Optical system alignment system and method with high accuracy and simple operation #>O+!IH
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Abstract 7o$S6Y;c4
A system for aligning of optical components includes an interferometer and a first diffractive alignment element. A housing is used for positioning a first optical element being aligned. A detector is used for detecting fringes produced by reflections off surfaces of the first optical element. A grating pattern on the first diffractive alignment element is designed to produce a retro-reflected wavefront or a wavefront transmitted or reflected in a predetermined direction when the first optical element is in alignment. The first diffractive alignment element includes a first region for alignment of the interferometer, a second region for alignment of one surface of the first optical element, and a third region for alignment of another surface of the first optical element. The first, second and third regions can be of any shape such as circular, rectangular, triangular, or the like. 2t:CK
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Inventors: Harned, Robert D.; (Redding, CT) ; Harned, Nora-Jean; (Redding, CT) ""x>-j4
Correspondence Name and Address: STERNE, KESSLER, GOLDSTEIN & FOX PLLC ^%}PRl9
1100 NEW YORK AVENUE, N.W. -02.n}u>
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Assignee Name and Adress: ASML Holding N.V. Sj@VOW
Veldhoven <L:}u!
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Serial No.: 938954 :zsMkdU
Series Code: 10 ?5rM'O2
Filed: September 13, 2004 r<EwtO+x
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U.S. Current Class: 356/508 fU^5Dl
U.S. Class at Publication: 356/508 4x?4[J~u[
Intern'l Class: G01B 009/02 s1
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Claims Y>I9o)KR
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What is claimed is: ?HP{>l0r
1. A system for aligning of optical components, comprising: an interferometer; a reference optic; a first diffractive alignment element having a grating pattern thereon; a housing that positions a first optical element being aligned; and a detector that detects fringes produced by reflections off surfaces of the first optical element, wherein the grating pattern produces diffracted wavefronts that align the first diffractive alignment element to the reference optic and that align the first optical element. tW"s^r=95
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2. The system of claim 1, wherein the first diffractive alignment element comprises: a first region that aligns the interferometer; a second region that aligns one surface of the first optical element; and a third region that aligns another surface of the first optical element. t9[%o=N~lD
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3. The system of claim 2, wherein the first, second and third regions are generally circular. ]`TX%Qni
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4. The system of claim 3, wherein the circular regions are non-concentric. `k~w
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5. The system of claim 3, wherein the circular regions are decentered relative to an optical axis of the system. m";?B1%x
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6. The system of claim 1, wherein the first diffractive alignment element is capable of being replaced by a second diffractive optical alignment that aligns a second optical component. smat6p[
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7. The system of claim 1, wherein the first diffractive alignment element comprises a plurality of regions, each region used for alignment of a different surface of a plurality of optical components being aligned within the housing. jP"yG#
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8. The system of claim 7, wherein at least one of the regions is used for alignment of an aspheric surface. Jb]22]
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9. The system of claim 1, wherein the plurality of regions correspond to a plurality of surfaces of a multi-element lens being aligned. 4\(|V
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10. The system of claim 1, wherein the first optical element is a reflective element. h#]LXs
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11. The system of claim 1, wherein the first optical element is a refractive element. rwY{QBSf
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12. The system of claim 1, wherein the first optical element is an off-axis optical element. z-;yDB:~t
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13. The system of claim 12, further comprising a second diffractive alignment optical element that produces interference fringes in the interferometer using a reflection off the off-axis optical element. Zie t-@}
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14. The system of claim 13, wherein the second diffractive alignment optical element is a transmissive grating. a\Dw*h?b~
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15. The system of claim 13, wherein the second diffractive alignment optical element is a reflective grating. `bcCj~j
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16. The system of claim 1, wherein the first optical component has a spherical surface. xoNn'LF#u
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17. The system of claim 1, wherein the first optical component has an aspheric surface. KhPDkD-
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18. The system of claim 1, further comprising any one of a transmission flat, a transmission sphere, or a lens between the interferometer and the first diffractive alignment element. @rxfOc0J#
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Description JbW!V Y
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CROSS-REFERENCE TO RELATED APPLICATIONS CB>O%m[1
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[0001] This application claims priority to U.S. Provisional Application No. 60/554,420, filed Mar. 19, 2004, titled "OPTICAL SYSTEM ALIGNMENT SYSTEM AND METHOD WITH HIGH ACCURACY AND SIMPLE OPERATION," which is incorporated herein by reference in its entirety. xZp`Ke!
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BACKGROUND OF THE INVENTION @4%x7%+[c
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[0002] 1. Field of the Invention :qO)^~x
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[0003] The present invention relates to alignment of optical components, and more particularly, to alignment of reflective and refractive optical components in high precision optical systems.
