JP2011215267A - Achromatic lens, method for manufacturing the same, and optical device equipped with achromatic lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an achromatic lens having high transmittance in a vacuum ultraviolet ray area.SOLUTION: Provided is the achromatic lens including a refractive lens and a diffraction lens having a plurality of binary shaped gratings. As for the achromatic lens, the refractive lens is made of a refractive lens material being at least one selected from a group consisting of lithium fluoride and magnesium fluoride and the diffraction lens is made of a diffraction lens material being at least one selected from a group consisting of lithium fluoride and magnesium fluoride, wherein the mean square deviation value (rms value) of the surface roughness of the grating of the diffraction lens is 5 nm or less. The achromatic lens corrects the color aberration of the vacuum ultraviolet ray area from 200 nm to an absorption edge wavelength on a short wavelength side, where the refractive indexes of the lithium fluoride and the magnesium fluoride are drastically changed.

Description

  The present invention relates to an achromatic lens that can be used in the vacuum ultraviolet region and a manufacturing method thereof, and relates to an optical device such as a light source, a spectroscopic measuring device, and a radical measuring device equipped with the achromatic lens.

  Since optical materials generally have a wavelength dependency of refractive index, that is, wavelength dispersion characteristics, for example, when light of different wavelengths is passed through a lens system, an imaging position shift occurs. This shift is chromatic aberration, and correcting chromatic aberration is called achromatic. Achromatization can be performed by a lens system in which a plurality of lenses are arranged at appropriate intervals, or a lens system in which a plurality of lenses are bonded to form one lens. A lens system in which chromatic aberration is corrected is called an achromatic lens.

  As an achromatic lens in which a plurality of lenses are bonded to form a single lens, for example, a convex crown lens having a positive refractive power and a concave flint lens having a negative refractive power are bonded, An achromatic lens in which chromatic aberration of two wavelengths is corrected has been proposed. Further, there has been proposed an achromatic lens in which a diffractive lens and a refractive lens, each having a plurality of gratings arranged concentrically with respect to the optical axis and having a rectangular shape formed on each grating, are combined. Such an achromatic lens is disclosed in Patent Document 1, Patent Document 2, and Non-Patent Document 1, for example. The achromatic lens disclosed in Patent Document 1 and Patent Document 2 is described as being capable of correcting chromatic aberration in a wavelength region of about 400 nm or more. As described in Patent Document 2, the rectangular shape of the diffractive lens is formed by cutting.

JP-A-8-304741 Japanese translation of PCT publication No. 8-508116

Scientific American, May 1992, 50-55, “Binary Optics”

  In the vacuum ultraviolet region (10 to 200 nm), it is difficult to realize a suitable achromatic lens, and surface reflectors are often used for condensing a light source, a collimator for a spectroscope, and the like. However, when a surface reflecting mirror is used, the surface reflecting mirror cannot be brought close to other optical elements, and the optical system is bent by the surface reflecting mirror, so that the optical device is often large.

  There is a need for an achromatic lens capable of correcting chromatic aberration in the vacuum ultraviolet region where the wavelength is 200 nm or less. Many optical materials have a large absorption in the vacuum ultraviolet region, and the refractive index increases rapidly in the vicinity of a wavelength (absorption edge) where light is not completely transmitted. In general, an optical material having a wavelength near the absorption edge has a similar wavelength dispersion characteristic of the refractive index. For this reason, optical materials having absorption wavelengths close to each other have little change in the refractive index difference with respect to the wavelength, and a refractive lens having a positive refractive power and a refractive lens having a negative refractive power, such as an achromatic lens, are attached. When using a combined achromatic lens, it is necessary to increase the curvature of each refractive lens, which increases the thickness of the achromatic lens.

  On the other hand, a diffractive lens has a wavelength dispersion characteristic opposite to that of a refractive lens, and a diffractive lens using an optical material that transmits vacuum ultraviolet rays is combined with a refractive lens using an optical material that transmits vacuum ultraviolet rays in the same manner. If used as an achromatic lens, an achromatic lens with a small thickness can be realized.

  However, the shorter the wavelength, the greater the loss of light due to scattering. For this reason, in order to ensure high efficiency in the vacuum ultraviolet region having a short wavelength, it is necessary to suppress scattering by smoothing the surface of the diffractive lens. Conventionally, a diffractive lens is processed by scraping the material of the diffractive lens by a method of mechanically cutting or grinding the lens material or a method of etching the lens material, and these methods transmit vacuum ultraviolet rays. When processing an optical material, the surface roughness required to ensure the efficiency of the diffractive lens cannot be obtained.

  On the other hand, if a resin material is used as the material of the diffractive lens, it is relatively easy to ensure the processing accuracy of the diffractive lens. However, since the resin material cannot transmit ultraviolet rays of 280 nm or less, a resin diffractive lens is used. An achromatic lens combined with a refractive lens cannot correct chromatic aberration in the vacuum ultraviolet region.

  In view of the above problems, an object of the present application is to provide an achromatic lens that has little absorption loss and scattering loss and can be suitably used in the vacuum ultraviolet region.

  The present invention is an achromatic lens comprising a refractive lens and a diffractive lens having a plurality of binary-shaped gratings, wherein the refractive lens is at least one selected from the group consisting of lithium fluoride and magnesium fluoride The diffractive lens is formed using a diffractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride. The mean square deviation value (rms value) of the surface roughness of the grating is 5 nm or less, and the chromatic aberration in the vacuum ultraviolet region having a wavelength longer than the absorption edge on the short wavelength side where the refractive indexes of lithium fluoride and magnesium fluoride change greatly. An achromatic lens that corrects for the above is provided. In this specification, a lens material used as a refractive lens material is particularly referred to as a refractive lens material, and a lens material used as a diffractive lens material is particularly referred to as a diffractive lens material.

  In the present application, the “achromatic lens” may be a lens system having a plurality of lenses sharing the optical axis, or may be a single lens.

