Optical element and method for producing same

Abstract

This method for manufacturing an optical element includes a resist layer formation step for forming a resist layer on one surface of a substrate (20) comprising an optically isotropic body, an exposure step for exposing the resist layer, and a resist pattern formation step for developing the exposed resist layer and forming a first resist pattern having a periodic lattice structure in a region functioning as an optical element. In the resist pattern formation step, a second resist pattern having a periodic lattice structure is formed also in a region of the substrate (20) not functioning as an optical element.

Description

Optical element and manufacturing method thereof

[0001] The present disclosure relates to an optical element that, when irradiated with light of a specific wavelength, reflects, diffuses, polarizes, diffracts, or otherwise affects incident light, and a method for manufacturing the same.

[0002] Light travels due to the regular temporal and spatial oscillations of electric and magnetic fields, and the polarization state that characterizes the light is determined by the state of these oscillations. Generally, the light we see in our daily lives, such as from fluorescent lamps, is unpolarized, vibrating randomly. However, in the fields of optics and engineering, light is often used in linearly polarized, circularly polarized, elliptically polarized, or other states by adjusting the state of electric and magnetic field oscillations depending on the purpose or application. One optical element used to obtain arbitrary polarization is a wave plate.

[0003] Traditionally, wave plates have been made from birefringent materials, such as quartz, which have a difference in refractive index between two orthogonal axes. When light passes through a birefringent material, a phase difference occurs between the polarized component along the fast axis (i.e., the fast axis) and the polarized component along the slow axis (i.e., the slow axis), converting the polarization state of the light. On the other hand, optical materials that do not exhibit birefringence can be engineered to function as wave plates by using microfabrication technology to create a periodic lattice structure with irregularities on the surface of the material, thereby artificially creating a refractive index difference in orthogonal directions. While the latter wave plates are more difficult to manufacture, they have the advantage of being able to obtain any polarization state by adjusting the shape of the lattice structure.

[0004] In the latter wave plate, photolithography processes, dry etching processes, etc. are generally used to obtain a fine grating structure that generates a phase difference. For example, to use it as a wave plate, a highly accurate and uniform periodic grating structure must be formed in an area larger than the size of the incident light passing through the wave plate. In this case, structures designed using theoretical calculations, optical simulators, etc. are assumed to have a completely uniform periodic grating structure within the plane. However, in reality, variations in quirks or shape depending on the manufacturing method, process conditions, etc. often occur, resulting in characteristics that differ from the design values. To prevent this, the manufacturing method is appropriately selected depending on the substrate material, the size of the grating structure, etc., and the process conditions are further optimized. This reduces the quirks or shape variations due to the manufacturing method, making it possible to form a periodic structure with high in-plane uniformity.

[0005] In Patent Document 1, in the photolithography process, one shot is defined as the area that the exposure system can irradiate with light in one go, and a resist pattern is formed by performing multiple exposure and development processes centered on the central part of the substrate so that the area of ​​each shot fits entirely on the substrate, and a concave-convex pattern is then formed on the substrate by a subsequent process such as a dry etching process.

[0006] JP 2015-99898 A

[0007] In the technology described in Patent Document 1, no pattern is formed in the exposure and development processes near the edges of the substrate where the shot area does not fit, leaving the resist layer intact. To manufacture optical elements such as wave plates with highly accurate and uniform concave-convex patterns, a resist pattern having a periodic grating structure formed by a photolithography process is used as an etching mask. Depending on the dry etching conditions in the subsequent dry etching process, the resist may peel off from areas of the resist layer where the pattern is not formed and scatter as debris during the dry etching process. If this debris reaches the area with the periodic grating structure pattern during the dry etching process, the debris acts as an etching mask that interferes with processing, resulting in a problem of partially preventing the desired processing depth. Particularly in optical elements that require highly accurate and uniform concave-convex patterns over a large area to achieve the designed optical characteristics, suppressing in-plane shape variations due to processing interference is required.

[0008] The present disclosure has been made in consideration of the above, and aims to provide a method for manufacturing an optical element that can form a more highly accurate and uniform uneven pattern using dry etching than conventional methods.

[0009] In order to solve the above-mentioned problems and achieve the object, the method for manufacturing an optical element according to the present disclosure includes a resist layer formation step of forming a resist layer on one surface of a substrate made of an optically isotropic body, an exposure step of exposing the resist layer to light, and a resist pattern formation step of developing the exposed resist layer to form a first resist pattern having a periodic grating structure in an area that functions as an optical element. In the resist pattern formation step, a second resist pattern having a periodic grating structure is also formed in an area of ​​the substrate that does not function as an optical element.

[0010] According to the present disclosure, it is possible to achieve an effect of forming a concave-convex pattern that is more accurate and uniform than conventional methods using dry etching.

[0011] 10 and 11 are used to etch a substrate.

[0012] Optical elements and manufacturing methods thereof according to embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. The embodiments will be described below using a transmission wave plate, which is one type of optical element, as an example.

