Patent Number: 
Section: description

1. Field of the Invention The invention relates to an extreme UV radiation focusing mirror for focusing of extreme UV radiation which is emitted by a high density and high temperature plasma, and an extreme UV radiation source device using this extreme UV radiation focusing mirror. The invention relates especially to an extreme UV radiation focusing mirror which focuses extreme UV radiation of the above described plasma with high efficiency and which, moreover, can make the far-field distribution of the light beam at the focus point uniform, and an extreme UV radiation source device using this extreme UV radiation focusing mirror. 2. Description of the Prior Art According to the miniaturization and increased integration of integrated semiconductor circuits, an increase in resolution is required in a projection exposure device for its manufacture. To meet this requirement, the wavelengths of the exposure radiation source are being increasingly shortened. As a semiconductor exposure radiation source of the next generation in succession to an excimer laser device, an extreme UV radiation source device (hereinafter also called an EUV radiation source device) is being developed which emits extreme UV radiation (extreme ultra violet radiation; hereinafter also called EUV radiation) with wavelengths from 13 nm to 14 nm, especially with a wavelength of 13.5 nm. A primary goal of the invention is to eliminate the above described disadvantages in the prior art. Therefore, a primary object of the invention is to devise an EUV focusing mirror in which the far-field distribution of the EUV radiation which has been focused by the EUV focusing mirror can be made advantageous and the nonuniformity (scattering) of the illuminance can be suppressed, and to devise an EUV radiation source device using such an EUV focusing mirror. The EUV focusing mirror in accordance with the invention, like a conventional EUV focusing mirror, has several concave mirrors in a rotation shape with different diameters. The concave mirrors comprising the EUV focusing mirror are coaxially arranged such that their axes of revolution agree with one another such that the focus positions essentially agree with one another. The EUV focusing mirror is made such that the EUV radiation with a grazing incidence angle from 0° to 25° can be advantageously reflected, and moreover, focused by this arrangement of the interlaced high precision concave mirrors. The above described object is also achieved in accordance with the invention as follows: (1) For an extreme UV radiation focusing mirror of the grazing incidence type in which there are several nested concave mirrors with different diameters, on the end of the respective concave mirror on the side which is not the reflection surface, a bevel is formed such that it has a given angle with respect to the radiation reflection surface. This means that a bevel is formed on the end on the radiation incidence side of the respective concave mirror of the EUV focusing mirror and/or on the end on the radiation exit side thereof. Specifically, on the end on the radiation incidence side and on the end on the radiation exit side of the respective concave mirror, a bevel is formed such that the end on the radiation incidence side and/or the end on the radiation exit side of the above described concave mirror drops out of the away region of the incidence EUV radiation which is focused by the EUV focusing mirror, and of the away region of the exit radiation thereof. The area which contains this bevel is called the edge area. Furthermore, the state in which on the end a bevel is formed such that its thickness becomes smaller, the more it approaches the tip, is called a knife edge type. Also, the end on which this bevel is formed is called the knife edge part. (2) In (1), the shape of the reflection surface of the above described concave mirror is one of the shapes of an ellipsoid of revolution, a paraboloid of revolution, or a Wolter shape. In this connection, the focal points of the respective concave mirrors essentially coincide with one another. (3) In (1) and (2), the angle of the bevel on the radiation incidence side of the end of the above described concave mirror is set such that it is a positive angle in the clockwise direction with respect to the running direction of the extreme UV radiation which is incident on the above described radiation incidence end. (4) In (1) and (2), the angle of the bevel on the radiation exit side of the end of the above described concave mirror is set such that it is a negative angle in the clockwise direction with respect to the running direction of the extreme UV radiation which emerges from the above described radiation exit end. (5) In (1) and (2), the angle of the bevel on the radiation incidence side of the end of the above described concave mirror is set such that it is a positive angle in the clockwise direction with respect to the running direction of the extreme UV radiation and furthermore the angle of the bevel on the radiation exit side of the end of the above described concave mirror is set such that it is a negative angle in the clockwise direction with respect to the running direction of the emerging UV radiation. (6) In an extreme UV radiation device which comprises the following:                a vessel;        a raw material supply means for supply of raw material which contains an extreme UV radiation fuel and/or a compound of an extreme UV radiation fuel to this vessel;        a means for heating and excitation which heats and excites the supplied raw material in the above described vessel and in which a plasma is produced that emits radiation which contains extreme UV radiation;        a focusing optical