  The achromatic lens according to the present application includes a refractive lens and a diffractive lens. Since the mean square deviation value (rms value) of the surface roughness of the grating of the diffractive lens is 5 nm or less, lithium fluoride and Provides an achromatic lens for correcting chromatic aberrations in the vacuum ultraviolet region with less light absorption and scattering loss in the vacuum ultraviolet region of wavelengths longer than the absorption edge on the short wavelength side where the refractive index of magnesium fluoride changes greatly can do. This achromatic lens can reduce light absorption loss and scattering loss even when used for the purpose of correcting chromatic aberration in the vacuum ultraviolet region, particularly in the vacuum ultraviolet region having a wavelength of 120 to 200 nm.

The lattice spacing of the plurality of binary-shaped gratings is wide on the side close to the optical axis of the achromatic lens and narrow on the side far from the optical axis of the achromatic lens, and the plurality of binary-shaped gratings are it may have a stepped shape of 2 ^x stages partitioned by a vertical plane parallel to the curved surface to the optical axis to the optical axis of the achromatic lens.

  The mean square deviation value (rms value) of the surface roughness of the grating of the diffractive lens is preferably 3 nm or less. The scattering loss of the achromatic lens can be further reduced.

  The binary shape of the diffractive lens is obtained by forming a mold made of an organic material mainly composed of a resin on the surface of the substrate formed of the refractive lens material or the diffractive lens material, and using the mold to place the substrate at room temperature. Alternatively, it is preferably formed by laminating the diffractive lens material on the surface of the substrate while heating at a substrate temperature of 100 ° C. or less and removing the mold. Provides a diffractive lens with low scattering loss in the vacuum ultraviolet region including the wavelength region where the refractive index of lithium fluoride and magnesium fluoride causes anomalous dispersion, with the rms value of the surface roughness of the diffractive lens being 3 nm or less can do.

  The binary shape of the diffractive lens forms a sacrificial layer as a template of an inorganic material mainly composed of silicon or silica (silicon oxide) on the surface of the substrate formed of the refractive lens material or the diffractive lens material. The substrate is preferably formed by laminating the diffractive lens material on the surface of the substrate while heating the substrate at 200 ° C. or more and 1200 ° C. or less using the mold and removing the sacrificial layer. The rms value of the surface roughness of the diffractive lens can be reduced to 3 nm or less, and a diffractive lens grating with little scattering loss can be realized. Accordingly, it is possible to provide an achromatic lens with less loss in a vacuum ultraviolet region including a wavelength region in which the refractive indexes of lithium fluoride and magnesium fluoride cause anomalous dispersion.

  In the achromatic lens according to the present application, a binary diffraction grating may be formed on at least one of the front surface and the back surface of the refractive lens.

  The achromatic lens according to the present application can be suitably used for a vacuum ultraviolet light source, a spectroscopic measurement device, and a radical measurement device. This makes it possible to achieve both improvement in efficiency by reducing absorption loss and scattering loss in the vacuum ultraviolet region of the optical device, and downsizing of the optical device such as a measuring device.

  The present invention can also provide a method of manufacturing an achromatic lens including a refractive lens and a diffractive lens. A first method for producing an achromatic lens according to the present invention includes a refractive lens forming step of forming a refractive lens using a refractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride, A diffractive lens forming step of forming a diffractive lens by using at least one diffractive lens material selected from the group consisting of lithium fluoride and magnesium fluoride. The diffractive lens forming step includes a mold forming step of forming a mold of an organic material mainly composed of a resin on a surface of a substrate formed of the refractive lens material or the diffractive lens material, and the mold using the mold. A lamination step of laminating the diffractive lens material on the surface of the substrate by sputtering while heating the substrate at room temperature or 100 ° C. or less; and a mold removal step of removing the mold after the heat treatment step.

  The achromatic lens according to the present invention can also be manufactured by using the second manufacturing method. A second method for producing an achromatic lens according to the present invention includes a refractive lens forming step of forming a refractive lens using a refractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride, A diffractive lens forming step of forming a diffractive lens by using at least one diffractive lens material selected from the group consisting of lithium fluoride and magnesium fluoride. The diffractive lens forming step is a sacrifice that forms a sacrificial layer as a template of an inorganic material mainly composed of silicon or silica (silicon oxide) on the surface of the substrate formed of the refractive lens material or the diffractive lens material. A layer forming step, a laminating step of laminating the diffractive lens material on the surface of the substrate while heating the substrate at 200 ° C. to 1200 ° C. using the mold, and a sacrificial layer removing step of removing the sacrificial layer including.

  According to the present invention, it is possible to provide an achromatic lens for correcting chromatic aberration in the vacuum ultraviolet region, which has high efficiency due to low absorption loss and scattering loss in the vacuum ultraviolet region.

It is the figure which looked at the achromatic lens concerning an embodiment from the diffraction lens side. It is the II-II sectional view taken on the line of FIG. FIG. 3 is a diagram showing optical characteristics of the achromatic lens of Example 1. It is a conceptual diagram which shows the radical measuring apparatus using an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 1st manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 1st manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 1st manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 1st manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 1st manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 1st manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 1st manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 1st manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens. It is a figure explaining the diffraction lens formation process concerning the 2nd manufacturing method of an achromatic lens.

(Achromatic lens)
FIG. 1 is a view of the achromatic lens 10 according to the embodiment as viewed from the diffraction lens 11 side, and FIG. 2 is a cross-sectional view taken along the line II-II in FIG. As shown in FIGS. 1 and 2, the achromatic lens 10 includes a diffractive lens 11 and a refractive lens 12. The diffractive lens 11 is a diffractive lens having a binary shape formed on a plane on the front side and a plane on the back side. The refractive lens 12 is a refractive lens having a convex lens shape on the front surface side and a flat surface on the back surface side. The back surface of the diffractive lens 11 and the back surface of the refractive lens 12 are arranged so as to be in contact with each other at the contact surface 13. In the achromatic lens 10, the diffractive lens 11 and the refractive lens 12 are formed of the same lens material that is at least one selected from the group consisting of lithium fluoride (LiF) and magnesium fluoride (MgF ₂ ). . Since lithium fluoride and magnesium fluoride are materials having excellent vacuum ultraviolet transmittance at wavelengths longer than 120 nm, the achromatic lens 10 has excellent vacuum ultraviolet transmittance at wavelengths longer than 120 nm. When lithium fluoride is used as a lens material, the absorption short wavelength is around 105 nm, and the refractive index change region in which the refractive index changes greatly (the refractive index in the vicinity of twice the absorption edge wavelength from the absorption edge wavelength on the short wavelength side is sharp) The wavelength region that changes to 110-200 nm. When magnesium fluoride is used as the lens material, the absorption short wavelength is around 110 nm, and the refractive index change region in which the refractive index changes greatly is 115 to 200 nm. However, both substances have large absorption from the wavelength of the absorption edge to less than 120 nm, and their practicality is low.

