Abstract:
A mirror includes: a substrate having a surface; and a thin-film finish layer provided over the surface of the substrate and having on a side thereof opposite from the substrate a surface with a polished finish. According to a different aspect of the invention, a method of making a mirror includes: providing a substrate having a surface thereon; forming a thin-film finish layer over the surface of the substrate using thin-film techniques, the finish layer having a surface on a side thereof opposite from the substrate; and polishing the surface of the finish layer.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates in general to mirrors and, more particularly, to the structure and fabrication of high precision mirrors.  
         BACKGROUND OF THE INVENTION  
         [0002]    There are a variety of optical systems in which high precision mirrors are needed. Examples include ultraviolet lithography condenser mirrors, reflective laser scanner mirrors, exo-atmospheric reflecting optics, systems in which low scattering is needed for visible radiation, and a variety of other applications.  
           [0003]    One existing technique for making precision metal mirrors is to take a substrate of an aluminum alloy, such as that commonly known in the art as aluminum 6061-T6, and to carry out single point diamond turning (DPT) of a surface on the substrate, which then serves as the reflective surface. This alloy is lightweight, is easily machined by DPT, and has good long-term stability. Unfortunately, however, this aluminum alloy contains alloy elements such as zinc, chromium and iron, which leave defects or artifacts after DPT that effectively limit the surface finish achievable with DPT to approximately 80 Angstroms RMS. A surface finish of this quality provides adequately low scatter for many applications in which the reflected radiation of interest has a relatively long wavelength, such as a wavelength greater than 3 microns. At shorter wavelengths, however, such as the wavelength of visible radiation, a surface finish of 80 Angstroms RMS produces a level of scatter which is too high for many applications.  
           [0004]    In attempt to address this problem, one prior technique took the approach of plating electroless nickel over the DPT surface on the Aluminum 6061-T6 substrate, subjecting this nickel layer to DPT, and then post-polishing the nickel layer to improve the surface finish. This alternative approach had several different disadvantages.  
           [0005]    In particular, for all such mirrors, there is an inherent mismatch between the coefficient of thermal expansion (CTE) for the aluminum alloy substrate and the CTE for the plated nickel layer. Consequently, and due to the thickness of the nickel layer, there can be a bimetallic effect between the substrate and nickel layer, which causes bending of the mirror surface across the operational temperature range of the mirror. Such bending is undesirable in the context of a high precision mirror surface, because it changes optical characteristics of the mirror surface. In order to keep such bimetallic bending to an acceptable level over normal temperature variations, the plated nickel has to be a thin and uniform layer with a thickness of approximately 0.0005 inch to approximately 0.001 inch. In order to achieve this thin and uniform layer, the above-mentioned DPT operation was needed, in addition to the DPT operation for the aluminum. The need for this second DPT operation added cost and cycle time to the fabrication process.  
           [0006]    An alternative technique uses electro-deposited high purity aluminum plating instead of the nickel plating, in order to avoid the thermal mismatch. This process creates a hard aluminum oxide (sapphire) outer layer on the electro-deposited aluminum layer, and this oxide layer damages the diamond tool during cutting. In addition, this high purity aluminum is very soft, and tends to build up on the diamond tool during cutting, which makes the DPT operation difficult. Also, the soft aluminum surface is easily scratched, and difficult to clean. Using this approach, the DPT surface finish is improved to around 40 Angstroms RMS, but the grooves from the DPT are still present, and create undesirable scatter at visible wavelengths.  
           [0007]    One final method for improving surface finishes on 6061-T6 aluminum mirrors is to polish the aluminum. This is difficult because of the softness of the aluminum, although some techniques have demonstrated surface finishes of 10 Angstroms RMS. However, bi-directional reflective distribution function (BRDF) scatter testing shows that the resulting aspheric surface on the polished 6061-T6 aluminum layer effectively performs like a 60 Angstroms RMS surface finish, because the surface peak-to-valley variations remain very high as a result of impurities. Obviously, performing comparable to a 60 Angstroms RMS surface finish is not a significant improvement over an 80 Angstroms RMS surface finish, especially in view of the added difficulty and cost involved.  
