Abstract:
An illumination system has a light source, an optical train, and a wavelength beam splitter. The optical train focuses light from the light source into a defined geometrical pattern on a surface. The wavelength beam splitter transmits light of a first wavelength and redirects light of a second wavelength. One of these wavelengths is included by the light from the light source, while the other is an emission wavelength generated by thermal excitation of the surface by the focused geometrical pattern.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates generally to a combined illumination and imaging system. More specifically, this application relates to an illumination and imaging system configured for use with a thermal substrate processing apparatus. 
         [0002]    There are numerous examples of applications in which a substrate may be processed thermally, including thermal annealing processes and chemical-vapor-deposition processes, among others. A general structure of an apparatus that may be used for such thermal processes is illustrated schematically in  FIG. 1A . The apparatus comprises an illumination system  104 , a stage  128  adapted to receive a substrate  124 , and a translation mechanism  132 . The illumination system comprises  104  comprises an electromagnetic source  108  that produces illumination that is shaped by an optical arrangement  116  to generate a narrow elongated beam  120  as a line of radiation incident on the substrate  124 . 
         [0003]    The stage  128  may comprise a chuck or other mechanism for securely holding the substrate  124  during processing. For instance, a frictional, gravitational, mechanical, and/or electrical system may be provided for grasping the substrate  124 . The translation mechanism  132  is configured to translate the stage  128  and the beam  120  relative to each other, through movement of the stage  128 , movement of the illumination system  104 , or movement of both. Any suitable translation mechanism may be used, including a conveyor system, rank-and-pinion system, or the like. The translation mechanism  132  is operated by a controller  136  to define the scan speed of the line of radiation relative to the stage  128 . 
         [0004]    A more detailed description of specific structures that may be used in implementing the thermal processing apparatus of  FIG. 1A  and of various alternative and equivalent variations to such a structure, is provided in published PCT application WO 03/089,184, the entire disclosure of which is incorporated herein by reference for all purposes. 
         [0005]      FIG. 1B  provides a top view of the substrate  124  overlying the stage  128 . The line of radiation  140  provided by the narrow elongated beam  120  may extend across the entire diameter of the substrate  124 . The relative geometry of the illumination system  104  and the translation mechanism  132  are such that the line of radiation  140  traverses the substrate  124  in a direction perpendicular to its length, i.e. the line  140  remains parallel to a fixed chord  144  of the substrate  124 . 
         [0006]    There are numerous considerations that may affect the effectiveness of such a system. This application describes embodiments of structures that may be used for the illumination system in such a thermal processing apparatus. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Embodiments of the invention thus provide methods and systems of illuminating a surface that may find application in thermal processing apparatus. In a first set of embodiments, an illumination system is provided for illuminating a surface. The illumination system comprises a light source, an optical train, and a wavelength beam splitter. The optical train is disposed along an optical path between the light source and the surface. The optical train has elements configured to focus light from the light source into a defined geometrical pattern on the surface. The wavelength beam splitter is disposed in the optical path. The wavelength beam splitter is adapted to transmit light of a first wavelength and to redirect light of a second wavelength. One of the first and second wavelengths is comprised by the light from the light source. The other of the first and second wavelengths is an emission wavelength generated by thermal excitation of the surface by the focused geometrical pattern and is not comprised by the light from the light source. 
         [0008]    In some instances, the illumination system may be a combined illumination and imaging system that further comprises an imaging subsystem disposed to focus light of the second wavelength redirected by the wavelength beam splitter onto a second surface. 
