A manufacturing process for a semiconductor element such as an LSI or VLSI formed from a micropattern uses a reduction type projection exposure apparatus for transferring by reduction projection a circuit pattern drawn on a mask onto a substrate coated with a photosensitive agent. With an increase in the packaging density of semiconductor elements, demands have arisen for further micropatterning. Exposure apparatuses are coping with micropatterning along with the development of a resist process.
A means for increasing the resolving power of the exposure apparatus includes a method of changing the exposure wavelength to a shorter one, and a method of increasing the numerical aperture (NA) of the projection optical system.
As for the exposure wavelength, the 365-nm i-line has been replaced by a KrF excimer laser with an oscillation wavelength of around 248 nm. Also, an ArF excimer laser with an oscillation wavelength around 193 nm and a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm have been developed.
An ArF excimer laser with a wavelength around far ultraviolet rays, particularly, 193 nm, and a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm are known to have a plurality of oxygen (O2) absorption bands around their wavelength bands.
For example, a fluorine excimer laser has been applied to an exposure apparatus because of a short wavelength of 157 nm. The 157-nm wavelength falls within a wavelength region generally called a vacuum ultraviolet region. In this wavelength region, light is greatly absorbed by oxygen molecules, and hardly passes through air. Thus, the fluorine excimer laser can only be applied in an environment in which the atmospheric pressure is decreased to almost vacuum and the oxygen concentration is fully decreased.
According to a reference “Photochemistry of Small Molecules” (Hideo Okabe, A Wiley-Interscience Publication, 1978, p. 178), the absorption coefficient of oxygen to 157-nm light is about 190 atm−1cm−1. This means that, when 157-nm light passes through gas at an oxygen concentration of 1% at one atmospheric pressure, the transmittance per cm is onlyT=exp(−190×1 cm×0.01 atm)=0.150
Oxygen absorbs light to generate ozone (O3), and ozone promotes absorption of light, greatly decreasing the transmittance. In addition, various products generated by ozone are deposited on the surface of an optical element, decreasing the efficiency of the optical system.
To prevent this, the oxygen concentration in the optical path is suppressed to a low level of several ppm order, or less, by a purge mechanism using inert gas, such as nitrogen in the optical path of the exposure optical system of a projection exposure apparatus using a far ultraviolet laser, such as an ArF excimer laser or a fluorine (F2) excimer laser, as a light source.
In such an exposure apparatus using an ArF excimer laser beam with a wavelength around far ultraviolet rays, particularly, 193 nm, or a fluorine (F2) excimer laser beam with a wavelength around 157 nm, an ArF excimer laser beam or fluorine (F2) excimer laser beam is readily absorbed by a substance. The optical path must be purged to several ppm order or less. This also applies to moisture, which must be removed to the ppm order or less.
To ensure the transmittance or stability of ultraviolet rays, the ultraviolet path of the reticle stage or the like of an exposure apparatus or the like is purged with inert gas. For example, Japanese Patent Laid-Open No. 6-260385 discloses a method of spraying inert gas toward a photosensitive substrate. However, oxygen and moisture cannot be satisfactorily purged. Japanese Patent Laid-Open No. 8-279458 discloses a method of covering the whole space near a photosensitive substrate with a sealing member from the lower end of a projection optical system. However, this method is not practical because it is difficult to move the stage.
As described above, an exposure apparatus using ultraviolet rays, particularly, an ArF excimer laser beam or fluorine (F2) excimer laser beam suffers from large absorption by oxygen and moisture at the wavelength of the ArF excimer laser beam or fluorine (F2) excimer laser beam. To obtain a sufficient transmittance and stability of ultraviolet rays, the oxygen and moisture concentrations must be reduced.
From this, it is desired to develop an effective means for purging the ultraviolet path in an exposure apparatus, particularly, the vicinities of a wafer and reticle with inert gas.
However, vibration sources such as the motors and air compressors of various units installed on the floor and units to be isolated from vibrations, such as a projection optical system, an alignment system, a laser interferometer, and a stage, supported by a vibration isolating mechanism, coexist in an exposure apparatus. For this reason, if a space between the vibration sources and the units is purged with inert gas, vibrations are transmitted through a connecting member arranged between them to sustain airtightness. Consequently, the units supported by the vibrations isolating mechanism vibrate, thereby causing a reduction in exposure precision, and the like.
Assume that a bellows structural member is employed as the connecting member to sustain airtightness. In this case, the bellows structure member has flexibility in the compression direction and high rigidity in the shear and twist directions, but vibrations cannot be satisfactorily isolated.