Optical: systems and elements – Compound lens system – Telescope
Reexamination Certificate
1994-11-30
2001-05-01
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Compound lens system
Telescope
C359S362000, C359S619000, C359S642000, C351S057000, C351S158000
Reexamination Certificate
active
06226120
ABSTRACT:
This invention pertains to microstructures, and to methods for making microstructures whose three-dimensional shapes are not constrained by conventional lithographic limitations. For example, the novel methods may be used to make a novel x-ray lens, or a novel thin array of microtelescopes.
Microscopic machines, structures, devices, and integrated circuits (hereafter collectively called “microstructures” for simplicity) have wide application. Integrated circuits are used in devices too numerous to be recited. Microstructures other than integrated circuits, structures whose minimum dimensions are typically on the order of several hundred microns down to one micron, or even into the submicron range, also have a wide range of applications. Microstructures have been used in micromechanics, microoptics, integrated optics, sensors, actuators, and chemical engineering. Microstructures that have been built include such structures as gears, nozzles, chromatographic columns, acceleration sensors, microturbines, micromotors, and linear actuators.
Microstructures are usually manufactured through a lithographic process. In lithography, one or more “masks” are initially prepared, each mask incorporating all or part of the pattern to be formed on a workpiece. Transparent and opaque areas of the mask represent the desired pattern. Radiation, such as visible light, ultraviolet light, x-rays, an electron beam, or an ion beam, is transmitted through the mask onto a resist, such as a photoresist or an x-ray resist. After exposure, the resist (which may have either a positive tone or a negative tone) is developed to form the pattern. The pattern is then generally transferred to the workpiece in a subsequent step.
Workers in the field of x-ray lithography have long sought a highly efficient x-ray lens. Despite the importance that an efficient x-ray lens would have, there have been no prior, truly successful x-ray lenses. Previous attempts have used capillary arrays (the so-called “Kumakov lens”), and slumped multichannel arrays of square holes to collimate x-rays. These designs have inherent limitations in uniformity, throughput, and angle control. See M. Vartanian et al., “Polycapillary Collimator for Point Source Proximity X-Ray Lithography,” J. Vac. Sci. Technol. B., vol. 11, no. 6, pp. 3003-3007 (1993); and H. N. Chapman et al., “X-Ray Focusing Using Square Channel-Capillary Arrays,” Rev. Sci. Instrum., vol. 62, no. 6, pp. 1542-1561 (1991); Q. F. Xiao et al., “Polycapillary-Based X-Ray Optics,” pp. 376-383 in G. G. Long et al. (eds.),
Synchrotron Radiation Instrumentation
(1994); S. A. Hoffman et al., “Developments in Tapered Monocapillary and Polycapillary Glass X-Ray Concentrators,” pp. 384-389 in G. G. Long et al. (eds.),
Synchrotron Radiation Instrumentation
(1994); K. F. Voss et al., “A Capillary Concentrator for an X-Ray Microprobe,” pp. 390-396 in G. G. Long et al. (eds.),
Synchrotron Radiation Instrumentation
(1994); Q. F. Xiao et al., “Guiding Hard X Rays with Glass Polycapillary Fiber,” pp. 397-400 in G. G. Long et al. (eds.),
Synchrotron Radiation Instrumentation
(1994); J. B. Ullrich et al., “Potential for Concentration of Synchrotron Beams with Capillary Optics,” pp. 401-406 in G. G. Long et al. (eds.),
Synchrotron Radiation Instrumentation
(1994); and G. J. Chen et al., “Ray-Tracing of X-Ray Focusing Capillaries,” pp. 407-411 in G. G. Long et al. (eds.),
Synchrotron Radiation Instrumentation
(1994).
Compared to these prior structures, uniformity, throughput, and angle control would be greatly improved if an x-ray lens could be made of nested cones of suitable scale. However, a method for generating nested cones of suitable scale has not previously been available.
The imaging properties that a series of nested cones made of a reflective material would have are known (see FIG.
1
). “Nested cones” are a set of cones having a common axis and a common vertex. Rays parallel to the common axis are reflected to a focal point halfway between the base of the cones and their common vertex. As illustrated in
FIG. 1
, only a small portion of each cone is present, near its base, to insure that the rays undergo only one reflection. Multiple reflections are undesirable, as they would result in rays diverging from the focus.)
The resolution of a series of nested cones is limited by the separation between the cones. For an array of constant thickness (i.e., the depth of the cones measured in a direction parallel to their common axis), resolution increases with the radius of the largest cone of the set of nested cones.
Nested cones constructed so that all rays reflect at relatively shallow grazing angles (e.g., about 1° to about 5°, preferably about 2°) would be particularly useful for soft x-rays (e.g., those having a wavelength between about 5 Å and about 20 Å, preferably about 10 Å). Soft x-rays have high reflectivities at grazing angles. The ability to focus soft x-rays is very important, both in concentrating a parallel x-ray beam from a synchrotron to a high power density, and in collimating a diverging beam from a point source.
Despite the desirability of creating a nested cone x-ray lens, no one has previously been able to construct nested cones of suitable scale, primarily because nested cones have non-vertical boundaries. Features with non-vertical boundaries have been difficult or impossible to generate with prior lithographic techniques.
In making a microstructure (such as a miniature gear or motor), there has previously been a limited ability to sculpt the microstructure in three dimensions. In conventional lithography a perpendicular x-ray beam strikes a resist, forming vertical edges in the pattern. It is difficult to generate objects with curved edges or walls with such a method. Except for special cases, features curved in three dimensions cannot be constructed through prior methods.
Prior work on non-vertical boundaries between lithographic regions has been limited to a few instances using a constant-angle projection (or a small number of constant-angle projections) of x-rays onto a wafer. No prior work is known in which non-vertical lithographic boundaries have been made other than through such a constant-angle projection. Constant-angle projections greatly limit the types of non-vertical boundaries that may be formed, and in particular such projections do not allow the formation of nested cones.
In machining microstructures other than integrated circuits, it would often be desirable to be able to have the heights of features vary continuously (e.g., as in a sphere), rather than be limited to the current norm of flat planes and vertical steps. No prior method is known for creating lithographic features having a controllable and continuously variable height.
Conventional resist processing for microlithography has historically been based on the requirements of integrated circuit lithography, where vertical boundaries and complete removal of resist are desired for patterned areas. In principle, continuous variations in height could be obtained by varying the thickness of the absorber on the mask. Regions of thicker absorber would transmit fewer x-rays, resulting (in a positive resist such as PMMA, polymethylmethacrylate) in thicker remaining resist after exposure and development. But in practice such a mask would be difficult to make, and the process would be hard to control. For example, small changes in the x-ray wavelength can induce large changes in the thicknesses of the patterned features. No prior method for overcoming these problems is known.
PMMA is a commonly used x-ray resist. PMMA is a thermoplastic material that softens at temperatures above 100° C. The softening mechanism is largely unrelated to the chain scission mechanism by which a latent image is formed in an exposed PMMA resist. Consequently, a PMMA resist may be heated to deform it either before or after exposure, but before development. After the exposure, the resist may be returned to its original shape either before or after development. Deformation of the resist through heating
Board of Supervisors of Louisiana State University and Agricultu
Davis Bonnie J.
Nguyen Thong
Runnels John H.
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