JPH0525086B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a reflective optical system, and more particularly to a reflective optical system used in a projection exposure apparatus for manufacturing integrated circuits such as ICs and LSIs. Conventionally, a reflective optical system for printing patterns of integrated circuits such as ICs and LSIs onto silicon wafers using a projection exposure apparatus has been disclosed, for example, in Japanese Patent Application Laid-open No. 12039/1983.
This has been proposed in Japanese Patent Application Laid-Open No. 53-100230. The reflective optical systems used in these projection exposure apparatuses have extremely high resolving power. The shorter the wavelength used, the better the resolution of the projected image becomes. For this purpose, a light source that emits as short a wavelength as possible is used. In order to obtain high resolution not only in the center of the screen but also over a wide screen, an optical system is used that has almost completely corrected aberrations. An imaging optical system using a lens is usually constructed using a plurality of glass materials in order to correct chromatic aberration. Light on the short wavelength side is advantageous for obtaining high resolution, but chromatic dispersion is large on the short wavelength side, so it is difficult to properly correct chromatic aberration due to the design. For this reason, an optical system using a reflective mirror is suitable for the imaging optical system for printing IC patterns, etc., which requires high resolution. This is because it has the characteristics of being able to be used in a variety of ways, and that the transmittance of the entire optical system can be increased compared to when a lens system is used. An object of the present invention is to provide a reflective optical system that can obtain high resolution and is suitable for a projection exposure apparatus that takes advantage of the characteristics of a reflective optical system. This reflective optical system to achieve this purpose has a plurality of mirror systems that reflect light and form an image in the order of a concave mirror, a convex mirror, and a concave mirror. The magnification of at least one mirror system is reduced while the magnification of at least one other mirror system is configured to be located between a concave mirror and a convex mirror, and to form a reduced image by sequentially repeating imaging with each mirror system. The feature is that the magnification is set to enlarge. Next, embodiments of the present invention will be described with reference to each drawing. FIG. 1 is a schematic diagram of a reflective optical system according to an embodiment of the present invention. The reflective optical system in the same figure has _a center of curvature in the same direction as _shown in _FIGS .
A plurality of S′ are arranged so that they are located on substantially the same optical axis and the overall imaging magnification is reduced, thereby achieving a reflective optical system with high resolution. . In the embodiment shown in FIG. 1, the reflecting mirrors M ₁ to _{M 3} are used as the first mirror system S ₁ , the reflecting mirrors M ₄ to M ₆ are used as the second mirror system S ₂ ,
Third mirror system S ₃ with reflectors M ₇ to _{M 9} and reflector
_M10 to _M12 each constitute a fourth mirror system _S4 . Further, as shown in the figure, each mirror system is configured such that one concave mirror is located between the other concave mirror and convex mirror in the optical axis direction. Then, the object point P ₁ is sequentially mirrored by the mirror system S ₁ , S ₂ , S ₃ , S ₄
The images are repeatedly formed at each point, and the image is finally formed at an image point _P5 at an imaging magnification of 1/5. Next, the image forming state of the mirror system constituting the reflective optical system shown in FIG. 1 will be explained. In Fig. 2 and Fig. 3, three reflecting mirrors M ₁ ,
M ₂ and M ₃ are the image point after the light beam L ₁ from the object point P ₁ is reflected in the order of concave mirror M ₁ , convex mirror M ₂ and concave mirror M ₃
It is arranged so that the image is focused on P ₁ ′. In the mirror system S shown in Figure 2, the concave mirror _M3 is a convex mirror.
The mirror system S' _shown in FIG _. 3 is arranged such that the concave mirror _M1 is located between the convex mirror _M2 and the concave mirror _M3 .
