Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2000-06-14
2003-02-11
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C423S336000, C359S355000, C359S642000
Reexamination Certificate
active
06518210
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silica glass for photolithography, optical members including the glass, an exposure apparatus including the same, and a method for producing the glass. More particularly, it relates to a silica glass used in photolithography techniques together with light in a wavelength region of 400 nm or shorter or, more preferably, 300 nm or shorter, optical members such as lens and mirror including the glass, an exposure apparatus including the glass, and a method for producing the glass.
2. Related Background Art
In recent years, VLSI has been produced with a higher integration and a higher functionality. Particularly, in the field of logical VLSI, a larger system has been mounted on a chip, namely, system-on- chip technique has been in progress. In conjunction with such a trend, there is a demand for finer processability and higher integration on a wafer, such as that made of silicon, which constitutes a substrate for VLSI. In photolithography techniques according to which fine patterns of integrated circuits are exposed to light and transferred onto wafers such as of silicon, exposure apparatuses called stepper are used.
In the case of DRAM, as an example of VLSI, with the advance from LSI to VLSI, as its capacity gradually increases from 1 KB through 256 KB, 1 MB, 4 MB, and 16 MB to 64 MB, the processing line width required for the stepper correspondingly becomes finer from 10 &mgr;m through 2 &mgr;m, 1 &mgr;m, 0.8 &mgr;m, and 0.5 &mgr;m to 0.3 &mgr;m.
Accordingly, it is necessary for a projection lens of the stepper to have a high resolution and a great depth of focus. The resolution and the depth of focus are determined by the wavelength of the light used for exposure and the N.A. (numerical aperture) of the lens.
The angle of the diffracted light becomes greater as the pattern is finer, whereas the diffracted light cannot be captured when the N.A. of the lens becomes greater. Also, the angle of the diffracted light becomes smaller in the same pattern as its exposure wavelength A is shorter, thereby allowing the N.A. to remain small.
The resolution and the depth of focus are expressed as indicated by the following equations:
resolution=
k
1
·&lgr;/N.A.
depth of focus=
k
2
·&lgr;/N.A.
2
wherein k1 and k2 are constants of proportionality.
In order to improve the resolution, either the N.A. is increased or &lgr; is shortened. However, as can be seen from the above equations, it is advantageous, in terms of the depth of focus, to shorten &lgr;. In view of these points of view, wavelength of light sources becomes shorter from g-line (436 nm) to i-line (365 nm) and further to KrF excimer laser beam (248 nm) and ArF excimer laser beam (193 nm).
Also, since the optical system loaded in the stepper is constituted by a combination of numerous optical members such as lenses, even when each lens sheet has a small transmission loss, such a loss is multiplied by the number of the lens sheets used, thereby decreasing the amount of light at the irradiated surface. Accordingly, it is necessary for the optical member to have a high degree of transmittance.
Therefore, in the steppers using light in a wavelength region of 400 nm or shorter, optical glass made by a specific method in view of the shortening of wavelength as well as the transmission loss due to the combination of the optical members is used. Also, in the steppers using light in a wavelength region of 300 nm or shorter, it has been proposed to use synthetic silica glass and a fluoride single crystal such as CaF, (fluorite).
As a specific method for measuring internal transmittance, for example, a method of measuring transmittance of optical glass is known from JOGIS 17-1982. Here, the internal transmittance is calculated by the following equation:
log
τ
=
-
log
T1
-
log
T2
Δd
×
10
(
1
)
wherein &tgr; is internal transmittance of the glass when its thickness is 10 mm; d is difference in thickness of a sample; and T1 and T2 are spectral transmission factors of the glass having sample thickness values of 3 mm and 10 mm, respectively, including their reflection loss.
SUMMARY OF THE INVENTION
However, the inventors have found that, in the optical members composed of the conventional silica glass whose internal transmittance is defined in this manner, although a certain magnitude of the resolution is secured in terms of their specification, contrast of an image resulting therefrom may be so unfavorable that a sufficiently vivid image cannot be obtained.
Here, the contrast is defined by the following equation:
contrast
=
I
max
-
I
min
I
max
+
I
min
(
2
)
wherein Imax is maximum value of optical intensity on a wafer surface and Imin is minimum value of the optical intensity on the wafer surface.
The object of the invention is to provide a silica glass for photolithography which can overcome the foregoing shortcomings of the prior art and can realize a sufficiently fine and vivid exposure and transfer pattern with a favorable contrast.
Accordingly, the inventors have studied, among the transmission loss factors in the silica glass (optical member) used for photolithography techniques and the like, factors for decreasing the contrast of image. As a result, it has been found that not only the optical absorption at the silica glass but also its optical scattering causes the transmission loss and that the amount of loss in light based on such optical scattering (scattering loss amount) can be sufficiently suppressed when the structure determination temperature in the silica glass containing at least a predetermined amount of OH group is reduced at least to a predetermined level. Thus, the present invention has been accomplished.
The silica glass (fused silica, quartz glass) of the present invention is used for photolithography together with light in a wavelength region of 400 nm or shorter and is characterized in that it has a structure determination temperature of 1,200 K or lower and an OH group concentration of at least 1,000 ppm.
Further, the optical member (optical component) of the present invention is an optical member used for photolithography together with light in a wavelength region of 400 nm or shorter and is characterized in that it includes the above-mentioned silica glass of the present invention.
Furthermore, the exposure apparatus (exposing device) of the present invention is an exposure apparatus which uses light in a wavelength region of 400 nm or shorter as exposure light and is characterized in that it is provided with the optical member including the above-mentioned silica glass of the present invention.
Moreover, the method for producing the silica glass in accordance with the present invention is characterized in that it comprises the steps of heating a silica glass ingot having an OH group concentration of at least 1,000 ppm to a temperature of 1,200 to 1,350 K, maintaining the ingot at that temperature for a predetermined period of time, and then cooling the ingot to a temperature of 1,000 K or lower at a temperature-lowering rate (cooling rate) of 50 K/hr or less to anneal the ingot, whereby making it possible to produce a silica glass having a structure determination temperature of 1,200 K or lower and an OH group concentration of at least 1,000 ppm.
The “structure determination temperature” herein used is a factor introduced as a parameter which expresses structural stability of silica glass and will be explained in detail below. The fluctuation in density of silica glass at room temperature, namely, structural stability is determined by density of the silica glass in the state of melt at high temperatures and density and structure of the silica glass when the density and the structure are frozen at around the glass transition point in the process of cooling. That is, thermodynamic density and structure corresponding to the temperature at which the density and structure are
Hiraiwa Hiroyuki
Jinbo Hiroki
Komine Norio
Group Karl
Nikon Corporation
Pennie & Edmonds LLP
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