Optical: systems and elements – Having significant infrared or ultraviolet property – Lens – lens system or component
Reexamination Certificate
1998-11-25
2002-11-19
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Having significant infrared or ultraviolet property
Lens, lens system or component
C359S350000, C359S361000, C250S492200
Reexamination Certificate
active
06483639
ABSTRACT:
FIELD OF INDUSTRIAL APPLICATION
The present invention relates to an optical system for fabrication of an integrated circuit of a capacity of 64 M bits to 254 M bits. More particularly, the present invention relates to an optical system for fabrication of an integrated circuit in which an integrated circuit pattern image is projected on a wafer using ultraviolet laser light of a short wavelength having a narrowed spectral band width with the help of an optical system made of quartz glass material.
PRIOR ART
Heretofore, an optical photolithographic technique, in which a pattern formed on a mask is transferred onto a wafer by illumination, has widely been used as an optical system for fabrication of an integrated circuit, owing to its advantage in cost compared with other techniques, such as those using an electron beam, an X-ray or the like.
While a projection aligner has been developed in a conventional photolithographic technique, said projection aligner using an i-line light of a wavelength of 365 nm emitted from a high pressure mercury lamp for depicting a patterned image with a line width of 0.5 to 0.4 &mgr;m, such a projection aligner corresponds to an integrated circuit (IC) of a capacity of 16 M bits to or less.
An IC of 64 M bits to 256 M bits of the first generation requires a resolution of 0.25 to 0.35 &mgr;m and an IC of 1 G bits requires a resolution of 0.13 to 0.20 &mgr;m. A resolution of 0.35 &mgr;m is not achieved by a wavelength of an i-line light and therefore, KrF light is used as a light source. In a region of a resolution better than 0.2 &mgr;m, ArF light is used instead of KrF light, especially an ArF excimer laser.
There are various problems in an photolithographic technique using an ArF excimer laser and one of them is a problem associated with an optical material constituting lenses, mirrors and prisms, which is used to construct an optical system for projection.
While an optical material which shows a good transmittance in a wavelength of 193 nm emitted from an ArF laser is substantially limited to quartz glass, especially high purity synthetic quartz glass, ArF light adversely affects quartz glass, damaging it ten times or more than KrF light does.
An improved resistance to excimer laser irradiation of quartz glass is dependent on a hydrogen concentration contained therein, as described in EP-A 401 845. In the publication, it is disclosed that quartz glass may be used to construct an optical system of a projection aligner using a KrF excimer laser as a light source which was able to secure enough resistance to the laser light with a hydrogen concentration of 5×10
16
molecules/cm
3
or more contained therein.
However, since influence exerted on quartz glass by ArF laser light is extremely greater, compared with KrF laser light, as has been described above. It was found from an investigation into the degree of the damage, such as changes in transmittance and refractive index thereof produced in the bulk of synthetic quartz glass required hydrogen concentration for preventing occurrence of the damage of 100 to 1000 times or more as large as the hydrogen molecule concentration required in the case of a KrF excimer laser light, specifically 5×10
18
molecules/cm
3
or more.
There are two methods to make synthetic quartz glass incorporating hydrogen molecules. In one of the methods, wherein hydrogen molecules are incorporated into the synthetic quartz glass in atmospheric pressure, adjusting the environment in production, the maximum concentration of hydrogen molecule to be incorporated, is on the order of 5×10
18
molecules/cm
3
. In the other method, wherein hydrogen molecules are incorporated into the quartz glass by a heat treatment in a pressured hydrogen atmosphere, the upper limit of an incorporated hydrogen molecule concentration is the same 5×10
18
moleculeslcm
3
under a pressure of 10 atm as well, when the pressure is its maximum under Japanese Law of High Pressure Gas Control.
In such circumstances, if a hydrogen molecule concentration of 5×10
18
molecules/cm
3
or more is desired, it is required that quartz glass is subjected to a heat treatment at a temperature of 1000° C. or higher and in a high-pressure hydrogen atmosphere of 100 atm or higher as described in EP-A 483 752.
However, since a heat treatment at 1000° C. or higher under a pressure of 100 atm or higher provokes new defects in the bulk of quartz glass, a temperature of the heat treatment preferably is in the range of 200 to 800° C. (see Publication of Unexamined Japanese Patent Application No. Hei 6-166528). In the case where a great number of hydrogen molecules, which is 5×10
18
molecules/cm
3
, are incorporated into quartz glass by a heat treatment at a temperature in this range in a hydrogen atmosphere, there arises a fault that it takes very long to incorporate hydrogen molecules in the quartz glass to such a concentration, since a diffusion coefficient of a hydrogen molecule is not so large and in addition, to conduct a heat treatment in a high pressure atmosphere results in not only decrease of homogeneity in refractive index in the bulk of the quartz glass but also generation of strains in the bulk.
Therefore, if quartz glass is subjected to a high-pressure, high-temperature treatment, as a result another heat treatment becomes necessary for readjustment. These treatments make the process to produce quartz glass which is suitable for constituting an optical system of a projection aligner industrially very complex and time-consuming, which in turn makes the quartz glass having a hydrogen concentration of 5×10
18
molecules/cm
3
or more, a homogeneity in refractive index, a low level of strains and the like, highly expensive.
However even if the quartz glass having hydrogen concentration of 5×10
18
molecules/cm
3
or more, optical properties, such as a homogeneity in refractive index, a low level of strains and the like, is obtained, a volume constriction causing a change in refractive index of the quartz glass may arise during irradiation by ArF light.
A technique to combine synthetic quartz and fluorite in an optical system used for integrated circuit fabrication has been disclosed in Publication of Unexamined Japanese Patent Application No. Hei 8-78319 (hereinafter referred to as first prior art technique), but the technical concept is essentially different from that of the present invention.
The first prior art technique is to constitute the optical system for exposure with a diffraction optical element having a positive power, a quartz lens having a negative power and a fluorite lens having a positive power, which constitution is to correct a chromatic aberration. In the first prior technique, the chromatic aberration is corrected by a combination of a diffraction optical element and a refracting lens respectively having positive and negative powers, and a fluorite having a positive power, wherein a combination of optical elements, different from one another in optical dispersion corrects chromatic aberration. Such a combination specially realizes an optical system having an image formation property that a secondary spectrum of chromatic aberration is minor and thereby not only can a larger curvature radius of a lens, a larger NA, a larger field of view be realized to give room to improvement on a specification of the optical system but a tolerance of eccentricity in fabrication is also larger to promote easy fabrication.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical system for use with ultraviolet light of a short wavelength with a narrowed spectral band width, especially ArF laser light, as illumination in the projection aligner without degrading properties such as durability, optical transmittance and the like with a low cost and ease as a whole.
In the present invention the optical system is constructed of a combination of synthetic quartz glass and fluorite.
Therefore, since the first prior art technique is one in which a quartz glass lens having a negative power and a flu
Fujinoki Akira
Nishimura Hiroyuki
Heraeus Quarzglas GmbH
Milde & Hoffberg LLP
Nguyen Thong
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