abstract

A computer-assisted method for determining an element fraction of a determination element, in particular with a small atomic number, especially lithium, of an examination region 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 spectroscopy element composition of the examination region determined using an X-ray spectroscopy detector, such as an EDX detector, are obtained. A practicable quantitative determination can be achieved if a measured gray value SM determined from the backscattered electron signal is combined with element fractions of the spectroscopy element composition in order to determine a fraction of the determination element. A device for processing data and to a computer product for carrying out the method is also disclosed.

Claims

1. A computer-assisted method for determining an element fraction of a determination element of an examination region of a sample bombarded with primary electrons, comprising the steps of: capturing a backscattered electron signal using a backscattered electron detector, obtaining using an X-ray spectroscopy detector a spectroscopy element composition of the examination region, combining a measured gray value SM determined from the backscattered electron signal with element fractions of the spectroscopy element composition in order to determine a fraction of the determination element, calculating a reference gray value SV using the element fractions of the spectroscopy element composition, and determining the fraction of the determination element using a comparison of the measured gray value SM and reference gray value SV.

2. The method according to claim 1, wherein combining the measured gray value SM and the element fractions of the spectroscopy element composition occurs using backscatter coefficients of the element fractions.

3. The method according to claim 1, wherein one or more calibration samples are measured using the backscattered electron detector, in order to assign specific gray values to specific elements or element compositions.

4. The method according to claim 1, wherein a calibration sample formed with or from the determination element is measured using the backscattered electron detector, in order to assign a gray value to the determination element.

5. The method according to claim 1, wherein the examination region of the sample is bombarded with a primary electron beam and electrons backscattered from the examination region are detected using the backscattered electron detector in order to capture the backscattered electron signal, and X-rays emitted from the examination region are detected using the X-ray spectroscopy detector in order to determine the spectroscopy element composition.

6. The method according to claim 1, wherein the determination element has a small atomic number.

7. The method according to claim 6, wherein the determination element is lithium.

8. The method according to claim 1, wherein the captured backscattered electron signal is a backscattered electron image.

9. The method according to claim 1, wherein the X-ray spectroscopy detector is an EDX detector.

10. The method according to claim 1, wherein the comparison is based on a difference between the measured gray value SM and reference gray value SV.

11. The method according to claim 5, wherein the X-ray spectroscopy detector is an EDX detector.

12. An electron microscope, comprising: a device having at least one processor for processing data; a backscattered electron detector; an X-ray spectroscopy detector; and wherein the electron microscope is configured to carry out the method according to claim 1.

13. A non-transitory computer-readable recording medium for use in a computer, the recording medium having a computer program recorded thereon for causing the computer in combination with a backscattered electron detector, and an X-ray spectroscopy detector to carry out the method according to claim 1.

Description

(1) Additional features, advantages, and effects follow from the exemplary embodiments described below. In the drawings which are thereby referenced:

(2) FIG. 1 shows a schematic illustration of a conceptional sequence of method for determining a fraction of lithium as a determination element;

(3) FIG. 2 shows an illustration of a backscattered electron image of an LX410 alloy sample;

(4) FIG. 3 shows an illustration of a backscattered electron image of an LSX2021 alloy sample;

(5) FIG. 4 and FIG. 5 show calibration curves determined -using calibration standards.

(6) A gray value of a backscattered electron image, referred to as the measured gray value S.sub.M, of a surface of a sample is a function of a mean atomic element of the surface of the sample. Elements having a low atomic number, such as lithium for example, decrease the gray value. By calculating a reference gray value S.sub.V based on an element composition of the surface determined by means of X-ray spectroscopy, in which composition lithium is not represented due to inadequate sensitivity of X-ray spectroscopy to lithium, the Li content of the surface of the sample can be determined by combining the measured gray value S.sub.M of the backscattered electron image with the reference gray value S.sub.V. Typically, the measured gray value S.sub.M and reference gray value S.sub.V are thereby respectively stated as an averaged value. The backscattered electron image, customarily referred to in the art as a BSE image, is typically captured using a backscattered electron detector, or BSE detector. The element composition of the surface is normally determined by means of energy-dispersive X-ray spectroscopy, referred to as EDX.

(7) FIG. 1 shows a conceptual basic concept by way of example of a ternary MgLiAl alloy. A gray value S.sub.M, where S.sub.M=f(.sub.Li, .sub.rest, x.sub.Li), can be assigned to a backscattered electron image (BSE image) of an examination region of a sample, with a backscatter coefficient .sub.Li for Li and a backscatter coefficient .sub.rest for a mixture of Mg and Al at a ratio y. In this case, x.sub.Li signifies the ratio of Li to said mixture of Mg and Al. Furthermore, it also holds that .sub.rest=f(.sub.Al, .sub.Mg, y) with the backscatter coefficients .sub.Al and .sub.Mg for Al and Mg. The backscatter coefficients .sub.Al, .sub.Mg, and .sub.Li can be determined from the literature or by means of simulation. y can be determined by means of X-ray spectroscopy, and S.sub.M can be determined from the backscattered electron image. By determining a reference gray value S.sub.V from fractions of Al and Mg determined by means of EDX, a functional relation can be mathematically solved. This can preferably take place using backscattered electron images of calibration standards in order to precisely determine the reference gray value S.sub.V.

(8) As a result, x.sub.Li can be calculated, and a fraction of Li in the examined sample surface can thus be determined.

