Optical: systems and elements – Mirror – Plural mirrors or reflecting surfaces
Reexamination Certificate
2002-09-16
2004-06-08
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Mirror
Plural mirrors or reflecting surfaces
C359S850000, C359S869000, C359S900000
Reexamination Certificate
active
06746128
ABSTRACT:
The present invention relates to ultra-high resolution imaging devices, including devices using light or other sorts of electromagnetic waves for high resolution lithography for, say, semiconductor or microchip manufacture. The invention involves a combination of an ultra-high numerical aperture imaging system in conjunction with a suitably structured mirror and other associated components designed to result in the boundary conditions to the wave equation arising from the device more nearly approximating to that required to generate an imploding dipole solution to Maxwell's equations. The invention also provides an ultra-high numerical aperture imaging system using two suitably shaped reflectors, of relatively simple construction which have further potential uses beyond those conventionally relating to ultra-high resolution.
In the context of the present invention “ultra-high resolution” means having a resolving power better than that implied by the Rayleigh resolution criterion and “ultra-high numerical aperture” means that the range of angles that rays make when striking the image plane (if the device is being used to concentrate light) span a high proportion of the total 2&pgr; it steradians possible for light falling onto one side of a plane. To create sharp images (at least for small objeds), an optical system needs to be aplanatic. Geometrical optical theory indicates that such a system must have at least two surfaces at which the waves are deflected, see e.g. Schulz, G. “Higher order aplanatism”,
Optics Communications
, 41, No 5, 315-319 (1982). The invention provides a two-mirror aplanatic lens arrangement that simultaneously facilitates ultra-high resolution and achieves a very high angle span into a plane.
Attempts to achieve a complete angle span using a combination of a mirror and a refracting surface, rather than two mirrors, have previously been described by Benitez, P. and Miñano, J. C. Ultrahigh-numerical-aperture imaging concentrator,
J. Opt. Soc. Am. A
14, No 8, 1988-1997 (1997), and in other papers by the same authors. However, the mirror plus refractor arrangement they describe requires the image plane to be embedded within a material with refractive index greater than unity, which is considerably less practical than an approach in which both deflecting surfaces are mirrors.
Benitez and Miñano appear to have developed their ideas from non-imaging systems of relatively similar layout that were able to achieve very high concentrations for sources that were not very small. Their imaging layouts are in effect limiting cases of their non-imaging systems when the (far away) source object becomes very small. Other more traditional forms of non-imaging system are known, such as the Compound Parabolic Concentrator (CPC) described in Welford, W. T. & Winston, R.
High collection nonimaging optics
(Academic Press, 1989). However, the present invention differs from these systems in that it is imaging rather than non-imaging and, as is apparent from a cross-section taken through an axis of symmetry, a device according to the present invention comprises two separate deflecting surfaces not one, as is the case with a CPC. Additionally, in the limit for the CPC as the (far away) source becomes small, the CPC simply becomes arbitrarily long. A device according to the present invention is readily distinguishable.
Some aplanatic two-mirror arrangements have also been previously described. These include:
(a) Siemens-Reiniger-Werke Aktiengesellschaft “Improvements in or relating to optical mirror systems having aspherical surfaces”, in GB Patent No GB 0 717 787 (1952). This patent describes a two-mirror aplanalic device, without explicitly specifying any limitation on the numerical aperture involved. However, it does not indicate how to achieve an ultra-high numerical aperture, nor do the Figures that it contains envisage such a device. The patent relates primarily to the design of X-ray telescopes which, because of the physical nature of reflection of X-rays, would not work if the device involved had a very high numerical aperture. Furthermore, despite making reference to a two-mirror aplanatic device, the Siemens-Reiniger-Werke Aktiengesellschaft patent does not indicate how to define the shape of the two mirrors involved.
(b) Mächler, Glück, Sclemmer and Bittner “Objective with aspheric surfaces for imaging microzones”, in U.S. Pat. No. 4,655,555 (1984) concentrates on mirrors that use total internal reflection. It includes reference to a special case of an aplanatic two-mirror arrangement involving two confocal equally-sized ellipsoids. It concentrates on other confocal mirror arrangements (as does Hunter “Confocal reflector system” in U.S. Pat. No. 4,357,075 (1980)), although these mirror lay-outs are not actually aplanatic except in the special case of the two confocal equally-sized ellipsoids). However, U.S. Pat. No. 4,655,555 also refers to an article by Lawrence Mertz entitled “Geometrical Design for Aspheric Reflecting Systems”.
Applied Optics
, 18, pages 4182-4186 (1979), which does appear to describe (in its FIG. 10) a very high numerical aperture aplnatic two-mirror arrangement, again focusing on microscopy. Pioneer “Manufacture of reflective type multiple-degree aspherical optical control system” in Japanese Patent JP 57141613 (1981) refers to an efficient means of producing a two mirror aplanatic arranement using a grip and press work plated by aluminium by vapour deposition.
(c) Döring in German Patent DE 2916741 notes that such arrangerrents can be used as optical collectors for solar cells, and the figures suggest reference to aptanatic rather than merely confocal arrangements.
However, none of the above indicate how the precise positioning of the mirrors can be identified. The present invention therefore embodies a significant departure from and advance over the various prior art systems not only because it refers to ultra-high resolution devices but also because it provides a simple methodology for identifying the precise positioning of such aplanatic mirror pairs. In certain preferred embodiments it also incorporates other refinements not described in the above references.
According to one aspect of the present invention a high numerical aperture imaging device comprises first and second axially-symmetric curved mirrors for focussing the image of an object onto an image plane, wherein the first and second curved mirrors are arranged to effectively create inwardly imploding dipole-like solutions to the applicable wave equation, to concentrate the light flux arriving at the image plane from a given point in the object more than would be possible were the image formation to be subject to the diffraction limits that generally apply to far field devices.
In a preferred embodiment, the device further comprises a plane mirror, wherein the plane mirror is partially transparent and is positioned in or closely adjacent to the image plane.
A device according to the invention may further comprise a wave attenuation element and/or wave polarisation-rotating element to attenuate and/or rotate the polarisation of the waves traversing the device so that the spatial distribution of the amplitude and polarisation of a wavefront as it approaches the plane mirror is rendered more closely consistent with that required to generate dipole-like solutions to the wave equation.
Two mirror ultra-high numerical aperture imaging devices according to the invention may have practical application for several possible uses, including, for example:
(a) use to concentrate sunlight to a high temperature, indeed the second law of thermodynamics indicates that the temperatures reached could be dose to the temperature of the sun's photosphere, i.e. to in excess of 4,000° K. At such temperatures, unusual ways of converting sunlight to electric power (e.g. use of thermionic emission) could be facilitated by a device according to the invention;
(b) as solar concentrators made out of lightweight mirrors (for example, using thin films whose shapes remain stable because of rotation &ls
Dergosits & Noah LLP
Nebb Richard A.
Robinson Mark A.
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