JPH0562722B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field of the Invention) The present invention relates to a reflective optical system for projecting and exposing a pattern of a mask (original plate) onto a wafer coated with photoresist when manufacturing integrated circuits such as ICs and LSIs. (Background of the Invention) Conventionally, various configurations have been proposed as this type of reflective optical system, but since all of them have unit magnification, when used as an optical system of a projection exposure apparatus, a mask ( The original plate had to be made to the same size as the actual integrated circuit, creating difficulties in mask manufacturing. In addition, there are optical systems that perform reduction projection using only a refractive system without using a reflective surface, but these are generally made up of a dozen or more glass members, which causes the problem that the absorption of light by the glass members increases. have. A catoptric reduction projection optical system capable of solving such problems was proposed in Japanese Patent Application No. 1983-1983 by the same inventor as the present application.
It was previously filed by the same applicant as No. 152502. The catoptric reduction projection optical system disclosed in this prior application has a concentric reflecting surface and a refractive surface, and a curvature arranged at an object point position on the optical axis or an image point position on the optical axis of the entire optical system, or a position conjugate thereto. It was composed of a refracting surface with an infinite radius. Assuming that the materials used as refractive members have the same refractive index N, each time an aerial image is formed in the optical system, it is reduced (or enlarged) by ±1/N times (or ±N times). ) is capable of obtaining magnification, and consists of a partial optical system with 3 reflections by concentric reflecting surfaces and 1
By combining it with a double-reflection partial optical system, the Petzval sum can be corrected well and excellent imaging performance can be maintained. However, in the configuration disclosed in this prior application, in order to obtain a higher reduction ratio (enlargement ratio),
The disadvantage is that the partial optical system must be stacked in several stages, making the configuration complicated and large. (Object of the Invention) An object of the present invention is to solve the above-mentioned problems, and to create a catoptric reduction projection optical system that has a relatively simple and compact configuration and is capable of reducing projection at a higher magnification. Our goal is to provide the following. (Summary of the Invention) The catoptric reduction projection optical system according to the present invention uses an aplanatic surface in addition to a concentric reflecting surface to enable projection at a higher reduction ratio (higher magnification ratio). That is, the catoptric reduction projection optical system according to the present invention is basically a coaxial optical system in which a plurality of reflecting surfaces and refractive surfaces are arranged with their centers of curvature located on the same optical axis. The refractive surface includes an aplanatic refractive surface, and the apparent object point position on the optical axis of each of the plurality of reflective surfaces substantially coincides with the center of curvature of each of the reflective surfaces,
Are the refractive surfaces other than the aplanatic refractive surface configured such that the center of curvature of each refractive surface coincides with the apparent position of an object point on the optical axis for each refractive surface,
Alternatively, it is composed of a plane refractive surface disposed at the object point position, image point position, or a position conjugate thereto of the optical system. In addition, the apparent on-axis object point related to a certain surface in the present invention corresponds to the on-axis image point of the optical surface in front of that surface, and means the apparent on-axis object point seen from that surface. . The specific requirements for the configuration of the present invention are summarized as follows. (Requirement 1) All magnifications related to reflective surfaces must be -1x. That is, the position of the object point on the optical axis and the position of the image point on the optical axis for each reflecting surface are the same. (Requirement 2) Regarding the refractive surface, it must be one of the following three types. Refracting surface No. 1: The position of the object point on the optical axis of the refractive surface coincides with the center of curvature of the refractive surface. That is, they form concentric refractive surfaces. Refracting surface 2: Let the object point distance related to the refractive surface be s, the image point distance be s', the refractive surface in the front space along the ray traveling direction of the refractive surface be N, and the refractive index in the rear space be When N', the relationship R=Ns/N+N'=N's'/N+N' is satisfied with