[0013] 1 is a cross-sectional view showing a schematic example of the configuration of a transmission wave plate. The wave plate 100 includes a flat substrate 20, a grating structure 10 provided on one surface 20a of the substrate 20, and an anti-reflection layer 30 provided on the other surface 20b of the substrate 20 opposite the surface 20a on which the grating structure 10 is provided.

[0014] The substrate 20 is a flat plate-shaped member made of an optically isotropic material. An optically isotropic material is a material in which light waves propagate in all directions at the same speed, and is an amorphous material or a cubic material. In one example, a cubic compound semiconductor material can be used as the substrate 20. Here, the substrate 20 is made of zinc sulfide (ZnS) as an example. The substrate 20 may be a single crystal substrate or a polycrystalline substrate. The substrate 20 supports the grating structure 10 and the anti-reflection layer 30.

[0015] The grating structure 10 is a periodic structure provided on one surface 20a of the substrate 20. In this example, the grating structure 10 has grating protrusions 11 that protrude from one surface 20a of the substrate 20, and grating recesses 12 that are spaces between adjacent grating protrusions 11. In this example, the grating protrusions 11 extend in a direction perpendicular to the paper surface. In other words, the grating structure 10 formed on one outermost surface of the wave plate 100 has a concavo-convex structure in which the grating protrusions 11 and the grating recesses 12 are arranged at a one-dimensional pitch P. The pitch P is the average period across multiple grating protrusions 11.

[0016] The grating protrusions 11 are formed of the same material as the substrate 20, in this example, ZnS, a cubic compound semiconductor material. In one example, the grating structure 10 is formed by performing a photolithography process and a dry etching process on the substrate 20 to remove ZnS from the surface of the substrate 20. The width of the grating protrusions 11 in a cross section perpendicular to the extension direction is designated W, and the height from one surface 20a of the substrate 20 is designated D. The cross-sectional shape perpendicular to the extension direction of the grating protrusions 11 is not limited to a rectangular shape in which the sidewalls of the grating protrusions 11 are perpendicular to the flat portion of one surface 20a of the substrate 20, but may also be a trapezoidal or triangular shape in which the sidewalls of the grating protrusions 11 are inclined. The width of the grating recesses 12 is designated P-W, and the depth is designated D.

[0017] The antireflection layer 30 is composed of a combination of a low refractive index material and a high refractive index material. In one example, the antireflection layer 30 is a laminated film in which low refractive index materials and high refractive index materials are alternately stacked on the substrate 20. The antireflection layer 30 is provided on the other surface 20b of the substrate 20 on which the grating structure 10 is not formed. The antireflection layer 30 suppresses reflection loss when light passing through the substrate 20 is emitted from the other surface 20b into the air, or when light is incident from the air onto the substrate 20 via the other surface 20b. The antireflection layer 30 does not necessarily have to be provided.

[0018] Next, a method for manufacturing an optical element having such a structure will be described. Figures 2 to 7 are cross-sectional views schematically showing an example of the steps of the method for manufacturing an optical element according to the first embodiment. First, as shown in Figure 2, a flat substrate 20 on which the wave plate 100 is to be formed is prepared. The substrate 20 is a ZnS substrate. The ZnS substrate can be manufactured by, for example, chemical vapor deposition or sintering.

[0019] Next, as shown in FIG. 3 , a resist layer formation process is performed in which photoresist is applied to the entire surface of one surface 20 a of the substrate 20 to form a resist layer 40. Methods for applying photoresist to the substrate 20 include spin coating. After applying the photoresist to the substrate 20, the substrate 20 is pre-baked on a hot plate or in a thermostatic chamber to remove the solvent in the photoresist, thereby forming the resist layer 40. Note that, before applying the photoresist to the substrate 20, the surface of the substrate 20 may be subjected to a surface treatment to improve adhesion between the surface of the substrate 20 and the resist layer 40. Examples of surface treatments include plasma treatment or the addition of an adhesion layer. An example of an adhesion layer is a layer formed of hexamethyldisilazane.

[0020] In the following steps, a photolithography process is performed on the resist layer 40 to form a fine pattern in the resist layer 40 .

[0021] As shown in FIG. 4 , an exposure process is performed to expose the resist layer 40. In this example, the resist layer 40 formed on the surface of the substrate 20 is exposed using a stepper device equipped with a reticle 50. For example, ultraviolet light 60 is used for the exposure. One surface of the reticle 50 is formed with light-shielding portions 51 that do not transmit the ultraviolet light 60 and light-transmitting portions 52 that transmit the ultraviolet light 60. In other words, the reticle 50 has a pattern. This pattern corresponds to a grating pattern, which is the pattern of the grating structure 10 to be formed on the substrate 20. For example, the light-shielding portions 51 are formed of a chromium film. By performing exposure through the reticle 50, it is possible to selectively irradiate the resist layer 40 with ultraviolet light 60 in accordance with the pattern formed by the light-shielding portions 51 and the light-transmitting portions 52.