         [0009]    The light source may comprise a monochromatic or quasimonochromatic light source. In one embodiment, the light source comprises a plurality of laser diodes that emit light at the first wavelength. For instance, the first wavelength may be approximately 808 nm and the second wavelength may be greater than 850 nm. At least some of the optical elements may sometimes include an antireflective coating that is antireflective at the first wavelength and at the second wavelength. One antireflective coating comprises a plurality of layers of Ta 2 O 5  interleaved with a plurality of layers of SiO 2 . 
         [0010]    In some embodiments, the light source comprises a plurality of light sources, with the optical train comprising a spatial interleaver disposed to interleave light generated by a first set of the plurality of light sources with light generated by a second set of the plurality of light sources. Some embodiment comprise an isolation beamsplitter and a polarization-rotation element disposed in the optical path. The isolation beamsplitter is adapted to transmit light of a specified linear polarization and to redirect light not of the specified linear polarization from the optical path. The polarization-rotation element is adapted to change the specified linear polarization of light incident on the polarization-rotation element to a circular polarization. The isolation beamsplitter and the polarization-rotation element are disposed to be encountered by light emanating from the light source and by light reflected from the surface. In one example, the polarization-rotation element comprises a quarter waveplate. 
         [0011]    The optical train may comprise a cylinder array and a plurality of spherical lenses having optic axes along the optical path. The optical train may also comprise a first coupling cylinder having an optic axis along the optical path. In some embodiments, the optical train further comprises a second coupling cylinder having an optic axis along the optical path and spaced apart from the first coupling cylinder. 
         [0012]    In a second set of embodiments, methods are provided of illuminating a surface. First light comprising a first wavelength is generated with a light source. The first light is focused with an optical train disposed along an optical path between the light source and the surface into a defined geometrical pattern on the surface. Second light comprising a second wavelength is received along the optical path and redirected from the optical path. The second light is generated by thermal emission from the surface in response to focusing the first light on the surface. The second wavelength is not comprised by the light generated with the light source. 
         [0013]    In some embodiments, the redirected second light is focused onto an imaging surface to generate an image of the surface. The first light may be monochromatic or quasimonochromatic. In one embodiment, the first wavelength is approximately 808 nm and the second wavelength is greater than 850 nm. 
         [0014]    The first light may sometimes be generated by generating a first set of rays with a first set of a plurality of light sources and generating a second set of rays with a second set of the plurality of light sources. The first set of rays is then interleaved with the second set of ray. In some instances, the first light may be focused in a slow-axis direction with focusing the first light in a fast-axis direction. 
         [0015]    In certain embodiments, third light is received along a reverse direction of the optical path and corresponds to light reflected from the surface. The third light is redirected from the optical path. In such embodiments, light of a specified linear polarization may be transmitted along the optical path and light not of the specified linear polarization redirected from the optical path. The specified linear polarization of the transmitted light may be changed to a circular polarization before it encounters the surface. The third light then has the circular polarization of the first light. This permits the third light to be redirected from the optical path by changing the circular polarization of the third light to a linear polarization different from the specified linear polarization, and again redirecting light not of the specified linear polarization from the optical path. 
         [0016]    In a third set of embodiment, an apparatus is provided for thermally processing a substrate. The apparatus comprises a stage, a combined illumination and imaging system, and a translation mechanism. The stage is disposed to support the substrate. The combined illumination and imaging system is adapted to illuminate the substrate with a line of electromagnetic radiation extending partially across a surface of the substrate. It is also adapted to provide an image of a portion of the substrate from an emission from the portion of the substrate generated by thermal excitation of the portion of the surface by the line of electromagnetic radiation. The translation mechanism is adapted to translate the stage and the line of electromagnetic radiation relative to each other. 
