Patent Number: 051827638
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, showing an exposure apparatus according to an embodiment of the present invention, denoted at 1 is a radiation source such as an X-ray source, for example, adapted to produce a radiation beam such as X-rays, for example, which contains different wavelengths and which has a relatively wide bandwidth and a relatively small divergent angle. Denoted at 2 is a scan and diffusion device having a swingable mirror; at 3 is a mask on which a circuit pattern is formed; at 4 is a semiconductor wafer having a resist applied to the surface thereof; at 30 is a mask stage for holding the mask 3; and at 40 is a wafer stage for holding the wafer 4. The radiation source 1, the scan and diffusion device 2 and the stages 30 and 40 are disposed along a horizontal axis. The scan and diffusion device 2 is arranged to reflect the X-ray beam 101 with its mirror and to scanningly deflect the X-ray beam in a direction or directions as denoted by a double-head arrow S in FIG. 1. In this manner, it produces a diffused X-ray beam (reflection beam) with which the mask 3 is illuminated and, thus, the resist of the wafer 4 is exposed to the mask pattern with the X-ray beam. Although not shown in FIG. 1, the X-ray beam is propagated within a tube or a chamber the inside of which is evacuated. If desired, the inside of such a tube or a chamber is filled with He gas, for example. The swingable mirror of the scan and diffusion device 2 has a substrate and a reflection film formed on the substrate, and the reflection film is provided by a multilayered film. Each layer constituting the multilayered film has a thickness which gradually increases with an increase in the distance from the radiation source 1, in the plane of incidence of the radiation beam (X-ray beam) 101. In designing such a mirror, first the angle of incidence of the radiation beam such as the X-ray beam 101 to the mirror (reflection surface) is taken into account, and the necessary angle of swinging movement of the mirror is determined in accordance with the size of a required illumination zone. Then, the optimum multilayered structure of the reflection film is determined. As for the material of the substrate of the reflection mirror, silica (SiO.sub.2) may be the best, particularly in light of the lapping precision and the like. However, any other material may be used. For example, silicon carbide (SiC) or other metal materials may be used. Typically, the multilayered structure film comprises alternately accumulated different-material layers. As regard the materials, preferably two different materials having a large difference in the real portion of the complex index of refraction, with respect to the wavelength of X-rays in question, but having small imaginary portions of the complex index of refraction, may be selected. For the method of formation of such a film, a high frequency magnetron sputtering method or an ion beam sputtering method, for example, is effective. However, any other method such as any one of various sputtering methods, electron beam deposition methods, chemical vapor deposition methods, for example, is usable. In order to ensure a continuous change in the period of the multilayered film (i.e. the thickness of each layer), during the film formation, a method wherein a shutter means is disposed before the substrate and the open time period of the shutter is controlled, can be used. More specifically, a shutter means is disposed before the substrate surface on which the film is to be formed, and the opening of a shutter blade of the shutter means is initiated from that side of the substrate surface at which a larger thickness film is to be formed. The opening movement of the shutter blade is made continuously. By repeating such an operation for all the layers to be formed, the resultant multilayered film has a continuously changing period (film layer thickness). However, the film forming method is not limited to this, and any other method may be used. FIG. 2 shows details of an example of a scan and diffusion device 2 of the FIG. 1 embodiment. In this example, the device 2 has a reflection mirror 104 having a substrate with a flat surface on which a multilayered film is formed. More particularly, the surface of the substrate 103 is finished by lapping and a multilayered film 102 is formed on this surface. In the horizontal direction along the surface of the substrate 103, the multilayered film 102 has a continuously changing period. More specifically, each layer of the multilayered film 102 has a thickness which increases linearly, in a direction from the X-ray beam input side toward the output side. Here, the thickness of each layer at a given point on the substrate 103 surface is determined so that a quantity "d x sin.theta..sub.x " is maintained substantially constant, where .theta..sub.x is the angle of incidence of the X-ray beam 101 which changes with the angular position of the mirror 104 and d is the thickness at a particular position within the section of the input X-ray beam 101 (for example, at the center of the section of the X-ray beam). In this example, the mirror swinging mechanism is such as illustrated, and it comprises an actuator 109, a driver 110, a controller 111 and a system control computer 112, wherein the actuator 110 is driven through the system control computer 112, the controller 111 and the driver 110. By this drive, a movable portion 108 of the actuator is swingingly moved along an arcuate path, about a rotational shaft 113, whereby the mirror 