JP7756232B2 - Diffraction grating manufacturing method and diffraction grating - Google Patents

Description

The present invention relates to a method for manufacturing a diffraction grating and a diffraction grating, and in particular to a method for manufacturing a diffraction grating that is suitable for faithfully reproducing the groove shape in a manufacturing method in which the groove shape of a diffraction grating formed on a photoresist applied to a silicon wafer is transferred to a metal film.

A diffraction grating is an optical element used in spectrometers in analytical equipment to separate light containing a mixture of various wavelengths (such as white light) into narrow bands of wavelengths.It consists of fine grooves engraved on the surface of an optical material with a reflective film vapor-deposited on it.

Diffraction gratings include, for example, blazed (sawtooth) diffraction gratings, sinusoidal diffraction gratings, and laminar diffraction gratings. Blazed diffraction gratings are manufactured by creating a master diffraction grating by rubbing grooves one by one on a glass substrate coated with a metal film using a processing machine such as a ruling engine, and then transferring the rubbing groove shape to a resin or metal. A method for manufacturing sinusoidal diffraction gratings using photolithography is disclosed, for example, in Patent Document 1 (JP 2003-172639 A).

The wavelength resolution performance of a sinusoidal diffraction grating is determined by the number of grooves engraved on the diffraction grating (number of grooves per mm), so a diffraction grating with a large number of grooves is required to increase the wavelength resolution of the device.

In addition, since laminar diffraction gratings have lower diffraction efficiency than blazed diffraction gratings and sinusoidal diffraction gratings, high efficiency is required for diffraction gratings used in the soft X-ray and vacuum ultraviolet regions.

Japanese Patent Application Laid-Open No. 2003-172639

Conventional methods using machining, such as ruling engines, require a tool to scribe each groove one by one. This makes manufacturing time consuming, which is an issue. Because the groove shape that can be scribed is determined by the tool, it is difficult to produce a sinusoidal diffraction grating with a large number of scribed lines. Furthermore, when attempting to transfer the shape to a metal film using conventional replica technology, the metal does not reach the bottom of sinusoidal grooves with a high aspect ratio, making it difficult to transfer the grooves. When metal does not reach the bottom of the sinusoidal grooves, the shape of the grooves on the surface of the reflective metal film is deformed and not sinusoidal, resulting in reduced diffraction efficiency.

The present invention was conceived to solve these problems, and its purpose is to improve the performance of diffraction gratings.

A brief overview of a representative embodiment disclosed in this application is as follows:

One embodiment of a method for manufacturing a diffraction grating includes the steps of: (a) preparing a substrate having a first surface and a second surface opposite the first surface; (b) forming a photoresist film on the first surface of the substrate, the photoresist film having a sinusoidal first groove on a third surface opposite the substrate; and (c) using electroless plating to form a first metal film on the third surface of the photoresist film that fills the sinusoidal first groove, thereby forming a diffraction grating having the first metal film.

In one embodiment, the diffraction grating comprises a substrate and a first metal film formed on the substrate and having sinusoidal grooves on the surface opposite the substrate, and the groove width of the sinusoidal grooves in the first metal film is 100 nm or less.

The present invention can improve the performance of diffraction gratings.

3 is a flowchart showing a method for manufacturing a diffraction grating according to the first embodiment. 1 is a flowchart showing a manufacturing method for forming a groove in a photoresist film according to the first embodiment. 5A to 5C are cross-sectional views showing a manufacturing method for forming a groove in a photoresist film according to the first embodiment. 4 is a cross-sectional view showing a manufacturing method subsequent to FIG. 3. 5 is a cross-sectional view showing the manufacturing method subsequent to FIG. 4. 6 is a cross-sectional view showing a manufacturing method subsequent to FIG. 5. 5A to 5C are cross-sectional views showing a method for manufacturing a diffraction grating according to the first embodiment. 8 is a cross-sectional view showing the manufacturing method subsequent to FIG. 7. 9 is a cross-sectional view showing the manufacturing method subsequent to FIG. 8. 10A to 10C are cross-sectional views showing the manufacturing method following FIG. 9. 11A to 11C are cross-sectional views showing the manufacturing method following FIG. 10 . 12 is a cross-sectional view showing the manufacturing method subsequent to FIG. 11 . 13 is a cross-sectional view showing the manufacturing method subsequent to FIG. 12. 1A to 1C are diagrams showing a manufacturing method in which heat is applied to a photoresist to deform the photoresist according to the first embodiment; 15 is a cross-sectional view showing the manufacturing method subsequent to FIG. 14. 10 is a flowchart showing a method for manufacturing a diffraction grating according to a second embodiment. 10A to 10C are cross-sectional views showing a method for manufacturing a diffraction grating according to a second embodiment. 18 is a cross-sectional view showing the manufacturing method subsequent to FIG. 17. FIG. 2 is a schematic diagram of a cross-sectional shape of a groove of a blazed diffraction grating. FIG. 2 is a schematic diagram of a cross-sectional shape of a groove of a sinusoidal diffraction grating. FIG. 1 is a schematic diagram of a cross-sectional shape of a groove of a laminar diffraction grating. FIG. 10 is a diagram showing a state in which a reflective metal film is uniformly formed on a sinusoidal groove made of photoresist. FIG. 10 is a diagram showing a state in which a reflective metal film is formed in a tip-concentrated manner in a sinusoidal groove made of photoresist. 10 is a graph showing the diffraction efficiency when a reflective metal film is uniformly formed on a sinusoidal groove made of photoresist, and when a reflective metal film is uniformly formed on a sinusoidal groove made of photoresist.