The diffractive lens 11 and the refracting lens 12 constituting the achromatic lens 10 are circular lenses centered on the axis Z shown in FIGS. 1 and 2, and the cross-sectional diagram shown by the oblique lines in FIG. The shape is rotated around the center. The diffractive lens 11 is a diffractive lens in which a binary (stepped) grating is formed concentrically around an axis Z. One concentric circle shown in FIG. 1 represents one lattice, and the lattice interval becomes narrower as the distance from the axis Z increases. This lattice interval is the distance between the highest portions of adjacent lattices (for example, the distance D shown in FIG. 2). In the diffractive optical element, the diffraction angle becomes smaller as the grating interval is larger. Therefore, the light incident on the diffractive lens 11 has a smaller diffraction angle on the side closer to the axis Z (the side where the grating interval is larger). The farther side (the side with the smaller grating interval), the larger the diffraction angle. As a result, the diffractive lens 11 functions as a lens having the axis Z as the optical axis. In the diffractive lens 11, each of the plurality of gratings descends from the optical axis (axis Z) of the achromatic lens 10 toward the peripheral edge, and is partitioned by a plane perpendicular to the optical axis and a curved surface parallel to the optical axis. It has a 2 ^x stage of the staircase shape that. Thus, when the staircase shape descends from the optical axis toward the peripheral edge, the diffractive lens has positive refractive power. The lattice height of each lattice (the distance from the top surface of the highest step of the staircase shape to the lowest step) is H, and they are all the same. The grating height H is designed to be an integral multiple of the wavelength. In each lattice, the width of the plurality of steps is the same, and the height difference between adjacent steps is the same.

  The achromatic lens 10 according to this embodiment is an achromatic lens having a positive refractive power in which a diffractive lens 11 having a positive refractive power and a refractive lens 12 having a positive refractive power are combined. Note that the achromatic lens may be an achromatic lens having negative refractive power. In this case, a diffractive lens whose binary shape is rising from the optical axis toward the peripheral portion contrary to the diffractive lens 11 may be used as the diffractive lens, and a concave lens-shaped refracting lens may be used as the refracting lens.

Although FIG. 2 illustrates a case where the number of stages of each lattice is four, the present invention is not limited to this. The number of stages may be 2 to the power of x (2 ^x , where x ≧ 1). For example, the number of stages may be 2, 4, 8, or 16. The diffraction efficiency increases as the number of stages of each grating increases. However, when the number of stages is 16 or more, the diffraction efficiency of a grating having a saw-tooth shape with a smooth slope gradually approaches. According to Non-Patent Document 1, when the diffraction efficiency of the saw-shaped grating is 100%, the maximum value of the diffraction efficiency when the number of stages is two is 41%, and when the number of stages is four, 81%, In the case of 8 stages, it is 95%, and in the case of 16 stages, it is 99%. In the saw-shaped grating, the maximum value of the diffraction efficiency is about 85% when the grating interval is 10 times the wavelength, and the maximum value of the diffraction efficiency is 80 when the grating interval is 5 times the wavelength. When the grating interval is four times the wavelength, the maximum value of the diffraction efficiency is about 75%, and when the grating interval is three times the wavelength, the maximum value of the diffraction efficiency is about 60%. (See Proc. SPIE 5005, 8-19, (2003), “Optimization of a Volume Phase Holographic Grism for Astronomical Observation using the Photopolymer”). The diffraction efficiency of the diffraction lens 11 having a binary-shaped grating is obtained by multiplying the diffraction efficiency of the saw-shaped grating described above by the ratio of the diffraction efficiency due to the number of steps of the binary shape when the saw-shaped grating is 100%. Can be determined by For example, when the grating interval is 5 times the wavelength and the number of stages of the binary shape is 8, a diffraction efficiency of about 76% obtained by multiplying 80% and 95% can be obtained. It is not necessary to make all the grids the same number of stages. For example, the diffractive lens 11 may include at least one grating having four, eight, and sixteen stages. In this case, it is preferable to design so that the number of steps is larger as the lattice is closer to the axis Z (the lattice having a larger lattice interval). For example, one or a plurality of gratings located on the side closest to the axis Z has 16 stages, and one or a plurality of gratings located on the outer side thereof has eight stages, and the side farthest from the axis Z (outermost circumference) One or more gratings located in can be arranged in four stages.

  Scattering not only lowers the efficiency of the optical system, but also contributes to stray light, so it is desirable that the scattering loss be smaller. When the scattering increases, an error may occur in the measured value, and in practice, the scattering loss of the optical element is often required to be 1% or less to 24% or less depending on the purpose. In spectroscopic measurement or the like when there is particularly strong background light, an achromatic lens with less scattering loss is required. On the other hand, in the absorbance measurement using a monochromatic light source, even an achromatic lens having a scattering loss of about 50% can be used practically.

The scattering loss Ls in the diffractive lens 11 of the achromatic lens 10 increases as the mean square deviation value (rms value) of the surface roughness increases when measured at an interval of 1 nm with an atomic force microscope (AFM). Become. If the rms value of the surface roughness is σ and the wavelength λ is given, the scattering loss Ls (%) can be obtained by the following equation (1).
Ls = [1-exp {-(4πnσcos θ / λ) ² }] × 100 (1)
Here, θ is the incident angle of the light beam, and n is the refractive index of the medium.