         [0017]    In various embodiments, the combined illumination and imaging system may comprise structures like those described above. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sublabel is associated with a reference numeral and follows a hyphen to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sublabel, it is intended to refer to all such multiple similar components 
           [0019]      FIG. 1A  shows a side view of an apparatus for thermal processing of a substrate; 
           [0020]      FIG. 1B  shows a top view of a substrate being processed with the apparatus of  FIG. 1 ; 
           [0021]      FIG. 2  shows an arrangement of optical elements that forms an illumination system that may be used with the apparatus of  FIG. 1A  in one embodiment; 
           [0022]      FIG. 3  shows an arrangement of an alternative illumination system that may be used with the apparatus of  FIG. 1A  in an alternative embodiment; 
           [0023]      FIG. 4A  provides a fast-axis view of the propagation of light in the illumination system of  FIG. 2 ; 
           [0024]      FIG. 4B  provides a side-axis view of the propagation of light in the illumination system of  FIG. 2 ; 
           [0025]      FIG. 5  shows mechanical dimensions for the illumination system of  FIG. 2  in a specific embodiment; 
           [0026]      FIG. 6A  illustrates a structure of a nonreflective coating that may be comprised by some of the optical elements in the illumination system of  FIG. 2  or in the illumination system of  FIG. 3 . 
           [0027]      FIG. 6B  shows the reflectance properties of the nonreflective coating of  FIG. 6A  as a function of wavelength; and 
           [0028]      FIG. 6C  shows the reflectivity of the nonreflective coating of  FIG. 6A  as a function of incidence angle for two different wavelengths. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    Embodiments of the invention provide an illumination system. While the illumination systems described herein were developed for use with a thermal processing system like that illustrated in  FIG. 1A , they may find utility in other applications. The description of uses in thermal processing applications is thus intended merely to be exemplary and is not intended to limit the scope of the invention. In certain embodiments, the illumination system comprises an imaging system to provide a combined illumination and imaging system, although this is not required in all embodiments. 
         [0030]    An overview of the optical structure of the illumination system in an embodiment is provided with the schematic diagram of  FIG. 2 . In this drawing, the optical axis from one or more electromagnetic sources to a surface is designated as the z axis. The slow axis (“SA”) of the system in this drawing is identified, with the fast axis (“FA”) being orthogonal to the page. In certain embodiments, divergences of light through the system along the slow axis are less than 7.5° and are less than 0.2° along the fast axis. 
         [0031]    In the illustrated embodiment, light is provided by a plurality of light sources  201  and  203 , identified as “D 1 ” and “D 2 ” in the drawing, and directed to a surface  268 , identified as “S 1 ” in the drawing. In embodiments where the illumination system is part of a thermal processing system, the surface  268  S 1  may comprise the surface of a substrate undergoing processing and disposed within a processing chamber. Light may enter the chamber through a chamber window  264 , identified as “W 2 ” in the drawing. 
         [0032]    The light sources  201  and  203  may comprise any form of electromagnetic source in different embodiments. In some instances, the light sources  201  and  203  comprise monochromatic or quasimonochromatic sources such as laser diodes (“LDs”) or light-emitting devices (“LEDs”). For example, each of the light sources  201  and  203  might itself comprise a plurality of a laser-diode bars. For thermal processing applications, suitable wavelengths for such sources may be between 190 and 950 nm, with a particular application using illumination at 808 nm, but other wavelengths may be used for other types of applications. In some embodiments, the light sources  201  and  203  are capable of providing illumination continuously for a period of time that exceeds 15 seconds. 
         [0033]    Respective coupling cylinders  204  and  208 , identified in the drawing as “C 1 ” and “C 2 ,” are oriented to focus light down in the slow-axis direction. The coupling cylinders  204  and  208  have optical power in only one direction; the focusing that they provide in that direction ensures that most or all of the energy provided by the light sources  201  and  203  is retained in the optical system. 
