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  Using isotopically specified fluids as optical elementsUSPTO Application #: 20070195302Title: Using isotopically specified fluids as optical elements Abstract: Fluidic optical elements and systems use isotopically specified fluids for processing light passing therethrough. The isotopic composition of the fluid may be adjusted to vary the optical properties. The properties of the isotopically specified fluid may be monitored and adjusted to obtain the desired optical characteristics of the fluidic optical element. In one embodiment, a method of optically processing light includes directing light through an optical element that includes an isotopically specified fluid disposed in a confined space. The isotopically specified fluid is selected to provide a preset desired effect on the light directed therethrough for optically processing the light. (end of abstract)
Agent: Oliff & Berridge, PLC - Alexandria, VA, US Inventor: Michael Sogard USPTO Applicaton #: 20070195302 - Class: 355053000 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20070195302. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This is a divisional of U.S. patent application Ser. No. 11/311,247 filed Dec. 20, 2005, which in turn is a continuation of International Application No. PCT/US2004/021159 filed Jun. 30, 2004, which claims the benefit of U.S. Provisional Patent Application No. 60/484,276 filed Jul. 1, 2003. The disclosures of these applications are incorporated herein by reference in their entireties. BACKGROUND [0002] The invention relates generally to optical systems and elements and, more particularly, to the use of isotopically specified, fluids as optical elements including applications in immersion lithography. [0003] An exposure apparatus is one type of precision assembly that is commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, an optical assembly (sometimes referred to as a projection lens), a wafer stage assembly that retains a semiconductor wafer, a measurement system, and a control system. The resist coated wafer is placed in the path of the radiation emanating from a patterned mask and exposed by the radiation. When the resist is developed, the mask pattern is transferred onto the wafer. In microscopy, extreme ultraviolet (EUV) radiation is transmitted through a thin specimen to a resist covered plate. When the resist is developed, a topographic shape relating to the specimen structure is left. [0004] Immersion lithography is a technique that can enhance the resolution of projection lithography by permitting exposures with numerical aperture (NA) greater than one, which is the theoretical maximum for conventional "dry" systems. By filling the space between the final optical element and the resist-coated target (i.e., wafer) with an immersion fluid, immersion lithography permits exposure with light that would otherwise be totally internally reflected at an optic-air interface. Numerical apertures as high as the index of the immersion liquid (or of the resist or lens material, whichever is least) are possible. Liquid immersion also increases the wafer depth of focus, i.e., the tolerable error in the vertical position of the wafer, by the index of the immersion liquid compared to a dry system with the same numerical aperture. Immersion lithography thus has the potential to provide resolution enhancement equivalent to the shift from 248 to 193 nm. Unlike a shift in the exposure wavelength, however, the adoption of immersion would not require the development of new light sources, optical materials, or coatings, and should allow the use of the same or similar resists as conventional lithography at the same wavelength. In an immersion system where only the final optical element of the optical assembly and its housing and the wafer (and perhaps the stage as well) are in contact with the immersion fluid, much of the technology and design developed for conventional tools in areas such as contamination control, carry over directly to immersion lithography. [0005] The immersion fluid in an immersion lithography system serves as a fluidic optical element. Fluids can also be used to form optical elements such as those inside the optical assembly of an exposure apparatus. The use of a fluid as an optical element raises certain issues and challenges but also provides new opportunities. SUMMARY [0006] Embodiments of the invention are directed to fluidic optical elements and systems using isotopically specified fluids for processing light passing therethrough. The isotopic composition of the fluid may be adjusted to vary the optical properties. The properties of the isotopically specified fluid may be monitored and adjusted to obtain desired optical characteristics of the fluidic optical element. Examples of such properties include the isotopic composition, index of refraction, temperature, and pressure. Water has optical properties that are suitable as the immersion fluid in an immersion lithography system. Water further has mechanical properties such as low viscosity and surface tension characteristics that render it