  When the scattering loss Ls of the diffractive lens is calculated using the equation (1), the scattering loss in the case where the normal incidence (θ = 0), the refractive index n = 1, and the rms value σ of the surface roughness is 1 nm at a wavelength of 120 nm. Ls is 1%, the scattering loss Ls is 10% when the surface roughness rms value σ is 3 nm, the scattering loss Ls is 24% when the surface roughness rms value σ is 5 nm, and the surface roughness rms. The scattering loss Ls when the value σ is 10 nm is 67%. When an achromatic lens is used in the vacuum ultraviolet region where the wavelength is short, the rate at which the transmittance of vacuum ultraviolet light decreases due to the influence of scattering increases as the surface roughness increases. When the diffractive lens 11 is measured by AFM at 1 nm intervals, the rms value of the surface roughness is 5 nm or less, and therefore the scattering loss at a wavelength of 120 nm is 24% or less. Since the scattering loss increases as the wavelength becomes shorter, the diffractive lens 11 has a scattering loss of 24% or less in vacuum ultraviolet rays having a wavelength of 120 nm or more and 200 nm or less, and is practically used in vacuum ultraviolet rays having a wavelength of 120 nm or more and 200 nm or less depending on applications. Can be used. The rms value of the surface roughness of the diffractive lens 11 is more preferably 3 nm or less, and particularly preferably 1 nm or less. If the rms value of the surface roughness of the diffractive lens 11 is 3 nm or less, the scattering loss in vacuum ultraviolet rays having a wavelength of 120 nm or more and 200 nm or less can be reduced to 10% or less, and if the rms value of the surface roughness is 1 nm or less, Scattering loss can be reduced to 1% or less.

Next, Example 1 that further embodies the achromatic lens according to the embodiment will be described.
FIG. 3 shows a result of conducting a simulation and examining the optical characteristics of an example of the achromatic lens 10 according to the embodiment. The horizontal axis indicates the wavelength of light transmitted through the achromatic lens or the like, and the vertical axis indicates the focal length. The simulated achromatic lens 10 is made of magnesium fluoride, the refractive lens 12 has a radius of curvature of 28 mm, and the diffraction lens 11 has a radius of 6.25 mm with a lattice spacing of 696 nm. 101. For comparison, the simulation result when the diffractive lens is used is indicated by reference numeral 151, and the simulation result when the refractive lens is used is indicated by reference numeral 152. The diffractive lens used as a comparative example is a diffractive lens with a grating interval of 420 nm at a radius of 6.25 mm, the refractive lens is a plano-convex lens with a radius of curvature of 11.6 mm, and both lenses have a focal length of 20 mm at 125 nm. Designed to be

Note that the diffractive lens or the diffractive lens is designed so that light emitted from the diffraction grating has a path difference that is an integral multiple of the wavelength. That is, when the diffraction order is m, the wavelength is λ, the grating interval is d, the incident angle is θ1, and the diffraction angle is θ2, the equation of the diffraction grating represented by the following equation (2) is satisfied.
mλ = d (sin θ1 + sin θ2) (2)
If m = 1 and θ1 = 0 (normal incidence),
λ = dsinθ2 (3)
become. On the other hand, the diffraction angle θ2 by the grating having the radius r of the diffractive lens is calculated from the focal length f as follows: r / f = tan θ2 (4)
Given by. Given a diffraction angle θ2 due to a grating having a wavelength λ and a radius r of the diffractive lens, from equation (3) and equation (4), the grating interval d is d = λ / sin {tan ⁻¹ (r / f)}. (5)
It can be obtained more. That is, by using Expression (5), the relationship between the wavelength and the focal length can be obtained from the diffraction lens or the lattice spacing at a specific radius of the diffraction lens.

When the focal length of the refractive lens (refractive lens) is f ₁ and the focal length of the diffractive lens (diffractive lens) is f ₂ , the focal length f ₀ of the achromatic lens when the refractive lens and the diffractive lens are in close contact is as follows. (6).
_{1 / f 0 = 1 / f} 1 + 1 / f 2} ...... (6)
Further, when the outermost radius is r, the numerical aperture on the imaging side when the parallel light beam is incident is N.P. A. Can be obtained by the following equation (7).
N. A. = Sin {tan ^-1 (r / f)} (7)

  As shown in FIG. 3, in the comparative examples (151 and 152), the wavelength dependence of the focal length is large. When the refracting lens (151) is used, the shorter the wavelength, the shorter the focal length. When the diffractive lens (152) is used, the shorter the wavelength, the longer the focal length. On the other hand, when the achromatic lens according to the example is used, the wavelength dependency of the focal length is small, and the chromatic aberration is corrected.

  As described above, the achromatic lens 10 according to the present embodiment is excellent in the transmittance of vacuum ultraviolet rays, and can be suitably used as an achromatic lens capable of correcting chromatic aberration in the vacuum ultraviolet region. The achromatic lens according to the present embodiment can be used, for example, in an optical device using vacuum ultraviolet rays, and can be suitably used in a spectroscopic measurement device that performs measurement using vacuum ultraviolet rays.

  The achromatic lens 10 according to this embodiment is particularly a radical measurement that measures the absolute number of hydrogen radicals (H), nitrogen radicals (N), oxygen radicals (O), carbon radicals (C), etc. present in the plasma. It can use suitably for an apparatus. The wavelengths at which electron transition absorption lines in the ground state of these radicals can be observed are 120 nm for nitrogen radicals, 121.6 nm for hydrogen radicals, 130.4 nm for oxygen radicals, and 165.7 nm for carbon radicals. In such a radical measuring device, when measuring nitrogen radicals, hydrogen radicals, and oxygen radicals, electron transition absorption lines in the vacuum ultraviolet region of 120 nm to 130.4 nm are measured. Furthermore, when it is also desired to measure carbon radicals, electron transition absorption lines in the vacuum ultraviolet wavelength region of 120 nm to 165.7 nm are measured. Examples of materials that transmit vacuum ultraviolet rays in such a wavelength region include magnesium fluoride and lithium fluoride. Conventionally, these materials are used to combine a refractive lens and a diffractive lens, thereby forming a vacuum ultraviolet region. It has not been possible to provide an achromatic lens capable of correcting chromatic aberrations at the same time. This is because the surface roughness of the grating produced by the conventionally proposed methods of cutting and grinding lens materials and etching methods is not practical due to the large scattering loss of vacuum ultraviolet rays. This is considered to be due to the fact that a method for realizing a diffractive lens capable of obtaining the above has not been proposed.

