Abstract:
An apparatus includes a workpiece support, a source for emitting a plume of coating material that flows toward the workpiece support, and plume influencing structure between the source and the workpiece support. The plume influencing structure includes a shield with plural openings extending therethrough approximately parallel to a general direction of flow of the plume away from the source. According to a different aspect, a method includes emitting from a source a plume of coating material that flows toward a workpiece support, and adjusting the flow of the plume with a shield between the source and the workpiece support, the shield having plural openings extending therethrough approximately parallel to a general direction of flow of the plume.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to coating techniques and, more particularly, to techniques for coating surfaces. 
       BACKGROUND 
       [0002]    When fabricating optical components such as lenses, it is very common to form a coating on a surface of the component, where the coating provides desired optical 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. These coatings often include multiple layers of different materials that collectively provide the desired characteristic(s). 
         [0003]    One problem with conventional coating techniques is that any given layer in a coating may have a thickness that is not uniform throughout the layer. As one example, when a coating is on a relatively highly curved surface, it is not unusual for a given layer of the coating to have a peripheral region that is as much as 30% to 50 W thinner than a central region of that layer, or even more than 50% thinner. 
         [0004]    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 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, rather than 1064 nm. 
         [0005]    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 the peripheral region than in the central region. Consequently, the ratios of thicknesses of different layers in the peripheral region can be different from the ratios of the thicknesses of those same layers in the central region. 
         [0006]    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 
         [0007]    A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawing, in which: 
           [0008]      FIG. 1  is a diagrammatic sectional side view of a coating apparatus that embodies aspects of the invention. 
           [0009]      FIG. 2  is a diagrammatic bottom view showing a shield and a workpiece from  FIG. 1 . 
           [0010]      FIG. 3  is a diagrammatic bottom view similar to  FIG. 2 , but showing the workpiece with a shield that is an alternative embodiment of the shield of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      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 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. 
         [0012]    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 . The axles  23  and  24  are spaced circumferentially from each other about the primary axle  17 , and each rotate about a respective 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 each could 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 a larger number of workpiece support members with respective axles, where the axles for all of the workpiece support members are spaced circumferentially from each other about the primary axle  17 . 
         [0013]    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 not-illustrated planetary gearing mechanism of a well-known type is provided 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 numbers  21  and  22  each undergo planetary movement about the primary axis  18  with respect to the housing  12 . The primary axle  17 , the support part  19 , the additional axles  23  and  24 , and the workpiece support members  21  and  22  collectively serve as a workpiece support mechanism. 
         [0014]    Each of the workpiece support members  21  and  22  is configured to removably support a respective workpiece  41  or  42 . In  FIG. 1 , the workpieces  41  and  42  each have a convex curved surface  43  or  44  on a lower side thereof, and have a concave curved surface  46  or  47  of similar shape on the upper side thereof. The apparatus  10  is used to form respective coatings  51  and  52  on the surfaces  43  and  44  of the workpieces  41  and  42 , in a manner discussed later. In the disclosed embodiment, 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. Further, it should be understood that 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. 
         [0015]    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 disclosed embodiment, the coatings  51  and  52  each happen to include a plurality of different layers, 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 and/or physical 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. 
         [0016]    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 characteristics. For example, a given coating might provide an anti-reflection characteristic as to radiation within one range of wavelengths, such as visible light, while also filtering out radiation in a different range of wavelengths, such as energy from a laser. 
         [0017]    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 . 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. 
         [0018]    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, which is shown diagrammatically. 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 disclosed embodiment, 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 . 
         [0019]    The source  62  is a device of a type well known in the art, and is therefore described here only briefly. More specifically, 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. 
         [0020]    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  forms a layer of the coatings  51  and  52  as the workpieces  41  and  42  pass above the source  62 . 
         [0021]    Several support wires  81 - 84  have their upper ends coupled to the support part  19 , and extend vertically downwardly from the support part. Although four of these support wires are visible at  81 - 84  in  FIG. 1 , there could be additional support wires, for example behind the wires that are visible in  FIG. 1 . The workpiece support member  21  is disposed between the wires  81  and  82 , and the workpiece support member  22  is disposed between the wires  83  and  84 . A perforated shield  87  is coupled to and extends horizontally between the lower ends of the wires  81  and  82 , and a perforated shield  88  is coupled to and extends horizontally between the lower ends of the wires  83  and  84 . It would be possible for the shields to be different but, in the disclosed embodiment, the shields  87  and  88  are identical. Therefore, only the shield  87  is described below in detail. 
