Patent Number: 051669625
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray mask for use in X-ray exposure and, more particularly, to an X-ray mask in which an X-ray transmitting thin film is improved. 2. Description of the Related Art Recently, as the degree of integration of semiconductor devices has been increased, the extent of micropatterning of a circuit pattern of LSI elements which constitute a semiconductor device has been increased. In order to form a very fine pattern on the order of so-called subhalfmicrons, high-resolution exposure transfer techniques are essential. As one of these techniques, there is proposed an exposure transfer technique using X-rays having a wavelength by far shorter than that of currently widely used ultraviolet rays. To put such an X-ray exposure transfer technique into practical use, a large number of technical problems must be solved. Of these problems, the formation of an X-ray mask is of most concern. FIG. 10 shows a representative sectional structure of an X-ray mask. The main portion of this X-ray mask is composed of a support frame 1, an X-ray transmitting thin film 2 serving as a mask substrate, and an X-ray absorber pattern 3. Of these parts, the X-ray transmitting thin film 2 is required to have a sufficient transmittance of X-rays used in exposure, a sufficient radiation resistance against intense X-rays used in exposure, and a sufficient transmittance of visible light (wavelength 633 nm) used in an alignment between a mask and a wafer. The film 2 is also required to have a sufficient mechanical strength and a small tensile stress so that a fine X-ray absorber pattern does not cause a displacement. In many cases, the X-ray transmitting thin film 2 is formed on a substrate, such as an Si wafer, which constitutes the support frame 1. For this reason, the step of forming the film on the substrate is an important step which determines the characteristics of the X-ray transmitting thin film 2. Note that unnecessary portions of the substrate are removed by etching. Conventionally, BN, Si, SiN, and SiC, for example, have been examined as the material of the X-ray transmitting thin film, and a vacuum vapor deposition method, a sputtering method, a CVD method, and the like have been studied as the formation method of the film. However, it is difficult to obtain a film which completely satisfies the above conditions. For example, the use of BN or SiN makes it difficult to form a film having a sufficient radiation resistance against intense X-rays used in exposure. Although Si is satisfactory in radiation resistance, a film having a high visible light transmittance is difficult to form by using this material. SiC is a substance which satisfies the above conditions comparatively well, but it has the following problem. That is, although SiC is a material originally having a high visible light transmittance, crystal defects are easily produced upon film formation using this material, and this makes it impossible to obtain a high visible light transmittance. In order to solve the above problems, the use of a stacked composite film consisting of two or more layers of different types of materials has been examined. In this method, however, it is necessary to use different source gases upon film formation performed by a CVD method. In addition, a technique of using different film formation methods for a single material has been studied. In this technique, for example, an amorphous film is formed by a plasma CVD or ECR-CVD method on a polycrystalline film formed by a thermal CVD method, and this composite film is used. However, this technique requires a plurality of different types of film formation apparatuses. As described above, it is difficult to obtain a sufficiently high visible light transmittance when SiC is used as the material of the X-ray transmitting thin film. In addition, the formation of a stacked composite film proposed as the X-ray transmitting thin film complicates the manufacturing steps and makes it difficult to easily improve the visible light transmittance. SUMMARY OF THE INVENTION The present invention has been made in consideration of the above situation and has as its object to provide an X-ray mask having a high visible light transmittance. It is another object of the present invention to provide a method of manufacturing an X-ray mask, which can form an X-ray transmitting thin film serving as a mask substrate by simple manufacturing steps and can improve a visible light transmittance. It is still another object of the present invention to provide an exposure method using the X-ray mask according to the present invention. The characteristic feature of the X-ray mask according to the present invention is that the overall visible light transmittance is improved by constituting an X-ray transmitting thin film serving as a mask substrate by a stacked film consisting of layers having different compositions. In addition, in order to form this stacked film, the film formation is performed under different film formation conditions in a CVD method. Suppose the film formation of SiC is performed by a CVD method under conditions in which the ratio of C atoms to Si atoms contained in source gases is changed. In this case, when the ratio is small, each crystal grain has high quality and the film has a high visible light transmittance. However, since unevenness in grain boundaries or on a film surface is enlarged, light is scattered on the surface of the film to cause the visible light transmittance to decrease. When the ratio is large, on the other hand, light is less scattered because unevenness in grain boundaries or on a film surface is small. However, since an inclusion which absorbs light is mixed in the film, the visible light transmittance decreases. For this reason, by stacking a plurality of layers having different ratios of C atoms to Si atoms, it is possible to form a good X-ray transmitting thin film having advantages of the respective layers. For example, both the major surfaces of an X-ray transmitting thin film are formed under a condition in which the ratio of C atoms to Si atoms is more (as compared with an average value of the entire thin film), whereas the central portion of the film is formed under a condition in which the ratio of C atoms to Si atoms is less. In this case, a good thin film having small undulations on its surface and containing a little inclusion which absorbs light can be obtained. In addition, according to the present invention, a desired X-ray transmitting thin film can be formed by properly selecting the flow rate of a source gas during film formation. Therefore, an X-ray transmitting thin film can be easily formed in a single reactor. Furthermore, since an X-ray transmitting thin film having a high visible light transmittance is realized, an alignment between a mask and a wafer can be performed with a high precision. Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.