Like the achromatic lens 10 according to this embodiment, the step shape of the diffractive lens 11 descends from the optical axis toward the peripheral edge, and is partitioned by a plane perpendicular to the optical axis and a curved surface parallel to the optical axis. If there are a plurality of binary-shaped gratings having a step shape of 2 ^x steps, a mold is formed on the surface of the substrate made of a lens material used for the planar portion of the diffractive lens 11, and the substrate is formed using this mold. The lens material used as the staircase shape of the diffractive lens 11 is laminated on the surface and a lift-off process for removing the mold can be performed easily and accurately. If the diffractive lens is molded using this mold and then the mold is removed, the shape of the diffractive lens can be sufficiently secured, and the binary-shaped surface roughness of the diffractive lens can be reduced. When the diffractive lens 11 is measured at an interval of 1 nm by AFM, the achromatic lens 10 having a surface roughness rms value of 5 nm or less can be realized, so that the light scattering loss can be reduced and the vacuum ultraviolet ray can be reduced. The transmittance can be improved. Further, the substrate may be heat treated. The heat treatment is preferably performed under the condition that the substrate temperature is 200 ° C. or higher and 1200 ° C. or lower, which makes it possible to further improve the transmittance of vacuum ultraviolet rays.

  If the achromatic lens according to the present embodiment is used in an optical device using vacuum ultraviolet light, a light source of vacuum ultraviolet light, or a spectroscopic measurement device that performs measurement in a wavelength region including vacuum ultraviolet light, these devices can be miniaturized. Become. For example, when applied to the radical measurement apparatus described above, it is possible to realize a miniaturized radical measurement apparatus by placing the vacuum ultraviolet light source unit and the spectroscopic measurement unit in the palm size. The radical measuring device thus miniaturized can be incorporated into various manufacturing apparatuses. For example, a radical measuring device can be incorporated into a semiconductor manufacturing apparatus, and radicals that are actually generated in the manufacturing process being executed can be measured while executing the manufacturing process. Since the manufacturing process conditions of the semiconductor device can be controlled based on the measured value of the radical, the manufacturing yield can be improved.

  FIG. 4 is a diagram schematically showing the radical measuring device 30 provided with the achromatic lens 10 according to the present embodiment. The radical measuring device 30 includes a vacuum ultraviolet light source 310, a vacuum ultraviolet spectrometer 320, and a vacuum chamber 330. The lens 312 of the vacuum ultraviolet light source 310 and the lens 322 of the vacuum ultraviolet spectrometer 320 are installed inside the plasma 340 generated in the vacuum chamber 330.

  When the achromatic lens 10 according to the present embodiment is used as the lens 312 and the lens 322, it is not necessary to adjust the focal position of the lens for each wavelength to be measured, and the vacuum ultraviolet light source 310 and the vacuum ultraviolet spectrometer 320 are made to be the palm size. Can be reduced in size. As described above, according to the achromatic lens 10 according to the present embodiment, the radical measuring device 30 can be miniaturized, and measurement can be performed quickly and efficiently even if the wavelength is changed.

(First manufacturing method of achromatic lens)
Next, a first manufacturing method of the achromatic lens 10 according to the embodiment will be described.
The first manufacturing method according to the present embodiment includes a refractive lens forming step for forming the refractive lens 12 and a diffractive lens forming step for forming the diffractive lens 11.
The refractive lens 12 according to the present embodiment is formed using a refractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride, and is easy to use using a general convex lens manufacturing method. Therefore, detailed description is omitted. Hereinafter, the diffractive optical element forming step for forming the diffractive lens 11 according to the present embodiment will be described with reference to FIGS.

  In the diffractive optical element forming step according to the first manufacturing method, the diffractive lens 11 is formed using at least one lens material selected from the group consisting of lithium fluoride and magnesium fluoride. In this specification, a lens material used as a refractive lens material is particularly referred to as a refractive lens material, and a lens material used as a diffractive lens material is particularly referred to as a diffractive lens material.

  In the diffractive lens forming step, the first lift-off step is repeated x times. Each first lift-off process performed x times is referred to as “i-th lift-off process” (i = 1, 2,..., x). The i-th first lift-off process includes a mold forming process, a stacking process, and a mold removing process. If necessary, a heat treatment step can be added after the template removal step or in the final step.

In the i-th first lift-off process, in the mold forming process, a mold is formed of a substrate (a diffractive lens material that is a material of the diffractive lens 11) using an organic material that is generally used as a resist and is mainly composed of a resin. Formed on the surface of the substrate). In this template forming step, when the binary lattice spacing is D, the templates are arranged at an interval of D / (2 ^i-1 ). In the laminating step, a diffractive lens material is laminated on the surface of the substrate by a sputtering method or the like using a resin mold. In the stacking step, the substrate temperature may be room temperature, or the substrate may be heated so that the substrate temperature becomes 100 ° C. or lower. In the mold removal step, the organic material mold is removed with an organic solvent or the like. You may perform a heat processing process as needed. In the heat treatment step, the substrate after the diffractive lens material is laminated is sintered at a substrate temperature of 200 ° C. or more and 1200 ° C. or less. The heat treatment process may be performed during the lift-off process or after all the x lift-off processes are completed. The mold forming process, the stacking process, the mold removing process, and the (heat treatment process) may be repeatedly performed by the same procedure.
Hereinafter, in the case of x = 2, that is, a four-stage binary shape is illustrated as an example, the diffractive lens forming step will be specifically described.