the radius of curvature R of the refracting surface. In other words, it forms an aplanatic refractive surface. Refracting surface 3: It is placed at the object point position on the optical axis or the image point position on the optical axis of the entire optical system, or at a position conjugate thereto, and its radius of curvature is infinite. The terms used in the above requirements shall be interpreted in a broad sense. That is, the term "appranatic" includes a state that is almost apranatic, and the term "concentric" includes a state that is almost concentric. Furthermore, in the present invention, reduction and enlargement are only differences when the positions of the image plane and object plane are reversed, and it goes without saying that they are substantially equivalent based on the principle of ray reversal. do not have. The magnifications of the refractive surface and the reflective surface, which are the basis of the present invention, will be explained below. As is easily understood from Gaussian optics, a concentric reflecting surface has a magnification of -1, and a refractive surface with an infinite radius of curvature has a magnification of +1. Also, the magnification β _c on a concentric refractive surface is calculated as β _c when the refractive index of the medium in the front space is N and the refractive index of the medium in the rear space is N′ along the traveling direction of the light ray incident on the surface. =N/N'. In other words, when light enters a concentric refracting surface made of a medium with a refractive index n from air, the magnification on this surface is 1/n, and conversely, when the concentric refractive surface made of a medium with a refractive index n When ejecting from this medium into the air, the magnification in this plane is n. On the other hand, the magnification β _a on an aplanatic refracting surface is β _a =(N/N') ² . That is, as shown in Fig. 2, the radius of curvature of the interface between a medium with a refractive index N and a medium with a refractive index N' is assumed to be R, and the image Q' of an axial object Q at an object distance s ₀ has an image distance of
s ₀ ′, and if this boundary surface is aplanatic for this object Q, s ₀ = (1+N′/N)R s ₀ ′=(1+N/N′)R holds true. Then, the magnification β _a regarding this aplanatic surface is the ratio of the object distance to the image distance, and is defined as β _a = s ₀ '/N'/s ₀ /N, so by substituting the above value, We obtain β _a = (N/N') ² . In other words, the image magnification of an aplanatic surface is equal to the square of the ratio of the refractive index of the medium in the front space to the refractive index of the medium in the rear space of the surface. Therefore, when the air enters an aplanatic surface made of a medium with a refractive index n, the medium on this surface becomes (1/n) ² , and conversely, the aplanatic surface made of a medium with a refractive index n When a surface is ejected from this medium into the air, the magnification on this surface is ⁿ² . The present invention focuses on the properties of such an aplanatic surface, and by combining the aplanatic surface with a concentric reflecting surface and a concentric refractive surface, a catoptric projection optical system with a higher reduction ratio or higher magnification is made possible. be. By the way, in reflective optical systems, the advantage of arranging optical surfaces concentrically and arranging object points and image points in a plane that includes the center and is perpendicular to the optical axis has been pointed out for a long time. Let's take a look at its properties in the structure of the present invention. In particular, third-order aberration coefficients will be used to represent aberrational properties. The symbol for the third-order aberration coefficient is based on the one described in "Lens Design Method" (written by Yoshiya Matsui, published by Kyoritsu Shuppan). That is, where I: spherical aberration, : comatic aberration, : astigmatism, : spherical field curvature, : distortion aberration, and P: Petzval sum, here, there is the following relationship: =+P. If the axial object point position of the reflective or refractive surface of interest coincides with the center of curvature of that surface, I==0. Furthermore, since the angle of incidence of the light beam in the direction of the sphere is always 90°, it simply passes through without undergoing regular reflection or refraction, so that =0. Next, for a refractive surface with an infinite radius of curvature located at the axial object point position or axial image point position of the entire optical system, or a position conjugate thereto, the incident height h of the paraxial marginal ray is n=0. Therefore, I===0 Also, since the radius of curvature is infinite, P=0 And, since =+P, =0. Therefore, in an optical system consisting of a concentric reflecting surface and a refractive surface, and a refractive surface with an infinite radius of curvature located at the object point position or image point position of the entire optical system, or at a position conjugate thereto, the concentric reflecting surface and the