Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2001-11-27
2003-12-02
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C065S017400, C065S017600, C065S030100, C355S053000
Reexamination Certificate
active
06656860
ABSTRACT:
RELATED APPLICATIONS
This is a Continuation-In-Part application of International Patent Application serial No. PCT/JP01/02567 filed on Mar. 28, 2001, now pending.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a synthetic silica glass member, to a photolithography apparatus and to a process for producing the photolithography apparatus. More specifically, it relates to a synthetic silica glass member for a photolithography apparatus used in an optical system with lenses, mirrors and the like for photolithography technology, in a specific wavelength range of no greater than 400 nm and preferably no greater than 300 nm, to a photolithography apparatus constructed using such optical members and to a process for producing the photolithography apparatus.
2. Related Background Art
Photolithography apparatuses known as “steppers” (projection exposure apparatuses) are conventionally used for photolithography techniques which accomplish exposure and transfer of intricate patterns of integrated circuits onto wafers of silicon or the like. The optical system of a stepper is constructed with an illumination optical system which uniformly irradiates light from a light source onto a reticle and a projection optical system which projects and transfers the integrated circuit pattern formed on the reticle onto a wafer at a ⅕ reduction, for example.
Recent higher integration of LSIs has required even shorter wavelengths for photolithography apparatus light sources, from g rays (436 nm) to i rays (365 nm) or even KrF (248.3 nm) or ArF (193.4 nm) excimer lasers. There has also been a corresponding demand for photolithography apparatuses allowing exposure with even finer minimum workability beam width. However, when the light source emits light with a wavelength range in the ultraviolet region, and particularly in the region below 250 nm, the light transmittance is too poor for practical use when employing lens materials used in illumination optical systems and projection optical systems designed for light with a longer wavelength region than i rays. Lens materials used in illumination optical systems and projection optical systems are therefore limited to silica glass and some crystal materials that exhibit high light transmittance.
When a silica glass member is used in the optical system of a photolithography apparatus, extremely high quality is required for exposure of the integrated circuit pattern with a large area at high resolution. For example, for a member with a relatively large diameter of about 200 mm, the refractive index distribution of the member must be on the order of no greater than 10
−6
. It is also necessary to reduce the birefringence, i.e. minimize internal strain of the member, and this improves the uniformity of the refractive index distribution and is also important for an the resolution of the optical system. Consequently, there have been strict limitations on silica glass members made of silica glass alone that can be used for photolithography apparatuses employing ultraviolet light from an excimer laser stepper as the light source.
In addition, silica glass members used for photolithography apparatuses using ultraviolet light as the light source must have high transmittance (small loss factor) in addition to the aforementioned conditions. This is because a very large number of lenses are provided in the illumination optical system or projection optical system of a photolithography apparatus to compensate for aberration, and the optical loss of each individual lens can lead to reduced transmittance of the apparatus as a whole.
Silica glass includes fused silica glass obtained by melting natural crystal powder and synthetic silica glass obtained by chemical synthesis, but synthetic silica glass is characterized by being of high purity with low metal impurities, and having high transmittance for ultraviolet light of a wavelength of 250 nm or smaller. Homogeneous synthetic silica glass with a large aperture can also be produced due to the nature of the production process.
SUMMARY OF THE INVENTION
Even with synthetic silica glass, however, the action of a high power ultraviolet beam or excimer laser beam produces a 260 nm absorption band due to structural defects known as NBOHCs (Non-Bridging Oxygen Hole Centers, having the structure ≡Si—O.) or a 215 nm absorption band due to structural defects known as E′ centers (having the structure ≡Si., where ≡ is not a triple bond but represents bonding to 3 oxygen atoms, and.represents an unpaired electron), and transmittance in the ultraviolet region is significantly reduced as a result. Because the center wavelengths of these absorption bands are close to the oscillation wavelength of the KrF excimer laser or ArF excimer laser used as the light source of the stepper, the transmittance of the optical system is considerably reduced by light absorption. A large 215 nm absorption band is also sometimes produced even with low total ultraviolet beam irradiation (=low irradiation energy or a short irradiation time). Since the absorption band is usually produced at the initial stage of irradiation of an excimer laser, the absorption is referred to as initial irradiation absorption. Thus, not all synthetic silica glass can necessarily be used for lens members of such apparatuses, and therefore synthetic silica glass with low initial irradiation absorption is necessary in order to satisfactorily guarantee practical performance for the apparatus.
To date it has been independently confirmed that the desired properties for an optical member used in a photolithography apparatus employing an ArF excimer laser or the like with a wavelength of 200 nm or less as the light source are a low loss factor before irradiation with an ArF excimer laser beam (wavelength: 193.4 nm) and low transmittance reduction even with prolonged irradiation with an ArF excimer laser (i.e. low prolonged transmittance variation); however, it has been difficult to obtain silica glass members satisfying both of these properties, and therefore no photolithography apparatus has existed with an ArF excimer laser employing such a member as the light source.
It is an object of the present invention, which has been accomplished in light of these problems, to provide a synthetic silica glass member which can increase the transmittance of a photolithography apparatus employing an ArF excimer laser as the light source and thereby exhibit adequate properties for practical use, as well as a high-resolution photolithography apparatus employing it and a process for its production.
The present inventors investigated the relationship between the properties of silica glass and the degree of the initial irradiation absorption in order to elucidate the cause of the initial irradiation absorption of silica glass for ultraviolet light. As a result, it was found that when silica glass is synthesized in a reducing atmosphere for doping of hydrogen in order to achieve enhanced durability, providing an atmosphere that is more strongly reducing than necessary results in silica glass containing a high degree of the structural defect ≡Si—H (which is easily cleaved by ultraviolet irradiation with low energy density to become an E′ center) which is a cause of lower transmittance of the silica glass member, or in other words, that silica glass with a greater hydrogen molecule concentration tends to exhibit high initial irradiation absorption. It was found at the same time that silica glass containing virtually no hydrogen molecules exhibits reduced transmittance with prolonged irradiation with an ArF excimer laser.
Thus, the synthetic silica glass of the invention is a synthetic silica glass member used in a photolithography apparatus employing light in a wavelength range of 400 nm or less, wherein hydrogen molecules are present at a certain level and the ≡Si—H concentration is lowered during synthesis by appropriately adjusting the hydrogen molecule concentration of the silica glass during synthesis in a
Jinbo Hiroki
Komine Norio
Yoshida Akiko
Bolden Elizabeth A
Nikon Corporation
Oliff & Berridg,e PLC
Sample David
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