JPH0735708A - Fluorescent x-ray analysis method - Google Patents

Abstract

PURPOSE:To prevent the analysis accuracy using X-rays diffracted fro a sample from being deteriorated by previously determining-the relationship between the rotational angle of a sample and the energy or wavelength of diffracted X-rays. CONSTITUTION:The relationship between the rotational angle of a sample having crystal structure and the energy or wavelength of X-rays B6 diffracted by the sample at that rotational angle is determined previously. The intensity of diffracted X-rays B6 causing noise is then determined based on the relationship thus determined and the true spectrum of fluorescent X-rays is obtained by subtracting the intensity of diffracted X-rays B6 from a measured spectrum. In other words, the intensity of diffracted X-rays B6 causing noise is determined by multiplying the intensity of effective incident X-rays by an intensity ratio determined by the rotational angle and then correcting the product while taking account of the deflection angle. Thus determined spectrum of diffracted X-rays B6 causing noise is then subtracted from the measured spectrum thus obtaining a true spectrum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluorescent X-ray analysis method suitable for analyzing a sample having a crystal structure such as a silicon wafer.

[0002]

2. Description of the Related Art Conventionally, for the analysis of the surface layer of a silicon wafer, total reflection fluorescence X in which primary X-rays are irradiated onto the surface of the sample at a minute incident angle and fluorescent X-rays from the surface layer of the sample are analyzed. A line analyzer is used (see, for example, JP-A-63-7).
(See Japanese Patent No. 8056). An example of this type of device is shown in FIG.

In FIG. 8, the target material 5 of the X-ray tube 5 is shown.
X-ray B1 emitted from 1 is a curved dispersive crystal (dispersive element)
Go to 1A. The characteristic X-ray of a predetermined wavelength of the X-ray B1 is diffracted by the dispersive crystal 1A, and is made into a monochromatic primary X-ray B1.
The surface 2a of the sample (silicon wafer) 2 is irradiated with 2 at a small incident angle γ (for example, about 0.05 ° to 0.20 °). The primary X-ray B2 incident on the sample 2 is totally reflected to become a reflected X-ray B4, and the sample 2 is excited as an excited X-ray to generate a fluorescent X-ray B5 peculiar to the element constituting the sample 2. The fluorescent X-ray B5 is incident on the X-ray detector 3 arranged so as to face the sample surface 2a. The X-ray detector 3 detects the X-ray intensity of the incident fluorescent X-rays B5, and then the target X-rays are detected by the multiple wave height analyzer 4 based on the detection signal a from the X-ray detector 3. A spectrum is obtained.

In this type of total reflection X-ray fluorescence analyzer, since the incident angle γ of the primary X-ray B2 is very small, the reflected X-ray B
4 and scattered X-rays are less likely to enter the X-ray detector 3, and there is an advantage that noise is smaller than the output level of the fluorescent X-ray B5 detected by the X-ray detector 3. In other words, a large S / N ratio can be obtained, so that there is an advantage that the analytical sensitivity is high and, for example, a trace amount of impurities can be detected. Therefore, this analysis method is effective as a surface contamination analysis method for silicon wafers and is widely adopted.

Further, in this prior art, since the X-ray B1 is monochromated by using the dispersive crystal 1A, the intensity of scattered X-rays and the like is reduced, so that the analysis accuracy is further improved.

However, when the dispersive crystal 1A is used, the primary X
There is a drawback in that the strength of line B2 is significantly reduced. Therefore, as shown in FIG. 7, a method of increasing the intensity of the primary X-ray B2 by using the artificial multilayer film grating 1 as a spectroscopic element can be considered. However, since the artificial multilayer film grating 1 cannot sufficiently convert the X-ray B1 into a single color, the primary X-ray B2 includes continuous X-rays. In this continuous X-ray, since the sample 2 is a single crystal made of a silicon wafer, the X-rays of a specific wavelength of the primary X-ray B2 are various lattice planes (for example, 311) of the sample 2.
Surface, 202 surface, etc.) and is incident on the X-ray detector 3. Therefore, the diffracted X-ray B6 diffracted by this sample 2
Becomes noise, which causes a problem that the analysis accuracy of the sample 2 is deteriorated.

As a technique for preventing a decrease in analysis accuracy due to the above-mentioned diffracted X-ray B6, a total reflection X-ray fluorescence analyzer disclosed in Japanese Patent Laid-Open No. 5-66204 is known. This analyzer monochromates the primary X-rays B2, obtains a specific rotation angle of the sample 2 in which no diffracted X-rays are generated, and at the position of this specific rotation angle, the monochromatic primary X-rays B1 are obtained. To the sample 2.

