JPS61169815A - Lighting optical device - Google Patents

Abstract

PURPOSE:To perform extremely uniform illumination by providing a generating member which generates an optical path difference among optical paths corresponding to plural light source images and a transmissivity correcting member which prevents irregularity in illumination. CONSTITUTION:Coherent parallel luminous flux from a light source 10 is passed through a stepped prism 30 which has steps as the optical path difference generating member to form light source images as many as the steps through a lenticular lens 4, and the light source images 11 illuminate a pattern on a reticle R to form its images on a wafer W. The prism 30 have the steps which differ in optical path length and degree of light absorption and irregularity in illumination is generated on an illuminated body, so the transmissivity correcting member 70 is made different in transmissivity at parts corresponding to the respective steps of the prism 30 to uniform the intensity of the transmitted light. Thus, extremely uniform illumination having no irregularity in illumination is performed.

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]

Proximity-type and projection exposure-type devices are conventionally known as photolithography devices used in the manufacture of semiconductors, and reduction projection-type exposure devices in particular have become indispensable for the manufacture of VLSIs. . In these photolithographic apparatuses, it is necessary to provide uniform illumination with high brightness, and an ultra-high pressure mercury lamp is generally used, and an elliptical reflecting mirror 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 because lasers have strong coherence, they form interference fringes on the illumination object surface, which is disadvantageous for uniform illumination.

[Purpose 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]

To achieve this objective, the illumination optical device according to the invention comprises:
a coherent light source; a light source image forming member that forms a plurality of light source images from a light beam supplied from the coherent light source;
an optical path difference generating member that gives an optical path difference to a plurality of optical paths corresponding to the plurality of light source images to prevent interference fringes from being formed on the surface of the object to be illuminated; Corrects the non-uniformity of transmittance that occurs between multiple optical paths,
The present invention is characterized by comprising a transmittance correction member that prevents uneven illumination from occurring on the surface of the object to be illuminated.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram showing the principle of the present invention as described above. Coherent light a! The coherent parallel light flux supplied from 10 passes through a staircase prism 3 having a plurality of steps as an optical path difference generating member, and then is reflected by a lenticular lens 4 having lens blocks of the same number as the number of steps of the staircase prism 3. The same number of light source images as the number of steps of the step prism 3 are formed near the exit surface. The light beams from the plurality of light source images l' illuminate the object surface 6 in a superimposed manner via the condenser lens 5.

In the figure, 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 lenticular lens 4 is composed of eight lens blocks arranged in parallel, and each lens block produces eight light source images.
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. Each produces an optical path difference.

Here, the optical path length from the point light source 1 to an arbitrary point i on the light source image 1' is assumed to be Ji, and the optical path length from this point i to an arbitrary point p on the object plane 6 is assumed to be li'.

Since point p is illuminated by each of the plurality of light source images, similarly for any other arbitrary point j different from point i in the light source image, the optical path length from point light source 4 is 1j, and the distance to point p on the object plane is When the optical path length is pj', the optical path difference Δl of each optical path passing through the two points i and j on the light source image 1' and reaching the point p on the object plane is Δl= (6i+j! l') −(N j+g 3
')... (1) If this optical path difference Δl is larger than the coherence distance determined by the light from the coherent light source, interference will not occur at point p on the object plane depending on the light beams from the pair of points i and j on the light source image. Therefore, for any two points i and j on the light source image, and for any point p on the illuminated object surface, 1(j!!+'quantity') - (#j41j' )
I>L... (2) When the following condition is satisfied, it is possible to avoid the formation of interference fringes on the illuminated object surface.

Now, assuming that a pair of points i and j are adjacent on the light source image 1', and rewriting the above equation (1), the optical path difference ΔL is Δ1 = (1i-j!j) + (j!i '−j!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 now S, the optical path difference caused by this is expressed as the refractive index n of the step prism 3. (n-1
) ・Represented as S, for adjacent points i and j, CI! i-1j) = (n 1) ・S.

Furthermore, if the distance between each lens block of the Lentiki error lens 4 is d, and its numerical aperture is NA, then the optical path difference from the light source image 1' to the periphery of the object to be illuminated is expressed as d-NA, and C1l'-13')-d-NA. In addition, the numerical aperture NA of the lenticular lens 4
is defined as sin θ, assuming that its exit angle is 20.

