Abstract:
A workpiece support member can rotate about an axis relative to a source, and the source can emit a plume of coating material that flows toward the workpiece support member approximately parallel to the axis. A plume-influencing shield can rotate with the workpiece support member, and has a plurality of openings extending therethrough approximately parallel to the general direction of flow of the plume. According to a different aspect, a method involves: rotating a workpiece support member about an axis relative to a source; emitting from the source a plume of coating material that flows toward the workpiece support member approximately parallel to the axis; and influencing the plume with structure that includes a shield rotating with the workpiece support member, the shield having a plurality of openings extending therethrough approximately parallel to the general direction of flow of the plume.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to coating techniques and, more particularly, to techniques for coating surfaces. 
     BACKGROUND 
     When fabricating an optical component such as a lens, it is very common to form a coating on a surface of the component, where the coating provides desired optical properties and/or physical properties. For example, the coating may provide an anti-reflective (AR) characteristic, a filtering characteristic, physical protection for the component, some other characteristic, or a combination of two or more characteristics. Coatings often include multiple layers of different materials that collectively provide the desired characteristic(s). 
     One known coating technique is to place a workpiece such as an optical component in a vacuum chamber with an evaporator. The evaporator generates a plume of coating material, which travels to and is deposited on a surface of the workpiece. Where the surface is relatively highly curved, for example highly concave or convex, it is not unusual for a given layer of the coating to have a peripheral region that is as much as 30% to 50% thinner than a central region of that layer, or even more than 50% thinner. This is due in part to the fact that, when a surface is highly curved, the plume of coating material will typically impinge on a central portion of the surface approximately perpendicular to the surface, and thus with a low angle of incidence, but will impinge on a peripheral portion of the same surface with a high angle of incidence. As a result, more coating material will be deposited on the central portion of the surface than on the peripheral portion. Consequently, the resulting layer of coating material will be significantly thinner in its peripheral region than in its central region. 
     In the case of an optical component, variations in the thickness of a coating layer can affect the optical performance of the coating. For example, if the coating is designed to pass light from a 1064 nm laser, it may do so properly in its central region, where the thicknesses are correct. But a 35% thickness variation in the peripheral region can cause a corresponding variation in the wavelengths passed in the peripheral region, such that the peripheral region passes wavelengths of about 676 nm to 709 nm, and not the intended wavelength of 1064 nm. 
     A further consideration is that different layers in the same coating often have different variations in thickness. For example, one layer may be 30% thinner in a peripheral region than in a central region, while another layer may be 50% thinner in a peripheral region than in a central region. Consequently, the ratios of thicknesses of different layers in the peripheral region of the coating can be different from the ratios of the thicknesses of those same layers in the central region. 
     Thus, even assuming that the layers all have the proper thicknesses and ratios of thickness in the central region of the coating, the thicknesses and the ratios of thicknesses in the peripheral region will typically not be correct. As a result, the coating may provide desired characteristics in the central region, but may fail to provide these desired characteristics in the peripheral region, or may at least exhibit a degradation of the desired characteristics in the peripheral region. Consequently, although pre-existing coating techniques have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a diagrammatic sectional side view of a coating apparatus that embodies aspects of the invention. 
         FIG. 2  is a diagrammatic fragmentary sectional side view, in an enlarged scale, of a portion of the apparatus of  FIG. 1 , including a workpiece support fixture and a workpiece. 
         FIG. 3  is a diagrammatic bottom view of a workpiece, a perforated shield and a shield support arrangement that are components of the embodiment of  FIGS. 1 and 2 . 
         FIG. 4  is a diagrammatic bottom view similar to  FIG. 3 , but showing the workpiece with a shield and a shield support arrangement that are an alternative embodiments of the shield and shield support arrangement shown in  FIG. 3 . 
