Patent Number: 062122525
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray mask and a method of manufacturing the same, and particularly an X-ray mask provided with an alignment mark, which is used for position detection in a process of forming a circuit pattern for transfer, as well as a method of manufacturing the same. 2. Description of the Background Art In a processes of manufacturing semiconductor devices, a lithography technology utilizing ultraviolet rays has been primarily used for transferring a circuit pattern such as an interconnection pattern onto a semiconductor substrate. For example, the semiconductor devices have been integrated to a higher extent and, for example, DRAMs (Dynamic Random Access Memories) have been improved to have higher densities and higher storage capacities of, e.g., 1 gigabit (Gbit). As a result, it is now required to miniaturize circuit patterns for interconnections or the like to a higher extent. It is now expected that a lithography technology using X-rays can be a useful technology for transferring such fine circuit patterns. According to the lithography technology using X-rays, X-rays which are used for exposure have a wavelength (soft X-ray: wavelength=5-20 nm) shorter than that of ultraviolet rays which have been used in the prior art, and therefore can transfer fine circuit patterns of a higher resolution than that transferred by the conventional lithography using ultraviolet rays. The lithography technology using X-rays uses an X-ray mask provided with a circuit pattern for transfer. FIG. 27 is a cross-sectional view showing a structure of a conventional X-ray mask. Referring to FIG. 27, the conventional X-ray mask will be described below. In FIG. 27, the conventional X-ray mask includes a substrate 102, a membrane 103, an X-ray absorber 104 and a support ring 101. Membrane 103 is a substrate allowing passage of X-rays, and is formed on substrate 102. Membrane 103 has a film thickness of 1-3 .mu.m. X-ray absorber 104 is made of a material intercepting transmission of X-rays, and is formed on membrane 103. Substrate 102 is provided with a window 111. A rear surface of membrane 103 is exposed through window 111. In a region located on window 111, X-ray absorber 104 includes a portion 110 for forming a transfer circuit pattern, i.e., a circuit pattern for transfer. Support ring 101 is arranged under substrate 102. The circuit pattern formed in transfer circuit pattern forming portion 110 of X-ray absorber 104 must have a high position accuracy and a high size accuracy. The transfer circuit pattern is usually produced through the following steps. First, a resist (not shown) is applied onto X-ray absorber 104. The transfer circuit pattern is written on the resist with an electron beam lithography system. Development is effected on this resist to form a mask pattern of the transfer circuit pattern. Using this mask pattern as a mask, etching is performed to remove X-ray absorber 104, whereby the transfer circuit pattern layer is formed. In the above step of writing the transfer circuit pattern on the resist with the electron beam lithography system for forming the transfer circuit pattern, the accuracy of position detection significantly affects the accuracies of the position and size of the transfer circuit pattern of the X-ray mask. The step of writing the transfer circuit pattern with the electron beam includes the following specific steps. First, the X-ray mask including a resist applied over X-ray absorber 104 is attached to a jig called an EB cassette. Then, the X-ray mask is conveyed together with the EB mask into a load-lock chamber (i.e., an area provided for keeping a vacuum around the X-ray mask prior to writing so that the X-ray mask can be conveyed into an area, where writing with the electron beam can be performed, while keeping a vacuum). Then, a vacuum is produced in the load-lock chamber. Subsequently, the X-ray mask is conveyed together with the EB cassette from the load-lock chamber onto a stage in the area for the electron beam writing. The circuit pattern for transfer is written on the resist of the X-ray mask with the electron beam. The writing of the transfer circuit pattern with the electron beam is performed while detecting and referring to positions of alignment marks situated on the EB cassette or the stage. Usually, there is a difference in temperature between the X-ray mask, the interior of the load-lock chamber and the area for the electron beam writing. When producing a vacuum in the load-lock chamber, the temperatures of the EB cassette and the X-ray mask lower. After the X-ray mask is conveyed onto the stage in the area for the electron beam writing, therefore, the temperatures of the EB cassette and the X-ray mask change until they reach an equilibrium with the temperature of the area where the electron beam writing is performed. As a result, the EB cassette and the X-ray mask conveyed onto the stage expand or shrink due to this change in temperature. The temperature of the X-ray mask is stabilized to some extent as a certain time elapses. However, a long time is required until the temperature of the EB cassette carrying the X-ray mask or the stage is stabilized. Therefore, it is impossible to write accurately the transfer circuit pattern with the electron beam before the temperature of the entire area for the electron beam writing reaches an equilibrium. In the conventional process, therefore, the X-ray mask and the EB cassette which were conveyed onto the stage are left standing on the stage for a long time until the entire area for the electron beam writing reaches an equilibrium. The electron beam writing of the transfer circuit pattern is performed after the entire area for the electron beam writing reaches an equilibrium. The fact that the standing state must be kept