JP2008286632A - Structure analysis method of trace element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a structure analysis method of a trace element capable of suppressing measurement inhibition of a fluorescent X-ray signal caused by generation of a strong diffraction X-ray at a specific angle from a sample, when performing structure analysis of the trace element included in a single crystal sample by utilizing a fluorescent XAFS method. <P>SOLUTION: In this structure analysis method of the trace element, the sample is irradiated with an X-ray, while changing X-ray energy, and a fluorescent X-ray emitted from the sample is detected, and information on an atomic structure of the trace element in the sample is analyzed from a relation between the X-ray energy and the fluorescent X-ray. In the method, the crushed single crystal sample (crushed sample S) is irradiated with the X-ray. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a structure analysis method for trace elements.

  As one of the analysis methods using X-rays, a sample is irradiated with X-rays while changing the X-ray energy. From the obtained X-ray absorption spectrum, the chemical bonding state, electronic state, and local state of the element to be analyzed in the sample are measured. An XAFS (X-ray Absorption Fine Structure) method for structural analysis is known. Among them, as a method for obtaining an X-ray absorption spectrum, there is a fluorescence method for measuring the intensity of fluorescent X-rays emitted from a sample along with X-ray absorption. For example, Non-Patent Document 1 discloses a technique for measuring fluorescent X-rays under conditions where a sample totally reflects incident X-rays.

"X-ray absorption spectroscopy-XAFS and its application-" p227-235 edited by Toshiaki Ota, IPC Corporation, published on June 28, 2002

  However, the above technique has the following problems when a single crystal material is used as a sample.

(1) Inhibition of normal measurement due to mixing of diffraction lines from a sample When an X-ray absorption spectrum is obtained by a fluorescence method, if the sample is a single crystal semiconductor material, etc. A diffraction line is generated in a specific narrow direction with high probability. When this diffracted ray is incident on a fluorescent X-ray detector, it becomes a strong noise and prevents normal measurement of the fluorescent X-ray. In particular, a sample with a low concentration of the element to be analyzed has a very high incident X-ray intensity compared to the intensity of the fluorescent X-ray, so that the measurement of fluorescent X-rays is possible even with weak diffraction that does not completely satisfy the diffraction conditions. Affects.

  As a method for avoiding the influence of diffraction lines during measurement, there is a method of adjusting the energy of incident X-rays. However, since XAFS irradiates X-rays while changing the energy, a strong diffraction line enters the detector with any energy, and cannot be an effective measure.

  As another countermeasure, there is a technique of rotating a sample to prevent a strong diffraction line from entering the detector. However, in order to rotate the sample, high accuracy is required for the installation of the rotating shaft. On the other hand, if the rotating shaft is decentered, the measurement result will vary. In particular, when the X-ray irradiation is performed under the total reflection condition, the incident angle of the X-ray with respect to the sample must be controlled extremely minutely. At this time, if the rotation axis is decentered, the penetration depth of the X-ray into the sample changes, the signal intensity of the measured fluorescent X-ray changes, and in some cases, vibration that is not originally detected appears in the measurement result. .

(2) Low measurement sensitivity When an additive element such as a semiconductor material is to be analyzed, the sample is usually very thin and the concentration of the additive element is extremely small. For such a sample, the X-ray incident angle is lowered to increase the irradiation area of the incident X-ray sample. However, for example, when the sample is irradiated with X-rays under total reflection conditions, the depth at which the X-rays penetrate from the surface of the sample is only a few nm at most. Therefore, since the number of atoms of the analysis target element irradiated with X-rays is small and the signal intensity of the obtained fluorescent X-rays is low, the measurement sensitivity is inevitably lowered.

  The present invention has been made in view of the above circumstances, and one of its purposes is to identify a trace element contained in a single crystal sample using a fluorescent XAFS method. An object of the present invention is to provide a trace element structure analysis method capable of suppressing the inhibition of measurement of a fluorescent X-ray signal due to the generation of diffracted X-rays with a strong angle.

  Another object of the present invention is to provide a trace element structural analysis method capable of improving the measurement sensitivity when performing a structural analysis of trace elements contained in a single crystal sample using a fluorescent XAFS method. Is to provide.

