JP7593172B2 - Thin film X-ray diffraction method - Google Patents

Description

The present invention relates to an X-ray diffraction method for identifying the crystalline phase of a thin film.

Thin films are used in various devices. The characteristics of these devices often depend on the crystalline structure of the thin film. X-ray diffraction is generally used to identify the crystalline phase of the thin film. When the thin film is on the order of a few nm thick, the in-plane XRD (X-ray diffraction) method or the GI-XRD (Grazing incidence X-ray diffraction, also called Out-of-Plane GI-XRD) method, which have a small angle of incidence, are used. These X-ray diffraction methods are measurement techniques that selectively obtain information about the thin film by fixing the X-ray incidence angle (ω) of the X-ray diffraction device used for the measurement to 0.1° to 1°.

For example, Patent Document 1 discloses a method in which X-rays are irradiated onto a sample in which a thin film to be measured is formed on a substrate, the X-rays are totally reflected at the interface between the thin film and the substrate, the diffracted X-rays are measured with an X-ray detector, and the X-ray detection intensity is counted. In this case, it is described that when two materials are in contact with each other at a sufficiently smooth interface, the total reflection phenomenon of X-rays occurs when X-rays are incident at a small angle to the interface from the material with low electron density to the material with high electron density. The X-rays are irradiated with energy that causes them to be totally reflected by the substrate, but not by the surface of the thin film. Furthermore, glass is used as the substrate, and a thin film of metal or organic material is formed on top of it.

Patent Document 2 discloses a method for calculating the density of a sample, which involves irradiating an X-ray onto a film on the surface of the sample at a predetermined angle, measuring the intensity of the reflected X-ray with a first detector while moving the sample in a direction perpendicular to the surface, measuring the intensity of the diffracted X-ray among the incident X-rays with a second detector, calculating the film thickness from the difference between the height position of the sample at which the differential signal of the intensity of the diffracted X-ray is maximized and the height position of the sample at which the intensity of the reflected X-ray is maximized, and calculating the density of the film on the surface of the sample based on the film thickness and the intensity of the X-ray at each position. In this case, it is described that the material forming the substrate is silicon or glass with a density of 2.0 to 2.3 g/cm ³ , and the material forming the film is carbon nanotubes or an organic material with a density of about 1 g/cm ³ .

Patent No. 2591650 Patent No. 6171940

However, if the density of the film formed on the measurement target is low, the X-rays will penetrate to the substrate and detect information about the substrate. For example, if glass is used as a substrate for an extremely thin film material with a low film density, the X-rays will penetrate the film and detect not only the X-ray diffraction peaks from the sample but also the halo (ring-shaped diffraction image) from the glass at the same time.
In this regard, both patent documents irradiate X-rays at a critical angle at which total reflection occurs. However, in the region where the diffraction angle is small, the X-rays are affected by the halo of the substrate. Therefore, the application of these documents is limited to objects that can be measured at a diffraction angle 2θ that is not affected by this (e.g., carbon as described in Patent Document 2).

The present invention was made in consideration of these circumstances, and aims to provide a thin film X-ray diffraction method that reduces the influence of the substrate, accurately analyzes the object to be measured, and can expand the range of application of the object.

The thin film X-ray diffraction method of the present invention involves forming an undercoat film made of a material with a higher density than the thin film to be measured on the surface of a substrate, forming the thin film on top of the undercoat film, and irradiating X-rays onto the thin film at an angle of incidence between the total reflection critical angle of the undercoat film and the total reflection critical angle of the thin film to identify the crystalline phase of the thin film.

When forming a film of the object to be measured on the surface of a substrate, only objects with a density smaller than that of the substrate can be used to totally reflect X-rays on the surface of the substrate. Therefore, a base film made of a material with a higher density than the thin film to be measured is formed on the surface of the substrate, and total reflection occurs at the interface between the base film and the thin film. By adopting this method, it is possible to reduce the influence from the substrate and accurately capture diffracted X-rays for accurate analysis. In addition, by appropriately selecting the material of the base film, it is possible to make the angle of incidence larger than when the surface of the substrate is used as the interface, expanding the range of applicable objects to be measured.

In this thin film X-ray diffraction method, the undercoat film may be an indium tin oxide film, which is widely used as a transparent electrode and can be easily formed by deposition or the like.
In addition, since an indium tin oxide film has a higher density than glass (for example, glass has a density of 2.5 g/ ^cm3 , while indium tin oxide has a density of 7.18 g/ ^cm3 ), by using it as a base layer on the surface of a glass substrate, it is possible to accurately analyze a thin film of a material with a higher density than glass.

In this thin film X-ray diffraction method, the thin film may be a film made of copper oxide. The density of a film made of copper oxide is, for example, about 3 g/ ^cm3 , and if the film is directly formed on a glass substrate and subjected to X-ray diffraction, it will be affected by the halo of the glass. However, by using an indium tin oxide film as the base film, the copper oxide of the measurement object can be accurately analyzed.

