Patent Publication Number: US-2007116423-A1

Title: Arrays of optical elements and method of manufacturing same

Description:
FIELD OF INVENTION  
      The present application relates to arrays of optical elements and methods of manufacturing arrays of optical elements.  
     BACKGROUND  
      Optical elements and semiconductor elements having at least one dimension of less than a few millimeters are currently fabricated by a number of processes. These processes include molding, lapping individual elements, casting the optical elements from a sol-gel followed by sintering, microreplication, and processes using surface tension or shrinkage to form desired shapes. Of these processes, only lapping allows the production of precise shapes from refractory or crystalline materials. However, for producing a large number of optical elements, lapping is one of the slowest and most expensive processes because each shaped element must be handled individually.  
     SUMMARY  
      The present application discloses arrays of optical elements and methods of manufacturing arrays of optical elements. In one aspect, an array of optical elements is disclosed, wherein the array of optical elements has a lepped input aperture surface. In another aspect, a mechanically stable array of optical elements is disclosed. The mechanically stable array of optical elements comprises an array of optical elements, wherein each optical element has a sidewall, and wherein the sidewalls of adjoining optical elements form channels in the array and a protective material at least partially filling the channels in the array to form the mechanically stable array.  
      In another aspect, a manufacturing method is disclosed. The method comprises providing an array of roughly shaped optical elements, wherein each roughly shaped optical element has a sidewall, and wherein the sidewalls of adjoining roughly shaped optical elements form channels in the array, filling the channels with a removable protective material to form a mechanically stable array, and lapping the mechanically stable array to a final shape and surface finish to form the array of optical elements.  
      The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. These and other aspects of the present application will be apparent from the detailed description below. In no event should the above summaries be construed as limitations on the claimed subject matter. The claimed subject matter is defined solely by the attached claims, which may be amended during prosecution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, where like reference numerals designate like elements. The appended drawings are intended to be illustrative examples and are not intended to be limiting.  
       FIG. 1  is a schematic side view illustrating an optical element and an LED die in one embodiment.  
       FIGS. 2   a - c  are perspective views of exemplary shapes of optical elements.  
       FIG. 3  is a schematic side view of an array of optical elements.  
       FIGS. 4   a - b  are bottom views of two alternative arrays of optical elements.  
       FIGS. 5   a - c  are schematic side views of an exemplary array of optical elements in three steps of a first embodiment of the manufacturing method.  
       FIG. 6  is a block diagram illustrating an exemplary manufacturing method according to a second embodiment.  
       FIGS. 7   a - f  are schematic side views of an array of optical elements during the manufacturing steps shown in  FIG. 6 .  
       FIG. 8  is a block diagram illustrating additional steps in a third embodiment of the manufacturing method.  
       FIGS. 9   a - c  are schematic side views of an array of optical elements during the manufacturing steps shown in  FIG. 8 .  
       FIG. 10  is a schematic side view of a light emitting article produced by the disclosed embodiments. 
    
    
     DETAILED DESCRIPTION  
      The present application discloses methods of manufacturing arrays of optical elements. These methods include providing an array of roughly shaped optical elements, filling the spaces between adjoining optical elements with a removable protective material to form a mechanically stable array of optical elements, and lapping the mechanically stable array to impart a desired shape and surface finish to the optical elements.  
      The presently disclosed methods are particularly useful for manufacture of optical elements such as those used for light extraction in light emitting devices (LEDs). When lapping is desired for an optically smooth final surface finish, typically such optical elements are manufactured as individual elements. When assembled together with an LED die, handling individual LED die/optical element pairs is slow and cumbersome. The present application discloses methods of manufacturing arrays of optical elements in a way that allows for assembly together with arrays of LED dies, thus creating a multitude of individual LED die/optical element pairs. By assembling the array of optical elements together with the array of LED dies, before separating into individual pairs of lighting elements the process is faster and more cost effective.  
       FIG. 1  is a schematic side view illustrating a configuration of an optical element  20  and an LED die  10  in an exemplary embodiment. The optical element  20  is transparent and preferably has a relatively high refractive index.  