means which is located in the above described vessel to focus the radiation which has been emitted from the above described plasma; and        a radiation extracting part from which the focused radiation is extracted from the vessel,the above described focusing optical system is the extreme UV radiation focusing mirror which was described in (1) to (5).Action of the Invention         As was described above, the following effects can be obtained in accordance with the invention. (1) By the measure that, on the side of the end on the radiation incidence side and/or of the end on the radiation exit side which does not constitute the reflection surface of the respective concave mirror of the EUV focusing mirror, a bevel in the form of a knife edge is formed, the disadvantage of formation of nonuniformity/scattering of the illuminance on the workpiece by the shielded region can be prevented by the amount of thickness of the substrate material of the respective concave mirror. (2) By the measure that the angle of the bevel on the radiation incidence side of the end of the concave mirror is set such that it is a positive angle in the clockwise direction with respect to the running direction of the extreme UV radiation which is incident on the radiation incidence end and that the angle of the bevel on the radiation exit side of the end of the concave mirror is set such that it is a negative angle in the clockwise direction with respect to the running direction of the extreme UV radiation emerging from the radiation exit end, the shielding ratio of the grazing incident EUV radiation onto the end of the respective concave mirror can be reduced to a dramatic degree and the region with reduced light intensity for far-field distribution can be reduced. It becomes possible to reduce the nonuniformity of the illuminance on the workpiece. The invention is further described below with reference to the accompanying drawings. FIGS. 1 & 2 schematically show the arrangement of a focusing mirror in accordance with the invention, FIG. 1 being a cross section of the focusing mirror through a plane which passes through the optical axis (A-A cross section as shown in FIG. 2). FIG. 2 shows the focusing mirror as viewed from the radiation exit side. In the figures, a case is shown in which the above described shape of the reflection surface is of the Wolter type. However, the reflection shape can also be an ellipsoid of revolution, a paraboloid of revolution or the like. FIGS. 1 & 2 show an arrangement in which there are, for example, five nested Wolter mirrors a, b, c, d, and e of different diameters. The reflection surface of the respective Wolter mirrors a, b, c, d, and e, proceeding from the radiation incidence side, have the shape of a hyperboloid and the shape of an ellipsoid. Furthermore, knife edge parts are formed by bevels on the ends of each of the Wolter mirrors b through e on the sides which do not constitute the reflection surfaces. As is shown in FIG. 2, on the radiation exit sides of the respective Wolter mirror a to e, there is a spider 2a which is comprised, for example, of a spider flange 2b of copper and spokes 2c. The ends of the respective Wolter mirrors a to e, on the radiation exit side, as shown in FIG. 1, are installed in a groove of the spoke 2c, and for example, are attached by means of a cement or a fixing brace or the like. FIG. 3(a) is a schematic of the geometric locations of the light beams of one embodiment of the focusing mirror in accordance with the invention. In the figure, as also in FIG. 7(a), the solid lines a1, b1, c1, d1 and e1 show the geometric locations of the light beams which pass proceeding from the plasma position through the radiation incidence ends and radiation exit ends of the respective Wolter mirrors a, b, c, d, and e. This means that the solid lines a1, b1, c1, d1 and e1 represent the geometric locations of the light beams in the case of the maximum incidence angle on the respective Wolter mirrors a, b, c, d, and e of the radiation which has been focused by the respective Wolter mirrors a, b, c, d, and e onto given sites. On the other hand, the double dot-dash lines a3, b3, c3, d3 and e3, as also in FIG. 7(a), are the geometric locations of the light beams in the case of reflection on the boundaries between the hyperbolas and the ellipsoids for the respective Wolter mirrors a, b, c, d, and e. This means that the double dot-dash lines a3, b3, c3, d3 and e3 are the geometric locations of the light beams in the case of the minimum incidence angle of the radiation which is being focused by the respective Wolter mirrors a, b, c, d and e onto given sites, into the respective Wolter mirrors a, b, c, d and e. It also means that the rays which have been focused by the respective Wolter mirrors a, b, c, d and e pass through a space between the solid lines a1, b1, c1, d1, e1 and the double dot-dash lines a3, b3, c3, d3 and e3. The radiation which has been focused by the Wolter mirror e passes through a space between the light beams e1 and the light beams e3 while the light beams which pass outside this space are not focused. The geometric locations of the light beams which are focused by the respective Wolter mirrors a, b, c, d, e therefore appear as follows. The light beams proceeding from the plasma emission point until incidence on the hyperbolas of the respective Wolter mirrors a, b, c, d, e pass through the region which is formed by the following: the distance between the plasma and the incidence ends of the respective Wolter mirrors a, b, c, d, and e for the solid lines a1, b1, c1, d1, and e1; the distance between the plasma and the boundaries between the hyperbolas and the elliptical surfaces of the respective Wolter mirrors a, b, c, d, and e for the