Optical: systems and elements – Mirror – Plural mirrors or reflecting surfaces
Reexamination Certificate
1998-02-17
2001-05-15
Spyrou, Cassandra (Department: 2812)
Optical: systems and elements
Mirror
Plural mirrors or reflecting surfaces
C359S850000, C359S852000, C359S853000
Reexamination Certificate
active
06231198
ABSTRACT:
1. Field of the Invention
This invention relates generally to an apparatus and method of producing an array of point source images to improve the illumination uniformity of an illumination system. More specifically, this invention relates to an apparatus and method of producing an array of point sources using mirror segments based on an off-axis section of a parent parabolic surface.
2. Background of the Invention
The goal of an optical illumination system is to provide efficient energy transfer from a source to another location so that an image can be projected to a remote location. Specifically, a projection/illumination condenser produces an image of the light source in the pupil of a projection lens to produce uniform illumination across the projection lens field. The maximum illumination possible based on the brightness of the source and the diameter of the projection lens pupil occurs when the source image fills the projection lens pupil. This condition of maximum illumination is called Köhler illumination. In projection exposure lithography systems, the Köhler illumination condition is used to provide uniform illumination across the exposure field.
A basic illumination optical system consists of a source and a condenser lens. In this configuration the condenser lens forms an image of the source in the pupil of the projection optic. Higher performance illumination systems add additional components to collect and project more light into the pupil and also add components to increase the illumination uniformity across the pupil.
In the field of integrated circuit manufacturing, reduction projection lithography systems are being used to fabricate smaller and smaller linewidths. In such systems, it is crucial that the illumination intensity over the exposure area be extremely uniform because the photoresist used to record such intricate patterns is sensitive to illumination intensity. In addition, as integrated circuit linewidths continue to be reduced, so must the exposure wavelength of the reduction projection lithography systems. Currently, lithography systems have been proposed that use reflective optics to handle the exposure radiation to produce linewidths that are smaller than the linewidths that are theoretically possible with refractive optics. As those skilled in the art will appreciate, reflective optics are required when the wavelength of the exposure radiation drops below that which can be transmitted by conventional refractive optics (approximately 130 nm).
In current ultraviolet projection lithography systems a combination of mirrors and lenses is used to provide uniform illumination over the exposure area of the integrated circuit pattern that is recorded by the photoresist. One common method of providing a uniform exposure area has been to use a “fly's eye” lens array. The fly's eye lens array is composed of nearly identical lenslets arranged in a two-dimensional array with the optical axes of each of the lenslets essentially parallel to a common optical axis. When the fly's eye lens array is illuminated with collimated radiation, each lenslet in the array forms a secondary source image over a planar two-dimensional area. The secondary source image array increases the intensity uniformity over an extended area and is imaged by a condenser optic assembly to the pupil of the projection optical system. As discussed above, this type of illumination is Köhler illumination and provides efficient uniform illumination over the field of a projection lens.
The fly's eye array is useful in producing multiple secondary image sources where suitable refractive lens materials are available to transmit and refract the incident radiation. However, with x-rays a refractive material that transmits and refracts x-ray energy has not been found which is suitable. A reflective equivalent for the fly's eye array has been proposed in U.S. Pat. No. 5,581,605 to Murakami et al. The system proposed by Murakami et al. uses reflective spherical or toroidal mirror segments to generate secondary x-ray sources over a wide region. However, the uniformity of the secondary x-ray sources produced by such mirror segments is limited by optical aberrations. Spherical aberration is generated from the use of spherical or toroidal mirror segments and coma aberration is generated by the off-axis illumination of these mirror segments.
Accordingly, there is a need to provide an array of point sources located over a wide planar area that are not afflicted with either spherical or coma aberration. Preferably, such an array of point sources would be capable of providing suitable performance for exposure radiation that cannot be accommodated by conventional refractive optics.
SUMMARY OF THE INVENTION
According to the present invention, the foregoing and other advantages are attained by an optical integrator made up of a two dimensional array of mirror segments each based on an off-axis section of a parent parabolic surface. In one embodiment, the mirror segments are arranged so that the focus of each mirror segment is in a plane perpendicular to the propagation direction of incident coaxial collimated radiation. In a second embodiment, the mirror segments are arranged so that the focus of each mirror segment is in a plane that is tilted with respect to the optical axis of the parent parabolic surface.
The optical integrator of the present invention reflects the incident radiation to produce an array of secondary point sources. By choosing the appropriate off-axis segment of the parent parabolic surface from which the mirror segments are based, an extremely uniform array of secondary point sources is obtained. Such an array of point sources may be utilized in various optical systems, including an illumination system for use in a projection optical system. The use of the off-axis segment of the parent parabolic surface ensures that the resultant point sources will be substantially free of coma and spherical aberration.
These and other advantages of the present invention will become more apparent upon a reading of the detailed description of the preferred embodiments that follows, when considered in conjunction with the drawings of which the following is a brief description. It should be clear that the drawings are merely illustrative of the currently preferred embodiments of the present invention, and that the invention is in no way limited to the illustrated embodiment. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications in various obvious aspects, all without departing from the scope of the invention. The present invention is best defined by the claims appended to this specification.
REFERENCES:
patent: 3645606 (1972-02-01), La Vantine
patent: 4087682 (1978-05-01), Kolodziej
patent: 5377049 (1994-12-01), Mueller et al.
patent: 5442436 (1995-08-01), Lawson
patent: 5993010 (1999-11-01), Ohzawa et al.
Calculus With Analytic Geometry, Earl Swokowski, Prindle Weber &Schmidt (1977), p 262-266, Jun., 1977.
Cherry Euncha
Morrison & Foerster / LLP
Nikon Corporation
Spyrou Cassandra
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