Patent Number: 055725643
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflection-type photo mask principally for use in X-ray projection exposure during the manufacturing process of semiconductor devices and the like. 2. Description of the Prior Art Attention has been focused recently on lithography utilizing the X ray of a shorter wave length compared with ultraviolet ray. X-ray lithography is considered advantageous in that it can achieve higher resolution. This high resolution is theoretically impossible for lithography utilizing ultraviolet rays. According to conventional X-ray lithography, so-called transmission-type X-ray masks have been used in which a desirable pattern is formed with a thin film member comprising an X-ray absorbing material onto a membrane of a thickness of about 2 .mu.m and comprising a material with relatively good X-ray transmission, for example silicone nitride. With X-ray irradiation these masks, a projection image of the mask pattern by the X ray transmitting through the part except for the pattern part is generated on the photo resist layer on the surface of wafer, which is employed for effecting lithography. The following problems have been suggested about such transmission-type X-ray masks because thin membranes are employed in such masks. 1. Because the membranes per se are thin, such masks are readily breakable, and handling is difficult. PA0 2. Because the membranes are in the form of a thin film, deformation is induced in the membranes due to the inner stress in X-ray absorbing materials, with resulting occurrences of positional distortion in the projected pattern image. PA0 3. On X-ray irradiation, the temperature of the masks is raised due to the X ray energy absorbed into the membranes per se and the X-ray absorbing materials, and thermal expansion of the membrane and x-ray absorbing matter develops positional distortion in the pattern. PA0 (a) a process of forming an X-ray reflectable multilayer on a substrate formed from a material; PA0 (b) a process of forming a desirable resist pattern on the multilayer; PA0 (c) a process of irradiating an ion beam onto the multilayer on which is formed the resist pattern, from a preliminarily defined slant direction, to effect etching of the multilayer in the form of the resist pattern; and PA0 (d) a process of removing the resist pattern after the completion of the etching process. In order to solve these problems, reflection-type X-ray masks have been developed. In reflection-type X-ray masks, substrates in the form of thick board are used, instead of thin membranes. FIG. 2a shows one example of such masks such that an X-ray reflectable multilayer 101 is formed on the entire surface thereof and a pattern is formed with X-ray absorbing material 105. FIG. 2b shows that on X-ray unreflectable substrate 102 a desirable pattern is formed with X-ray reflectable multilayer 101. By means of an optical system, the light transmitted through a mask generates an image on the surface of a substrate in transmission-type X-ray masks, while the light reflected on the pattern surface of a mask generates an image on a substrate in such reflection-type X-ray masks. Because a thin membrane is not utilized in such reflection-type masks, the individual problems described above can be solved which are encountered with transmission-type masks. However, a newly developed problem concerning such reflection-type X-ray masks has been suggested. When using a reflection-type X-ray mask 106, as is shown in FIG. 3, it is required that the optical axis of the incident light 103 be arranged not to be co-incident with the optical axis of reflection light 104, in order that optical system 107 which works to generate an image on substrate 108 from the reflected light 104 on the mask 106 should not interfere with the incident light 103 into the mask 106. Therefore, the incidence of the X ray into the mask 106 is absolutely required to be arranged in the form of grazing incidence, not in the form of vertical incidence. The X ray incident into multilayer 101 (FIGS. 2a and 2b) of a mask penetrates into the multilayer in the depth direction thereof at some degree, and thereafter the X ray is partially on the interface of the individual layers within the multilayer. Hence, the partially overlaid reflection on a large number of the individual layers within the multilayer is utilized as the reflected X ray for generating an image. For this reason, the intensity of reflected X ray 104 is lowered near the edge on the opposite side to the direction of X-ray incident direction of a pattern 105 of an absorbing material, where the pattern of the absorbing material 105 is formed on the X-ray reflectable multilayer (see FIG. 2a). FIG. 4a, concerns the reflected X ray which exits from the multilayer exposing part near the edge. The part of the X ray should be incident to multilayer and should essentially form reflected X ray disturbed by the pattern 104 of an absorbing material. Consequently, no overlap is observed in some of the reflected X ray 104 on the individual interfaces at the portion of the pattern. This produces consequent a lowered reflection intensity. Therefore, the distribution of intensity I of reflected X ray, on position P in the pitch direction of the mask pattern, has a diversity in the form of exponential curve. In FIG. 4a W represents the width of a pattern. When a pattern comprising X-ray reflectable multilayer 101 is formed on an X-ray unreflectable substrate 102 (see FIG. 2b), a problem is suggested about the reflection intensity in the proximity of both edges on the incident and exit sides to the incident direction