         [0034]    Light from the different light sources  201  and  203  may be interleaved, providing better alignment control. Such interleaving is achieved with a spatial interleaver  216 , identified as “IL” in the drawing. Light is directed to the spatial interleaver  216  from each of the light sources  201  and  203 . In the illustrated embodiment, light from source  201  is directly incident on the spatial interleaver  216  after passing through coupling cylinder  204 , while light from source  203  is directed to the spatial interleaver by a folding element  212 , such as a mirror identified as “M” in the drawing. More generally, any combination of light-direction elements may be used to direct the light from the sources  201  and  203  to the spatial interleaver  216 . 
         [0035]    The principal optical elements in the illumination system  104  that generate that the image of the light onto surface  268  SI include a cylinder array  228 , identified in the drawing as “A,” and an array of spherical lenses  272 . This combination of optical elements is sometimes referred to in the art as a “flyby homogenizer” and acts to generate multiple images of the light on an image plane and to focus the light into the desired shape on the surface  268  SI. In one embodiment, the multiple images comprise on the order of several tens of images. The cylinder array  228  comprises a micro-optic array of cylindrical lenses or lenslet pairs that overlay their aperture onto surface  268  S 1 . The radiance produced on surface  268  SI is the sum of the radiance from each of the component lenses or lenslet pairs, and the variability of the illumination on surface  268  SI is the average of the variability from the component lenses or lenslet pairs. 
         [0036]    In the embodiment illustrated in  FIG. 2 , the spherical lens array  272  comprises six lenses  236 ,  240 ,  244 ,  248 ,  252 , and  256 , identified as “L 1 ”-“L 6 ” in the drawing. The invention is not limited to this particular number of lenses and alternative embodiments may include a different number of lenses. The specific optical characteristics of each of the lenses and the way in which they are combined may define the shape of the overlaid images provided on surface  268  S 1 . In embodiments where the illumination source is a component of a thermal processing apparatus like that described in connection with  FIGS. 1A and 1B , the spherical lens array  272  may have a prescription that provides a narrow elongated beam of light onto surface  268  S 1 . 
         [0037]    The illumination system  104  may comprise one or more beamsplitters in different embodiments to manage the backtransmission of light through the system. For example, the interleaved light is directed to an isolation beamsplitter  220 , identified as “BS 1 ” in the drawing. This element acts as a polarizing beamsplitter so that light that passes along the optical axis z has a defined polarization. A waveplate  224 , identified in the drawing as “WP,” retards the polarization of the transmitted light by a defined amount; for instance, the waveplate  224  may comprise a quarter waveplate so that light that linearly polarized light that passes through the beamsplitter  220  becomes circularly polarized. Alternative polarization-rotation structures may be used, such as in embodiments that use a Faraday rotator. 
         [0038]    After passing through the remainder of the illumination system  104 , some of this light may be reflected from surface  268  S 1  back through the system. During such backtransmission, the second encounter of the light with the waveplate  224  causes the light again to become linearly polarized, but rotated by 90°. Upon its second encounter with the polarizing beamsplitter  220 , this reflected light is directed to a light dump and prevented from propagated back to the light sources  201  and  203 . Many of the light sources  201  and  203  that may be used are susceptible to damage from bright incident light, so such a configuration acts to protect the light sources  201  and  203  from potential damage. 
         [0039]    In embodiments where the illumination system  104  is a component of a thermal processing apparatus, a second beamsplitter  232  may advantageously be included as part of an imaging capability of the illumination system. The second beamsplitter  232  is identified as “BS 2 ” in the drawing and is selective according to wavelength (rather than according to polarization like beamsplitter  220 ). This selectivity may be exploited by recognizing that an increase in temperature of surface  268  S 1 , particularly when the surface  268  S 1  is comprised by a substrate being processed, may produce thermal radiation that backpropagates through the system. If the second beamsplitter (sometimes referred to herein as a “pyro beamsplitter”) is selective at a wavelength different than the wavelength of the illumination provided by the light sources  201  and  203 , that thermal radiation may be used to image the surface  268  S 1 . For example, in an embodiment where the light sources  201  and  203  provide light at 808 nm, the pyro beamsplitter  232  could be configured to redirect light having a wavelength of 950 nm or more. 