[0022] In the exposure process using a stepper device, by using a reticle 50 engraved with several different patterns, it is possible to arbitrarily combine the types and arrangements of patterns to be exposed onto the resist layer 40 formed on the substrate 20. This allows for a resist pattern area of ​​a desired shape and size to be obtained. In particular, for optical elements such as a wave plate 100, the same grid pattern can be formed over a large area by performing multiple exposures, with one shot being the area that can be exposed at one time by the stepper device, and arranging the same multiple patterns formed in one shot seamlessly in a checkerboard pattern.

[0023] Next, as shown in FIG. 5 , a resist pattern formation process is performed in which the exposed resist layer 40 is developed to form a resist pattern 70 with a periodic grating structure in the region where the wave plate 100 will be formed. During the development process, only the portions of the resist layer 40 that were irradiated with light during the exposure process are removed. Examples of development processes include a dipping process in which the resist layer 40 applied to the substrate 20 is immersed in a developer, or a spraying process in which the developer is sprayed onto the resist layer 40. The development process forms an etching mask for forming the grating projections 11. In other words, the resist layer 40 remaining after the development process serves as the etching mask. Note that FIG. 5 illustrates an example in which a positive-type photoresist is used, in which the irradiated portions are dissolved by the developer. However, depending on the pattern to be formed, a negative-type photoresist, in which the unirradiated portions are dissolved by the developer, may also be used. After the development process, a post-baking process is performed using a hot plate or a thermostatic bath. This post-baking process removes residual developer, moisture, and the like, and improves the etching resistance of the resist layer 40 in the next process. In this way, a resist pattern 70 is formed in which the portions where the grating convex portions 11 are to be formed are masked. The resist pattern 70 corresponds to the first resist pattern.

[0024] 6, an etching step is carried out in which the exposed surface of the substrate 20 is processed by a dry etching process using the resist pattern 70 as an etching mask. As a result, grating recesses 12 are formed in the etched portions, and grating protrusions 11 are formed in the portions masked by the resist pattern 70.

[0025] One method for efficiently processing ZnS using a dry etching process is the reactive ion etching (RIE) process, which uses a mixed gas containing methane (CH4), as described in References 1 and 2 below. In the RIE process, the gas used is converted into plasma using externally applied high-frequency power, and the generated ions, radicals, etc. are used to remove the surface of the object to be processed. Due to its nature, this RIE process has both physical etching, which utilizes the collision energy of ions and radicals, and chemical etching, which utilizes chemical reactions. When processing ZnS using an RIE process using CH4 gas, as in this case, the ZnS surface is removed by generating volatile substances according to the reaction formulas expressed by the following formulas (1) and (2): (Reference 1) S. H. Su et al., Reactive Ion Etching of ZnS Films using a Gas Mixture of Methane / Hydrogen / Argon. Japan Journal of Applied Physics. 1998, 37, pp. 1764-1767. (Reference 2) J. Xing et al., Study of Inductively Coupled Plasma Etching of ZnS Materials. Advances in Engineering Research. 2018, 137, pp. 370-375.

[0026] Zn 2+ +2CH3 - →Zn(CH3)2...(1) S 2- +2H + → H2S ... (2)

[0027] In the first embodiment, a mixed gas of CH4 and argon (Ar) is used as the gas species for processing ZnS. The purpose of introducing Ar is to stabilize the generated plasma and to improve the processing rate of ZnS by adding physical etching with Ar ions. While References 1 and 2 describe processing ZnS using a mixed gas of three types of gases—CH4, hydrogen (H2), and Ar—as described above, the first embodiment uses only two types of gases, CH4 and Ar, without including H2. This eliminates the need for H2 gas, which is a flammable gas, and eliminates the need for a H2 gas cylinder and equipment for storing the gas cylinder. This has the advantage of reducing manufacturing costs compared to the use of three types of gases as in References 1 and 2.

[0028] Examples of plasma generation methods include capacitively coupled plasma (CCP), inductively coupled plasma (ICP), electron cyclotron resonance (ECR), etc. From the viewpoint of shortening the processing time and improving the controllability of the processed shape, it is desirable to use the ICP or ECR method, which can obtain high-density plasma and independently control the ion flux and energy.

[0029] RIE process conditions include the flow rate of each gas used, antenna power, bias power, chamber temperature, process pressure, stage temperature, and processing time. When the resist pattern 70 shown in FIG. 5 is used as an etching mask, it is necessary to ensure that the resist pattern 70, which protects the non-processed region that will ultimately become the grating top, does not disappear until the target is etched to the desired depth. Therefore, the above process conditions are appropriately set while taking into account their correlation so that conditions are obtained that efficiently etch the ZnS target and minimize damage to the resist pattern 70. One example of a method for minimizing damage to the resist pattern 70 is to deposit polymers caused by CH4 gas on the surface of the resist pattern 70 during the etching process. However, since the polymer deposits not only on the resist surface but also on the ZnS surface, the processing rate of the ZnS may decrease, so care must be taken when adjusting the amount of polymer deposition.

