Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-10-05
2003-09-30
Font, Frank G. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S055000, C355S067000, C355S077000, C250S492200, C250S492220, C430S005000, C430S030000
Reexamination Certificate
active
06628371
ABSTRACT:
INCORPORATION BY REFERENCE
The disclosure of the following priority application is herein incorporated by reference: Japanese Patent Application No. 11-288399 filed Oct. 8, 1999.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure apparatus employed to transfer a pattern of an original such as a mask or a reticle (hereafter referred to as a mask) onto a photosensitive substrate such as a wafer during a photolithography process implemented in the fabrication of a semiconductor device such as an LSI or a semiconductor device in an image-capturing element such as a CCD, a liquid crystal display element or a thin-film magnetic head, a method of exposure implemented by utilizing this exposure apparatus and a method for manufacturing a semiconductor device.
2. Description of the Related Art
Keeping pace with the increasingly higher integration achieved for semiconductor devices, significant progress has been made in the area of exposure apparatuses employed during the photolithography process that is crucial in the fabrication of semiconductor devices. The resolving power achieved by a projection optical system mounted at an exposure apparatus is expressed through the relational expression R=k×&lgr;/NA, known widely as Rayleigh's formula. In this relational expression, R represents the resolving power of the projection optical system, &lgr; represents the wavelength of the exposing light, NA represents the numerical aperture at the projection optical system and k represents a constant which is determined by process-related factors as well as the resolving power of the resist.
The resolving power required of the projection optical system to support higher integration in the semiconductor device may be achieved by reducing the wavelength of the light from the exposing light source or by increasing the numerical aperture at the projection optical system as the relational expression above indicates. Thus, continuous efforts to achieve a higher NA value have been made. In recent years, by using an exposure apparatus that employs an argon fluoride excimer laser (ArF excimer laser) having an output wavelength of 193 nm as an exposing light source, fine processing down to 0.18 &mgr;m~0.13 &mgr;m has become possible.
Since there are at present only two materials, i.e., synthetic quartz glass and calcium fluoride (fluorite), that may be used to constitute the lenses while achieving a satisfactory transmittance in the wavelength range of the output wavelength (193 nm) of the argon fluoride excimer laser, tireless efforts are being made to develop an optical material achieving sufficient transmittance and sufficient internal consistency to be used in this type of exposure apparatus. Currently, synthetic quartz glass achieves an internal transmittance of 0.995/cm or higher, and calcium fluoride has reached a point at which the level of internal absorption can be disregarded.
In addition, the intense efforts made in development of materials to constitute an anti-reflection film coated on the surfaces of optical members are beginning to show (tangible results and at present.) the levels of losses at the individual lens surfaces have been lowered to 0.005 or less.
In the wavelength range of ArF excimer laser light, problems occur in that moisture and organic matter may become adhered to the surfaces of the optical elements constituting the optical systems (the illumination optical system and the projection optical system) in the exposure apparatus to lower the transmittances at the optical systems and in that the glass material itself that constitutes the optical elements becomes degraded due to laser irradiation to result in poor transmittances at the optical systems. The first problem is caused by gases within the spaces enclosed by a plurality of optical elements, or moisture or organic matter originating from the inner walls or the like of the lens barrel supporting the optical systems becoming adhered to the surfaces of the optical systems. The second problem, on the other hand, is attributable to the degrading phenomenon that takes place at the glass material itself while laser light is irradiated at the glass material.
FIG. 13
illustrates time-varying transmittance characteristics in an optical system. The figure presents the optical system transmittance, which represents the ratio of the illuminance of the exposing light between the laser light source and the mask and the illuminance of the exposing light on the wafer measured over specific intervals while irradiating pulse laser light continuously from the laser light source during the laser irradiation and is calculated for each measuring time point. As
FIG. 13
indicates, the transmittance at the optical system temporarily becomes lowered immediately after the start of the laser light irradiation (to be referred to as a short-term fluctuation of the transmittance since it occurs over a short period of time relative to the overall change in the transmittance) and then the transmittance gently rises to reach a near saturated state after a certain period of time (to be referred to as a long-term fluctuation of the transmittance which occurs gradually over a long period of time within the overall transmittance change). The degradation occurring at the glass material constituting the optical system causes the transmittance of the optical system to become lowered shortly after the start of the laser light irradiation. Whereas the transmittance at the optical system recovers at a large time constant after the laser light irradiation starts since moisture and organic matter adhering to the surfaces of the optical system are gradually removed from the optical system surfaces through the laser light irradiation. This phenomenon is known as optical cleaning.
It is desirable to expose a photosensitive substrate in a state in which the transmittance at the optical system does not fluctuate greatly, in order to achieve good control of the exposure quantity on the substrate. Accordingly, a near saturated state may be achieved for the transmittance by irradiating the exposing laser light over a specific period of time prior to an exposure operation. However, if an exposure operation is performed following such irradiation, a reduction in the throughput occurs, and, in addition, since the laser is oscillated over a long period of time prior to the exposure operation, the durability of the laser light source becomes lowered.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a method for exposure and an exposure apparatus that maintain the illuminance of exposing light on a photosensitive substrate (exposure object) at a target value at all times, unaffected by the change in the attenuation rate of the exposing light at an optical system occurring over time.
A second object of the present invention is to provide a method for manufacturing a semiconductor device that achieves an improvement in the yield by estimating the change in the attenuation rate of the exposing light at an illumination optical system or a projection optical system occurring over time when exposing the circuit pattern or the like onto a semiconductor substrate.
In order to achieve the objects described above, a method for exposure according to the present invention comprises a step in which a first attenuation rate of the exposing light passing through an optical system is measured before transferring an image of a pattern illuminated with the exposing light onto a first specific surface via the optical system provided within the optical path of the exposing light, a step in which a second attenuation rate of the exposing light passing through the optical system is measured after the image of the pattern is transferred onto the first specific surface, a step in which a third attenuation rate of the exposing light passing through the optical system is measured before transferring an image of the pattern onto a second specific surface different from the first specific surface and a step in wh
Brown Khaled
Font Frank G.
Nikon Corporation
Oliff & Berridg,e PLC
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