JPH03101708A - Reflected image forming optical device - Google Patents

Abstract

PURPOSE:To form a flat image forming plane over a wide area with simple constitution consisting of only spherical surfaces and to enable reduced projection by arranging a spherical reflecting mirror in the vicinity of at least one intermediate image position of an intermediate image which is formed through plural spherical reflection optical systems. CONSTITUTION:The spherical reflecting mirror R3 is arranged in the vicinity of the intermediate image a1 at least one place of the intermediate image a1 formed through the spherical reflecting optical systems R1 - R5. In concrete, what is called a Schwarzschild optical system is used as concentric optical systems S1 and S2 and the concave reflecting mirror which has the spherical surface for field curvature compensation is arranged in the vicinity of the intermediate image a1. Therefore, the curvature of field which is generated by at least one couple of the concentric optical systems S1 and S2 and can not be compensated can be compensated excellently by the spherical mirror R3 arranged in the vicinity of the intermediate image formation plane. Consequently, a flat image with high resolution can be obtained over a wide area although the simple constitution consisting of only the spherical surfaces is employed, and the reduced projection is attained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a reflective imaging optical system, particularly a reflective imaging optical device capable of reduction projection. [Prior Art] Conventionally, semiconductor devices such as LSI and VLSI In the exposure apparatus used in the production of lithography, a mercury lamp has been used as the light source. In order to cope with the further miniaturization of semiconductor device patterns, excimer lasers with shorter wavelengths are also being used as exposure light sources. However, as long as ordinary light is used, there is a natural limit to achieving a single wavelength, and in order to expose a pattern with a line width of 0.2 μm or less, it is necessary to use X-rays.

Exposure apparatuses using X-rays have already been developed, but all of these are so-called proximity type in which a shadow image of a mask is transferred onto a wafer, and the projection magnification is the same. For this reason, it is essential to finish the mask itself, which serves as a projection master, into an extremely fine pattern.
It becomes necessary to make a mask with a line width of 2 μm and a positional accuracy of 0.5 μm or less. It is extremely difficult to process a mask having such a fine pattern, and even if X-rays were used, the proximity method was not practical.

[Problem to be solved by the invention]

Therefore, there is a desire for an apparatus that can perform reduction projection exposure using X-rays, but ordinary lenses cannot be used because they do not transmit X-rays, and only a reflective system can be used as an optical system. Of course, such an X-ray reflection reduction projection optical system can be designed by using an aspheric reflection system. however,
Extremely high machining accuracy is required in the X'a region, so
It is impossible to manufacture an aspherical reflective system.

Therefore, the only reflective surface that can be used practically is a spherical surface.

The Nyuwarzschild type has been known for a long time as a reflective optical system composed only of spherical surfaces. As shown in FIG. 8, this is an optical system composed of two spherical reflecting mirrors, a concentric concave mirror 1 and a convex mirror 2, and is an extremely excellent optical system. However, the only drawback is that since it is a concentric optical system, the object plane and image plane are spherically curved around the spherical center of the reflecting mirror.

If the object surface is taken as a flat surface, the image becomes even more curved. For this reason, it cannot be used in semiconductor manufacturing equipment that requires a wide exposure area.

SUMMARY OF THE INVENTION An object of the present invention is to provide a reflective imaging optical device that has a simple configuration consisting of only spherical surfaces, yet can form a flat imaging surface over a wide area, and is capable of reduction projection.

[Means for solving problems]

The present invention provides an optical device having a plurality of spherical reflective optical systems including at least one set of concentric optical systems, in which a spherical surface is provided near at least one intermediate image position of an intermediate image formed by the plurality of spherical reflective optical systems. It is equipped with reflective mirrors. Specifically, a so-called Nywarzschild optical system is used as a concentric optical system, and a concave reflecting mirror with a spherical surface for correcting field curvature is arranged near the intermediate image thereof.

[Effect]

According to the present invention, the curvature of field that cannot be corrected due to at least one set of concentric optical systems can be favorably corrected by the spherical reflecting mirror disposed near the intermediate imaging surface. The design method of this spherical mirror for correcting field curvature is as follows.

Now, for example, two concentric optical systems S. like the first embodiment shown in FIG. ．． S! shall be combined.

Then, the reduction projection magnification of the entire system is set to I/M, and the first optical system S with a magnification subtraction of 1 and the second optical system S with a magnification of 1/m2 are combined to reduce the reduction by 1/M. Let us obtain a projection optical system.

That is, M=m+Xm*.