[0030] The depth and aspect ratio of the grating formed in the grating structure 10 of the ZnS wave plate 100 are, for example, 8.1 μm and 1.98, respectively. Here, the aspect ratio of the grating indicates the depth / pitch ratio, which is the ratio of the depth to the pitch. By forming a grating shape within the range presented here with high precision and uniformity on the ZnS substrate 20, for example, practical polarization characteristics can be obtained for light in the mid-infrared region.

[0031] 7, a resist layer removal process is performed to remove the resist pattern 70 used as an etching mask. This results in the formation of a grating structure 10 consisting of grating protrusions 11 made only of ZnS and grating recesses 12 formed between adjacent grating protrusions 11. Methods for removing the resist pattern 70 include plasma ashing using primarily oxygen gas, or immersion in a solution. Note that ZnS reacts with strong acids to produce toxic hydrogen sulfide, so it is desirable to select a solution other than a strong acid when performing the immersion process.

[0032] Finally, an anti-reflection layer 30 is formed on the other surface 20b of the substrate 20 on which the grating structure 10 is not formed. The anti-reflection layer 30 suppresses reflection loss of light incident from the air. Methods for forming the anti-reflection layer 30 include vacuum deposition and sputtering. Then, an area that will become one shot in the exposure process is cut out from the substrate 20. Note that if the wave plate 100 is to be used as is without being cut into individual pieces, the cutting out from the substrate 20 is omitted. In this manner, the wave plate 100 shown in FIG. 1 is manufactured.

[0033] FIG. 8 is a traced scanning electron microscope image of the cross section of an optical element manufactured by the optical element manufacturing method according to the first embodiment. Hereinafter, the scanning electron microscope image will be referred to as an SEM (Scanning Electron Microscope) image. FIG. 8 shows an example of a ZnS grating structure 10 formed using an ICP-type RIE apparatus using a mixed gas of CH4 and Ar, based on the series of steps shown in the optical element manufacturing method in FIGS. 2 to 7, after which the resist pattern 70 serving as the etching mask is removed. FIG. 8 also shows an example of a case in which no processing abnormality region occurs in the grating structure 10. From FIG. 8, it can be seen that the grating protrusions 11, each having a triangular cross section, are formed with high precision and uniformity.

[0034] When forming the grating structure 10 using the etching process shown in FIG. 6 , foreign matter may adhere to the grating structure 10 while the ZnS substrate 20 is being removed. In another example, foreign matter larger than the pitch P may adhere, spanning adjacent grating protrusions 11 and covering the grating recesses 12. In this case, the processing of the ZnS is hindered, resulting in a shallower depth for the grating structure 10, i.e., the grating recesses 12. If the depth of the grating structure 10 is too shallow, it may deviate from the tolerances and fail to achieve optical characteristics sufficient for practical use as an optical element. Furthermore, deviations from an ideal grating structure may result in locally higher reflectivity than the design value. This may result in stray light being generated outside the intended optical path. For these reasons, it is desirable for the grating shape to be within the design range within the effective area through which light passes in the wave plate 100, and for the processing depth to have minimal variation.

[0035] To obtain the desired characteristics calculated by an optical simulator or numerical analysis, shape parameters, including the depth of the lattice structure, must be uniform across the surface of the ZnS substrate 20. Here, the region within the processing surface where the depth of the lattice structure 10 is shallower than the depth of the surrounding area is defined as a "processing abnormality region." The ratio of the area of ​​the processing abnormality region to the area of ​​the measurement target region is defined as the "processing abnormality area ratio." The measurement target region is the range within the processing surface for which the processing abnormality area ratio is calculated. When calculating the processing abnormality area ratio using an optical microscope from above the processing surface, this is the region imaged using an optical microscope. The measurement target region may also be multiple regions imaged using an optical microscope from above the processing surface. Figure 9 is a traced SEM image of a case where a processing abnormality region occurs in the lattice structure. Examples of cases where a processing abnormality region 25 does not occur and where it does occur in the lattice structure 10 formed on the surface of the ZnS substrate 20 are shown in Figures 8 and 9, respectively. 9, even though the entire surface was processed under the same conditions for the same amount of time, the depth of the lattice recesses 12 in the center is shallower than the regions on the right and left sides of the SEM image. This shallow portion of the lattice recesses 12 is the abnormal processing region 25. As mentioned above, this is thought to be due to foreign matter generated during the etching process.

[0036] Causes of foreign matter generation during the etching process include scattering of foreign matter remaining in the vacuum chamber of the RIE apparatus, redeposition of ZnS that was physically removed, redeposition of resist pattern 70 that was physically removed, and localized polymer deposition.

