JPH05297209A - Manufacture of unequally spaced diffraction grating - Google Patents

Abstract

PURPOSE:To provide an unequal space diffraction grating that has been optimized in the desired diffraction orders respectively in the different regions inside the diffraction grating surface, and whose sectional shape has been formed in sawteeth. CONSTITUTION:In the first process, an unequally spaced diffraction grating (L') having a sawtooth-shaped sectional shape and in which the diffraction efficiency has been optimized with regard to higher order diffracted light is formed. In the second process, the sectional structure is divided by etching while keeping the sawtooth shape in an arbitrary region inside the diffraction grating surface to form a region where the diffraction efficiency of the lower order diffracted light has been optimized and also the desired diffraction angle has been achieved by the diffracted light, so that an unequal space diffraction grating (L) can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a non-equidistant diffraction grating having a saw-toothed cross-sectional shape, and particularly optimizes the diffraction efficiency of diffracted light of a desired order in different regions in the diffraction grating plane. The present invention relates to a method for manufacturing a non-uniformly spaced diffraction grating.

[0002]

2. Description of the Related Art It is conventionally known that diffraction efficiency can be improved by forming a diffraction grating into a sawtooth cross-sectional shape. FIG. 4 is a front view of a diffractive lens in which the cross-sectional shape is formed in a sawtooth shape and the distance is adjusted so as to have a lens action to process a diffraction grating, and FIG. 5 is a schematic cross-sectional view thereof. When the cross-sectional shape of the lens is completely serrated, a diffraction efficiency close to 100% can be achieved at a wavelength satisfying the blaze condition. Further, when giving a lens effect to the diffractive lens, it is necessary to make the diffraction angle near the outer circumference of the concentric circle larger than the diffraction angle near the inner circumference.
In this case, as shown in FIG. 5, the lattice spacing P2 in the region II near the outer periphery is equal to the lattice spacing P1 in the region I near the inner periphery.
It is generally smaller.

However, in the case of actually manufacturing such a diffractive lens, the grating interval P2 in the region II depends on the processing size accuracy of the processing apparatus, and therefore, there is a limit in lens design due to the processing accuracy limit. Was there. This limitation has been an obstacle to manufacturing a higher performance diffractive lens.

[0004]

By the way, the relationship between the diffraction angle of the diffraction grating and the grating spacing is as follows: incident light angle to the diffraction grating is θ, exiting light ray angle is θ ′, grating spacing is P, incident light wavelength is λ. Is given by the following equation (1). sin θ′−sin θ = mλ / P (1) where m is the diffraction order (0, ± 1, ± 2, ...). From this equation (1), if higher order diffracted light is used, the same relationship between the incident ray angle θ and the exit ray angle θ ′ when lower order diffracted light is used has a wider lattice spacing. You can see that it can be obtained with. That is, it is shown that the diffraction grating optimized with higher-order diffracted light can have a wider grating interval over the entire area of the diffraction grating, and the above-mentioned design limitation can be relaxed.

On the other hand, regarding the thickness of the diffraction grating, in the diffraction grating having a sawtooth cross-sectional shape as shown in FIG. 5, for example, the thickness of the diffraction grating, that is, the depth of the surface relief structure of the diffraction grating is extremely thin (shallow). Then, the diffraction efficiency of the diffracted light can be set to 100% regardless of the order within a certain order range. Therefore, if the diffraction grating is optimized with the highest order diffracted light within this range of orders, the restriction on processing accuracy will be relaxed due to the spread of the grating spacing, and optimization will also be performed for lower order diffracted light. Can be done, which is more advantageous in design. However, in reality, the diffraction grating optimized for high-order diffracted light has to have a large grating thickness, so that the diffraction efficiency decreases.

As described above, the diffractive lens utilizing the high-order diffracted light has both the advantage that the grating spacing can be widened and the disadvantage that the grating thickness becomes thick. As a method of improving the light-collecting efficiency of the lens and relaxing the design limitation, low-order diffracted light is used in a region where the lattice spacing is sufficiently wide and processing is ready, and in a region where the lattice spacing is narrow and processing is difficult. There is a method of using high-order diffracted light. This will be described with reference to FIG. 5. For example, the first-order diffracted light is used in a region I near the lens center where the grating spacing is wide, and the second-order diffracted light is used in a region II near the outer periphery where the grating spacing is narrow. By doing so, the lattice spacing in the region II can be processed at twice the spacing as compared with the case where it is optimized by using the first-order diffracted light in this region.
It is possible to improve the manufacturing margin. Also, region I
Since the first-order diffracted light is used in (1), the light-collecting efficiency can be made higher than in the case where the entire diffractive lens is configured with the second-order diffracted light.