(First first lift-off process)
(First mold forming process)
First, as shown in FIG. 5, a resist 820 which is an organic material mainly composed of a resin is formed on the surface of a substrate 800 made of a diffractive lens material (for example, MgF ₂ substrate), and the resist 820 is interposed through a mask 830. The exposure is performed. Since the mask 830 includes the opening 831 and the light shielding portion 832, light is irradiated only on a portion of the resist 820 positioned below the opening 831. When the mask 830 is viewed in plan, the light shielding portion 832 is formed concentrically, and the mask 830 is arranged so that the concentric axis of the light shielding portion 832 coincides with the axis Z that is the concentric axis of the grating of the diffraction lens 12. . When the binary lattice spacing is D, the pitch between the opening 831 and the light shielding portion 832 is D.

  When development is performed after exposure, the resist 820 can be patterned as shown in FIG. The resist 820 is patterned in an annular shape having the axis Z of the diffractive lens 12 as a concentric axis.

(First lamination process)
When the diffractive lens material is laminated on the substrate in the state of FIG. 6, as shown in FIG. 7, the diffractive lens material 802 is laminated on the surface of the substrate 800, and the diffractive lens material 803 is laminated on the surface of the resist 820. . In the laminating step, the diffractive lens material is laminated up to the height of two steps of the step shape of the diffractive lens. In the first stacking step, the first resist formation described above is performed so that the diffractive lens material layer 803 formed on the surface of the resist 820 does not come into contact with the diffractive lens material layer 802 stacked on the surface of the substrate 800. The thickness of the resist 820 formed in the process is adjusted. The diffractive lens material can be laminated by, for example, a sputtering method. When the lens material is stacked by the sputtering method, the temperature of the substrate 800 may be normal temperature, or the substrate 800 may be heated so as to be 100 ° C. or lower.

(First mold removal process)
Next, when the diffractive lens material 803 is removed together with the resist 820 with an organic solvent, the state shown in FIG. 8 is obtained.

  If necessary, the substrate in the state of FIG. 8 is heat-treated. The heat treatment is preferably performed under a condition where the temperature of the substrate 800 is 200 ° C. or higher and 1200 ° C. or lower. By performing a heat treatment of the lens material at a substrate temperature of 200 ° C. or higher and 1200 ° C. or lower, it may be possible to further increase the vacuum ultraviolet transmittance of the lens material.

  Thus, the first first lift-off process is completed, and then the second first lift-off process is performed.

(Second first lift-off process)
(Second mold forming process)
Next, a second template forming step is performed. A resist 822 which is an organic material containing a resin as a main component is formed on the surface of the substrate in the state shown in FIG. 8, and the resist 822 is exposed through a mask 840. Since the mask 840 includes the opening 841 and the light shielding portion 842, light is irradiated only to a portion located below the opening 841 of the resist 820. When the mask 840 is viewed in plan, the light shielding portion 842 is formed concentrically, and the mask 840 is disposed so that the concentric axis of the light shielding portion 842 coincides with the axis Z that is the concentric axis of the grating of the diffractive lens 12. . When the binary lattice spacing is D, the pitch between the opening 841 and the light shielding portion 842 is D / 2. The width of the opening 841 is half that of the opening 831, and the width of the light shielding portion 842 is half that of the light shielding portion 832.

  When development is performed after exposure, the resist 822 can be patterned as shown in FIG. The resist 822 is patterned in an annular shape having the axis Z of the diffractive lens 12 as a concentric axis.

(Second lamination process)
When a diffractive lens material as a diffractive lens material is laminated on the substrate in the state of FIG. 10, the diffractive lens material 804 is laminated on the surfaces of the substrate 800 and the diffractive lens material 802 as shown in FIG. A diffractive lens material 805 is laminated on the surface. In the laminating step, the diffractive lens material is laminated by sputtering or the like up to the height of one step of the step shape of the diffractive lens. In the second layering step, the second resist described above is used so that the diffractive lens material layer 805 formed on the surface of the resist 822 does not come into contact with the diffractive lens material 804 stacked on the surface of the diffractive lens material 802. The thickness of the resist 822 formed in the formation process is adjusted.

(Second mold removal step)
As in the first mold removal step, the second mold removal step is performed, and when the diffractive lens material 805 is removed together with the resist 822 with an organic solvent, the state shown in FIG. 12 is obtained. If necessary, the substrate in the state of FIG. 12 may be heat-treated.

  Thus, the second first lift-off process is completed, and the second process of forming the binary shape of the diffractive lens is completed. As shown in FIG. 12, a diffractive lens having a four-stage binary shape can be formed.

As described above, the binary shape having a 2 ^x stepped shape of the diffractive lens is formed by forming an organic material mold mainly composed of resin on the surface of the substrate formed of the diffractive lens material. Then, while heating the substrate at room temperature or below 100 ° C., the diffractive lens material is laminated on the surface of the substrate by sputtering, etc., and the mold is removed from the substrate after the diffractive lens material is laminated, and the mold is removed if necessary. After that, the first lift-off process in which the substrate is heat-treated at a substrate temperature of 200 ° C. or more and 1200 ° C. or less can be formed by repeating x times. Of the first lift-off processes repeated x times, in the i-th first lift-off process, the mold is arranged at an interval of D / (2 ^i-1 ), where D is the binary lattice spacing. . If necessary, the substrate after repeating the first lift-off step x times may be heat-treated.

  The diffractive lens 11 can be formed by the diffractive lens forming process of the procedure described above, and the rms value of the surface roughness when the diffractive lens 11 is measured at an interval of 1 nm with an atomic force microscope (AFM). Can be 3 nm or less. When a conventionally proposed method of cutting or grinding a lens material or a method of etching is used, the rms value of the surface roughness when measured at 1 nm intervals with an atomic force microscope is 5 nm or less (vacuum) Although it was very difficult and practically impossible to manufacture an achromatic lens having a diffractive lens having a practically usable rms value in the ultraviolet region, according to the first manufacturing method described above, An achromatic lens having a smaller rms value (3 nm or less) than that can be manufactured by a simple manufacturing method. Furthermore, since the material that becomes the material of the diffraction lens is densified by heat treatment at a substrate temperature of 200 ° C. or higher and 1200 ° C. or lower and internal scattering is less (higher transmittance), an achromatic lens with less scattering loss is manufactured. can do.