The condition of equation (2) is l(n-1)・S+d-NAI>L, and the optical path difference for each optical path caused by the step prism 3 as the optical path difference generating member is (n-1)・S> d
-NA+L-・ ・ ・ ・ (3) is given.

For the prism blocks of the stair prism 3 that are spaced m apart from each other, it is necessary to satisfy (n-1) ・m-S>m-d-NA+L, but this condition is (3). It is clear that if the formula is satisfied, then the condition is necessarily satisfied.

Incidentally, when 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=λ8/Δλ (4).

Although the generation of interference fringes is prevented by the above principle, the staircase prism 3, which is a central component for this purpose and serves as an optical path difference generating member, causes uneven illumination on the illuminated object due to its structure. It has the disadvantage of In other words, each stage of the stair prism 3 is made of the same material, and each stage has a different optical path length, so each stage has a different degree of light absorption, resulting in uneven transmittance and uneven illumination. . As a means for preventing the occurrence of this illumination unevenness, a transmittance correction member 7 is provided on the step-shaped end face of the step prism 3. This correction member 7 adjusts the transmittance of the remaining stages so that the transmittance of each stage of the step prism 3 matches the stage with the lowest transmittance, that is, the stage with the longest optical path length. , a magnetic film, a metal film, or a semiconductor film is attached to the stepped end face of the prism 3 except for the step with the longest optical path length by a method such as vapor deposition. Since the transmittance of each stage is made to match the stage with the lowest transmittance, the loss of light amount due to the correction member 7 can be minimized.

FIG. 2 is a schematic diagram of an embodiment applied to a reduction projection type exposure apparatus for semiconductor manufacturing.

The parallel light beam from the coherent light source 10 passes through the step prism 30, which is an optical path difference generating member and has a plurality of steps, and reaches the lenticular lens 40, which has the same number of lens blocks as the number of steps in the step prism 30. The same number of light source images 11 as the number of steps of the step prism 30 are formed. The light flux from the light source image 11 is transmitted through the condenser lens 50
is guided to the reticle R as an object to be illuminated, and the reticle R is illuminated. The predetermined pattern formed on the reticle R is then projected onto the wafer W by the projection objective lens 12. An image 11' of the light source image 11 is placed on the entrance pupil of the projection objective 12 by means of a condenser lens 50, i.e., on the aperture 12a of the objective 12 as shown.
In FIG. 0, which is formed at the position where so-called Koehler illumination is performed, 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 light source image are shown as broken lines.

The transmittance correction member 70 is a so-called ND filter for transmittance adjustment, and is arranged between the step prism 30 and the Lentiki error lens 40.

The transmittance correction member 70 has portions with various transmittances facing each step of the step prism 30 in a planar manner so as to equalize the intensity of light passing through each step of the step prism 30. are arranged in Therefore, each particle that has passed through each step of the step prism 30 passes through each part of the opposing correction member 70, has a uniform intensity, and enters the opposing lens block of the lenticular lens 40, respectively.

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

This rare gas halide laser has a center wavelength λ=308n
m, wavelength width Δλ=0.5 ns. Therefore, the coherence length of this coherent light beam is approximately 0.2 mm according to equation (4). Further, the numerical aperture NA of the lenticular lens 40 is 0.2.
Assuming that the distance between each lens block is 6.0 wm, the minimum value of the optical path difference of the step prism 30 given by the condition of equation (3) is 1.4 fi. Therefore, the optical path difference caused by the step prism 40 is (1, 51) x4. Omm=2. 0
-1, which 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 configuration of the lenticular lens 40 nor the stair prism 30 was mentioned, but in practice, for example, a stair prism with a configuration as shown in the perspective view of FIG. 31, and the lenticular lenses must also be arranged three-dimensionally.

The stair prism 31 shown in FIG.
Consisting of four quadrangular prisms 312 to 312,
The prisms are arranged in order with the length up to the longest square prism 31J 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, each quadrangular prism of the stepped prism can be arranged with respect to the optical path of each lens block of the lenticular lens, regardless of its front and back sides. is important.

It is desirable that the above conditions be strictly satisfied, but when a large number of light source images are formed by a lenticular lens, one or several pairs of points on the light source image may be Even if interference fringes are formed, they are weak relative to the overall light intensity, so that practically sufficient uniform illumination is possible. In this case, it is better if the positions of the interference fringes on the surface of the object to be illuminated, which are generated by some pairs of light source images, are somewhat random in the illumination area. According to such a configuration, it is possible to suppress the optical path difference between each of the optical paths corresponding to a large number of light source images, or it is possible to eliminate the optical path difference between some pairs of the large number of optical paths. Since the difference between the longest optical path and the shortest optical path can be made relatively small, the optical path difference generating member can be configured compactly.

