JPH05150107A - Irregularly spaced diffraction grating - Google Patents

Abstract

PURPOSE:To improve diffraction efficiency under the limitation of the min. plotting size by a producing apparatus. CONSTITUTION:The gratings are formed of 4 stages of staircase shapes to an approximately saw tooth shape at the narrow inter-grating spacing Pb near the outermost periphery of the diffraction type lens consisting of the irregularly spaced diffraction gratings. The gratings are formed of 8 stages of the staircase shapes to the approximately saw tooth shape at the relatively wide inter-grating spacing Pb near the center. The min. working size is larger than the staircase width of the inter-grating spacing Pb. As a result, the diffraction efficiency of the grating region increased in the number of the staircases is improved and the improvement in the diffraction efficiency over the entire part of the lens and the decrease in detrimental light are attained. The performance of the irregularly spaced diffraction gratings and the degree of freedom in designing the diffraction gratings are improved under the present fine working technique.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-uniformly spaced diffraction grating having a grating cross section approximated to a sawtooth shape by a step shape.

[0002]

2. Description of the Related Art In recent years, diffractive optical elements have been used in a wide range of fields, and accordingly, a diffraction grating having higher diffraction efficiency has been required. FIG. 3 is a front view of a diffractive lens constituted by concentric non-equidistant diffraction gratings in which the grating intervals are not uniform and are appropriately adjusted so as to have a lens effect. In order for such a non-equidistant diffraction grating to function as a 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 order to realize this, it is necessary to form the lattice spacing in the outer peripheral region smaller than the lattice spacing in the inner peripheral region.

FIG. 4 is a sectional view showing a case where the diffraction grating pattern surface of this diffractive lens is processed into a sawtooth shape. FIG. 9A shows a partial cross-sectional view of the diffractive lens in the region E near the center of the concentric circle, and the lattice spacing Pa is sufficiently large. Further, FIG. 6B shows a partial cross-sectional view of the diffractive lens in the outermost peripheral region F, in which the lattice spacing Pb is the smallest. Then, as shown in this figure, when the cross-sectional shape of each grating is completely saw-toothed, it is 10 at a wavelength satisfying the blaze condition.
A diffraction efficiency close to 0% will be obtained.

However, since the diffraction grating is extremely fine and forms several thousand or more gratings at an interval of 1 mm, it is a technical technique to form its cross-section into a completely serrated shape. Is impossible. Therefore, the grating cross section is formed in a stepped shape to approximate a sawtooth shape, and the diffraction efficiency is improved as much as possible. Various proposals have been made regarding a diffraction grating whose cross-sectional shape is processed into a shape closer to a sawtooth shape in order to improve diffraction efficiency. For example, in Japanese Unexamined Patent Publication No. 2-1109, the diffraction gratings at equal intervals are formed in an eight-step staircase shape to form an approximately sawtooth shape. Then, as a method for realizing the diffraction grating having this structure, a photolithography process using a laser writer is performed.
A method of performing the times is disclosed. As described above, in order to manufacture a diffraction grating having a fine substantially sawtooth cross-sectional shape, a photolithography method is widely used, and a device for making the cross-sectional shape close to a sawtooth shape has been made in order to improve diffraction efficiency. ..

However, the diffraction grating in this conventional technique is one in which the gratings are formed at equal intervals. Since the lattice spacing of the non-equidistant diffraction grating differs depending on the region, this structure cannot be adopted as it is, particularly in relation to the region where the lattice spacing is narrow. A conventional diffractive lens using a non-uniform spacing diffraction grating is configured as shown in FIG. That is, the conventional diffractive lens of this type is configured by approximating the gratings of the entire area of the diffraction grating into a saw-tooth shape by four steps, and the area E near the center of the concentric circle shown in FIG. , The lattice spacing Pa is sufficiently large, and the lattice spacing Pb in the outermost region F shown in FIG. Therefore, the widths of the steps of each stairway in these two areas are significantly different.
When the saw-tooth shape is formed approximately like this, the light-collecting efficiency is lowered to 81% as compared with the above-mentioned diffractive lens of the ideal unequal-spaced diffraction grating.

Theoretically, the light collection efficiency can be improved by increasing the number of steps in one lattice interval.
When actually manufacturing a diffraction grating by photolithography, processing limitations occur. For example, when a resist pattern is drawn using a laser beam, the limit of drawing a fine pattern is determined by the diameter of the spot of the laser beam. In other words, the upper limit of the number of steps in the case of approximating a sawtooth shape with a staircase shape is determined by the condition that the width of one step (with the lattice spacing) is equal to or larger than the spot diameter of the laser beam. Therefore, in the actual fabrication of the unequal-spaced diffraction grating, it is possible to increase the number of steps of the stairs to a certain number or more even though the diffraction efficiency is lowered and the optical characteristics of the diffraction grating are insufficient. It is usually not possible.

