Diffraction grating for soft x-ray and extreme ultraviolet ray

Abstract

This diffraction grating for soft X-rays and extreme ultraviolet rays comprises a metal base reflection layer and a non-metal enhanced reflection layer, wherein peeling between the base reflection layer and the enhanced reflection layer is prevented and high diffraction efficiency is achieved. A diffraction grating (2) for soft X-rays and extreme ultraviolet rays according to the present invention comprises: a substrate (21) having a periodic surface relief on the surface; a base reflection layer (22) made of Au and formed on the surface of the substrate; an intermediate layer (23) made of any one of Nb, Cr, Ti, W, or an alloy of two or more thereof and formed on the base reflection layer; and an enhanced reflection layer (24) made of either B4C or C and formed on the intermediate layer.

Description

Diffraction Grating for Soft X-rays and Extreme Ultraviolet Rays

[0001] The present invention relates to a diffraction grating corresponding to electromagnetic waves in the soft X-ray and extreme ultraviolet regions.

[0002] There is an increasing need for electromagnetic wave spectroscopy in the vicinity of Li-K emission (22.83 nm) for state analysis and trace analysis of lithium battery materials and EUV (extreme ultraviolet) lithography materials. This electromagnetic wave is called both soft X-ray and extreme ultraviolet, but in this specification, it is called soft X-ray / extreme ultraviolet, and electromagnetic waves in the wavelength range of 1 nm to 100 nm (energy range of 12 eV to 1.2 keV) (hereinafter also referred to as light) are targeted.

[0003] In order to obtain a high diffraction efficiency in a diffraction grating, generally, the incident angle α measured from the normal direction perpendicular to the diffraction grating surface satisfies the total reflection condition of the mirror surface, sinα≧n M (α≧π / 2-{2(1-n M )} 1 / 2 )). Here, n M is the refractive index of the substance (layer) present on the surface of the diffraction grating. The refractive index n M of the substance laminated as a reflective layer on the surface of the diffraction grating used in the soft X-ray / extreme ultraviolet region is slightly smaller than 1. Therefore, the incident angle α becomes a value slightly smaller than 90°, and the incident light is incident at an angle almost parallel to the surface of the diffraction grating.

[0004] The energy of the light diffracted by the diffraction grating is dispersed into zero-order light and many orders of light that satisfy the specular reflection condition, and there is also a component absorbed by the surface substance. Therefore, the intensity of the first-order light (or -first-order light) used for measurement becomes very weak compared to the intensity of the incident light. Therefore, various methods have been developed to minimize absorption on the surface and increase the reflectivity.

[0005] One method for obtaining high diffraction efficiency in the soft X-ray and extreme ultraviolet regions is to use a multilayer diffraction grating having a multilayer film formed by stacking multiple bilayers on the surface of the diffraction grating, each consisting of a low-density material layer and a high-density material layer with a higher density than the low-density material layer. In this method, the diffracted light is strengthened by interference between the light diffracted by the high-density material layer. To maximize this effect, it is necessary to allow the incident light to penetrate into the interior of the multilayer film. However, under total internal reflection conditions in the soft X-ray and extreme ultraviolet regions, the penetration depth is small, preventing light from penetrating deep enough to fully utilize the effect of the multilayer film. This has made it difficult to obtain high diffraction efficiency with soft X-ray and extreme ultraviolet multilayer diffraction gratings (see Patent Document 1).

[0006] In contrast, numerical calculations and experiments have shown that diffraction efficiency can be improved by forming a metallic basic reflective layer on the surface of a diffraction grating, which is made of conventionally used materials such as gold (Au), platinum (Pt), and nickel (Ni), and then stacking an enhanced reflective layer made of amorphous carbon (diamond-like carbon, DLC), which has low light absorption, on top of it with an optimal thickness (see Patent Document 2).

[0007] Japanese Patent Publication No. 2011-141129 Japanese Patent Publication No. 2015-094892

[0008] The amorphous carbon reflective layer, which is deposited on top of the metallic base reflective layer, is formed by sputtering. However, because the base reflective layer and the reflective layer are made of completely different materials, their adhesion is not sufficient, and the reflective layer may peel off from the base reflective layer after deposition. Even slight peeling of the reflective layer significantly reduces its reflectivity (diffraction efficiency).

[0009] The present invention was made to solve the above-mentioned problems, and the problem that the present invention aims to solve is to prevent the peeling of the basic reflective layer and the enhanced reflective layer in a diffraction grating for soft X-rays and extreme ultraviolet rays, which comprises a basic reflective layer made of metal and an enhanced reflective layer, and to obtain high diffraction efficiency.

