Patent Number: 053944518
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be explained in conjunction with the drawings. Referring first to FIGS. 1A and 1B showing the first embodiment of the present invention, synchrotron radiation light produced by a synchrotron ring (SOR ring) 1 emanates from an emission point 2. The emitted radiation light is collected and collimated by a first mirror 5a and a second mirror 6a. Denoted at 9 is an exposure shutter for adjusting the amount of exposure. The radiation light then illuminates a mask (original) 3 by which the mask pattern is transferred to a resist applied to a wafer (substrate) 4. The wafer 4 can be displaced by a stage 8, for adjustment of the relative positional relationship between the mask 3 and the wafer 4. Here, each of the first and second mirrors 5a and 6a has formed on its reflection surface a multilayered film or a single-layer film, comprising SiC or Pt. The first mirror 5a has a toroidal shape of curvature, concave in the X direction and convex in the Y direction. The second mirror 6a has a cylindrical shape of curvature, infinite in the X direction and concave in the Y direction. The synchrotron radiation light is reflected by the first mirror 5a, by which it is collected with respect to the X direction. Thus, it is retrieved or accepted with a large angle and is collimated. Then, the synchrotron radiation light is reflected by the second mirror 6a, by which it is collimated with respect to the Y direction. The thus collimated synchrotron radiation light illuminates the mask 3, for transferring the mask pattern (circuit pattern) to the wafer 4. Table 1 below shows specifications of the first and second mirrors. TABLE 1 ______________________________________ X DIRECTION Y DIRECTION RADIUS RADIUS OF OF CURVA- FOCAL CURVA- FOCAL MIRROR TURE LENGTH TURE LENGTH ______________________________________ 1ST 0.279 4.0 -9.97 -0.174 2ND INFINITE INFINITE 238.4 4.167 ______________________________________ Unit:meter (Primary Incidence Angle: 88 deg.) In Table 1, each positive curvature radius corresponds to a concave surface while the negative curvature radius corresponds to a convex surface. The distance from the emission point 2 to the first mirror 5a is 4 m; the distance from the first mirror 5a to the second mirror 6a is 4 m; and the primary incidence angle (angle with respect to the normal to the mirror) of the synchrotron radiation light to the mirror is 88 deg., in both cases of the first and second mirrors 5a and 6a. The synthetic focal length is 4 m in the X direction and 100 m in the Y direction. If the size of the exposure region on the mask is 20 mm square, the angle of acceptance of synchrotron radiation light is 0.29 deg. in the X direction and 0.011 deg. in the Y direction. Namely, in the X direction the light collection is 25 times as much as in the Y direction. As a consequence, the illuminance on the mask increases and it is possible to reduce the exposure time considerably. FIGS. 2A and 2B show the second embodiment of the present invention, wherein like numerals as those in FIGS. 1A and 1B denote corresponding elements. This embodiment differs from the first embodiment in that: first mirror 5b has a toroidal shape of curvature, concave both in the X and Y directions, while second mirror 6b has a toroidal shape of curvature, concave in the X direction and convex in the Y direction. Table 2 below shows specifications of the first and second mirrors. TABLE 2 ______________________________________ X DIRECTION Y DIRECTION RADIUS RADIUS OF OF CURVA- FOCAL CURVA- FOCAL MIRROR TURE LENGTH TURE LENGTH ______________________________________ 1ST 0.101 1.923 17.34 0.227 2ND 0.098 1.875 363.8 4.762 ______________________________________ Unit:meter (Primary Incidence Angle: 88.5 deg.) In Table 2, the distance from the emission point to the first mirror 5b is 5 m; the distance from the first mirror 5b to the second mirror 6b is 5 m; and the primary incidence angle of the light to the mirror is 88.5 deg., in both cases of the first and second mirrors 5b and 6b. The synthetic focal length is 3 m in the X direction and 100 m in the Y direction. If the size of the exposure region on the mask is 20 mm square, the angle of acceptance of synchrotron radiation light is 0.38 deg. in the X direction and 0.011 deg. in the Y direction. Namely, in the X direction the light collection is 33 times as much as in the Y direction. As a consequence, the illuminance on the mask increases considerably. The reflectivity to the X-rays largely depends on the angle of incidence. In an ordinary reflecting mirror using metal or the like, a high reflectivity is attainable provided that the angle of incidence is slightly smaller than 90 deg. If the angle of incidence becomes small, the reflectivity decreases largely. Accordingly, the light has to be projected with an angle of grazing incidence. If a curved surface mirror is used with grazing light incidence and when the radius of curvature of the mirror is R and the angle of incidence is .theta., then the focal length F in a plane defined by a straight line parallel to the projected light and the projection thereof onto the tangential plane to the mirror surface is given by: EQU F=(R.times.cos.theta.)