      In some embodiments, the optical element can be shaped in the form of a taper as shown in  FIG. 1 . A tapered optical element can have numerous forms, including without limitation those shown in  FIGS. 2   a ,  2   b , and  2   c . A tapered optical element is a particularly advantageous shape of the optical element. In  FIG. 2   a , a tapered optical element  20   a  has an output aperture  130   a  that is larger than an input aperture  120   a . Tapered shapes, including a truncated inverted pyramid (TIP) shown in  FIG. 2   a , a truncated cone shown in  FIG. 2   b , and a shape with parabolic sidewalls as shown in  FIG. 2   c , and combinations thereof, provide the additional benefit of collimating light and are referred to herein as optical collimators. Using an optical collimator to extract light out of an LED die is particularly advantageous because it provides control over the angular distribution of light emitted. Additional shapes for optical collimators will be apparent to those skilled in the art. For example, a TIP shape, shown in  FIG. 2   a  can be modified to have curved sidewalls similar to those shown in  FIG. 2   c . Other variations are contemplated. For example, a sidewall can comprise a series of linear segments, a series of curved segments or a combination thereof. When made of high index materials, such optical elements increase light extraction from the LED die due to their high refractive index and collimate light due to their shape, thus modifying the angular emission of light. It will be understood by those skilled in the art that when collimation is less important or is not desired other shapes of optical elements may be used.  
      In  FIG. 1 , the LED die  10  is depicted generically for simplicity, but can include conventional design features as known in the art. For example, LED die  10  can include distinct p- and n-doped semiconductor layers, buffer layers, substrate layers, and superstrate layers. A simple rectangular LED die arrangement is shown, but other known configurations are also contemplated, e.g., angled side surfaces forming a truncated inverted pyramid LED die shape. Electrical contacts to the LED die  10  are also not shown for simplicity, but can be provided on any of the surfaces of the die as is known. In exemplary embodiments the LED die has two contacts both disposed at the bottom surface. This LED die design is known as a “flip chip”. The present disclosure is not intended to limit the shape of the optical element or the shape of the LED die, but merely provides illustrative examples.  
      The tapered optical elements have an input aperture  120 , an output aperture  130 , and at least one intermediate sidewall  140  disposed between the input aperture and the output aperture. If the optical element is shaped in the form of a truncated inverted pyramid, as shown in  FIG. 2   a , then such an optical element  20   a  contains four intermediate side walls  140   a . If the optical element is rotationally symmetric, then it can have a single side wall. For example if the optical element is shaped as an inverted cone as shown in  FIG. 2   b  or shaped with parabolic sidewalls as shown in  FIG. 2   c , then such an optical element  20   b  or  20   c  has a single sidewall  140   b  or  140   c , respectively. Other shape variations can be used. Each optical element depicted in  FIGS. 2   a ,  2   b , and  2   c  contains an input aperture  120   a ,  120   b , and  120   c  and an output aperture  130   a ,  130   b , and  130   c , respectively. The shapes and cross sections of the input apertures and the output apertures can vary. Exemplary shapes include square, rectangular, or circular apertures, or combinations thereof. The cross sections can vary in shape between the input and output apertures (e.g. an optical element having a square input aperture and a rectangular output aperture or a circular input aperture with a square output).  
       FIG. 3  shows an array  30  of tapered optical elements  20  made of a substrate material  50 . A plurality of individual optical elements  20  form the array, while the sidewalls  140  of adjoining optical elements form channels  142  in the array. Such an array of optical elements can be made by molding glass or by abrading a workpiece into an array of roughly shaped elements. When made by abrading, a workpiece typically contains the substrate material  50  and a carrier  52 . Optionally, the substrate and carrier materials can be integral. When molded from glass, the substrate and carrier material can be glass. Optical elements can be molded using other materials such as glass-ceramic materials, or fine grain polycrystalline ceramics via injection molding, or sol-gel derived glass or crystalline materials.  
      Suitable substrate materials include optical materials such as inorganic glasses and ceramics (e.g. calcite, sapphire, zinc oxide, silicon carbide, diamond, zirconia) or combinations thereof. Particularly useful glasses include, without limitation, lead-free glasses having refractive indexes greater than about 1.7 and glass transition temperatures less than 750° C., preferably glass transition temperature less than 650° C. Glasses with lower coefficients of thermal expansion are preferred. Exemplary glasses include n-LAF7, n-LAF3, n-LAF33, and n-LASF46 all available from Schott (Germany) and S-NPH2 available from Ohara Corporation (Japan).  
