Transmission type diffraction grating and method for manufacturing same

Abstract

The present invention provides a manufacturing method for a transmission type diffraction grating and a transmission type diffraction grating with high diffraction efficiency obtained by the manufacturing method, the manufacturing method making it possible to manufacture groove shapes having a uniform groove height and vertical side walls even when high-aspect-ratio shapes having different groove widths and different intervals between grooves according to locations are manufactured. A transmission type diffraction grating 100 has a plurality of grooves extending in one direction. The plurality of grooves are disposed parallel to each other, each groove has a substantially rectangular cross-sectional shape in a direction orthogonal to the extension direction, and the plurality of grooves are disposed with different widths and / or at different intervals according to locations where the grooves are provided.

Description

Transmission diffraction grating and its manufacturing method

[0001] The present invention relates to a transmission type diffraction grating and a method for manufacturing the same.

[0002] Diffraction gratings that refract light are known as optical elements. Furthermore, in recent years, it has become known to fabricate a surface structure (subwavelength structured surface: SWS) with a period shorter than the wavelength of light, as described in Non-Patent Document 1, for example, and this technology can be used to realize optical elements with new functions. When the period of a structure is shorter than the wavelength of light, it becomes equivalent to a medium with a certain refractive index for light. This allows the refractive index to be controlled by the shape of the microstructure, making it possible to realize any refractive index distribution.

[0003] Furthermore, by giving the structure directionality, an optical anisotropy called structural birefringence occurs, making it possible to freely control the strength and direction of the birefringence. Furthermore, by adjusting the structural period, it is possible to control the wavelength dispersion characteristics, and it is also possible to generate strong reflection or diffraction at specific wavelengths or angles of incidence by utilizing the optical resonance phenomenon within the structure.

[0004] By changing the occupancy rate of this SWS, an arbitrary refractive index distribution can be obtained, and a highly efficient diffraction grating can be realized.

[0005] Semiconductor manufacturing technologies such as electron lithography and plasma etching are used to fabricate SWSs. Interference exposure methods using ultraviolet lasers are also used to create simple patterns such as anti-reflection structures. Nanoimprinting technology has also been developed to transfer SWSs from a mold.

[0006] Furthermore, International Publication No. 2008 / 114466 (Patent Document 1) discloses a technique for forming a rectangular pillar shape on a silicon substrate by patterning a resist on the silicon substrate and then etching the silicon substrate.

[0007] WO 2008 / 114466

[0008] "Introduction to Diffractive Optical Elements," supervised by the Optical Design Research Group of the Japan Society of Applied Physics, published by Optronics Co., Ltd. on May 20, 1997

[0009] SWS diffraction gratings are expected to have high diffraction efficiency. However, to achieve high diffraction efficiency in SWS diffraction gratings, a manufacturing technique is required to form rectangular cross-sectional grooves of uniform height, with widths and spacings that vary from one location to another.

[0010] For example, in the case of a transmission type SWS diffraction grating used in the near-infrared to infrared band, it is necessary to form rectangular grooves on a silicon substrate with a minimum dimension of 70 nm or less, a groove height of 800 nm or more, and an aspect ratio of 10 or more.

[0011] The conventional semiconductor manufacturing technology, interference exposure using ultraviolet lasers, lacks the processing precision required to create groove shapes of the minimum dimensions mentioned above. Furthermore, while electron beam lithography, which can create fine patterns, can achieve patterning of the above dimensions, plasma etching of silicon substrates results in variations in groove height due to differences in SWS width, making it impossible to provide a diffraction grating with high diffraction efficiency.

[0012] In International Publication No. 2008 / 114466 (Patent Document 1), a silicon substrate is subjected to resist patterning, followed by etching to form rectangular pillars on the silicon substrate. However, since no technology is disclosed for forming grooves with different groove widths and groove spacing depending on the location, it is difficult to provide a transmission-type SWS diffraction grating with high diffraction efficiency.

[0013] The present invention aims to solve the problems of the conventional techniques described above and to provide a transmission diffraction grating manufacturing technique that enables the production of groove shapes with uniform groove heights and vertical sidewalls, even when producing shapes with high aspect ratios in which the groove width and groove spacing vary from place to place, and a transmission diffraction grating with high diffraction efficiency.