/2. On the other hand, in a plane perpendicular to that plane and containing a straight line parallel to the projected light, the focal length F is given by: EQU F=R/(2.times.cos.theta.). Also, if plural reflecting mirrors are used and when the focal length of a first mirror (5b) is F1 while the focal length of a second mirror (6b) is F2 and the distance from the first mirror to the second mirror is L, then the synthetic focal length FO of this system is given by: EQU FO=(F1.times.F2)/(F1+F2-L). On the basis of these relations, it is possible to design a collimating optical system with different synthetic focal lengths in the X and Y directions while using plural grazing incidence reflecting mirrors, as in the first and second embodiments. Referring to FIGS. 3A and 3B showing the third embodiment, like numerals as those in FIG. 1A and 1B are assigned to corresponding elements. In this embodiment, a first reflecting mirror 5d has a toroidal shape of curvature, concave in the X direction and convex in the Y direction. Second reflecting mirror 6d has a cylindrical shape of curvature, infinite in the X direction and concave in the Y direction. Since the light is collected by the first reflecting mirror 5d in the X direction, the radiation light can be accepted with a large angle. Table 3 shows specifications of the mirrors of this embodiment. TABLE 3 ______________________________________ X DIRECTION Y DIRECTION RADIUS RADIUS OF OF CURVA- FOCAL CURVA- FOCAL MIRROR TURE LENGTH TURE LENGTH ______________________________________ 1ST 0.419 3.0 -2.75 -0.096 2ND INFINITE INFINITE 88.6 3.09 ______________________________________ Unit:meter (Primary Incidence Angle: 86 deg.) Here, the distance from the emission point to the first mirror is 3 m; the distance from the first mirror to the second mirror is 3 m; and the primary incidence angle of the light to the mirror is 86 deg., in both cases of the first and second mirrors. The synthetic focal length is 3 m in the X direction and 100 m in the Y direction. In this embodiment, each of the mirrors 5d and 6d has formed thereon a multilayered film. More specifically, tungsten and carbon materials are accumulated alternately (each five layers) to provide a multilayered film. In an ordinary reflecting mirror using metal or the like, a high X-ray reflectivity is attainable provided that the angle of incidence (angle with respect to a normal) is about 90 deg. However, with a decreasing incidence angle, the reflectivity reduces largely. To the contrary, if a multilayered film is provided on the surface of a reflecting mirror, it becomes possible to obtain a high reflectivity even with a small angle of incidence. FIGS. 4A and 4B are graphs each showing the wavelength vs. reflectivity characteristic of a reflector where the angle of incidence to the reflector is 86 deg. FIG. 4A corresponds to a multilayered film mirror according to this embodiment wherein tungsten and carbon materials are accumulated alternately (each five layers). FIG. 4B corresponds to a reflecting mirror with a single gold layer. It is seen that the reflecting mirror of this embodiment assures a high reflectivity, as compared with the reflecting mirror of a single gold film. Using a multilayered film reflecting mirror in an exposure apparatus assures the following advantages: (1) The angle of incidence of X-rays can be made small and, therefore, it is possible to use a small size reflecting mirror for illuminating the exposure region. This facilitates reduction in size and cost of the exposure apparatus. PA1 (2) Since the angle of incidence of X-rays can be small, the surface precision of the reflecting mirror becomes less influential. This leads to improved parallelism of illuminating light and, thus, to improved precision of the exposure apparatus. PA1 (a) A mask can be illuminated with substantially collimated (parallel) synchrotron radiation light. Thus, it is possible to avoid or reduce the run-out error. This