         FIG. 5  is a diagrammatic bottom view of a further perforated shield that can be used in place of the shield and shield support arrangement shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagrammatic sectional side view of a coating apparatus  10  that embodies aspects of the invention. The coating apparatus  10  includes a housing  12  with a chamber  13  therein. During a typical coating operation, a vacuum is maintained in the chamber  13  by a conventional and not-illustrated vacuum pump. The housing  12  supports a primary axle  17  for rotation about a primary vertical axis  18 . A support part  19  is supported on the axle  17  within the chamber  13  for rotation with the axle about the axis  18 . In the disclosed embodiment, the support part  19  is disk-shaped, but it could alternatively have any other suitable shape. 
     The support part  19  rotatably supports two workpiece support members  21  and  22 . More specifically, two additional vertical axles  23  and  24  are each rotatably supported on the support part  19 . These additional axles are spaced circumferentially from each other about the primary axle  17 , and each rotate about a respective additional vertical axis  26  or  27 . The two support members  21  and  22  are each supported on a respective one of the axles  23  and  24  for rotation therewith about the associated axis  26  or  27 . In the disclosed embodiment, the support members  21  and  22  are disk-shaped, but they could each alternatively have any other suitable shape. Although  FIG. 1  shows two workpiece support members  21  and  22  with respective axles  23  and  24 , it would alternatively be possible to have one or more additional workpiece support members with respective axles, where the axles for all workpiece support members are spaced circumferentially from each other about the primary axle  17 . 
     A drive mechanism  31  such as an electric motor is coupled to the axle  17 , in order to effect rotation of the axle  17  and the support part  19 . A planetary gearing mechanism of a well-known type is shown diagrammatically at  33  and, in response to rotation of the support part  19  with respect to the housing  12 , effects rotation of the additional axles  23  and  24  with respect to the support part  19 . Thus, the workpiece support members  21  and  22  each undergo planetary movement about the primary axis  18  with respect to the housing  12 . Each of the workpiece support members  21  and  22  has thereon a respective workpiece support fixture  36  or  37 . The workpiece support fixtures  36  and  37  will be described in more detail later. The primary axle  17 , the support part  19 , the additional axles  23  and  24 , the workpiece support members  21  and  22 , and the workpiece support fixtures  36  and  37  collectively serve as a workpiece support mechanism. For simplicity and clarity,  FIG. 1  shows each of the workpiece support members  21  and  22  with just one workpiece support fixture  36  or  37  thereon. However, it would alternatively be possible for each of the workpiece support members  21  and  22  to have a plurality of workpiece support fixtures thereon. 
     Each of the workpiece support fixtures  36  and  37  is configured to removably support a respective workpiece  41  or  42 . The workpieces  41  and  42  each have a highly-curved concave surface  43  or  44  on a lower side thereof. The apparatus  10  is used to form respective coatings  51  and  52  on the concave surfaces  43  and  44  of the workpieces  41  and  42 , in a manner discussed later. In the embodiment of  FIG. 1 , the workpieces  41  and  42  with the coatings  51  and  52  are each an optical component of a well-known type, such as a lens. Therefore, they are described here only briefly, to the extent necessary to facilitate an understanding of various aspects of the present invention. 
     For clarity and simplicity, the workpieces  41  and  42  are identical in the embodiment of  FIG. 1 . However, it would alternatively be possible for the workpieces  41  and  42  to be different, such that the apparatus  10  simultaneously coats different types of workpieces. A further consideration is that, although the surfaces  42  and  44  in the embodiment of  FIG. 1  are both steeply curved concave surfaces, they could alternatively be less steeply curved surfaces, and/or could be convex or have some other shape. Still another consideration is that, although the illustrated workpieces  41  and  42  are optical components, the coating apparatus  10  is not limited to use for coating optical components, but instead can be used for coating any of a wide variety of other types of workpieces. 
     It would be possible for each of the coatings  51  and  52  to be only a single layer of a single material. But in the embodiment of  FIG. 1 , the coatings  51  and  52  each happen to include a plurality of different layers that are not separately illustrated, involving the use of one material for some layers, another material for other layers, and so forth. By interleaving different layers of different materials in a known manner, the coatings  51  and  52  can each be given certain desired optical characteristics. For example, the coatings  51  and  52  may each provide an anti-reflective (AR) characteristic that causes little or no reflection of a selected range of wavelengths, such as a range corresponding to visible light. 
