JPS60230629A - illumination optical device - Google Patents

Abstract

PURPOSE:To realize lighting superior in uniformity by providing an optical path difference generating member which generates optical path differences in optical path length corresponding to secondary light sources between a coherent light source and a secondary light source forming member. CONSTITUTION:Parallel luminous flux from the coherent light source 10 is transmitted through a stepwise prism 10 as the optical path difference generating member to reach a lenticular lens 40, at which plural secondary light sources 11 are generated. Then, luminous flux from the secondary light sources 11 is guided to a reticle R as a body to be lighted through a condenser lens 50 to light up the reticle R. This invention uses the coherent light source such as a laser, but provides lighting extremely superior in uniformity; and brighter uniform lighting than before is realized by a high-brightness and high-efficiency light source such as an excimer laser.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a lighting device capable of providing high brightness and uniform illumination.

(Background of the Invention) Conventionally, proximity method and projection exposure method have been known as devices for performing photolithography used in semiconductor manufacturing.
It has become essential for the production of SI. In the equipment used to perform these photolithography, it is necessary to provide high brightness and uniform illumination, and an ultra-high pressure mercury lamp is generally used, and an elliptical reflector is often used to collect the light. . However, recently there has been a demand for even higher integration of VLSIs, and not only uniform illumination but also higher luminance illumination is required. Lasers have long been known as high-intensity light sources, but since lasers have strong coherence, they form interference fringes on the surface of the illuminated object, which is disadvantageous for uniform illumination.

(Objective of the Invention) An object of the present invention is to provide an illumination optical device that can perform illumination with excellent uniformity while using a coherent light source such as a laser.

(Summary of the Invention) An illumination optical device according to the present invention is provided with a coherent light source and a secondary light source forming member for forming a plurality of secondary light sources from a luminous flux supplied from the coherent light source. An optical path difference generating member is provided between the secondary light source and the light source forming member to provide an optical path difference to the optical path length of the optical path corresponding to each secondary light source. That is, when forming a plurality of secondary light sources from a coherent light source,
A partial optical path difference is given to the optical path of the light beams forming each secondary light source, so that the light beams from the plurality of secondary light sources do not interfere with each other on the surface of the object to be illuminated. This forms an incoherent surface light source. For this purpose, specifically, the light beam from the coherent light source is separated into multiple optical paths and guided onto the secondary light source surface at a predetermined position, the optical path length is changed for each optical path, and this optical path length difference is made into a coherent light source. It is larger than the distance.

FIG. 1 is a schematic diagram showing the principle of the present invention as described above. A coherent parallel light flux supplied from a coherent light source (10) passes through a step prism (3) as an optical path difference generating member, and then passes through a lenticular lens (4).
) to form a plurality of secondary light sources (1°) near the exit surface. Then, a condenser lens (5) illuminates the object surface (6) with light beams from a plurality of secondary light sources in a superimposed manner. Since the coherent light source (10) is equivalent to a point light source (1) collimated by a positive lens (2), it is illustrated as a point light source for convenience. The illustrated lenticular lens (4) is configured by, for example, eight lens blocks arranged in parallel, and each lens block forms eight secondary light sources. The step prism (3) has eight steps to produce a different optical path length for each optical path corresponding to each lens block of the lenticular lens (4), and has eight steps for each of the eight secondary light sources. This creates an optical path difference.

Here, the optical path length from the point light source (1) to any one point i on the secondary light source is 21, and the optical path from one point i on this secondary light source to any point P on the object plane (6) Let the length be zi+.

Since point P on the object plane is illuminated by each of the plurality of secondary light sources, similarly, for any other arbitrary point j that is different from point i on the secondary light source, calculate the optical path length from point light source (1). 7! j, and when the optical path length to point P on the object plane is lj゛, the optical path difference Δ of each optical path passing through two points i and j on the secondary light source (1゛) and reaching point P on the object plane is ! is, ΔIt= (j!i +ni') (7!j +Nj"
)(1). If this optical path difference Δl is larger than the coherence distance determined by the light from the coherent light source, the light beams from the pair of points i and j on the secondary light source will not interfere at point P on the object plane. It won't happen.

Therefore, for any two points i and j on the secondary light source, and for any point P on the illumination object region, l (j!i + βi') (j!j +1j') l
>t, it is possible to avoid the formation of interference fringes on the illuminated object region if the condition (2) is satisfied.