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[0004] 2. Related Art J5*( PxDF
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[0005] Most multiple lens assemblies are currently aligned using one (or more) of the following methods: \>(S?)6
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[0006] (a) Mechanical indicators are used for either (or both) centering the outside diameter and minimizing the apparent wedge between lens surfaces relative to the lens cell; L*Q#!_K0P
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[0007] (b) Alignment telescopes can be used for aligning centers of curvatures of the lens elements to a common optical axis; n%}Vd
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[0008] (c) Fabricating the lens elements and the lens cell to very tight optical and mechanical tolerances, so that a "slip fit" of the elements in the cell results in an aligned system; and &Un^
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[0009] (d) Coarsely assembling the lens, measuring the lens' wavefront and distortion across its field of view, and calculating the adjustments required to each lens element to minimize the wavefront error and distortion. DF~{i{
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[0010] For optical systems requiring diffraction-limited performance (as needed for lithography optics), the first three of these techniques do not have the necessary alignment accuracy. To even get close to diffraction-limited performance, state-of-the-art mechanical and optical measuring systems are required. Optimizing the alignment using measured wavefront and distortion data requires either of the first two alignment methods to be performed as a starting point. The alignment process that uses the measured wavefront and distortion data is an iterative process. Because of cross-coupling of errors in the optical system, several measurements and alignment adjustments are required to successfully align a system. The exact number of iterations required to align a system depends on the designed quality. ]^?V8*zL]
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[0011] Aligning an optical system using mechanical indicators does not account for homogeneity errors that can have the same effect as a mechanical wedge. Mechanical indicators and their related tooling (air bearing rotary tables, etc.) do not have the required accurately to align high quality optical systems, such as lithography optical systems. Because a mechanical probe or an air gauge must either be in contact, or be in very close proximity, to the lens element being aligned, there are frequently mechanical interferences with the lens cell structure. The probe is actually measuring an extremely small region on the lens surface. This region may not accurately represent the full optical surface. %2`geN<
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[0012] An alignment telescope's sensitivity is limited by the angular resolution of its optical system, the distance between the lens being aligned and the alignment telescope, and how well the alignment telescope optics are aligned. Commercially available alignment telescopes do not have the required accuracy. A custom-designed and fabricated alignment telescope has a limited range over which it can be used, because it works only for a limited range of lens radii of curvatures. This results in the need to build at least several alignment telescopes (or additional optical elements and mechanical components to an existing alignment telescope), each of which has to be aligned to tolerances close to what is required for a lithography lens. Alignment telescopes are difficult to use on short radii of curvature lens surfaces, due to the small amount of light captured by the alignment telescope aperture. Alignment telescopes are also not usable with lenses and mirrors that have aspheric surfaces. The asphericity causes the image reflected off the surface being aligned to be badly aberrated, making it impossible to achieve fine alignment tolerances. qW'5Zk
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[0013] Measuring an optical systems wavefront and distortion, and then back-calculating the alignment errors, is very time consuming and difficult, unless one starts with the optical system being relatively close to the optimum alignment condition. Multiple alignment iterations are required because of the cross coupling of the alignment aberrations between all the surfaces. 7T-}oNaJA\
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[0014] Accordingly, there is a need in the art for a fast and simple method of aligning optical surfaces. JBvP {5
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SUMMARY OF THE INVENTION #\r5Q>
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[0015] The present invention relates to an optical system alignment system and method with high accuracy and simple operation that substantially obviates one or more of the disadvantages of the related art. _^cFdP)8|
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[0016] More particularly, in an exemplary embodiment of the present invention, a system for aligning of optical components includes an interferometer and a first diffractive alignment element. A housing is used for positioning a first optical element being aligned. A detector, normally part of the interferometer system, is used for detecting fringes produced by reflections off surfaces of the first optical element. A grating pattern on the first diffractive alignment element is designed so if the element it is designed to align is in fact perfectly aligned then a "null" (or predetermined) interference pattern will be visible in the interferometer. A null (or predetermined) interference pattern indicates there is no optical path difference between the position of the optic being aligned and its ideal location in the X, Y, Z, azimuth, elevation and rotation axes. The first diffractive alignment element includes a first region for alignment of the interferometer, a second region for alignment of one surface of the first optical element, and a third region for alignment of another surface of the first optical element. The first, second and third regions can be any shape, such as circular, rectangular or some arbitrary shape. The grating pattern is designed to diffract rays so that they strike the surface being aligned at normal incidence, or at an angle that results the rays being transmitted or reflected in a particular direction. The first diffractive alignment element can be replaced by a second diffractive optical alignment for alignment of a second optical component. The first diffractive alignment element can include a plurality of regions, each region used for alignment of a different surface of a plurality of optical components being aligned within the housing. At least one of the regions is used for alignment of an aspheric surface. The plurality of regions correspond to a plurality of surfaces of a multi-element lens being aligned. The first optical element can be a reflective element or a refractive element. The first optical element can be an off-axis optical element. A second diffractive alignment optical element can produce interference fringes in the interferometer using a reflection off an off-axis optical element. The second diffractive alignment optical element can be a transmissive grating or a reflective grating. The first optical component can have a spherical surface or an aspheric surface. A transmission flat, a transmission sphere, or a lens can be between the interferometer and the first diffractive alignment element. gCc::[}\Y
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[0017] During the alignment process the fringe pattern is evaluated either visually or with an interferogram reduction program to assess the status of the alignment process. The element being aligned is adjusted until residual aberration level in the interference pattern is at an acceptable level. g7nqe~`{
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[0018] Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. u[2B0a
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