(Second manufacturing method of achromatic lens)
The achromatic lens 10 according to this embodiment can also be manufactured by a second manufacturing method described below. The second manufacturing method of the achromatic lens 10 is different from the first manufacturing method described above in the diffractive lens forming step of forming the diffractive lens 11.

  In the diffractive lens forming step according to the second manufacturing method, diffraction is performed by repeating the second lift-off step x times using at least one diffractive lens material selected from the group consisting of lithium fluoride and magnesium fluoride. The lens 11 is formed. Each second lift-off process performed x times is referred to as an “i-th second lift-off process” (i = 1, 2,..., x). The i-th second lift-off process includes a mold forming process for forming an organic material mold, a sacrificial layer forming process for forming a sacrificial layer as an inorganic material mold, and a mold removing process for removing the organic material mold. , Including a lamination step and a sacrificial layer removal step.

  In the organic material mold forming step, a resist mold, which is an organic material, is formed on the surface of the substrate formed of the diffractive lens material.

In the second manufacturing method, unlike the first manufacturing method, a sacrificial layer mainly composed of silicon or a silicon compound is formed by a lift-off method using an organic material mold, and this sacrificial layer is laminated with a diffractive lens material. It is used as a casting mold. In the sacrificial layer forming step for forming the sacrificial layer, when the binary lattice spacing is D, the sacrificial layers are arranged at a distance of D / (2 ^i-1 ). After the sacrificial layer forming step, a template removing step for removing the organic material template is performed. Then, a lamination process is performed. In the laminating step, the diffractive lens material is laminated on the surface by resistance heating vapor deposition or electron beam vapor deposition while heating the substrate at 200 ° C. to 1200 ° C. using this sacrificial layer. In the sacrificial layer removal step, the diffractive lens material that is excessively laminated is removed from the substrate after the diffractive lens material is laminated by isotropic etching together with the sacrificial layer.

  For example, as shown in FIGS. 13 and 14, a resist 920 is formed as a template of an organic material on the surface of the substrate 900, and the resist 920 is patterned at a distance D using a mask 930 having an opening 931 and a light shielding portion 932. (First organic material mold forming step). Thereafter, as shown in FIG. 15, silicon or silica is laminated by sputtering or the like to form sacrificial layers 950 and 951 (first sacrificial layer forming step). Thus, the sacrificial layer 950 is stacked on the surface of the substrate 900, and the sacrificial layer 951 is stacked on the surface of the resist 920. Thereafter, when the sacrificial layer 951 is removed together with the resist 920 with an organic solvent, the state shown in FIG. 16 is obtained (first organic material template removing step).

  As shown in FIG. 16, the sacrificial layer 950 formed on the surface of the substrate 900 is used as a mold, and as shown in FIG. 17, the diffractive lens material layer 902 and the height of two steps of the diffractive lens have a step shape. 903 is stacked. As a result, the diffractive lens material layer 902 is laminated on the surface of the substrate 900, and the diffractive lens material layer 903 is laminated on the surface of the sacrificial layer 950. When the sacrificial layer 950 is removed by isotropic etching, as shown in FIG. 18, the diffractive lens material layer 903 formed on the surface of the sacrificial layer 950 is removed (first sacrificial layer removing step).

  Next, as shown in FIGS. 19 and 20, a resist 922 is formed on the surface of the substrate 900, and the resist 922 is patterned at a distance D / 2 using a mask 940 having an opening 941 and a light shielding portion 942 (see FIG. 19 and FIG. 20). Second organic material mold forming step). Thereafter, in the second sacrificial layer forming step, as shown in FIG. 21, a sacrificial layer 952 is laminated on the surface of the diffractive lens material layer 902, and a sacrificial layer 953 is laminated on the surface of the resist 922 (second sacrificial layer). Forming step). Thereafter, when the resist 922 is removed, as shown in FIG. 22, the sacrificial layer 953 formed on the surface together with the resist 922 is removed (second organic material template removing step). After that, when the diffractive lens material is laminated by the second diffractive lens material laminating process using the sacrificial layer 952 as a mold, the diffractive lens material layer 904 is laminated on the surface of the substrate 900 and the diffractive lens material layer 902, and the sacrificial layer 952. A diffractive lens material layer 905 is laminated on the surface. Thereafter, when the sacrificial layer 952 is removed, as shown in FIGS. 23 and 24, the sacrificial layer 952 and the diffractive lens material layer 905 formed on the surface thereof are removed (second sacrificial layer removing step). Also according to the second manufacturing method, the rms value of the surface roughness can be reduced to 3 nm or less when the diffractive lens 11 is measured with an atomic force microscope (AFM) at 1 nm intervals. A diffractive lens can be realized in which the rms value of the surface roughness is even smaller than the rms value (5 nm or less) practically usable in the vacuum ultraviolet region.

  In the first manufacturing method and the second manufacturing method described above, since the diffractive lens 11 and the refractive lens 12 of the achromatic lens 10 are the same material, The same material can be used. The refractive lens material and the diffractive lens material may be the same material or different materials.

In the above embodiment, the diffractive lens and the refracting lens are separate lenses, and the achromatic lens 10 in which the planar back surfaces of the parallel diffractive lens 11 and the plano-convex refracting lens 12 are in contact with each other is illustrated. However, the present invention is not limited to this. For example, an achromatic lens having the following forms (a) to (e) may be used.
(A) If the diffractive lens and the refractive lens are arranged so as to share the optical axis, they may be arranged with the front surface and the back surface reversed. Further, the diffractive lens and the refractive lens may be disposed in close contact with each other or may be disposed apart from each other. By adjusting the distance between the diffractive lens and the refractive lens, chromatic aberration and focal length can be adjusted.
(B) The refractive lens is not limited to a plano-convex shaped refractive lens, and may be, for example, a biconvex lens, a biconcave lens, a meniscus lens, or the like.
(C) The diffraction lens is not limited to a parallel plane diffraction lens. For example, the diffractive lens may be formed on a convex surface or a concave surface.
(D) The back surface of the diffractive lens may be a convex surface or a concave surface.
(E) The achromatic lens may be a single lens in which a diffractive optical element and a refractive optical element are integrally formed.
(F) The achromatic lens is a single lens in which a diffractive optical element and a refractive optical element are integrally formed, and binary diffraction gratings may be formed on both the front and back surfaces of the refractive lens. .