Now, assuming that a pair of adjacent points on the secondary light source are i and j, and rewriting the above equation (1), the optical path difference Δl is Δ2 =, (βi-1!, j) + (Ili' -A
! j'). Here, the first term on the right side corresponds to the optical path difference caused by the step prism (3), so if each step of the step prism (3) is S, then
The optical path difference caused by this is expressed as (n-1).S, where n is the refractive index of the step prism, and (Ji -j!j) = (n-1).S.

Furthermore, if the distance between each lens block of the lenticular lens (4) is dl and the numerical aperture is NA, then the optical path difference from the secondary light source to the periphery of the illuminated object is expressed as d-N^, ( j2'i'-63') = d - N^. Note that the numerical aperture N^ of the lenticular lens (4) is defined as sin θ, assuming that its exit angle is 2θ. Therefore,
The condition of equation (2) is 1 (n-1)-3+d-NAI>
It becomes L. Therefore, the optical path difference for each optical path that should be generated by the step prism as the optical path difference generating member is (n-1)
・It is given as S > d-N^+L (3).

For prism blocks that are spaced apart by m, it is necessary to satisfy (n-1) ・m-3> m-d-NA+L, but this condition must satisfy equation (3). It is clear that this is necessarily satisfied.

Although it is desirable that the above conditions be strictly met, if a large number of secondary light sources are formed by a lenticular lens, one or more secondary light sources may be Even if interference fringes are formed by a pair of points, they are weak compared to the overall light intensity, so that practically sufficient uniform illumination can be achieved. In this case, it is only necessary that the positions of the interference fringes on the object plane generated by the pairs of lights on the secondary light source are random in the illumination area.

According to such a configuration, the difference between the longest optical path and the shortest optical path caused by the optical path difference generating member can be made relatively small, so that the optical path difference generating member can be configured compactly.

For details on the coherence distance, please refer to Max Born&
Written by Emit Wolf Pr1ncfples of
0ptics (4th edition) 7.5.8. (p 31
6-323) Pergamon Press, Oxfo
rd.

1970, where the center wavelength of the light beam supplied from the coherent light source is λ and the wavelength width is Δλ, the coherence length is given by L−λ2/Δλ (4).

(Example) Hereinafter, the present invention will be described based on illustrated examples. FIG. 2 is a schematic diagram of an embodiment in which the illumination optical device according to the present invention is applied to a reduction projection type exposure device for semiconductor manufacturing. The parallel light beam from the coherent light source (10) passes through the step prism (30) as an optical path difference generating member and reaches the lenticular lens (40), where a plurality of secondary light sources (11) are formed. The light beam from the secondary light source (11) is guided to the reticle R as an object to be illuminated by a condenser lens (50), and illuminates the reticle R.

The predetermined pattern formed on the reticle R is then projected onto the wafer W by a projection objective lens (12). The image (11') of the secondary light source (11) is placed by the condenser lens (50) onto the entrance pupil of the projection objective (12), i.e. at the aperture (12a) of the objective (12) as shown. This is called Koehler illumination. In the figure, the light rays showing the conjugate relationship between the reticle R and the wafer W are shown as solid lines, and the light rays showing the conjugate relationship of the secondary light source are shown as dotted lines.

Here, a staircase prism (30) as an optical path difference generating member
has a refractive index n = 1.5, and the step value is 4.0II1
It is m. As the coherent light source (10), a rare gas halide laser (XeC1), which is one of the excimer lasers and has high output and high efficiency, is used.

This XeC1 rare gas halide laser has a center wavelength λ-
It supplies a coherent light beam having a wavelength width of 308 nm and a wavelength width Δλ=0.5 nWl. Therefore, the coherence length of this coherent light beam is approximately 0.degree. 2 mm according to equation (4). Furthermore, assuming that the numerical aperture NA of the lenticular lens (40) is 0.2 and the interval between each lens block is 6.01, the minimum optical path difference of the step prism (30) given by the condition of equation (3) is The value is 1, 4++on. Therefore, the optical path difference caused by the step prism (40) is (1,5-1)X4. Omn+=2.0 mm is considerably larger than the minimum value of the optical path difference given by condition (3), and it is possible to perform extremely uniform illumination without forming any interference fringes on the reticle R.

In the above explanation, for the sake of simplicity, neither the three-dimensional structure of the lenticular lens (40) nor the step prism (30) was mentioned, but in practice, for example, as shown in the perspective view of FIG. It is necessary to use a step prism (31) of the configuration and also use lenticular lenses arranged three-dimensionally. The staircase prism (31) shown in Fig. 3 consists of 12 square prisms (31) of different lengths.
18 to 311), from the shortest rectangular prism (31a) to the longest rectangular prism (31A).
) are arranged in order with their lengths increasing by S.

