Abstract:
A coating apparatus includes support structure supporting a workpiece support member for rotation about an axis, and a drive structure for rotating the support member. A source is spaced along an imaginary line from the support structure, and emits a plume of coating material that flows away from the source toward the support structure. The axis extends at an angle with respect to an imaginary plane perpendicular to the imaginary line. According to a different aspect, a coating method includes rotating a workpiece support member about an axis, and emitting a plume of coating material from a source that is spaced along an imaginary line from the support structure, the plume flowing away from the source toward the support structure. The axis extends at an angle to an imaginary plane that is perpendicular to the imaginary line.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to techniques for coating surfaces and, more particularly, to techniques for coating curved 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 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. For example, where 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% thinner than a central region of that layer, or even more than 50% thinner. The reduced thickness in the peripheral region can result in a mechanical failure in the peripheral region of a layer. In the case of a highly curved surface, the coating material often arrives at peripheral regions of the surface with a glancing incidence, rather than perpendicular to the surface, and this is believed to also contribute to mechanical failure. 
         [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 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 of a coating all have the proper thicknesses and ratios of thickness in the central region, 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. 
     
    
     
       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, which is a diagrammatic sectional side view of a coating apparatus that embodies aspects of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    The drawing FIGURE is a diagrammatic sectional side view of a coating apparatus  10  that embodies aspects of the present invention. The coating apparatus  10  includes a housing  12  with a chamber  13  therein. The housing  12  supports a primary axle  17  for rotation about a vertical primary axis  18 . A support part  19  is supported on the axle  17  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. 
         [0009]    The support part  19  rotatably supports two workpiece support members  21  and  22 . In this regard, two additional 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 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 . The axes  26  and  27  each extend at an angle  29  with respect to an imaginary plane  28  that is perpendicular to the vertical axis  18 . The angle  29  is within a range of approximately 20° to 70°, and preferably within a range of approximately 35° to 55°. In the disclosed embodiment, the angle  29  happens to be about 45°. But in practice, the angle  29  will be selected in dependence on various factors, such as the curvature of the surface to be coated, whether the surface is concave or convex, the geometry of the coating chamber  13 , and so forth. In the disclosed embodiment, the support members  21  and  22  are each disk-shaped, but they could each alternatively have any other suitable shape. 
         [0010]    In the disclosed embodiment, the axes  26  and  27  are each oriented so that they intersect the vertical axis  18  at a common, not-illustrated point. Alternatively, however, the axes  26  and  27  could be oriented so that they intersect the axis  18  at different points, or so that they are skewed with respect to the axis  18  and do not intersect it at all. 
         [0011]    Although the drawing FIGURE shows two workpiece support members  21  and  22  that are supported by respective axles  23  and  24 , it would alternatively be possible to have one or more further workpiece support members that each have an axle, where all of the additional axles are spaced circumferentially from each other about the primary axle  17 . 
         [0012]    A drive mechanism  31  such as 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 members  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. 
         [0013]    Each of the workpiece support members  21  and  22  is configured to removably support a respective workpiece  41  or  42 . The workpieces  41  and  42  each have, on a side thereof opposite from the support member  21  or  22 , a relatively highly curved surface  43  or  44 . The apparatus  10  is used to form respective coatings  51  and  52  on the respective curved surfaces  43  and  44  of the workpieces  41  and  42 , in a manner discussed in more detail later. The surfaces  43  and  44  are equivalent to surfaces that would be swept out by rotating a segment of an arc or curve about the axis  26  or the axis  27 . Thus, the axis  26  extends through a central region of the surface  43  and a central region of the coating  51 , and the axis  27  extends through a central region of the surface  44  and a central region of the coating  52 . 
         [0014]    In the drawing FIGURE, the curved surface on workpiece  41  is concave, and the curved surface  44  on workpiece  42  is convex. This visually demonstrates that the coating apparatus  10  is suitable for use with a variety of different surface shapes, including both concave and convex surfaces. As a practical matter, during an actual coating operation, the workpieces in the coating apparatus  10  would typically be identical or very similar, and would thus have curved surfaces that are identical or very similar. 
         [0015]    The coating apparatus  10  is not limited to use for coating highly curved surfaces, and in fact can be used to coat surfaces having a variety of different shapes. However, the coating apparatus  10  is very effective when used to coat highly curved surfaces, such as those shown at  43  and  44 . 
