JPH0248090B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an illumination optical system for projecting a predetermined pattern on a mask onto a wafer.

Conventionally, various illumination optical systems used in IC pattern transfer devices are known. For example, as in US Pat. A predetermined area of the (mask surface) is illuminated with a parallel light beam within a predetermined collimation half angle (the maximum angle that the illumination light makes with the normal to the object surface). Here, the fly-eye lens is used to form multiple secondary light sources and illuminate the mask surface with multiple light beams to remove the diffraction image caused by the mask and to obtain even and uniform illumination. Such means are called optical integrators. In general, illumination optical systems using elliptical mirrors are
It is broadly divided into One is to focus a light source image near the optical integrator using an elliptical mirror, and the other is as described in the above-mentioned U.S. Patent No. 3,296,923.
The light beam from the elliptical mirror is guided to the optical integrator as parallel light using a diverging collimator lens. From the point of view of light collection efficiency, the latter is relatively better because there is less vignetting of the light in the optical integrator.
Even so, I still wasn't completely satisfied.
In particular, PMMA, a typical photoresist, has low sensitivity when using deep ultraviolet light, so increasing the light collection efficiency of the illumination optical system has been a major challenge.

The purpose of the present invention is to eliminate the influence of diffraction on the entire surface of the wafer due to the mask, to enable uniform illumination with higher light collection efficiency, and to improve efficiency even when changing the angle of the illumination light beam on the mask. It is an object of the present invention to provide a mask illumination optical system that does not deteriorate.

Hereinafter, the present invention will be explained based on examples. FIG. 1 is a schematic cross-sectional view showing an embodiment of the illumination optical system according to the present invention.

The arc tube 1 has a high-intensity light source a that emits ultraviolet rays and deep ultraviolet rays, and this light source a is located approximately at the first focal point of the elliptical mirror 2. Elliptical mirror 2 forms an image a' of light source a at its second focal point. The cold mirror 3 transmits most of the infrared light and reflects most of the deep ultraviolet light, and is generally coated with a multilayer film. The collimation lens 4, which consists of two positive lenses, uses its forward focus as a light source image.
It is arranged to match a', and converts the light from the light source image a' into a parallel beam. An optical integrator 5 is arranged to form a large number of secondary light sources in this parallel light beam, and substantially generates a large number of light beams. Specifically, the optical integrator 5 is a hexagonal prism of glass, with both end surfaces processed into convex lenses, which are bundled together in a honeycomb shape, as shown in the longitudinal cross-sectional view of FIG. 2a and the cross-sectional view of FIG. 2b. Front small lens 11 and rear small lens 12
The distance between them is equal to the rear focal length of the front lenslet 11, and of course the front focal length of the rear lenslet 12. Then, a small lens 11 on the front side and a small lens 12 on the rear side.
There is a one-to-one correspondence with each other, and the rear small lens 1
2 has the function of focusing the image of the corresponding front small lens 11 on the object plane (mask plane). Therefore, the shape of the aperture of the front small lens 11 and the illumination area on the object plane are similar, and here, since the individual small lenses are hexagonal in shape, the illumination area is also hexagonal. As shown in the longitudinal cross-sectional view of FIG. 3a and the cross-sectional view of FIG. It is also possible to configure it accordingly.

Returning to FIG. 1, a positive lens 6 is disposed immediately after the optical integrator 5, and an aperture 7 is disposed immediately after this, and the light beam reflected by the reflector 8 is directed to an object by a collimator lens 9. The light is focused on the surface 10. The aperture of the collimator lens 9 can be made small by the action of the positive lens 6, and the distance between the collimator lens 9 and the object plane 10 can be made small, so that the entire apparatus can be configured more compactly. However, the distance between the collimator lens 9 and the object surface is a so-called working distance, and if it is desired to increase the working distance for some purpose, a negative lens can be placed in place of the positive lens 6. The aperture 7 is used to change the collimation half angle of the illumination light by changing its aperture. The reflector 8 simply serves to bend the traveling direction of the light beam by 90 degrees. The collimator lens 9 is arranged so that its front focus matches the position of the secondary light source image formed by the rear surface of the optical integrator 5, that is, the front small lens 11 of the optical integrator. The diverging light beams from each of the lenses 12 are converted into parallel light beams, and the parallel light beams are completely superimposed on the object plane 10.

In the configuration of the present invention as described above, the light source a is not a complete point light source, but has a certain size in the light source image a', so the light flux exiting the collimation lens 4 corresponds to the size of the light source image a'. It becomes a divergent beam of light that spreads at an angle of . Here, collimation lens 4
Due to the convergence action of , an image b' of the aperture surface b of the elliptical mirror 2 is formed at a distance d behind the collimation lens 4, as shown in FIG. 4, so the optical integrator 5 is positioned at this position. By doing so, vignetting can be prevented. If the collimation lens is a concave diverging lens as in the prior art, the image b' of the aperture surface b of the elliptical mirror 2 will be formed in front of the collimation lens, making it impossible to prevent vignetting.