In this thin film X-ray diffraction method, it is preferable that the base film is amorphous. By making the base film amorphous, it is possible to prevent the base film from affecting the crystal growth and components of the object to be measured that is formed on top of the base film, and it is possible to accurately analyze the object to be measured.

In this X-ray diffraction method for a thin film, the substrate is made of glass, and the measurement includes diffraction peaks in the thin film having a diffraction angle 2θ of 20° or more and 35° or less.
When no undercoat film is used, a large halo from the glass substrate appears in the range of diffraction angles 2θ between 20° and 35°, and the slight peaks of the diffraction lines of the object to be measured are buried in the halo and cannot be identified. With this diffraction method, an undercoat film is formed on the surface of the glass substrate, eliminating the influence of the substrate halo and making it possible to capture the slight peak lines of the thin film.

In this thin film X-ray diffraction method, the surface roughness Ra of the undercoat film is 5 nm or less. If the surface roughness Ra of the undercoat film is 5 nm or less, scattering in the undercoat film is suppressed, making it easier to detect diffracted X-rays and allowing accurate analysis.

In this case, the thickness of the thin film can be 10 nm or less.
Even thin films of copper oxide can be accurately analyzed.

The present invention reduces the influence of the substrate, allowing accurate analysis of the measurement target and expanding the range of application of the target.

FIG. 2 is a perspective cross-sectional view showing a measurement sample in an embodiment of the present invention. This is an X-ray diffraction pattern measured after forming a thin film made of copper oxide on an undercoat film made of indium tin oxide.

Hereinafter, an embodiment of the present invention will be described.
In the embodiment, the substance to be measured is, for example, copper oxide. In the measurement sample shown in FIG. 1, a thin film 3 made of copper oxide is formed on the surface of a substrate 1 via an undercoat film 2 .
The substrate 1 can be suitably made of glass, which is generally used in the XRD diffraction method.
The undercoat film 2 is, for example, an indium tin oxide (ITO) film, which is an inorganic mixture of indium oxide (III) (In ₂ O ₃ ) and tin oxide (IV) (SnO ₂ ). Indium tin oxide is preferably amorphous so as not to affect the crystal growth of the measurement object formed on the undercoat film 2 (e.g., to prevent epitaxial growth or chemical reactions). The thickness of the undercoat film 2 is preferably 20 nm or more. In addition, the surface is preferably a mirror surface, and the arithmetic mean roughness Ra is preferably 5 nm or less.
The thin film 3 to be measured is formed on the surface of the base film 2 to a thickness of approximately 8 nm.

The crystal phase of the sample thus constructed is identified by the in-plane XRD method or the GI-XRD method.
In the in-plane XRD method or the GI-XRD method, in order to perform an accurate analysis, it is important to cause total reflection of X-rays between the thin film 3 and the undercoat film 2 of the measurement object.
It is known that the critical angle α of total reflection of a film can be calculated by multiplying the wavelength of the incident X-rays by the square root of the mass density of the target material. Specifically, the critical angle α for total reflection is expressed by the following formula:

The undercoat film 2 made of indium tin oxide is effective in causing total reflection at the interface with the copper oxide thin film 3. That is, the copper oxide thin film 3 has a density of approximately 3 g/ ^cm3 , and the undercoat film 2 made of indium tin oxide has a density of approximately 7 g/ ^cm3 . In the case of indium tin oxide, the total reflection critical angle α is approximately 0.4°. Therefore, if X-rays are incident at an angle smaller than this total reflection critical angle α, they will be totally reflected on the surface of the undercoat film 2. Note that since the refractive index of materials with respect to X-rays is approximately 1, the effect of refraction is ignored.
On the other hand, in the case of the thin film 3 of copper oxide, which is the substance to be measured, the total reflection critical angle α is approximately 0.25°.
Therefore, it is preferable to make the X-rays incident at an incident angle ω between the total reflection critical angle of the base film 2 and the total reflection critical angle of the thin film 3 to be measured, for example, 0.3° in this case.

In this way, by irradiating X-rays on the thin film 3 at an angle of incidence (ω) between the total reflection critical angle of the undercoat film 2 and the total reflection critical angle of the thin film 3 to be measured, it is possible to achieve almost total reflection at the interface between the undercoat film 2 and the thin film 3. When total reflection occurs, the X-rays generate evanescent waves parallel to the sample surface and hardly penetrate into the sample. In other words, by inducing total reflection on the surface of the undercoat film 2 formed directly below the measurement target, it is possible to reduce the generation of diffracted X-rays, amorphous halos, and characteristic X-rays by the undercoat film 2 and substrate 1. Therefore, the crystalline phase of the thin film 3 to be measured can be accurately identified.