         [0016]    In the disclosed embodiment, the workpieces  41  and  42  with the coatings  51  and  52  thereon are each an optical component of a type well known in the art, such as a lens. They are therefore described here only briefly, to the extent necessary to facilitate an understanding of the structure and operation of the coating apparatus  10 . Further, it should be understood that the coating apparatus  10  is not limited to use for coating optical components, but can alternatively be used for coating a wide variety of other types of workpieces. In the disclosed embodiment, since the workpieces  41  and  42  are each an optical component, they each have an optical axis, and the optical axis of each is coincident with the associated axis  26  or  27 . However, it is not a requirement that optical workpieces have their optical axes aligned with their rotational axes. 
         [0017]    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 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 characteristics. For example, the coatings  51  and  52  may each be anti-reflective (AR) coatings that provide little or no reflection of a selected range of wavelengths, such as a range corresponding to visible light. 
         [0018]    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 may provide an AR characteristic as to one range of wavelengths, such as visible light, while also filtering out wavelengths in a different range, such as a range associated with laser energy. 
         [0019]    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  thereon 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 AR characteristic, while also being physically harder than the material of the workpiece  41  or  42 , in order to help physically protect the workpiece. The discussion here of AR 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. 
         [0020]    In the multi-layer coatings  51  and  52 , the layers may all have the same thicknesses, or some layers may be intentionally be thicker than other layers. Ideally, it is desirable that the thickness of each layer be relatively uniform throughout the layer. In comparison to pre-existing coating systems, the disclosed coating apparatus  10  is configured to achieve significantly better uniformity of the thickness of each layer within the coatings  51  and  52 . 
         [0021]    The coating apparatus  10  includes a source  62  in a lower portion of the housing  12 . The source  62  is spaced from the support part  19  along an imaginary vertical line  71 . Although the drawing FIGURE 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 . Alternatively, however, it would be possible for the source  62  to be positioned at any of a variety of other locations within the housing  12 . The source  62  and the drive mechanism  31  are both controlled by a control unit  64 . 
         [0022]    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 different materials that it contains. But in some situations, the source may simultaneously evaporate two or more of the different materials. 
         [0023]    In a coating formed by a pre-existing coating system, each layer in the coating is often thinner in its peripheral region than in its central region, especially when the coating is formed on a highly curved surface. For example, it is not unusual for a given layer to have a peripheral region that is as much as 30% to 50% thinner than a central region of that same layer, or even more than 50% thinner. Consequently, in a pre-existing coating, the various different layers could all have the desired thicknesses and the desired ratios of thicknesses in the central region of the coating, but these same layers could have reduced thicknesses and different ratios of thicknesses in the peripheral region of the coating. As a result, the central region of the coating could accurately provide desired optical characteristics (such as filtering or anti-reflection), whereas the peripheral region of the same coating might fail to provide these optical characteristics, or might provide them with reduced performance. 
         [0024]    With reference to the disclosed coating apparatus  10 , when the source  62  is evaporating a material, a plume of the evaporated material travels upwardly, as indicated diagrammatically by arrows  81 - 86 . The plume  81 - 86  from the source  62  basically coats the surfaces  43  and  44  on the workpieces  41  and  42  as the workpieces pass directly above the source  62 . Due to the fact that the axles  23  and  24  each extend downwardly and outwardly at an angle with respect to the vertical axis  18 , as the workpieces rotate about their respective axes  26  and  27 , coating material is deposited relatively uniformly on the curved surfaces  43  and  44 , from the central region to the peripheral region of each surface. Moreover, even where the plume  81 - 86  happens to have a relatively wide dispersion angle  92 , a peripheral portion  81  of the plume will tend to pass between the workpiece supports  21  and  22 , without contacting workpieces that are not currently passing above the source  62 . This also helps to avoid undesired thickness variations. 
         [0025]    When coating highly curved surfaces, pre-existing coating systems deposit layers with thickness variations that typically average about 35%. In contrast, when coating highly curved surfaces, the disclosed coating apparatus  10  can deposit layers with thickness variations that average only about 3%. Due to this reduction in thickness variations, the resulting coatings have layers that are relatively uniform in thickness across the entire curved surface, and that have about the same thickness ratios in both the central region and the peripheral region of the coating. Accordingly, in the case of an optical component with an optical coating, the optical characteristics of the coating are very uniform in both the central and peripheral regions of the coating. Further, the layers of the coating have improved mechanical properties, with reduced susceptibility to mechanical failure. 
         [0026]    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.