of X ray, as is shown in FIG. 4b. There is less overlapping of reflected X ray on the interfaces of individual layers, because incident X ray 103 does not reach a sufficient depth within the multilayer 101 so that no reflection is induced from the reflection surface below. Therefore, the intensity of the reflected X ray 104 on the X-ray incident side has a distribution in the form of exponential curve as is shown in FIG. 4b. Thus, the reflection intensity on the pattern edge is low. The X-ray which is reflected within the multilayers 101 and exits from the edge part constitutes the pattern. The intensity of the X ray also has a distribution in the form of exponential curve, as is shown in FIG. 4b. With respect to FIG. 4b, it may be considered that the influences on both of the edges of the pattern work to negate each other. As is shown in the figure, however, a problem occurs in that the width Wr of the range in which the quantity of light I.sub.R to be required for exposure of resist generally is changed into a different one from the original pattern width W of a mask. The diversity in the form of exponential curve of the distribution of the reflection intensity on (and around) these edges is represented in either case approximately as exp(-.mu.X/sin.theta.) if X, .mu. and .theta. are defined as representing the position in coordinate in the same direction as P, a linear coefficient and incident angle, respectively. When the X ray of a wave length of 124 angstroms irradiates a multilayer comprising Mo/Si at an angle of 45.degree., for example, the position with a 1/e-fold reflection intensity corresponds to a position in distance by about 0.13 .mu.m apart from the pattern edge, which is never negligible in a projection image of a microfine circuit pattern. As has been described above, a serious problem has been suggested about conventional reflection type mask for X-ray exposure in that a pattern on a mask cannot be precisely transferred on a substrate in either type as in FIG. 2a (FIG. 4a) or FIG. 2b (FIG. 4b). SUMMARY OF THE INVENTION It is a major objective of the present invention to provide a reflection-type mask for X-ray exposure capable of accurate and clear transfer of a mask pattern, in particular as a high-contrast image of an edge bordering part, wherein it is prevented that the reflected X ray on the edge part of a mask pattern may have a diffuse intensity distribution in the form of exponential curve on the surface of a substrate; and to provide a method for manufacturing the same. The mask for X-ray exposure according to the concept of the present invention is a mask provided with a substrate composed of a material that does not reflect X-ray radiation along with a multilayer being patterned on the substrate and being reflectable of X ray. In order to achieve the objective described above, at least a part of the edge part of the pattern comprising the multilayer is formed preferably in smooth slant face approximately parallel to the exit direction of the X ray reflected on the interfaces between the individual layers within the multilayer, namely the exit direction of the X ray after its reflection on the interfaces among a plurality of layers within the multilayer. Since the reflection-type X-ray mask of the present invention is formed as described above, the reflected X ray which exits from the proximity of the pattern edge of the multilayer (inside the pattern) is composed of the X ray partially reflected on the interfaces of all of the layers within the multilayer, so the reflected X ray which exits from any part of the pattern has an identical reflection intensity. The edge surface may not necessarily be smooth slant face. However a slant face in a step-wise form corresponding to each layer in the etching of the multilayer may be satisfactory. As the slant face is as smooth as possible, there may be less X ray exiting from the edge surface of the pattern after the reflection thereof on any interface within the multilayer, leading to a drastic decrease to zero of the intensity of the reflected X ray on the outside of the pattern with the border of the pattern edge. This produces a highly-contrasted pattern image. Accordingly, the mask of the present invention if used can generate an accurate projection of a mask pattern on a substrate, by the exposure to the X ray in the incident direction preliminarily determined according to the direction of the slope of the slant face of the edge face, which enables transfer of the pattern in the accurate size thereof on wafer during the process of lithography. If the mask of the present invention is used, an extremely high contrast is obtained of the pattern image on a substrate, with the border of the pattern edge. An image in clear resolution can be obtained even for a microfine pattern. The process of manufacturing a mask for X-ray exposure, in accordance with the basic embodiment of the present invention, has each of the following processes; According to the manufacturing process, the adjustment of the incident angle of ion beam enables to form the edge faces of the multilayer pattern as the slant face along a desirable exit direction of reflected X ray. The exit direction of reflected X ray can absolutely be defined, individually depending on the configuration of each X-ray exposure system, such as X-ray optical system, mask holder or the like. The objectives described above and other objectives of the present invention, characteristic features and advantages thereof will now be understood further in the following detailed explanation with reference to attached drawings illustrating non-limiting examples.