      Optical materials may also include laminates of these materials, for example, silicon carbide bonded to glass, sapphire bonded to glass, calcite bonded to glass, and polymer films bonded to glass. Advantageous characteristics of optical materials include a thermal diffusivity of at least 0.01 cm 2 /s, transparency, a high refractive index, low color, and low toxicity.  
      The substrate material  50  may also comprise semiconductor material such as silicon or semiconductors deposited on silicon carbide or sapphire. Though the substrate material may include any type of optical and/or semiconductor material, abrading and polishing with a patterned abrasive is particularly advantageous for fragile, extremely hard, and/or temperature sensitive materials—materials that are very difficult to cut using conventional methods and are non-moldable. Carrier  52  can be made using any of a number of materials well known in the art. Suitable materials should be very mechanically stable. Carrier  52  can alternatively be the same material as the substrate material.  
       FIGS. 4   a  and  4   b  show bottom views of two exemplary arrays ( 30   a  and  30   b , respectively) of optical elements ( 20   a  and  21 , respectively).  FIG. 4   a  depicts an array of optical elements  20   a  each shaped as a truncated inverted pyramid having a square cross section. Each optical element  20   a  has four sidewalls  140   a , an input aperture  120   a , and an output aperture  130   a . The sidewalls  140   a  of adjoining optical elements form channels  142   a  in the array  30   a . For applications involving TV&#39;s, LCD monitors or displays, it may be advantageous to provide optical elements and LED dies having a rectangular cross section with an aspect ratio commonly used in those applications (e.g. 16:9 or 4:3).  FIG. 4   b  shows another embodiment of an array  30   b  of tapered optical elements  21 . In this array, individual optical elements  21  have a circular input aperture  121  and a square output aperture  131 . The sidewalls  141  are shaped to connect the input and output apertures accordingly. The sidewalls  141  of adjoining optical elements  21  form channels  142   b  in the array  30   b.    
       FIGS. 5   a - c  show one embodiment of the present manufacturing method. The first step in this embodiment is to provide an array  32  of roughly shaped optical elements  22 .  FIG. 5   a  shows a cross-sectional view of an exemplary array of three roughly shaped optical elements  22 . Each shaped element has one or more sidewalls  42 . The sidewalls  42  of adjoining optical elements  22  form channels  142  in the array  32 . The array of roughly shaped optical elements can be prepared by molding glass elements into the array of optical elements, by grinding or abrading a work piece into an array of roughly shaped optical elements, or by other methods known in the art. For example, an array of roughly shaped elements can be prepared from a work-piece, where the work-piece is abraded to at least partially form channels that define an array of shaped elements as described in U.S. patent application Ser. No. 10/977239 (Ouderkirk et al.), entitled “Process for Manufacturing Optical and Semiconductor Elements” (Attorney Docket No. 60203US002). Alternatively, an array of roughly shaped elements can be formed by molding or viscous sintering. For example a high index glass such as N-LASF46 (available from Schott North America, Inc., Elmsford, N.Y.) can be heated above its softening point and allowed to slump into a suitably shaped platinum coated tungsten carbide mold.  
      The second step in this embodiment is filling the channels of the array with a removable protective material to form a mechanically stable array.  FIG. 5   b  shows the array of roughly shaped optical elements  32  of  FIG. 5   a  filled with a removable protective material  40  to form a mechanically stable array  34 . Protective materials can be applied from a coating from a solvent, directly applied as liquids, or applied using a transfer tape. Some materials may require hardening after application. For example, curable materials such as thermosets could be hardened using an energy source such as heat, light, or a combination thereof. Thermoplastic materials could be hardened by cooling below their glass transition temperature.  
      Suitable protective materials should be mechanically robust, have good adhesion to the optical elements, and be removable from the array of optical elements without damaging the optical elements. Preferably the removable protective materials should not be soluble in any slurry material used during the lapping process (typically aqueous based). Some examples of suitable polymers include acrylics such as polymethyl methacrylate, polyphenyl acrylate, and polyisoborynl acrylate. Other useful polymers include polyolefins (like polyethylene and polypropylene), polystyrene, polyesters, polyamides (nylons), epoxy resins, polyurethanes, polyethers, and polycarbonates. Linear polymers are preferred. Low functionality oligomers can also be used. In this case oligomer is mixed with a suitable curing catalyst such as is known in the art, the mixture is deposited in the spaces between the elements and then is cured using heat, light, or a combination thereof. Certain sol-gels, other inorganic precursors, or low melting point metals can also be used provided they can be removed without damaging the optical elements. Curing can include increasing the average molecular weight, cross-linking, or other indicators of polymerization, as is known in the art.  