         SUMMARY OF THE INVENTION  
         [0008]    From the foregoing, it may be appreciated that a need has arisen for a high precision mirror and a method of making it, which avoid at least some of the disadvantages discussed above. One form of the invention involves an apparatus that includes a mirror with: a substrate having a surface; and a thin-film finish layer provided over the surface of the substrate and having on a side thereof opposite from the substrate a surface with a polished finish.  
           [0009]    Another form of the invention relates to a method of making a mirror, and includes: providing a substrate having a surface thereon; forming a thin-film finish layer over the surface of the substrate using thin-film techniques, the finish layer having a surface on a side thereof opposite from the substrate; and polishing the surface of the finish layer.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1 is a diagrammatic fragmentary sectional view of a mirror which embodies aspects of the present invention;  
         [0012]    [0012]FIG. 2 is a diagrammatic fragmentary sectional view similar to FIG. 1, but showing a mirror which is an alternative embodiment of the mirror of FIG. 1;  
         [0013]    [0013]FIG. 3 is a diagrammatic fragmentary sectional view similar to FIG. 1, but showing a mirror which is yet another alternative embodiment of the mirror of FIG. 1; and  
         [0014]    [0014]FIG. 4 is a flowchart showing a process which can be used to make the mirrors of FIGS.  1 - 3 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIG. 1 is a diagrammatic fragmentary sectional view of a high-precision mirror  10  which embodies aspects of the present invention. The mirror  10  has a high-precision surface  9  which can reflect radiation, as indicated diagrammatically by broken-line arrows  11 . The mirror  10  of FIG. 1 is designed for use with visible radiation, but could alternatively be used with other types of radiation. The surface  9  is an aspheric surface, although the portion of the mirror  10  which is visible in FIG. 1 is sufficiently small so that the curvature of the surface  9  is not apparent in FIG. 1. The invention has aspects which are advantageous in the context of an aspheric mirror surface, such as the surface  9 , but the invention is not limited to aspheric surfaces.  
         [0016]    The mirror  10  has a substrate  12 , which in the disclosed embodiment is an aluminum alloy that is readily commercially available from a number of suppliers as aluminum 6061-T6. However, the substrate  12  could alternatively be made from any other suitable material. The 6061-T6 alloy is primarily aluminum, with alloy elements of zinc, chromium and iron, and may also include elements such as magnesium and silicon.  
         [0017]    The substrate  12  has a surface  13  thereon. The surface  13  is a precision surface formed using a machining technique which is commonly referred to in the industry as single point diamond turning (DPT). Despite the use of this precision machining technique, the alloy elements in the substrate  12 , such as the zinc, chromium and iron, leave defects or artifacts in the finished surface which limit its smoothness to about 80 Angstroms RMS.  
         [0018]    After the DPT of the surface  13  has been completed, a finish layer  16  is deposited on the surface  13  using thin-film vapor deposition techniques of a type known in the art. In the embodiment of FIG. 1, the finish layer  16  is a nickel-chromium layer, and has a thickness of approximately 5,000 Angstroms. However, the layer  16  could alternatively be made of any other suitable material, and could have some other suitable thickness. For example, as discussed in more detail later, the layer  16  could alternatively be made of amorphous silicon.  
         [0019]    The finish layer  16  has a surface  17  on a side thereof opposite from the substrate  12 . The surface  17  is initially somewhat rough, because the finish layer  16  is a thin-film layer, and thus its upper surface will conform to some degree to the contour of the DPT surface  13  on the substrate  12 , which as mentioned above will have some defects or artifacts. Therefore, in order to remove peak-to-valley variations in the surface  17 , the surface  17  is polished.  
