Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2002-05-14
2003-03-25
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000, C355S068000, C355S069000, C355S071000, C250S492200, C430S030000
Reexamination Certificate
active
06538722
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a projection exposure method for exposing a pattern as an original print, such as a mask or reticle (hereinafter referred to collectively as a “mask”), to a photosensitive substrate, such as a wafer, in a photolithography process for manufacturing semiconductor elements, such as LSIs, image pickup elements, CCDs, liquid crystal display elements or thin film magnetic heads, to an projection exposure apparatus for use in the projection exposure method, and to methods for manufacturing and optically cleaning the exposure apparatus.
As a degree of integration of semiconductor elements becomes higher, rapid progress has also been made of projection exposure apparatuses for use in a photolithography process that is important for the manufacture of such semiconductors. The resolving power of a projection optical system to be loaded on a projection exposure apparatus may be represented, as it is well known as Rayleigh's formula, by the relationship as represented by R=k×&lgr;×NA, wherein R is the resolving power of a projection optical system, &lgr; is the wavelength of a light for use in exposure, NA is an aperture number of the projection optical system, and k is a constant to be determined by the process, in addition to the resolving power of a photoresist.
In order to realize the resolving power required for the projection optical system in correspondence with a high degree of integration of semiconductor elements, efforts have been continued to shorten the exposure light source or to enlarge the number of apertures of the projection optical system, i.e. to make the NA higher. Recent years, an exposure apparatus using krypton fluoride excimer laser beam (KrF excimer laser beam) with an output wavelength of 248 nm as an exposure light beam source and having the aperture number of 0.6 or larger of the projection optical system can be commercially available so that the exposure to a pattern as fine as 0.25 &mgr;m can be effected.
Further, an argon fluoride excimer laser (ArF excimer laser) with an output wavelength of 193 nm has recently been put to practical use after the krypton fluoride excimer laser. If an exposure apparatus could be realized which uses the argon fluoride excimer laser beam as an exposure light beam source, it can be expected that the processing as fine as 0.8 &mgr;m to 0.13 &mgr;m can be effected. Great efforts have currently been continued to make research to develop such an exposure apparatus that can use the such laser beam.
The material for use as a lens is currently restricted to only two materials, i.e., synthetic quartz glass and calcium fluoride (fluorite) glass, from the point of view of transmittance or the like in the wavelength region of the output wavelength (193 nm) of the argon fluoride excimer laser. Therefore, extensive efforts have been continuously made to develop optical material for use with an exposure apparatus of this kind, which has a sufficient level of transmittance and an inner material uniformity. Synthetic quartz glass having an inner transmittance of 0.995/cm or higher and calcium fluoride having a negligible level of inner absorption have currently been developed.
Material to be coated on the surface of an optical material for use in preventing reflection is very narrow in a selection scope and undergoes great restrictions upon the freedom of design, as compared with material in a wavelength region of the output wavelength (248 nm) of krypton fluoride excimer laser. However, the problems are being overcome by extensive development efforts, and the loss at each lens plane is being realized to a level below 0.005 or less.
In a wavelength region shorter than the wavelength of the KrF excimer laser beam, the problems may be caused to arise such that the transmittance or reflectance of an optical system is caused to be reduced (fluctuate) due to the phenomenon that if water or organic material would be attached to the surfaces of optical elements constituting the optical system (illumination optical system or projection optical system) in a projection exposure apparatus. In other words, the problems with a variation or fluctuation in an attenuation factor of the optical system may be caused to arise due to a variation or fluctuation in the transmittance or reflectance. The such problems may result from the attachment of impurities to the surface of the optical system, such impurities including water, hydrocarbons or organic substances for example, derived from gases present in a space interposed between plural optical elements, the inner wall of a lens barrel supporting the optical system, or adhesive.
FIG. 14
shows features of a periodical variation in transmittance of an optical system. The periodical variation is represented by measuring an illuminance of an exposure light beam on a portion between a laser beam source and a mask and an illuminance of the exposed light on a wafer at predetermined time intervals while emitting pulse laser beams continuously from the laser beam source and calculating a ratio of the illuminance of the exposure light beam to the illuminance of the exposed light beam at every measured time as a variation in transmittance of the optical system. As it is apparent from
FIG. 14
, the transmittance is caused to be reduced to a great extent immediately after the start of irradiation with a laser beam and thereafter the transmittance arises gradually to reach an almost saturated state as some time elapses. The reduction in transmittance immediately after the start of the irradiation with laser beam is caused by a fluctuation in the inner features of a glass material, and the transmittance is gradually recovered thereafter due to the fact that water or organic substances attached to the surface of the optical system is or are removed by the irradiation with laser beam.
Therefore, the transmittance of the illumination optical system or the projection optical system is caused to arise gradually during the exposure operation of the projection exposure apparatus, that is, upon the transfer of an image of a device pattern formed on a mask onto a photosensitive substrate in a step-and-repeat manner or a step-and-scan manner by illuminating the mask with an exposure light beam from the illumination optical system and projecting at least a part of the image of the device pattern formed on the mask onto the photosensitive substrate by means of the projection optical system. It can be noted herein that the rise of the transmittance represents the effect of temporarily cleaning an projection exposure apparatus, and the optical system with the surface of the optical element activated by the irradiation with an exposure light beam may become rather likely to attract water or organic material present in the surrounding once the irradiation with the light beam would be suspended. With the such phenomenon kept in mind, in the case where the irradiation with the exposure light beam (the exposure operation) is suspended for a long time or for a long period of time, it is considered that the transmittance is made nearly saturated as needed by effecting the optical cleaning by the irradiation with the exposure laser beam for a predetermined period of time prior to the start of exposure and thereafter the exposure operation has been started. This operation is undesirable, however, because it may cause decreasing throughput. Further, the oscillation of laser beam for a long period of time prior to exposure is likewise undesirable because it may result in a decrease in durability of a laser beam source, in addition to a reduction in throughput. Moreover, it is difficult to continue the irradiation with the exposure laser beam all the time, including the time for exchanging wafers and masks.
Then, a description will be made of the problems that may result from the fluctuation in the transmittance of the projection exposure apparatus adapted so as to be used by shifting an optical element as a part of the illumination optical system or the projection
Matsumoto Yukako
Mori Susumu
Ogata Taro
Adams Russell
Armstrong Westerman & Hattori, LLP
Brown Khaled
Nikon Corporation
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