[0008]

As described above, according to the above-mentioned prior art, it is intended to improve the analysis accuracy by preventing the generation of the diffracted X-rays from the sample 2, so that the sample is sufficiently monochromatic. It is necessary to use the primary X-ray B2. Therefore, when the primary X-ray B2 is not sufficiently monochromatic, it cannot be applied, and thus has no versatility.

The present invention has been made in view of the above problems, and a fluorescent X-ray analysis in which a sample having a crystal structure is irradiated with primary X-rays and the sample is analyzed based on the fluorescent X-rays generated from the sample. An object of the method is to provide a fluorescent X-ray analysis method capable of preventing a decrease in analysis accuracy due to diffracted X-rays diffracted by a sample regardless of whether primary X-rays are monochromatic.

[0010]

In order to achieve the above object, the present invention provides a rotation angle of a sample having a crystal structure,
The relationship between the energy or wavelength of the diffracted X-rays diffracted by the sample at the rotation angle is obtained in advance, and the intensity of the diffracted X-rays that cause noise is obtained based on the obtained relation. A spectrum of fluorescent X-rays from which the line intensity has been removed is obtained.

[0011]

The principle of the present invention will be described below. The wavelength and energy generated by the diffracted X-rays are determined by the lattice spacing d of the crystal and the diffraction angle θ, as can be seen from the Bragg equation below. 2dsin θ = nλ (1) where n: Order of reflection (1, 2, 3, ...) λ: X-ray wavelength (Å) (λ = 12.4 / E) E: Energy (KeV)

Since various lattice spacings d exist in one crystal structure, various diffracted X-rays are generated from the sample. However, by setting the direction of the lattice plane, the lattice plane spacing d is determined, and by setting the rotation angle of the sample from the reference plane, the diffraction angle θ is determined. Furthermore, since the intensity of the diffracted X-rays of the second order or higher (n = 2 or higher) is small, it does not significantly affect the analysis accuracy even if they are ignored. Therefore, the relationship between the rotation angle of the sample having the crystal structure and the energy of the diffracted X-ray diffracted by the sample at the rotation angle can be obtained in advance.

On the other hand, the intensity of the diffracted X-ray is, as is well known,
It can be determined from the crystal structure factor. Therefore,
Since the intensity of the diffracted X-ray generated at the above rotation angle can also be obtained, the fluorescence X obtained by removing the intensity of the diffracted X-ray from the measurement spectrum based on the relationship between the previously obtained rotational angle and the energy or wavelength of the diffracted X-ray. The spectrum of the line can be obtained.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. First, a method of obtaining the relationship between the rotation angle of the sample and the energy diffracted by the sample at the rotation angle will be described.

In FIG. 1, a sample 2 made of a silicon wafer is generally provided with a reference plane 2b called an orientation flat along the 110 direction. In the case of total reflection fluorescent X-ray analysis, the primary X-ray B2 is incident on the surface 2a of the sample 2 at an angle close to parallel, so the X axis is set in the incident direction of the primary X-ray B2, and the surface of the silicon wafer 2 is set. Set the XY plane to 2a. Further, the Z axis is set in a direction that is perpendicular to the surface of the sample 2 and coincides with the axis of the X-ray detector 3 (FIG. 7).

Now, the crystal plane 2c is set in the sample 2, the unit vector in the normal direction thereof is r ₁ , its component is x ₁ ,
Assuming y ₁ and z ₁ , x ₁ , y ₁ and z ₁ are the following (2) to
It is expressed by equation (4). x ₁ = (OA) ・ cos φ ₁ ＝ sin α ₁・ cos φ ₁ … (2) y ₁ ＝ (OA) ・ sin φ ₁ ＝ sin α ₁・ sin φ ₁ … (3) z ₁ ＝ cos α ₁ … (4) where α _{1 is the} angle between the vector r ₁ and the z axis φ ₁ is the angle between the line OA that is the projection of the vector r ₁ on the XY plane and the X axis. The thing shows the length of the line segment.