     In some cases, the multi-layer coatings  51  and  52  will be configured in a known manner to provide a combination of two or more desired optical characteristics. For example, a given coating might provide an anti-reflection characteristic as to one range of wavelengths, such as visible light, while also filtering out wavelengths in a different range of wavelengths, such as a range associated with laser energy. 
     As another example, if the optical workpiece  41  or  42  happens to be made of a relatively soft material that was selected because it provides certain desirable optical properties, the coating  51  or  52  may be configured to be physically harder than the associated workpiece  41  or  42 , in order to help physically protect the material of the workpiece  41  or  42 . Thus, a given coating  51  or  52  may provide an anti-reflection characteristic, while also being physically harder than the material of the associated workpiece  41  or  42 , in order to help physically protect the workpiece. The discussion here of anti-reflection characteristics, filtering characteristics and hardness characteristics is merely exemplary. The coatings  51  and  52  may each provide some or all of these characteristics, and/or any of a variety of other characteristics, separately or in combination. 
     The coating apparatus  10  includes a source  62  within the housing  12 , in a lower portion of the chamber  13 . The source  62  is spaced downwardly from the support part  19 . The source  62  and the drive mechanism  31  are both controlled by a control unit  64  of a known type. Although  FIG. 1  shows only a single source  62 , it would alternatively be possible to provide two or more sources in the apparatus  10 . In the embodiment of  FIG. 1 , the source  62  is spaced radially from the primary axis  18 , and is positioned approximately below the path of travel of the workpiece support members  21  and  22 . However, it would alternatively be possible for the source  62  to be positioned at any of a variety of other locations within the housing  12 . 
     The source  62  is a device of a type well known in the art, and is therefore described here only briefly. In the disclosed embodiment, the source  62  is a type of device commonly referred to as an electron beam evaporator. However, the source  62  could alternatively be any other suitable type of device. The source  62  contains two or more different materials that will be used to form respective layers in each of the multi-layer coatings  51  and  52 , and the source can selectively evaporate any of these different materials. At any given point in time, the source  62  will typically be evaporating only one of the multiple materials that it contains. But in some situations, the source may simultaneously evaporate two or more of these different materials. 
     When the source  62  is evaporating a material, a plume of the evaporated material travels upwardly, as indicated diagrammatically by arrows  71 - 74 . The plume  71 - 74  has a dispersion angle  76 . The plume  71 - 74  from the source  62  coats the surfaces  43  and  44  on the workpieces  41  and  42  as the workpieces pass above the source  62 . 
       FIG. 2  is a diagrammatic fragmentary sectional side view, in an enlarged scale, of a portion of the structure shown in  FIG. 1 , including the workpiece support fixture  36  and the workpiece  41 . The workpiece support fixture  36  includes a cylindrical annular sleeve  101 , with an inwardly-projecting annular flange  102  at its lower end. Although the sleeve  101  and flange  102  in the disclosed embodiment are cylindrical, they could alternatively have any other suitable shape, for example in dependence on the shape of the particular workpiece that is to be supported. It is desirable that the apparatus  10  form a coating that covers the entire curved surface  43 . It will be noted that, in  FIG. 2 , the surface  43  on the lower side of the workpiece  41  is approximately semispherical, and has a diameter that is less than the diameter of the circular opening through the flange  102 . Consequently, the entire curved surface  43  is exposed to the plume  71 - 74  of coating material ( FIG. 1 ), and the flange  102  does not block or otherwise interfere with coating of any portion of the curved surface. 