      The third step in this embodiment is lapping the mechanically stable array  34  to a desired shape and surface finish. In  FIG. 5   c , a single-sided lapping step is employed to lap the input aperture side of the optical elements  22 . Each roughly shaped optical element  22  of  FIG. 5   a  has a rough input aperture  24 . After lapping, each optical element in the array  36  of  FIG. 5   c  has a lapped input aperture  26 . The array of optical elements  36  has a lapped input aperture surface  27 . An array of optical elements having a lapped input aperture surface would typically have optical elements that are substantially co-planar and exhibit the same surface topology, e.g. surface roughness.  
      In some embodiments, the protective material fills the channels in the array in such a way that when lapped, both the protective material and the optical element material are lapped. In this case, the lapped surface  27  includes the plurality of optical elements together with the protective material filling the channels in the array, as shown in  FIG. 5   c . In other embodiments, the protective material can partially fill the channels in the array. In this case, the lapped surface  27  is formed by the plurality of optical elements alone.  
      Optionally, further steps can be added to the presently disclosed method. Exemplary additional steps are described in context of a second embodiment.  
       FIG. 6  is a block diagram illustrating the presently disclosed manufacturing method according to a second embodiment.  FIG. 7  shows schematic side views of an array of optical elements during the corresponding manufacturing steps of  FIG. 6 . In the embodiment shown in  FIGS. 6 and 7 , an array of roughly shaped optical elements can be prepared by abrading a workpiece comprising of a substrate material  350  and a carrier  352 .  FIG. 7   a  shows the workpiece before any shaping takes place.  
       FIG. 7   b  shows the array of roughly shaped optical elements  332 . Rough shaping (step  220  of  FIG. 6 ) can be done by grinding or abrading the workpiece shown in step  210 . Optical elements can be produced by making two sets of grooves, 90 degrees apart on one surface of the workpiece using a rough grinding process. The grooves can be made using a shaped diamond saw, or by form grinding using a shaped surface grinding wheel or a shaped fixed abrasive such as a 3M Trizact™ Diamond Tile (see U.S. patent application Ser. No. 10/977239 (Ouderkirk et al.), entitled “Process for Manufacturing Optical and Semiconductor Elements”, (Attorney Docket No. 60203US002)). Alternatively, rough shaping ( 220 ) can also be done by molding a glass blank such that one surface of the blank is flat while the other surface has a plurality of roughly shaped optical elements. Using either method, the roughly shaped optical elements can be made taller than the final optical elements to compensate for removal of the some of the material during the lapping step.  
      Step  230  of  FIG. 6  is an optional step of finishing the sidewalls of the optical elements to a desired surface finish. In this step the final geometry, both angles and surface roughness, of the angled sidewalls of the optical elements can be achieved. The finishing process can be a one or two step process depending on how close the rough grind or molding process was to final tolerance.  
      If the final geometric tolerances are not achieved during the rough shaping step  220 , the finishing step can include an optional fine grinding process, as well as an optional polishing process. Fine grinding of the optical elements can be accomplished by making two sets of grinding traces, 90 degrees apart on the structured surface of the workpiece, following the grooves from the rough grind or molding operation. The fine grinding operation can be done with either a shaped diamond wheel or methods described in co-pending U.S. application Ser. No. 10/977239 (Ouderkirk et al.), entitled “Process for Manufacturing Optical and Semiconductor Elements” (Attorney Docket No. 60203US002), commonly assigned with the present application. The amount of stock removal needed can be determined by calculating what is needed to achieve the geometrical tolerances on the angled sidewalls  342  of the optical elements. A polishing process can then be employed to achieve the final surface finish on each of the angled faces of the optical elements.  
      If final tolerance for angle has been met, only a polishing process can be used produce the final surface finish on the angled sidewalls of the optical elements. Polishing can be done using a number of conventional polishing techniques, including both loose and fixed abrasive polishing, as described in co-pending U.S. application Ser. No. 10/977239 (Ouderkirk et al.), entitled “Process for Manufacturing Optical and Semiconductor Elements” (Attorney Docket No. 60203US002), commonly assigned with the present application.  