         [0020]    In the disclosed embodiment, the polishing of the surface  17  is carried out using a not-illustrated polishing compound and a not-illustrated polishing element or pad. The polishing compound is a material available commercially from GE Micron Products of Deerfield Beach, Fla. under the tradename K-SPRAY DIAMOND ABRASIVE, type SJK-5, size 0.0-0.2 micron, formula K-285T. However, any other suitable material could alternatively be used as the polishing compound. The polishing element is a part available commercially under the tradename TAC&#39;N STIK REUSABLE ADHESIVE from Stockwell Office Products, Westborough, Mass. However, any other suitable element could alternatively be used as the polishing element. In the disclosed embodiment, polishing of the surface  17  removes approximately 500 Angstroms of the material of the finish layer  16 . After polishing, the resulting surface  17  has a surface finish of approximately 22 to 25 Angstroms RMS.  
         [0021]    A thin reflective layer  21  is then formed on the surface  17  of the finish layer  16 , using thin-film vapor deposition techniques of a type known in the art. In the disclosed embodiment, the reflective layer  21  has a thickness of about 2,000 to 5,000 Angstroms, but it could alternatively have any other suitable thickness. Further, the reflective layer  21  is silver, but it could alternatively be any other suitable reflective material, such as gold or aluminum. The outer surface of the reflective layer  21  serves as the reflective surface  9 . Since the reflective layer  21  is a thin-film layer provided on the surface  17  of the underlying finish layer  16 , the surface  9  on the reflective layer  21  has a surface finish comparable to that of the surface  17 , or in other words a surface finish of approximately 22 to 25 Angstroms RMS.  
         [0022]    A thin overcoat, which is not shown in the drawings, may be applied over the reflective layer  21 , in order to protect the reflective layer and/or to increase the reflectance within a selected waveband. For example, if the mirror  10  was intended for use with radiation in the visible and near infrared wavebands, an overcoat of silicon dioxide (SiO 2 ) with a thickness of about 1,500 Angstroms could be applied over the reflective layer  21  using well-known techniques. However, there is a wide variety of other known materials that, depending on the particular intended use for the mirror, could alternatively be used for the overcoat, including but not limited to silicon oxide (SiO), tantalum oxide (Ta 2 O 5 ), titanium oxide (TiO 2 ), zirconium oxide (ZrO 2 ).  
         [0023]    [0023]FIG. 2 is a diagrammatic fragmentary sectional view similar to FIG. 1, but showing a mirror  40  which is an alternative embodiment of the mirror  10  of FIG. 1. The mirror  40  of FIG. 2 is substantially identical to the mirror  10  of FIG. 1, except that the mirror  40  has a finish layer  46  which is made from amorphous silicon rather than nickel-chromium. Alternatively, the layer  46  could be made from germanium instead of silicon. The silicon layer  46  has a thickness of approximately 5,000 Angstroms, but could alternatively have any other suitable thickness. The procedure for making the mirror  40  is substantially identical to the procedure described above for making the mirror  10 , except that amorphous silicon, rather than nickel-chromium, is applied to the surface  13  of the substrate using thin-film vapor deposition techniques.  
         [0024]    The finish layer  46  of amorphous silicon in FIG. 2 is somewhat harder than the finish layer  16  of nickel-chromium in FIG. 1. Consequently, although the surface  47  on the finish layer  46  in FIG. 2 is polished in the same manner as the surface  17  on the finish layer  16  in FIG. 1, the surface  47  will exhibit a higher degree of smoothness. In particular, the surface  47  will have a surface finish of approximately 10 to 15 Angstroms RMS.  
         [0025]    [0025]FIG. 3 is a diagrammatic fragmentary sectional view similar to FIG. 1, but showing a mirror  70  which is yet another alternative embodiment of the mirror  10  of FIG. 1. The mirror  70  of FIG. 3 is substantially identical to the mirror  10  of FIG. 1, except that a thin-film intermediate layer  76  of amorophous silicon is provided between the substrate  12  and the finish layer  16 . More specifically, the surface  13  on the substrate  12  is finished with a DPT operation, as discussed above in association with FIG. 1. Then, the layer  76  of amorophous silicon is deposited on the surface  13  of the substrate  12  using thin-film vapor deposition techniques of a type known in the art. The layer  76  has a thickness of approximately 6,000 Angstroms, but could alternatively have some other suitable thickness. The finish layer  16  is then deposited on the surface  77  of the layer  76 , using thin-film vapor deposition techniques of a type known in the art, in the manner described above in association with FIG. 1.  