By providing a base film 2 in this way and measuring at an incidence angle ω, a more accurate analysis can be performed, and the measurement object can be applied not only to copper oxide but also to a wide range of materials that satisfy the incidence angle ω, such as nitride films.

The present invention is not limited to the configuration of the above embodiment, and various changes can be made to the details of the configuration without departing from the spirit of the present invention.
For example, although the substrate is made of glass, it may be made of silicon. Alternatively, a silicon wafer coated with amorphous silicon may be used as the substrate. In addition, since a base film is formed, the material of the substrate is not particularly limited as long as it can have a smooth surface.

Furthermore, the base film 2 may be any film having a large total reflection critical angle (for example, larger than that of glass), and in addition to the indium tin oxide film described in the embodiment, a film made of zinc oxide (ZnO), tin oxide (SnO ₂ ), titanium oxide (TiO ₂ ), niobium oxide (NbO ₂ ), etc. may be used.

In order to confirm the specific effects of the present invention, the results of a qualitative analysis of a thin film made of copper oxide using an in-plane XRD method are shown below.
The substrate was made of glass, and the undercoat film was made of indium tin oxide. The undercoat film had a thickness of 40 nm. The arithmetic mean roughness Ra of the surface was approximately 2 nm. A thin film made of copper oxide was formed on the undercoat film to a thickness of 8 nm. As a comparative example, a thin film made of copper oxide was also formed directly on a glass substrate by sputtering.

As shown in FIG. 1, the measurement angle range (angle of the scanning axis) 2θ was set to 10° to 90°, and the X-ray incident angle ω was set to 0.3°.
The diffraction patterns of the example and comparative example are shown in Figure 2. The horizontal axis is the diffraction angle (2θ), and the vertical axis is the diffracted X-ray intensity (counts: the number of diffracted X-rays captured by the detector per second). The diffraction pattern of the example is indicated by the symbol E, and the diffraction pattern of the comparative example is indicated by the symbol C. P1 to P6 indicate the positions of the diffraction peaks due to copper oxide.

In the case of the comparative example, a broad peak is observed in the region near 2θ=20° indicated by H due to the influence of the glass halo. This indicates that the X-ray penetration depth of the formed copper oxide thin film is about 400 nm when ω=0.3° (the material is Cu ₂ O, and the depth at which the diffracted X-ray intensity for the incident X-ray is 99% or more is calculated as the X-ray penetration depth), which is much greater than the film thickness of 8 nm, and therefore the X-ray penetrates all the way to the glass layer of the substrate.

In contrast, in the case of an embodiment in which an undercoat film made of amorphous indium tin oxide, a material that satisfies the above-mentioned requirements, is formed between a glass substrate and a thin film made of copper oxide, it is clear that the effect of the glass halo is not observed, as shown in Figure 2E. Furthermore, if diffraction lines were generated from the undercoat film, which is amorphous indium tin oxide, a broad peak should be observed near 2θ = 32°, but this is not seen. Also, the increase in background is suppressed.

Furthermore, in the case of the comparative example, due to the influence of the glass halo as described above, peaks assumed to be copper oxide diffraction peaks could not be seen in the region around 2θ = 10° to 35°, whereas in the case of the example, a small diffraction peak derived from Cu ₂ O in the region around 2θ = 30° (see P1) can be seen.

From these diffraction results, it can be said that the method of this embodiment is a technique that can measure copper oxide thin films of the nm order with high sensitivity by minimizing the influence of the substrate and undercoat film. Furthermore, as shown in the above formula (1), based on the relationship between the critical angle of total reflection of the indium tin oxide film, α ≒ 0.4°, and the critical angle of total reflection of the copper oxide thin film, α ≒ 0.25°, it is estimated that theoretically, total reflection occurs sufficiently at the X-ray incidence angle ω = 0.3° set in this experiment, and it was found that highly visible diffraction peaks can be obtained even for extremely thin film samples of 8 nm.

1 Substrate 2 Undercoat film 3 Thin film to be measured

Claims (4)

1. A method for X-ray diffraction of a thin film, comprising: forming an undercoat film on a surface of a substrate, the undercoat film being a material having a higher density than a thin film made of copper oxide and being made of any one of indium tin oxide, zinc oxide, tin oxide, titanium oxide, and niobium oxide; forming the thin film on the undercoat film; and irradiating X-rays onto the thin film at an incident angle between the total reflection critical angle of the undercoat film and the total reflection critical angle of the thin film to identify a crystalline phase of the thin film. 2. The method for X-ray diffraction of a thin film according to claim 1, wherein the undercoat film is amorphous. 3. The X-ray diffraction method for a thin film according to claim 1, wherein the substrate is made of glass, and the measurement includes diffraction peaks in the thin film having a diffraction angle 2θ of 20° or more and 35° or less. 4. The thin film X-ray diffraction method according to claim 1, wherein the undercoat film has a surface roughness Ra of 5 nm or less.