  The present invention irradiates a sample with X-rays while changing the X-ray energy, detects fluorescent X-rays emitted from the sample, and determines the atomic amounts of trace elements in the sample from the relationship between the X-ray energy and the fluorescent X-rays. It is a structure analysis method of trace elements for analyzing information on structure. In this method, X-ray irradiation is performed on a crushed single crystal sample.

  The structure analysis method of trace elements according to the present invention appears in a narrow range of, for example, a solid angle of 0.1 degrees or less if the sample is pulverized, that is, polycrystallized, and remains a single crystal sample that is not pulverized. Strong diffraction lines can appear almost uniformly distributed in the 360 ° radiation direction. Therefore, even if the sample is not rotated, the diffraction line intensity per unit angle in the fluorescent X-ray detector can be reduced, and the fluorescent XAFS analysis can be performed with the influence of the diffraction line being largely eliminated.

  Hereinafter, the present invention will be described in more detail.

<Sample to be analyzed>
The sample used for analysis in the present invention is a single crystal material to which a trace element as an analysis target element is added. For example, a single crystal semiconductor material can be used. The method of the present invention can be used particularly effectively when the trace element content is 1 mol% or less. When this content exceeds 1 mol%, the transmission XAFS method that is not affected by the diffraction line is applied, or even in the fluorescent XAFS method, the intensity of the fluorescent X-ray is high to some extent, so that measurement can be performed with a smaller detector. The X-ray fluorescence can be measured in such a manner that the radiation direction of the diffraction line deviates from the detector.

<Crushing process>
The sample is kept in a pulverized state prior to irradiation with X-rays. The method for pulverizing the sample is not particularly limited as long as the sample can be pulverized to such an extent that a strong diffraction line does not appear at a specific angle when the crushed sample is irradiated with X-rays. For example, a mechanical pulverization method such as pulverization by putting a sample in a mortar and hitting it can be suitably used.

  When a semiconductor material in which a plurality of crystal layers are stacked is used as a sample, this sample usually includes a single-crystal analysis layer containing a trace element to be analyzed, and a non-analysis layer stacked on the analysis layer. Will exist. In that case, it is preferable that the analysis layer and the non-analysis layer are separated, and the X-ray irradiation is performed on the pulverized sample of the analysis layer. By removing a non-analysis layer that is not an analysis target in advance, it is possible to perform measurement with higher sensitivity. For separation of the analysis layer and the non-analysis layer, a mechanical processing method such as polishing / cutting or a chemical processing method such as etching can be suitably used.

  In particular, in the case of semiconductor materials stacked in multiple layers, the thickness of each layer is thin, so that etching can be suitably used for separating the analysis layer and the non-analysis layer. When etching is performed, it is preferable to use an etching solution in which the non-analysis layer is dissolved but the analysis layer is not dissolved. However, in the case of an etching solution in which both the non-analysis layer and the analysis layer are dissolved, a mask layer that is not dissolved in the etching solution may be formed on the analysis layer and etched.

  Depending on the thickness and material of the analysis layer, the analysis layer may be pulverized during this etching. In other words, if the non-analytical layer is removed by etching, and the analytical layer inevitably becomes discrete particles, the particles in the etching solution can be removed without performing a separate pulverization step of the analyzing layer after this etching. What is necessary is just to collect and make it X-ray irradiation object. Filtration is preferably used as a method for collecting the particle pieces in the etching solution. Of course, even if the non-analysis layer is removed by etching, the analysis layer is not pulverized at all or the analysis layer is pulverized to some extent. However, if the pulverization level is insufficient, a separate pulverization step may be performed after the etching.

  And the average particle diameter of the ground sample before X-ray irradiation is preferably 10 μm or less. If the average particle size of the pulverized sample is 10 μm or less, it is possible to suppress generation of strong diffraction lines in a specific direction when the pulverized sample is irradiated with X-rays.

<Measurement of X-ray fluorescence and analysis of measurement results>
In the method of the present invention, a single crystal sample is pulverized, and the pulverized sample is irradiated with X-rays. At that time, the crushed sample may be spread on a plane and irradiated with X-rays. However, it is preferable that the crushed samples are gathered together to form an aggregate, and the aggregate is irradiated with X-rays. At this time, it is desirable to make the projection size of the aggregate viewed from the X-ray irradiation direction smaller than the X-ray irradiation area.