         [0026]    The amorophous silicon layer  76  is harder than the nickel-chromium layer  16 . Since the nickel-chromium layer  16  is a thin-film layer, it will benefit from the hardness of the silicon layer  76 . In particular, when the surface  17  of the thin-film nickel-chromium layer  16  in FIG. 3 is polished in the same manner described above in association with FIG. 1, the presence of the silicon intermediate layer  76  below the finish layer  16  will cause the surface  17  to have a higher degree of smoothness in the mirror  70  of FIG. 3 than in the mirror  10  of FIG. 1. In particular, the surface  17  in FIG. 3 will have a surface finish of approximately 10 to 15 Angstroms RMS.  
         [0027]    [0027]FIG. 4 is a flowchart showing a process which can be used to fabricate each of the mirrors  10 ,  40  and  70  of FIGS.  1 - 3 . The process begins at block  101 , where the substrate  12  is fabricated. As discussed above, the substrate  12  in each disclosed embodiment is made from well-known aluminum 6061-T6 material, but could alternatively be made from some other suitable material. At block  102 , the surface  13  on the substrate  12  is finished using known DPT techniques, thereby giving the surface  13  a surface finish of about 80 Angstroms RMS.  
         [0028]    Block  103  is shown in broken lines in FIG. 4, in order to indicate that block  103  is carried out during fabrication of the mirror  70  of FIG. 3, but is not carried out during fabrication of the mirrors  10  and  40  of FIGS. 1 and 2. In block  103 , the intermediate layer  76  of amorophous silicon is formed on the surface  13  of the substrate  12 , using thin-film vapor deposition techniques.  
         [0029]    In block  106 , the finish layer is formed using thin-film vapor deposition techniques. In the mirrors  10  and  70  of FIGS. 1 and 3, the finish layer is the nickel-chromium layer  16 , whereas in the mirror  40  of FIG. 2, the finish layer is the amorophous silicon layer  46 . In block  107 , the surface  17  or  47  of the finish layer is polished in the manner explained above. This yields a surface finish of approximately 20 to 25 Angstroms RMS for the surface  17  in the embodiment of FIG. 1, a surface finish of approximately 10 to 15 Angstroms RMS for the surface  47  in the embodiment of FIG. 2, or a surface finish of approximately 10 to 15 Angstroms RMS for the surface  17  in the embodiment of FIG. 3.  
         [0030]    In block  108 , the thin reflective layer  21  is formed over the surface of the finish layer, using thin-film vapor deposition techniques. As discussed above, the reflective layer  21  in each of the disclosed embodiments is made from silver, but could alternatively be any other suitable reflective material, such as gold or aluminum.  
         [0031]    The present invention provides a number of technical advantages. One such advantage involves the provision of a mirror having a reflective surface with a high degree of smoothness, for example in the range of about 10 to 25 Angstroms RMS, depending on the specific structure of the mirror. This degree of smoothness is particularly advantageous when the mirror is used for radiation with relatively short wavelengths less than about 3 microns, such as visible radiation. A further advantage is that such a mirror can be fabricated with only one diamond point turning operation, which reduces the time and cost involved in its fabrication. The mirror is thus easier and cheaper to make than pre-existing mirrors, but is also more accurate.  
         [0032]    Another advantage is that, by finishing the surface of the finish layer using a polishing operation rather than diamond point turning, the finish layer can be a thin-film layer which is sufficiently thin so that the finish layer and the substrate do not exhibit a bi-metallic effect that causes bending of the mirror in response to temperature variations. Still another advantage is that the single diamond point turning operation and the formation of the various thin-film layers can be carried out using known equipment and techniques, thereby avoiding the need for development and/or purchase of custom manufacturing equipment which is expensive and has no other meaningful use.  
         [0033]    Although selected embodiments have been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.