Patent Number: 052296164
Section: summary

Background of the Invention This invention relates to a lamp which is operable as an exposure light source of an exposure apparatus. An exposure apparatus of the type described is very often used in manufacturing a semiconductor device so as to project a mask pattern onto a semiconductor wafer and to expose the semiconductor wafer. To this end, the exposure apparatus comprises a lamp which is often called an exposure lamp and which is operable as an exposure light source in addition to an optical system which guides an optical beam from the lamp to the semiconductor wafer. It is noted that the lamp has major influence on the size of each of the products, namely, semiconductor devices, as known in the art. Specifically, a minimum pattern size of the semiconductor devices is dependent on the wavelength of the optical beam emitted from the exposure light source. Heretofore, a high pressure mercury lamp is used as such an exposure light source. In the high pressure mercury lamp, mercury which is enveloped in solid state form is vaporized and kept at a high pressure of several tens of atmospheres when it is excited by supply of an electric voltage. The high pressure mercury lamp can emit optical beams which have a spectrum distribution. The spectrum distribution includes spectra of specific wavelengths which are equal to 436 nm and 365 nm and which may be called g and i lines, respectively. The g line is suitable for manufacturing a dynamic random access memory of 4 megabits which may be referred to as 4 MDRAM and which has a minimum pattern size of 0.8 micron meters while the i line is used for manufacturing a dynamic random access memory of 16 megabits which may be referred to as 16 MDRAM and which has a minimum pattern size of 0.5 to 0.6 micron meters. Herein, it is mentioned that recent interest is mainly directed to a dynamic random access memory of 64 megabits which may be called a 64 MDRAM and which is considered as a very large scale integrated memory of the succeeding generation. In order to actually manufacture the 64 MDRAM, a line and space size, namely, a minimum pattern size should be equal to or smaller than 0.3 micron meter. This means that an exposure light source must emit a beam having an exposure wavelength of 250 nm or so within a far ultraviolet waveband. As a conventional exposure light source for emitting far ultraviolet light, an excimer laser of krypton-fluoride (KrF) has been known which emits a laser wavelength of 248 nm. It is mentioned here that the far ultraviolet light is projected onto a semiconductor wafer through an optical lens system of a reduction projection type. In such an optical lens system, use must be made of a lens material which exhibits a low absorptivity for the far ultraviolet light. The lens material may be, for example, fluorspar (CaF.sub.2) or quartz. However, fluorspar has difficulty as regards precise processing and temperature control. Under the circumstances, only synthetic quartz is practically used as the lens material of the optical lens system for the exposure light source. This shows that each lens of the optical lens system must be formed by a single material, namely, quartz which has an identical refraction coefficient. In general, an achromatic lens is formed by a combination of lenses which have different refraction coefficients. Therefore, the above-mentioned optical lens system in question can not include an achromatic lens or lenses and is structured as a monochromatic lens system. It is to be noted that quartz has a very large dispersion within the far ultraviolet region. Therefore, a spectrum bandwidth of an exposure wavelength must be adjusted in the optical lens system so that a half band width becomes equal to or less than 0.003 nm. When the excimer laser is used as the exposure light source to emit an optical beam, the optical beam emitted from a excimer laser has the spectrum bandwidth which is as wide as 0.3 nm. Such a wide spectrum bandwidth should be narrowed in some way so as to send the optical beam to the semiconductor wafer through the optical lens system, as mentioned above, and to expose the semiconductor wafer to the optical beam. Even if the optical beam has a narrow bandwidth, speckles are liable to occur on the semiconductor wafer, which makes it difficult to obtain a line and space of 0.3 micron meters. In addition, a halogen gas is inevitably used as a laser gas in the excimer laser. This necessitates large supplementary equipment for handling the halogen gas and evacuating it. Furthermore, running costs are very high as well as manufacturing costs. SUMMARY OF THE INVENTION It is an object of this invention to provide an exposure light source which is suitable for manufacturing a very large scale integrated memory without large-scale supplementary equipment. It is another object of this invention to provide an exposure light source of the type described, which is capable of producing a line and space necessary for the very large scale integrated memory. It is still another object of this invention to provide an exposure lamp which is capable of obtaining the high resolution necessary for a very large scale integrated memory without any supplementary equipment. It is yet another object of this invention to provide an exposure lamp of the type described which is comparatively inexpensive. According to this invention, a lamp is used in emitting far ultraviolet light to illuminate a substrate. The lamp envelopes a metal element which substantially consists of a single isotope such that the far ultraviolet light is emitted on vaporization of the single isotope.