Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-05-10
2003-11-25
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06653024
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure apparatus used when producing for example a semiconductor device, an imaging device, a liquid crystal display, a thin film magnetic head, or another microdevice and to a photomask and aberration correction plate used for the exposure apparatus and a process of production of such a microdevice.
2. Description of the Related Art
In the photolithographic process for producing a semiconductor device etc., use is made of an exposure apparatus to transfer a pattern image of a photomask (including reticle) on to a photosensitive substrate through a projection optical system. The photomask used for such an exposure apparatus is generally produced using silica glass (quartz glass). Silica glass has a low transmission loss of light, has resistant to temperature changes, is excellent in corrosion resistance and elastic performance, has a small coefficient of linear expansion (about 5.5×10
−7
/K), and has other superior properties. It therefore has the advantages of enabling formation of a pattern with a high precision and a good work efficiency. Semiconductor integrated circuits of increasing miniaturization are being developed. In the photolithographic process, the wavelengths of the light sources used are becoming increasingly shorter. Vacuum ultraviolet light, in particular, light of wavelengths shorter than 200 nm, for example, light of ArF excimer lasers (wavelength 193 nm) or F
2
lasers (wavelength 157 nm), etc. has now come into use as exposure light.
If ordinary silica glass is used for light of a wavelength less than about 200 nm, however, the transmission loss increases due to absorption and scattering. Further, the optical performance declines due to heat buildup resulting from absorption and to the fluorescence. The glass itself discolors along with time, that is, “color center” occurs, and changes in density, that is, “compaction”, occurs. These become more remarkable the shorter the wavelength. Therefore, when using ordinary silica glass, it has been thought that use for light of a wavelength about that of an ArF excimer laser (wavelength 193 nm) was the limit. It was generally considered difficult to use a photomask made of ordinary silica glass for light of shorter wavelengths. Therefore, production of a photomask using fluorite (CaF
2
), a material with a high transmittance even with respect to light of a wavelength less than about 200 nm, has been studied. Note that the use of fluorite is considered particularly effective when used for light of a wavelength less than 190 nm, but use of fluorite is also preferable in the case of the above ArF excimer laser from the viewpoint of the transmittance.
The Mohs hardness of fluorite, however, is 4. This is softer than the Mohs hardness 7 of quartz, the material used for photomask substrates in the past. Use as a photomask is problematic in that the photomask is susceptible to damage during transport and the dust generated from the damaged locations is liable to cause unwanted foreign matter to deposit on the pattern and prevent exposure of the desired pattern. Therefore, when producing a photomask using fluorite, measures have to be taken against this. Further, fluorite has a low resistance to temperature changes, is fragile and easily scratched, is damaged during the process of pattern formation, and has a higher coefficient of linear expansion compared with quartz, so special control and countermeasures are required for forming a mask pattern with a high precision. Further, when used for an exposure apparatus for actual exposure work, the temperature has to be controlled extremely strictly. Sometimes obstacles arise in transferring a pattern on a photosensitive substrate with a higher precision.
The above problems relate to the photomask per se. The exposure apparatus is sometimes equipped with a fixed or detachable transparent aberration correction plate in the path of the imaging light between the photomask and photosensitive substrate (for example, between the photomask and projection optical system) so as to correct distortion resulting from distortion of the projection optical system or other aberration. Similar problems arise as with the above photomask for this aberration correction plate as well. Further, the problems relating to the photomask and aberration correction plate hinder shortening of the wavelength of the exposure apparatus, make dealing with the increasing miniaturization of patterns to be formed on photosensitive substrates difficult, and make realization of stable exposure performance over time and provision of an exposure apparatus with a long service life hard.
SUMMARY OF THE INVENTION
Accordingly, a first object of the present invention is to provide a photomask produced using a material, such as fluorite, having a low hardness to enable the exposure wavelength to be shortened further and a finer pattern to be transferred, yet which is not damaged when transporting the photomask or at the time of scan exposure.
A second object of the present invention is to provide a photomask, used for an exposure apparatus using light of a particularly short wavelength (wavelength of not more than about 200 nm) for exposure, which is high in light transmittance, excellent in UV resistance, good in processability, and enables realization of a high precision.
A third object of the present invention is to provide an aberration correction plate, used for an exposure apparatus using light of a particularly short wavelength (wavelength of not more than about 200 nm) for exposure, which is high in light transmittance, excellent in UV resistance, good in processability, and enables realization of a high precision.
A fourth object of the present invention is to provide an exposure apparatus able to handle the increasing miniaturization of patterns formed on photosensitive substrates, feature low deterioration of the exposure performance along with time, and having a long service life.
A fifth object of the present invention is to provide a process for production of a microdevice of a good quality.
1. To achieve the first object, according to the present invention, there is provided a photomask including a substrate, a transfer pattern formed on the substrate, and a protective film formed on the substrate for protecting the substrate. The protective film may be formed, in a region of the outer surface of the substrate other than the region at which the transfer pattern is formed, at a contact surface with a member for holding the photomask and at least at a part of the area near the contact surface.
According to the photomask of the present invention, since a protective film is formed for protecting the substrate of the photomask, even if the photomask is produced using a material, such as fluorite, with a low hardness, the photomask will not be damaged during transport of the photomask or during scan exposure. The photomask is preferably irradiated with illumination light of a wavelength not more than 190 nm. In this case, it is possible to transfer an extremely finer pattern using extreme ultraviolet illumination light of a wavelength not more than 190 nm. The substrate of the photomask is preferably formed by calcium fluoride (CaF
2
). Note that fluorite is a typical example of this. In this case, it is possible to use extreme ultraviolet exposure light such as light of an F
2
excimer laser (wavelength 157 nm). The protective film is preferably comprised of chrome (Cr), chromium oxide (CrO), or silicon oxide (SiO
2
or SiO). In this case, it is possible to keep down the cost for forming the protective film for preventing damage of the photomask.
The protective film may be formed by the same material as the pattern. Further, the protective film may be provided at the surface of the substrate on which the pattern is formed or on an end face of the substrate.
2. To achieve the second object, according to the present invention, there is provided a photomask, used for an exposure apparatus having a light source emi
Owa Soichi
Shiraishi Naomasa
Takeuchi Hitoshi
Mohamedulla Saleha R.
Nikon Corporation
Young Christopher G.
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