Abstract:
A method involves forming a first surface on a substrate, applying to the first surface a layer of a material having a thickness less than approximately 10 microns, and precision polishing the layer of material to form a precision optical second surface on a side of the layer opposite from the substrate. A different aspect involves an apparatus that includes a substrate having a first surface, and a layer provided on the surface and having a thickness less than approximately 10 microns, the layer having on a side thereof opposite from the substrate a polished second surface with an RMS surface roughness less than approximately 10 Angstroms.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to optical surfaces and, more particularly, to fabrication of precision optical surfaces. 
       BACKGROUND 
       [0002]    Optical parts are often fabricated by machining a substrate such as aluminum, including machining an optical surface on the substrate. These types of optical parts may, for example, be used as precision mirrors for long-range telescopes, multi-band imagers, military or commercial surveillance systems, targeting systems, laser designator systems, or other types of systems. 
         [0003]    The performance and thus value of many optical systems is often limited in whole or in part by the accuracy and roughness of the optical surface on such an optical part. Existing techniques produce, at best, an optical surface with an RMS roughness of approximately 30 Angstroms. One effect of this degree of surface roughness is that, while existing systems may be used in the infrared (IR) range and, more recently, in the visible range, they are typically not suitable for use in the ultraviolet (UV) range. Also, some approaches require overcoat layers of significant thickness, but this can cause bi-material deformation in response to thermal changes. Accordingly, while existing techniques for fabricating optical surfaces have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which: 
           [0005]      FIG. 1  is a diagrammatic view, partly in section, showing an apparatus that includes an optical substrate and a diamond point turning (DPT) machine. 
           [0006]      FIG. 2  is a diagrammatic fragmentary sectional side view showing in a greatly enlarged scale a very small portion of the substrate of  FIG. 1  and an optical surface thereon, after completion of machining of the surface by the DPT machine of  FIG. 1 . 
           [0007]      FIG. 3  is a diagrammatic fragmentary sectional side view showing the substrate after the optical surface has been DPT-machined and cleaned, and showing an electroless plating apparatus that includes a container with the substrate and an electroless plating solution therein. 
           [0008]      FIG. 4  is a diagrammatic fragmentary sectional side view showing the substrate after the electroless plating forms an alloy layer on the optical surface, and showing a conventional oven having the substrate therein. 
           [0009]      FIG. 5  is a diagrammatic view, partially fragmentary and in section, showing the plated substrate after heat treatment in the oven of  FIG. 4 , and showing a metrology apparatus that is a full-surface optical interferometer capable of very accurately measuring the outer surface of the alloy layer. 
           [0010]      FIG. 6  is a diagrammatic view, partially fragmentary and in section, showing the plated substrate and showing a precision polishing apparatus that is a magnetorheological finishing (MRF) machine capable of very accurately polishing the surface of the alloy layer. 
           [0011]      FIG. 7  is a diagrammatic fragmentary sectional side view showing the substrate after completion of precision polishing of the surface on the alloy layer using the metrology apparatus of  FIG. 5  and the precision polishing apparatus of  FIG. 6 . 
           [0012]      FIG. 8  is a diagrammatic view, partially fragmentary and in section, showing a thin-film coating apparatus having therein the substrate of  FIG. 7  with two thin-film coatings on the polished surface. 
           [0013]      FIG. 9  is a flowchart summarizing the process depicted in  FIGS. 1 to 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  is a diagrammatic view, partly in section, of an apparatus  10  that includes an optical substrate  12  and a diamond point turning (DPT) machine  13 . In the disclosed embodiment, the substrate is made from  6061  aircraft-grade aluminum, but it could alternatively be made from any other suitable material. In  FIG. 1 , the substrate  12  is depicted before any machining by the DPT machine  13 . The substrate  12  as shown in  FIG. 1  is obtained by taking a solid block of material, and pre-machining the block using known techniques to obtain the illustrated starting shape, which is approximately its final shape. This pre-machining includes formation of an optical surface  16  and an optional annular reference surface  17 , where the surface  16  is approximately semi-spherical, and the reference surface  17  extends around the periphery of the surface  16 . The optical surface  16  could alternatively have any other desired shape, and could for example be flat, or irregular. 