[0014] In order to solve the above problems, a transmission type SWS diffraction grating as an example of the present invention is a transmission type diffraction grating having a plurality of grooves extending in one direction, the plurality of grooves being arranged parallel to one another, the grooves having a substantially rectangular cross-sectional shape in a direction perpendicular to the extension direction, and the plurality of grooves being arranged at different widths and / or different intervals depending on where they are provided.

[0015] (f) after step (e), etching the silicon wafer to form a plurality of grooves in the silicon wafer; and (g) after step (f), removing the oxide film to obtain a diffraction grating. The plurality of grooves are arranged parallel to one another, each groove has a substantially rectangular cross section perpendicular to its extension direction, and the plurality of grooves are arranged at different widths and / or different intervals for each location where they are provided.

[0016] The transmission diffraction grating of the present invention has grooves with different widths and intervals at different locations, uniform heights, and vertical sidewalls, and can provide a diffraction grating with high diffraction efficiency.

[0017] Furthermore, according to the method for manufacturing a transmission diffraction grating of the present invention, even when groove shapes having different widths or intervals at different locations are formed, groove shapes having vertical side walls with uniform heights can be manufactured, and a diffraction grating with high diffraction efficiency can be easily manufactured.

[0018] Fig. 1 is a diagram showing a method for manufacturing a diffraction grating according to a first embodiment of the present invention; Fig. 2 is a schematic diagram of a (110) silicon wafer; Fig. 3 is a diagram showing a method for adjusting the crystal direction of a (110) silicon wafer and the direction in which grooves of a diffraction grating are formed; Fig. 4 is a diagram showing a cross-sectional shape of a diffraction grating according to an embodiment of the present invention; Fig. 5 is a diagram showing a method for manufacturing a diffraction grating according to a second embodiment of the present invention;

[0019] The following describes in detail the embodiments of the present invention with reference to the drawings. In the following embodiments, it goes without saying that the components (including element steps, etc.) are not necessarily essential unless otherwise specified or considered to be obviously essential in principle.

[0020] First Embodiment A diffraction grating according to a first embodiment of the present invention and a method for manufacturing the same will be described below with reference to FIGS.

[0021] <Method for Manufacturing Diffraction Grating> The method for manufacturing a diffraction grating according to the first embodiment involves sequentially performing the operations of steps S11 to S17 shown below (FIG. 1). FIG. 1 shows the flow of each step in this manufacturing method and a schematic cross-sectional view at that time. By going through these steps, the diffraction grating according to the first embodiment can be manufactured.

[0022] S11: (110) The direction in which the silicon wafer 101 and the diffraction grating grooves are formed is adjusted. S12: (110) An oxide film 102 is formed on the entire surface of the silicon wafer 101. S13: A photoresist is applied to one side of the formed oxide film 102, and photolithography is used to pattern the photoresist 103 into the desired shape (a shape for forming the groove shape). S14: The oxide film 102 is removed from the exposed portion where the photoresist 103 is not formed, to form an oxide film 102a. S15: The photoresist 103 remaining on the oxide film 102a is removed. S16: (110) The silicon wafer 101 is etched to form a groove structure made of silicon, to form the diffraction grating 100. In this specification, the term "groove" refers to a portion formed as a convex portion.

[0023] S17: The oxide film 102a remaining on the top of the formed trench and the oxide film 102 on the other surface are removed.

[0024] <<About the (110) Silicon Wafer>> In manufacturing the above-mentioned diffraction grating, first, a silicon wafer for manufacturing the diffraction grating is prepared. The (110) silicon wafer used here will be explained using FIG. 2. First, the position of the (111) plane in the (110) silicon wafer 101, which is the material for manufacturing the diffraction grating, is as shown in FIG. 2. In FIG. 2, the arrow indicates the normal direction of the plane.

[0025] 2, it can be seen that in the (110) silicon wafer 101, the (111) plane has planes that are perpendicular to the (110) plane and planes that are oblique to it. Furthermore, the perpendicular (111) planes form an angle of 109.5° with each other. This shows the crystal planes that are formed during the growth of silicon crystals, and is unique to single-crystal silicon.