particularly desirable for use in immersion lithography. The index of refraction of an immersion fluid plays a role in the imaging during lithography. Generally, an immersion fluid having a higher index of refraction is more desirable. The heavier isotopes of water can provide a desired higher index of refraction. [0007] In accordance with an aspect of the invention, a method of optically processing light comprises directing light through an optical element that includes an isotopically specified fluid disposed in a confined space. The isotopically specified fluid is selected to provide a preset desired effect on the light directed therethrough for optically processing the light. [0008] In some embodiments, the isotopically specified fluid comprises one or more isotopes of a fluid. The isotopically specified fluid may comprise a plurality of isotopes of a fluid, and an isotopic composition of the fluid is monitored. The method further comprises adjusting amounts of the plurality of isotopes of the fluid to change the isotopic composition based on the monitored isotopic composition of the fluid. The method may also comprise adjusting amounts of the plurality of isotopes of the fluid to form a variable isotopic composition of the fluid to provide variable optical characteristics of the optical element. [0009] In specific embodiments, the isotopically specified fluid comprises one or more isotopes of water. The method further comprises positioning the optical element in an immersion lithography apparatus at a location between an optical assembly and a substrate to be processed by lithography, wherein the light is directed through the optical assembly and the optical element to the substrate. The isotopically specified fluid of the optical element has a first index of refraction and the confined space is provided in a container comprising a material having a second index of refraction that is different from the first index of refraction. The method further comprises recirculating the isotopically specified fluid between the confined space of the optical element and a recirculation reservoir. The method may also comprise controlling at least one of a temperature, a pressure, and an isotopic composition of the isotopically specified fluid in the optical element. The confined space is provided in a container which comprises fused silica. The light has a wavelength of about 193 nm or greater. [0010] In accordance with another aspect of the invention, an optical element comprises a container and an isotopically specified fluid disposed in the container. The isotopically specified fluid is selected to provide a preset desired effect on a light directed therethrough for optically processing the light. [0011] In accordance with another aspect of the present invention, an optical system for processing a substrate comprises an optical assembly spaced from a substrate by a space and an optical element disposed in the space between the optical assembly and the substrate. The optical element includes an isotopically specified fluid selected to provide a preset desired effect on a light directed therethrough for optically processing the light. BRIEF DESCRIPTION OF THE DRAWINGS [0012] The invention will be described in conjunction with the accompanying drawings of exemplary embodiments in which like reference numerals designate like elements and in which: [0013] FIG. 1 is a simplified schematic view of an optical assembly including an optical element having an isotopically specified fluid according to an embodiment of the invention; [0014] FIG. 2 is an elevational view of an optical element having an isotopically specified fluid according to another embodiment of the invention; [0015] FIG. 3 is a simplified schematic view of a fluidic system for an optical element illustrating a recirculation arrangement according to another embodiment of the invention; [0016] FIG. 4 is a simplified schematic view of a fluidic system for an optical element illustrating an open system arrangement according to another embodiment of the invention; [0017] FIG. 5 is a simplified schematic view of an immersion lithography system employing an isotopically specified fluid according to another embodiment of the invention; and [0018] FIG. 6 is a plot of refractive index versus D.sub.2O concentration in water based on calculations. DETAILED DESCRIPTION OF EMBODIMENTS [0019] FIG. 1 shows an optical assembly 10 including two solid optical elements 12, 14, and a fluidic optical element 16 disposed therebetween. In the specific embodiment shown, the solid optical elements 12, 14 are converging lenses and the fluidic optical element 16 is a diverging lens. The solid optical elements 12, 14 may be made of glass or the like. The fluidic optical element 16 includes an isotopically specified fluid 20 disposed in a confined space defined by the surfaces of the two solid optical elements 12, 14 and a side wall 18. The side wall 18 is sealed to the solid optical elements 12, 14. The side wall 18 may be made of a transparent material such as glass. Continue reading... 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