JP3495925B2 - Optical element manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical element in which periodic structures made of different materials are superposed.

[0002]

2. Description of the Related Art Conventionally, correction of chromatic aberration of an optical system has been performed by combining optical elements made of glass materials having different dispersions, and a diffractive optical system is used instead of a refractive optical system such as a lens. That is the document SPIE, Vo
l. 1354, volumes 24-37.

When a diffractive surface having a diffraction efficiency is added to an optical system having a spectral characteristic, it is important to keep the diffraction efficiency in the wavelength range to be used high. However, light of orders other than the designed order appears as defocus because the diffraction angle becomes larger as the order becomes farther away, and the difference in focal length becomes larger, and especially when there is a high-luminance light source, side lobes are present. May be seen.

A diffractive optical element having a two-layer structure can significantly reduce the diffraction efficiency in the vicinity of the designed order within the wavelength range of use in an optical system having a chromatic aberration correction effect. By doing so, a considerable improvement in image quality and information can be expected. But,
Although this diffractive optical element has improved optical performance, it has a problem that the manufacturing method is very complicated and the cost is high, and it is difficult to put it into practical use.

For this reason, a diffractive optical element having a multilayer structure of two or more layers is formed by a photolithography method which is a semiconductor manufacturing process, and this photolithography method uses a photosensitive resin called photoresist as a protective film. After the fine patterning, the etching process is performed to transfer the developed photoresist pattern onto the substrate.

FIG. 10 shows a schematic cross-sectional view of a manufacturing process of a stepped grating having an eight-step structure by a conventional photolithography method. In FIG. 10A, a photoresist 2 is applied to the substrate 1 by a spinner, and then a light flux L is irradiated to perform pattern exposure. In FIG. 10 (b), development, rinsing, and post-baking are performed, and after etching in FIG. 10 (c), cleaning is performed to remove the residual photoresist 2,
A two-step staircase shape is formed. In FIGS. 10 (d) to 10 (f), the same process as in FIGS. 10 (a) to 10 (c) is performed to form a four-step staircase shape, and the process is repeated in FIGS. 10 (g) to 10 (i). As a result, an 8-step staircase shape as shown in FIG. 10 (j) is completed.

A method for manufacturing a diffractive optical element having a two-layer structure, to which the method shown in FIG. 10 is applied, is known.
2 shows a sectional view of a step of forming a desired diffractive optical element having a two-layer structure as a diffractive lens. In FIG. 11 (a), the organic coating material 4 is placed on the quartz glass substrate 3, covered with the first mold 5 made of quartz glass, and irradiated with ultraviolet rays.
As described above, the staircase shape is formed by the organic coating material 4.
After that, ion etching is performed in FIG. 11C to form a silica glass diffraction grating 6 as shown in FIG. 11D.

In FIG. 12 (e), a TiO ₂ film 7 is formed on the quartz glass diffraction grating 6, and in FIG.
The organic coating material 4 is placed on the O ₂ film 7, and the second mold 8 made of quartz glass whose step shape is opposite to that of the first mold 5 is covered and irradiated with ultraviolet rays as shown in FIG. 12 (g). Then, the stepped shape of the organic coating material 4 is formed. Ion etching is performed again in FIG. 12 (h) to form a high-dispersion diffraction grating 9 of a TiO ₂ film on the silica glass diffraction grating 6 as shown in FIG. 12 (i).

[0009]

However, in the above-mentioned manufacturing method, in order to manufacture a diffractive optical element having a desired two-layer structure, the number of manufacturing steps is increased and the manufacturing cost is increased. It is difficult to popularize it as a device element.

A lithography method, which is an exposure method for transferring a normal planar structure, is also known.
If the height of the three-dimensional shape, which is the shape condition of the diffractive optical element, cannot be controlled to the desired distribution in each step, and if there is a different height (thickness) setting in the end, repeat the photolithography method multiple times. It is necessary to complete the desired lattice structure. Further, the height (thickness) of the diffraction grating formed on the second upper layer in FIG. 11 is not uniform with respect to the diffraction grating on the lower first layer, so that it is necessary to perform multiple steps. There's a problem.