         [0040]    This redirected light may be used to generate an image of the portion of the surface  268  S 1  that is radiating onto a second surface  288 , identified as “S 2 ” in the drawing. Imaging is achieved with an imaging subsystem  290  that comprises a plurality of optical elements. In this illustration, the imaging subsystem comprises four lenses,  276 ,  282 ,  284 , and  286 , identified as “L′ 1 ”-“L′ 4 ” in the drawing, and a folding element  280 , identified as “F” in the drawing. The lenses are configured to correct for chromatic aberration resulting from backtransmission through the spherical lens array  272  and the folding element  280  is included to simplify packaging of the illumination system  104 . 
         [0041]    Packing of the illumination system  104  may also include use of a replaceable window  260 , identified in the drawing as “W 1 ,” that protects the interior of the illumination system  104 . It is noted that in thermal processing applications, window  264  is generally larger than the substrate being processed. This is because access may be needed to all regions of the substrate as part of the processing. Such a constraint need not apply to window  260 , particularly when the substrate and/or illumination system  104  are configured for movement so that the light provided by the illumination system  104  may access all parts of the substrate being processed. 
         [0042]    An alternative embodiment for the illumination system is illustrated in  FIG. 3 . This embodiment has a generally similar structure as that shown in  FIG. 2 , although certain variations are highlighted below. In this illustration, light  308 - 1  from half the light sources is shown in a dark color and light  308 - 2  from half the light sources is shown in a light color. This illustrates the homogenization of light that is achieved by the assembly. Light incident on the initial coupling cylinder  304 , corresponding to C 1  and C 2  in  FIG. 2 , the different colors are distinctly separate. After transmission through the assembly to the surface  344 , corresponding to S 1  in  FIG. 2 , the different colors are well mixed. This homogenization is achieved with the use of a secondary coupling cylinder  320 , whose effect in combination with coupling cylinder  304  and with cylinder array  324  achieves good mixing of the light. The secondary coupling cylinder  320  also mitigates the amount of divergence in the light incident on the cylinder array  324 . Effectively, the combination of coupling cylinders  304  and  320  optimize the tradeoff with the divergence of light in the slow-axis direction. 
         [0043]    In addition to the use of an additional coupling cylinder  320 , the structure of the illumination system  300  of  FIG. 3  differs from that of  FIG. 2  by omitting an interleaver IL and by including a different number of lenses in the spherical lens array. In this illustration, the spherical-lens array has 8 lenses  332 , illustrating that the particular choice of optical structure for the spherical-lens array may vary in different embodiments. Other elements in common with the embodiment of  FIG. 2  include the use of an isolation beam splitter  312  and waveplate  316  to manage reflections from the surface  344 , and a pyro beamsplitter  328  to image portions of the surface that respond with thermal radiation. Details of the imaging subsystem are not shown in  FIG. 3 , but are usually included in embodiments that include a pyro beamsplitter  328  to redirect thermal emissions. The replaceable window  336  and chamber window  340  are similar to windows WI and W 2  described in connection with  FIG. 2 . 
         [0044]    To permit a comparison of light mixing in the embodiments of  FIGS. 2 and 3 ,  FIGS. 4A and 4B  provide fast-axis and side-axis views of light propagating through the illumination system of  FIG. 2 . As for  FIG. 2 , this illustration shows half the light in a dark color and half the light in a light color. The two light sources D 1  and D 2  shown in  FIG. 2  respectively provide the dark-color light and the light-color light. 
         [0045]    A detailed prescription for one embodiment of the invention is provided in Table 1, which refers to components of the system using the nomenclature introduced in the discussion of  FIG. 2 .  FIG. 5  provides a detailed specification of mechanical dimensions for the assembly in this illustrative embodiment. In that drawing, all of the dimensions are expressed in mm. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Prescription for Exemplary Embodiment 
               