until the entire area for the electron beam writing reaches an equilibrium results in increase in manufacturing period of the X-ray mask, and consequently increases a manufacturing cost of the X-ray mask. In order to reduce the time for which the standing state is kept before writing the transfer circuit pattern with the electron beam, the alignment mark may be formed on the X-ray mask itself instead of the EB cassette or the stage. This is because the alignment mark, which is formed on the X-ray mask, does not change its position once the temperature of only the X-ray mask is stabilized, even before the entire area for the electron beam writing reaches an equilibrium. Japanese Patent Laying-Open No. 2-166720 (1990) has disclosed an X-ray mask provided with an alignment mark, which is made of a material having a high electron beam reflection coefficient and is formed on the surface of the X-ray mask. FIG. 28 is a perspective view of the X-ray mask proposed in the above publication. Referring to FIG. 28, the X-ray mask already proposed will be described below. The proposed X-ray mask in FIG. 28 includes a support material 105, a X-ray transmissive material 106 allowing passage of X-rays, an X-ray absorber 104 and alignment marks 107. X-ray transmissive material 106 is formed on support material 105. X-ray absorber 104 is formed on X-ray transmissive material 106. Alignment marks 107 are made of gold films and are formed on X-ray absorber 104. For forming alignment marks 107 (see FIG. 28), this proposed X-ray mask requires an additional process for forming gold films on X-ray absorber 104. This complicates the process of manufacturing of the X-ray mask, and increases the manufacturing period. Japanese Patent Laying-Open No. 4-297016 has disclosed an X-ray mask, in which alignment mark providing reference positions are formed on an X-ray absorber. In the X-ray mask disclosed in Japanese Patent Laying-Open No. 4-297016, however, the alignment marks are formed in a region on a membrane where a mask pattern is formed. Therefore, the reference position of the alignment marks shifts due to distortion caused by a heat and a resist stress relief, which are generated or caused when a pattern for forming the mask pattern is written on a resist. SUMMARY OF THE INVENTION An object of the invention is to provide an X-ray mask which includes a transfer circuit pattern having a high position accuracy, and can be manufactured by a simplified manufacturing process. Another object of the invention is to provide a method of manufacturing an X-ray mask, which includes a transfer circuit pattern having a high position accuracy, and can be manufactured by a simplified process. According to an aspect of the invention, an X-ray mask includes a substrate, a membrane allowing passage of X-rays, and an X-ray absorber intercepting transmission of X-rays. The membrane allowing passage of X-rays is formed on the substrate. The X-ray absorber is formed on the membrane. The substrate includes a window exposing the membrane. The X-ray absorber includes a transfer circuit pattern, and an alignment mark formed in a region not overlapping with the window in a plan view. According to the above aspect, the alignment mark is formed on the X-ray absorber of the X-ray mask without an additional step, which is required for the X-ray mask already proposed and is performed for forming a film of gold or the like for the alignment mark on the X-ray absorber. Therefore, the X-ray mask provided with the alignment mark can be produced through a more simple manufacturing process than that in the prior art. In the step of forming the transfer circuit pattern on the X-ray mask, the alignment mark formed on the X-ray mask can be used for detecting the position of the X-ray mask. Therefore, it is not necessary to wait until the temperature of the whole area for the electron beam writing of the transfer circuit pattern reaches an equilibrium in contrast to the prior art, and the X-ray mask provided with the transfer circuit pattern having a high accuracy can be manufactured without the wait. According to the above aspect, the alignment mark is situated in the region, which is spaced from the region for forming the transfer circuit pattern, and is situated in the region covering the membrane overlying the substrate. Therefore, a significant variation in position of the alignment mark can be prevented even when the membrane located on the window vibrates in a step of writing a mask pattern for forming the transfer circuit pattern or a distortion occurs due to resist stress relief or heat caused by the mask pattern writing step. Consequently, it is possible to prevent deterioration of an accuracy of position detection of the X-ray mask which may be caused by a variation in position of the alignment mark or the like. Accordingly, the X-ray mask can have the transfer circuit pattern having a higher accuracy. In the X-ray mask according to the above aspect, the alignment mark may be an opening formed in the X-ray absorber. According to the above structure, the step of forming the alignment mark can employ a technique such as etching which is used in the step of forming the transfer circuit pattern. Therefore, the alignment mark can be formed easily without adding a complicated process. In the X-ray mask according to the above aspect, a protective film may be arranged between the substrate and the X-ray absorber. According to another aspect, the invention provides a method of manufacturing an X-ray mask, in which a membrane allowing passage of X-rays is formed on a substrate. An X-ray absorber intercepting transmission of X-rays is formed on the membrane. A window exposing the membrane is formed in the substrate. A first mask layer is formed on the X-ray absorber. A first mask pattern is written on the first