Embodiments of the present invention will be described in detail below with reference to the drawings. Note that in all drawings used to explain the embodiments, components having the same functions will be given the same reference numerals, and repeated explanations will be omitted. Furthermore, in the embodiments, explanations of identical or similar parts will not be repeated, unless specifically required.

ROOM FOR IMPROVEMENT
The room for improvement will be described in detail below with reference to FIGS.

Diffraction gratings are classified into several types depending on the shape of the grooves engraved into them. First, we will explain blazed (sawtooth) diffraction gratings. Figure 19 shows a schematic diagram of the cross-sectional groove shape of a blazed diffraction grating 201. Spectrophotometers and other spectroscopic analysis devices are required to make measurements with a high S/N ratio by efficiently utilizing the energy of the light source, so reflective blazed diffraction gratings are preferred because they can efficiently extract only diffracted light of specific diffraction orders. One method for manufacturing blazed diffraction gratings is to create a master diffraction grating by rubbing grooves one by one into a glass substrate coated with a metal film using a processing machine such as a ruling engine, and then transfer the rubbing groove shape to resin or metal.

Next, we will explain sinusoidal diffraction gratings. Figure 20 shows the cross-sectional groove shape of a sinusoidal diffraction grating 202. Sinusoidal diffraction gratings have different optical properties from blazed diffraction gratings, such as broad diffraction efficiency over a wide wavelength range and the peak wavelength of diffraction efficiency being determined by the groove depth and width. Sinusoidal diffraction gratings are used in devices such as inductively coupled plasma (abbreviated as ICP in the following text) optical emission spectroscopy. The wavelength resolution of an ICP optical emission spectroscopy device is determined by the number of grooves engraved on the diffraction grating (number of grooves per mm), so a diffraction grating with a large number of grooves is required to increase the wavelength resolution of the device.

Next, we will explain laminar diffraction gratings. Figure 21 shows a schematic diagram of the groove cross-section of a laminar diffraction grating 203. Laminar diffraction gratings have high reflectivity in the soft X-ray and vacuum ultraviolet (VUV) wavelength ranges of 0.1 nm to 200 nm, and are therefore used in spectrometers for soft X-rays and VUV, or in research applications such as material analysis in that wavelength range. Because laminar diffraction gratings have lower diffraction efficiency than blazed diffraction gratings and sinusoidal diffraction gratings, diffraction gratings used in the soft X-ray and vacuum ultraviolet ranges must be highly efficient. In other words, the first area of improvement is the challenge of creating a diffraction grating with high diffraction efficiency.

Furthermore, with methods using machining, such as a ruling engine, grooves are engraved one by one using a tool, which results in a time-consuming manufacturing process. Furthermore, diffraction gratings used in the soft X-ray and vacuum ultraviolet regions must have a large number of grooves (e.g., 10,000 lines/mm or more). In this case, since the grooves to be engraved are very fine, it is possible to manufacture a sinusoidal diffraction grating. However, with methods using machining, the groove shape that can be engraved is determined by the tool, making it difficult to manufacture a sinusoidal diffraction grating with a large number of grooves. Furthermore, when attempting to transfer a shape to a metal film using well-known replica techniques, metal does not reach the bottom of sinusoidal grooves with a high aspect ratio, making it difficult to transfer the grooves.

In other words, to use sinusoidal grooves with a high aspect ratio as a diffraction grating, it is necessary to uniformly deposit a reflective metal film 501 in the sinusoidal grooves created in the photoresist film 101, as shown in Figure 22. In other words, it is necessary to embed the reflective metal film 501 without any gaps along the sinusoidal grooves on the top surface of the photoresist film 101.