[0026] In the first embodiment, a rectangular shape with a high aspect ratio is formed by utilizing the difference in etching rate due to crystal anisotropy and the vertical (111) plane of silicon described above. That is, the characteristic that the (111) plane is less easily etched (has a slower etching rate) than other planes such as the (110) plane of silicon is utilized to form a desired groove shape with high precision.

[0027] Therefore, the silicon wafer used in this embodiment 1 is a wafer sliced ​​from an ingot so that the (110) plane is the surface (as shown in FIG. 2, silicon wafer 101 has the (110) plane as the surface shown as the horizontal plane).

[0028] <<Regarding Adjustment of the Direction in Which Grooves are Formed>> Then, the position of the prepared silicon wafer is adjusted so that the crystal planes are arranged in a predetermined position (S11). Specifically, with reference to FIG. 3 , a method for adjusting the (111) plane of the (110) silicon wafer 101 and the direction in which the grooves of the diffraction grating are formed in step S11 will be described.

[0029] In the first embodiment, a rectangular shape with vertical sidewalls is formed on a silicon substrate by utilizing the difference in etching rate between the (110) and (111) planes. Therefore, as shown in FIG. 3 , one of the (111) planes perpendicular to the (110) plane is aligned with the longitudinal direction of the diffraction grating. FIG. 3 shows the positional relationship between the silicon wafer 101 and a photomask 301 placed above the (110) plane. At this time, the photomask 301 has an opening formed according to the shape of the groove to be formed. This opening corresponds to the shape of the groove extending in one direction, and is formed as a rectangular opening corresponding to the extension direction (longitudinal direction) of the groove and the width direction (lateral direction) of the groove, which is perpendicular to the extension direction.

[0030] 3 shows a method for adjusting the longitudinal positions of the (111) plane of the (110) silicon wafer 101 and the grooves of the diffraction grating by relatively rotating a photomask 301 used in photolithography. That is, the (110) silicon wafer 101 is placed and fixed in a lithography apparatus so that the (111) plane is vertical, and then the photomask 301 placed above the (110) silicon wafer 101 is rotated relative to the opening shape of the photomask 301, so that the (111) plane and the longitudinal direction of the grooves of the diffraction grating are aligned.

[0031] In step S11, any method can be used to align the (111) plane with the longitudinal direction of the diffraction grating grooves. The same effect can also be achieved by fixing the photomask 301 and rotating the (110) silicon wafer relatively.

[0032] <<Regarding Oxide Film Formation>> Next, step S12 will be described. After the above-described position adjustment is completed, as shown in step S12 of FIG. 1 , an oxide film 102 is formed on the entire surface of the silicon wafer 101 to serve as a mask (protective film) during crystalline anisotropic etching. The oxide film 102 is formed by a known oxide film formation method for this type of process, and is not particularly limited. Examples of oxide film formation methods that can be used include thermal oxidation, vacuum deposition, and CVD (Chemical Vapor Deposition).

[0033] <<Regarding Patterning on Resist>> Next, step S13 will be described. First, photoresist (photosensitive organic material) 103 is applied to one side of a silicon wafer 101 on which an oxide film 102 has been formed. Next, the photoresist applied to the silicon wafer 101 is exposed to a mask pattern drawn on a photomask, and developed, thereby transferring the mask pattern to the photoresist. The transferred mask pattern obtained here has a shape (planar view shape) corresponding to the groove shape of the diffraction grating.

[0034] This photoresist 103 is broadly divided into negative and positive types based on how it reacts with light or electron beams. When a negative type is exposed to light, its solubility in a developer decreases, and the exposed portions remain after development. In contrast, when a positive type is exposed to light, its solubility in a developer increases, and the exposed portions are removed during development. The photoresist 103 used in this step S13 may be either negative or positive.

[0035] The wavelength of the exposure device used to expose the mask pattern includes (KrF excimer laser) and 193 nm (ArF excimer laser), but in this embodiment, an exposure device with any wavelength may be used. Also, although photolithography is used as a method for patterning the resist in step S13, a similar effect can be obtained by using an EB (electron beam) drawing device.