When the angle between the vector r ₁ and the X axis is θ ₁ , and the angle between the line OB obtained by projecting the vector r ₁ on the YZ plane and the Y axis is β, the following (5), (6) The formula holds. x ₁ = cos θ ₁ (5) y ₁ = (OB) ・ cos β = sin θ ₁・ cos β (6)

On the other hand, the unit vector in the outgoing direction of the diffracted X-ray B6 diffracted by the crystal plane 2c shown by the diagonal line is r,
Let the components be x, y, and z. Here, since the primary X-ray B2 is reflected equiangularly on the crystal plane 2c to generate the diffracted X-ray B6, the angle formed by the vector r and the X axis is 2θ ₁ . Further, since the X axis, the vector r _1, and the vector r are on the same plane, the angle formed by the line OC obtained by projecting the vector r on the YZ plane and the Y axis is the angle β formed by the line segment OB and the Y axis. be equivalent to. Therefore, x, y, and z are the following (7),
It is expressed by Eqs. (8) and (9). _{x = cos2θ 1 ... (7)} y = (OC) · cos β = sin2θ 1 · cos β ... (8) z = (OC) · sin β = sin2θ 1 · sin β ... (9)

Next, the angle between the vector r and the Z axis is α. This angle α is an angle at which the diffracted X-ray B6 is incident on the X-ray detector 3 (FIG. 7), and this angle α is hereinafter referred to as a deviation angle. Further, when the angle between the line OD obtained by projecting the vector r on the XY plane and the X axis is φ, the deviation angle α and the angle φ represent the emission direction of the diffracted X-ray B6. The angles α and φ have the relationships of the following expressions (10) and (11). z = cos α (10) x = (OD) ・ cos φ ＝ sin α ・ cos φ (11)

Now, when the type of crystal plane (plane index) is set,
Since the direction of the crystal plane is set, the angle φ ₀ and the angle α 1 formed by the line OA and the 110 direction are determined, and the rotation angle ω of the sample 2, that is, the angle ω formed by the 110 direction and the X axis is set. Then, since the angle φ ₁ is determined from the angle ω and the angle φ _0, x ₁ is determined from the equation (2), and the angle θ ₁ is determined from the equation (5).

Here, if the Bragg diffraction angle θ is represented by an angle θ ₁ , then θ = (π / 2) −θ ₁ , so the above Bragg equation is λ = 2dsin θ = 2dsin {(π / 2) −θ ₁ } = 2d cos θ (12) where λ = 12.4 / E. Further, if the type of crystal plane is set, the lattice plane spacing d is uniquely determined, and thus the relationship between the diffraction angle θ and the wavelength λ is determined from the equation (12). From this diffraction angle θ to the angle θ ₁
And the angle θ ₁ and (2), (3), (6), (7), (9), (1
The angles φ and α can be known from the equations 0) and (11), and by extension, the relationship between the rotation angle ω of the sample 2 and the energy E generated by the diffracted X-ray B6 can be known only by desk calculation.

FIG. 2 shows a part of the result of obtaining the energy of the diffracted X-ray B6 reflected by the sample 2 for each rotation angle ω of the sample 2 based on the above calculation formula. This calculation result agrees with the noise in the spectrum of FIG. 3 actually measured by the apparatus of FIG. 7, which confirms that the above calculation method is correct. It should be noted that the above calculation may be performed in the range of the rotation angle ω of 0 ° to 45 ° from the symmetry of the crystal, and ω + m · 90 when the rotation angle is 45 ° or more.
The energy and intensity ratio of the diffracted X-ray B6 at the rotation angles of ° and −ω + m · 90 ° (m is an integer) are
The same as the energy and intensity ratio of the diffracted X-ray B6 at.

Next, a method for obtaining the intensity I ₆ of the diffracted X-ray B6 will be described. I ₆ = I ₂₀ I ₀ (13) where I ₂₀ : X-ray (effective incident X-ray) intensity (Kcps) incident on the sample I ₀ : Relative intensity of diffracted X-ray B 6 with respect to I ₂₀ (anonymous number)

[0024]

[Equation 1]

In the above formula (13), the intensity I ₆ of the diffracted X-ray B6 is based on the assumption that the sample 2 in FIG. 1 has a minute spherical shape. Further, all of the diffracted X-rays B6 are the X-ray detector 3
(FIG. 7), for example, the deviation angle α
Becomes larger, as shown in FIG. 4, a part of the light beam of the rectangular diffracted X-ray B6 is incident on the circular X-ray detector 3, and the rest is X-ray.
It does not enter the line detector 3. Further, the deviation angle α in FIG.
For example, at a large angle exceeding 45 °, the diffracted X-ray B6
Does not enter the X-ray detector 3 (FIG. 7) at all. Therefore, in consideration of the shape of the sample 2 and the deviation angle α, it is necessary to perform correction as is well known to obtain the intensity of the diffracted X-ray B6.