In contrast, the method disclosed in Patent Document 1 uses vacuum deposition to deposit the metal film. With this method, when manufacturing a diffraction grating with, for example, 10,000 or more rulings per mm and a groove width (pitch) of 100 nm or less, the reflective metal film 501 concentrates at the tips of the grooves, forming a mushroom shape, as shown in Figure 23. In other words, the reflective metal film 501 is not embedded in the bottoms of the sinusoidal grooves on the top surface of the photoresist film 101, and instead is deposited concentrated at the tips of the irregularities on the top surface of the photoresist film 101.

Figure 24 shows a graph of the relationship between wavelength and diffraction efficiency. The horizontal axis of Figure 24 is wavelength, and the vertical axis is diffraction efficiency. In Figure 24, the solid line shows the diffraction efficiency when the reflective metal film 501 is uniformly applied to a sinusoidal groove (see Figure 22), and the dashed line shows the diffraction efficiency when the reflective metal film 501 is concentrated at the tip of the groove and forms a mushroom shape (see Figure 23). As shown in Figure 23, when the reflective metal film 501 is concentrated at the tip of the groove and forms a mushroom shape, the diffraction efficiency decreases compared to when the reflective metal film 501 is uniformly applied to the groove in Figure 23 because the surface shape of the reflective metal film 501 is deformed rather than sinusoidal.

Thus, as a second area for improvement, when attempting to manufacture a diffraction grating with a line count of 10,000 lines/mm or more and a groove width of 100 nm or less, it is difficult to form the surface of the reflective metal film 501 into a sinusoidal wave shape, which poses the problem of reduced diffraction efficiency.

Therefore, in each embodiment of the present application, we have implemented measures to address the room for improvement mentioned above. Below, we will explain the technical concept behind these implemented embodiments.

(Embodiment 1)
This embodiment will be described below with reference to Figures 1 to 15. Note that the groove width (pitch) of a diffraction grating in the following embodiments refers to the period of the repeated irregularities in a direction when multiple grooves are formed on the surface of the diffraction grating aligned in that direction. For example, in the case of a sinusoidal diffraction grating, the groove width of the diffraction grating refers to the distance between the centers (vertices) of adjacent convex portions in that direction.

Figure 1 shows the manufacturing flow of the diffraction grating of this embodiment. The manufacturing method of the diffraction grating of this embodiment consists of steps S11 to S17 shown below.

That is, first, as shown in FIG. 7, grooves are formed on a substrate (silicon substrate) 102 using a photoresist film 101 (Step S11). Next, as shown in FIG. 8, a reflective metal film (electroless plating film) 103 is formed on the photoresist film 101 using electroless plating (Step S12). Next, as shown in FIG. 9, adhesive 104 is applied to the reflective metal film 103 (Step S13). Next, as shown in FIG. 10, the reflective metal film 103 is bonded to a glass substrate 105 (Step S14). Next, as shown in FIG. 11, the substrate 102 is peeled off from the photoresist film 101 (Step S15). Next, as shown in FIG. 12, the photoresist film 101 remaining on the reflective metal film 103 is removed (Step S16). Next, as shown in FIG. 13, a protective film 106 is formed on the surface of the reflective metal film 103 where the grooves are formed (Step S17). This essentially completes the diffraction grating of this embodiment.

Here, step S11, that is, the process of forming a photoresist film 101 having sinusoidal grooves on its surface, will be explained using Figures 2 to 6. Figure 2 shows the manufacturing flow of the photoresist film 101.

First, as shown in Fig. 3, a photoresist film 101 is applied to a substrate 102 (step S1). Next, as shown in Fig. 4, rectangular diffraction grating grooves are formed using photolithography (step S2). Next, as shown in Fig. 5, the photoresist film 101 is heated to deform the grooves (step S3). Next, as shown in Fig. 6, a water immersion treatment is performed on the outermost surface of the photoresist film 101 (step S4). As a result, a photoresist film 101 having sinusoidal grooves on its surface is formed.
Next, each step will be described in detail.

Regarding step S1, the substrate 102 is a silicon substrate having a first surface and a second surface opposite the first surface. Photoresists are broadly divided into negative and positive types based on how they react with light or electron beams. When negative types are exposed to light, their solubility in a developer decreases, and the exposed portions remain after development. In contrast, when positive types are exposed to light, their solubility in a developer increases, and the exposed portions are removed during development. The photoresist film 101 used in step S1 may be either negative or positive.