Under such a condition that the number of steps of the stairs is determined from the processing limit of the manufacturing apparatus, the number of steps of the stairs, that is, the diffraction efficiency is limited as shown in FIG. 5 (B). It is a pattern in the vicinity of the outermost periphery where the lattice spacing is smaller and finer processing is required. In this case, the minimum processing dimension of the diffractive lens in which the sawtooth-shaped lattice spacing is approximately formed by four steps is shown by using the lattice spacing Pb in the outermost peripheral region F where the lattice spacing is the smallest. Then, it becomes about Pb / 4.

[0008]

By the way, in a diffraction grating having a low diffraction efficiency, diffracted light generated in many orders other than the desired order has a non-negligible intensity. This unnecessary diffracted light often acts as harmful light, which causes deterioration of the performance of the diffraction grating or the entire optical system including the diffraction grating. When the sawtooth shape is approximately formed by steps, the width of one step of the steps is made smaller and the number of steps is increased, so that the steps become closer to the sawtooth shape and higher diffraction efficiency can be obtained. .. However, when manufacturing such a staircase shape by the method of photolithography, there is a limit to the minimum drawing size depending on the manufacturing apparatus, and therefore there is a limit to the number of steps that can be manufactured within one lattice interval.

In view of the above problems, the present invention is capable of improving the diffraction efficiency and the performance of the diffraction grating even when the above-mentioned minimum drawing size is limited in manufacturing the diffraction grating. It is an object to provide an evenly spaced diffraction grating.

[0010]

In the unequal-spaced diffraction grating, the limitation of the number of steps of the stairs by the above-described manufacturing apparatus is not necessarily uniform in the entire area of the diffraction grating, and in the area where the grating spacing is wide, Furthermore, there is a processing margin to increase the number of steps of the stairs. The present invention has been made by paying attention to this point, and the unequal-interval diffraction grating according to the present invention has at least two non-equidistant diffraction gratings each having a grating cross section similar to a sawtooth shape by a step shape. It is characterized in that the number of staircase-shaped steps is different from each other in certain lattice areas. The number of staircase-shaped steps in the lattice region with a wide lattice spacing is larger than the number of staircase-shaped steps in the lattice region with a narrow lattice spacing.

By making the number of steps of the stairs different according to the position of the grating region, it is possible to more approximate the sawtooth shape, and the diffraction efficiency is improved. In particular, the diffraction efficiency of the entire diffraction grating can be improved by increasing the number of steps of the stairs in the grating region having a wide grating interval so as to more approximate the sawtooth shape.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to FIG. FIG. 1 is an enlarged cross-sectional view of a main part of a diffraction grating of a diffractive lens. In the area E near the center of the diffraction grating shown in FIG. 4A, the stepped shape forming the substantially sawtooth shape with one grating spacing Pa is from 4 steps to 8 steps in the prior art.
The number of steps has been increased. Further, in the region F near the outermost periphery of the diffraction grating shown in FIG. 3B, the stepped shape forming the substantially sawtooth shape with one grating interval Pb is 4 as in the conventional technique.
It is composed of steps.

If the relation between the lattice spacing Pa in the region E near the center and the lattice spacing Pb in the region F near the outermost circumference is set as Pa> 2 × Pb, the lattice spacing Pa in the region E will be 8 steps. The width of the steps when formed by steps, that is, the minimum processing dimension is Pa / 8> Pb / 4. Therefore, in the area inside the area E that satisfies this condition, the minimum processing dimension for setting the substantially sawtooth-shaped lattice by the steps of 8 steps is the minimum processing size when the vicinity of the outermost periphery is formed by the steps of 4 steps. It becomes larger than the dimension Pb / 4. Therefore, in a region satisfying the above-mentioned condition, that is, a region inside the annular zone satisfying the condition that the lattice spacing P is P> 2 × Pb, the conventional diffractive lens constructed as shown in FIG. 5 is manufactured. Even if the manufacturing apparatus, which was the limit of the above, is used, it is possible to manufacture a substantially sawtooth-shaped region with eight steps, and it is possible to easily realize the diffraction type lens of the diffraction grating as shown in this embodiment. In the area E near the center, when the sawtooth shape with one lattice spacing is approximately configured with 8 steps, the diffraction efficiency in that area is 9
Since it is 5%, the diffraction efficiency is improved by 14% and the light collection efficiency is improved in the region where the number of steps of the staircase shape is eight.