      For example, polishing can be accomplished by using a soft, resilient pad material (shaped or flat) with an abrasive in a slurry form. Alternatively, polishing can be done using a soft, resilient fixed abrasive pad (shaped or flat). A shaped polishing pad may be preferred when deep channels are desired. In the case of hard ceramic materials it may be desirable to use the polishing techniques using an abrasive article comprising precisely shaped abrasive composites having a resin phase and a metal phase, as described in co-pending U.S. application Ser. No. 11/254614 (Lugg et al.), entitled “Abrasive Article and Method of Modifying the Surface of a Workpiece” (Attorney Docket No. 61340US002), commonly assigned with the present application. In the case of glass materials it may be desirable to use conventional polishing pads and slurries (e.g. porous polyurethane pad using an abrasive slurry). Cerium oxide abrasives can be used for silica containing materials. Alumina or diamond abrasives can be used for harder ceramic materials such as sapphire.  
       FIG. 7   c  shows the array  333  of optical elements having finished sidewalls  344 .  
      If the sidewalls  342  of the roughly shaped optical elements in step  220  ( FIG. 7   b ) are of acceptable angle and finish, step  230  can be omitted. Alternatively, step  230  can be preformed after the lapping step  250 , provided that the protective material was removed before the finishing step.  
      In step  240  of  FIG. 6 , the channels of the array are back-filled with a protective material  340 , such as a soluble polymer, to form a mechanically stable array  334 .  FIG. 7   d  shows the array of optical elements with the protective material  340  filling the channels  342  in the array. In this process, the optical elements are surrounded by the protective material that facilitates subsequent finishing operations. One such material could be a soluble polymeric material such as a solvent developable photoresist. The purpose of encapsulating the angled sides of the optical elements is to provide support for them during subsequent lapping operations. This material should be mechanically robust enough to withstand the lapping during step  250 . Preferably, the channels  342  are filled only to the tops of the optical elements to avoid the need for an additional grinding step to remove excess encapsulant prior to the lapping step. Alternatively, the channels  342  can be partially filled, provided that the amount of protective material still serves to mechanically stabilize the array of optical elements. The protective material should not be soluble in the slurry material used in step  240  but should still be easily removed, for example using a suitable solvent, as described below.  
      In some embodiments, the mechanically stable array  334  of optical elements together with the protective material can be provided for further processing. Examples of further processing include attaching the optical elements to a corresponding array of LED dies, which will be described below. In other embodiments, the protective material can be removed prior to further processing steps.  
      In step  250  of FIGS.  6  and shown in  FIG. 7   e , the mechanically stable array  334  formed in step  240  is lapped to provide a lapped surface  327 . Lapping provides a desired surface finish to the input apertures  326  of the optical elements. In this process, the extra material underneath the optical elements is removed, the final height of the optical elements is achieved, and the desired surface finish is produced on the wafer bonding or input aperture surface  326 . Lapping can be performed using methods known in the art. For example lapping can be performed with either fixed abrasives (e.g. 3M Trizact™ Diamond Tile) or loose abrasives (e.g. alumina or diamond) on a metallic plate (e.g. cast iron). For hard ceramic materials or very hard glasses it may be desirable to employ methods described in co-pending U.S. application Ser. No. 11/191722 (Fletcher et al.), entitled “Self-Contained Conditioning Abrasive Article” (Attorney Docket No. 60707US002), co-pending U.S. application Ser. No. 11/191711 (Fletcher et al.), entitled “Abrasive Agglomerate Polishing Method” (Attorney Docket No. 61094US002), and co-pending U.S. application Ser. No. 11/254614 (Lugg et al.), entitled “Abrasive Article and Method of Modifying the Surface of a Workpiece” (Attorney Docket No. 61340US002), all commonly assigned with the present application.  
      In step  260  of  FIG. 6  the output aperture side of the mechanically stable array is lapped.  FIG. 7   f  shows the mechanically stable array having a lapped output aperture surface  325 . The thickness of the glass or ceramic material between the individual optical elements will be very small (possibly zero) after this step. The optical elements will be held together primarily by the protective material  340  applied in step  240 .  
