Patent Number: 054854970
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 10 shows a sectional structure of a reflection mask which is an example of an optical element of the present invention, and the relationship between the position of a pattern 22 formed from a multilayer, which is a region having a high reflectivity, and the reflected radiation intensity of incident radiation when vacuum ultraviolet radiation or X-radiation 411 is introduced at an incidence angle .theta. (.theta.&gt;0.degree.; this applies similarly in the following description) to the reflection mask. Here, since a reflection optical system is employed for an exposure optical system, preferably the wavelength of the vacuum ultraviolet radiation or X-rays 411 is equal to or higher than about 3 nm but equal to or lower than 150 nm. When the direction in which the vacuum ultraviolet radiation or X-radiation 411 having the incidence angle .theta. is regularly reflected from the pattern 22 formed from a multilayer is parallel to the direction of a right-hand side boundary 47 of the pattern 22 as seen from FIG. 10, since reflected radiation does not escape from the outer side of the right-hand side boundary 47 of the pattern 22, the reflected radiation intensity exhibits a rectangular distribution at the right-hand side boundary 47 of the pattern 22. Where the width of the pattern is represented by T, the thickness by a, the angle of regular reflection by .theta. and the angle between a normal line to the outermost surface of the pattern 2223 and the side face of the pattern by .THETA., preferably the angle .THETA. is determined so that the value of a.cndot..vertline.tan.theta.-tan.THETA./T may range from 0 to 0.05. This is because, in order to allow resolution of a pattern substantially sufficiently, it is necessary that the region in which the intensity of reflected radiation is decreased be within 5 percent with respect to the pattern width T. Another example of improvement in structure of an optical element is shown in FIG. 11. FIG. 11 shows a sectional structure of a reflection mask, which is another example of an optical element of the present invention, and the relationship between the position of a pattern 221 formed from a multilayer, which is a region having a high reflectivity, and the reflected radiation intensity of incident radiation when vacuum ultraviolet radiation or X-radiation 411 is introduced at an incidence angle .theta. to the reflection mask. The reflection mask of FIG. 11 is constructed such that, in order to reduce or prevent emergence of reflections from a right-hand side boundary 47 of the pattern 221 formed from a multilayer when the vacuum ultraviolet radiation or X-radiation 411 is introduced at the incidence angle .theta., an absorber 2211 is provided on the right-hand side boundary 47. Since no reflected radiation escapes from the outer side of the right-hand side boundary 47 of the pattern 221 formed from a multilayer, the reflected radiation intensity at the boundary of the pattern 221 is improved similarly as in the reflection mask shown in FIG. 10. The reflected radiation intensity of the reflection mask of FIG. 11 is improved, when the incidence angle .theta. and the angle .THETA. between a normal line to the outermost surface of the pattern 221 and a side face of the pattern 221 are different from each other or when the angle .THETA. does not meet the requirement of 0.ltoreq.a.vertline.tan.theta.- tan.THETA..vertline./T&lt;0.05 after formation of the pattern 221, by applying the absorber 2211 so that reflected radiation may not escape from the right-hand side boundary 47 of the pattern 221. Consequently, the yield of accepted products of reflection masks as optical elements of the present invention is raised. A further example is shown in FIG. 12. FIG. 12 shows a sectional structure of a reflection mask and the relationship between the position of a pattern 221 formed from a multilayer, which is a region having a high reflectivity, and the reflected radiation intensity of incident radiation when vacuum ultraviolet radiation or X-radiation 411 is introduced at an incidence angle .theta. to the reflection mask. An ion beam or a like beam is introduced at an angle .THETA. to the multilayer so that the angle .THETA. may be equal to an angle .theta. at which the vacuum ultraviolet radiation or X-radiation 411 is introduced into the reflection mask is regularly reflected to form a region 222 to which the ion beam has been introduced. Another region to which the ion beam has not been introduced makes a pattern 22 formed from a multilayer which is a region in which the reflectivity is high. The multilayer in the region 222 in which the ion beam has been introduced is free from steep interfaces between the at least two kinds of films forming the multilayer, and consequently, the reflectivity thereof is deceased remarkably. Since the region 222 into which the ion beam has been introduced is present on a right-hand side boundary 47 of the pattern 22 formed from a multilayer, reflected radiation does not likely escape from the outer side of the right-hand side boundary 47 of the pattern 22. Since the incidence angle of an ion beam or a like beam