FIG. 4 is a plan view showing such an example of the step prism 31', and the numbers in the figure represent the order of the lengths of the square prisms (the smaller the number, the longer). That is, in this case, the longest rectangular prism and the shortest rectangular prism are arranged adjacent to each other to make the length of the optical path difference generating member as a whole uniform. In the example of the stepped prism shown in FIGS. 3 and 4, columnar prism blocks with a square cross-sectional shape are bundled, but the projection pattern formed on the reticle R is generally rectangular. This is effective for illuminating a rectangular area. Such a configuration has the effect of not only preventing the occurrence of interference fringes but also suppressing the occurrence of uneven illumination, and the transmittance correction member may be omitted.

In the example of the stair prism 31# shown in the plan view of FIG.
Each columnar prism has a fan-shaped cross section and is arranged in a spiral pattern with the longest prism at the center.The numbers in the figure represent the order of the length of each columnar prism (the smaller the number, the longer the prism ).

In this case, in accordance with the optical system that is generally rotationally symmetrical, the optical path difference generating member is also almost rotationally symmetrical, and a long columnar prism is placed in the center where a relatively high-intensity light beam exists. Since the transmittance correction member is arranged, it is more effective for uniform illumination, and the transmittance correction member may be omitted.

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. The entrance surface 32a of the step prism 32 is formed in a step shape, and the exit surface 32b is provided with a lens having a positive lens action corresponding to each prism block of the step prism. A cular lens is formed.

Further, the optical path difference generating member is not limited to the prism described above, but an optical fiber 33 whose side surface is shown in FIG. 7 can also be used. In the illustrated example, both the entrance surface 33a and the exit surface 33b are flat, and the length of each fiber bundle is changed according to the required optical path difference.

The location of this optical fiber 33 is
The location is not limited to between the coherent light source 10 and the lenticular lens 40, but may be at or near the imaging position of the light source image 11 formed by the lenticular lens 40.

In order to create an optical path difference, each prism block can be made of optical materials having different refractive indexes as well as different prism lengths. Furthermore, although a Lentiki error lens with a positive lens effect was used as the light source image forming member in the above example, it can be similarly used with a negative lens effect, and in this case, the light source image is formed as a virtual image. be done.

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.

The position of the transmittance correction member is not limited to the position in the above embodiment, but may be any position as long as the plurality of lights forming each light source image formed to prevent interference fringes do not intersect. good.

Further, the transmittance correction member is not limited to a form that is independent of other components (the optical path difference generating member, the light source image forming member) as in the above embodiments, but, for example, the transmittance correction member is The transmittance correction member may be configured by using an optical material having a different transmittance for each portion through which a plurality of lights forming each light source image are transmitted, and fused with other components.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to provide extremely uniform illumination even though a coherent light source such as a laser is used. Bright, uniform illumination is possible. If used as an illumination system for a device that performs photolithography, which is essential for semiconductor manufacturing,
This makes it possible to provide extremely high-intensity illumination while maintaining the same uniformity as conventional methods, resulting in an improvement in throughput, and it also makes it possible to perform photolithography using shorter wavelength light using a laser. Super L
It is also useful for further miniaturization of SI 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 of the present invention, FIG. 3 is a perspective view showing an example of an optical path difference generating member, and FIG. FIG. 5 is an explanatory view showing 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] 10... Coherent light source 3.30.31.31', 31', 32.33...
..... Optical path difference generating member 4.40 ..... Light source image forming member 6 ..... Object to be illuminated

Claims (1)

[Scope of Claims] A coherent light source, a light source image forming member that forms a plurality of light source images from a light flux supplied from the coherent light source, and a plurality of optical paths corresponding to the plurality of light source images, each of which has an optical path difference. and an optical path difference generating member that prevents the formation of interference fringes on the illuminated object surface, and corrects non-uniformity in transmittance occurring between the plurality of optical paths by the optical path difference generating member,
An illumination optical device comprising: a transmittance correction member that prevents uneven illumination from occurring on the surface of the object to be illuminated.