Patent Number: 060382791
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to X-ray apparatuses, suitable for use in exposure systems, as well as in semiconductor device production. 2. Description of the Related Art A method known as an X-ray demagnification (reduced magnification) projection exposure method has been suitably used in the production of devices having fine patterns, such as semiconductor circuit elements. According to this method, a mask carrying a circuit pattern formed thereon is illuminated with X-rays so that the mask image, i.e., the circuit pattern, is projected at a prescribed demagnification onto a wafer surface so that a resist on the wafer surface is exposed, whereby the circuit pattern is transferred to the resist in a reduced scale. FIG. 10 shows an example of a conventional X-ray demagnification projecting exposure apparatus. This apparatus has, as its major components, an X-ray source 109, 110, 101, 111, an illuminating optical system 103, a reflective mask 104, a projection optical system 105, a stage 108 carrying a wafer 106, an alignment mechanism (not shown) for precisely aligning the positions of the mask 104 and the wafer 106, and a vacuum vessel and evacuating system (not shown) which cooperate with each other in maintaining a vacuum atmosphere around the entire optical system so as to prevent attenuation of t he X-rays. Laser-excited plasma, for example, is used as the X-ray source. A laser source 109 emits laser beam in the form of pulses, which hit a target 111 so that laser plasma is generated. X-rays emitted from a luminescent point 101 of the laser plasma are collected on the reflective mask 104 through a collecting lens (not shown). The luminescent point 101 of the laser plasma has a size on the order of several hundreds of .mu.m and, hence, can be regarded as being a point source. The X-rays 102 emitted from the luminescent point 101 pass through filter 112 and are collimated by a parabolic mirror 103 having a focal point located on the luminescent point 101. The projection optical system 105 includes a plurality of multi-layered reflective mirrors which demagnify (reduce the magnification of) the pattern on the mask 104 so as to project the pattern image of a reduced size on the surface of the wafer 106. The projection optical system 105 is usually constituted by a telecentric system. Conventional X-ray demagnifying projection exposure apparatuses, however, have suffered from the following problems. Namely, these conventional apparatuses could not provide resolution and focal depth which would be sufficient for projecting extremely delicate and fine patterns on masks onto wafers, thus failing to transfer such patterns with a desired high degree of precision. In order to obviate this problem, it has been proposed to improve the image forming performance by using a phase shift effect offered by a phase shift mask. However, it has been impossible to fully enjoy the effects of such a phase shift mask. These problems encountered with conventional devices are attributable to the following reasons. One of the characteristic parameters of an illuminating system is a coherence factor .sigma.. The coherence factor .sigma. is expressed as follows, representing the mask-side numerical aperture of the projection optical system by NA.sub.p1 and that of the illuminating optical system by NAi: EQU .sigma.=NAi/NA.sub.p1. The optimum value of the coherence factor .sigma. is determined based on the levels of resolution and contrast required in the transfer of the pattern. In general, a too small value of the coherence factor .sigma. allows an interference pattern to appear at edges of the fine pattern image projected on the wafer, while a too large coherence factor .sigma. reduces the contrast of the projected image. An illumination system having a coherence factor .sigma. of 0 (zero) is referred to as a "coherent illumination system". Such a coherent illumination system exhibits a constant optical system transfer factor (OTF) when the spatial frequency does not exceed a value given by NA.sub.p2 /.lambda. where NA.sub.p2 and .lambda. respectively indicate the wafer-side numerical aperture of the projection optical system and the wavelength of the X-rays. However, at higher spatial frequencies, the transfer factor OTF is zero, so that resolution of the image is impossible. In contrast, an illumination system having a coherence factor of 1 is referred to as an "incoherent illumination system". In this type of an illumination system, the transfer factor OTF decreases in accordance with an increase in the spatial frequency, but is not reduced to zero until the spatial frequency reaches a value which is given as 2.times.NA.sub.p2 /.lambda.. Thus, resolution is possible to a greater degree of fineness as compared with the coherent illuminating system. The conventional X-ray demagnifying projection exposure apparatuses, however, are designed such that the coherence factor .sigma. approximates 0 and, hence, operate under substantially coherent illuminating conditions. These apparatuses, therefore, have limited resolution and cannot transfer patterns having a high degree of fineness. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an X-ray generating apparatus and method capable of providing improved image forming performance over conventional devices, as well as a high-performance exposure apparatus and an improved device production method which make use of such an X-ray generating apparatus and method. To this end, according to a preferred form of the present invention, a light source is provided in the form of an aggregate of a plurality of luminescent points and X-rays emitted from these luminescent points are collected through an illuminating optical system so as to effect Kohler's-illumination on an object such as a mask. X-rays emitted from different luminescent points of the source impinge upon the mask at different incident angles. Therefore, when the entire light source composed of an aggregate of luminescent points has a finite size, the X-rays illuminating the mask also have a finite angular divergence. Furthermore, when the source has a non-uniform luminescent intensity distribution, the X-rays illuminating the mask also have