Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – With measuring – controlling – sensing – programming – timing,...
Reexamination Certificate
2000-12-19
2004-01-06
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
With measuring, controlling, sensing, programming, timing,...
C065S413000, C065S421000, C065S422000, C065S424000, C065S426000, C065S104000, C065S117000, C065S900000, C501S905000
Reexamination Certificate
active
06672109
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a silica glass member, a method for using the same, and a projection aligner using the same. In further detail, the present invention relates to a silica glass member for use as a lens of imaging optics for patterning desired mask patterns on substrates by utilizing light sources in the ultraviolet region or vacuum ultraviolet region such as excimer laser radiations, a photomask such as a reticle for transferring circuit patterns of integrated circuits, a diffraction optical element (DOE), an Etalon plate for light sources, or a blank thereof, to a method for producing said silica glass member, and to a projection aligner using the same.
BACKGROUND ART
As a projection aligner for use in producing semiconductor devices, conventionally used are those having the structure shown in FIG.
16
A and FIG.
16
B.
More specifically, referring to a projection aligner
800
shown in
FIG. 16A
, a beam emitted from a light source
501
such as a mercury arc lamp, etc., is converged by an elliptical mirror
502
, and is converted into parallel beams through a collimator lens
503
. The parallel beams then pass through a fly-eye lens
504
consisting of an assembly of optical elements
504
a
each having a square cross section shown in
FIG. 16B
to form a plurality of light source images on the light emitting side. An aperture stop
505
having a round opening portion is provided at the position of the light source images. The light beams emitted from the plurality of the light source images are then converged by a condenser lens
506
, and a reticle R, which is the object to be irradiated, is uniformly illuminated by the superimposed beams.
The pattern on the reticle R, which is uniformly illuminated by the illuminating optics as above, is projection aligned on a wafer W coated with a resist by a projection optical system
507
consisting of a plurality of lenses. The wafer W is mounted on a wafer stage WS provided movable in two dimension; in case of a projection aligner
800
shown in
FIG. 16A
, the alignment is performed in the so-called step-and-repeat system (stepper), in which the wafer stage is sequentially moved two-dimensionally for the exposure of the next shot region after the exposure is completed for a one-shot area on the wafer.
Recently, furthermore, there is proposed a scanning type aligner system capable of transferring the pattern of a reticle R onto a wafer W at high throughput, in which a square or an arc beam is irradiated to the reticle R while scanning, in a predetermined direction, the reticle R and the wafer W that are provided in a conjugate arrangement with respect to the projection optics
507
.
In order to transfer finer mask pattern images on the wafer plane, i.e., to further increase the resolution, it is further proposed recently to shorten the wavelength of the light source. For instance, radiations with shorter wavelength such as KrF (248 nm) and ArF (193 nm) excimer lasers are being used instead of the conventionally used g-line (436 nm) or i-line (365 nm).
As an optical member for use in the optical system inside the projection aligners above, it is preferred that the member yields a high transmittance for the radiation having the wavelength corresponding to that of the light source being used in the system. This is because, since the optical system of the projection aligner is constructed of a plurality of optical members, even if the optical loss per 1 lens should be small, the total accumulation of the loss for the total number of the optical members leads to a large drop in transmittance. If an optical member having a poor transmittance is used, the exposure light is absorbed by the optical member as to increase the temperature thereof, and this results in a heterogeneous distribution in refractive index inside the optical member; furthermore, the local thermal expansion of the optical member leads to a deformation in the polished plane. Thus, a deterioration in optical performance is induced. In the case of a projection optical system, in particular, it is required that the optical member yields a highly uniform refractive index to obtain fine and clear projection exposure patterns, because a fluctuation in refractive indices leads to a retardation that greatly affects the imaging performance of the projection optical system.
In general, in case of a projection aligner using a light having a longer wavelength than an i-line, a optical glass made from multi-component optical glass is being used for the lens member of the illumination optical system or the projection optical system. However, in case a radiation having a wavelength shorter than the i-line is used for such optical glasses, the internal transmittance for such a radiation abruptly decreases, and in case of radiations having a wavelength not longer than 250 nm, in particular, such optical glasses can no longer exhibit transmittance for the radiations. Thus, as a material for use in an optical member for use in an optical system of a projection aligner equipped with a light source emitting radiations in the ultraviolet region not longer than 400 nm in wavelength, generally employed is a highly uniform silica glass or a single crystal of calcium fluoride that yield high transmittance for radiations in the ultraviolet region. These two materials are required in case of correcting color aberration in the imaging optics of excimer lasers.
Of the optical materials above, a silica glass not only yields a high optical transmittance, but also has superior properties, for instance, it is characterized in that it has an excellent resistance against excimer lasers; that it yields stability to change in temperature; that it has excellent corrosion resistance and elastic properties; and that it has small linear expansion coefficient (about 5.5×10
−7
/K) at temperatures in the vicinity or room temperature. Thus, attempts are being made to apply the silica glass as a material constituting the optical members such as reticles, in which some optical properties are required when used in projection aligners, such that it has an excellent UV durability, and that it generates less heat and thereby low thermal expansion occurs.
The application of a single crystal of calcium fluoride to such optical members is being studied, because it has high optical transmittance and an excellent resistance against ultraviolet radiations when used as a material of an optical member, particularly, for radiations having a specified wavelength of 190 nm or shorter.
DISCLOSURE OF INVENTION
However, even in an optical member such as a lens, a photomask substrate, etc., made of silica glass, it was found to yield insufficient optical transmittance or resistance against ultraviolet radiations for light having a specified wavelength of 250 nm or shorter. As described above, this is ascribed to the fact that the optical transmittance of the optical system which consists of a plurality of lenses (a group of lenses) as a whole is the accumulation of each of the optical transmittances of the lenses, and that there are problems such as an increase in transmitting loss of the silica glass attributed to the internal absorption and the internal scattering of the light, a generation of color centers due to laser induction, a decrease in optical performance due to heat generation and phosphorescence, a change in density due to compaction, etc. This tendency becomes particularly distinct in case the optical system is used with a light having a wavelength of 190 nm or shorter.
More specifically, when the optical members were used in a projection aligner using a light source such as an ArF excimer laser (having a wavelength of 193 nm), a F
2
laser (having a wavelength of 157.6 nm), etc., there was found to induce a problem of line width deviation and the like in the pattern transfer process, thereby making it extremely difficult to achieve a high resolution.
Furthermore, a single crystal of calcium fluoride was disadvantageous in that it suffered breakage during the process of forming
Colaianni Michael
Nikon Corporation
Oliff & Berridg,e PLC
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