         [0015]    The pre-machining also includes formation in a known manner of a number of recesses  21  in a side of the substrate  12  opposite from the surface  16 . The creation of the recesses  21  is commonly referred to as “light-weighting”. In order to avoid excessive stress within the substrate  12 , formation of the light-weighting recesses  21  is carried out progressively, through a number of iterations. During each iteration, some material of the substrate  12  is removed, and then the substrate  12  is annealed. This ensures that the substrate  12  will be stable over time, and does not have residual stresses that might later cause deformation of the resulting optical part, notwithstanding variations in temperature or other environmental conditions. At the end of the pre-machining procedure, the substrate  12  should be very close to its final net shape, in order to minimize the processing required in later stages, enhance the performance of the final optical part, and reduce costs. 
         [0016]    After pre-machining is completed, the substrate  12  is mounted in the DPT machine  13 . The DPT machine  13  is itself a conventional device, and includes a DPT tool  31 . The DPT machine  13  includes a part support and movement section  33  that supports the substrate  12 , and also moves the substrate relative to the tool  31 , for example by rotating the substrate about a not-illustrated axis. The DPT machine  13  also includes a tool control section  34  that effects movement of the DPT tool  31  in relation to the substrate  12 . In particular, the tool control section  34  can selectively move the tool  31  in two or three orthogonal directions with respect to the substrate. The DPT machine  13  is used to carry out single-point diamond turning (SPDT) of the optical surface  16 , and also any optional reference surface  17  that may be present. The SPDT machining operation will reduce the surface roughness of the surfaces  16  and  17 , for example so that the root mean square (RMS) surface roughness is in the range of 60 to 100 Angstroms. The smaller the surface roughness, the better. Factors that contribute to the residual roughness include microscopic imperfections in the aluminum material, and “tearout” of aluminum material caused by the DPT tool  31 . In the disclosed embodiment, state-of-the-art SPDT techniques known in industry are used with the goal of achieving an RMS surface roughness near the lower end of the 60 to 100 Angstrom range, in order to minimize the processing required in later stages, enhance the performance of the final optical part, and reduce costs. 
         [0017]    For clarity, the discussion that follows will typically refer only to the surface  16 , but it should be understood that if the optional reference surface  17  is also present, then the surface  17  will be treated in the same manner as the surface  16 .  FIG. 2  is a diagrammatic fragmentary sectional side view of a very small portion of the substrate  12  of  FIG. 1  with the surface  16  thereon, in a greatly enlarged scale and after completion of SPDT machining of the surface  16  by the DPT machine  13 . The roughness of the surface  16  is visible in  FIG. 2 , and is within the above-mentioned range of 60 to 100 Angstroms RMS. Due to the degree of enlargement from  FIG. 1  to  FIG. 2 , the curved surface  16  of  FIG. 1  appears to be almost flat in  FIG. 2 . 
         [0018]    After completion of the DPT machining, the surface  16  is cleaned in a known manner in preparation for electroless plating, using a series of cleaning chemicals such as bases and acids. The acids remove scaling, and other chemicals remove oils. Application of each pre-treatment chemical is followed by two to three water rinses, for example with de-ionized water, in order to remove any of the chemical that happens to be adhering to the surface  16 . 
         [0019]      FIG. 3  is a diagrammatic sectional and fragmentary side view showing an electroless plating apparatus that includes a container  51  having therein an electroless plating solution  52  and the DPT-machined substrate  12  of  FIG. 2 , after the above-described cleaning of the surface  16 . The electroless plating solution  52  of  FIG. 3  is known in the art, and carries out an auto-catalytic chemical process that deposits a layer  56  of nickel-phosphorus alloy on the DPT-machined surface  16  of the substrate  12 . As is known in the art, this process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO 2 H 2 .H 2 O). The reducing agent reacts with metal ions to deposit the nickel-phosphorus alloy. Unlike electroplating, it is not necessary to pass an electric current through the solution in order to form the layer  56 . The plating process is carefully controlled in a known manner so as to ensure purity of the deposited alloy layer  56 , and avoid the presence of gas bubbles trapped within the layer. 
         [0020]    At the end of the electroless plating process, the alloy layer  56  will have a thickness in the range of approximately 1 to 10 microns, and more typically within a range of 2 to 3 microns, but the thickness could be larger or smaller. Due to the fact that the layer  56  is relatively thin, the outer surface of this layer tends to conform to the shape or roughness of the surface  16 . 