assures improved positional precision of a transferred pattern. PA1 (b) Since the synchrotron radiation light can be accepted efficiently, the strength of illumination is high. This effectively reduces the exposure time and increases the throughput of the exposure apparatus. Referring to FIG. 5 showing the fourth embodiment of the present invention, like numerals as those of FIGS. 1A and 1B are assigned to corresponding elements. Two flat reflecting mirrors 7--7 are provided to divide synchrotron radiation light into three beams. For each of the three divided beams, there are provided two mirrors 5c and 6c so as to collect and collimate the corresponding beam. The three radiation beams each being collected and collimated are directed to three exposure units, respectively, for executing mask pattern transferring operations in these units, respectively. Each mirror 5c or 6c is of the same structure as that described above with reference to one of the first to third embodiments of the present invention. In an arrangement according to this embodiment, a plurality of exposure units can operate simultaneously in conjunction with a single port and, therefore, the efficiency of utilization of synchrotron radiation light is high. Also, the area of floor necessary for the exposure apparatus as a whole can be made small. The present invention is not limited to the forms of the embodiments described above. Any combination of mirrors may be used to collect light in the X and Y directions, provided that the absolute value of its synthetic focal length in the X direction is smaller than that in the Y direction. With the above-described structure of the present invention, various advantageous effects are obtainable. Examples are as follows: It is to be noted that the present invention is not limited to an exposure apparatus. The invention is applicable also to an X-ray microscope, an optical CVD apparatus, or an optical etching apparatus, for example. Referring now to FIGS. 6 and 7, description will be made of a semiconductor device manufacturing method according to an embodiment of the present invention, which uses one of the exposure apparatuses of the first and fourth embodiments. FIG. 6 is a flow chart of the sequence of manufacturing a semiconductor device such as a semiconductor chip (e.g. IC or LSI), a liquid crystal panel or a CCD, for example. Step 1 is a design process for designing the circuit of a semiconductor device. Step 2 is a process for manufacturing a mask on the basis of the circuit pattern design. Step 3 is a process for manufacturing a wafer by using a material such as silicon. Step 4 is a wafer process which is called a pre-process wherein, by using the so prepared mask and wafer, circuits are practically formed on the wafer through lithography. Step 5 subsequent to this is an assembling step which is called a post-process wherein the wafer processed by step 4 is formed into semiconductor chips. This step includes assembling (dicing and bonding) and packaging (chip sealing). Step 6 is an inspection step wherein operability check, durability check and so on of the semiconductor devices produced by step 5 are carried out. With these processes, semiconductor devices are finished and they are shipped (step 7). FIG. 7 is a flow chart showing details of the wafer process. Step 11 is an oxidation process for oxidizing the surface of a wafer. Step. 12 is a CVD process for forming an insulating film on the wafer surface. Step 13 is an electrode forming process for forming electrodes on the wafer by vapor deposition. Step 14 is an ion implanting process for implanting ions to the wafer. Step 15 is a resist process for applying a resist (photosensitive material) to the wafer. Step 16 is an exposure process for printing, by exposure, the circuit pattern of the mask on the wafer through the exposure apparatus described above. Step 17 is a developing process for developing the exposed wafer. Step 18 is an etching process for removing portions other than the developed resist image. Step 19 is a resist separation process for separating the resist material remaining on the wafer after being subjected to the etching process. By repeating these processes, circuit patterns are superposedly formed on the wafer. The semiconductor device manufacturing method of this embodiment uses the optical arrangement described with reference to any one of the first to fourth embodiments of the present invention described hereinbefore. It is therefore possible to produce semiconductor devices of higher degree of integration. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.