Patent Number: 055725643
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic cross sectional view of the pattern part of a reflection-type mask for X-ray exposure, in accordance with the present invention, with a graph of the distribution of X-ray intensity I in the position P in the pitch direction of the mask pattern corresponding to it. In the reflection-type mask for X-ray exposure shown in FIG. 1, a desirable pattern composed of X-ray reflectable multi layer 1 is formed partially on substrate 2 composed of a material that does not reflect X-ray radiation. At the essential use of such mask, the incident X ray 3 into the multilayer 1 is reflected on the interface of each layer within the multilayer and exits as reflected X ray 4; the cross section of the multilayer 1, represented on the paper face of FIG. 1, is a face where the reflected X ray 4 is included when the mask of the present invention is used, while the individual edge faces of the multilayer pattern are formed as slant faces 1a and 1b, in almost parallel with the exit direction of X ray. In the example, if viewed from the parallel cross section to the incident face, the edge faces 1a and 1b on the incident and exit sides of the cross section have separately a slope approximately consistent with the exit direction of the X ray after the reflection thereof on the interface of each layer within the multilayer 1. Consequently, the cross sectional form of the multilayer 1 represented on the paper face of FIG. 1 is parallelogram with the oblique sides being parallel with the progressing direction of the reflected X ray 4. By such a configuration, all of the layers within the multilayer (i.e. the interfaces) contribute to overlapping of reflection light, even in the proximity of the pattern edges (inside the pattern) on the incident side of X ray, so no reduction in reflection intensity occurs, as is observed in conventional examples. In the proximity of the pattern edges on the exit side of X ray (outside of the pattern), no exit X ray from the part of the pattern edge face is generated, as is observed in conventional examples. Consequently, at the pattern edge part, no diversity in the form of exponential curve of the intensity distribution of reflected X ray reaching the side of a substrate. One example of manufacturing such a reflection-type mask for X-ray exposure is illustrated below. On substrate 2 composed of Si as a material that does reflect X-ray radiation, 50 periods of Mo layer and Si layer, were formed to form multilayer 1 of a period length of 88 angstroms. A line-and-space pattern of 1 .mu.m was formed on multilayer 1 as a resist pattern, according to photolithography. Thereafter, etching of the multilayer 1 was effected by an ion milling system as shown in FIG. 5. During the etching, the substrate 2 with the multilayer 1 being formed thereon was arranged, in slant manner with an angle of 45.degree. to the incident direction of ion beam 10 from ion beam generator 9. The pattern direction of the mask was set up to be vertical to the incident direction of the ion beam (the paper face of FIG. 5), whereby the edge faces of the pattern line part, individually formed by the multilayer 1, became slant faces having an angle of 45.degree. to the substrate 2. If viewed in the cross sectional forms, a pattern of the multilayer, in the form of parallelogram with the oblique sides having a slope of 45.degree., was formed. Finally, the resist remaining was removed to complete a reflection-type mask for X-ray exposure as shown in FIG. 1. The types of ions for the irradiation in etching may be inert gases or reactive gases, or a mixture of inert gases and reactive gases. The inert gases include Ar (argon), Kr (krypton), Xe (xenon) and the like. In this case, physical sputter-etching is effected. The inert gases if used have advantageous in that etching can be effected, notwithstanding any of the types of the materials of the multilayer. The reactive gases include CF.sub.4, CCl.sub.4, CHF.sub.3, O.sub.2 and the like. In this case, chemical etching involving chemical reaction is effected. The reactive gases if used can promote the etching rate, although the material of the multilayer is necessarily changed into a volatile substance such as halides. If the ratio of a material etching rate to a resist etching rate is defined as a selectable ratio and the selectable ratio is not satisfactory, the resist forming the pattern is more rapidly etched than the material so the material cannot be etched in a desirable pattern. In etching, therefore, a multilayer resist process realizing a