      Alternatively, lapping steps  250  and  260  can be combined into one double-sided lapping step  265 . With double-sided lapping, the final surface finish (e.g. an optically smooth finish) can be produced simultaneously on both the input aperture and output aperture sides of the optical elements. The double-sided lapping process is very fast and makes it significantly easier to prepare large numbers of high quality optical elements in high yield. For example, with a small taper angle and narrow gaps between the optical elements it could be very difficult to polish or lap right to the top of the channels, even with a shaped abrasive. With the presently disclosed methods, the initial optical element height can be made somewhat larger than the final product and then a portion of one or both the top and bottom of the optical element can be removed through single- or double-sided lapping. The pitch of the optical elements can still be minimized in this process, maximizing the yield per wafer. Doubled sided lapping also yields high quality optical surfaces on the input apertures, ideal wafer bonding or optical coupling to the emitting surface of the LED die.  
      Optionally, the lapped surface(s) can also be polished to provide an optically smooth finish. Polishing can be performed on a porous polyurethane pad using an abrasive slurry. Cerium oxide abrasives can be used for silica containing materials. Alumina or diamond abrasives can be used for harder ceramic materials such as sapphire. Alternatively, silica abrasives (preferably colloidal silica) can be used for final polishing of sapphire (via a chemical mechanical polishing operation).  
      For some applications, it may be desirable to produce an array of optical elements bonded to wafer or LED die elements.  FIG. 8  shows additional optional steps that could be used with the presently disclosed methods. Additional processing steps shown in  FIG. 8  include attaching the optical elements to a wafer carrier (step  270 ), wafer bonding with the array to an epi-wafer (step  280 ), and dicing the wafer to produce individual optical elements bonded to LED die elements (step  290 ). The methods disclosed herein provide such bonded optical element/LED die pairs in a single manufacturing operation. For some applications the size of the LED die and the size of the output aperture of the optical element can be designed to match. This is advantageous for high volume production.  
       FIGS. 9   a - 9   c  show schematic side views of an array of optical elements during the manufacturing steps of  FIG. 8 .  
      Step  270  of  FIG. 8  and the corresponding  FIG. 9   a  show an optional step of attaching the optical elements to a wafer carrier  370 . A suitable wafer carrier material can be attached to the output aperture side of the array of optical elements. The attached wafer carrier  370  provides support for the optical elements as the protective material  340  is removed prior to bonding to an epi-wafer  380  (step  280 ). The wafer carrier  370  can be attached using an adhesive. For example, a 3M Wafer Support System which employs a unique, UV-curable 3M adhesive to bond wafers to a rigid, uniform support surface can be used. This would minimize stress on the optical element array during wafer bonding (step  280 ) and singluation of the optical elements (step  290 ).  
      Next, the protective material can be removed to expose the array of individual optical elements  338 . Depending on the particular removal process, suitable steps can be taken to preserve the finish quality of the optical element surfaces during the removal process. When using photoresist, standard photoresist removal processes such as ashing or chemical etching can be employed to remove the protective material. Other suitable methods for removal of the protective material include, without limitation, heating (e.g. to melt or soften a thermoplastic material or low melt metal), plasma ashing, pyrolysis, and degradation by laser. Alternatively, the protective material  340  can be removed from the array of optical elements before attaching to a wafer carrier.  
       FIG. 8  at step  280  and  FIG. 9   b  show the step of wafer bonding the array of optical elements to an epi-wafer  380 . The epi-wafer  380  comprises an array of LED dies. During this step the input aperture surface of the optical element array can be bonded to the surface of the epi-wafer  380  using suitable bonding techniques. When using a flip chip LED design, alignment of the optical elements to the LED dies on the epi-wafer  380  can be accomplished prior to bonding. For example, the array of optical elements can be aligned to the array of flip-chip LEDs using a mask aligner such as is used in conventional photolithography. Since the substrate and semiconductor layers are transparent in the visible, one could image through epi-wafer, identify the etched metal contacts or other opaque fiducial markings on the back side of the epi-wafer and align these axes with the cross-hairs on the microscope image. The microscope objective could then be focused on the plane of the array of optical elements and the array could similarly be aligned (e.g. centered and rotated by using the x, y and θ controls on an aligner stage) with the cross hairs on the microscope or machine vision system. Finally, the array of optical elements and the epi-wafer can be brought into intimate contact while performing fine adjustments on x, y and θ and bonded using techniques described previously.  