(9) A determination of a fraction of lithium as a determination element is described below by way of example with the aid of two MgLi-based alloys LAX410 and LSX2021. For this purpose, one backscattered electron image each of an examination region, customarily referred to in the art as ROI or region of interest, of alloy samples of the respective alloy is determined, and one spectroscopy element composition of the respective examination region is determined by means of energy-dispersive X-ray spectroscopy, or EDX. The nominal compositions of LAX410 and LSX2021 are shown in wt % in Table 1. The alloy samples were produced using an electric induction furnace under argon protective gas atmosphere. To distinguish the alloy samples, a field-emission scanning electron microscope was used, wherein a silicon drift detector was used for EDX and a four-quadrant semiconductor detector was used to capture the backscattered electron image.

(10) TABLE-US-00001 TABLE 1 Nominal composition of LAX410 and LSX2021 in wt %. Mg Li Al Si Ca LAX410 Remainder 4.0 0.8 0.3 LSX2021 Remainder 20.0 2.0 1.0

(11) FIG. 2 shows a backscattered electron image of an LAX410 alloy sample, wherein three examination regions, denoted by ROI 1, ROI 2, and ROI 3, are marked which were examined in greater detail in terms of their element composition. In a corresponding way, FIG. 3 shows a backscattered electron image of an LSX2021 alloy sample, wherein. two examination regions, ROI 1 and ROI 2, are marked. The backscattered electron images are in this case embodied as gray value images with a gray value spectrum of 0 to 255.

(12) In order to calculate a respective mean reference gray value S.sub.V based on the element composition determined by means of EDX, also referred to as spectroscopy element composition, of the respective examination regions, calibration samples formed from essentially pure Al, Mg, and Si were measured. Related calibration curves with measured calibration gray values S and backscatter coefficients determined by means of simulation are illustrated in FIG. 4 and FIG. 5. In this case, a ratio of the calibration gray value S and backscatter coefficient is expediently illustrated over the atomic number Z in FIG. 4, in order to form a linear relation between the calibration gray value and the atomic number. FIG. 5 illustrates both gray values S and backscatter coefficients over the atomic number Z, wherein for Li a calculated gray value is indicated over the atomic number Z. In Table 2, the spectroscopy element composition, formed from Mg, Al, and Ca, measured by means of EIDX of the respective examination regions is stated for the LAX410 alloy sample, and that composition for the LSX2012 alloy sample, formed from Mg, Si, and Ca, is stated in Table 3. Furthermore, in Table 2 and Table 3, the respective mean reference gray value S.sub.V calculated from the spectroscopy element composition and a mean measured gray value S.sub.M of the respective examination region of the related backscattered electron image are stated.

(13) Based on the formula

(14) $x=\frac{{\stackrel{\_}{S}}_M-{\stackrel{\_}{S}}_V}{{\stackrel{\_}{S}}_B-{\stackrel{\_}{S}}_V},$
the fraction x of Li in the respective examination region was thus calculated, wherein S.sub.M signifies a mean measured gray value, S.sub.V signifies a mean reference gray value, and S.sub.B signifies a mean determination element gray value, in this case that for Li.

(15) TABLE-US-00002 TABLE 2 For LAX410: Fractions of Mg, Al, and Ca in wt % determined by means of X-ray spectroscopy; mean measured gray value S.sub.M of the backscattered electron image; calculated mean reference gray value S.sub.V and calculated fraction of Li in wt %. ROI 1 ROI 2 ROI 3 Mg 99.3 84.1 60.6 Al 0.6 8.2 18.1 Ca 0.2 7.7 21.3 S.sub.V (calculated) 139 151 171 S.sub.M (measured) 136 148 178 Li (calculated) 2.8 2.2

(16) TABLE-US-00003 TABLE 3 For LSX2021: Fractions of Mg, Si, and Ca in wt % determined by means of X-ray spectroscopy; mean measured gray value S.sub.M of the backscattered electron image; calculated mean reference gray value S.sub.V and calculated fraction of Li in wt %. ROI 1 ROI 2 Mg 98.5 69.8 Si 0.5 13.1 Ca 0.9 17.2 S.sub.V (calculated) 140 167 S.sub.M (measured) 119 169 Li (calculated) 17.5

(17) As can be seen in Table 2 and Table 3, an Li fraction of 2.8 wt % and 17.5 wt %, respectively, was determined for the ROI 1 of the alloy samples. For both the LAX410 alloy sample and the LSX2021 alloy sample, the ROI 1 relates to a matrix region of the respective alloy morphology. The Li fractions determined for the ROI 1 are somewhat lower than, but very close to, the nominal Li fractions according to Table 1. As can be seen in Table 2 and Table 3, the ROI 2 of the LAX410 alloy sample and LSX2O2I alloy sample have slightly higher calculated mean reference gray values S.sub.V than mean measured gray values S.sub.M, which indicates a lack of Li. For ROI 2 of the LAX410 alloy sample, the Li fraction was determined to be 2.2 wt %. The related microstructure appears to comprise a dendritic structure with Li-free intermetallic compounds, such as Al.sub.2Ca or Mg.sub.2Ca Laves phases, and a matrix formed with Li. The ROI 3 of the LAX410 alloy sample is embodied to be essentially Li-free.

(18) It is thus shown that, with the method according to the invention, a quantitative determination of elements having a smaller atomic number, in particular lithium, is enabled with high accuracy in that backscattered electron signals, in particular backscattered electron images, and X-ray spectroscopy measurement results are synergistically combined. A bonding state of the determination element thereby advantageously plays a secondary role, since a backscattering or a backscatter coefficient of backscattered electrons is typically dominated by the nucleus of the respective element.