larger thickness of resist film is utilized if the selectable ratio of a monolayer resist is not satisfactory. Examples therefor are depicted in FIGS. 7a and 7b. As is shown in FIG. 7a, multilayer 21 is formed on substrate 22, on which resin layer 23 of a first resist, polyimide or the like is formed. Furthermore, SiO.sub.2 layer 24 is formed as an intermediate layer on the resin layer 23, and a second resist layer 25 is simultaneously formed further on the SiO.sub.2 layer 24. The first resist (or polyimide resin) layer 23 should have a film thickness sufficient enough to work as a mask in the etching of multilayer 21. The SiO.sub.2 layer 24 is for patterning the first resist (or polyimide resin) layer 23. In ion milling, the SiO.sub.2 layer 24 has a higher selectable ratio relative to the resist, so the film thickness thereof may be thin, satisfactorily. The second resist layer 25 is a resin layer similar to the first resist layer, which works as a mask for effecting the etching of the SiO.sub.2 layer 24 as the layer below thereof. The etching of the SiO.sub.2 layer 24 is effected with the reactive gases, so the thickness of the second resist layer 25 may be thin. During the etching, patterning is effected by photolithography in order that the second resist layer 25 acquires a desirable mask pattern (FIG. 7a). Then, a reactive gas such as CHF.sub.3 and the like is used to effect etching of the SiO.sub.2 layer 24 (FIG. 7b). After the completion of the etching of the SiO.sub.2 layer 24, the second resist layer 25 remaining on the surface is removed. Subsequently, the irradiation of ion beam on the substrate 22 is effected in the state of grazing incidence. The ion then may be an inert gas ion such as Ar , or a gas for chemical etching of a multilayer material, such as CF.sub.4, CCl.sub.4 and the like. Through the process, etching of the first resist (or polyimide resin) layer 23 is effected in a slant form. (FIG. 7c) When the surface of the multilayer 21 is subsequently exposed, the multilayer 21 in a pattern following the resist pattern is etched in a slant form in the cross section of the mask. During the process, the SiO.sub.2 layer 24 on the surface is also etched little by little, which layer is almost eliminated at the completion of the etching of the multilayer 21. (FIG. 7d) Finally, the first resist (or polyimide resin) layer 23 remaining is removed to form a reflection-type mask for X ray exposure of a desirable pattern. (FIG. 7e) With reference to FIG. 6, there will follow a description of the results of exposure tests which were carried out by using the reflection-type mask for X-ray exposure in accordance with the present invention. As is shown in FIG. 6, the X-ray beam of a wave length of 124 angstroms, which was obtained through spectroscopy of radiation, was made incident into the reflection-type mask for X-ray exposure 6 at an angle of 45.degree.. Then, an image with reflected X ray 4 was transferred to Si wafer 8 coated with positive-type PMMA resist. In this case, the pitch direction of the line-and-space pattern of the mask was arranged to be parallel to the incident face (paper face of FIG. 6). Because the mask was herein slanted at 45.degree., the pattern width of the mask if projected on the wafer can be converted into a line-and-space of 0.7 .mu.m. When the pattern of the formed wafer was actually measured, a line-and-space of 0.7 .mu.m was accurately transferred onto the wafer. For comparison, an identical experiment was carried out by using a reflection-type mask for X-ray exposure manufactured following conventional examples, wherein the edge faces of a multilayer pattern were not slanted. On measuring the outcome of the transfer, the width of the pattern part got as wide as 0.8 .mu.m, and the width of the space part got as narrow as 0.6 .mu.m. That is, no accurate transfer of the pattern on the mask onto the wafer could be achieved. In the present example, no special reduction of the pattern image through an optical system was effected. For the actual use in a process, however, a pattern image is reduced about 1/10 fold, by passing the reflected X ray on mask 6 through a reduction optical system 7 composed of a multilayer mirror and the like, as is explained in FIG. 3, thereby enabling accurate transfer of such pattern. The individual examples described insofar are those for illustrating the present invention, and should not be interpreted to a limitation. The present invention includes various versions or modifications, possibly employed by the skilled person in the art in a variety of manners not shown herein.