Photocopying – Projection printing and copying cameras – With temperature or foreign particle control
Reexamination Certificate
2000-11-15
2003-12-23
Font, Frank G. (Department: 2877)
Photocopying
Projection printing and copying cameras
With temperature or foreign particle control
C355S053000, C355S067000, C355S068000, C359S362000, C356S399000, C399S004000, C399S032000, C399S051000
Reexamination Certificate
active
06667796
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus, and more particularly, to an exposure method and an exposure apparatus used when a mask pattern is transferred onto a light-sensitive substrate during a photo lithography process for producing semiconductor devices, image pickup devices (CCDs etc.). liquid crystal display devices, plasma displays, thin film magnetic heads and the like. The present invention is suitably used for the exposure apparatus using ultraviolet light such as an excimer laser or the like as an exposure beam.
2. Description of the Related Art
As an integrated circuit is further reduced in size, in a full field exposure type projection exposure apparatus such as a stepper or a scan and exposure type projection exposure apparatus such as a step-and-scan apparatus, it is required to enhance resolution. One method for enhancing the resolution is to shorten the wavelength of illumination light (exposure light) for exposure. For this reason, the wavelength of exposure used for a projection exposure apparatus is shortened year after year. The currently mainstream exposure light is KrF excimer laser light (wavelength of 248 nm). At the present, ArF excimer laser light (wavelength of 193 nm) of shorter wavelength and F
2
laser light (wavelength of 157 nm) are also under test. Harmonics of metal vapor laser light and YAG laser light and the like are also under test.
As excimer laser light sources used as exposure light sources, there are a narrow-band laser light source in which the full width at half maximum of a spectrum of the laser light is 2 to 3 pm or smaller, and a broadened-band laser light source in which the full width at half maximum of a spectrum of the laser light is 100 pm or more. When illumination light having short wavelength equal to or shorter than ultraviolet region such as excimer laser light is used as exposure light, only quartz (SiO
2
) and fluorite (CaF
2
) are presently known as a glass material for refractor having practical transmittance, and the narrow-band laser light source has an advantage of being easy to carry out achromatization of a projection optical system as compared with the broadened-band laser light source. However, the band of the excimer laser light is originally broadened band, and in order to use the excimer laser as the narrow-band laser light source, it is necessary to carry out injection-locking or the like for narrowing the band of the oscillation spectrum, and the broadened-band laser light source has advantages in terms of laser output, durability (life), and manufacturing cost. Therefore, attempts are recently made for the projection optical system to have a structure in which achromatization is easily carried out, so that the broadened-band laser source can be used.
That is, as a projection optical system used for a projection exposure apparatus, there are a refracting-type projection optical system comprised of a plurality of refractors alone, and a catadioptric projection optical system such as disclosed in Japanese Patent Application Laid-open No. 6-132191 comprising a combination of a catadioptric element such as a concave mirror and refractors. In the former refracting projection optical system, a proportion of lenses made of fluorite is increased to broaden an achromatic width, thereby making it possible to use the broadened-band laser light source. In the latter catadioptric projection optical system, since the concave mirror disposed between the refractors has no chromatic aberration and it is easy to achromatize, and it is possible to use the broadened-band laser light source.
In recent years, a pattern steps tends to move from on the order of conventional 1 &mgr;m to 0.1 &mgr;m or less with CMP technique (Chemical Mechanical Polishing) which flattens a surface of a wafer. A film thickness of a resist can also be made thinner in accordance with this trend. Therefore, when a projection optical system having the numerical aperture of 0.7 or more is used for example, it is possible to obtain a resolution of about 80 to 130 nm. In the case of the catadioptric projection optical system, if a narrow-band ArF excimer laser light having a full width at half maximum of about 0.5 to 1 pm is used for example, it has been found that it is possible to design an optical system having a maximum effective field diameter of a lens of about 300 mm and numerical aperture of 0.7 or more by using several aspheric optical elements.
As described above, laser light in a far ultraviolet region such as an ArF excimer laser is used in a recent projection exposure apparatus so as to enhance the resolution. However, ultraviolet light is originally absorbed by ozone (O
3
) and if the wavelength becomes about 200 nm or shorter, the absorption amount of the light by oxygen (O
2
) is increased. Thereupon, when the laser light of such an ultraviolet region is used, it is preferable that a gas having a low absorptance with respect to ultraviolet light and having no effect on photoresist such as an ozone-free air or a nitrogen (N
2
) gas is circulated on the optical path in the illuminating optical system or projection optical system to suppress the reduction in exposure amount. However, since the nitrogen gas has a high index of refraction, when the nitrogen gas is circulated in the projection optical system, the index of refraction is varied by a slight variation in temperature or pressure and the projection image is wandered, resulting in an inconvenience that the exposure precision (superposing precision or the like) is lowered. Especially in a catadioptric projection optical system designed to have a numerical aperture of 0.7 or greater, even though the number of lenses is as small as 15 to 25, since a distance between a reticle and a wafer is long, space between lenses is great, and a wandering effect on the projection image due to temperature variation is increased.
In order to suppress the wandering of the projection image, it is effective that a gas such as helium (He) having a small index of refraction as small as about ⅛ of that of nitrogen gas and having small variation amount thereof is circulated in the projection optical system. However, the projection optical system is provided with a driving mechanism for driving predetermined lens or the like for excellently keeping the image-forming characteristics, and with an opening for bringing in and out an optical filter or the like which was optimized with respect to the pattern shape. Since inside and outside of the projection optical system are not completely insulated from each other, it was difficult to constantly keep purity of the helium gas in the projection optical system and to maintain excellent image-forming characteristics. Further, it may be possible to keep supplying helium gas so that the pressure of the helium gas becomes positive pressure with respect to the pressure outside the projection optical system, thereby constantly keeping the purity of the helium gas in the projection optical system. However, since the helium gas is expensive under present circumstances, there is an inconvenience that if the flow rate of the helium gas is increased in this manner, working cost is increased.
SUMMARY OF THE INVENTION
In view of the above circumstances, it is a first object of the present invention to provide an exposure method capable of suppressing deterioration in image-forming characteristics due to variation in a temperature or a purity of a gas in a projection optical system, and capable of maintaining excellent image-forming characteristics. It is a second object of the present invention to provide an exposure method capable of preventing the transmittance of an exposure beam in a projection optical system from being lowered, or capable of suppressing the deterioration in image-forming characteristics. Further, it is another object of the present invention to provide an exposure apparatus capable of carrying out the above-described exposure methods.
According to a first aspect
Brown Khaled
Font Frank G.
Nikon Corporation
Oliff & Berridg,e PLC
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