             
          
           
               
                   
                 Radius 
                   
                 Aperture SA × FA 
                   
                 Edge Thickness 
               
               
                 Surface 
                 (mm) 
                 Thickness (mm) 
                 or Diameter (mm) 
                 Material 
                 (mm) 
               
               
                   
               
             
          
           
               
                 D1, D2 
                   
                 3.000 
                 13.0 
                 Air 
                   
               
               
                 C1 Surface 1 
                   
                 5.000 
                 20 × 30 
                 Silica 
                 3.644 
               
               
                 C1 Surface 2 
                 −44.704 
                   
                 20 × 30 
                 Air 
               
               
                 C2 Surface 1 
                   
                 6.000 
                 20 × 30 
                 Silica 
                 3.692 
               
               
                 C2 Surface 2 
                 −40.2971 
                 14.000 
                 20 × 30 
                 Air 
               
               
                 M Surface 1 
                   
                 3.000 
                 25 × 30 
                 Silica 
                 3.000 
               
               
                 M Surface 2 
                   
                   
                 25 × 30 
                 Air 
               
               
                 IL Surface 1 
                   
                 25.000 
                 25 × 25 
                 Silica 
                 25.000 
               
               
                 IL Surface 2 
                   
                 3.000 
                 25 × 25 
                 Air 
                 3.000 
               
               
                 BS1 Surface 1 
                   
                 20.000 
                 20 × 20 
                 Silica 
                 20.000 
               
               
                 BS1 Surface 2 
                   
                 2.000 
                 20 × 20 
                 Quartz 
                 2.000 
               
               
                 WP Surface 1 
               
               
                 WP Surface 2 
                   
                 3.000 
                 20 × 20 
                 Air 
                 3.000 
               
               
                 A Surface 1 
                   
                 0.900 
                 25 × 35 
                 Silica 
                 1.000 
               
               
                 A Surface 2 
                   
                 3.000 
                 25 × 35 
                 Air 
                 3.000 
               
               
                 BS2 Surface 1 
                   
                 20.000 
                 20 × 20 
                 Silica 
                 20.000 
               
               
                 BS2 Surface 2 
                   
                 5.800 
                 20 × 20 
                 Air 
                 2.021 
               
               
                 L1 Surface 1 
                 −22.56282 
                 5.000 
                 12.5 
                 Silica 
                 11.554 
               
               
                 L1 Surface 2 
                 44.6786 
                 8.300 
                 15.5 
                 Air 
                 3.214 
               
               
                 L2 Surface 1 
                 −87.6808 
                 10.000 
                 20.0 
                 Silica 
                 4.000 
               
               
                 L2 Surface 2 
                 −35.9791 
                 1.000 
                 23.0 
                 Air 
                 12.161 
               
               
                 L3 Surface 1 
                 134.112 
                 16.000 
                 27.5 
                 Silica 
                 1.856 
               
               
                 L3 Surface 2 
                 −39.12616 
                 1.000 
                 27.5 
                 Air 
                 17.905 
               
               
                 L4 Surface 1 
                 70.2056 
                 9.000 
                 27.5 
                 Silica 
                 3.390 
               
               
                 L4 Surface 2 
                   
                 1.000 
                 25.0 
                 Air 
                 9.624 
               
               
                 L5 Surface 1 
                 40.54856 
                 9.000 
                 25.0 
                 Silica 
                 7.965 
               
               
                 L5 Surface 2 
                 30.1498 
                 12.487 
                 20.0 
                 Air 
                 12.487 
               
               
                 L6 Surface 1 
                 30.1498 
                 8.000 
                 20.0 
                 Silica 
                 3.706 
               
               
                 L6 Surface 2 
                 50.8254 
                 4.500 
                 18.0 
                 Air 
                 1.206 
               
               
                 W1 Surface 1 
                   
                 3.000 
                 20.0 
                 Silica 
                 3.000 
               
               
                 W1 Surface 2 
                   
                 8.000 
                 20.0 
                 Air 
                 8.000 
               
               
                 W2 Surface 1 
                   
                 6.000 
                   
                 Silica 
                 6.000 
               
               
                 W2 Surface 2 
                   
                 20.500 
                   
                 Air 
                 20.500 
               
               
                 S1 
                   
                   
                  10 × 0.1 
                 Silicon 
               
               
                   
               
             
          
         
       
     