refractive surface are By appropriately selecting the radius of curvature with respect to the refractive surface so that P=0, an optical system where I===P==0 can be realized. The above is the content disclosed in the aforementioned earlier application (Japanese Patent Application No. 152502/1982). Next, aberrations in the aplanatic refractive surface, which is a feature of the present invention, will be explained. In an aplanatic refractive surface, I===0. Therefore, by selecting the radius of curvature of the aplanatic refracting surface so that the Petzval sum due only to the aplanatic refractive surface cancels out and becomes almost zero, if P=0, the spherical truncated area due only to the aplanatic refractive surface can be reduced. Since the field curvature is =+P, it becomes =0. It is also necessary that a plurality of aplanatic refractive surfaces be provided in the optical system of the present invention, and that the Petzval sums concerning only these aplanatic surfaces be independently corrected. That is, the radius of curvature of the aplanatic refracting surface is R _i , the refractive index of the space in front along the direction of propagation of the ray is N _i , and the refractive index of the space behind it is
Assuming that there is an aplanatic refractive surface K in the entire optical system as N _i ′, the condition of _K 〓 ⁱ⁼¹ N _i ′−N _i /R _i N _i N _i ′=0 is approximately satisfied. This is also a requirement of the present invention. It is also necessary that the Petzval sums for only refractive surfaces and reflective surfaces excluding aplanatic refractive surfaces be corrected. That is, the radius of curvature of the reflective and refractive surfaces other than the aplanatic refracting surface is r _i
If the entire optical system includes a reflective surface and a refractive surface other than the aplanatic refractive surface, then
Assuming that the refractive index is determined in the same manner as above, it is also a requirement of the present invention that the condition _L 〓 ⁱ⁼¹ N _i ′−N _i /r _i N _i N _i ′=0 is substantially satisfied. (Example) Hereinafter, the present invention will be described in detail based on the configuration of the illustrated example. In the following, a case will be described in which the object plane is placed on the reduction side for convenience in design, and an image of this object is projected in an enlarged manner. As mentioned above, whether it is a reduction or an enlargement, the direction in which the light ray travels is simply reversed, and they are substantially equivalent. The configuration of the first embodiment shown in FIG. 1A is the basic configuration of a catoptric reduction projection optical system according to the present invention, and FIG. 1B is a partially enlarged view. This first embodiment is a reflective reduction projection optical system with a reduction ratio of approximately 1/3.4, and includes a first partial optical system _K1 and a second partial optical system K1.
It has a partial optical system _K2 . Here, an enlarged image of the object on the object plane O is formed on the image plane I by the first partial optical system _K1 , and from this enlarged object image, the image plane I is formed by the second partial optical system _K2 . The explanation will be given assuming that a further enlarged object image is formed on . In this case, the configuration is such that the image plane I of the first partial optical system coincides with the object plane O' of the second partial optical system. This first embodiment is composed of a total of 12 refractive surfaces and reflective surfaces, and the first partial optical system _K1 includes a first concentric refractive surface whose center of curvature coincides with the on-axis object point _C0 . A first applanatic lens L _A1 having R _C1 and a first applanatic surface R _A1 that is applanatic with respect to the on-axis object point _{C0 ;}
A concave first reflecting surface M ₁ having the center of curvature at C ₁ , a second concentric refractive surface R _C2 having the same center as the first reflecting surface M ₁ , and an on-axis image by the second concentric refractive surface R _C2 The second aplanatic for point C ₁
It consists of a second aplanatic lens L _A2 having an aplanatic surface R _A2 . Further, the second partial optical system K ₂ has a third optical system centered on the on-axis image point C ₂ formed by the second applanatic surface R _A2 in the first partial optical system K ₁ .
a concentric lens L _C having a concentric refractive surface R _C3 and a fourth concentric refractive surface R _C4 ; a concave second reflecting surface M ₂ having a center of curvature at an axial image point C ₂ formed by the concentric lens L _C ; a convex _third reflective surface _M3 whose center of curvature coincides with the reflective surface M2; a concave fourth reflective surface _{M4 whose} center of curvature coincides with the third reflective surface _M3 ; The third reflective surface M ₃ is arranged to face the fourth reflective surface M ₄ . A fifth reflective surface whose center of curvature coincides with the fourth reflective surface _M4 .