Then, consider a l-th optical system Sl with a magnification of 1, and assume that its object surface 01 is a plane. Next, the second magnification of 1/m7
The image plane (2) of the optical system S2 is a plane, and the size of the image a is 1/M of the object a0 of the first optical system. The arrangement of objects and images in the second optical system S is opposite to that of the first optical system, and the optical axes of each optical system are made to coincide with each other, so that the image plane ■ of the first optical system S1 and the object plane O of the second optical system S2 are aligned. , and coincide on the optical axis. Then, as shown in Fig. 2, the image plane (1) of the first optical system and the object plane (O) of the second optical system are each approximately spherically curved, and although their sizes are the same, the degree of curvature is different. are different.

If we consider the curved surface M formed by connecting the midpoints of the corresponding points of the image plane L of the first optical system and the object surface 02 of the second optical system, this will also be approximately spherical, and if this surface M is a mirror surface, then The image plane I1 of the first optical system and the object plane O of the second optical system become conjugate via the reflective surface M.

As a result, the plane object a0 on the object plane 01 of the first optical system is imaged as a 1/M plane image a+ on the image plane I of the second optical system, and the flat image of the plane object is It is formed at a desired magnification without curvature of field.

Note that, when an object plane and an image plane are given, the optical axis A in the concentric optical system is defined as a straight line passing through the concentric center and perpendicular to the object plane and the image plane.

By the way, to explain the design of the spherical mirror for field curvature correction in a different way from the above, an optical system without field curvature is an optical system with a Petzval sum of O. Now, in each concentric optical system, the radius of curvature of each reflecting surface is rl. r2.・・・・・・
, r7, the Petzval sum P is P=1/r++l/r,+1/rs+-・----+
1/r. (11, so the value of this equation (1) is 0
This is the condition for no surface curvature.

Therefore, if each concentric optical system is a Schwarzschild optical system consisting of two concentric reflecting surfaces, let r+, rz be the first
When the radius of curvature of the Schwarzschild optical system, r, is the radius of curvature of the second Schwarzschild optical system, the radius of curvature r of the spherical surface MM for image plane correction placed near the intermediate image position is given by It will be done.

This f11 formula has the refractive index on the incident light side of the k-th surface as N1, the refractive index on the emitted light side as Nk', and the refractive index on the incident light side of the correction reflecting surface as N(0, the refractive index on the emitted light side). When N1 is set, more precisely, it is as follows.

k-1,, where the refractive index N. ', Nk, etc. are 1 or = 1,
The sign is positive when the direction of the light ray is to the left, and negative when the direction of the light ray is to the right, similar to the interval between reflective surfaces described later.

Of course, this spherical surface coincides with the image plane correction reflecting mirror described above in FIG.

Similarly, the radius of curvature of the reflecting surface of the entire system including the concentric optical system is r
l+ rl+ . It is given by r, k-+4,,.

In practice, it is necessary to modify the radius of curvature of the reflecting surface for image plane correction determined by the above formula (31) as appropriate, depending on the balance with residual aberrations in the Schwarzschild optical system and the optical system combined with it. In that case, the final value r of the radius of curvature of the reflective surface for image plane correction is (2
) or (3), it is effective to set it within the range of λ.

Here, λ is the wavelength, Y is the image height of the final image, and NA is the numerical aperture corresponding to the aperture angle of the imaging light beam at the final image plane.

Condition (4) above is the amount of deviation Δ=-Y'(N
k'P) and the jQ point depth Δ in the diffraction-limited optical system.
This is a condition determined based on the relationship with Z=±λ/2(NA)', and if it exceeds the above range, the higher-order aberrations generated on the reflective surface for image plane correction will become too significant, so it is not good. It becomes difficult to obtain a clear image.

〔Example〕

Hereinafter, the present invention will be explained in detail based on examples.

FIG. I is a cross-sectional optical path diagram showing the structure of the first embodiment of the present invention. The chief ray emitted from the axial point of is the final image a+
It shows how it reaches.

As shown in the figure, in the first embodiment, the first reflecting mirror R. and a concave mirror second reflecting mirror R, a first Schwarzschild concentric optical system S, and an image plane correction MM.
, and a second Schwarzschild concentric optical system S2 consisting of a fourth reflecting mirror R4, which is a concave mirror, and a fifth reflecting mirror R, which is a convex mirror, with ct as the spherical center. . Here, the first Schwarzschild type concentric optical system is a reduction system, and the second Schwarzschild type concentric optical system is an expansion system.