[0036] <<Removal of Oxide Film from Silicon Surface>> Next, step S14 will be described. In step S14, a portion of the oxide film 102 is removed so as to leave only the oxide film 102 underlying the patterned photoresist 103. By removing this oxide film, an oxide film 102a corresponding to the patterned shape remains on the silicon wafer 101. The oxide film 102 can be removed by any oxide film removal method known in this type of process, and is not particularly limited. For example, the oxide film 102 can be removed by dry etching using a plasma etching apparatus to expose the silicon surface. Alternatively, the oxide film 102 may be removed by wet etching using hydrofluoric acid.

[0037] <<Removal of Photoresist>> Next, step S15 will be described. In step S15, the photoresist 103 patterned in step S13 is removed. The photoresist 103 can be removed by any known method for removing the photoresist 103 in this type of process, and is not particularly limited. Examples of the method for removing the photoresist 103 include a method using a plasma ashing device or an organic solvent.

[0038] <<Formation of Vertical Sidewalls on Silicon Surfaces>> Next, step S16 will be described. As shown in Fig. 1, in step S16, a (110) silicon wafer 101 is subjected to crystal anisotropic etching to form the diffraction grating 100. The crystal anisotropic etching of the silicon wafer 101 can be performed using an alkaline aqueous solution such as KOH (potassium hydroxide) or TMAH (tetramethylammonium hydroxide) as an etching solution.

[0039] During etching with an alkaline aqueous solution, etching proceeds on the band-shaped (exposed) silicon surface free of oxide film, forming multiple convex grooves with vertical side surfaces on the silicon substrate. The convex portions thus formed have a substantially rectangular cross-sectional shape in the direction of extension of the groove and in a direction perpendicular to the direction of extension. That is, multiple grooves protruding in the shape of a rectangular parallelepiped are formed on one surface of the silicon substrate. At this time, since the positioning is adjusted in the above-mentioned step S11 so that the tops of the rectangular shapes are (110) planes and the side surfaces are (111) planes, the groove shapes can be efficiently formed in the desired shape.

[0040] By performing predetermined position adjustment and applying crystalline anisotropic etching as in this embodiment, it is possible to manufacture a diffraction grating (SWS structure) with a high aspect ratio and small variations in groove height even when etching shapes with different groove widths or groove spacings for each location to be formed. The diffraction grating obtained in this manner has high diffraction efficiency.

[0041] <<Removal of Oxide Film on Top of the Trench>> Next, step S17 will be described. In step S17, the oxide film 102a on the top of the trench used as an etching mask is removed. A known oxide film removal method for this type of process is used to remove the oxide film 102a, and there is no particular limitation. For example, etching using hydrogen fluoride or dry etching using a plasma etching device can be used to remove the oxide film 102a.

[0042] <Diffraction Grating> Next, a diffraction grating according to the first embodiment will be described. This diffraction grating is manufactured by the diffraction grating manufacturing method described above. In this case, as described above, even when etching shapes in which the groove width or the spacing between grooves varies depending on the formation location, there is little variation in groove height, and the diffraction grating has a characteristic structure obtained by utilizing the ability to manufacture a high aspect ratio diffraction grating (SWS structure).

[0043] That is, the diffraction grating according to the first embodiment is a transmission type diffraction grating having a plurality of grooves extending in one direction, the plurality of grooves being arranged parallel to one another, the grooves having a substantially rectangular cross section in a direction perpendicular to the direction of extension, and the plurality of grooves being arranged with different widths and / or different intervals depending on where they are provided. Note that the diffraction grating may have a structure in which some of the grooves are arranged with the same width or at the same intervals.

[0044] In the diffraction grating according to the first embodiment, even when the grooves have different widths and / or different spacings at different locations, the cross-sectional shape of the grooves can be stably formed to be approximately rectangular. Furthermore, the groove height can be made uniform, resulting in a diffraction grating with high efficiency. Note that a transmission type diffraction grating can be formed by forming a surface structure (SWS) such that each of the grooves has a period shorter than the wavelength of light to be used.