It consists of a third applanatic lens L _A3 having a concentric refractive surface R _C5 and a third applanatic surface R _A3 that is applanatic with respect to the axial image point C ₂ formed by the fifth concentric refractive surface. Therefore, in the above configuration, if we consider the conjugate relationship regarding the three aplanatic surfaces R _A1 , R _A2 , and R _A3 , the optical system consists of a reflecting surface and a refractive surface that are substantially concentric with each other. It can be said that it is a system. In this embodiment, the object O side has a telecentric configuration. In the configuration of the first embodiment shown in FIG. 1, the second reflective surface M ₂ and the fourth reflective surface M ₄ are on the same curved surface, resulting in a simple configuration, but the configuration is not limited to this. It's not something you can do. Further, although the third reflecting surface M ₃ and the exit surface R _C4 of the concentric lens L _C have the same radius of curvature, it is not necessary that these also match. Specific numerical examples of the first embodiment are shown in Table 1 below. The following tables, including Table 1, show the radius of curvature, surface spacing, and refractive index of each curved surface in the order from the object surface O side toward the final image surface I'. In the table, the radius of curvature and refractive index of each surface are defined as positive in the direction of travel of the ray from left to right in the figure, and their positive and negative values are determined based on this. It is assumed that it is positive in a certain medium, and negative in a medium in which the traveling direction of the ray is negative. Therefore, the illustrated first
In the configuration of the embodiment, for example, since the light ray from the object plane O first proceeds from left to right, the distance between the object plane O and the first concentric refractive surface R _C1 and the refractive index therebetween are given as positive values. It will be done. In addition, the image magnification on each surface is also listed in the table.

【table】

[Table] Here, the rays are displayed in the order in which they were traced in the opposite direction, so the magnification is 1/3.375.
This is an optical system that can perform double reduction projection. The focal length of the entire system in the first embodiment is f=243.789, which is normalized to f=1.0 and the third-order aberration coefficients are calculated. Table 2 below shows the results. The inclination angle of the paraxial marginal ray on the object plane required for aberration calculation is α ₀ and the height from the optical axis is h ₀ , and the inclination angle of the paraxial principal ray on the object plane _{is 0} and the height from the optical axis. When is set to ₀ , and the object plane is considered to be the 0th plane, h ₀ =0. In addition, since it is used in a telecentric state on the object side, the value of α ₀ , ₀ only has the function of multiplying the aberration coefficient by a certain constant, so here, α ₀ = −1, The third-order aberration coefficient was calculated using the following initial values: h ₀ = 0, ₀ = ₀ , 0 = 1. In the table, Σ represents the total of the entire system.

[Table] From the values of the third-order aberration coefficients described above, it is clear that the optical system of the first example is well corrected for the four aberrations related to sharpness, except for distortion among Seidel's five aberrations. FIG. 3A is a diagram showing the basic configuration of a second embodiment of the present invention, and FIG. 3B is a partially enlarged view. In addition to the configuration of the first embodiment described above, this embodiment has a planar refractive surface that coincides with or is close to the object surface. A catoptric reduction projection optical system having a plane refractive surface near the object plane or image plane is disclosed in the previous patent application No. 59-152502.
However, this second embodiment enables a higher reduction ratio by adding an aplanatic refractive surface to the structure disclosed in the above-mentioned earlier application. Figures 3A and 3B, Figures 1A and 1B
The same symbols are attached to the members having the same functions as those shown in the drawings. This second embodiment is a catoptric reduction projection optical system with a reduction ratio of approximately 1/7.6.
This second embodiment also includes a first partial optical system _K1 and a second partial optical system K1.
It has a partial optical system _K2 . Each partial optical system is basically similar to the configuration of the first embodiment, but instead of concentric refractive surfaces, each system has an infinite radius of curvature near the object plane and its conjugate surface. This is a reflective optical system having a refractive member having a flat refractive surface. The position of this plane refractive surface is placed near the object plane or image plane on the reduction side of each partial optical system, so that the image plane of the first partial optical system coincides with the object plane of the second partial optical system. It is structured as follows. Here, too, an enlarged image of the object on the object plane O is formed on the image plane I by the first partial optical system, and from this enlarged object image, the image on the image plane is formed by the second partial optical system _K2 .