double dot-dash line a3, b3, c3, d3 and e3, and a hyperbola. Light beams which pass outside this region are not focused. On the other hand, the light beams which are reflected by the elliptical surfaces of the respective Wolter mirrors a, b, c, d, and e pass through a region which is formed by the following: a distance after reflection from the elliptical surfaces for the solid lines a1, b1, c1, d1, and e1; a distance after reflection from the boundaries between the hyperbolas and the elliptical surfaces for the double dot-dash lines a3, b3, c3, d3 and e3; and solid lines a1, b1, c1, d1, and e1; double dot-dash lines a3, b3, c3, d3 and e3; an elliptical surface. Light beams which pass outside the region are not focused. Conventionally, the above described double dot-dash lines a3, b3, c3, and d3 were shielded by the thick parts of the incidence ends of the respective Wolter mirrors b, c, d, e. The geometric locations of the light beams in the case of a minimum incidence angle on the respective Wolter mirrors a, b, c, d, e were therefore the dot-dash lines a2, b2, c2 and d2, as is shown in FIG. 7(b). For the EUV focusing mirror in accordance with the invention, the sides of the radiation incidence ends of the respective Wolter mirrors b to e which do not constitute reflection surfaces, are made in the form of knife edges so that the above described double dot dash lines a3, b3, c3, and d3 are not shielded by the thick parts of the ends on the radiation incidence sides (radiation incidence ends) of the respective Wolter mirrors b, c, d, e. This means that, on the radiation incidence ends of the respective Wolter mirrors b, c, d, e, there are knife edge parts bin, cin, din, ein, as is shown in FIG. 3(b). The angle of the edge side of the knife edge part (plane including the bevel) is set such that it becomes an identical or positive angle with respect to the directions in which the light beams a3, b3, c3, d3 (shown using the double dot-dash lines run), when the clockwise direction is considered positive. Likewise, the radiation exit ends of the respective Wolter mirrors b, c, d, e are made in the form of a knife edge so that the above described double dot-dash lines a3, b3, c3, and d3 are not shielded. This means that, on the radiation exit and incidence ends of the respective Wolter mirrors b, c, d, e, there are knife edge parts bout, cout, dout, eout, as is shown in FIG. 3(c). The angle of the edge side of the knife edge part (plane including the bevel) is set such that it becomes an identical or negative angle with respect to the directions in which the light beams a3, b3, c3, d3 (shown using the double dot-dash lines) run when the clockwise direction is considered positive. FIG. 4 shows the far-field distribution when using the EUV focusing mirror in accordance with the invention in an EUV radiation source device. When the far-field distribution which is shown in the figure is compared to the far-field distribution when using a conventional EUV focusing mirror for an EUV radiation source device (see, FIGS. 7(a) and (b)), the degree of reduction of the light intensity is smaller than in the conventional case, even if regions with reduced light intensity are present isolated. In FIG. 4, the regions with reduced light intensity are regions which depend on the thickness of the tip areas of the knife edge parts on the radiation incidence and exit end of the respective Wolter mirrors b, c, d, e. FIG. 5 shows the arrangement of one example when using the invention for a focusing mirror in the form of an ellipsoid of revolution. FIG. 5 is a cross section of the focusing mirror through a plane through the optical axis. In FIG. 5 only the arrangement of the mirrors to one side with respect to the optical axis is shown, the mirrors being arranged symmetrically to the optical axis. FIG. 5 shows that, for example, there are four nested mirrors a, b, c, d with different diameters. The reflection surfaces of the respective Wolter mirrors a, b, c, d are in the form of an ellipsoid. In this embodiment, as in the above described embodiment, the ends on the sides of the respective mirrors b to d which do not constitute reflection surfaces, are made in the form of a knife edge at the radiation incidence side and the radiation exit side. In this way, it is possible to prevent the disadvantage that nonuniformity/scattering of the illuminance is formed by the shielded regions as a result of the thickness of the substrate material of the respective concave mirror on the workpiece. In the case of a conventional EUV focusing mirror, for adequate cooling, a thickness of roughly 1 mm of the substrate material of the respective concave mirror of the EUV focusing mirror is needed. The grazing incident EUV radiation is shielded by this amount of thickness, as a result of which a region with an extremely reduced light intensity occurs for the far-field distribution. The nonuniformity of the illuminance on the workpiece is therefore increased. On the other hand, in the case of an EUV focusing mirror in accordance with the invention, on the two ends of the respective concave mirror of the EUV focusing mirror there are knife edge parts. The bevel of the above described knife edge part is set such that the grazing incident EUV radiation is not shielded. Furthermore, the thickness of the tip of the knife edge part can be fixed to the μm order, therefore very thin. Therefore, the ratio of the shielding of the grazing incident EUV radiation from the ends of the respective concave mirror can be made very small, by which a reduction in the size of the region with a reduced light intensity occurs for the far-field distribution. As a result, it becomes possible to reduce the nonuniformity of the illuminance on the workpiece.