In step S2, the groove shape is formed using photolithography technology, which is used in semiconductor manufacturing. In photolithography, a photoresist (photosensitive organic material) film 101 is applied to the substrate 102 (step S1), and the mask pattern drawn on a photomask is exposed to the photoresist film 101, which is then developed to transfer the mask pattern to the photoresist film 101 (step S2).

The wavelengths of the exposure equipment used to expose the mask pattern include 248 nm (KrF excimer laser) and 193 nm (ArF excimer laser), but any wavelength of exposure equipment can be used here and there is no limitation on the wavelength of the exposure equipment used. Here, the film thickness of the photoresist film 101 and the exposure amount of the exposure equipment are adjusted to expose the photoresist film 101 at the bottom of the grooves so that it remains. The exposed shape is, for example, a rectangular shape as shown in Figure 14. While Figure 14 shows a rectangular shape as an example, other examples of shapes include a trapezoidal shape. A trapezoidal shape can also achieve the same effect as a rectangular shape. That is, here, multiple rectangular or trapezoidal grooves are formed on the surface of the photoresist film 101.

In step S3, the diffraction grating of the developed photoresist film 101 is heated to melt the resist and cause it to flow in the direction indicated by the arrow in Figure 14, thereby deforming the photoresist film 101 into a sinusoidal shape as shown in Figure 15. That is, the multiple rectangular or trapezoidal grooves arranged on the surface of the photoresist film 101 are deformed by heat treatment to form sinusoidal grooves. When heating the photoresist film 101, heating is performed, for example, until the glass transition temperature of the photoresist is reached.

The heating device used in step S3 may be, for example, a hot plate that generates heat by passing an electric current through a heating wire to heat a plate, or an oven that heats the air in a heating chamber with gas or a heater. However, any device having a heating mechanism that can heat and fluidize the photoresist film 101 is sufficient, and is not limited to hot plates and ovens. As a result, the distance between the apex and the bottom surface of the sinusoidal groove formed in the photoresist film 101, i.e., the depth of the sinusoidal groove, is, for example, about 100 nm.

The hydrophilic treatment in step S4 involves an ashing process using oxygen plasma. By performing the ashing process, a hydrophilic surface 107 containing functional groups (OH groups) such as carbonyl groups and carboxyl groups can be formed, thereby chemically improving the hydrophilicity of the photoresist film 101.

Step S12 uses an autocatalytic electroless plating method. Because electroless plating forms a metal film in a solution, the metal film does not concentrate on the tips of the convex portions, as occurs when vacuum deposition is used in the comparative example described with reference to Figure 23. In other words, electroless plating can form the reflective metal film 103 uniformly, conforming to the irregularities on the surface of the photoresist film 101, faithfully reproducing the sinusoidal groove shape created on the surface of the photoresist film 101. Examples of materials used for the formed reflective metal film 103 include gold (Au), copper (Cu), and silver (Ag).

The adhesive 104 used in step S13 is, for example, epoxy resin.

The material of the glass substrate 105 used in step S14 is, for example, synthetic quartz glass or borosilicate glass (BK7).

In steps S15 and S16, the substrate 102 and photoresist film 101 are removed to expose the sinusoidal surface of the reflective metal film 103.

The protective film 106 in step S17 is made of, for example, magnesium fluoride, which has a wide transmission wavelength band and is resistant to thermal and mechanical shocks. The protective film 106 can be formed, for example, by vapor deposition. For example, if magnesium fluoride is used as the protective film for a diffraction grating for vacuum ultraviolet (wavelengths of 110 nm to 200 nm), it can prevent a decrease in diffraction efficiency due to the effects of metal film oxidation. The effect of magnesium fluoride in preventing a decrease in diffraction efficiency can be achieved in any wavelength band, including ultraviolet, visible, and infrared, as long as it is within the transmission wavelength band (wavelengths of approximately 110 nm to 7500 nm). The protective film 106 is transparent to such light.

The diffraction grating of this embodiment formed by the above process has a glass substrate 105 with a first surface and a second main surface opposite the first main surface, as shown in Figure 13. A reflective metal film 103, which is a reflective metal film, is formed on the first surface of the glass substrate 105 via an adhesive 104. The surface of the reflective metal film 103 opposite the adhesive 104 side has a plurality of irregularities arranged in a predetermined direction along the first main surface of the glass substrate 105, and these irregularities form sinusoidal grooves with convex portions and concave portions arranged repeatedly. The surface of the reflective metal film 103 on which the sinusoidal grooves are formed is covered with a protective film 106.

