Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
2001-03-30
2004-12-07
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Lens
With field curvature shaping
C359S569000, C359S364000, C359S730000, C355S053000
Reexamination Certificate
active
06829099
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a projection optical system, a projection exposure apparatus having a projection optical system, and a device manufacturing method. More particularly, the invention concerns a catadioptric projection optical system which uses a concave reflection mirror, for example, in a projection optical system for printing, by projection exposure, a reticle pattern on a semiconductor wafer.
Recent advancement in semiconductor device manufacturing technology is quite notable, and micro-processing technology following it also has advanced remarkably. Particularly, in the photo-processing technology, reduction projection exposure apparatuses having a resolution of submicron order and called steppers or scanners, are used widely. For further improvements of resolving power, enlargement of the numerical aperture (NA) of the optical system or shortening of the exposure wavelengths are attempted.
As regards imaging optical systems used in projection exposure apparatuses for printing a semiconductor device pattern such as an IC or LSI on a silicon wafer, for example, a very high resolving power is required. Generally, the resolving power of an imaging optical system is better as the wavelength used is shorter. For this reason, light sources which emit light of shorter wavelengths as much as possible are used. As an example of such a short wavelength light source, excimer lasers are known. These excimer lasers use KrF or ArF, for example, as the laser medium. Also, there is an F
2
laser which is expected as a next generation laser of the ArF laser.
In relation to the wavelength regions of these light sources, glass materials usable as a lens material are limited to quartz and fluorite. This is mainly because of the decrease in the transmission factor. Further, even with such quartz or fluorite usable in the wavelength regions of these light sources, as discussed in Japanese Laid-Open Patent Application, Laid-Open No. 79345/1998, for example, if the optical system consists of refraction lenses only and the number of lenses is large so that the total glass material thickness is large, there may occur problems such as a shift of the focal point position, for example, due to heat absorption of the lenses. Further, in recent projection optical systems, a larger numerical aperture and a wider exposure range are strongly desired, and this raises the necessity of further increasing the number of lenses used. This results in a decrease of the transmission factor and an increase of the cost of glass materials. Further, if the band-narrowing of a laser is insufficient, correction of chromatic aberration must be made. This needs achromatism based on a combination of refracting lenses in an optical system, for the correction of chromatic aberration. Also, this leads to a further increase of the number of lenses used.
Japanese Laid-Open Patent Application, Laid-Open No. 331941/1994 corresponding to U.S. Pat. No. 5,623,365 and Japanese Laid-Open Patent Application, Laid-Open No. 128590/1995 corresponding to U.S. Pat. No. 5,555,497, show an optical arrangement in which, for correction of chromatic aberration, a diffractive optical element is introduced into a projection optical system comprising dioptric systems. In this optical arrangement, a diffractive optical element having a dispersion inverse to that of an ordinary refracting lens is introduced and placed adjacent to a pupil of a dioptric projection optical system, by which axial chromatic aberration is mainly corrected. Also, by means of an aspherical surface effect of the diffractive optical element, aberrations such as spherical aberration and coma are mainly corrected.
The diffractive optical element is an optical element for converting an incident wavefront into a predetermined wavefront. It has unique features which refracting lenses do not have. For example, since it has a dispersion value inverse to a refracting lens or it has substantially no thickness, the optical system can be made very compact, as an example.
As a method producing a diffrative optical element having such features very precisely, binary optics have attracted attention, for example. This is because a semiconductor process used in the manufacture of an LSI, for example, can be applied to it by approximating a Kinoform shape by a step-like shape, such that even a very small pitch can be produced easily and very precisely.
Japanese Laid-Open Patent Application, Laid-Open No. 78319/1996 corresponding to U.S. Pat. No. 5,754,340 shows an optical system having diffractive optical elements, quartz lenses and fluorite lenses, in which at least one diffractive optical element has a positive refractive power, at least one quartz lens has a negative refractive power, and at least one fluorite lens has a positive refractive power. This is intended particularly to reduce a secondary spectrum of chromatic aberration.
Japanese Laid-Open Patent Application, Laid-Open No. 17720/1996 shows an optical system in which a diffractive optical element is introduced into a catoptric system. This optical system includes diffractive optical elements and reflecting members each having a curved reflection surface. The diffractive optical element is provided on the reflection surface. It is stated in this document that the role having been taken by a refracting lens is played by a diffractive optical element, by which a projection optical system of a reduced magnification is accomplished only by the combination of reflection surfaces and diffractive optical elements. Also, it is stated that, since the diffractive optical element has a dispersion corresponding to the bandwidth of light to be used for the projection exposure, in the paraxial region, it is desirable to use the same while keeping its refractive power nearly at zero, that is, at an infinite focal length. Thus, this structure proposes an optical system which can be used in a short wavelength region in which no refracting lens can be used.
Further, many proposals have been made with respect to a combination of a dioptric system and a catoptric system, that is, a catadioptric system. These optical systems are intended to correct chromatic aberration or any other aberrations by a combination of a mirror and a refracting lens, and no diffractive optical element is used.
Among them, Japanese Laid-Open Patent Application, Laid-Open No. 304705/1996 corresponding to U.S. Pat. No. 5,691,802 shows an optical system constituted by a double-imaging (twice-imaging) system, in which a first imaging system includes one concave mirror and a refracting lens so that an intermediate image of a reticle formed by the first imaging system is imaged upon a wafer by a second imaging system which comprises refracting lenses.
According to the structure of this document, a flat mirror is disposed adjacent to the intermediate image formed by the first imaging system, to deflect the advancement direction (optical axis) of the light by 90 degrees toward the second imaging system. Also, a reflection mirror is provided in the second imaging system so that the wafer surface and the reticle surface are held parallel to each other. This optical system accomplishes scanning exposure by using an abaxial light beam and by scanning the reticle and the wafer in synchronism with each other.
The optical system shown in Japanese Laid-Open Patent Application, Laid-Open No. 331941/1994, mentioned above, in which a diffractive optical element is introduced into a dioptric system, needs a large number of lenses, due to the necessity for aberration correction. Thus, there is a possibility that, due to the influence of thermal aberration or the like, the performance of the projection optical system is degraded. Further, when the wavelength of the exposure light is shortened much more, the influence of the thermal aberration or the like becomes much more notable.
The optical system shown in Japanese Laid-Open Patent Application, Laid-Open No. 128590/1995 mentioned above needs a smaller number of elements, but the exposure range is nar
Ishii Hiroyuki
Kato Takashi
Terasawa Chiaki
Amari Alessandro
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Robinson Mark A.
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