In the total reflection X-ray fluorescence analysis, the incident angle γ in FIG.
Is extremely small. Moreover, since the sample 2 of FIG. 7 is rotated by the goniometer, the incident angle γ is not constant, and the incident angle γ is extremely small (about 0.05 ° to 0.2 °), so the incident angle γ is slightly different. As a result, the total reflectance is significantly different. Therefore, as will be described later, the effective incident X-ray of FIG. 6B at the incident angle γ with respect to the sample 2 to be measured, that is, the effective incident X actually incident on the sample 2.
It is necessary to determine the line intensity I ₂₀ .

Diffracted X-ray B6 which becomes noise in FIG. 6 (c)
Of the effective incident X-rays of FIG. 6B has the energy of FIG. 2, and the intensity of the diffracted X-ray B6 is as shown in FIG.
The intensity can be obtained by multiplying the intensity of the effective incident X-ray of (b) by the intensity ratio of FIG. 2 and further correcting by considering the deviation angle α and the like. By subtracting the spectrum of the diffracted X-ray B6 which becomes the noise of FIG. 6C from the measured spectrum of FIG. 6A, the true spectrum of FIG. 6D is obtained.

The method for actually obtaining the true spectrum will be described below. First, various rotation angles ω of the sample 2 and the energy, intensity ratio and directly above the diffracted X-ray B6 diffracted by the sample 2 at the rotation angles ω are calculated by using the above equations (2) to (12). The relationship with the angle α is shown in FIG.
As shown in (d), it is stored in the storage element of the computer. The pitch of the rotation angle ω may be adjusted to the actually measured pitch instead of the 5 ° pitch as shown in FIGS. 2A to 2D, or may be a 1 ° pitch. Further, without storing the specific numerical values in FIG. 2, the above (2) to (1
The equation (2) may be stored in a storage element for later calculation.

The intensity of the generated diffracted X-ray B6 and X
The relationship with the intensity of the diffracted X-ray B6 incident on the line detector 3 is
The storage element stores the mathematical expression represented by the function of the upright angle α.
For example, when α> 45 °, the intensity of the diffracted X-rays incident on the X-ray detector 3 is set to 0.

Next, using the total reflection X-ray fluorescence analyzer of FIG. 7 and a blank wafer (single crystal of silicon before forming a thin film on the surface), the reference effective incident X-rays incident on the blank wafer of FIG. Find the spectrum of. The method of obtaining this spectrum will be briefly described. Various rotation angles ω
At (for example, every 1 °), the X-ray spectrum of FIG. 3 is measured. Then, the diffraction X on any lattice plane
The intensity of the reference effective incident X-ray at each energy is obtained by correcting the X-ray intensity of the ray in consideration of the deviation angle α and the reflectance, and the spectrum of the reference effective incident X-ray of FIG. 5 is obtained. For example, 202 in FIGS.
The intensity of the diffracted X-ray on the plane is plotted for each energy position, the plotted X-ray intensity is corrected in consideration of the deviation angle α and the reflectance, and further, another lattice plane (for example, 313 plane). Is similarly obtained, the spectrum of the reference effective incident X-ray of FIG. 5 is obtained. The calculation of the intensity ratio in FIG. 2 and the spectrum of the reference effective incident X-ray incident on the brown wafer in FIG. 5 described above are performed after the adjustment of the apparatus and periodically thereafter.

Next, a silicon wafer to be actually measured,
That is, the sample 2 having an unknown component is set in the analyzer of FIG. 7, and the X-ray spectrum of FIG. 6A detected by the X-ray detector 3 is temporarily stored.

The Si-Kα intensity of this measurement spectrum and S in the measurement spectrum of the blank wafer of FIG.
From the intensity ratio of i-Kα, the incident angle γ (FIG. 7) at the time of spectrum measurement of the unknown sample is obtained. From this calculated incident angle γ,
By correcting the reference effective incident X-ray of FIG. 5 in consideration of the critical angle and the reflectance for each energy, the sample 2 was actually incident on the sample 2 without being totally reflected on the sample 2 (FIG. 6B).
Of the effective incident X-ray of Note that the incident angle γ
Measures the intensity of the reflected X-ray B4 in FIG.
It may be determined from the strength of 4.