[0010] The soft X-ray and extreme ultraviolet diffraction grating according to the present invention, which was developed to solve the above problems, comprises: a substrate having periodic irregularities on its surface; a basic reflective layer made of gold (Au) deposited on the surface of the substrate; an intermediate layer made of niobium (Nb), chromium (Cr), titanium (Ti), tungsten (W), or an alloy of two or more thereof, deposited on the basic reflective layer; and an enhanced reflective layer made of boron carbide (B4C) or carbon (C), deposited on the intermediate layer.

[0011] In this invention, the diffraction grating for soft X-rays and extreme ultraviolet rays has an intermediate layer made of Nb, Cr, Ti, W, or an alloy of two or more of them, between a basic reflective layer made of Au and an enhanced reflective layer made of either B4C or C. Therefore, each layer does not peel off from the adjacent layer. As a result, a high reflectivity (diffraction efficiency) can be obtained.

[0012] This is a cross-sectional view of a diffraction grating that is one embodiment of the present invention. This is a photograph of a diffraction grating in which a basic reflective layer made of Au has been deposited on a substrate. This is a photograph of a diffraction grating in which a basic reflective layer made of Au, an intermediate layer made of W, and an enhanced reflective layer made of B4C have been deposited on a substrate in this order from bottom to top. This is a photograph of a diffraction grating in which a basic reflective layer made of Au and an enhanced reflective layer made of B4C have been deposited on a substrate in this order from bottom to top. This is a graph showing the numerical simulation results of the diffraction efficiency of a diffraction grating according to an embodiment. This is a graph showing the numerical simulation results of the diffraction efficiency of a diffraction grating according to Modification 1. These are graphs showing the numerical simulation results of the diffraction efficiency of diffraction gratings according to Modifications 2 and 3.

[0013] The inventors used numerical simulations to search for diffraction gratings that exhibit high diffraction efficiency in the soft X-ray and extreme ultraviolet regions, and found that a diffraction grating comprising a basic reflective layer made of Au and an enhanced reflective layer made of either B4C or C, in that order from the bottom, on a substrate with periodic irregularities, is suitable. However, since Au has a much larger coefficient of thermal expansion than B4C or C, it was found that when an enhanced reflective layer made of B4C or C is actually deposited on the basic reflective layer made of Au by sputtering, a large stress acts on the interface between the basic reflective layer and the enhanced reflective layer, causing the enhanced reflective layer to peel off from the basic reflective layer.

[0014] The inventors then conducted further research and discovered that a diffraction grating in which an intermediate layer made of Nb, Cr, Ti, W, or an alloy of two or more of these materials (with a thermal expansion coefficient greater than B4C or C but less than Au) is deposited between the basic reflective layer and the enhanced reflective layer exhibits diffraction efficiency comparable to that of a diffraction grating without an intermediate layer, while preventing delamination between adjacent layers. These materials are considered suitable as intermediate layers because their relatively high density results in a relatively high refractive index, which contributes to the improvement of diffraction efficiency.

[0015] The specific configuration and manufacturing method of diffraction grating 2, which is one embodiment of the diffraction grating for the soft X-ray and extreme ultraviolet regions according to the present invention, will be described below with reference to Figure 1.

[0016] As shown in Figure 1, the diffraction grating 2 comprises a substrate 21 having periodic irregularities on its surface, a basic reflective layer 22 formed on the substrate 21, an intermediate layer 23 formed on the basic reflective layer 22, and an enhanced reflective layer 24 formed on the intermediate layer 23.

[0017] The substrate 21 is manufactured using a known substrate material such as SiO2, by a known method for manufacturing the substrate. The substrate 21 can be, for example, a substrate for a laminar-type diffraction grating. In this case, the peak wavelength of the diffraction efficiency is determined by the number of grooves (groove count), depth, and duty cycle formed on the surface of the diffraction grating 2. In light of the spirit of the present invention, in order to obtain high diffraction efficiency in the soft X-ray and extreme ultraviolet regions, it is preferable to set the number of grooves of the substrate 21 or diffraction grating 2 to 120 to 3600 grooves / mm, and in order to obtain high diffraction efficiency near Li-K emission (22.83 nm), it is more preferable to set the number of grooves to 300 to 1500 grooves / mm. Furthermore, it is preferable to set the groove depth of the substrate 21 or diffraction grating 2 to 10 to 50 nm. Furthermore, it is preferable to set the duty cycle of the substrate 21 or diffraction grating 2 to 0.3 to 0.5.

[0018] The basic reflective layer 22 is made of Au and is formed on the substrate 21 by a known method of forming a thin film on the surface of the diffraction grating. The thickness of the basic reflective layer 22 is sufficient if it is greater than the penetration depth of electromagnetic waves in the soft X-ray and extreme ultraviolet regions. That is, the thickness of the basic reflective layer 22 is preferably 20 to 50 nm.