An object of the present invention is to solve the above problems,
The present invention relates to an optical element manufacturing method for manufacturing a highly accurate optical element.

[0012]

A two-layer diffractive optical element according to the present invention for achieving the above object is an optical element in which a second periodic structure is formed on a first periodic structure by molding. The method is characterized in that the alignment mark on the substrate on which the first periodic structure is formed is used to perform alignment of a mold for molding the second periodic structure.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail with reference to the embodiments shown in FIGS. FIG. 1 shows the eighth embodiment.
A cross-sectional view of a diffractive optical element having a step structure is shown, showing a glass substrate 1.
A first diffraction grating (first periodic structure) 11 is formed on 0, and a second diffraction grating (second periodic structure) 12 is formed on the first diffraction grating 11. The first diffraction grating 11 is formed of a photo-curing resin whose main component is a modified epoxy acrylate having a high refractive index and large dispersion, and the diffraction grating 12 of the second layer is formed.
Is formed of a low-dispersion acrylate-based UV curable resin. The selection of each resin is determined by the combination of two or more kinds of resin materials depending on the optical design, and therefore can be arbitrarily selected depending on the application, and the order of stacking can also be arbitrarily selected.

As a design example of a two-layer composite diffractive optical element,
The following resins are used. Main component of first layer material: Refractive index after curing modified epoxy acrylate 1.598 Abbe number 28 Main component of second layer material: Refractive index after curing urethane modified polyester acrylate 1.525 Abbe number 50.8

Here, in order to have a lattice structure in which the luminous flux in the used wavelength region is concentrated in a specific order, for example, a wavelength of 486.
It is necessary to obtain high diffraction efficiency in the C line of 13 mm and the F line of 656.27 nm, and the shape must satisfy the following conditions. 656.27 / 8 = | (Na _F −1) · da− (Nb _F −1) · db | 486.13 / 8 = | (Na _C −1) · da− (Nb _C −1) · db |

Where Na _F is the refractive index of the first layer for F rays, Nb _F is the refractive index of the second layer for F rays, Na _C is the refractive index of the first layer for C rays, and Nb _C is the second layer for C rays. Is the step difference of the diffraction grating of the first layer db is the step difference of the diffraction grating of the second layer

The geometrical dimensions for satisfying such conditions are as follows, for example. da = 2355nm db = 2818nm

FIG. 2 is a sectional view of the manufacturing process of the two-layer composite diffractive optical element. First, a silane coupling is applied to the surface of the quartz glass substrate 10 for forming the first diffraction grating 12 in FIG. 2 (a). After uniformly applying with a spinner, it is dried in an oven. The coupling is a substrate 1 made of quartz glass at the time of release.
0 and the adhesiveness between the first layer resin material 13 shown in FIG. It has a function of sufficiently fixing the resin material 13 thus obtained onto the quartz glass substrate 10.

In FIG. 2 (b), the first mold 14 of quartz glass designed so that the quartz glass has a desired shape is used for molding.
Irradiate ultraviolet rays to cure. Then, when the first mold 14 is released, the first diffraction grating 11 is formed as shown in FIG. 2 (c).

Next, in FIG. 2D, the resin material 15 of the second layer is placed on the first diffraction grating 11 and is molded and cured by the second mold 16 in the same manner as described above. As a result, FIG.
A two-layer composite diffraction grating as shown in (e) is formed.

The diffraction grating 12 of the second layer needs to be aligned with the pitch of the diffraction grating 11 of the first layer with high accuracy, and if there is a deviation at this time, the correction effect designed in units of pitch only decreases. Not only that, the original diffraction is disturbed and the function as a lens is lost. Therefore, in this embodiment, as shown in FIG. 3 and FIG.
The alignment mark 11 a is previously transferred onto the substrate 10 by the concave shape 14 a of the second mold 16
By aligning the concave shapes 16a, the highly accurate alignment is performed.