The light flux from object a6 is reflected by convex mirror R+ as the first reflecting surface.
, an intermediate image a is reflected by the concave mirror R2 serving as the second reflecting surface.
, is a form of precept. Although the intermediate image a1 is curved, the intermediate image a
1 is further bent by a concave mirror R, which serves as a third reflecting mirror, and the image a! to give form to. The light beam that has been reflected by the concave mirror R, which is the third reflecting mirror, and which is placed near the intermediate image a, is reflected by the concave mirror R, which is the fourth reflecting surface, and the convex mirror R, which is the fifth reflecting surface. final (l a
Precept the form of t.

As explained with reference to FIG. 2, the intermediate image a1 is moderately bent as the reflected image a2 by the third reflection MR as the image plane correction mirror M, so that the images generated in the first and second Schwarzschild optical systems are The surface curvature is corrected and the final image a, is formed on a plane. In such a structure, in order to further improve the telecentricity of the image feed so that the chief ray forming the final image a is almost parallel to the optical axis, a third reflection mirror as an image plane correction mirror is used. It is effective to tilt the mirror R slightly.

In the first embodiment described above, the first Schwarzschild optical system is used as a reduction system and the 1g2 Schwarzschild type concentric optical system is used as an enlargement system in order to reduce the final image. It is not limited to the following combinations. In other words, the magnification and reduction of the Schwarzschild type concentric optical system should be selected and combined as appropriate, and it is also possible to use the so-called Offner type concentric optical system with a same magnification consisting of a concave reflecting mirror and a convex reflecting mirror. It is.

The configuration of the second embodiment shown in FIG. 3 is a combination of two Schwarzschild type concentric optical systems as a reduction system. That is, object a. Along the optical path, an intermediate image a1 is formed by the first Schwarzschild optical system Sl consisting of a convex reflecting mirror R as a first reflecting mirror and a concave mirror R as a second reflecting surface having a concentric center C. , a second Schwarzschild optical system consisting of a concave surface M R + as a fifth reflecting surface that intersects the convex reflection #IR1 as the fourth reflecting mirror and the concentric center C through reflection at the image plane capturing mirror M. S2
A final image al is formed.

The structure of the third embodiment shown in FIG. 4 is a combination of the first Schwarzschild optical system SI as an enlargement system and the second Schwarzschild optical system S2 as a reduction system. That is, along the optical path from the object a0, there is a second mirror having a concentric center C, with the concave reflecting mirror R1 as the first reflecting mirror.
An intermediate image al is formed by the first Schwarzschild optical system Sl consisting of a convex mirror R as a reflecting surface, and through reflection by an image plane correction mirror M, a convex reflection M as a fourth reflecting mirror is formed.
A final image a is formed by a second Schwarzschild optical system S consisting of a concave mirror R as a fifth reflecting surface having a concentric center Ct with RI.

In these embodiments as well, the radius of curvature of the reflection MM for image plane correction is given by the above equation (2), and a flat reduced image a of the planar object a0 is clearly formed.

The fourth embodiment shown in FIG. 5 uses a reduction system Nyuwarzschild optical system S1 on the object side, and a same-magnification concentric system S on the image side.
is used. The same size system is not a Schwarzschild type,
The so-called Offner type is used, which consists of a combination of a concave mirror and a convex mirror, and reflects the light once before and after the concave mirror. Specifically, along the optical path from object a, Schwarzschild optics as a first concentric optical system consisting of a convex reflecting mirror R as a first reflecting mirror and a concave mirror Rl as a second reflecting surface having a concentric center C1. An intermediate image a1 is formed by the system SI, and after being reflected by the image correction mirror M,
Concave reflecting mirror R as a fourth reflecting surface having a concentric center C,
．． A final image a is formed by the Offner type second concentric optical system S2 consisting of the convex reflection itRS and the fifth reflection surface itRS. Here, since the second concentric optical system S, is a same-magnification Offner optical system, the final image al is located off the optical axis, but while the intermediate image is curved, the final image As in the previous embodiment, the image is formed as a flat image located within the plane of a.

In this fourth embodiment, it is effective to tilt the object plane a0 with respect to the optical axis in order to maintain the principal ray reaching the final image a perpendicular to the image plane. be.

As described above, a projection optical device with a desired reduction magnification is constructed by combining a plurality of concentric optical systems, but the configuration of the first embodiment shown in FIG. 1 can obtain a flat image over the largest area. This is advantageous in increasing the exposed area.

FIG. 6 shows a concrete example of the combination of the two Schwarzund type concentric optical systems shown in FIG. 1.