  For example, when XAFS analysis is performed on a thin-film single crystal sample that is not pulverized by the fluorescence method, the analysis target region that is actually irradiated with X-rays is only a small part of the sample. However, if the crushed samples are gathered together to form an aggregate, the aggregate can be irradiated with X-rays to greatly increase the area to be analyzed, and XAFS analysis can be performed on thin film samples that are not pulverized. Compared to the case where the X-ray was performed, far stronger fluorescent X-rays can be detected. Therefore, it is possible to measure fluorescent X-rays with high sensitivity. In particular, if the projection size of the aggregate viewed from the X-ray irradiation direction is made smaller than the X-ray irradiation area, it corresponds to the case where the entire thin-film single crystal sample is set as the analysis target area. Measurements can be made.

  The incident angle of the X-ray with respect to the sample is preferably set to a low angle. If the incident angle is set to a low angle, the X-ray irradiation area on the sample can be earned.

  When the crushed sample is irradiated with X-rays, X-ray absorption occurs in the atoms of the analysis target element in the sample. At this time, the inner shell electrons of the analysis target atoms are excited and emitted to the outside of the shell. Holes are generated. In order to fill these vacancies, electrons transition from the outer shell to stabilize the energy balance of the atoms to be analyzed. Corresponding to the difference in energy level at this time, characteristic X-rays (fluorescent X-rays) are generated.

  There are two types of detection of fluorescent X-rays: a wavelength dispersion type in which fluorescent X-rays are detected by a detector using a spectral crystal, and an energy dispersion type in which a semiconductor detector is used. The method of the present invention can be suitably used in an energy dispersion type using a semiconductor detector such as an SSD (Solid-state Detector) or a silicon drift detector (SDD).

  Then, XAFS is performed by obtaining an absorption spectrum that is a correlation between the energy of incident X-rays and the intensity of fluorescent X-rays.

  An example of the present invention will be described with reference to the drawings, taking as an example the case where Zn in the InP layer on the outermost surface of a semiconductor wafer is used as an element to be analyzed.

First, a semiconductor wafer 10 for a light receiving element in which an InGaAs layer 12 having a thickness of 1 μm and a Zn-doped InP layer 13 having a thickness of 1 μm are sequentially formed on an InP substrate 11 having a thickness of 350 μm is prepared (FIG. 1A). )). The Zn content in the Zn-added InP layer 13 is 0.005 mol%. Next, a mask layer M that is not dissolved by later etching is formed on the surface of the InP layer 13 (analysis layer) of the wafer 10 (FIG. 1B). Here, SiN is used as the mask layer M. Next, the wafer is etched using an etching solution having a volume ratio of HCl: H ₂ O = 1: 1, and only the InP substrate 11 (non-analytical layer) is removed, leaving the other three layers (FIG. 1 ( C)). Subsequently, the remaining three layers are etched by using H ₃ PO ₄ : H ₂ O ₂ : H ₂ O = 5: 1: 40 by volume as an etchant, and an InGaAs layer 12 (non-analytical layer) is etched. ) Only (FIG. 1D). Further, the thin film remaining after etching is scooped out and pulverized in a mortar to form particles (FIG. 1E). The average particle size of the obtained particle pieces was 10 μm or less. And after filtering this particle piece, it wash | cleaned one by one by ethanol and the pure water, and it was set as the grinding | pulverization sample S for analysis.

  When the pulverized sample S is obtained, the crushed sample S is collected and placed on a flat surface, and X-ray irradiation is performed while changing the X-ray energy to the aggregate of particles (FIG. 2). At this time, the incident angle of the incident X-ray Xi was set to a low angle where the projection size of the aggregate viewed from the X-ray irradiation direction was smaller than the X-ray irradiation area. Furthermore, diffraction lines D are also generated by irradiating the pulverized sample S with X-rays. The diffraction lines D are emitted from the sample S while being dispersed in almost all directions. Then, the fluorescent X-ray Xf generated with the X-ray irradiation is measured by the semiconductor detector Xd. Further, as a comparative example, fluorescent X-rays were similarly measured on the same wafer without removing the non-analytical layer by etching and crushing the analytical layer.