  By combining the above (b), (c), (f) and these, the numerical aperture N.I. A. A large achromatic lens and a wide wavelength band can be realized.

  In the above, the achromatic lens 10 in which the diffractive lens 11 and the refractive lens 12 are separate lenses is exemplified, and the first manufacturing method and the second manufacturing method have been described, but the above (a) to (f) ) And an achromatic lens combining these, the first manufacturing method and the second manufacturing method can be used effectively, and the diffraction lens was measured at 1 nm intervals by an atomic force microscope (AFM). In this case, the rms value of the surface roughness can be 3 nm or less. In the first manufacturing method and the second manufacturing method, depending on the form of the achromatic lens, the template forming step is performed using the substrate of the diffractive lens material. However, the substrate formed of the refractive lens material is used. A person skilled in the art can naturally understand that the first template forming step may be performed. For example, in the case of manufacturing an achromatic lens in which a diffractive lens and a refractive lens are integrated, a first mold forming step is performed using a substrate formed of a refractive lens material, and then, by a laminating step, A diffractive lens material may be laminated on a refractive lens material substrate. Further, the refractive lens material and the diffractive lens material may be the same material or different materials.

  As mentioned above, although the Example of this invention was described in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

  The technical elements described in this specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or the drawings can achieve a plurality of objects at the same time, and has technical usefulness by achieving one of the objects.

DESCRIPTION OF SYMBOLS 10 Achromatic lens 11 Diffraction lens 12 Refractive lens 13 Joint surface 800,900 Board | substrate 802,803,804,805,902,903,904,905 Lens material layer 820,822,920,922 Resist 830,840,930,940 Masks 831, 841, 931, 941 Opening portions 832, 842, 932, 942 Light shielding portions 950, 952, 953 Silicon layers

Claims (11)

An achromatic lens comprising a refractive lens and a diffractive lens having a plurality of binary-shaped gratings,
The refractive lens is formed using a refractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride,
The diffractive lens is formed using a diffractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride,
The mean square deviation value (rms value) of the surface roughness of the grating of the diffractive lens is 5 nm or less,
An achromatic lens that corrects chromatic aberration in the vacuum ultraviolet region from 200 nm where the refractive index of lithium fluoride and magnesium fluoride changes greatly to the absorption edge wavelength on the short wavelength side. The lattice spacing of the plurality of binary-shaped gratings is wide on the side close to the optical axis of the achromatic lens and narrow on the side far from the optical axis of the achromatic lens,
2. The achromatic lens according to claim 1, wherein the plurality of binary-shaped gratings have a 2 ^× stepped shape that is partitioned by a plane perpendicular to the optical axis of the achromatic lens and a curved surface parallel to the optical axis.   The achromatic lens according to claim 1 or 2, wherein the value of the mean square deviation (rms value) of the surface roughness of the grating of the diffractive lens is 3 nm or less. The binary shape of the diffractive lens is
On the surface of the substrate formed of the refractive lens material or the diffractive lens material, an organic material mold mainly composed of a resin is formed,
Laminating the diffractive lens material on the surface of the substrate while heating the substrate at room temperature or 100 ° C. or less using the mold,
The achromatic lens according to any one of claims 1 to 3, which is formed by removing the template. The binary shape of the diffractive lens is
Forming a sacrificial layer as a template of an inorganic material mainly composed of silicon or silica (silicon oxide) on the surface of the substrate formed of the refractive lens material or the diffractive lens material;
The diffractive lens material is laminated on the surface of the substrate while heating the substrate at 200 ° C. or more and 1200 ° C. or less using the mold,
The achromatic lens according to any one of claims 1 to 3, which is formed by removing the sacrificial layer.   The achromatic lens according to any one of claims 1 to 5, wherein a binary diffraction lens is further formed on at least one of the front and back surfaces of the refractive lens.   A vacuum ultraviolet light source comprising the achromatic lens according to claim 1.   A spectroscopic measurement device comprising the achromatic lens according to any one of claims 1 to 6 and including a vacuum ultraviolet region in a measurement range.   A radical measuring device comprising the achromatic lens according to any one of claims 1 to 6 and including a vacuum ultraviolet region in a measuring range. A method of manufacturing an achromatic lens comprising a refractive lens and a diffractive lens,
A refractive lens forming step of forming a refractive lens using a refractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride;
A diffractive lens forming step of forming a diffractive lens using a diffractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride, and
The diffractive lens forming step includes:
A mold forming step of forming a mold of an organic material having as a main component on the surface of the substrate formed of the refractive lens material or the diffractive lens material;
A lamination step of laminating the diffractive lens material on the surface of the substrate by sputtering while heating the substrate at room temperature or 100 ° C. or less using the mold;
A method for producing an achromatic lens, comprising: a mold removing step of removing the mold after the heat treatment step. A method of manufacturing an achromatic lens comprising a refractive lens and a diffractive lens,
A refractive lens forming step of forming a refractive lens using a refractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride;
A diffractive lens forming step of forming a diffractive lens using a diffractive lens material that is at least one selected from the group consisting of lithium fluoride and magnesium fluoride, and
The diffractive lens forming step includes:
A sacrificial layer forming step of forming a sacrificial layer as a mold of an inorganic material mainly composed of silicon or silica (silicon oxide) on the surface of the substrate formed of the refractive lens material or the diffractive lens material;
A laminating step of laminating the diffractive lens material on the surface of the substrate while heating the substrate at 200 ° C. or more and 1200 ° C. or less using the mold;
And a sacrificial layer removing step of removing the sacrificial layer.

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