[0037] FIG. 10 is a top view showing an example of an etching mask used in the manufacturing method of an optical element according to the first embodiment. This etching mask 80 is formed by a resist pattern 70 formed by forming a pattern on a photoresist applied to the surface of a ZnS substrate 20. In this example, the etching mask 80 has a resist pattern 70 including a one-dimensional grating structure arranged in parallel. That is, FIG. 10 shows an etching mask 80 in a state in which a resist pattern 70 having a periodic grating structure in which linear patterns extending in one direction are spaced apart in a direction perpendicular to the extending direction is formed on the entire surface of one surface 20a of the ZnS substrate 20. Here, the region in which the resist patterns 70 shown in FIG. 5 are collectively formed is defined as a resist pattern portion 75. In other words, a collection of resist patterns 70 formed in multiple shot regions constitutes the resist pattern portion 75. The resist pattern portion 75 corresponds to the first resist pattern. In the example of FIG. 10, the resist pattern 70 is formed on the entire surface of the substrate 20, and the etching mask 80 coincides with the resist pattern portion 75 forming the grating structure. In this example, the pitch P and filling factor FF of the periodic grating structure in the resist pattern portion 75 of the etching mask 80 are, for example, 4.1 μm and 0.12, respectively. Here, the filling factor FF represents the width W of the resist pattern 70 relative to the pitch P of the resist pattern 70 as shown in FIG. 1 , and is defined by the following equation (3): If the width W of the resist pattern 70 is not constant in the thickness direction of the resist pattern 70, the width W of the resist pattern 70 is defined as the width of the resist pattern 70 at the midpoint in the thickness direction of the resist pattern 70. The etching mask 80 is formed by the method described with reference to FIGS. 3 to 5 .

[0038] FF=W / P...(3)

[0039] As described above, the exposure process using a stepper apparatus to form the resist pattern 70 involves multiple exposures to obtain the desired pattern size. To achieve high-precision patterns using the stepper apparatus, the autofocus function is used to measure and adjust the height and tilt of the target substrate 20 surface before each exposure. When performing adjustments using the autofocus function, a problem occurs in that the exposure process is interrupted if the upper or lower limits of the adjustment range set specifically for the apparatus are exceeded. Substrates 20 used in the semiconductor and optical fields often have chamfered edges to prevent chipping or cracking. When performing full-surface exposure, if autofocus is performed on this chamfered portion or if the autofocus point deviates from the substrate 20, the exposure process may be interrupted. To address this issue, when using a stepper apparatus to expose photoresist on a ZnS substrate 20, appropriate autofocus settings must be selected. For example, if the autofocus does not sufficiently reflect the light irradiated onto the substrate 20, a method may be used in which the autofocus value previously obtained on the same substrate 20 is used as a reference.

[0040] FIG. 11 is a top view showing an example of an etching mask used in a conventional optical element manufacturing method. In the example shown in FIG. 11 , the etching mask 85 has a resist pattern portion 75 with a lattice structure in the center of the substrate 20, and a non-patterned layer 90, which is an unpatterned resist layer 40, near the edge of the substrate 20, where light is not irradiated and the portion does not function as a wave plate 100. With this shape of etching mask 85, if the adhesion between the substrate 20 and the resist pattern portion 75 is weak, the non-patterned layer 90 adjacent to the resist pattern portion 75 can be expected to serve as an anchor to suppress pattern peeling or pattern tipping during the development process shown in FIG. 5 . This is because the non-patterned layer 90 has a larger adhesion area with the substrate 20 than the resist pattern portion 75 and therefore has stronger adhesion to the substrate 20 than the former. Therefore, one method for forming a high-precision resist pattern 70 with a high aspect ratio, for example, an aspect ratio of 1 or more, is to leave the non-patterned layer 90 as shown in FIG. 11 . Furthermore, in order to avoid the process interruption due to the autofocus and to shorten the exposure process time, exposure may not be performed near the edge of the substrate 20, as shown in FIG. 11 . Note that "does not function as a wave plate 100" means that even if cut out from the substrate 20, it cannot be used as a wave plate 100. In other words, if there is a portion that protrudes from the substrate 20 in one shot area during the exposure process, or if a piece cut out from the substrate 20 is missing a portion of the periodic lattice structure that functions as a wave plate 100, this portion or the region of the substrate 20 corresponding to this piece cannot be used as a wave plate 100 and can be said to not function as a wave plate 100. Furthermore, if the wave plate 100 is used as is without being cut out into pieces, when light smaller than the diameter of the wave plate 100 is transmitted through the wave plate 100, the outer portion that is not hit by the light can be said to not function as a wave plate 100, as described above.