is easy to control, there is an advantage that the angle .THETA. of the pattern 22 of the mask formed from a multilayer can be controlled comparatively readily. Consequently, the yield of accepted products of reflection masks as optical elements of the present invention is raised. If the arrangement of the incidence angle .theta. and a pattern of a reflection mask 81 is taken into consideration as shown in FIG. 13, then worsening of the reflected radiation intensity of an image pattern formed on a plane of a wafer or the like can be greatly decreased. Patterns 22 of a reflection mask 81 are disposed so that, when the reflection mask 81 on which at least one kind of predetermined pattern is carried is exposed to or irradiated by vacuum ultraviolet radiation or X-rays 411 at an incidence angle .theta., shadows 361 which are formed (that is, side face portions 361 of the patterns 22 at which the reflection ratio is decreased) may be minimized. Consequently, the worsening of the reflected radiation intensity of an image pattern formed on the plane of the wafer or the like can be greatly reduced. Further, the worsening of the reflected radiation intensity of an image pattern formed on the plane of the wafer or the like can be reduced by disposing the patterns 22 of the reflection mask 81 so that, in the positional relationship between the reflection mask 81 and the incident radiation 411, the side face portions 361 which make longitudinal shadows of those of the at least one kind of patterns 22 of the reflection mask 81 which have comparatively small line widths may be minimized as shogun in FIG. 13. According to the present method, since a pattern which has a comparatively small width and is liable to be worsened in resolution by a shadow does not make a shadow, the resolution of the small width pattern is not worsened at all. Furthermore, the worsening of the reflected radiation intensity of an image pattern formed on the plane of the wafer or the like can be reduced by disposing the patterns 22 of the reflection mask 81 so that those of the at least one kind of patterns 22 which have comparatively small line widths may coincide with the tangential direction of the incident radiation 411 as much as possible as shown in FIG. 13 so as to minimize side face portions which make the shadows 361. Further, when the reflection mask 81 is scanned in the tangential direction in synchronism with a wafer in order to expand the irradiation area of the reflection mask 81 and the exposure area of the wafer with such an optical system as described hereinabove with reference to FIG. 1, vibrations are sometimes produced in the synchronous scanning direction so that worsening of the reflected radiation intensity of an image pattern is caused. The resolution of an image pattern is worsened by the synchronous scanning due to synergetic effects of the worsening of the reflected radiation intensity of an image pattern formed on the plane of the wafer or the like and worsening of the reflected radiation intensity of an image pattern by the shadows. When the incident radiation 411 is introduced to the reflection mask 81 in FIG. 13 to replicate the patterns 22 onto a wafer (not shown) serving as a substrate, the patterns 22 on the reflection mask 81 are disposed so that the direction in which the reflection mask 81 and the wafer are scanned synchronously, and the direction in which side portions which make the longitudinal shadows 361 of those of the at least one kind of patterns 22 on the reflection mask 81 which have comparatively small line widths are minimized, may coincide with each other in order to greatly reduce the worsening of the reflected radiation intensity of an image pattern on the wafer. A pattern 2224 which does not extend in parallel to any of the tangential and sagittal directions in FIG. 13 makes shadows 361 by an amount defined by the cosine of the angle .alpha. of the pattern with respect to the sagittal direction, that is cos .alpha.. In order to minimize the shadows 361, preferably attention is paid to the arrangement of the pattern 2224 on the reflection mask 81 so that the angle .alpha. of the pattern having a comparatively small line width with respect to the sagittal direction may approach 90.degree.. Further, if a pattern of an optical element to be replicated or imaged is designed so that the width thereof may be adjusted in advance so as to correct the worsening of the reflectivity which takes place at a side face portion of a pattern 22 or to correct the escape of reflected radiation from the side face of the pattern, then the difference between a designed value of a width of the pattern and a width of a pattern replicated or actually imaged is improved. Where the reduction ratio is represented by r and the width of the pattern of the reflection mask by T in FIG. 10, ideally the width of an image pattern on a wafer is given by T.cndot.r. Actually, since worsening of the reflected radiation intensity of an image pattern on a wafer is caused by worsening of the reflectivity at the location spaced by the distance 2a.cndot.tan.theta. from the left-hand side face 45 of the pattern 22, the width of the image pattern on the wafer is given by (T-.DELTA.T).cndot.r. If the width of the pattern 22 of the reflection mask is set in advance to T+.DELTA.T to effect correction, then the image pattern formed on the wafer will have the width T.cndot.r. Accordingly, the difference between a designed value of the width of a pattern of a reflection mask and a width of an image pattern replicated or actually imaged is eliminated. Example 1 An example of a process of manufacturing a reflection X-ray mask which is an example of an optical element of the present invention will be described. FIGS. 14(a) to 14(d) show different steps of the manufacturing process. Nickel (Ni) films each having a thickness of 1.27 nm and carbon (c) films each having a thickness of 1,27 m are formed alternately on a silicon or SiC substrate 11 by a magnetron sputtering method until a total of 160 layers of each of the two films are reached to make up a multilayer 21 (FIG. 14(a)). A resist is applied to the upper surface of the multilayer 21, and a resist pattern 38 is formed (FIG. 14(b)) by electron beam lithography, which is but one possible pattern forming method. Then, the multilayer 21 is removed by reactive ion etching using the resist pattern 38 as a mask (FIG. 14(c)), and thereafter, the resist pattern is removed to form a multilayer pattern 22 (FIG. 14(d)). In this instance, oblique etching is performed by disposing a Si wafer 12 which is a specimen and a substrate such that, as shown in FIG. 15, the lower face of an opposing electrode 71 of an etching apparatus 78 and the plane of the Si wafer 12 define therebetween an angle .theta. equal to the angle of incident radiation which is used for irradiation or exposure of a reflection mask. Preferably, the pressure of gas used is made as low as possible and the directivity of a reactive ion beam 72 for etching is strengthened. Subsequently, the resist is removed by an oxygen plasma asher. A reflection mask is thus produced which carries thereon the pattern 22 which makes an angle .theta. with respect to a normal line to the outermost surface of the pattern. Alternatively, oblique etching may be performed by reactive ion etching by means of an ion beam employing electron cyclotron resonance. Example 2 Another embodiment of a reflection X-ray mask which is an example of an optical element of the present invention will be described. FIGS. 16(a) to 16(e) show different steps of the manufacturing process of the reflection X-ray mask. As in Example 1 described above, nickel (Ni) films each having a thickness of 1.27 nm and carbon (C) films each having a thickness of 1.27 nm are formed alternately on a silicon or SiC substrate 11 by a magnetron sputtering method until a total of 160 layers of each of the two layers are reached to make up a multilayer 21 (FIG. 16(a)). A resist is then applied to the upper surface of the multilayer 21, and a resist pattern 38 is formed (FIG. 16(b)) by optical lithography, for example. Then, the multilayer 21 is removed by reactive ion etching using the resist pattern 38 as a mask to form a multilayer pattern 221 (FIG. 16(c)). Thereafter, Ta 2211 is vapor deposited to a thickness of 0.1 .mu.m in an oblique direction (FIG. 16(d)), and then the Ta on the surface of the multilayer pattern 221 is removed in a vertical direction by ion milling 721 to form a reflection X-ray mask (FIG. 16(e)) having a structure wherein reflected radiation is not able to escape from a side face of the pattern. Example 3 A further embodiment of a reflection X-ray mask which is an example of an optical element of the present invention will be described. FIGS. 17(a) and 17(b) show different steps of the manufacturing process of the reflection X-ray mask. As in Example 1 described above, nickel (Ni) films each having a thickness of 1.27 nm and carbon (C) films each having a thickness of 1.27 nm are formed alternately on a silicon or SiC substrate 11 by a magnetron sputtering method until a total of 160 layers of each of the two layers are reached to make up a multilayer 21 (FIG. 17(a)). Then, a focus ion beam 721 formed from Be ions and having a high directivity is introduced obliquely into the multilayer 21 at an angle .theta. equal to the angle at which incident radiation for exposure of a reflection mask is reflected regularly to form a desired pattern 222. In this instance, the kind of incidence ions and the incidence energy of the ion beam 721 must necessarily be selected suitably in accordance with the thickness of the multilayer 21. The element of the incidence ions may be Be, N, O, C, Ar, Kr, P, Xe, F, Cl, B or some other suitable elements. The steepness of interfaces between the Ni films and the C films is removed from the multilayer in a region 222 to which the ion beam 721 has been introduced. Consequently, a reflection X-ray mask is formed which has a structure wherein reflected radiation is not able to escape from a side face of the pattern (FIG. 17(b)). Example 4 A reflection mask 81 formed according to any one of the Examples 1 to 3 is loaded in position into an X-ray projection exposure apparatus shown in FIG. 18, and the pattern of the reflection mask 81 is replicated onto a wafer 82 which defines a plane. The reflection mask 81 and the wafer 82 on which