When molding the diffraction grating 12 of the second layer,
If the thickness of the optical resin material 15 is smaller than the height of the diffraction grating 11 of the first layer, the second mold 16 has to be fitted to the UV-cured resin grating structure on quartz, and the alignment accuracy and Not only is it difficult to set the fitting dimension, but also the second mold 16 and the diffraction grating 11 of the first layer may be damaged. In order to avoid this, theoretically, the order of layers does not affect the design of the diffractive optical element, so that the layer having a small grating height is set on the base side.

Since the diffraction grating 12 of the second layer must be in complete contact with the diffracted photons 11 of the first layer after releasing, so that the adhesive strength between the resins is sufficient for releasing. The mold is treated with a mold release agent to improve the mold releasability. And, the mold release treatment, after soaking in a sufficiently diluted mold release agent,
It is necessary to take care so that excess mold release agent does not disturb the fine shape by steam cleaning or the like.

The thickness of the resin molded into two layers is actually thicker than the total thickness of each layer, and even if the viscosity is reduced by heating or pressure molding is performed, the thickness of the lattice portion alone is not obtained. . However, if the thickness in the plane of the grating is uniform, the effect on the entire transmitted light flux will be equal and there will be no hindrance to the performance as a diffraction grating, so it is possible to make the thickness of each layer uniform. It becomes important. Further, since the angle of view becomes large in a diffractive optical element having a short focal point, if the resin thickness is large, the direction of the light beam shifts inside the element due to the angle of view, and the correction effect is reduced. Therefore, it is important to design the element by making the resin thickness as small as possible and sufficiently considering the angle of view and the resin thickness.

A mold of resin material made of quartz is manufactured by a photolithography process. For example, when the shape of the mold to be transferred is a step-like lattice, the photolithography process can be repeated a plurality of times as in the conventional example shown in FIG. Further, since generally high refractive resins have poor weather resistance, it is important for determining the constitution to use a relatively stable resin for the second layer so as to maintain the performance.

FIG. 5 is a sectional view of the second embodiment, in which a diffraction grating 17 formed by etching lanthanum glass is
The replica diffraction grating 12 is molded.

Main component of the first layer material: lanthanum glass Refractive index 1.678 Abbe number 55.3 Main component of the second layer material: modified epoxy acrylate Refractive index after curing 1.598 Abbe number 28

The lanthanum glass is processed by a method of performing photolithography and a method of forming a coating material on a lanthanum glass substrate and performing anisotropic etching to transfer the shape to the surface of the lanthanum glass. The latter method is advantageous in consideration of sex. In any of the methods, the diffraction grating 17 is formed, and at the portion other than the diffraction grating 17,
By forming the alignment mark 11a as shown in FIGS. 3 and 4 and aligning the mold of the replica diffraction grating 12 with the alignment mark 11a, highly accurate alignment is performed.

Similar to the first embodiment, the second diffraction grating is made of resin. At this time, when the shape satisfying the correction condition is calculated, it becomes as follows.

Lattice step of the first layer: da = 2042 nm Second layer grating step: db = 2204nm

With respect to glass materials other than lanthanum glass, a desired diffractive optical element can be manufactured by the same optical design.

Further, the present invention can be applied to a two-layer diffractive optical element having a blazed type grating structure, and the combination of the materials of the first embodiment satisfies the following relationship. The effect of the two layers can be maintained. Λ _D , λ _F , and λ _C are D line, F line, and
The wavelength of C line, Nb _D is the refractive index of the second layer for D line.

Mλ _D = (Na _D −1) · da− (Nb _D −1) · db mλ _F = (Na _F −1) · da− (Nb _F −1) · db mλ _C = (Na _C − 1) ・ da- (Nb _C -1) ・ db

As an example, the height is set as follows, and the mold is cut and processed by a diamond cutting tool.