[0041] The resist layer 40 near the edge of the substrate 20 in the etching mask 80 shown in FIG. 10 is patterned at a constant pitch P over the entire surface of the ZnS substrate 20. Therefore, the size of the remaining resist layer 40 is smaller than that of the etching mask 85 having the non-patterned layer 90 shown in FIG. 11 . Therefore, the size of resist-induced foreign particles generated during the etching process using the etching mask 80 shown in FIG. 10 is considered to be smaller than that of resist-induced foreign particles generated during the etching process using the etching mask 85 shown in FIG. 11 . In other words, the size of the resist scattered during the etching process for deep etching the substrate 20 is reduced. Furthermore, due to the patterning, the absolute amount of resist remaining near the edge of the substrate 20 is smaller in FIG. 10 than in FIG. 11 . This also reduces the overall processing abnormality area and processing abnormality area ratio. In other words, by approaching the ideal lattice structure used in an optical simulator, etc., optical components with high in-plane uniformity and optical properties close to analytical results can be manufactured with a high yield. 10, in the first embodiment, a resist pattern 70 having a periodic grating structure is formed even in an area that does not function as the wave plate 100. In this case, the resist pattern 70 having a periodic grating structure formed in an area where an optical element is formed, i.e., an area that functions as the wave plate 100, corresponds to the first resist pattern, and the resist pattern 70 having a periodic grating structure formed in an area where an optical element is not formed, i.e., an area that does not function as the wave plate 100, corresponds to the second resist pattern.

[0042] The present disclosure aims to reduce the amount and size of resist-induced foreign matter generated during the etching process, thereby minimizing the processing abnormality region and processing abnormality area rate. Therefore, the optical element and manufacturing method thereof according to the present disclosure differ from a method of forming sub-patterns only on the edges of a main pattern, which aims to prevent the collapse of a main pattern that has been cut off. The optical element and manufacturing method thereof according to the present disclosure forms a resist pattern 70 over the entire surface of one side 20 a of the substrate 20, i.e., in the regions of the substrate 20 where optical elements are to be formed and in the regions where optical elements are not to be formed.

[0043] FIG. 12 is a diagram showing an example of the measurement results of the processing abnormality area rate when a substrate is etched using the etching masks shown in FIGS. 10 and 11. In this diagram, the vertical axis represents the processing abnormality area rate. Measurement targets are four samples each for the etching mask 80 in FIG. 10 and the etching mask 85 in FIG. 11. In FIG. 12, the average value of the processing abnormality area rate of the four samples is plotted, and the maximum and minimum values ​​of the processing abnormality area rate are represented by error bars. The measurement was performed immediately after the etching step shown in FIG. 6. The measurement target area in this case was a 25.8 mm × 25.8 mm area centered on the center of the substrate 20.

[0044] 12, it can be confirmed that the processing abnormality area rate is reduced by reducing the non-patterned layer 90. The reason why the processing abnormality area rate differed even though etching was performed under the same conditions is thought to be because the shapes of the etching masks 80 and 85 were changed to reduce the absolute amount of the non-patterned layer 90, which could be a source of foreign matter.

[0045] In the example shown in FIG. 10 , an etching mask 80 with a lattice structure having the same pitch P is formed on the entire surface of the substrate 20. However, in order to reduce resist-induced foreign matter, it is not necessary for the entire surface of the substrate 20 to have a lattice structure pattern with the same pitch P, as long as the area of ​​the non-patterned layer 90 can be reduced compared to the case of FIG. 11 and the absolute amount of the resist layer 40 can be reduced. Therefore, in the example of FIG. 11 , a lattice structure pattern with a pitch greater than or less than the pitch P may be formed outside the resist pattern portion 75 in order to reduce the area of ​​the non-patterned layer 90. Furthermore, the absolute amount of resist may be reduced by using the same pitch P and reducing only the filling factor FF near the edge of the substrate 20, which is not irradiated with light and does not function as the wave plate 100. In this way, the period of the lattice structure of the resist pattern 70 in the region that does not function as the wave plate 100 may be the same as or different from the period of the lattice structure of the resist pattern 70 in the region that functions as the wave plate 100.

[0046] Furthermore, although the above description has been given of a case where a one-dimensional periodic structure is formed on the substrate 20, the same applies to a case where a two-dimensional periodic structure is formed on the substrate 20. An optical element having a two-dimensional periodic structure can exhibit not only phenomena such as reflection and refraction, but also actions such as polarization and diffraction. In this case, a resist pattern 70 having a grating structure pattern with the same pitch P as the region that functions as an optical element may be formed in the region that does not function as an optical element, or a resist pattern 70 having a grating structure with a pitch P different from that of the region that is used as an optical element may be formed. Furthermore, a resist pattern 70 having a one-dimensional periodic structure may be formed in the region that does not function as an optical element.

[0047] In the above description, an example has been given in which a plurality of optical elements such as wave plates 100 are cut out from a single substrate 20, but it is also possible to use the substrate 20 having a pattern of a periodic grating structure formed thereon as an optical element such as wave plates 100 without cutting it out. Figure 13 is a cross-sectional view schematically showing an example of the configuration of an optical element according to embodiment 1. Here, a transmissive wave plate 100A is taken as an example of the optical element. Furthermore, the same components as those in Figure 1 are designated by the same reference numerals, and their description will be omitted.