[0019] The intermediate layer 23 is made of Nb, Cr, Ti, W, or an alloy of two or more of them, and is deposited to a thickness of 1 nm or more by a known method of forming a thin film on the surface of the diffraction grating, such as magnetron sputtering. On the other hand, the thicker the intermediate layer 23 is deposited, the more of the incident or diffracted electromagnetic waves will be absorbed by the intermediate layer 23. Also, the thicker the intermediate layer 23 is deposited, the greater the stress acting on the interface with the adjacent layer becomes, making it easier for the intermediate layer to delaminate at the interface. For this reason, it is preferable that the thickness of the intermediate layer 23 be limited to the depth of the grooves of the diffraction grating 2.

[0020] The reflective enhancement layer 24 is made of either B4C or C and is formed by a known method of forming a thin film on the surface of the diffraction grating, such as magnetron sputtering. The thickness of the reflective enhancement layer 24 is preferably about 50 nm when combined with the thickness of the intermediate layer 23. For example, if the intermediate layer 23 is formed to a thickness of 5 nm, the thickness of the reflective enhancement layer 24 is preferably about 45 nm.

[0021] The following describes a specific embodiment of the diffraction grating 2. To avoid redundant explanations and diagrams, this embodiment will use the same reference numerals and diagrams as those used in the previous description of the diffraction grating 2.

[0022] The diffraction grating 2 of this embodiment is a laminar-type diffraction grating comprising: a substrate 21 made of SiO2 with 1000 grooves / mm, a groove depth of 30 nm, and a duty cycle of 0.4; a basic reflective layer 22 made of Au deposited on the substrate to a thickness of 30 nm; an intermediate layer 23 made of W deposited on the basic reflective layer to a thickness of 5 nm; and an enhanced reflective layer 24 made of B4C deposited on the intermediate layer to a thickness of 45 nm.

[0023] Figure 2 is a photograph of a substrate 21 with a basic reflective layer 22 deposited on it, and Figure 3 is a photograph of the diffraction grating 2 made in this embodiment. Figure 4 is a photograph of a substrate 21 with a basic reflective layer 22 deposited on it, and then with an enhanced reflective layer 24 deposited directly on top of the basic reflective layer 22 (i.e., without depositing an intermediate layer 23). Hereafter, the object shown in the photograph in Figure 4 will be referred to as diffraction grating 1. As can be seen by comparing Figure 2 and Figure 4, the surface of the diffraction grating with a basic reflective layer 22 made of Au deposited on the substrate 21 was smooth, but the surface of diffraction grating 1 with an enhanced reflective layer 24 made of B4C deposited on top of the basic reflective layer 22 had wrinkles, and the enhanced reflective layer 24 was partially peeled off. In contrast, comparing Figure 2 with Figure 3, and Figure 3 with Figure 4, it can be seen that in the diffraction grating 2 in which an intermediate layer 23 made of W is deposited on the basic reflective layer 22, and an enhanced reflective layer 24 made of B4C is deposited on top of that, the surface is smooth, and there was almost no delamination between the intermediate layer 23 and the basic reflective layer 22 and enhanced reflective layer 24.

[0024] Figure 5 is a graph showing the diffraction efficiency of diffraction gratings 1 and 2 at each wavelength, obtained by numerical simulation. The horizontal axis of the graph represents wavelength (nm), and the vertical axis represents diffraction efficiency. The dotted line corresponds to diffraction grating 1, and the solid line corresponds to diffraction grating 2. As shown in Figure 5, the diffraction efficiency curves of diffraction grating 2 and diffraction grating 1 peak at almost the same wavelength, and it can be seen that the diffraction efficiency of diffraction grating 2 at that peak wavelength is slightly lower than that of diffraction grating 1, but still comparable. In particular, since the diffraction efficiency around the Li-K wavelength (22.83 nm) is almost the same, it can be said that diffraction grating 2 is useful for electromagnetic wave spectroscopy near the Li-K wavelength.

[0025] Note that the diffraction efficiency obtained by numerical simulation is the value assuming that the basic reflective layer, the intermediate layer, and the enhanced reflective layer are in close contact, or that the basic reflective layer and the enhanced reflective layer are in close contact. Since diffraction gratings without an intermediate layer are prone to wrinkles on their surface, the actual diffraction efficiency will be lower than the value obtained by numerical simulation. Therefore, diffraction grating 2 shows superior diffraction efficiency compared to diffraction grating 1, which has wrinkles. [Modification]

[0026] The above-described examples are merely illustrations of the present invention, and it goes without saying that, in accordance with the spirit of the invention, and specifically according to the numerical ranges and material selections described above, appropriate modifications can be made.