If this system is corrected so that the astigmatism becomes zero off the optical axis 0-0', the object in this example will have an arc-shaped portion Q ₁ as an effective surface as shown in Fig. 4. There is. For this reason, the external shapes of the three reflecting mirrors M ₁ , M ₂ , and M ₃ do not necessarily have to be circular, but unnecessary parts are deleted, for example, the lower part of the concave mirror M ₁ is deleted as shown in Fig. 3. However, it may also have a shape as shown in the figure. As a result, a configuration can be adopted in which the concave mirror _M3 is placed to the right of the concave mirror _M1 . In this embodiment, the light beam from the object point P is divided into 1 by using a plurality of mirror systems consisting of a concave mirror M ₁ , a convex mirror M ₂ , and a concave mirror M ₃ , that is, reflecting mirrors with positive, negative, and positive refractive powers. Various aberrations generated by the single mirror system, especially coma aberration and field curvature, are reduced, resulting in good overall imaging performance. Figure 5 shows two mirror systems S ₁ , part of Figure 1;
FIG. 3 is an explanatory diagram of an imaging light beam when the optical path of S ₂ is developed. In the figure, concave mirror M ₁ , convex mirror M ₂ and concave mirror M ₃ are connected to the first mirror system S ₁ , concave mirror M ₄ and convex mirror
M ₅ and concave mirror M ₆ each constitute a second mirror system S ₂ . The light flux L ₁ from the object point P ₁ is transformed into an image point by the mirror system S ₁
P ₁ ′, that is, the object point P ₂ of the mirror system S ₂ , and the mirror system S ₂ focuses the light beam L ₂ from the object point P ₂ on the image point P ₂ ′. Although not shown below, the object image is similarly reduced by sequentially repeating imaging using mirror systems S ₃ and S ₄ . _In this _{way ,} in this embodiment, as _shown in _{FIG . P2}
to the image point P ₂ ′, that is, to the object point P ₃ of the mirror system S _3. In the same way, images are sequentially formed by the mirror system S ₃ and then the mirror system S ₄ , and finally the object point P ₁ is transformed to the image point P ₄ ′. It is focused on. In this embodiment, the mirror systems S ₁ , S ₂ , and S ₃ are configured as a reduction system, and the mirror system S ₄ is configured as an expansion system. Table 1 specifically shows the imaging magnification of each mirror system.
It's like this.

[Table] In this example, in order to achieve an overall imaging magnification of 1/5, aberration correction is performed in a well-balanced manner by configuring the four mirror systems to have the imaging magnification shown in Table 1. It's on. In particular, by making mirror system S ₄ an enlargement system, coma aberration, field curvature, distortion, etc. occurring in mirror systems S ₁ to _{S 3} are favorably corrected. In this embodiment, the vignetting of the light beam that occurs when a reflective optical system is configured with a plurality of mirror systems is suppressed between the first mirror system _S1 and the second mirror system _S2 , and between the second mirror system _S2 . A total reflection mirror between the third mirror system _S3 and
This is prevented by arranging H ₁ and H ₂ . In this embodiment, in order to particularly reduce vignetting of the luminous flux and obtain good overall optical performance, the concave mirrors constituting each of the four mirror systems S ₁ , S ₃ , and S ₄ are
When the radius of curvature of M ₁ , convex mirror M ₂ and concave mirror M ₃ is R ₁ , R ₂ , R ₃ respectively, R ₁ /R ₂ ≧2 ...(1) R ₁ /R ₃ >1 ...(2) It is preferable to satisfy the following conditions. Conditional expressions (1) and (2) are used to reduce the size of the entire mirror system while suppressing the occurrence of off-axis aberrations when the effective screen of the object is a part of an arc shape as shown in Figure 4. This is to allow the light beam to form an image at a predetermined position without vignetting. If conditional expression (1) is not satisfied, coma flare will increase and vignetting of the luminous flux will increase. If conditional expression (2) is not satisfied, the radius of curvature of concave mirror _M3 becomes too large compared to concave mirror _M1 , making it difficult to obtain a predetermined refractive power while downsizing the mirror system as a whole. In particular, in this embodiment, in order to reduce field curvature and obtain a high contrast object image, the radius of curvature R ₁ ,
R ₂ and R ₃ are further set as |R ₁ |>|R ₃ |>|R ₂ |...(3). If this condition is violated, the curvature of field increases, the vignetting of the light beam increases, and it becomes difficult to perform good aberration correction in a reflective optical system that combines a plurality of mirror systems. A reflective optical system with high resolution can be easily achieved by configuring the mirror system so as to satisfy each of the above conditional expressions, but it is more preferable to configure the mirror system S ₂ , S ₃ ,
When the radius of curvature of each reflecting mirror of S ₄ is R ₄ to R ₁₂ in order, 1.18<R ₄ /R ₅ <1.26 ...(4) 1.02<R ₆ /R ₅ <1.17 ...(5) 2.03< R ₇ /R ₈ ＜2.65 …(6) 1.35＜R ₉ /R ₈ ＜1.47 …(7) 2.03＜R ₁₀ /R ₁₁ ＜2.15 …(8) 1.18＜R ₁₂ /R ₁₁ ＜1.26 … ...(9). Conditional expression (4) is for properly correcting coma aberration, and if the upper or lower limit of conditional expression (4) is exceeded, coma flare increases. Conditional expressions (5) and (7) are intended to increase the contrast of the sagittal image plane, and if the upper or lower limits of conditional expressions (5) and (7) are exceeded, the field curvature increases in the positive or negative direction. I don't like it because it comes. Conditional expressions (6) and (8) are intended to reduce the flare component of the meridional image plane, and if the upper or lower limits of conditional expressions (6) and (8) are exceeded, the field curvature increases in the positive or negative direction. It's coming. Conditional expression (9) is preferable because it reduces the astigmatism difference over the entire screen when the object has an arc-shaped effective screen, and if the upper or lower limit of conditional expression (9) is exceeded, the astigmatism difference becomes large. do not have. In addition, in the present invention, the imaging adjustment of the object image is performed using a mirror system.