As described above, according to this embodiment, a sawtooth shape can be partially formed in a stepped shape of eight steps using a conventional diffraction grating manufacturing apparatus, and the diffraction efficiency of the entire lens can be improved. Improvement and reduction of harmful light can be achieved. Therefore, conventionally,
The problem of the limit of microfabrication, which has been an obstacle in manufacturing, is improved, and the degree of freedom in designing the diffraction grating is increased, which makes it possible to develop a higher performance optical system.

Next, a structural example of an optical system using the diffractive lens according to this embodiment will be described with reference to FIG. Figure 2
FIG. 3 is a configuration diagram of a projection optical system using a diffractive lens according to the present embodiment, and an object 1 is illuminated by an illumination system 2 to generate diffracted light 3. The diffracted light 3 is composed of a 0th-order diffracted light portion 3a near the center (optical axis) and a diffracted light portion 3b other than the 0th-order diffracted light in the outer peripheral area thereof. A projection lens 4 having a plurality of lenses is arranged on the side of the object 1 opposite to the illumination system 2, and a diffractive (convex) lens 5 which is an embodiment of the present invention is located at the pupil position thereof. Further, a diaphragm 6 for limiting the light flux 3 is arranged on the side of the object 1 adjacent to the diffractive lens 5, and an image of the object 1 is projected onto an image plane 7 by a projection lens 4.
It is projected on.

Here, the diffraction grating pattern surface 5a provided on the image surface 7 side of the diffractive lens 5 has a central region C which is approximately formed by a stepped shape of eight saw teeth,
The outer peripheral region D has a saw-tooth shape which is approximately formed by four steps. Then, at least the 0th-order diffracted light portion 3a of the object 1 is transmitted through the central region C near the optical axis of the diffraction grating pattern surface 5a, and the diffracted light portion 3b other than the 0th-order diffracted light is transmitted through the peripheral region D outside the central region C. Will be done. Also, the diffractive lens 5 is
Together with the optical system before and after that, it has a function of keeping the aberration balance of the entire projection lens 4 good.

Since the present embodiment is configured as described above, the diffracted light 3 generated by the object 1 by being illuminated by the illumination of the illumination system 2 is diffracted by the diaphragm 6 and the diffracted light portion 3 other than the 0th diffracted light is generated.
b is limited, passes through the projection lens 4 including the diffractive lens 5, and an object image is formed on the image plane 7.

Here, in the peripheral area D of the diffraction grating pattern surface 5a of the diffractive lens 5, the number of steps of the substantially sawtooth step is smaller than the number of steps of the central area C, so that the diffraction efficiency is inferior to the area C. Therefore, in this region D, diffracted light other than the desired diffraction order creates flare, and can become harmful light that reduces the contrast of the image on the image plane 7. However, the energy of the light beam 3b passing through the region D is overwhelmingly smaller than the energy of the light beam 3a near the center.
That is, since the intensity of the diffracted light by the object 1 is overwhelmingly higher than that of the other diffracted light, the diffracted light intensity does not include the zero-order diffracted light at all as compared with the energy of the light flux 3a including all the zero-order diffracted light. The energy of the light beam 3b passing through the peripheral region D which does not exist is extremely small. Therefore, the energy of the harmful light generated in the peripheral area D is practically negligible, so that the image contrast is not deteriorated.

As described above, according to the present embodiment, by disposing the diffractive lens 5 according to the present invention, the influence of flare is hardly exerted, the light collection efficiency can be improved, and a clearer image is projected. be able to.

Although the diffractive lens 5 is arranged at the pupil position of the lens system in the above embodiment, the invention is not limited to this, and the diffractive lens 5 is arranged at a position different from the pupil position of the projection lens 4. It is also possible to let. However, in this case, the area of the region through which the light flux 3b passes is relatively larger than that of the region through which the light flux 3a passes, so that the peripheral region D
Will be reduced relative to the central region C.
Therefore, the peripheral region D having a small number of steps and easy to manufacture is reduced and the central region C having a large number of steps is increased, resulting in an increase in manufacturing cost. Therefore, it is most preferable to arrange the diffractive lens 5 near the pupil position of the projection lens 4.