     It is also desirable that the coatings  51  and  52  each have a thickness that is relatively uniform across the entire coating. It will be noted from  FIG. 1  that, in the absence of any corrective measure, the upwardly-traveling plume  71 - 74  of coating material would impinge on a central portion of the surface  43  approximately perpendicular thereto and thus with a low angle of incidence, but would impinge on a peripheral portion of the same surface with a high angle of incidence. As a result, in the absence of any corrective measure, more coating material would be deposited on the central portion of the surface than on the peripheral portion. That is, the resulting layer of coating material would be significantly thinner in its peripheral region than in its central region. However, in the embodiment of  FIG. 1 , in order to achieve more uniformity in the thickness of the coating  51 , a corrective measure is provided in the form of a perforated shield  111 . 
     In more detail, and with reference to  FIGS. 1 and 2 , the perforated shield  111  is supported below the curved surface  43  of the workpiece  41  by a shield support arrangement  113 . Similarly, a further perforated shield  112  is supported below the curved surface  44  on the workpiece  42  by a shield support arrangement  114 . In the embodiment of  FIG. 1 , the shields  111  and  112  are effectively identical, and the shield support arrangements  113  and  114  are effectively identical. Therefore, only the shield  111  and shield support arrangement  113  will be described below in detail. 
       FIG. 3  is a diagrammatic bottom view of the workpiece  41 , the perforated shield  111  and the shield support arrangement  113  from  FIGS. 1 and 2 . For clarity and simplicity, the coating  51  and the workpiece support fixture  36  have been omitted in  FIG. 3 . With reference to  FIGS. 2 and 3 , the shield support arrangement  113  includes two elongate metal wires  121  and  122 , which each have a very small diameter. Although the embodiment of  FIGS. 1-3  uses two wires  121  and  122 , it would alternatively be possible to use a different number of wires. In the illustrated embodiment, the wires  121  and  122  are each made of stainless steel, and each have a diameter of about 0.0055 inches. However, it would alternatively be possible to use wires that are made of some other suitable material, or that have some other diameter. In fact, it would be possible to use a shield support arrangement made from something other that wires. 
     With reference to  FIGS. 2 and 3 , the wire  121  has a horizontally-extending central portion  126 , and two end portions  127  and  128  that are each bent to extend upwardly at a right angle with respect to the central portion  126 . Similarly, with reference to  FIG. 3 , the wire  122  has a horizontally-extending central portion  131 , and two end portions  132  and  133  that are bent to extend upwardly at a right angle to the central portion  131 . The wires  121  and  122  are oriented so that their central portions  126  and  131  extend perpendicular to each other. The central portions  126  and  131  each have a length that is slightly longer than the outside diameter of the workpiece  41 . 
     With reference to  FIG. 2 , a short strip of a flexible, vacuum-compatible tape  136  is provided, and releasably secures the end portion  127  of the wire  121  to a cylindrical inner surface of the workpiece support fixture  36 . A similar short strip of tape  137  is used to secure the opposite end portion  128  of the wire  121  to the cylindrical inner surface of the workpiece support fixture  36 . Two other short strips of the tape, which are not visible in the drawings, are used to respectively secure the end portions  132  and  133  of the wire  122  to the inner surface of the workpiece support fixture  36 . Each strip of tape includes an elongate flexible strip or carrier, and an adhesive layer on one side of the flexible strip. 
     In the illustrated embodiment, these four strips of tape are each a product that is obtained commercially under the trademark KAPTON® from C.S. Hyde Company, Inc. of Lake Villa, Ill. KAPTON® is commercially available in a variety of widths, and with a variety of different levels of adhesion. The particular tape selected for use will depend on a variety of different factors, such as the size and weight of the particular workpiece  41  that is to be coated. 
     Although the illustrated embodiment uses KAPTON® tape, it would alternatively be possible to use any other suitable tape, or some other suitable material. Also, depending on various factors, such as the size of the workpiece  41 , the diameter and stiffness of the wires  121  and  122 , and the size and weight of the shield  111 , it may be possible to optionally omit the strips of tape, including the strips shown at  136  and  137 . 