Although it is significant to make it possible to utilize higher-order diffracted light in a partial region of the non-equidistant diffraction grating as described above, such non-equidistant diffraction grating is actually manufactured. There has never been a method.

The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to optimize the desired diffraction orders in different regions in the diffraction grating plane. Another object of the present invention is to provide a non-uniformly spaced diffraction grating having a sawtooth cross section.

[0009]

According to the method of manufacturing an unequal-spaced diffraction grating according to the present invention, unequal-spaced diffraction gratings having a sawtooth-shaped cross section satisfying a predetermined diffraction efficiency for diffracted light of an arbitrary order. A first step of forming a diffraction grating, and a first step
By dividing or combining the cross-sectional structure while maintaining the saw-tooth shape in any region of the non-equidistant diffraction grating formed in the step of d, the diffraction efficiency of lower-order or higher-order diffracted light is optimized. And a second step of forming a region where the desired diffraction angle is achieved by the diffracted light of the low order or the high order.

The second step of dividing or joining the sectional structure of the diffraction grating is to reduce the thickness of a partial region of the diffraction grating by etching or to increase it by depositing a suitable substance. It is characterized by a photolithography process.

[0011]

In order to improve the diffraction efficiency in a region where the grating interval P is sufficiently wide, in a non-equidistant diffraction grating in which the diffraction efficiency is optimized with higher-order diffracted light, when optimizing in this region, the light beam The diffracted angle θ of the m-th order diffracted light when it is incident perpendicularly on is given by the following equation (2) from the above equation (1). sin θ = mλ / P (2) If the diffraction grating having a sawtooth cross-sectional shape is optimized for the m-th order diffracted light, the grating thickness D at that time is
It is given by the following equation (3). D = mλ / (n−1) (3) where n is the refractive index of the substrate on which this diffraction grating is formed. Therefore, in order to form a region optimized by the first-order diffracted light without changing the diffraction angle θ, equation (2),
From (3), the lattice spacing P and the lattice thickness D in this region are 1 /
It may be multiplied by m.

FIG. 1 is a diagram for explaining the principle of the present invention. FIG. 1A shows a part of a non-equidistant diffraction grating in which the sectional shape formed in the first step is processed into a sawtooth shape. A schematic cross-sectional view showing a region, (b) shows a second cross section of this diffraction grating.
FIG. 3C is a schematic cross-sectional view of the diffraction grating showing the step of, and FIG. 7C is a schematic cross-sectional view showing a partial region of the diffraction grating formed in the second step. In the figure, L'is a diffraction grating formed in the first step, L '
Is the diffraction grating formed in the second step, P is the grating spacing, and D is
Is the grid thickness. Here, assuming that this diffraction grating is optimized by the diffracted light of the third order in the first step, in order to form an area optimized by the diffracted light of the first order, FIG.
The sectional structure of one period of the diffraction grating shown in (1) is divided as shown in FIG. 2B, and the portions 1 having the same shape may be formed at three locations along the serrated slope. At this time, the shape of the portion 1 corresponds to the sawtooth shape for one cycle shown in FIG. The bottom portion 2 of the portion 1 in FIG. 2B is arranged in the same plane as shown in FIG.
This corresponds to the grating pitch P and the grating thickness D of the diffraction grating formed in the first step multiplied by 1/3, respectively, and such an operation was performed from the above equations (2) and (3). It can be seen that the region is optimized by the first-order diffracted light without changing the diffraction angle.