[0048] The wave plate 100A has a first pattern 13A having a periodic grating structure disposed in a first region R1 of the substrate 20 that functions as the wave plate 100A, and a second pattern 13B having a periodic grating structure disposed in a second region R2 of the substrate 20 that does not function as the wave plate 100A. "Functioning as the wave plate 100A," i.e., functioning as an optical element, means that light is irradiated onto the substrate 20 when the wave plate 100A is used as a product. "Not functioning as the wave plate 100A," i.e., not functioning as an optical element, means that light is not irradiated onto the substrate 20 when the wave plate 100A is used as a product.

[0049] As shown in FIG. 13 , the grating structure 10 has a first pattern 13A and a second pattern 13B. The first pattern 13A has grating protrusions 11A and grating recesses 12A. That is, the first pattern 13A has a structure in which the grating protrusions 11A are periodically arranged one-dimensionally or two-dimensionally. A grating recess 12A is arranged between adjacent grating protrusions 11A and 11A. The second pattern 13B has grating protrusions 11B and grating recesses 12B. That is, the second pattern 13B has a structure in which the grating protrusions 11B are periodically arranged one-dimensionally or two-dimensionally. A grating recess 12B is arranged between adjacent grating protrusions 11B and 11B. The period of the grating structure of the second pattern 13B may be the same as or different from the period of the grating structure of the first pattern 13A. In addition, in the example of Figure 13, the first pattern 13A and the second pattern 13B are shown as lattice structures with a one-dimensional periodic shape, but the first pattern 13A and the second pattern 13B may be lattice structures with a two-dimensional periodic shape, or one of the first pattern 13A and the second pattern 13B may be a lattice structure with a one-dimensional periodic shape and the other may be a lattice structure with a two-dimensional periodic shape.

[0050] In one example of a manufacturing method for such a wave plate 100A, one shot in an exposure process using a stepper apparatus has approximately the same size as the substrate 20. In this case, this one shot includes patterns for forming the first pattern 13A and the second pattern 13B over the entire surface of the substrate 20. In other words, one shot includes a pattern for forming the first pattern 13A in the first region R1, which is a region of the wave plate 100A that will be irradiated with light when used as a product, and a pattern for forming the second pattern 13B in the second region R2, which is a region other than the first region R1 that will not be irradiated with light when used as a product. In another example of a manufacturing method for the wave plate 100A described above, the patterns for forming the first pattern 13A and the second pattern 13B may be separate shots rather than the same single shot. In other words, a shot including a pattern for forming the first pattern 13A and a shot including a pattern for forming the second pattern 13B may be used. Furthermore, the process for forming first pattern 13A is not limited to one shot, and first pattern 13A may be formed by seamlessly arranging a plurality of shots each having an area smaller than substrate 20, as described with reference to Figures 4 and 10. The process for forming second pattern 13B is similar.

[0051] As described above, the method for manufacturing an optical element according to the first embodiment includes a resist layer formation step of forming a resist layer 40 on one surface of the substrate 20 made of an optically isotropic material, an exposure step of exposing the resist layer 40 to light, and a resist pattern formation step of developing the exposed resist layer 40 to form a first resist pattern having a periodic grating structure in an area that functions as an optical element. In the resist pattern formation step, a second resist pattern having a periodic grating structure is also formed in an area of ​​the substrate 20 that does not function as an optical element. This reduces the amount and size of the resist that scatters during the etching step for deeply digging the substrate 20, thereby reducing the total area where in-plane variations in shape occur due to processing interference.

[0052] That is, the substrate 20 is etched by a dry etching process using an etching mask 80 having a resist pattern 70 with the same pitch P as the region functioning as an optical element or a different pitch from the region functioning as an optical element in the region on one surface 20a of the substrate 20. This reduces the possibility that foreign matter originating from the resist, which is generated during the dry etching process of the region not functioning as an optical element, will adhere to the etching mask 80 in the region functioning as an optical element. As a result, the processing abnormality area rate in the region functioning as an optical element can be reduced, and a more highly accurate and uniform uneven pattern can be formed compared to conventional methods.

[0053] Furthermore, the grating structures of the first resist pattern and the second resist pattern may be one-dimensionally or two-dimensionally periodic. This allows the manufactured optical element to exhibit not only phenomena such as reflection and refraction, but also polarization and diffraction, thereby realizing a more highly functional optical element. In particular, by applying the method for manufacturing an optical element according to the first embodiment to a case where a grating structure is formed by deeply digging a substrate 20 having a one-dimensional and two-dimensional periodic grating structure with an aspect ratio of 1 or more, a more highly accurate and uniform uneven pattern can be formed using dry etching compared to conventional methods.

[0054] Furthermore, the method for manufacturing an optical element according to the first embodiment further includes an etching step of processing the substrate 20 by a dry etching process using the first resist pattern and the second resist pattern as an etching mask 80. This eliminates the need for an additional step, such as depositing an additional film on the substrate 20 to form a grating structure, and thus has the effect of facilitating the manufacture of optical elements.