104 which is held by a base holder 105 and placed within a chamber 106 is swingingly moved along an arcuate path, about the shaft 113. Denoted at 107 are bellows for intercepting the inside of the chamber 106 from the driving part (108, 109 and 113) and to sealingly close the chamber 106. The swinging mechanism is not limited to this, and any one that can swing the mirror 104 is usable. An example is that: a supporting means for supporting the mirror is provided, and a driving means is coupled to the mirror or the supporting means to drive the same. The illustrated example has a computer-control function. Another form of the scan and diffusion device 2 is illustrated in FIG. 3. In FIG. 3, like numerals as those of FIG. 2 are assigned to similar elements. Denoted at 108 are movable arms, and denoted at 119 is an actuator which operates to move the arms 108 upwardly and downwardly. An end of each arm 108 is coupled to a base 105, such that the base 105 and the mirror 104 are supported by the arms 108. In this example, the substrate 103 of the mirror 104 has a convex shape quadratically curved surface (cylindrical surface) having a very large curvature radius R, and a multilayered film 102 is formed on this surface. Basically, the film 102 has a similar multilayered structure as that of the mirror 104 of FIG. 2, and the thickness of each layer increases with an increase in the distance from the radiation source 1 (FIG. 1). In this example, the mirror 104 is oscillatingly moved upwardly and downwardly so as to traverse the path of the X-ray beam 101, by means of the actuator 119 and the arms 108. As a result, the mirror 104 scans the X-ray beam while reflecting the same. Thus, the mirror 104 produces a diffused reflection beam (X-ray beam). The scan and diffusion device 2 may be modified in a manner, different from those shown in FIGS. 2 and 3. As an example, a convex mirror such as shown in FIG. 3 may be mounted to the support 105 of FIG. 2, such that the mirror may be moved along an arcuate path. A practical example of reflection mirror 104 which may have a flat or convex reflection surface will now be explained. A silica substrate 103 having its surface finished by lapping to an order of a flatness .lambda./20 (.lambda.=6328 .ANG.) and a surface roughness 4.6 .ANG.rms and having a size 40.times.40.times.15 (mm) may be used. On such a substrate, four layers of ruthenium (Ru) and three layers of aluminum (A1) may be alternately accumulated in accordance with the ion sputtering method, to provide a multilayered film 102 with a total of seven layers. The thickness of each layer may be continuously changed rectilinearly, in accordance with the aforementioned shutter control method. For the ruthenium layer, it may have a thickness changing from 17.2 .ANG. to 51.4 .ANG.. For the aluminum layer, it may have a thickness changing from 7.8 .ANG. to 78.9 .ANG.. In the shutter control method, a rectangular shutter may be provided before the substrate 103 and may be moved at a constant speed in a horizontal direction along the surface of the substrate. Assuming that such a mirror 104 was disposed in a mirror chamber, simulations were made on the assumption that the mirror was swung along an arcuate path with a rotational radius of 1 m; a beam of X-rays from a synchrotron orbit radiation (SOR) source was inputted to the mirror; the angle of swinging movement was.+-.0.86 deg.; the X-ray beam had an angle of incidence of 4 deg. upon the mirror as the mirror's angular position was zero (0) deg.; and the point of measurement for measuring the produced reflection beam (X-ray beam) was set at a distance of 1 m from the mirror 104. From the results, it has been confirmed that the input X-ray beam having a vertical expansion of 5 mm just before the mirror 104 can be expanded and projected to illuminate an illumination zone of 30 mm. Also, it has been confirmed that the center wavelength of the projected X-ray beam can be maintained substantially at 9 .ANG., at each of the upper edge, the center and the lower edge of the illumination zone, and that throughout the zone the variation of wavelength can be suppressed sufficiently. FIG. 4 illustrates an exemplary wavelength distribution at the upper edge (solid line) and that at the lower edge (broken line) of the illumination zone. As a comparative example, similar simulations were made on the assumption that a multilayered film having an optimized design with respect to light of 9 .ANG. was formed on a substrate with a uniform layer thickness; the substrate was made of silica; each ruthenium layer had a uniform thickness of 33.8 .ANG. throughout the substrate surface; each aluminum layer had a uniform thickness of 38.9 .ANG. throughout the substrate surface; and the total number of the layers and the angle of swinging movement of the mirror as well as the angle of incidence of the input X-ray beam were the same as those of the simulations made with regard to the mirror of the present invention. From the results, it has been confirmed that, although a similar illumination zone of 30 mm can be obtained at a distance of 1 m from the mirror, the center wavelength of the X-ray beam is 11 .ANG. at the upper edge of the zone and 7 .ANG. at the lower edge of the zone, and that there is produced non-uniformness of wavelength of about 4 .ANG. throughout the illumination zone. FIG. 5 shows an exemplary wavelength distribution at the upper edge (solid line) and that at the lower edge (broken line) when such a multilayered film with a uniform layer thickness is used. Another practical example of a reflection mirror of the present invention will now be explained. A silica substrate 103 having a cylindrical surface finished by lapping to an order of surface precision .lambda./10 and surface roughness 5.0 .ANG.rms and having a size 350.times.80.times.50 (mm) and a radius R=50 m, may be used. On this cylindrical surface, eight layers of ruthenium (Ru) and seven layers of aluminum (A1) may be accumulated alternately in accordance with the high frequency magnetron sputtering method, to provide a multilayered film 102 with a total of fifteen layers. The thickness of each layer may be changed continuously from one end to the other end, in accordance with the shutter control method described hereinbefore. Within a range of .