  The measurement results are shown in FIG. FIG. 3 is an X-ray absorption spectrum expressed by the relationship between the energy (keV) of the incident X-ray Xi and the count number by the detector Xd. As apparent from the comparison of the spectra, in the spectrum of the example (normal), no abnormal peak is observed, and it is considered that the fluorescent X-ray is measured without being disturbed by the diffraction line. On the other hand, in the spectrum of the comparative example (abnormal peak), an abnormal peak that seems to be an influence of diffraction lines is recognized, and it can be seen that accurate measurement of fluorescent X-rays is inhibited.

  That is, when measuring fluorescent X-rays in the examples, the X-ray irradiation target is, so to speak, a polycrystalline pulverized sample S, so that diffraction lines are dispersed in almost all directions without being strongly generated in a specific direction. A strong diffracted ray enters the device Xd and measurement of the fluorescent X-ray Xf is not hindered. Of course, since it is not necessary to rotate the crushed sample S, a highly accurate measurement result can be obtained. Furthermore, since the aggregate of the pulverized samples collected is irradiated with X-rays, the entire Zn-added InP layer 13 before pulverization can be substantially made the analysis target region, so that the signal intensity is high. A fluorescent X-ray signal can be obtained, and highly sensitive measurement can be performed.

For example, even in the case of a sample having a diameter of 2 inches and an analysis layer having a thickness of 1 μm, the measurement target of the XAFS method is at most several mm width × several nm depth. However, by removing the non-analytical layer by etching and making the particles into aggregates, the entire volume of 2 inches in diameter × 1 μm can be made the analysis target region. Under total reflection conditions, when the analysis target area of a wafer that is not crushed is 5 mm x 2 inches x 3 nm, the analysis area is 7.6 x 10 ^-7 cm ³ whereas the analysis area when aggregated is 2 inches Since it was φ × 1μm, it was 2.0 × 10 ⁻³ cm ³ , and it was also found that a signal from an area about 2700 times wider could be obtained.

  The trace element structural analysis method of the present invention can be suitably used for structural analysis of additive elements in semiconductor materials.

FIG. 2 is a schematic explanatory diagram of steps required to obtain a pulverized sample in the method of the present invention according to Example 1. FIG. 2 is a schematic explanatory diagram of a step of irradiating a ground sample with X-rays in the method of the present invention according to Example 1. In Example 1, it is a graph which shows the acquired X-ray absorption spectrum.

Explanation of symbols

10 Semiconductor wafer 11 InP substrate 12 InGaAs layer 13 Zn-doped InP layer
M mask layer
S Ground sample D Diffraction line Xf X-ray fluorescence Xd Semiconductor detector
Xi Incident X-ray Xo Reflected X-ray

Claims (5)

The sample is irradiated with X-rays while changing the X-ray energy, the fluorescent X-rays emitted from the sample are detected, and information on the atomic structure of the trace elements in the sample is obtained from the relationship between the X-ray energy and the fluorescent X-rays. A method for analyzing a structure of a trace element to be analyzed,
A method for analyzing a structure of a trace element, wherein the X-ray irradiation is performed on a crushed single crystal sample. The sample includes a single crystal analysis layer containing a trace element and a non-analysis layer laminated on the analysis layer.
Separating the analysis layer from the non-analysis layer,
2. The trace element structure analysis method according to claim 1, wherein the X-ray irradiation is performed on a sample whose analysis layer is pulverized. The separation of the analysis layer and the non-analysis layer is performed by removing the non-analysis layer by etching,
The method for analyzing a structure of a trace element according to claim 2, wherein the analysis layer is pulverized by this etching. The separation of the analysis layer and the non-analysis layer is performed by removing the non-analysis layer by etching,
3. The trace element structure analysis method according to claim 2, wherein the analysis layer is mechanically pulverized after the etching. Gathered pieces of crushed pieces into an aggregate,
5. The trace element structure analysis method according to claim 1, wherein the aggregate is irradiated with X-rays.

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