The effective incident X-ray spectrum actually incident on the sample 2 is multiplied by the intensity ratio of FIG. 2 at the rotation angle ω of the sample 2 when the primary X-ray B2 is actually irradiated on the sample 2, The intensity of the diffracted X-ray that becomes noise in FIG. 6C is obtained. That is, the intensity of the effective incident X-ray of FIG. 6B at the energy position of the diffracted X-ray of FIG. 2A and the intensity at the energy position of the diffracted X-ray of FIG. 2 at the specific rotation angle ω. The ratio is multiplied with the ratio to obtain the intensity of the diffracted X-ray that becomes noise in FIG. 6C at the specific rotation angle.

After that, the spectrum of the diffracted X-ray which becomes noise of FIG. 6C is subtracted from the measured spectrum of FIG. 6A, and the diffracted X-ray diffracted by the sample 2 is removed.
Find the true spectrum of (d). Quantitative analysis of the element of sample 2 is performed based on the obtained true spectrum.

The present invention can be applied to fluorescent X-ray analysis other than the total reflection fluorescent X-ray analysis and is included in the scope of the present invention. In that case, it is necessary to make the above equations (2) to (12) into versatile equations using the angle between the primary X-ray B2 and the X axis in FIG. 1 as a parameter. Further, it can be applied to samples having a crystal structure such as gallium arsenide (GaAs) other than silicon wafers.

Further, in the above embodiment, the case where the artificial multilayer film grating 1 of FIG. 7 is used as the spectroscopic element has been described, but the present invention is also applied to the case where the dispersive crystal 1A of FIG. 8 is used. That is, the present invention is also applied to the case where the primary X-ray B2 is completely monochromatic light.

In the above embodiment, the case where the primary X-ray B2 incident on the sample 2 from the outside of the sample 2 is diffracted in the sample 2 has been described. However, the present invention provides a source of diffracted X-rays. It is not limited. For example, the present invention can be applied even when the characteristic X-ray generated in the sample 2 serves as a light source to cause diffraction in the sample 2.

[0038]

As described above, according to the present invention, the intensity of the diffracted X-rays is removed from the measured spectrum based on the relationship between the rotation angle of the sample and the energy or wavelength of the diffracted X-rays which is obtained in advance. Since the spectrum of the fluorescent X-rays can be obtained, it is possible to prevent the deterioration of the analysis accuracy due to the diffracted X-rays, which is noise, and improve the analysis accuracy.
In particular, the present invention does not prevent the diffracted X-rays from being generated from the sample, but removes the diffracted X-rays generated from the sample by correction. Therefore, whether or not the primary X-rays are monochromatic is determined. Nevertheless, there is an advantage that the analysis accuracy can be improved.

[Brief description of drawings]

FIG. 1 is a perspective view illustrating a method for obtaining the intensity of a diffracted X-ray according to the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a rotation angle of a sample and energy of diffracted X-rays generated.

FIG. 3 is a characteristic diagram showing a measured spectrum of a blank wafer.

FIG. 4 is the diffraction X incident on the X-ray detector in relation to the angle directly above.
It is a top view which shows the light flux of a line.

FIG. 5 is a characteristic diagram showing a spectrum of effective incident X-rays incident on a brown wafer.

FIG. 6A is a measurement spectrum of an actual unknown sample,
(B) is a spectrum of effective incident X-rays incident on an unknown sample, (c) is a spectrum of diffracted X-rays that become noise,
(D) is the true spectrum.

FIG. 7 is a schematic configuration diagram showing an example of an analysis device used in the analysis method of the present invention.

FIG. 8 is a schematic configuration diagram showing an example of a general X-ray fluorescence analyzer for total reflection.

[Explanation of symbols]

2 ... sample, B2 ... primary X-ray, B5 ... fluorescent X-ray, B6 ... diffraction X-ray, ω ... rotation angle.

Claims (1)

[Claims] 1. A fluorescent X-ray analysis method in which a sample having a crystal structure is irradiated with primary X-rays and the sample is analyzed based on the fluorescent X-rays generated from the sample. The relationship between the energy or wavelength of the diffracted X-rays diffracted by the sample at an angle is obtained in advance, and the intensity of the diffracted X-rays that cause noise is obtained based on the obtained relation, thereby obtaining the intensity of the diffracted X-rays from the measured spectrum A fluorescent X-ray analysis method, which comprises obtaining a fluorescent X-ray spectrum from which is removed.