         [0022]      FIG. 2  is a diagrammatic bottom view of the shield  87  and the workpiece  41 , where the coating  51  has been omitted from the workpiece  41 . The perforated shield  87  is a flat and approximately rectangular strip of wire mesh material. The openings between adjacent wires serve as the perforations through the shield  87 . In the disclosed embodiment, the shield  87  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 a suitable filter cloth. 
         [0023]    The strip of mesh material forming the shield  87  has a length  91  that is greater than the diameter of the workpiece  41 . Further, the strip has a width  92  that is less than the diameter of the workpiece  41 , and that is uniform along most of the length of the strip. In  FIG. 2 , the width  92  is approximately one-third of the diameter of the workpiece  41 . The axis of rotation  26  of the workpiece  41  intersects the shield  87  at approximately the center of the shield. Although the workpiece  41  happens to be circular, a shield similar to that shown at  87  can be used with other workpieces that have other shapes. Typically, the length of the shield would be greater than the largest transverse dimension of the workpiece (as viewed from the source  62 ), and the width of the shield would be less than the same transverse dimension of the workpiece. 
         [0024]    The shield  87  has a recess  94  in one side. The inner edge of the recess  94  is arcuate. The length  96  of the recess is less than the radius of the workpiece  41 . The depth  97  of the recess is less than the radius of the workpiece  41 , less than half the width  92  of the shield, and less than half the length  96  of the recess. In the disclosed embodiment, the length  96  of the recess is approximately three to four times the depth  97 . 
         [0025]    As the workpiece  41  is rotated with respect to the shield  87  during a coating operation, the recess  94  will influence coating of an annular region of the workpiece. This annular region is disposed outwardly of the broken-line circle  98  and inwardly of the broken-line circle  99 , where the circle  99  has a diameter greater than the diameter of circle  98 , and less than the diameter of the workpiece  41 . 
         [0026]    With reference to  FIGS. 1 and 2 , as the plume  71 - 74  of coating material travels upwardly, portions of the plume will not encounter the shield  87  or  88 , and will travel to and be deposited on the workpiece  41  or  42 . Other portions of the plume  71 - 74  will encounter the shield  87  or the shield  88 , but the perforations in the shield will permit a portion of that coating material to pass through the shield and then be deposited on the workpiece. The shields  87  and  88  are each sufficiently thin so that they do not tend to collimate the coating material as it flows through the perforations therein. In the disclosed embodiment, the filters  87  and  88  each pass approximately 50% of the coating material impinging on them. In experiments using the shields  87  and  88  of  FIGS. 1 and 2 , it was found that these shields reduced variations in the thickness of each coating layer from about 40 W to about 2%. 
         [0027]      FIG. 3  is a diagrammatic bottom view similar to  FIG. 2 , but showing the workpiece  41  with a shield  187  that is an alternative embodiment of the shield  87  of  FIG. 2 . The shield  187  is identical in all respects to the shield  87 , except that the arcuate recess  94  has been replaced with a triangular recess  194  having an inner edge that is V-shaped. The recess  194  has a depth that is approximately one-fourth of its length. 
         [0028]      FIGS. 2 and 3  each show a perforated shield having an overall shape that is approximately rectangular, except for a single recess in one side thereof, but it would alternatively be possible to use perforated shields with a wide variety of other shapes. In general, according to one approach for designing a suitable shield, a determination is made of the degree of blocking required in the center region of a workpiece. The material of the shield is then selected with perforations that provide slightly more than this degree of blocking, for example about 10% more blocking. Then, the width of the shield is selected so that the coating thickness at the edge region of the workpiece is matched to the coating thickness at the center. Next, the shape of the shield is altered if necessary so as to even out the uniformity of the coating at points between the center and edge regions of the workpiece. (For example, this is the purpose of the recesses  94  and  194  in  FIGS. 2 and 3 ). The shield does not need to have a shape that is straight, uniform or symmetric, so long as the shield provides the desired degree of uniformity in the resulting coating. 
         [0029]    In the embodiments depicted in the drawings, each shield is configured so that the size and density of the perforations is 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, in the case of the workpiece  41  shown in  FIGS. 1 and 2 , a shield might have spaced first and second portions that are respectively aligned with the central and peripheral regions of the workpiece, and a third portion disposed between the first and second portions. The third portion might have perforations that are larger and/or more dense than the perforations in each of the first and second portions. 
         [0030]    The drawings depict workpieces on which the surfaces to be coated are relatively highly curved convex surfaces. However, perforated shields can also be used to coat surfaces having a wide variety of other shapes, including but not limited to concave surfaces and flat surfaces. 
         [0031]    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.