      Step  290  of  FIG. 8  and  FIG. 9   c  show the singulation or wafer dicing step in which a plurality of individual optical elements  328  bonded to LED die elements  382  are produced. During this step, the epi-wafer  380  is diced to produce an array of individual LED die elements  382 . The LED die containing epi-wafer  380  can be singulated into individual LED die elements  382  using methods known in the art, including without limitation, abrasive sawing using resin or metal bonded diamond saws, dry laser scribing, water jet guided laser dicing, and wet or dry etching. The resulting array of LED die element—optical element pairs remain bonded to the wafer carrier  370  after this step.  
      Steps  270  through  290  are not required if the optical element is going to be used in a non-bonded configuration with the LED die. Referring to  FIG. 1 , an optical element can be optically coupled to the LED die without bonding. In a non-bonded configuration, the optical element  20  can be held in place over the LED die  10  using a clamp while optical contact is achieved via an air gap  150  or a thin optically conducting layer such as an index matching fluid or gel, as described in U.S. patent application Ser. No. 10/977249 (Connor et al.), entitled “LED Package with Non-bonded Optical Element”, (Attorney Docket No. 60216US002).  
      Each optical element—LED die pair forms a light emitting article in the array. After dicing, the light emitting articles can be removed from the wafer carrier. As mentioned above, for some applications the size of the LED die and the size of the output aperture of the optical element can be designed to match. The methods disclosed herein are particularly suited for high volume production of such light emitting articles.  
       FIG. 10  shows a single light emitting article  200  produced by the presently disclosed methods. The optical element  28  has an output aperture  130  characterized by an output aperture size b. Similarly, the LED die element  82  is characterized by an LED die size b. The size can be a one-dimensional measurement, e.g. length, width, or diameter. Alternatively, size can refer to a surface area. The presently disclosed methods produce light emitting articles in which the LED die size is substantially equal in size to the output aperture size. For example, if an optical element has an output aperture that is square, but the LED die is rectangular, the dicing step can be adapted so that only one (e.g. the x-direction) of the two planar (x-y) dimensions substantially match. Alternatively, the dicing step can also be adapted to provide an LED die surface area size matched to the surface area of the output aperture of the optical element.  
       FIG. 5   c  shows an array of optical elements  36  having a lapped input aperture surface  27 . In some applications, it may be desirable to provide an array of optical elements wherein the array has a total thickness variation (TTV) of less than 100 ppm expressed as a percentage of a characteristic lateral dimension of the array (e.g. diameter). For example, a thickness variation of 5 μm measured for an array having a characteristic lateral dimension of 50 mm, would be expressed as a TTV of 100 ppm. In other applications, it may be desirable to provide an optical element with a finished input aperture that has a surface roughness of less than a desired tolerance, e.g. peak to valley surface roughness of less than 50 nm. Alternatively, it may be desirable to provide an array of optical elements wherein the input apertures and output apertures are parallel to each other within a certain tolerance, e.g. parallel to within 1°.  
      Although the presently disclosed methods have been described in detail in context of an optical element composed of a single material, these methods are also applicable to optical elements comprising two or more materials. For example, the methods can be used to manufacture compound optical elements as disclosed in U.S. patent application Ser. No. 10/977225 (Ouderkirk et al.), entitled “High Brightness LED Package with Compound Optical Element(s)” (Attorney Docket No. 60218US002), commonly assigned with the present application. Similarly, the presently disclosed methods can be used to provide a plurality of optical elements to be combined with a single LED die, as described in U.S. patent application Ser. No. 10/977248 (Ouderkirk et al.), entitled “High Brightness LED Package with Multiple Optical Elements” (Attorney Docket No. 60219US002), commonly assigned with the present application.  
      The presently disclosed methods can also be used to provide an array of optical elements which is then combined with other elements prior to combining with LED dies. For example, an array of optical elements can be placed in optical contact with a patterned low refractive index layer as described in U.S. patent application Ser. No. 10/977577 (Ouderkirk et al.), entitled “High Brightness LED Package” (Attorney Docket No. 60217US002), commonly assigned with the present application. Similarly, the array of optical elements can be placed in optical contact with a birefringent material or a reflective polarizer as describe in U.S. patent application Ser. No. 10/977582 (Wheatley et al.), entitled “Polarized LED” (Attorney Docket No. 60202US002), commonly assigned with the present application.  
      While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and the detailed description. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.