[0055] The configurations shown in the above embodiments are merely examples, and may be combined with other known technologies, and parts of the configurations may be omitted or modified without departing from the spirit of the invention.

[0056] Various aspects of the present disclosure are summarized below as appendices.

[0057] [Supplementary Note 1] A method for manufacturing an optical element, comprising: a resist layer formation step of forming a resist layer on one surface of a substrate made of an optically isotropic body; an exposure step of exposing the resist layer to light; and a resist pattern formation step of developing the exposed resist layer to form a first resist pattern having a periodic grating structure in an area that functions as an optical element, wherein in the resist pattern formation step, a second resist pattern having a periodic grating structure is also formed in an area of ​​the substrate that does not function as an optical element. [Supplementary Note 2] A method for manufacturing an optical element according to Supplementary Note 1, wherein the grating structures of the first resist pattern and the second resist pattern are one-dimensionally or two-dimensionally periodic. [Supplementary Note 3] A method for manufacturing an optical element according to Supplementary Note 2, wherein the period of the grating structure of the second resist pattern is the same as or different from the period of the grating structure of the first resist pattern. [Supplementary Note 4] A method for manufacturing an optical element according to any one of Supplements 1 to 3, wherein the substrate is made of a compound semiconductor material. [Appendix 5] The method for manufacturing an optical element according to any one of Appendices 1 to 4, further comprising an etching step of processing the substrate by a dry etching process using the first resist pattern and the second resist pattern as etching masks. [Appendix 6] An optical element comprising: a first pattern having a periodic grating structure disposed in a first region of a substrate made of an optically isotropic body that functions as an optical element; and a second pattern having a periodic grating structure disposed in a second region that does not function as an optical element. [Appendix 7] The optical element according to Appendices 6, wherein the first pattern and the second pattern have one-dimensional or two-dimensional periodic grating structures. [Appendix 8] The optical element according to Appendices 7, wherein the period of the grating structure of the second pattern is the same as or different from the period of the grating structure of the first pattern. [Appendix 9] The optical element according to any one of Appendices 6 to 8, wherein the substrate is made of a compound semiconductor material.

[0058] 10 Grating structure portion, 11, 11A, 11B Grating convex portion, 12, 12A, 12B Grating concave portion, 13A First pattern, 13B Second pattern, 20 Substrate, 20a, 20b Surface, 25 Processing abnormality area, 30 Anti-reflection layer, 40 Resist layer, 50 Reticle, 51 Light-shielding portion, 52 Transmitting portion, 60 Ultraviolet light, 70 Resist pattern, 75 Resist pattern portion, 80, 85 Etching mask, 90 Non-patterned layer, 100, 100A Wave plate, R1 First region, R2 Second region.

Claims

1. A method for manufacturing optical elements, comprising: a resist layer formation step of forming a resist layer on one side of a substrate made of an optically isotropic body; an exposure step of exposing the resist layer to light; and a resist pattern formation step of developing the exposed resist layer to form a first resist pattern having a periodic lattice structure in an area that functions as an optical element, wherein in the resist pattern formation step, a second resist pattern having a periodic lattice structure is also formed in an area of ​​the substrate that does not function as an optical element.

2. The method for manufacturing an optical element according to claim 1, wherein the grating structures of the first resist pattern and the second resist pattern are one-dimensionally or two-dimensionally periodic shapes.

3. The method for manufacturing an optical element according to claim 2, wherein the period of the grating structure of the second resist pattern is the same as or different from the period of the grating structure of the first resist pattern.

4. The method for manufacturing an optical element according to any one of claims 1 to 3, wherein the substrate is made of a compound semiconductor material.

5. A method for manufacturing an optical element according to any one of claims 1 to 4, further comprising an etching step of processing the substrate by a dry etching process using the first resist pattern and the second resist pattern as an etching mask.

6. The method for manufacturing an optical element described in claim 5, further comprising: a resist layer removal step of removing the etching mask; and a cutting step of cutting out an area that will become one shot in the exposure step from the substrate from which the etching mask has been removed, wherein the area of ​​the substrate that does not function as an optical element includes an area in one shot area in the exposure step that includes a portion that protrudes from the substrate.

7. A method for manufacturing an optical element according to any one of claims 1 to 6, characterized in that in the resist pattern forming step, the first resist pattern and the second resist pattern are formed over the entire surface of the substrate.

8. An optical element comprising: a first pattern having a grating structure with a periodic shape arranged in a first region that functions as an optical element of a substrate made of an optically isotropic body; and a second pattern having a grating structure with a periodic shape arranged in a second region that does not function as the optical element.

9. The optical element according to claim 8, wherein the first pattern and the second pattern have a grating structure with a one-dimensional or two-dimensional periodic shape.

10. The optical element according to claim 9, wherein the period of the grating structure of the second pattern is the same as or different from the period of the grating structure of the first pattern.

11. The optical element according to any one of claims 8 to 10, wherein the substrate is made of a compound semiconductor material.

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