[0027] For example, in the diffraction grating 2 of the example, a modified example 1 can be implemented in which the material constituting the intermediate layer is changed from W to Cr. Figure 6 is a graph showing the diffraction efficiency of diffraction grating 1, diffraction grating 2 of the example, and diffraction grating of modified example 1, obtained by numerical simulation. The horizontal axis of the graph represents wavelength (nm), and the vertical axis represents diffraction efficiency. The dotted line corresponds to diffraction grating 1, the solid line corresponds to diffraction grating 2 of the example, and the dashed line corresponds to the diffraction grating of modified example 1. As shown in Figure 6, the diffraction efficiency curve of the diffraction grating of modified example 1 is in close agreement with the diffraction efficiency curve of diffraction grating 2 of the example, and it is recognized that it has an equivalent effect.

[0028] Furthermore, in the diffraction grating 2 of the example, a modified example 2 can be implemented in which the material constituting the enhanced reflection layer is changed from B4C to C. In addition, a modified example 3 can be implemented in which the material constituting the intermediate layer of modified example 2 is changed from W to Cr. Figure 7 is a graph showing the diffraction efficiency of the diffraction grating in which the enhanced reflection layer of diffraction grating 1 is changed from B4C to C (hereinafter referred to as the diffraction grating without an intermediate layer), the diffraction grating of modified example 2, and the diffraction grating of modified example 3, obtained by numerical simulation. The horizontal axis of the graph represents wavelength (nm), and the vertical axis represents diffraction efficiency. The dotted line corresponds to the diffraction grating without an intermediate layer, the solid line corresponds to the diffraction grating of modified example 2, and the dashed line corresponds to the diffraction grating of modified example 3. The diffraction efficiency curves of the diffraction gratings of modified examples 2 and 3 both peak at almost the same wavelength as the diffraction efficiency curve of the diffraction grating without an intermediate layer, and although the diffraction efficiency of the diffraction gratings of modified examples 2 and 3 at that peak wavelength is slightly lower than that of the diffraction grating without an intermediate layer, it can be seen that the diffraction efficiency around the Li-K wavelength (22.83 nm) is comparable.

[0029] Although not shown in the diagrams, it was visually confirmed that in the diffraction gratings of modified examples 1, 2, and 3, the presence of an intermediate layer made of W or Cr makes delamination less likely to occur between adjacent layers and prevents the formation of wrinkles on the surface of the enhanced reflection layer.

[0030] [Embodiments] It will be apparent to those skilled in the art that the exemplary embodiments described above are specific examples of the following embodiments.

[0031] (Section 1) A diffraction grating for soft X-rays and extreme ultraviolet rays according to one aspect of the present invention comprises a substrate having periodic irregularities on its surface, a basic reflective layer made of Au deposited on the surface of the substrate, an intermediate layer made of any of Nb, Cr, Ti, W or an alloy of two or more thereof deposited on the basic reflective layer, and an enhanced reflective layer made of B4C or C deposited on the intermediate layer.

[0032] (Article 2) The diffraction grating for the soft X-ray and extreme ultraviolet region according to Article 2 is the diffraction grating for the soft X-ray and extreme ultraviolet region according to Article 1, wherein the intermediate layer is made of W.

[0033] According to the diffraction gratings for soft X-rays and extreme ultraviolet regions described in paragraphs 1 and 2, an intermediate layer made of Nb, Cr, Ti, W, or an alloy of two or more of these materials is present between the basic reflective layer made of metal and the enhanced reflective layer made of either B4C or C. Therefore, each layer does not peel off from the adjacent layer. As a result, a high reflectivity (diffraction efficiency) can be obtained.

[0034] (Article 3) The diffraction grating for the soft X-ray and extreme ultraviolet region according to Article 3 is the diffraction grating for the soft X-ray and extreme ultraviolet region according to Article 1 or Article 2, wherein the thickness of the intermediate layer is less than or equal to the depth of the grooves of the irregularities.

[0035] According to the diffraction grating for soft X-rays and extreme ultraviolet radiation described in paragraph 3, by making the thickness of the intermediate layer less than or equal to the depth of the groove, the amount of incident or diffracted electromagnetic waves absorbed by the intermediate layer can be reduced. In addition, the stress acting on the interface between the intermediate layer and the adjacent layer can be reduced.

[0036] 1, 2...Diffraction grating 21...Substrate 22...Basic reflective layer 23...Intermediate layer 24...Enhanced reflective layer

Claims

1. A diffraction grating for soft X-rays and extreme ultraviolet rays, comprising: a substrate having periodic irregularities on its surface; a basic reflective layer made of Au deposited on the surface of the substrate; an intermediate layer made of Nb, Cr, Ti, W, or an alloy of two or more thereof, deposited on the basic reflective layer; and an enhanced reflective layer made of B4C or C deposited on the intermediate layer.

2. A diffraction grating for the soft X-ray and extreme ultraviolet region according to claim 1, wherein the intermediate layer is made of W.

3. A diffraction grating for the soft X-ray and extreme ultraviolet region according to claim 1, wherein the thickness of the intermediate layer is less than or equal to the depth of the grooves of the irregularities.

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