Abstract:
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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [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. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [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.  
         [0004]     2. Related Art  
         [0005]     Most multiple lens assemblies are currently aligned using one (or more) of the following methods: 
        (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;     (b) Alignment telescopes can be used for aligning centers of curvatures of the lens elements to a common optical axis;     (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     (d) Coarsely assembling the lens, measuring the lens&#39; wavefront and distortion across its field of view, and calculating the adjustments required to each lens element to minimize the wavefront error and distortion.        
 
         [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.  
         [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.  
         [0012]     An alignment telescope&#39;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.  
         [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.  
         [0014]     Accordingly, there is a need in the art for a fast and simple method of aligning optical surfaces.  
       SUMMARY OF THE INVENTION  
       [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.  
         [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.  
         [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.  
         [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.  
         [0019]     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0020]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:  
         [0021]      FIGS. 1A, 1B  and  1 C show an alignment system according to the present invention for use in lens alignment.  
         [0022]      FIGS. 2A and 2B  illustrate an alignment system according to the present invention that may be used to align a multi-element lens.  
         [0023]      FIG. 3  illustrates how a single diffractive alignment element may be used to align multiple lenses.  
         [0024]      FIGS. 4A and 4B  illustrates the use of the present invention with off-axis reflective systems. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0026]     The proposed lens alignment technique uses an interferometer and diffractive optics, with specially designed alignment zones, to align optical systems containing lenses, mirrors and diffractive optics to sub-arc-second angular and sub-micron displacement tolerances. A diffractive alignment element is written preferably using lithographic technologies on a substrate. The grating pattern that is required is easily designed using commercially available optical design programs. The actual alignment process and data analysis is the same as used when testing spherical, aspherical optics using diffractive optics.  
         [0027]      FIGS. 1A, 1B  and  1 C show an alignment system according to the present invention for use in lens alignment.  FIG. 1A  shows the overall system, and  FIGS. 1B and 1C  show exemplary diffractive elements that can be used in such an alignment system.  
         [0028]     As shown in  FIG. 1A , the following optical elements are used: an interferometer  102 , a transmission flat, or a transmission sphere or a lens  104 , a diffractive alignment element  106 , a lens housing or cell  108 , and the lens being aligned  110 .  FIG. 1  also illustrates the various beams used in the optical alignment process. Illustrated in  FIG. 1  is the test beam exiting the interferometer: A. Beam A 2  (and then B 2  and C 2  represents the optical axes for the interferometer, reference optic (transmission sphere, etc.) and diffractive alignment optics). A portion of beam A reflects off element  104  back toward the interferometer where it is used to align element  104  to the interferometer. The portion of the beam not reflected passes through the optical transmission flat  104 , becoming B. After passing through the diffractive alignment element  106 , it splits to become C 1 , such that it is perpendicular to the front surface  110 A of the lens  110 , so that it is reflected exactly back on itself. Beam C 2  functions in the same manner where it is designed to hit surface  110 B at normal incidence, so that it also reflects exactly back on itself if the lens  110  is properly aligned.  
         [0029]      FIG. 1B  illustrates how the diffractive optical element looks in a plan view. In one embodiment, the diffractive alignment element  106  can have an outer annulus  106 A, used to align the interferometer  106 . An inner annulus  106 B is used to align a concave surface, in other words, the surface  110 A. An inner region  106 C is used to align the rear surface  10 B (in this case, a convex surface).  
         [0030]     Note that the regions need not be concentric as shown in  FIG. 1B , and any number of arrangements of these regions are possible, as shown in  FIG. 1C . In the case of  FIG. 1B , the grating can be a circular grating, rather than a grating that uses parallel rulings. Note that the circles (if circles are used) need not be concentric, and may not all be concentric, and may also not all be centered in the center of the diffractive alignment element  106 . The alignment zones do not have to be concentric regions as shown in the lower left figure. The case shown in the lower right figure illustrates how the different alignment zones can be placed on different areas on the diffractive element  106 . Either of these types of designs can be developed using commercially available software. The exact pattern of the grating of a diffractive optical element  106  will depend on the parameters (size, radius of curvature, aspheric profile, etc.) of the lens  110  being aligned, the parameters of the interferometer  102 , the transmission flat  104 , and the distances between the components. One of ordinary skill in the art will readily understand how to produce such diffractive optical elements  106 , given the description herein.  
         [0031]     Thus, as described above, the diffractive optical element  106  (whether one shown in  FIG. 1B , or  FIG. 1C , or some other configuration) has several different alignment zones, or regions,  106 A- 106 C formed on it. One zone ( 106 A) is used to align the alignment element  106  to the interferometer  102 . This alignment step can be done in up to 6 axes if required. The wavefront from the interferometer alignment zone  106 A is used to align the diffractive alignment element  106  in tilt and/or location with respect to the interferometer  102 . The second alignment zone  106 B is designed to focus at the center of curvature of the front lens surface  111 A. The third alignment zone  106 C focuses at the center of curvature of the rear lens surface  110 B, taking to account the lens  110  curvature and lens  110  material thickness. The third alignment zone also takes in to account the aberrations introduced into beam C 2  by surface  110 A and the refractive index of the lens material. The shape and area of the different alignment zones  106 A- 106 C is selected based on the radii of curvature of the lens  110  (or mirror surfaces, if a reflective element is being aligned) and the alignment accuracy that needs to be achieved. The fringe pattern viewed using the interferometer detector system appears differently depending on the state of the alignment of the different surfaces. Examples of the appearance of fringe patterns that result from misalignment can be found in optics textbooks.  
         [0032]     Multi-element optical systems consisting of lenses and/or mirrors can also be aligned using diffractive alignment elements.  FIG. 2A  illustrates one arrangement that may be used to align a multi-element lens. As shown in  FIG. 2A , a lens element  112 , in this case, a second lens element, may be added to the system of  FIG. 1 . In this case, a different diffractive alignment element  106  may be used, one that is optimized to align the second lens element  112 , given its desired optical characteristics and position relative to other optical components of the system.  
         [0033]     As shown in  FIG. 2B  the diffractive alignment element  106  used in aligning the first element can be replaced by a different one (element  206 , with alignment zones  206 A,  206 B,  206 C), designed to align the second lens  112 . Alternatively, a second diffractive element can be added (not shown in the figure). The second (or different) diffractive alignment element is aligned to the interferometer same as the first element  106 , thus giving both diffractive alignment elements a common datum.  
         [0034]      FIG. 3  illustrates how a single diffractive alignment element  106  may be used to align multiple lenses. The diffractive alignment element  106  can be divided up into different regions, each of the regions used for a particular lens. For example, as shown in  FIG. 3 , some of the regions may be used for alignment of spherical, as well as aspherical components, as well as for alignment of on axis versus off-axis components. The corresponding software that interprets the interferometric fringes can be easily modified to recognize only those portions of the fringe pattern that relate to the particular lens being aligned at the moment.  FIG. 3  shows a diffractive alignment element  106  with alignment zones for multiple lens elements. This eliminates the need to replace the diffractive alignment element for each element being designed. How many lenses a single diffractive alignment element can align depends on the optical assembly&#39;s alignment tolerances.  
         [0035]     An example of how the diffractive alignment element is used with a mirror-based system is shown in  FIG. 4A .  FIG. 4B  shows an exemplary diffractive alignment element that may be used in this application. Alignment configurations for a mirror exist using both one and two diffractive alignment elements. The configuration selected depends on the accuracy requirements and the number of elements in the optical system being aligned.  
         [0036]      FIG. 4A  is illustrative of the case of off-axis reflective systems. This is regarded as a particularly difficult problem in the art. As shown in  FIG. 4A , in order to align the off-axis asphere  402 , two diffractive alignment elements may be used—the first element  106 , similar to what is shown on  FIG. 1A , and a second diffractive alignment element  404 , positioned as shown in  FIG. 4A . The general principle regarding the operation of these diffractive elements  106 ,  404  is as described above with reference to  FIGS. 1A and 2A . In this case, the beam from the interferometer  102  pass through the transmission flat  104 , When the beam reaches the diffractive alignment element  106 , part of the beam is diffracted towards the off-axis asphere  402  and part is diffracted (or transmitted) toward the second diffractive alignment element  404 . The part of the beam that is diffracted toward the off-axis aspheric mirror  402  is reflected of the mirror surface in the direction of the second diffractive alignment element  404 . This beam is then diffracted by the second diffractive element  404  back to the off-axis asphere  402 , or can be transmitted towards the first diffractive alignment element  106 . In other words, the diffractive element  404  can be either reflective or transmissive, and is usually a grating.  
         [0037]     Note that, as in the case of  FIGS. 2 and 3 , where a single diffractive alignment optical element can have multiple zones used for alignment of different components, similarly multiple off-axis components can be aligned using the same two diffractive elements  106 ,  404 , shown in  FIG. 4 , in the same manner as discussed above with reference to a multi-element lens.  
         [0038]     The present invention has the a number of advantages. Optical assemblies (lenses or mirrors) can be aligned to better accuracy than is currently achievable using mechanical and alignment telescope-based processes. Also, an optical assembly can be aligned more accurately than is currently achieved using the assembled system wavefront and distortion measurement process. This is a result of the individual elements being able to be aligned more accurately during the lens assembly integration process as compared to the standard techniques employed during the typical assembly level alignment optimization.  
         [0039]     The alignment process is much faster then either the mechanical indicating or alignment telescope process. An alignment check on a surface or element can be made in the time it takes to take a standard interferometric measurement, which is less than 10 seconds. Also, off-the-shelf interferogram reduction software can be used to analyze the interference patterns over the alignment zones in the diffractive alignment element. The interferogram reduction software can be used to determine the aberration content, which in tern can be used to calculate the required motions of the optic to bring it in to perfect alignment.  
         [0040]     On and off-axis aspheric surfaces can be aligned as easily as spherical surfaces. For aspheric surfaces the alignment zones can be designed to be the equivalent of a null-corrector so spherical wavefronts, not distorted ones, are being used during the alignment process.  
         [0041]     The alignment process can take into account the effects of lens material in-homogeneity by making alignment measurement through a lens. Axial spacing of an optical surface can be determined by measuring power in the wavefront reflected off or transmitted through the surface being aligned.  
         [0042]     The technology required for fabricating the diffractive alignment elements is well developed and readily available. Substrates the alignment-grating pattern is written on can be fabricated to exceptionally high qualities using the MRF (Magnetorheological Finishing) or CCOC (Computer Controlled Optical Surfacing) polishing process. Any number of integrated circuit reticle manufacturers can manufacture the grating pattern on the diffractive alignment element. Diffractive alignment zones can be designed using most commercially available optical design programs. The optical design programs can easily output the design in a format suitable for grating manufacturers.  
         [0043]     Having thus described a preferred embodiment of a system and method, it should be apparent to those skilled in the art that certain advantages of the described method and apparatus have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.