[0045] Fig. 4 shows a schematic diagram of the cross-sectional shape of an example diffraction grating according to the first embodiment. The diffraction grating 100 shown in Fig. 4 has a plurality of grooves 100a extending in one direction (the extension direction is the depth direction in Fig. 4), and these grooves are arranged parallel to one another. The cross-sectional shape of these grooves 100a in a direction perpendicular to the extension direction (the vertical cross-sectional shape of the diffraction grating) is approximately rectangular.

[0046] Furthermore, these grooves 100a show an example in which a structure with a period g smaller than the wavelength is repeatedly provided at a period G larger than the wavelength while changing the groove width and groove spacing from place to place. Here, within one period G, grooves of different widths are arranged so that the widths increase in order from left to right in FIG. 4 .

[0047] Furthermore, the diffraction gratings used in analytical instruments are mainly those that maximize the diffraction efficiency of a specific wavelength of first-order diffracted light. In order to maximize the diffraction efficiency of a specific wavelength of first-order diffracted light with an SWS diffraction grating, it is necessary to gradually change the width of the grooves 100a from a narrow width to a wider width, as shown in Figure 4.

[0048] For example, in a transmission type SWS diffraction grating for the near-infrared to infrared band, the width of the groove 100a at its narrowest point is 70 nm or less, and the width at its widest point is more than three times the narrowest point. Also, the height of the groove 100a needs to be 800 nm or more.

[0049] That is, it is preferable that the minimum processing dimension (minimum width) MDmin of the width of the cross-sectional shape of the groove 100a in FIG. 4 is 70 nm or less, and the height H of the groove 100a is 800 nm or more. Here, by making the maximum processing dimension (maximum width) MDmax of the width of the groove 100a three times or more of MDmin, an SWS diffraction grating for the near-infrared to infrared band can be obtained. In this case, it is also preferable to have grooves with a substantially rectangular cross-sectional shape in which the ratio of the long side to the short side of the rectangle (aspect ratio) is 10 or more, because this can further improve efficiency.

[0050] The diffraction grating according to the present embodiment is not limited to the above description, and when designing the shape of this diffraction grating, an optimal solution may be found for the groove width, groove spacing, and groove height of the SWS diffraction grating using a calculation technique such as RCWA (Rigorous Coupled-Wave Analysis) method, so as to obtain high diffraction efficiency at a desired wavelength.

[0051] [Embodiment 2] Next, a method for manufacturing a diffraction grating according to embodiment 2 of the present invention will be described. The method for manufacturing a diffraction grating according to embodiment 2 generally involves sequentially performing the operations of steps S21 to S27 shown below (FIG. 5). FIG. 5 shows the flow of each step in this manufacturing method. By going through these steps, the diffraction grating according to embodiment 2 can be manufactured.

[0052] S21: (110) The direction in which the silicon wafer 101 and the diffraction grating grooves are formed is adjusted. S22: (110) A nitride film 501 is formed on the entire surface of the silicon wafer 101. S23: Photoresist 103 is applied to one side of the formed nitride film 501, and photolithography is used to pattern the photoresist 103 into the desired shape (a shape for forming the groove shape). S24: The exposed portions of the nitride film 501 on which the photoresist 103 is not formed are removed to form nitride film 501a. S25: The photoresist 103 remaining on the nitride film 501a is removed. S26: (110) The silicon wafer 101 is etched to form a groove structure made of silicon, resulting in the diffraction grating 100. S27: The nitride film 501a remaining on the top of the formed grooves and the nitride film 501 on the other side are removed.

[0053] The method for manufacturing a diffraction grating according to the second embodiment will be described below. Note that in the second embodiment, steps S21, S23, and S25 to S26 are the same as the corresponding steps S11, S13, and S15 to S16 in the first embodiment. Therefore, the following description of the method for manufacturing a diffraction grating will be omitted, and the differences will be mainly described.