[0035] Height of first layer grating: h1 = 21.07 μm Height of second layer grating: h2 = 22.54 μm

FIG. 6 is a sectional view of a triangular wave-shaped two-layer diffractive optical element of the third embodiment. In the second embodiment, with respect to the replica diffraction grating 12 formed on the diffraction grating 17 formed by etching lanthanum glass, the triangular diffraction grating 18 is formed instead of the stepped grating.
The fine processing of the glass can be achieved by forming the photoresist into a triangular wave shape calculated from the etching rate and then performing anisotropic etching. After that, as in the second embodiment, the second diffraction grating 19 is formed on the resin. To mold.

FIG. 7 is a front view of the two-layer diffractive optical element 20. Only one pitch boundary of one grating is shown by a solid line, and the boundary line of the staircase structure within one grating pitch is omitted. Since the two-layer diffractive optical element 20 can increase the diffraction efficiency of the diffracted light of the designed order over the entire wavelength range used, it can exhibit good optical performance. It is to be noted that the sawtooth cross section is approximated by an 8-level staircase structure.
It is possible to manufacture with an approximation other than the level, for example, 4 level or 16 level.

FIG. 8 is a front view of a camera having the two-layer diffractive optical element 20, and FIG. 9 is a side view. Camera body 2
1, a photographing optical system 22 and a finder optical system 23 are arranged, and the two-layer diffractive optical element 20 can be provided at any position in the photographing optical system 32 and the finder optical system 33. In this way, by using the two-layer diffractive optical element 20 in the optical system of an optical device such as a camera, the optical performance of the optical device can be improved.

[0039]

As described above, the method of manufacturing an optical element according to the present invention has a low cost when the second periodic structure is formed by using the mold on the substrate on which the first periodic structure is formed. Therefore, high-performance devices can be mass-produced.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a two-layer step diffraction grating.

FIG. 2 is a cross-sectional view of the manufacturing method.

FIG. 3 is an explanatory diagram of alignment marks.

FIG. 4 is an explanatory diagram of alignment marks.

FIG. 5 is a sectional view of a two-layer diffractive optical element made of glass and resin.

FIG. 6 is a sectional view of a two-layer blazed diffraction grating.

FIG. 7 is a front view of a two-layer diffractive optical element.

FIG. 8 is a front view of a camera having a two-layer diffractive optical element.

FIG. 9 is a side view.

FIG. 10 is a sectional view of a method for manufacturing a conventional eight-step stepped diffraction grating.

FIG. 11 is a cross-sectional view of a method for manufacturing a two-layer 8-step stepped diffraction grating.

FIG. 12 is a cross-sectional view of a method for manufacturing a two-layer 8-step stepped diffraction grating.

[Explanation of symbols]

10 Quartz glass substrate 11, 12, 17 Step diffraction grating 13,15 Resin material 14, 16 Mold 18, 19 triangular wave diffraction grating 20 Two-layer diffraction grating 21 camera 22 Shooting optical system 23 Finder optical system

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-9-127322 (JP, A) JP-A-7-108612 (JP, A) JP-A-8-29769 (JP, A) JP-A-59- 143110 (JP, A) Special Table 10-513575 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02B 5/18 G02B 3/08

Claims (6)

(57) [Claims] 1. A method of manufacturing an optical element, comprising forming a second periodic structure on a first periodic structure by molding, comprising using an alignment mark on a substrate on which the first periodic structure is formed. A method of manufacturing an optical element, which comprises performing alignment of a mold for molding the second periodic structure. 2. The method of manufacturing an optical element according to claim 1, wherein the alignment is performed by fitting a predetermined portion of a mold for molding the second periodic structure to the alignment mark. 3. The method of manufacturing an optical element according to claim 1, wherein the alignment mark is a convex portion provided on a substrate on which the first periodic structure is formed. 4. The method of manufacturing an optical element according to claim 1, wherein the height of the first periodic structure is higher than the height of the second periodic structure. 5. The method for manufacturing an optical element according to claim 1, wherein the first and second periodic structures are diffraction gratings. 6. An optical element manufactured by the method according to any one of claims 1 to 5.