The following explanation assumes that a further enlarged object image is formed on I′. The second embodiment is composed of a total of 12 refractive surfaces and reflective surfaces, and the first partial optical system _K1 includes a first plane refractive surface R _P1 that coincides with the object plane and an on-axis object point C _a first refractive member P ₁ having a first applanatic surface R _{A1 that is} applanatic with respect to ₀ ;
a reflective surface M ₁ , a first concentric refracting surface R _C1 having the same center as the first reflective surface M ₁ , and a second aplanatic refracting surface R C1 that is aplanatic with respect to the axial image point C ₁ by the first concentric refracting surface R _C1 ;
It consists of a first aplanatic lens L _A1 having an aplanatic surface R _A2 . Further, the second partial optical system K ₂ has a second planar refractive surface R _P2 that coincides with the axial image point C ₂ formed by the second aplanatic surface R _A2 in the first partial optical system K ₁ and is perpendicular to the optical axis. , a second refractive member _P2 having _{a second concentric refractive surface R C2 whose} center of curvature coincides with _the on-axis image point _C2 ; a second reflective surface _M2 ; a convex third reflective surface _M3 whose center of curvature coincides with the second reflective surface M2; and a concave fourth reflective surface _{M4 whose} center of curvature coincides with the third reflective surface _M3 . , the third reflective surface M3 is disposed to face the second reflective surface _M2 and the _fourth reflective surface _M4 . A third reflective surface whose center of curvature coincides with the fourth reflective surface _M4 .
It consists of a second applanatic lens L _A2 having a concentric refractive surface R _C3 and a third applanatic surface R _A3 that is applanatic with respect to the axial image point C ₂ formed by the third concentric refractive surface. Therefore, in the above configuration, except for the plane refractive surface arranged almost coincident with the object plane or image plane, considering the conjugate relationship regarding the three aplanatic surfaces R _A1 , R _A2 , R _A3 , , it can be said that it is an optical system consisting of a reflective surface and a refractive surface that are substantially concentric with each other. In this example, the object O
The side has a telecentric configuration. In the configuration of the second embodiment shown in FIG. 3, the object planes O and O' of each partial optical system are configured to coincide with the incident planes of the respective refractive members P ₁ and P ₂ , but only slightly. It is also possible to separate the
Particularly at the final image plane of the optical system, a working distance is required to place the wafer, so it is effective to provide an air gap of several millimeters. Specific numerical examples of the second embodiment are shown in Table 3 below. Table 3 is also shown as an enlarged projection system for convenience of design, and the radius of curvature, interplanar spacing, and refractive index of each curved surface are shown in the order from the object plane O side to the final image plane I'.

[Table] Here, too, the rays are displayed in the order in which they were traced in the opposite direction, so the magnification is 1/7.5938.
This is an optical system that can perform double reduction projection. In this embodiment as well, the Petzval sum _PZA due to the aplanatic refractive surfaces, that is, the second, fifth, and twelfth surfaces (R _A1 , R _A2 , R _A3 ), is zero as shown below. P _ZA =Σi1/R _i (1/N _i -1/N _i ') =1/-30 (1/1.5-1/1) +1/-40 (1/-1.5-1/-1) +1/ 120 (1/1.5 - 1/1) = 0 The focal length of the entire system in the first embodiment is f = -184.051, but by normalizing this to f = 1.0, Similarly, Table 4 below shows the results of calculating the third-order aberration coefficients using the following initial values: α ₀ =−1, h ₀ =0, ₀ = 0, ₀ =1. In the table, Σ represents the total of the entire system.

[Table] From the values of the third-order aberration coefficients above, it can be seen that the optical system of the second example also has well corrected the four aberrations related to sharpness, except for distortion among Seidel's five aberrations. Also, from the table of third-order aberration coefficients above, in both the first and second embodiments, the Petzval sum by the aplanatic refractive surfaces, that is, the second, fifth, and twelfth surfaces,
It is also confirmed that the Petzval sums for other surfaces are independently zero. (Effects of the Invention) As described above, according to the present invention, it is possible to achieve a reduction projection type reflective optical system with a higher reduction ratio, which is advantageous for manufacturing integrated circuits. Then, by correcting the Petzval sum independently for the aplanatic refractive surfaces, and by correcting the Petzval sum independently for other refractive surfaces and reflective surfaces, the optical system as a whole can eliminate distortion among Seidel's five aberrations. Except for this, it is possible to satisfactorily correct the other four aberrations related to image sharpness.