In FIG. 6, MS is a projection object such as a mask (reticle) corresponding to the object a0, and WF is an exposure surface such as a wafer surface onto which the final image a is transferred. M (Rs.
) is the field curvature correction surface, and other members having the same functions as those in FIG. 1 are given the same symbols. In the configuration shown in Figure 6,
A diaphragm S is arranged in the optical path between the third and fourth reflecting surfaces for image plane correction. Further, the plane mirror M0 is provided to prevent the final image from mechanically contacting the first reflecting mirror R1.

As shown in Figure 6, the direction in which the light ray travels from left to right is positive, the radius of curvature r of the reflecting surface with the convex surface facing the left is positive, and the radius of curvature r of the reflecting surface with the concave surface facing the left is negative. and the surface spacing d
is assumed to be positive in a medium in which the direction of propagation of light rays is positive, and negative in a medium in which the direction of propagation of light rays is negative, and the values of its specifications are shown in the table below. In the table, d0 is the distance from the object plane to the apex of the first reflecting surface, and d6 is the distance from the apex of the fifth reflecting surface to the final image plane, as shown in FIG.

Magnification - 1/3 NA = 0.02 Image height Y = 201am j, = 595.27679 r2 = 2163.40741 r = -300. 90422 (M: reflective surface for image plane correction) r+= 4347.90820 rs= 426.12529 d. = 2059.02029 d = -1575.26720 dz = 2634.39929 ds = -222.74000 d4 = -4459.64835 ds = 3945.38501 d* = -238.79334 Image plane size φ = 20 + nn + Specific numbers above In the example, the object surface is a plane, the image plane is also a plane, and the radius of curvature R1 of the image tube by the first concentric optical system SI. is 410.625, and the radius of curvature R of the object surface for the second concentric optical system S. ．． is -236
．． It is 213. Therefore, it can be seen that the value of the radius of curvature r3 of the third reflective surface serving as the reflective surface for correcting curvature of field is approximately an intermediate value among the radii of curvature described above.

Furthermore, regarding the condition of the above equation (4) related to the Petzval sum, the value of the radius of curvature rw of the reflecting surface for image plane correction determined by the equation (3) is r M =299. It becomes 90502. In this example, the wavelength λ=100
Person, final image height Y = 20 mm, numerical aperture of final imaging light beam NA = 0. 02, the value on the right side of the above equation (4) is as follows.

λ rv' = 5.6 mm Y ''(NA)'' On the other hand, as described above, the radius of curvature r of the image plane correction reflecting surface (third reflecting surface). is -300. 90422, and the value on the left side is r-rw =0.9992, so it can be seen that the above condition (4) is satisfied.

Astigmatism for the final image in the configuration of the above specifications,
Distortion aberration and lateral aberration are shown in FIG.

As can be seen from Fig. 7, the image plane is almost well corrected over an image height of 20 mm, distortion and lateral aberrations are very well corrected, and excellent resolution is achieved even at a one-third reduction magnification. It is clear that the image quality is improved.

〔Effect of the invention〕

As described above, according to the present invention, a reflective imaging optical device has been achieved which is capable of obtaining a flat, high-resolution image over a wide area and capable of reduction projection even though it has a simple configuration consisting of only a spherical surface. This is extremely useful as an xvin optical device that cannot use a refraction system. Furthermore, by tilting the correction spherical reflecting mirror or the object surface, it is possible to improve the telecentricity on the image side, making it more suitable as an exposure apparatus.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of the first embodiment according to the present invention, FIG. 2 is a diagram for explaining the function of a reflecting mirror for correcting field curvature, FIG. The figure is a configuration diagram of the third embodiment,
FIG. 5 is a block diagram of the fourth embodiment, FIG. 6 is a diagram showing a specific numerical configuration example of the first embodiment shown in FIG. 1, FIG. 7 is a diagram of various aberrations, and FIG. 1 is a diagram illustrating a known Schwarzschild optical system. [Explanation of symbols of main parts] S1...first concentric optical system S,...second concentric optical system R,, R*, R s. R +, R i...
・Spherical reflector M...Reflector for image plane correction

Claims (1)

[Scope of Claims] 1) In an optical device having a plurality of reflective spherical optical systems including at least one set of concentric optical systems, at least one intermediate image position of intermediate images formed by the plurality of spherical reflective optical systems. A reflective imaging optical device characterized by having a spherical reflective mirror placed nearby. 2) The reflective imaging according to claim 1, wherein the spherical reflecting mirror is a concave reflecting surface, and the radius of curvature thereof is a value corresponding to a field curvature due to a composite system of the plurality of spherical optical systems. optical equipment.