+-.35 mm about the center (peak) of the substrate 103 and along an arcuate (cylindrical surface), each ruthenium layer may have a thickness changing from 15.0 .ANG. to 35.5 .ANG. (from one end to the other end), while each aluminum layer may have a thickness changing from 9.5 .ANG. to 106.5 .ANG.. Assuming that such a convex mirror 104 was disposed in a mirror chamber, simulations were made on the assumption that the mirror was swung along an arcuate path with a rotational radius of 1 m; a beam of X-rays from a synchrotron orbit radiation (SOR) source was inputted to the mirror; the angle of swinging movement was .+-.0.86 deg.; the X-ray beam had an angle of incidence of 5 deg. upon center (peak) of the mirror as the mirror's angular position was zero (0) deg.; and the point of measurement for measuring the produced reflection beam (X-ray beam) was set at a distance of 1 m from the mirror 104. From the results, it has been confirmed that the input X-ray beam having a vertical expansion of 5 mm just before the mirror 104 can be expanded and projected to illuminate an illumination zone of 93 mm. Also, it has been confirmed that the center wavelength of the projected X-ray beam can be maintained substantially at 10 .ANG., at each of the upper and lower edges of the illumination zone, and that throughout the zone the variation of wavelength can be suppressed sufficiently. It will be understood from the foregoing description that the reflection mirror of the present invention has an advantage that the produced reflection beam has small non-uniformness of wavelength, in addition to the advantage of transforming light of a small divergent angle into light of sufficient expansion to illuminate a wide zone. When such a mirror is used in an X-ray exposure apparatus, for example, it is possible to avoid or reduce the correction of non-uniform exposure dependent upon the spectral sensitivity of a resist used and, therefore, there is no necessity for complicated apparatus for correction based on characteristics of individual resist materials. Also, when such a mirror is used, uniform exposure of a resist on a wafer is easily attainable even in a case when the illuminance in the exposure region is not uniform, by setting the exposure time periods different to different portions of the exposure region in accordance with the non-uniformness in illuminance. For this exposure control, a known method such as discussed in a paper "Summary of Society of Japanese Applied Physics, 1988, Spring, 31a-k-9", by Nihon Denki Kabushiki Kaisha, may be used. While on that occasion it is necessary to measure the strength of the illumination light by some means, the strength of the illumination light should correspond to the absorption characteristics (wavelength dependence) of the resist used. On the other hand, recently, attention has been paid to the damage by radiation of a radiation optical element. Since in the arrangement shown in FIGS. 1, 2 or 3 the mirror 104 is swingingly or oscillatingly moved, the position of incidence of the radiation upon the mirror changes with time. Therefore, the possibility of local damage of the mirror surface is very small. Further, by appropriately selecting the shape of the mirror reflection surface or the angle of swinging movement, for example, it is possible to adjust easily the distance or the angle of view and, by appropriately selecting the materials, the number of layers, the layer thickness or the layer thickness distribution, for example, of the multilayered film, it is possible to obtain easily a desired wavelength distribution. In another aspect, the present invention is applicable to a reflection mirror which is to be fixedly secured. When such a mirror is to be used in an exposure apparatus, preferably it may have a convex reflection surface. A multilayered film may be formed on the convex reflection surface, each layer having a thickness changing with position on the reflection surface. This is effective to suppress non-uniformness in wavelength of a produced reflection beam from the reflection surface, resulting from different angles of incidence of different portions of an input radiation beam, impinging upon different positions of the reflection surface of the mirror. Accordingly, it is possible to obtain easily a reflection beam having sufficient expansion and sufficiently reduced non-uniformness in wavelength. The radiation beam to be used in the present invention is not limited to X-rays or vacuum ultraviolet rays. For the reflection of ultraviolet rays, visible rays or infrared rays of a wavelength of 200 nm or larger, for example, the present invention can be used. The present invention is particularly effective, when it is used with a radiation beam having a relatively wide bandwidth. 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.