         [0021]    The as-deposited electroless nickel-phosphorus layer  56  is typically too soft to facilitate good polishing. Therefore, according to the disclosed process, the layer  56  is annealed in order to harden it before subsequent polishing. In this regard,  FIG. 4  is a diagrammatic sectional and fragmentary side view showing the plated substrate  12  within a conventional oven  71 . In the disclosed embodiment, the temperature within the oven is progressively raised from ambient or room temperature to approximately 750° F. over a 45-minute time interval, and then is maintained at 750° F. for a time interval of about one hour. This hardens the alloy layer  56  on the substrate  12 . 
         [0022]    After the heat treatment, the plated substrate  12  is allowed to cool, and is removed from the oven  71 . The alloy layer  56  is then subjected to precision polishing that removes much of the layer  56 . This precision polishing involves an iterative process, where each iteration includes use of precision metrology followed by an advanced technique of precision polishing. At the start of this precision polishing, the surface  83  should have less than a few fringes of light error in form, or else the precision polishing may introduce higher spatial frequency form errors, while removing lower spatial frequency form errors typically present in the machined surface. The roughness of the surface  83  should be less than a maximum of about 0.1 microns, and typically about 0.006 to 0.01 microns (60 to 100 Angstroms), or else the surface may not become suitably smooth during the precision polishing process. 
         [0023]      FIG. 5  is a diagrammatic view, partially fragmentary and in section, showing the plated substrate  12  and a metrology apparatus that, in the disclosed embodiment, is a full-surface optical interferometer  81  of a known type capable of very accurately measuring the outer surface  83  of the alloy layer  56 . In the disclosed embodiment, the interferometer  81  is a model GPI XP/D available commercially from Zygo Corporation of Middlefield, Conn. Alternatively, however, it would be possible to utilize any other suitable metrology apparatus. 
         [0024]    The interferometer  81  is supplied with a computer file containing a definition  86  of the ideal shape of the final optical surface desired on the alloy layer  56 . The interferometer  81  measures the actual surface  83  on the alloy layer  56 , compares it to the surface definition  86 , and outputs a computer file containing measurement data  87  that indicates how the alloy layer  56  should be machined in order to bring the actual surface  83  into closer conformity with the ideal surface definition  86 . 
         [0025]      FIG. 6  is a diagrammatic view, partially fragmentary and in section, showing the plated substrate  12  and a precision polishing apparatus of a known type that is a magnetorheological finishing (MRF) machine  101 . In the disclosed embodiment, the MRF machine  101  is a model Q22Y machine available commercially from QED Technologies of Rochester, N.Y. However, it would alternatively be possible to use any other suitable precision polishing machine. The machine  101  is known in the art and is therefore discussed here only briefly, and not in detail. 
         [0026]    The machine  101  includes a rotating spherical wheel  106 . A polishing slurry  107  is applied to the surface of the wheel  106 , and is entrained and travels over the top of the wheel as the wheel rotates. The slurry  107  contains abrasive particles, for example a diamond powder. The machine  101  supports the plated substrate  12  in an inverted orientation just above the rotating wheel  106 , and can move the substrate with two or three degrees of freedom relative to the wheel. This multi-axis control allows non-symmetrical (free-form) surface shapes to be accurately polished, in addition to spherical or other rotationally-symmetric forms. The moving slurry  107  engages and polishes the surface  83  on the alloy layer  56 . The machine  101  receives the measurement data  87  from the interferometer  81  of  FIG. 5 , and moves the substrate  12  in relation to the rotating wheel  106 , so as to accurately polish the surface  83  in a manner specified by the measurement data  87 . More material will be removed in some regions than in other regions, based on the measurement data  87 . As discussed above, the precision polishing process is an iterative process, where each iteration involves precision measurement using the interferometer  81  of  FIG. 5 , followed by advanced polishing using the MRF machine  101  of  FIG. 6 . 
         [0027]      FIG. 7  is a diagrammatic fragmentary sectional side view showing the substrate  12  with the polished alloy layer  56  thereon, after completion of the precision polishing process. In particular, the surface  83  on the alloy layer  56  has been polished until it is very smooth, and conforms very closely to the ideal surface definition  86  ( FIG. 5 ). In the disclosed embodiment, the polished alloy layer  56  will have a thickness less than approximately 10 microns, typically less than 5 microns, and more typically less than 1 micron. The polished surface  83  will have an RMS roughness less than approximately 60 Angstroms, typically less than 30 Angstroms, and more typically less than 10 Angstroms, (or even less if intended for use with ultraviolet radiation). Any error in shape (form error) will be a fraction of the 0.6 micron wavelength of the red light typically used for interferometric testing. This error is on the order of 1/10 of a wavelength, or in other words less than about 0.06 microns. Consequently, the surface  83  is sufficiently smooth that it can be accurately used not only for visible and infrared radiation, but also for shorter wavelengths such as ultraviolet radiation. 