Also, by the reverse process of FIGS. 9A to 9C, that is, by combining the sawtooth slopes of the adjacent gratings, for example, a diffraction grating optimized for low-order diffracted light. The region near the outer periphery can be optimized by higher-order diffracted light without changing the diffraction angle.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. First Embodiment FIG. 2 is a schematic cross-sectional view of a diffraction grating showing a step of forming a diffraction lens composed of diffraction gratings of unequal intervals according to the first embodiment. The diffractive lens of the present embodiment attempts to use the first-order diffracted light in the region I near the center with a wide lattice spacing and the second-order diffracted light in the region II near the outer periphery with a narrow lattice spacing. It is a thing.

First, as shown in FIG. 1A, a diffractive lens L'having a sawtooth cross-section whose diffraction efficiency is optimized for the second-order diffracted light is formed on a glass substrate. In the figure, 2d indicates the grating thickness. As a method of processing the cross-sectional shape into a sawtooth shape in this manner, for example, Japanese Patent Laid-Open No. 3-1
As disclosed in Japanese Patent No. 20501, a known method such as machining or machining with a convergent beam may be used.

In order to form a region optimized by the first-order diffracted light in a partial region of the diffractive lens optimized by the second-order diffracted light, the grating thickness and the grating interval are set in that region. Each may be multiplied by 1/2, and this is realized by a photolithography method. That is, the photoresist 3 is applied to the surface of the diffraction grating shown in FIG. 7A, and only the region I optimized by the first-order diffracted light has the same period as the underlying pattern as shown in FIG. To form a binary pattern with a duty ratio of 1: 1.

After this substrate is anisotropically etched to a depth of d and the resist is stripped off, finally, as shown in FIG. 3C, a region I near the center with a wide lattice spacing is formed.
1st order diffracted light is used, and the 2nd order diffracted light can be used in the outer peripheral region II where the grating spacing is narrow. Thus, a diffractive lens L with adjusted grating spacing and grating thickness is formed.

Second Embodiment FIG. 3 is a schematic sectional view of a diffraction grating showing a step of forming a diffractive lens composed of diffraction gratings of unequal intervals according to the second embodiment. The diffractive lens of this embodiment attempts to use the first-order diffracted light in the region I near the center with a wide grating spacing and the third-order diffracted light in the region II near the outer periphery with a narrow grating spacing. It is a thing.

First, as shown in FIG. 3A, a diffractive lens L'having a sawtooth cross-section whose diffraction efficiency is optimized for the third-order diffracted light is formed on a glass substrate.
In the figure, 3d indicates the lattice thickness. The method for processing the cross-sectional shape is the appropriate method described in the first embodiment.

In order to form a region optimized for the first-order diffracted light in a partial region of the diffractive lens optimized for the third-order diffracted light, the grating thickness and the lattice spacing are set in that region. Each one should be multiplied by 1/3. In order to realize this by the photolithography method, the photoresist 4 is applied to the surface of the diffraction grating shown in FIG. 7A, and only the region I optimized by the first-order diffracted light is used in FIG. ), A binary pattern having the same cycle as the background pattern and a duty ratio of 1: 2 is formed.

After this substrate is anisotropically etched to a depth of 2d and the resist is peeled off, a lattice pattern having a sectional shape shown in FIG. Furthermore, the same figure (c)
Photoresist 5 is applied to the surface of the pattern shown by, and 1
Only in the region I to be optimized by the next diffracted light, a binary pattern having the same cycle as the underlying pattern and a duty ratio of 1: 2 is formed, as shown in FIG. Then, after anisotropic etching of depth d is applied to this substrate, the resist is peeled off, and finally, in the region I near the center where the lattice spacing is wide, as shown in FIG. Diffracted light is used, and in the outer peripheral region II where the grating spacing is narrow, a diffractive lens L is formed in which the grating spacing and the grating thickness are adjusted so that the third-order diffracted light can be used.

In the above-described embodiment, anisotropic etching was used as a method for dividing the cross-sectional structure, but this step is performed by depositing a material having the same optical property as that of the substrate so that the sawtooth shape of the adjacent lattice is formed. Similar results can be obtained by replacing the slopes of the above step. Further, in the present invention, there is no limitation on the method of processing the cross-sectional shape of the diffraction grating into a sawtooth shape.