     With reference to  FIGS. 2 and 3 , the perforated shield  111  is a flat and approximately circular piece of wire mesh material. The openings between adjacent wires serve as the perforations through the shield  111 . In the embodiment of  FIGS. 1-3 , the shield  111  is a stainless steel wire mesh material purchased commercially from Ferrier Wire Goods of Toronto, Ontario as Type 304 wire mesh, 80×80, woven. The wires in this particular material have a diameter of about 0.0055 inches, and the space between adjacent wires is about 0.007 inches. However, it would alternatively be possible to use any other suitable perforated material, such as filter cloth. 
     As evident from the bottom view of  FIG. 3 , the circular shield  111  in the illustrated embodiment has a diameter that is approximately half the diameter of the curved surface  43  that is to be coated. The circular shield  111  is supported on the wires  121  and  122  so that, in the bottom view of  FIG. 3 , the shield  111  is approximately concentric with the surface  143 . In order to avoid movement of the shield  111  relative to the workpiece  41  and the wires  121  and  122 , the wires  121  and  122  may each be threaded through a portion of the wire mesh of the shield  111 . Alternatively, the wire mesh of the shield  111  could be formed so that two of the wires integral to the wire mesh are significantly longer than all of the other wires in the mesh, and serve as the support wires  121  and  122 . 
     During a coating operation, the workpiece  41  undergoes planetary movement (as explained above), and the wires  121  and  122  and the shield  111  undergo this same planetary movement move with the workpiece. Stated differently, the shield  111  and the wires  121  and  122  do not move relative to the workpiece  41  during a coating operation. With reference to  FIG. 1 , as the plume  71 - 74  of coating material travels upwardly, portions of the plume will not encounter the shield  111 , and will travel to and be deposited on an annular peripheral portion of the curved surface  43  that is being coated. Other portions of the plume  71 - 74  will encounter the shield  111 , but the perforations in the shield will permit a portion of that coating material to pass through the shield and then be deposited on a central region of the surface  43 . In the illustrated embodiment, the shield  111  passes approximately 50% of the coating material arriving at the shield. The shield  111  is sufficiently thin so that the perforations therein do not tend to collimate the coating material that flows through the perforations. Since the shield  111  is spaced from the workpiece surface  43 , and since the shield and workpiece are rotating relative to the moving plume of coating material, the coating material that does pass through the shield tends to be diffused somewhat. Consequently, there is no tendency for the resulting coating to have thickness variations that image the perforated and non-perforated portions of the shield. 
     As discussed above, if the perforated shield  111  was not present while the coating  51  was being formed on the curved surface  43 , the annular peripheral portion of the coating would be as much as 40% thinner than the central portion of the coating. In contrast, when the perforated shield  111  is present during formation of the coating  51 , the coating  51  will be relatively uniform in thickness across its entirety, with variations in thickness being less than about 2%. 
     In the embodiment of  FIGS. 1-3 , the support fixture  36  supports both the workpiece  41  and the associated shield  111 . Similarly, the support fixture  37  supports both the workpiece  42  and the associated shield  112 . Alternatively, however, other structure could be provided to support the shields  111  and  112 . For example, a not-illustrated annular member could be provided on the workpiece support member  21 , could concentrically encircle the fixture  36 , and could support the shield support  113  and the shield  111  below the workpiece  41 . Similarly, a not-illustrated annular member could be provided on the workpiece support member  22 , could concentrically encircle the fixture  37 , and could support the shield support  114  and the shield  112  below the workpiece  42 . 
     For clarity and simplicity,  FIG. 1  shows only the shields  111  and  112  that are supported on and rotate with the workpiece support members  21  and  22 . However, it would alternatively be possible to other shields in addition to the shields  111  and  112 . For example, as shown diagrammatically by broken lines in  FIG. 1 , a shield  151  could be supported on the support part  19 , so that it is disposed below the workpiece  42 . The shield  151  would rotate with the part  19 , but the workpiece  42  and shield  112  would rotate relative to the shield  151 . Similarly, as also shown by broken lines in  FIG. 1 , a stationary shield  152  could be supported between the source  62  and the workpiece support mechanism. The shield  152  and/or the shield  151  would serve to provide a degree of rough, preliminary plume shaping, and then the shield  111  or  112  would then provide an additional level of influence on the plume that is customized for the particular workpiece. 