         [0028]    After the precision polishing, the alloy layer  56  is extremely thin, and is intrinsically interlocked into the surface roughness of the surface  16  on the aluminum substrate  12 . This avoids the potential for a bi-material bending effect in response to temperature changes or other environmental variations. 
         [0029]      FIG. 8  is a diagrammatic view, partially fragmentary and in section, showing the substrate  12  supported within a thin-film coating apparatus  131  of a known type. The coating apparatus  131  is used to first apply a thin-film reflective layer  141 . The reflective layer  141  could optionally be a multi-layer coating but, for purposes of the present discussion, it is assumed that the reflective layer  141  involves only a single layer. In the disclosed embodiment, the reflective layer  141  is one of gold, aluminum or silver. Gold works well in the infrared range. Aluminum works well in the visible range (except it is slightly less reflective to wavelengths just below 1 micron). Silver has a flatter response than aluminum, but is not as reflective in the shorter visible wavelengths (such as blue light). Alternatively, however, and depending on the intended application, the reflective layer  141  could alternatively be made from any other suitable material. 
         [0030]    If the reflective layer  141  is made from gold, it will have a thickness within a range of 70 to 200 nm, and more typically within a range of 80 to 100 nm. Alternatively, if the reflective layer  141  is made of silver, it will have a thickness within a range of 90 to 200 nm, and more typically within a range of 100 to 120 nm. As still another alternative, if the reflective layer  141  is made of aluminum, it will have a thickness within a range of 50 to 200 nm, and more typically within a range of 100 to 120 nm. 
         [0031]    After application of the reflective coating  141 , the thin-film coating apparatus  131  (or some other suitable coating apparatus) is used to form a thin-film protective layer  146  over the reflective layer  141 . The protective layer  146  could optionally be a multi-layer coating but, for purposes of the present discussion, it is assumed that the protective layer  146  involves only a single layer. In the disclosed embodiment, the protective layer  146  is thin in comparison to the range of wavelengths at which it will be used. The protective layer  146  may, for example, be made of zinc sulfide or silicon dioxide, or any other suitable material. If the reflective layer  141  is made of gold, it may be advantageous to omit the protective layer  146 , provided that the gold layer  141  can be suitably protected from the environment in which it will be used. The reflective layer  141  and the protective layer  146  (if present) provide desired spectral performance, mechanical performance, and/or environmental durability performance. 
         [0032]    The thickness of the protective layer  146  may be a function of the material from which it is made, and also a function of the range of wavelengths that it is to reflect. For example, if the protective layer  146  is made of zinc sulfide and is to reflect wavelengths in the visible range, it may have a thickness of approximately 100 nm. Alternatively, if the protective layer  146  is made of silicon dioxide and is to reflect wavelengths in the visible range, it may have a thickness of approximately 170 nm. 
         [0033]      FIG. 9  is a flowchart summarizing the process discussed above in association with  FIGS. 1 to 8 . The process begins in block  201 , and proceeds to block  202 , where the substrate  12  is pre-machined to the pre-DPT shape shown in  FIG. 1 , including the iterative light-weighting and stress relief to form the cavities  21 , and including the formation of the surfaces  16  and  17 . Next, in block  203 , the surfaces  16  and  17  are subjected to the SPDT machining operation using the DPT machine  13  of  FIG. 1 . Then, in block  206 , the DPT-machined surfaces  16  and  17  are subjected to the pre-treatment that cleans them in preparation for electroless plating. 
         [0034]    Next, in block  207 , the thin nickel-phosphorus alloy layer  56  is formed on the surfaces  16  and  17  of the substrate using the electroless plating process depicted in  FIG. 3 . Then, this alloy layer is subjected to the heat treatment that hardens it, as shown in  FIG. 4 . Next, the alloy layer  56  is subjected to the precision polishing involving the iterative process that alternates precision metrology according to  FIG. 5  with advanced polishing according to  FIG. 6 . Thereafter, in block  212 , the thin-film reflective layer  141  is applied to the polished surface  83  using the coating apparatus  131  of  FIG. 8 . Then, in block  213 , the thin-film protective coating  146  is optionally applied over the reflective coating  141  using the coating apparatus  131  of  FIG. 8 . The process then concludes at block  216 . 
         [0035]    Although a selected embodiment has been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.