Patent Number: 051724030
Section: summary

FIELD OF THE INVENTION AND RELATED ART This invention relates to an X-ray exposure apparatus for lithographically transferring onto a semiconductor wafer a fine pattern of a semiconductor integrated circuit, by using soft X-rays. As a light source of such an X-ray exposure apparatus, known are a bulb type one which produces X-rays by electron beam excitation, one which uses X-rays produced from plasma, one that uses synchrotron orbit radiation, and the like. All of these X-ray sources produce X-rays in vacuum. Accordingly, an X-ray source is usually disposed in a vacuum and gas-tight X-ray source accommodating chamber, and produced X-rays are projected to a mask or wafer through a blocking window made of a material (usually, beryllium (Be)) having a high X-ray transmission factor. If in this case atmosphere is present in the path of X-ray transmission from the blocking window to a wafer, X-rays are absorbed by the atmosphere, resulting in an increase in exposure time and thus in a decrease of the throughput. Considering an X-ray exposure apparatus as an industrial productive machine, the decrease in throughput is a critical problem. In an attempt to solve this, a proposal has been made, in accordance with which an alignment mechanism for a mask and a wafer is disposed in a vacuum and gas-tight container chamber (hereinafter "stage accommodating chamber") and such stage accommodating chamber is filled with a particular gaseous fluid (usually, helium (He) gas) of reduced pressure, lower than the atmospheric pressure, having little X-ray absorbency (Japanese Patent Application No. 63-49849). In this type of X-ray exposure apparatus, however, the amount of X-ray transmission through the path from the blocking window to the wafer is greatly affected by the helium gas ambience within the stage accommodating chamber. If the purity or pressure of helium introduced into the stage accommodating chamber changes largely, the amount of X-ray exposure changes, which results in reduction in precision of an exposure apparatus. Additionally, it is necessary to retain the purity of helium at a high level and also to keep the variation thereof small. If it is desired to control the purity by using some purity detecting means, use of a very high precision detection and control means is necessary. In that respect, an X-ray exposure apparatus has been proposed in U.S. patent application Ser. No. 07/417,054 filed Oct. 4, 1989, now copending U.S. patent application Ser. No. 07/733,977, filed Jul. 22, 1991, assigned to the same assignee of the subject application, which apparatus is arranged so that, even after the inside gas of a stage accommodating chamber is replaced by helium, a constant amount of helium is continuously supplied into the chamber to compensate for the reduction in the purity due to introduction of an impure gas into the stage accommodating chamber. FIG. 6 shows this X-ray exposure apparatus. In FIG. 6, a barrel 5 is coupled to an X-ray source (not shown) and, to this barrel 5, a stage accommodating chamber 19 is coupled. The barrel 5 is equipped with a beryllium blocking window 6 and the X-rays passing therethrough are used. The stage accommodating chamber 19 accommodates therein a mask 13, a mask chuck 14, a wafer 15, a wafer chuck 16 and a wafer stage 18. To the stage accommodating chamber 19, a low vacuum pump 11 such as an oil rotation pump, for example, is coupled by way of a variable valve 10. The valve 10 is adapted to change the opening (conductance) thereof automatically in response to a signal from a controller 9. By means of a pressure sensor 8 and a pressure detecting port 7, the pressure within the stage accommodating chamber 19 can be detected and, on the basis of this detection, the controller 9 controls the opening of the variable valve 10. By this, the inside pressure of the stage accommodating chamber 19 is controlled and maintained constant. Denoted at 1 is a helium tank and denoted at 2 is a valve the opening of which can be adjusted manually. The He gas can be supplied through a He supply port 3. In this type of X-ray exposure apparatus, first the inside of the stage accommodating chamber 19 is vacuum evacuated to a predetermined pressure by using the low vacuum pump 11 and, thereafter, a helium gas is supplied to fill the stage accommodating chamber 19 with a reduced pressure helium ambience, and the exposure process is executed in this ambience. SUMMARY OF THE INVENTION In this example, the control of the helium ambience (purity and pressure) is conducted with respect to the entirety of helium gas in the stage accommodating chamber 19. However, since in the stage accommodating chamber 19, many elements such as the mask 13, the wafer 15, the mask chuck 14, the wafer chuck 16, the wafer stage 18 and the like are accommodated, it is not easy to correctly predict the flow of helium within the stage accommodating chamber 19. Also, when the wafer stage 18 moves, there is a possibility of the production of a regional flow. If this occurs, it is not possible to maintain the entire helium ambience in the stage accommodating chamber 19 uniform and, in the X-ray projection path from the beryllium blocking window 6 to the wafer 15 (to which the control is actually required), the purity of helium is liable to be degraded or the pressure thereof tends to change. It is accordingly a primary object of the present invention to provide an X-ray exposure apparatus by which in the X-ray projection path the helium ambience can be maintained at a predetermined purity and a predetermined pressure, whereby high-precision X-ray exposure is ensured. In accordance with an aspect of the present invention, a helium supplying port and a helium discharging port as well as a pressure detecting port are provided in the neighborhood of an X-ray projection path from a blocking window, to be passed by the X-rays, to the wafer. This makes it possible to execute weighted control of the helium ambience in the X-ray projection path in which the control is actually required. Additionally, a thin film may be provided between a mask and the blocking window to suppress vibration of the mask due to the gas flow of helium, to thereby ensure high precision exposure. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.