         [0046]    In embodiments where thermal radiation is backpropagated through the system, it is advantageous to provide a coating on certain of the optical elements comprised by the illumination system  104  with a coating that is antireflective at both the wavelength of the light generated by the light sources  201  and  203  and of the thermal emission. For example, in embodiments where the light provided by the light sources  201  and  203  is at a wavelength of 808 nm and the thermal emission is at a wavelength around 950 nm, it may be advantageous to apply a coating that is antireflective from about 800 nm to about 1000 nm on all elements of the spherical lens array  272 , the replaceable window  260 , the chamber window  264 , and the pyro beamsplitter. Such an antireflective coating could also be applied to other optical elements in the system, although it could be sufficient to apply a coating that is antireflective only around 950 nm to the components of the imaging subsystem  290  and to apply a coating that is antireflective only around 810 nm to the cylinder array  228 , the waveplate  224 , the isolation beamsplitter  220 , the interleaver  216 , the mirror  212 , and the cylinder arrays  204  and  208 . 
         [0047]    The inventors have developed a coating that is antireflective from about 800 nm to about 1000 nm. This coating comprises interleaved layers of Ta 2 O 5  and SiO 2 . In certain embodiments, the number of interleaved layers is six. A specific configuration is shown in  FIG. 6A  for a coating  600  in a particularly embodiment, specifying the thicknesses of each of the interleaved layers.  FIG. 6B  shows a reflectance profile for the coating detailed in  FIG. 6A . These results show the substantially zero reflectance at wavelengths between about 800 nm and 1000 nm for light that is directly incident. The angular variation of the reflectivity is shown in  FIG. 6C , with curve  610  showing the reflectivity at 810 nm and curve  620  showing the reflectivity at 975 nm. The relevance of these results in evaluating the effectiveness of the coating may be further understood with reference to Table II, which provides angle-of-incidence values that define the extreme angle of incidence for different components where the coating may be applied in the embodiment having the specific structure detailed in  FIG. 5  and in Table I. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Angle of Incidence for Exemplary Embodiment 
               
             
          
           
               
                   
                 FA 
                   
                 Angle of Incidence (deg) 
               
             
          
           
               
                 Surface 
                 SA Intersect (mm) 
                 Intersect (mm) 
                 Z sag   
                 Angle 1 
                 Angle 2 
                 Extreme 
               
               
                   
               
             
          
           
               
                 BS1 Surface 1 
                 5.88 
                 8.82 
                   
                 7.26 
                 4.99 
                 7.26 
               
               
                 BS1 Surface 1 
                 7.63 
                 8.93 
                   
                 4.99 
                 7.26 
                 7.26 
               
               
                 L1 Surface 1 
                 7.93 
                 8.95 
                 −3.43 
                 27.29 
                 18.39 
                 27.29 
               
               
                 L1 Surface 2 
                 10.24 
                 10.51 
                 2.48 
                 33.35 
                 53.03 
                 53.03 
               
               
                 L2 Surface 1 
                 12.40 
                 12.21 
                 −1.74 
                 22.62 
                 15.35 
                 22.62 
               
               
                 L2 Surface 2 
                 14.56 
                 13.98 
                 −6.19 
                 7.77 
                 11.33 
                 11.33 
               
               
                 L3 Surface 1 
                 17.84 
                 16.37 
                 2.20 
                 33.74 
                 22.47 
                 33.74 
               
               
                 L3 Surface 2 
                 18.66 
                 16.93 
                 −9.19 
                 28.12 
                 43.22 
                 43.22 
               
               
                 L4 Surface 1 
                 18.50 
                 15.94 
                 4.38 
                 17.51 
                 11.95 
                 17.51 
               
               
                 L4 Surface 2 
                 18.06 
                 15.39 
                   
                 8.70 
                 12.70 
                 12.70 
               
               
                 L5 Surface 1 
                 17.03 
                 14.04 
                 6.53 
                 20.76 
                 14.12 
                 20.76 
               
               
                 L5 Surface 2 
                 14.8 
                 11.79 
                 6.68 
                 20.05 
                 29.88 
                 29.88 
               
               
                 L6 Surface 1 
                 13.79 
                 10.17 
                 5.35 
                 25.84 
                 17.45 
                 25.84 
               
               
                 L6 Surface 2 
                 12.65 
                 9.04 
                 2.44 
                 2.76 
                 4.01 
                 4.01 
               
               
                 W1 Surface 1 
                 12.22 
                 8.55 
                   
                 17.53 
                 11.96 
                 17.53 
               
               
                 W1 Surface 2 
                 11.80 
                 8.07 
                   
                 11.96 
                 17.53 
                 17.53 
               
               
                 W2 Surface 1 
                 10.13 
                 6.17 
                   
                 17.53 
                 11.96 
                 17.53 
               
               
                 W2 Surface 2 
                 9.30 
                 5.22 
                   
                 11.96 
                 17.53 
                 17.53 
               
               
                   
               
             
          
         
       
     
         [0048]    Having described several embodiments, it will be recognized by those of skill in the art that further modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.