       FIG. 4  is a diagrammatic bottom view similar to  FIG. 3 , but showing the workpiece  41  with a shield  171  and shield support arrangement  172  that are an alternative embodiment of the shield  111  and shield support arrangement  113  of  FIG. 3 . The primary difference is that the shield  171  has the shape of a star, rather than a circle. In particular, the shield  171  has the shape of a star with eight pointed legs. The point at the outer end of each leg is located adjacent an outer peripheral edge of the curved surface  43 . A further difference is that, although each support wire for the shield  171  is similar to the support wire shown at  121  in  FIG. 3 , the shield support arrangement  172  has four support wires rather than two support wires. 
       FIG. 5  is a diagrammatic bottom view of a further perforated shield  186 , which can be used in place of the shield  111  and the shield support arrangement  113  shown in  FIG. 3 . The shield  186  is made from a very thin piece of a platelike material. In  FIG. 5 , the shield  186  is made from a thin piece of metal, such as stainless steel, but it could alternatively be made of any other suitable material. The shield  186  has an outside diameter that is approximately equal to the outside diameter of the workpiece  41  ( FIG. 3 ). The shield  186  has a plurality of sector-shaped openings therethrough, including an inner ring of sector-shaped openings  187 , and an outer ring of sector-shaped openings  188 . 
     In a sense, the shield  186  has approximately the shape of a wheel, with a plurality of spokes  191  that extend radially outwardly in respective different directions, from a hub  192  to a circumferential rim  193 . A circular rib  194  has a diameter that is approximately half the diameter of the rim  193 . The circular rib  194  is concentric to the hub  192 , and is integrally coupled to each spoke  191 , at a location about halfway along the radial length of each spoke. For clarity, the widths of the rib  194  and the spokes  191  are exaggerated in  FIG. 5 . The spokes  191  and the rib  194  each have a very thin width. Also, the number of spokes  191  could be different. For example, the number of spokes could be larger. 
     In operation, the shield  171  of  FIG. 4  and the shield  186  of  FIG. 5  do not move relative to the associated workpiece. Instead, each moves with the associated workpiece, and each reduces the amount of coating material deposited in the central region of a curved surface relative to the amount deposited in the peripheral portion of that surface, thereby yielding greater uniformity in thickness of the resulting coating. 
     Although  FIG. 5  shows a single circular rib  194 , it would alternatively be possible to provide a plurality of such ribs, and/or to provide a rib or ribs with a shape that is other than circular. For example, there might be a plurality of concentric circular ribs, and the radial spacing between them might be either uniform or non-uniform. As another example, a rib might be circular, but offset so that it is not concentric to the axis of rotation of the workpiece. As still another example, a rib might have an oval shape, or an egg shape, rather than being circular. 
     With reference to  FIGS. 3 and 4 , the shields  111  and  171  happen to be configured so that the size and density of the perforations are approximately uniform throughout the shield. However, it would alternatively be possible to vary the sizes and/or the density of the perforations in different portions of a shield. For example, the shield  111  might have relatively small perforations in the central region of the shield, and slightly larger perforations near the peripheral edge of the shield. 
     The disclosed shields  111 ,  112 ,  171  and  186  each have a configuration that is customized to a particular workpiece surface that is to be coated. The customized configuration may involve factors such as the shape of the peripheral edge of the shield, the sizes of the perforations in the shield, and/or the locations of the perforations. 
     In the drawings, the surfaces to be coated are depicted as relatively highly curved concave surfaces. However, the invention is not limited to highly-curved concave surfaces, and could also be used to coat a wide variety of other surface shapes, including but not limited to convex surfaces and flat surfaces. 
     Although selected embodiments have 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.