It is preferable for aberration correction and magnification adjustment to be performed by moving at least one of the mirror systems S ₁ to S ₄ . Next, various numerical values of the embodiment shown in FIG. 1 will be shown. Ri
is the radius of curvature of the i-th reflecting mirror counting from object point P ₁ , and D is the distance between each reflecting mirror when measured from left to right along the direction of light travel, and the opposite is negative. Shown as The effective screen of the object is 3 mm in slit width, and the effective F number is 2.8 (NA - 0.18). The effective screen width of the object point _P1 is within a height range of 207 to 210 mm from the optical axis. R D 1-626.9 -280 2 -134.4 224.875 3 -386.5 -990 4 576.518 330 5 472.86 -292.3 6 516.98 540.15 7 -366.935 -118.4 8 -147.45 141.75 9 - 208 −578.35 10 435.96 400 11 209.22 −148.494 12 252.51 or more According to the present invention, a high-resolution reflective optical system can be achieved by appropriately combining four mirror systems.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention;
2, 3, and 5 are explanatory diagrams of a part of FIG. 1, FIG. 4 is an explanatory diagram of the effective screen of an object according to the present invention, and FIG. 6 is an explanatory diagram of various aberrations of the optical system of FIG. 1. It is a diagram. In the figure, Y is the object height, S ₁ to _{S 4} are mirror systems, and M ₁ to
M ₁₂ each indicates a reflecting mirror.

Claims (1)

[Scope of Claims] 1 It has a plurality of mirror systems that reflect light and form an image in the order of a concave mirror, a convex mirror, and a concave mirror, and each mirror system is arranged so that one concave mirror connects the other concave mirror and convex mirror in the optical axis direction. The magnification of at least one mirror system is set to reduce and the magnification of at least one other mirror system is set to enlarge so that a reduced image is formed by sequentially repeating imaging with each mirror system. A reflective optical system featuring 2. The plurality of mirror systems include mirror systems S ₁ , S ₂ , S ₃ , S ₄ in order from the object side, and mirror systems S ₁ , S ₂ ,
_{The magnification} of S _{3 is} set to reduction, and the magnification of mirror system S _{4 is} set to expansion. The radius of curvature of the concave mirror, convex mirror, and concave mirror of S ₂ is
R ₄ , R ₅ , R ₆ , the radius of curvature of the concave mirror, convex mirror, and concave mirror of mirror system S ₃ is R ₇ , R ₈ , R ₉ , the radius of curvature of the concave mirror, convex mirror, and concave mirror of mirror system S ₄ is R ₁₀ , R ₁₁ ,
The reflective optical system according to claim 1, characterized in that, when _R12 , the following conditions are satisfied. R ₁ /R ₂ ≧2 R ₁ /R ₃ ＞1 ｜R ₁ ｜＞｜R ₃ ｜＞｜R ₂ ｜ 1.18＜R ₄ /R ₅ ＜1.26 1.02＜R ₆ /R ₅ ＜1.17 2.03＜R ₇ /R ₈ <2.65 1.35 <R ₉ /R ₈ <1.47 2.03 <R ₁₀ /R ₁₁ <2.15 1.18 <R ₁₂ /R ₁₁ <1.26