In addition, by using a reflective optical system that can perform projection at such a high reduction ratio, it is possible to create a mask (original plate) for an integrated circuit pattern larger than the actual size of the circuit, making mask manufacturing extremely difficult. In addition to being easy to manufacture, it is also very effective in manufacturing integrated circuits with finer patterns. Furthermore, even when using light in the far ultraviolet region as the exposure wavelength, there is a small number of refractive members such as glass, so there is an advantage that the transmittance is not deteriorated due to absorption by these members. Furthermore, most of the refractive power of the entire optical system depends on convex and concave reflective surfaces, so when designing with chromatic aberration in mind, it is difficult to remove chromatic aberration in the case of an optical system consisting only of a refractive system. It also has the advantage of being relatively easy. The aplanatic lens of the present invention also has the advantage that the centering process in the manufacturing stage thereof is easier than the concentric meniscus lens as in the embodiment described in the previous application.

[Brief explanation of the drawing]

FIG. 1A is an optical configuration diagram of a first embodiment consisting of the basic configuration of a catoptric projection optical system according to the present invention;
Figure 1B is a partially enlarged view of the first embodiment;
The figure is an explanatory diagram of magnification on an aplanatic refracting surface, FIG. 3A is a block diagram of a second embodiment according to the present invention, and FIG. 3B is a partially enlarged view of the second embodiment. Explanation of the symbols of the main parts, R _A1 , R _A2 , R _A3 ... Aplanatic refracting surface, R _C1 , R _C2 , R _C3 , R _C4 ,
R _C5 , ... concentric refractive surface, O, O' ... object surface, I,
I'... Image plane, M ₁ ... Concave first reflective surface, K ₁ ...
First partial optical system, M ₂ ... concave second reflective surface, K ₂
...Second partial optical system, M ₃ ...Convex third reflective surface,
M ₄ ... concave fourth reflective surface.

Claims (1)

[Scope of Claims] 1. In a coaxial optical system in which a plurality of reflective surfaces and refractive surfaces are arranged with their centers of curvature located on the same optical axis, the plurality of refractive surfaces include an aplanatic refractive surface. , the centers of curvature of the plurality of reflecting surfaces substantially coincide with the apparent on-axis object point positions of the respective reflecting surfaces, and the centers of curvature of each of the refractive surfaces other than the aplanatic refractive surface coincide with the respective refractive surfaces. A catoptric reduction projection optical system characterized in that it is configured to match such an apparent on-axis object point position. 2. In the catoptric reduction projection optical system according to claim 1, the Petzval sum at the aplanatic refracting surface is almost independently corrected by a plurality of aplanatic refracting surfaces, and A catoptric reduction projection optical system characterized in that the Petzval sum on a refractive surface and a reflective surface is almost independently corrected by each of these surfaces. 3. In a coaxial optical system in which a plurality of reflective surfaces and refractive surfaces are arranged with their centers of curvature located on the same optical axis, and an object surface and an image surface are on a plane perpendicular to the optical axis, The plurality of refractive surfaces include an aplanatic refractive surface and a planar refractive surface provided near the image plane or the object surface or a conjugate position thereof and substantially parallel to the surface, and the center of curvature of the plurality of reflective surfaces is coincides with the apparent on-axis object point position of each reflecting surface, and the center of curvature of each refractive surface other than the planar refractive surface is on the apparent optical axis of each refractive surface. A catoptric reduction projection optical system characterized in that it is configured to match the position of an object point. 4. In the catoptric projection optical system according to claim 3, the Petzval sum at the aplanatic refracting surface is almost independently corrected by a plurality of aplanatic refracting surfaces, and A catoptric reduction projection optical system characterized in that the Petzval sum on a refractive surface and a reflective surface is almost independently corrected by each of these surfaces.