[0054] <Regarding the Deposition of Nitride Film> Step S22 will be described. The second embodiment differs from the first embodiment in that a nitride film 501 (silicon nitride: SiN) is deposited. Silicon nitride films have a high selectivity with silicon and are not etched by alkaline solutions such as KOH and TMAH. Therefore, compared to methods using oxide films, even if a silicon nitride film is deposited very thinly, it is possible to form deep trenches.

[0055] The nitride film 501 can be formed by a known method without any particular limitation, and the nitride film 501 can be formed by, for example, a CVD method.

[0056] By forming a thin nitride film 501, etching of the nitride film 501 in the next step, step S24, can be completed in a short time. That is, since the photoresist 103 in the preceding step can also be formed thinly, the precision of the formed resist pattern can be improved.

[0057] <Description of Nitride Film Removal> Next, step S27 will be described. In step S27, the nitride film 501a on the upper part of the trench is removed. To remove the nitride film 501a, a nitride film removal method known in this type of process is used, and there is no particular limitation. The nitride film 501a can be removed by etching using hot phosphoric acid heated to 150°C or dry etching using a plasma etching device.

[0058] As described above, the second embodiment also has a characteristic structure that utilizes the fact that, even when etching shapes in which the groove width or the spacing between grooves varies depending on the formation location, the variation in groove height is small and a diffraction grating (SWS structure) with a high aspect ratio can be manufactured. Furthermore, the transmission type diffraction grating obtained in the second embodiment has the same configuration as the transmission type diffraction grating obtained in the first embodiment, and therefore a description thereof will be omitted.

[0059] It should be noted that the present invention is not limited to the above-described embodiments and includes various modifications. The above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. The configuration of one embodiment can be added to, deleted from, or substituted for the configuration of another embodiment.

[0060] 100 Diffraction grating 100a Groove 101 (110) Silicon wafer 102 Oxide film 102a Oxide film 103 Photoresist 301 Photomask 501 Nitride film 501a Nitride film

Claims

1. A transmission type diffraction grating having a plurality of grooves extending in one direction, the plurality of grooves being arranged parallel to one another, the grooves having a substantially rectangular cross-sectional shape in a direction perpendicular to the extension direction, and the plurality of grooves being arranged with different widths and / or different intervals for each location where they are provided.

2. A transmission type diffraction grating according to claim 1, wherein the grooves are formed from silicon, and the tops of the grooves are (110) surfaces and the side surfaces of the grooves are (111) surfaces.

3. A transmission type diffraction grating according to claim 1, wherein the plurality of grooves include grooves having a width of 70 nm or less and a height of 800 nm or more.

4. A transmission type diffraction grating according to claim 3, wherein the maximum processing dimension of the width of said grooves in said plurality of grooves is three times or more the minimum processing dimension of the width of said grooves.

5. A transmission type diffraction grating according to claim 3, wherein the plurality of grooves include grooves having a ratio (aspect ratio) of a long side to a short side of the substantially rectangular cross-sectional shape of 10 or more.

6. A method for manufacturing a transmission grating having a plurality of grooves extending in one direction, comprising: (a) a step of aligning a direction in which the grooves are formed with respect to a crystal plane of a silicon wafer; (b) after step (a), a step of forming a protective film on a surface of the silicon wafer; (c) after step (b), a step of forming a photoresist film on one of the surfaces on which the protective film is formed, and forming a resist pattern of a desired shape by photolithography; (d) after step (c), a step of removing the protective film exposed from the resist pattern; (e) after step (d), a step of removing the resist pattern; (f) after step (e), a step of etching the silicon wafer to form a plurality of grooves in the silicon wafer; and (g) after step (f), a step of removing the oxide film to obtain a diffraction grating, wherein the plurality of grooves are arranged parallel to one another, and each of the grooves has a substantially rectangular cross-sectional shape in a direction perpendicular to the extension direction, and the plurality of grooves are arranged at different widths and / or different intervals for each location where they are provided.

7. A method for producing a transmission type diffraction grating according to claim 6, wherein in step (a), a photomask for forming the grooves is rotated with respect to a crystal plane of the silicon wafer.

8. The method for producing a transmission type diffraction grating according to claim 6, wherein the protective film is an oxide film or a nitride film.

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