an image pattern is to be formed are carried on a mask stage 83 and a wafer stage 84, respectively. First, the relative positions of the reflection mask 81 and the wafer 82 are detected using an alignment apparatus 85, and they are positioned relative to each other by way of driving apparatus 87 and 88, respectively, under the control of a control apparatus 86. X-rays 411 radiated from an X-ray source 89 are collimated by a reflecting mirror 90 and illuminate a ring field on the reflection mask 81. Here, the arrangement of the pattern of the reflection mask 81 is set so that side faces in a lateral direction of those of the multilayer patterns on the reflection mask 81 which have comparatively small widths, and the direction of the X-rays 411 reflected regularly from the multilayer patterns, may extend in parallel to each other. In particular, the positional relationship between the reflection mask 81 and the incident X-rays 411 is set so that, as shown in FIG. 13, the lateral direction of the pattern 22 having the comparatively small width may coincide with the sagittal direction of the incident X-rays 411 while the longitudinal direction of the pattern having the comparatively small width coincides with the tangential direction of the incident X-rays 411. The X-rays reflected from the reflection mask 81 have a wavelength near 5 nm, and are imaged at a magnification of 1/5 on the wafer 82 by way of an imaging optical system 95 consisting of reflecting mirrors 91, 92, 93 and 94. Each of the reflecting mirrors 91, 92, 93 and 94 has a Ni/C multilayer similar to that of the reflection mask 81 vapor-deposited thereon. The cycle of each of the layers of the multilayer is adjusted so that it may coincide with the incidence angle to the corresponding optical system. The reflection mask 81 and the wafer 82 are synchronously scanned in the directions indicated by a double-sided arrow 96 in accordance with the magnification to replicate the pattern of the entire surface of the reflection mask 81 onto the wafer 82. By such a method, a pattern having a width of 0.05 .mu.m can be obtained in a 30 nm square area on the wafer 82. Example 5 As a multilayer for a reflection mask, rhodium (Rh) films each having a thickness of 1.8 nm and boron nitride (BN) films each having a thickness of 1.8 nm are alternately formed by a magnetron sputtering method until a total of 150 layers of each of the two films are reached to form a reflection X-ray mask similar to those of Examples 1, 2 and 3. Then, the reflection X-ray mask is exposed and illuminated using the X-ray projection exposure apparatus shown in FIG. 18 to image and replicate the pattern of the reflection mask onto a wafer. Each of the reflection mirrors 91, 92, 93 and 94 has a Rh/BN multilayer similar to that of the mask that is vapor-deposited thereon. The X-rays reflected from the mask have a wavelength near 7 nm. When imaging and replication are performed similarly as in Example 4, a pattern having a width of 0.07 .mu.m can be obtained. Example 6 As a multilayer for a reflection mask, molybdenum (Mo) films each having a thickness of 3.37 nm and silicon carbide (SIC) films each having a thickness of 3.37 nm are alternately formed by a magnetron sputtering method until a total of 50 layers of each of the two films are reached to form a reflection X-ray mask similar to those of Examples 1, 2 and 3. Then, the reflection X-ray mask is exposed and illuminated using the X-ray projection exposure apparatus shown in FIG. 18 to image and replicate the pattern of the reflection mask onto a wafer. Each of the reflection mirrors 91, 92, 93 and 94 has a Mo/SiC multilayer similar to that of the mask that is vapor-deposited thereon. The X-rays reflected from the mask have a wavelength near 13 nm. When imaging and replication are performed similarly as for Example 4, a pattern having a width of 0.1 .mu.m can be obtained. While, in the examples described above, only the cases wherein the material of a multilayer is Ni/C, Rh/BN and Mo/SiC are described, the present invention is not limited to such materials, but can be put into practice using any material from which a suitable multilayer can be formed, such as, for example, NiCr/C, Ni/V, Ni/Ti, W/C, Ru/C, Rh/C, Ru/BN, Rh/B.sub.4 C, RhRu/BN, Ru/B.sub.4 C, Mo/Si, Pd/BN, Ag/BN, Mo/SiN, Mo/B.sub.4 C, Mo/C, Ru/Be, Cu/C, Co/C, Fe/C or Mn/C. Further, while only a reflection mask is described in the examples, the present invention is not limited to a reflection mask but can be applied to any optical element which has a refined pattern on a reflecting surface thereof such as a diffraction grating or a linear zone plate. As is apparent from the foregoing description, according to the present invention, an optical element which is improved in reflected radiation intensity of an image pattern and can replicate or image a refined pattern, and a projection exposure apparatus employing the optical element, can be provided. Further, a refined pattern can be replicated onto a plane of an element such as a wafer since an image pattern which is improved in reflected radiation intensity can be imaged by way of an imaging optical system by irradiating the optical element of the present invention.