a correspondingly non-uniform angular distribution, thus realizing modified illumination. The X-ray source composed of a plurality of luminescent points can be realized by applying a plurality of laser beams onto a plurality of points on a target. Alternatively, a laser beam from a single laser source is deflected so as to impinge upon a plurality of points on the target in a time-series manner. The detail of the methods of creating such an X-ray source will be described later in connection with the description of the preferred embodiments. Thus, the use of an aggregate of a plurality of luminescent points as the source of the X-rays makes it possible to widen the angular distribution of the X-rays illuminating a mask so as to provide a greater value of the coherence factor .sigma., and to achieve modified illumination such as ring illumination, oblique illumination and so forth. It is therefore possible to improve the image forming performance in terms of resolution and focal depth. When a phase shift mask is used, it is possible to make full use of the effect to improve the image forming performance offered by such a mask. In another preferred form of the present invention, an area X-ray source of finite size is used as the X-ray source. The X-rays emitted from this area source are collected on an object such as a mask through an illuminating optical system, thus effecting Kohler's-illumination on such an object. X-rays emitted from different points on the planar source irradiate the mask at different angles. Since the area X-ray source has a finite size, the X-rays illuminating the mask also have a correspondingly finite angle of divergence value. It is possible to vary the coherence factor by varying the form of the X-ray generating section, i.e., the configuration, size and position of the source. The planar X-ray source may have a non-uniform luminescent intensity distribution. In such a case, the X-rays illuminating the mask have a non-uniform angular distribution, thus realizing a modified illumination. It is thus possible to optimize the coherence factor .sigma. of the illumination system and to achieve modified illumination such as ring illumination or oblique illumination, by controlling the angular distribution of the X-rays illuminating the mask, by suitably setting the form of the X-ray source. Consequently, illuminating conditions can be optimized in accordance with the conditions of the exposure to be performed, contributing to improvement in the image forming performance in terms of resolution and focal depth. In one aspect, the present invention provides an X-ray generating device comprising a laser source for generating a laser beam and a plurality of points on a target for receiving the laser beam and for generating X-rays from portions of high temperature plasma on the target. In another aspect, the present invention provides an X-ray generating device comprising a laser source for generating a laser beam, an aperture, having a variable shape, for receiving the laser beam and for defining a secondary laser beam and a target for receiving the secondary laser beam and for generating X-rays from portions of high temperature plasma on the target. In yet another aspect, the present invention provides an X-ray irradiating apparatus comprising (i) an X-ray generating device comprising a laser source for generating a laser beam and a plurality of points on a target for receiving the laser beam and for generating X-rays from high temperature plasma on the target, and (ii) irradiating means for irradiating an object with the X-rays generated by the X-ray generating device. In yet another aspect, the present invention provides an X-ray irradiating apparatus comprising (i) an X-ray generating device comprising a laser source for generating a laser beam, an aperture, having a variable shape, for receiving the laser beam and for defining a secondary laser beam and a target for receiving the secondary laser beam and for generating X-rays from portions of high temperature plasma on the target, and (ii) irradiating means for irradiating an object with the X-rays generated by the X-ray generating device. In still another aspect, the present invention provides an X-ray generating method including generating at least one laser beam from at least one laser source, providing a plurality of points on a target for receiving the at least one laser beam and generating X-rays from portions of high temperature plasma on the target. In still another aspect, the present invention provides an X-ray generating method comprising generating a laser beam from a laser source, providing an aperture, having a variable shape, for receiving the laser beam and for defining a secondary laser beam, providing a target for receiving the secondary laser beam and generating X-rays from portions of high temperature plasma on the target. In yet another aspect, the present invention provides a method of producing a semiconductor device, and includes steps of generating at least one laser beam from at least one laser source, providing a plurality of points of high temperature plasma on a target for receiving the at least one laser beam, generating X-rays from the points on the target, irradiating a mask with the X-rays from the portions and projecting an image of the pattern carried by the irradiated mask onto a wafer. In still another aspect, the present invention provides a method of producing a semiconductor device, and includes steps of generating a laser beam from a laser source, providing an aperture, having a variable shape, for receiving the laser beam and for defining a secondary laser beam, providing a target for receiving the secondary laser beam, generating X-rays from portions of high temperature plasma on the target, irradiating a mask with X-rays from the portions and projecting an image of a pattern carried by the irradiated mask onto a wafer. The present invention can be applied to a wide variety of apparatuses which need illumination or irradiation with X-rays, such as, for example, an X-ray exposure apparatus, X-ray microscopes, X-ray examination apparatus, X-ray processing apparatus, and clinical X-ray apparatuses. The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.