Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2000-01-24
2002-12-10
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C355S053000, C356S400000
Reexamination Certificate
active
06492649
ABSTRACT:
The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 9-199710
Japanese Patent Application No. 9-337104
Japanese Patent Application No. 10-67021
TECHNICAL FIELD
The present invention relates to a projection exposure apparatus employed to expose a pattern of an original such as a mask or a reticule (hereafter referred to as a mask) onto a photosensitive substrate such as a wafer during a photolithography process implemented during the fabrication of a semiconductor device such as an LSI, an image-capturing element such as a CCD, a liquid crystal display element, or a semiconductor device such as a thin film magnetic head, a projection exposure method utilizing this exposure aparatus, an optical cleaning method employed to clean the optical systems in the projection exposure apparatus and a method of fabricating a semiconductor device.
BACKGROUND ART
Keeping pace with the increasingly higher integration achieved for semiconductor devices, significant progress has been made in the area of projection exposure apparatuses employed during the photolithography process that is crucial in fabrication of semiconductor devices. The resolving power achieved by a projection optical system mounted at a projection 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, exposure aparatuses that use a krypton fluoride excimer laser (KrF excimer laser) having an output wavelength of 248 nm as an exposing light source with the numerical aperture at 0.6 or higher achieved at the projection optical system have been put into practical use to enable exposure of extremely fine patterns of down to 0.25 &mgr;m.
An argon fluoride excimer laser (ArF excimer laser) having an output wavelength of 193 nm has been attracting much attention recently as a light source to replace the krypton fluoride excimer laser. Since it is expected that by realizing an exposure apparatus using this argon fluoride excimer laser as the exposing light source, ultra-fine processing down to 0.18 &mgr;m~0.13 &mgr;m will be possible, concentrated efforts are being made in research and development.
Since there are at present only two materials, i.e., synthetic silica 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 silica 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.
The choice of material to constitute the anti-reflection film that is coated on the surface of the optical material, too, is extremely limited compared to the range of materials from which selection can be made to constitute an anti-reflection film used in the output wavelength range (248 nm) of the krypton fluoride excimer laser, and this also greatly restricts the degree of freedom afforded in design. However, thanks to the intense efforts made in development this problem, too, is being overcome. At present, the levels of losses at the individual lens surfaces (e.g., losses through the absorption of light by the coating, scattering of light, reflection at the interface of the coating and the optical material and reflection at the coating surface) have been lowered to 0.005 or less (light loss of 0.5% or less).
DISCLOSURE OF INVENTION
In wavelength ranges shorter than the wavelength of KrF excimer laser light, moisture and organic substances may adhere to the surfaces of the optical elements constituting the optical systems (illumination optical system, projection optical system) in the projection exposure apparatus, resulting in a reduction in transmittance of the optical systems. This problem is attributable to gas trapped within the space enclosed by a plurality of optical elements or moisture and organic substances generated from the inner walls of the lens barrel or the like supporting the optical systems becoming adhered to the surfaces of the optical systems.
FIG. 17
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. The figure also presents a similar optical system transmittance during a time period in which the laser is stopped that is obtained by irradiating laser over appropriate time intervals and calculated at each laser irradiation. As
FIG. 17
illustrates, after the start of laser light irradiation, the transmittance gradually increases and when a specific length of time has elapsed, a near-saturated state is achieved. This phenomenon of the optical system transmittance gradually recovering is due to moisture and organic substances adhering to the optical system surface being removed from the optical system surfaces by the laser irradiation. For this reason, it is conceivable to start an exposure operation after a near-saturated state of transmittance is achieved by irradiating exposing laser light over a specific period of time prior to the start of the exposure. However, this would cause a reduction in throughput. In addition, oscillation of the laser over a long period of time prior to the exposure would lead to poor durability of the laser light source and it is, therefore, not desirable. Furthermore, it is difficult to continuously irradiate exposing laser light at all times, including during replacement of the wafer or the mask.
A first object of the present invention is to provide a projection exposure method and a projection exposure apparatus that make it possible to sustain the illuminance of the exposing light on a photosensitive substrate at a target value at all times regardless of time-varying transmittance of the optical system.
A second object of the present invention is to provide a projection exposure method and a projection exposure apparatus that controls the accumulated light quantity (exposure dose) of the exposing light on a photosensitive substrate at a correct value that corresponds to the sensitivity of the photosensitive substrate even when the transmittances at the illumination optical system and the projection optical system change.
A third object of the present invention is to provide an optical cleaning method for cleaning the optical systems by predicting time-varying transmittance at the illumination optical system and the projection optical system.
A fourth object of the present invention is to provide a method of fabricating a semiconductor device that achieves an improvement in the yield by exposing a circuit pattern or the like on a semiconductor substrate by predicting time-varying transmittance at the illuminati
Nei Masahiro
Ogata Taro
Allen Stephone
Nikon Corporation
Oliff & Berridg,e PLC
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