The diffraction grating of this embodiment can have a line count of 10,000 lines/mm or more. That is, the groove width (pitch) W is 100 nm or less. Furthermore, the distance between the apex and the bottom surface of the sinusoidal groove in the direction perpendicular to the first main surface of the glass substrate 105 (vertical direction, longitudinal direction), i.e., the depth D of the sinusoidal groove, is, for example, approximately 100 nm.

(Effects of the embodiment)
In this embodiment, after forming a photoresist film 101 with sinusoidal grooves as shown in FIG. 7, a reflective metal film 103 is formed by electroless plating as shown in FIG. 12. In this case, since the reflective metal film 103 can be formed along the sinusoidal surface of the groove, a diffraction grating with fine sinusoidal grooves having a groove count of 10,000 lines/mm or more and a groove width of 100 nm or less can be realized. Therefore, a diffraction grating with high diffraction efficiency can be realized, and the first and second room for improvement described above can be eliminated. Therefore, in this embodiment, the performance of the diffraction grating can be improved.

(Embodiment 2)
The second embodiment will be described below with reference to Figures 16 to 18. Figure 16 shows a manufacturing flow of the diffraction grating according to this embodiment.

First, as shown in FIG. 7, a groove is formed on the substrate 102 using a photoresist film 101 (step S21). Next, as shown in FIG. 8, a reflective metal film (electroless plated film) 103 is formed on the photoresist film 101 using electroless plating (step S22). Next, as shown in FIG. 17, a reflective metal film (electrolytic plated film) 204 is formed on the reflective metal film 103 using electrolytic plating (step S22). The subsequent steps are the same as steps S13 to S17 shown in FIG. 1. That is, next, adhesive 104 is applied to the reflective metal film 204 (step S24). Next, the reflective metal film 204 is bonded to the glass substrate 105 (step S25). Next, the substrate 102 is peeled off from the photoresist film 101 (step S26). Next, the photoresist film 101 remaining on the reflective metal film 103 is removed (step S27). 18, a protective film 106 is formed on the surface of the reflective metal film 103 on which the grooves are formed (step S28). With the above steps, the diffraction grating of this embodiment is almost completed.

This embodiment differs from the first embodiment in that, in step S22, a reflective metal film 103 is formed on the photoresist film 101 using electroless plating, and then, in step S23, a reflective metal film 204 is formed on the reflective metal film 103. The material for the reflective metal film 204 is, for example, gold (Au). By forming the reflective metal film 204 using electrolytic plating, it is possible to form a thicker metal film than in the first embodiment.

By depositing a thick metal film, for example in a soft X-ray diffraction grating, the amount of soft X-rays that pass through the metal film can be reduced, thereby improving optical performance such as diffraction efficiency and stray light.

The invention made by the inventors has been specifically described above based on its embodiments, but the present invention is not limited to the above embodiments and can be modified in various ways without departing from the spirit of the invention.

The present invention can be widely used in diffraction grating manufacturing methods and diffraction gratings.

101 Photoresist film 102 Substrate 103, 204, 501 Reflective metal film 104 Adhesive 105 Glass substrate 106 Protective film 107 Hydrophilic surface 201 Blazed diffraction grating 202 Sinusoidal diffraction grating 203 Laminar diffraction grating

Claims (4)

(a) providing a substrate having a first surface and a second surface opposite the first surface;
(b) forming a photoresist film on the first surface of the substrate, the photoresist film having a sinusoidal first groove on a third surface opposite to the substrate side;
(c) forming a first metal film on the third surface of the photoresist film by an autocatalytic electroless plating method, the first metal film filling the sinusoidal first groove;
(d) after the step (c), removing the substrate and the photoresist film to expose a fourth surface of the first metal film;
(e) forming a protective film covering the fourth surface of the first metal film by a vapor deposition method;
and
The step (b) comprises:
(b1) forming the photoresist film on the first surface of the substrate, the photoresist film having a plurality of rectangular or trapezoidal third grooves aligned on the third surface;
(b2) heating the photoresist film to deform the third grooves, thereby forming the first grooves in a sinusoidal shape on the third surface;
and
the first metal film includes copper or silver;
the protective film contains magnesium fluoride;
A method for manufacturing a diffraction grating, which includes forming a diffraction grating including the first metal film. 2. The method for manufacturing a diffraction grating according to claim 1,
a fourth surface of the first metal film in contact with the third surface of the photoresist film has a second groove having a sine wave shape;
The method for manufacturing a diffraction grating, wherein the groove width of the sinusoidal second grooves is 100 nm or less. (delete) 2. The method for manufacturing a diffraction grating according to claim 1,
(f) after the step (c), forming a second metal film on the first metal film by electroplating;
The method for manufacturing a diffraction grating further comprises: