Abstract:
An imaging system for obtaining interferometric measurements from a sample spherical surface has a light source for providing an incident light beam, a beamsplitter disposed to direct the incident light beam toward the sample spherical surface and to direct a test light reflected from the sample spherical surface and a reference light reflected from a reference spherical surface toward an interferometric imaging apparatus. There is a lens assembly in the path of the incident light beam, with at least one lens element, wherein one of the at least one lens elements has an aspheric surface and wherein one of the at least one lens elements further provides the reference spherical surface facing the sample spherical surface. A reference plate is temporarily disposed in the path of the incident light beam for measuring the aspheric surface itself and is removable from the path of the incident light beam for obtaining interferometric measurements from the sample spherical surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 61/130,242 filed on May 29, 2008. 
     
    
     FIELD 
       [0002]    This invention generally relates to optical metrology and more particularly relates to an apparatus and method for design and use of an aspheric lens to provide a spherical waveform in a Fizeau interferometer. 
       BACKGROUND 
       [0003]    In interferometry, highly precise surface measurement is obtained for various types of optical components using interference fringes that are generated between light reflected from a reference surface and light from a surface under test. The Fizeau interferometer is one instrument of this type that is advantaged for measurement of various optical surfaces, particularly for spherical or planar surfaces having relatively large diameters. 
         [0004]    The schematic block diagram of  FIG. 1  shows components of a conventional Fizeau interferometer  10 . A laser  12  or other highly coherent light source directs light through a beamsplitter  14  and towards a test surface  20 , the sample surface to be measured, as well as toward a reference surface provided within Fizeau optics  18 . A collimator  16  and Fizeau optics  18  condition the path of light directed toward the target and reference surfaces and bend this light toward the proper angles for the surface being measured. The optical path of target and reference beams is identical through Fizeau optics  18  and collimator  16 . Beamsplitter  14  then redirects the returned light to an interference pattern imaging apparatus  24  for display and analysis. 
         [0005]    Fizeau interferometry, using the overall pattern of  FIG. 1 , is advantaged as a method for the interferometric surface inspection of precise spherical and nearly spherical surfaces because the Fizeau reference surface and the surface under test are within close proximity of each other. This reduces the likelihood of optical path disparities and helps to reduce retracing errors experienced by the light reflecting from the Fizeau reference surface and from the surface under test. Ideally, the interference that occurs during Fizeau interferometry only includes known errors on the Fizeau reference surface and errors on the surface under test, because all other sources of wavefront error are common to both paths. 
         [0006]    The schematic diagram of  FIG. 2  shows a function of Fizeau optics  18  for accurate surface measurement. Incoming light at L  1  is substantially planar, but must be redirected so that it arrives as spherical light at test surface  20 , a spherical surface. Under the desired conditions for interferometry, as shown at enlargements E 1  and E 2 , the light heading toward test surface  20 , traced along exemplary rays R 1 , R 2 , R 6 , and R 8  in  FIG. 2 , has a specific angular relationship to a reference surface  22 . At any point along reference surface  22 , this light that is directed toward the sample surface-under-test, that is, toward test surface  20 , exits at a normal to surface  22 . Reference surface  22  is also termed the Fizeau reference surface. In addition, when test surface  20  has the proper shape, the returning light, as test light, follows the exact same path and is incident on reference surface  22  at a normal. Reference light L 2  that is reflected back from Fizeau reference surface  22  is also returned along the same path as the returning test light. 
         [0007]    Fizeau objectives that deviate from perfect sphericity will cause the reference and test optical paths to vary slightly from one another upon return through the optical system, creating propagation errors. The greater the deviation of the wavefront from sphericity the greater the propagation error. Also, errors of larger slope, that is, of higher spatial frequency, will also cause similar propagation errors. 
         [0008]    Complex optics designs are typically used to create a nearly perfect spherical wavefront from an incoming planar wavefront and to allow the spherical wavefront to be nearly perfectly normal to the Fizeau reference surface as described with reference to  FIG. 2 . For example, to bend the light to an appropriate angle θ, as shown in  FIG. 2 , a number of conventional designs provide Fizeau objectives with as many as 4 or 5 or more lens elements, often where the numerical aperture is 0.5 or greater. Conventional solutions have not provided Fizeau interferometry systems that feature reduced parts count, smaller size, lower weight, and reduced cost, at the same time. At best, conventional solutions typically address one of these factors at the expense of others. For example, the use of one or more diffractive optical elements (DOE) has been proposed as one way to simplify lens design and reduce parts count. However, due to the relatively high cost and complexity of DOE device design itself, little or no cost advantage is obtained using this approach. As is well known to those familiar with optics fabrication, the complex lens optics conventionally used for Fizeau interferometry require considerable expense and skill in manufacturing and assembly. Even the slightest errors in surface quality, thickness, radius, and alignment can have a significant effect on the measurement accuracy of these optical assemblies. 
         [0009]    The use of an aspheric lens component is one solution that has been advanced for reducing the number of lens elements in the Fizeau optics. For example, commonly assigned U.S. Pat. No. 5,797,493 entitled “Interferometer with Catadioptric Imaging System Having Expanded Range of Numerical Aperture” to Vankerkhove disclosed the use of one or more aspheric lenses in the optical path of a Fizeau interferometer that has refractive elements and a curved reflective surface for beam direction. Similarly, U.S. Pat. No. 7,342,667 entitled “Method of Processing an Optical Element Using an Interferometer Having an Aspherical Lens That Transforms a First Spherical Beam Type into a Second Spherical Beam Type” to Freimann et al. discloses the use of one or more aspheric lenses in a Fizeau interferometer that uses refractive elements. 
         [0010]    Although aspheric lenses are known to offer certain advantages, however, there can be practical hurdles that complicate their deployment or diminish their usefulness in various different applications. The need for precision fabrication, testing, and validation of the aspheric surface is a widely recognized problem to the optics designer and can present complex difficulties that are not easily or inexpensively addressed. In the &#39;667 Freimann et al. patent, for example, a second interferometer apparatus is used in order to characterize or calibrate the aspheric lens for its use in a first interferometer apparatus. Design and use of a special-purpose second interferometer as a test fixture for using an aspheric lens in a first interferometer is a costly solution that adds time and complexity to interferometer manufacture, substantially eroding many of the potential advantages of using an aspheric lens in the first place. 
         [0011]    Thus, it can be seen that there would be significant advantages to apparatus and methods for inexpensively testing and using an aspheric lens, thereby reducing parts count, size, and complexity of Fizeau interferometry optics. 
       SUMMARY 
       [0012]    It is an object of the present invention to advance the art of optical measurement instrumentation. With this object in mind, the present invention provides an imaging system for obtaining interferometric measurements from a sample spherical surface, comprising: 
         [0013]    a light source for providing an incident light beam; 
         [0014]    a beamsplitter disposed to direct the incident light beam toward the sample spherical surface and to direct a test light reflected from the sample spherical surface and a reference light reflected from a reference spherical surface toward an interferometric imaging apparatus; 
         [0015]    a lens assembly in the path of the incident light beam and comprising at least one lens element, wherein one of the at least one lens elements has an aspheric surface and wherein one of the at least one lens elements further provides the reference spherical surface facing the sample spherical surface; and 
         [0016]    a reference plate that is temporarily disposed in the path of the incident light beam for measuring the aspheric surface itself and that is removable from the path of the incident light beam for obtaining interferometric measurements from the sample spherical surface. 
         [0017]    It is a feature of the present invention that it employs an aspherical surface within the objective lens of Fizeau interferometer optics. 
         [0018]    It is an advantage of the present invention that it provides interferometry optics solutions capable of having reduced parts count. 
         [0019]    These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein: 
           [0021]      FIG. 1  is a block diagram of a Fizeau interferometer; 
           [0022]      FIG. 2  is a schematic block diagram that shows the path of light at the reference surface of Fizeau interferometer optics; 
           [0023]      FIG. 3  is a block diagram showing a Fizeau optical assembly having aspheric and spherical lenses; 
           [0024]      FIG. 4  is a block diagram showing a Fizeau optical assembly having a single aspheric lens; 
           [0025]      FIG. 5A  is a schematic diagram showing a test setup for measuring the aspheric Fizeau objective in a single-lens embodiment; 
           [0026]      FIG. 5B  is a schematic diagram showing a test setup for measuring the aspheric Fizeau objective in a multi-lens embodiment; 
           [0027]      FIG. 6A  is a close-up of optical elements in the test setup of  FIG. 5A ; 
           [0028]      FIG. 6B  is a close-up of optical elements in the test setup of  FIG. 5B ; and 
           [0029]      FIG. 7  is a schematic block diagram showing a Fizeau interferometer according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. For example, conventional interferometer beam-generating, beamsplitting, and collimation optics, and techniques for handling and interpreting the interference patterns obtained from test and reference light are well known to those skilled in the optical metrology art and, unless related to changes effected by methods and apparatus of the present invention, are not described herein. 
         [0031]    Figures shown and described herein are provided in order to illustrate key principles of operation and component relationships along their respective optical paths according to the present invention and are not drawn with intent to show actual size or scale. Some exaggeration may be necessary in order to emphasize basic structural relationships or principles of operation. Some conventional components that would be needed for implementation of the described embodiments, such as various types of optical mounts, for example, are not shown in the drawings in order to simplify description of the invention itself. In the drawings and text that follow, like components are designated with like reference numerals, and similar descriptions concerning components and arrangement or interaction of components already described are omitted. 
         [0032]    A distinction may be made between spherical and planar with respect to light provided in various embodiments and with respect to surface shapes of optical components. However, it is known that a planar light can be considered to be a type of spherical light, that is, spherical light having a radius of curvature that is considered to be at infinity. 
         [0033]    The term “oblique angle”, as used in the present disclosure, describes an angle that is other than normal, that is, other than an integral multiple of 90 degrees. 
         [0034]    Embodiments of the present invention provide the light bending (to angle θ, as shown in  FIG. 2 ) that is required for Fizeau interferometry using a lens element that has an aspheric surface and a spherical surface, singly, or in combination with other lens elements. In addition, embodiments of the present invention provide a mechanism for facilitating testing and validation of the aspheric surface without requiring that a separate test fixture be designed and fabricated. 
         [0035]    Referring to the schematic diagram of  FIG. 3 , an embodiment of Fizeau optics  18  has a lens assembly that redirects an incident collimated light from collimator  16  toward a focal point, shown as target t, at angle θ. In this embodiment, a lens element  30  in the lens assembly has an aspherical surface at surface  38  and a surface  39  that is spherical. A spherical lens element  32 , has surfaces  31  and  33 . Lens element  33  provides the reference spherical surface that faces the sample spherical surface for Fizeau measurement. This embodiment uses only two optical elements and has a numerical aperture of 0.5, providing an f/1.0 Fizeau objective. It should be observed that there are other embodiments in which the aspherical surface is one of the other surfaces  31  or  39  of the lens assembly. 
         [0036]    It can be appreciated that there would be advantages to embodiments that use a single aspheric lens, rather than using an aspheric lens in combination with one or more spherical lenses. The additional optics may be necessary, however, in order to increase the overall cone angle (shown in these figures as angle θ) when the desired angle cannot be achieved using a singlet solution. 
         [0037]    There can be, however, applications in which a singlet solution is possible.  FIG. 4  shows an alternative Fizeau optics arrangement using a single aspherical lens element  40 . Lens element  40  receives the collimated light at its aspheric surface  42  and directs the focused light from a spherical surface  44  toward the test target at the needed normal angle described earlier with reference to  FIG. 2 . A single-element design of this type has a numerical aperture of 0.21 and provides an f/2.3 Fizeau objective. 
         [0038]    The aspheric solutions shown in  FIGS. 3 and 4  reduce the parts count, hence size and weight, of Fizeau optics and can provide improved performance over standard designs that use spherical lenses. However, as is well known to those skilled in the optical design arts, the jobs of testing and validation of an aspheric surface, at the accuracy needed, can present significant challenges. In conventional practice, for example, specially designed correction optics such as reflective, refractive, or diffractive “null” lenses or null correctors are typically used for asphere testing and a separate test fixture, such as a separate interferometer, is used. The development of specialized lenses and testing equipment, however, can add considerably to the time and expense required for asphere fabrication. 
         [0039]    The apparatus and methods of the present invention address the problem of aspheric use and testing for an interferometer by providing a way to test the aspheric surface directly in the interferometer itself. That is, in embodiments of the present invention, components of the Fizeau objective can advantageously be used as the test fixture apparatus for test of the asphere surface of lens element  30  ( FIG. 3 ) or of lens element  40  ( FIG. 4 ). Moreover, this same test apparatus arrangement can also be used to measure other problems that exist in the optical path. 
         [0040]    Referring to  FIG. 5A , there is shown a test apparatus  50  that is formed by adding a planar reference plate  48  to the interferometer system described earlier. Components shown outside the dashed box are those parts of the interferometer that provide incident collimated light that is directed toward the test surface and read back the test and reference light signals in conventional use. A mount  54  is added to the interferometer components, provided for temporarily seating reference plate  48  in position as part of this test setup. Reference surface  48 , slightly wedge-shaped as shown in the enlarged view of  FIG. 6A , is positioned between the aspheric lens element of Fizeau optics  18  and the source of collimated light from upstream optics in the interferometer. The wedge shape provides a slight tilt to a surface  51 , the first surface that receives incident collimated light, so that this surface is oblique to the incident light and deflects unwanted back-reflection from this surface away from the signal path; any detected back reflection from this surface could interfere with the signals of interest. 
         [0041]    For measurement of aspheric surface  42  using test apparatus  50  in  FIGS. 5A and 6A , an interference pattern is obtained by a combination of the reference reflected light (Ref) reflected back from a reference surface  52  with test reflected light (Test) that is reflected back from spherical surface  44 . 
         [0042]      FIGS. 5B and 6B  show the similar test apparatus  50  arrangement that is used for a more complex embodiment in which Fizeau lens  18  has one or more lens elements in addition to the aspherical lens, shown in these examples as lens  30 . It can be appreciated that this embodiment adds another level of testing capability, since the optical path of the test reflected light includes the additional lens surfaces and, therefore, allows correction or compensation for problems caused by imperfections in these surfaces or their materials. For example, the test setup of  FIG. 6B  can help to measure other optical factors in addition to aspheric curvature, such as inhomogeneity of the lens material for either lens, surface contour, lens radii, lens center thickness, and airspace errors between lens elements. 
         [0043]    As with other interferometry methods, the arrangements shown in  FIGS. 5A-6B  eliminate the need to profile aspheric surface  42  or, for alternate embodiments, the appropriate aspheric surface  31 ,  38 , or  39  directly, such as using contact profile methods. Instead, this measurement method uses the handling of light by lens element  18  and spherical waves as well as the support image processing components of the interferometer itself to ascertain surface shape and quality for the aspherical surface as it is being formed and finished. An iterative process of shaping and testing can then be followed to achieve the proper aspherical surface that is needed, including providing a surface that corrects for other defects in the Fizeau optics, as described with reference to the FIG.  5 B/ 6 B embodiment. Reference plate  48  can then be removed from mount  54  once Fizeau lens  18  is suitably formed and is ready for use in interferometry. 
         [0044]    This straightforward method for aspheric testing can then be used in conjunction with any of a number of suitable techniques for asphere fabrication, including, for example, magnetorheological finishing (MRF), computer-controlled polishing, and ion beam figuring. These techniques, known to those skilled in the optical fabrication and metrology arts, allow deterministic forming and finishing of a high-quality aspheric surface. 
         [0045]    Referring to  FIG. 7 , the interferometer  10  apparatus of the present invention allows one or more aspheric lenses  30 ,  40  to be used directly as the Fizeau lens  18  of the interferometer or at another suitable point along the optical path for measuring the sample spherical surface of test surface  20 . Embodiments of the present invention further allow testing of the aspheric surfaces of lenses  30 ,  40  to help in surface fabrication. 
         [0046]    Aspheric lens elements themselves can be formed from any suitable material, such as various types of glass or polymer. In general, high-index optical glasses are well suited for use as the aspheric lens. 
         [0047]    Any of a number of suitable finishing techniques can be used in final surface preparation of the Fizeau optics of the present invention. For example, finishing processes can include polishing or application of a coating, such as a partially reflective coating, anti-reflective coating, or protective coating. 
         [0048]    The interferometry apparatus of the present invention is capable of measurement for planar and spherical surfaces and is particularly well-suited for measurement applications of larger diameter lenses. As was noted earlier, the planar surface can be considered to be a special type of spherical surface, for the case in which the radius of curvature is effectively at infinity. 
         [0049]    Unlike earlier solutions that require fabrication of a separate interferometer for testing and validating the aspheric surface, the method and apparatus of the present invention use the interferometer itself as the “test fixture” needed for aspheric surface testing. The only change required to interferometer components is the addition of reference plate  48  to the optical path, whether the reference plate is provided a temporary mounting bracket or fixture within the interferometer for this purpose, or whether an external mechanical device is temporarily provided for positioning the reference plate appropriately while the aspheric surface is checked. That is, the reference plate can be mechanically pivoted or otherwise disposed in place as needed, without the requirement for a separate mount  54 . 
         [0050]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, light source  12  can be a laser or laser diode, other solid-state light source that provides an incident light beam. Collimator  16  is an optional component and can be formed from a single lens element or from multiple refractive or reflective optical components. Any of a number of different types of lens mounts or brackets could be used as mount  54 , including those that would allow reference plate  48  to be pivoted or otherwise temporarily moved into place in the optical path from outside the interferometer during device manufacture, so that no additional built-in fixture is needed in the interferometer hardware. 
         [0051]    Thus, what is provided is an apparatus and method for design of an aspheric lens for providing a spherical waveform in a Fizeau interferometer.