[0023]

As described above, according to the present invention, no matter what method is used to process the cross-sectional shape of the diffraction grating into the sawtooth shape, the diffraction efficiency is optimized in the desired region with the diffracted light of the desired order. It is possible to manufacture a nonuniformly spaced diffraction grating. This indicates that the conventional manufacturing process requires little modification. Further, according to the present invention, it is possible to design a non-equidistant diffraction grating with a specific order and then change the diffraction order in an arbitrary area, although the diffraction order varies depending on the area in the diffraction grating surface. Therefore, the troublesomeness in design can be reduced. Therefore, it is possible to easily provide the unequal-spaced diffraction gratings each having a desired diffraction order and optimized in different regions in the diffraction grating surface, and the cross-sectional shape of which is processed into a sawtooth shape.

[Brief description of drawings]

1 (a) to 1 (c) are respectively unequal shapes in which a sectional shape is processed into a sawtooth shape, showing a process in which a partial region of a diffraction grating is optimized by diffracted light of an arbitrary order according to the present invention. It is a schematic sectional drawing of an interval diffraction grating.

FIGS. 2A to 2C are steps of optimizing a partial region of a diffraction grating optimized with second-order diffracted light according to the first embodiment of the present invention with first-order diffracted light. FIG. 3 is a schematic cross-sectional view of a non-equidistant diffraction grating whose sectional shape shown in FIG.

3 (a) to (e) are steps of optimizing a partial region of a diffraction grating optimized with a 3rd order diffracted light according to the second embodiment of the present invention with a 1st order diffracted light. FIG. 3 is a schematic cross-sectional view of a non-equidistant diffraction grating whose sectional shape shown in FIG.

FIG. 4 is a front view of a diffractive lens in which the cross-sectional shape is formed in a sawtooth shape and a diffraction grating is processed by adjusting an interval so as to have a lens effect.

5 is a schematic cross-sectional view of the diffractive lens shown in FIG.

[Explanation of symbols]

1 ... Site 2 ... Bottom 3, 4, 5 ...
-Resist L '... Diffraction grating formed in first step L ... Diffraction grating formed in second step

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[Procedure amendment]

[Submission date] December 2, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction content]

FIG. 1 is a diagram for explaining the principle of the present invention. FIG. 1A shows a part of a non-equidistant diffraction grating in which the sectional shape formed in the first step is processed into a sawtooth shape. A schematic cross-sectional view showing a region, (b) shows a second cross section of this diffraction grating.
FIG. 3C is a schematic cross-sectional view of the diffraction grating showing the step of, and FIG. 7C is a schematic cross-sectional view showing a partial region of the diffraction grating formed in the second step. In the figure, L'is a diffraction grating formed in the first step, L '
Is the diffraction grating formed in the second step, P is the grating spacing, and D is
Is the grid thickness. Here, assuming that this diffraction grating is optimized by the diffracted light of the third order in the first step, in order to form an area optimized by the diffracted light of the first order, FIG.
The sectional structure of one period of the diffraction grating shown in 1 is divided as shown in FIG. 2B, and the portions 1 having the same shape may be formed at three locations so that the bottom portions 2 are aligned in the same plane. At this time, the shape of the part 1 is 1/3 of the sawtooth shape for one cycle in FIG.
It corresponds to the size reduced to. The bottom part 2 of the part 1 in FIG. 2 (b) is aligned in the same plane.
However, this corresponds to the grating spacing P and the grating thickness D of the diffraction grating formed in the first step multiplied by 1/3, respectively, and from the above equations (2) and (3), It can be seen that the region that has been subjected to such an operation is optimized by the first-order diffracted light without changing the diffraction angle.

Claims (2)

[Claims] 1. A first step of forming a non-equidistant diffraction grating having a sawtooth cross-sectional shape that satisfies a predetermined diffraction efficiency for diffracted light of an arbitrary order, and the first step. By dividing or combining the cross-sectional structure while maintaining the saw-tooth shape in any region of the formed nonuniform grating, the diffraction efficiency of lower-order or higher-order diffracted light is optimized, and A second step of forming a region in which a desired diffraction angle is achieved by the low-order or high-order diffracted light. 2. A second step of dividing or joining the sectional structure of a diffraction grating is photolithography, in which the thickness of a partial region of the diffraction grating is reduced by etching, or increased by depositing a suitable material. The manufacturing method according to claim 1, which is a process.