mask layer. Development is effected on the first mask layer to form a first mask pattern layer. The X-ray absorber is removed using the first mask pattern layer as a mask, whereby an opening functioning as an alignment mark is formed in a region not overlapping with the opening in a plan view. According to the above method, it is not necessary to form a film of gold or the like on the X-ray absorber, which is required for forming the alignment mark in a conventional X-ray mask. Therefore, the X-ray mask provided with the alignment mark can be produced through simplified steps, compared with the X-ray mask proposed in the prior art. According to the above aspect, the alignment mark is formed in the region, under which the substrate is present with the membrane therebetween. Therefore, a significant variation in position of the alignment mark can be prevented even when the membrane located on the window vibrates in a step of writing a mask pattern for forming the transfer circuit pattern or a distortion occurs due to resist stress relief or heat caused by the mask pattern writing step. Consequently, it is possible to prevent deterioration of an accuracy of position detection of the X-ray mask which may be caused by a variation in position of the alignment mark or the like. Accordingly, the X-ray mask can have the transfer circuit pattern having a higher accuracy. In the method of manufacturing the X-ray mask according to the above aspect, the first mask pattern layer may be removed and a second mask layer may be formed on the X-ray absorber. A second mask pattern may be written on the second mask layer while performing position detection of the second mask layer using the opening as the alignment mark. Development may be effected on the second mask layer to form a second mask pattern layer. The X-ray absorber may be removed to form a transfer circuit pattern, using the second mask pattern layer as a mask. According to the above method, the opening is used as the alignment mark when performing the position detection. Therefore, it is not necessary to wait until the temperature of an EB cassette for the electron beam writing of the transfer circuit pattern reaches an equilibrium in contrast to the prior art, and the X-ray mask provided with the transfer circuit pattern having a high position accuracy can be produced through a simplified manufacturing process. In the method of manufacturing the X-ray mask according to the above aspect, a second mask pattern may be written on the first mask layer while performing position detection of the first mask layer using the opening as an alignment mark. A second mask pattern layer may be formed by effecting development on the first mask layer. The transfer circuit pattern may be formed by removing the X-ray absorber, using the second mask pattern layer as a mask. According to this method, both the first and second mask patterns are written on the first mask layer. Therefore, the steps of manufacturing the X-ray mask can be simplified compared with the case where a mask layer other than the first mask layer is formed for writing the second mask pattern. In the method of manufacturing the X-ray mask according to the above aspect, the step of writing the first mask pattern may include an exposure step using light, and the step of writing the second mask pattern may include a writing step using an electron beam. According to the above manner, the step of writing the first mask pattern employs the exposure method using light which requires a shorter exposing time than the exposure method using an electron beam. Therefore, the time required for manufacturing the X-ray mask can be shorter than that in the case where the electron beam exposure method is used for writing the first and second mask patterns. In the method of manufacturing the X-ray mask according to the above aspect, the step of forming the second mask layer may include a step of forming a second mask layer exposing the opening on the X-ray absorber. According to this method, a resist or the like is not present on the opening functioning as the alignment mark even when an electron beam is emitted to the opening for position detection of the opening. Therefore, a problem which may arise in the case where the resist is present on the opening can be prevented and, more specifically, such a problem can be prevented that characteristics of the resist are deteriorated due to irradiation with the electron beam and thereby the position detection of the opening with the electron beam is impeded. As a result, position detection of the alignment mark can be performed accurately. Thereby, it is possible to write the second mask pattern having a high position accuracy. Consequently, the X-ray mask provided with the transfer circuit pattern having a high accuracy can be manufactured easily. In the method of manufacturing the X-ray mask according to the above aspect, the step of forming the opening may include a first etching step of removing the X-ray absorber, and the step of forming the transfer circuit pattern may include a second etching step of removing the X-ray absorber. The first and second etching steps may be performed under different conditions, respectively. The above method can employ etching conditions suitable to the size of the opening functioning as the alignment mark as well as etching conditions suitable to the sizes of the interconnection pattern and others of the transfer circuit pattern, even if the former and latter sizes are different from each other. Consequently, both the opening and the transfer circuit pattern can be formed with high position accuracies. Therefore, the X-ray mask provided with the transfer circuit pattern having a high size accuracy and a high position accuracy can be produced. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.