Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2002-02-22
2004-03-30
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000
Reexamination Certificate
active
06714280
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention pertains to a projection exposure apparatus and method which may be employed, for example, where photolithographic techniques are used for manufacture of semiconductor integrated circuits, charge coupled devices and other such image pickup elements, liquid crystal display devices, thin-film magnetic heads, and other such microdevices, and pertains as well to a projection optical system suitable for use in such a projection exposure apparatus or method. The present invention permits a projection optical system to be provided which is capable of high-resolution projection of a highly detailed pattern while permitting satisfactory correction of chromatic aberration and without incurring inordinate increase in cost. Furthermore, the present invention permits a projection exposure apparatus and a projection exposure method to be provided which permit satisfactory transfer of an image of an extremely detailed pattern from a mask to a substrate.
More particularly, one or more embodiments of the present invention employ a combination of techniques for facilitating correction of chromatic aberration in the context of a projection optical system comprising one or more refractive optical members collectively comprising two or more fluoride substances. Still more particularly, one or more embodiments of the present invention utilize one or more design conditions pertaining to an illumination optical system (including light source) and/or a projection optical system for economical and/or effective allocation of such fluoride substances.
One application of projection exposure apparatuses is in photolithographic operations for manufacture of semiconductor elements or the like, where an image of a pattern on a mask, reticle, or the like (hereinafter referred to collectively as “mask,” “reticle,” etc., these terms being used interchangeably where not otherwise specified) is projected by way of a projection optical system to expose resist or other such photosensitive material on a wafer, glass or other such plate, substrate, workpiece, or the like (hereinafter referred to collectively as “substrate,” “wafer,” “workpiece,” etc., these terms being used interchangeably where not otherwise specified). Accompanying the desire to achieve increased circuit density of semiconductor elements and the like, higher and higher resolutions are being required of the projection optical systems used in projection exposure apparatuses.
Resolution of an optical system is in general determined by Rayleigh's equation, or
R
=
k
×
λ
N
A
,
where &lgr; is the exposing wavelength, NA is the image-side numerical aperture of the projection optical system, and k is a constant which is in this case determined by resist resolution and so forth. It is clear from the above equation that resolution can be increased by decreasing the wavelength of the actinic light or radiation responsible for exposure (“light” and “radiation” are used interchangeably herein and without intention to limit either to wavelengths which are visible or invisible or the like; “actinic” light or radiation as used herein refers to light or radiation used for exposure without regard to whether such exposure occurs by a chemical, physical, or other process; “exposure” as used herein refers to any change due to receipt of such actinic light or radiation at the wafer or other such substrate or workpiece) or by increasing numerical aperture. Based on this fact, the mercury lamp i-line light sources (wavelength 365 nm) which had previously been favored by the industry have been largely replaced by the KrF excimer laser (wavelength 248 nm), and the still-shorter-wavelength ArF laser (wavelength 193 nm) is well on its way to practical application. In addition, with the goal of even further reduction in the wavelength of the light used for exposure, attempts are underway to develop an exposure apparatus utilizing an F
2
laser (wavelength 157 nm).
However, increasing the numerical aperture of a projection optical system decreases it depth of focus. This in turn places stringent demands on the projection optical system with respect to correction of chromatic aberration. Furthermore, as a result of developments in resist and other peripheral technologies, the magnitude of k in the above equation has grown smaller over time. Minor aberrations and small errors in exposure dose can therefore have a large effect on resolution, and chromatic aberration must be even more tightly controlled.
One strategy which has been proposed for accomplishing such goals is the use of an exposure apparatus employing actinic radiation having a narrowed linewidth. However, where the refractive optical members in the projection optical systems of such proposed exposure apparatuses are formed from a single substance there will be a limit as to how far chromatic aberration can be corrected, making such apparatuses incapable of providing the resolutions now in demand. Furthermore, achieving narrowed linewidth is not an easy matter, and it is only with great difficulty that narrowing sufficient to permit reduction of chromatic aberration to the desired level can be achieved in the context of a projection optical system employing refractive optical members composed of a single substance. There is therefore a need for a projection optical system having optical members formed from a plurality of substances to permit further improvement in ability to correct for chromatic aberration.
However, with a light source employing an F
2
laser, there are only a limited number of materials which are effective in reducing chromatic aberration, permit achievement of satisfactory transmittance, and do not present significant problems with respect to fabrication and endurance. At present, one set of materials that satisfies all of the above requirements is a combination of calcium fluoride and barium fluoride. However, barium fluoride has high specific gravity and does not lend itself to fabrication into parts having good homogeneity, and its high solvability with respect to water makes it less than suitable for fabrication. In light of the foregoing, there has been a problem in that any increase in the amount of barium fluoride in an attempt to correct chromatic aberration would lead to increased cost.
SUMMARY OF THE INVENTION
The present invention pertains to a projection exposure apparatus and method which may be employed, for example, where photolithographic techniques are used for manufacture of semiconductor integrated circuits, charge coupled devices and other such image pickup elements, liquid crystal display devices, thin-film magnetic heads, and other such microdevices, and pertains as well to a projection optical system suitable for use in such a projection exposure apparatus or method. The present invention permits a projection optical system to be provided which is capable of high-resolution projection of a highly detailed pattern while permitting satisfactory correction of chromatic aberration and without incurring inordinate increase in cost. Furthermore, the present invention permits a projection exposure apparatus and a projection exposure method to be provided which permit satisfactory transfer of an image of an extremely detailed pattern from a mask to a substrate.
More particularly, one or more embodiments of the present invention employ a combination of techniques for facilitating correction of chromatic aberration in the context of a projection optical system comprising one or more refractive optical members collectively comprising two or more fluoride substances. Still more particularly, one or more embodiments of the present invention utilize one or more design conditions pertaining to an illumination optical system (including light source) and/or a projection optical system for economical and/or effective allocation of such fluoride substances.
In order to solve one or more of the foregoing or related problems, a projection optical system associated with one aspect of the present invention is capabl
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