In correspondence with Fig. 2, the upper part of Fig. 3 is the incident light side,
The lower side is the exit light side, but as an optical path difference generating member, it is important that each square prism of the stepped prism is placed with respect to the optical path of each lens block of the lenticular lens, regardless of whether it is front or back. It is.

In the staircase prism (31) shown in Figure 3, the length distribution of the square prisms is uneven, so there is a risk of uneven illumination due to absorption of light in the prism. It is necessary to make the array have less variation in length. FIG. 4 is a plan view showing the step prism (31") of this example, and the numbers in the figure represent the order of the length of each square prism. In this case, the longest square prism The prisms are arranged every other time in order from 1 to 3, and the longest rectangular prisms and the shortest rectangular prisms are arranged adjacent to each other, so that the length of the optical path difference generating member as a whole is made uniform.Third. In the example of the step prism shown in Fig. 4 and Fig. 4, columnar prism blocks with a rectangular cross section are bundled together.
This is because the projection pattern formed on the reticle R is generally rectangular, and is effective for illuminating a rectangular area.

In the example of the step prism (31") shown in the plan view of Fig. 5, columnar prisms each having a fan-shaped cross section are arranged in a spiral shape with the longer one at the center, and the numbers in the figure indicate each columnar prism. It represents the order of the lengths of the prisms. In this case, the optical path difference generating member is also almost rotationally symmetrical in accordance with the optical system that is generally configured to be rotationally symmetrical.
Furthermore, since a long columnar prism is placed in the center where a relatively high-intensity light beam exists, it is more effective for uniform illumination.

Now, in the above embodiment, the optical path difference generating member and the lenticular lens are arranged separately, but it is also possible to integrate them. FIG. 6 is a side view showing an example of this, in which the entrance surface (32B> of the staircase prism (32) is formed on the staircase, and the exit surface (32b) has a shape corresponding to each prism block of the staircase prism (32). A lenticular lens with positive lens action is formed.

Furthermore, the optical path difference generating member is not limited to the above-mentioned prism, but an optical fiber (33) as shown in the side view of FIG. 7 can also be used.

In the illustrated example, the entrance surface (33a) and the exit surface (33
(b) and (b) are both planes, and the length of each fiber bundle is changed according to the required optical path difference. In order to create an optical path difference, each prism block can be made of optical materials with different refractive indexes as well as prism lengths.

Furthermore, although a lenticular lens with a positive lens effect was used as the secondary light source forming member in the above example, a lens with a negative lens effect can also be used in the same way.In this case, the secondary light source can be used as a virtual image. It is formed.

The lenticular lens may have a lens surface formed on its entrance side, a lens surface formed on its exit side, or a lens surface formed on both sides.

(Effects of the Invention) As described above, according to the present invention, it is possible to provide extremely uniform illumination while using a coherent light source such as a laser. This enables brighter and more uniform illumination than ever before. If used as an illumination system for equipment for photolithography, which is essential for semiconductor manufacturing,
This makes it possible to provide extremely high-intensity illumination while maintaining the same uniformity as before, resulting in an improvement in throughput, and it also makes it possible to perform photolithography using shorter wavelength light using a laser. , super LS
It is also useful for further miniaturization of I patterns.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the principle of an illumination optical device according to the present invention, FIG. 2 is a schematic configuration diagram of an embodiment according to the present invention, FIG. 3 is a perspective view showing an example of an optical path difference generating member, and FIGS. FIG. 5 is an explanatory view of another example of the optical path difference generating member, and FIGS. 6 and 7 are side views showing examples of the optical path difference generating member, respectively. [Explanation of symbols of main parts] lO... Coherent light source 3. 30. 31. 31", 31", 32.33... Optical path difference generating member 4.40... Secondary light source forming member 6...・Illuminated object applicant: Takao Watanabe, agent of Nippon Kogaku Kogyo Co., Ltd. - 11111111” N4 Figure 6 Figure 5 Figure 7 Figure 3

Claims (1)

[Claims] A coherent light source and a secondary light source forming member for forming a plurality of secondary light sources from the light flux supplied from the coherent light source are provided, and each secondary light source is provided between the coherent light source and the secondary light source forming member. An illumination optical device characterized in that an optical path difference generating member is provided for providing an optical path difference between the optical path lengths of corresponding optical paths.