As shown in Figure 4, the collimation lens 4
When the focal length of is f and the distance between the aperture b of the elliptical mirror 2 and the collimation lens 4 is l, the distance d between the collimation lens 4 and the image b' of the aperture b of the elliptical mirror 2 is: As is well known, it is expressed as d=f·l/l−f.

As mentioned above, vignetting can be almost completely prevented by arranging the optical integrator at a position conjugate with the aperture surface of the elliptical mirror with respect to the collimation lens 4; Uniformity is an issue. That is, in an actual condensing system using an elliptical mirror, at a position that is conjugate with the aperture surface b of the elliptical mirror 2 with respect to the collimation lens 4, the light amount distribution is annular, reflecting the light amount distribution of the aperture surface b of the elliptical mirror 2. Therefore, non-uniformity also occurs in the illumination onto the object plane 10. Figure 5 is an open aperture image of the collimation lens 4 and the elliptical mirror.
This figure shows the light amount distribution at four locations A, B, C, and D between the point and the point b'. The light intensity along the beam cross section at each position of A, B, C, and D is shown with the reference lines x ₁ , x ₂ , x ₃ , and x ₄ at each position as zero intensity, and the left side as high intensity. . As shown in the figure, at position D, where image b' of the aperture surface of the elliptical mirror is formed, the light intensity is almost zero at the center, but it is somewhat averaged at position C, and at position B, which is approximately in the middle, the intensity at the center is almost zero. increases considerably, and becomes almost uniform at position A. For this reason, it is desirable that the optical integrator 5 be placed as close to the collimation lens 4 as possible. However, since the light source has a size, the closer the light source is to the collimation lens 4, the larger the luminous flux width becomes, making it necessary to make the aperture of the optical integrator 5 larger.
In reality, it is desirable to arrange the optical integrator 5 at a position closer to the collimation lens 4 than at the intermediate C position. That is, the collimation lens 4 and the optical integrator 5
It is preferable that the distance D from the distance D satisfies the following condition: D<d/2=f·l/2(l−f).

In the above configuration, the collimation half angle can be changed by changing the aperture diameter of the diaphragm, but when increasing the diaphragm diameter as shown in Figure 6a, the collimation lens has a large focal length. In addition, when reducing the aperture diameter as shown in FIG. 6b, by replacing the aperture with one with a smaller focal length, the highest light collection efficiency can always be maintained without loss.

As described above, according to the present invention, a mask illumination optical system with uniform and extremely high light collection efficiency is achieved while sufficiently eliminating the influence of diffraction on the wafer surface due to the mask, and the angle of the illumination light beam on the mask can be adjusted. There is no risk of reducing efficiency even when changes are made.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view showing an embodiment of the illumination optical system according to the present invention, FIG. 2 a is a vertical cross-sectional view of an optical integrator, FIG. 2 b is a cross-sectional view thereof, and FIG. 3 a is an optical integrator. 3b is a cross-sectional view thereof, and FIG. 4 is an explanatory diagram in which the image of the aperture surface of the elliptical mirror is formed at a distance behind the collimation lens.
Figure 5 is an explanatory diagram showing the light intensity distribution at four locations A, B, C, and D between the collimation lens and the aperture image of the elliptical mirror, and Figure 6a is an explanatory diagram showing the light intensity distribution at four locations A, B, C, and D between the collimation lens and the aperture image of the elliptical mirror. , FIG. 6b is an explanatory diagram showing the types of collimation lenses used when reducing the aperture diameter. [Explanation of symbols of main parts], 1... Arc tube, 2
...Elliptical mirror, 3...Cold mirror, 4...Collimation lens, 5...Optical integrator.

Claims (1)

[Claims] 1. An elliptical mirror having first and second focal points, and an arc tube disposed at the first focal point of the elliptical mirror, and an annular light amount on the aperture surface of the elliptical mirror. a light source device for condensing the light beam from the arc tube onto a second focal point with a distribution; a parallel light beam conversion means for converting the light beam from the light source device into a parallel light beam; In the illumination optical system, the illumination optical system includes an optical integrator that forms multiple light beams based on a secondary light source image and the secondary light source image, and a convergent lens that guides the light beam from the optical integrator to an object to be illuminated, wherein the parallel light beam conversion is performed. An illumination device characterized in that the means is a convergent collimation lens, and the convergent collimation lens is configured to be switchable to different focal lengths in order to change the angle of the illumination light beam on the object to be illuminated. Optical system.