In the above-mentioned embodiment, the peripheral region D has a stepwise shape of four steps and the central region C has a stepwise shape of eight steps with respect to the diffraction grating pattern surface of the diffractive lens.
Without being limited to this, an appropriate number of different stairs can be set, and in short, the number of stairs with one lattice spacing in the peripheral region and the number of stairs with one lattice spacing in the central region may be different.

[0022]

As described above, the uneven diffraction grating according to the present invention is configured to have a sawtooth shape by varying the number of steps in at least two grating regions.
Conventionally, the problem of the limit of fine processing, which has been an obstacle in manufacturing, is solved, and the optical performance of the diffraction grating having a cross-sectional shape similar to a sawtooth shape can be easily improved by the step shape. Since this increases the degree of freedom in designing the diffraction grating, it becomes possible to develop a higher-performance optical system using this diffraction grating.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view of a diffractive lens that is an embodiment of the present invention, in which (A) is a cross-sectional view near the center,
(B) is a cross-sectional view near the outermost periphery.

FIG. 2 is a schematic configuration diagram of a projection optical system using a diffractive lens that is an example of the present invention.

FIG. 3 is a schematic front view of a general diffractive lens.

FIG. 4 is a partial cross-sectional view of a diffractive lens having an ideal diffraction grating pattern surface, (A) is a cross-sectional view near the center,
(B) is a cross-sectional view near the outermost periphery.

5A and 5B are partial cross-sectional views of a conventional diffractive lens having a non-equidistant grating, where FIG. 5A is a cross-sectional view near the center and FIG. 5B is a cross-sectional view near the outermost periphery.

[Explanation of symbols]

 5 Diffractive lens C Central area D Peripheral area

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[Procedure amendment]

[Submission date] March 16, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction content]

However, since the diffraction grating is extremely fine and forms several thousand or more gratings at an interval of 1 mm, it is a technical technique to form its cross-section into a completely serrated shape. Is impossible. Therefore, the grating cross section is formed in a stepped shape to approximate a sawtooth shape, and the diffraction efficiency is improved as much as possible. Various proposals have been made regarding a diffraction grating whose cross-sectional shape is processed into a shape closer to a sawtooth shape in order to improve diffraction efficiency. For example, in Japanese Unexamined Patent Publication No. 2-1109, the diffraction grating has an approximately sawtooth shape by forming each grating cross section into eight steps. Then, as a method of realizing a diffraction grating having this structure, a method of performing a photolithography process using a laser writer three times is disclosed. As described above, in order to manufacture a diffraction grating having a fine substantially sawtooth cross-sectional shape, a photolithography method is widely used, and a device for making the cross-sectional shape close to a sawtooth shape has been made in order to improve diffraction efficiency. ..

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0005

[Correction method] Change

[Correction content]

FIG. 5 shows a structure of a conventional diffractive lens using a non-uniformly spaced diffraction grating to which this method is applied. The conventional diffractive lens of this type is configured by approximating the gratings of the entire area of the diffraction grating in a sawtooth shape by four steps, and in the area E near the center of the concentric circle shown in FIG. Is
The lattice spacing Pa is sufficiently large, and the lattice spacing Pb in the outermost region F shown in FIG. Therefore, the widths of the steps of each stairway in these two areas are significantly different. When the saw-tooth shape is formed approximately like this, the light-collecting efficiency is lowered to 81% as compared with the above-mentioned diffractive lens of the ideal unequal-spaced diffraction grating.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0020

[Correction method] Change

[Correction content]

Although the diffractive lens 5 is arranged at the pupil position of the lens system in the above embodiment, the invention is not limited to this, and the diffractive lens 5 is arranged at a position different from the pupil position of the projection lens 4. It is also possible to let. However, in this case, at least the area of the region through which the 0th-order diffracted light from the object passes becomes relatively large, so that the peripheral region D is relatively reduced with respect to the central region C. For this reason,
The peripheral area D, which has a small number of steps and is easy to manufacture, is reduced, and the central area C, which has a large number of steps, is increased, resulting in an increase in manufacturing cost. Therefore, it is most preferable to arrange the diffractive lens 5 near the pupil position of the projection lens 4.

Claims (2)

[Claims] 1. An unequal-spaced diffraction grating having a grating cross section approximated to a sawtooth shape by a staircase shape, wherein the number of staircase-shaped steps is different in at least two of the grating regions. lattice. 2. The unequal spacing according to claim 1, wherein the number of stair-shaped steps in the lattice region having a wide lattice spacing is larger than the number of stair-shaped steps in the lattice region having a narrow lattice spacing. Diffraction grating.