Abstract:
In one embodiment, a method includes forming an array of recesses in a substrate, depositing spacer material in the recesses, forming photonic elements on the spacer material, and separating the structure previously formed into individual dies that each include a photonic element, spacer material and substrate.

Description:
BACKGROUND 
   One method for fabricating light emitting photonic devices includes forming a photonic crystal structure on a sacrificial layer that extends over substantially all of the silicon substrate. A release etch removes the sacrificial layer so that the photonic crystal structure can be peeled off the substrate, cut into individual photonic elements and mounted onto a light fixture. This method requires handling the small individual photonic elements multiple times which can damage elements and reduce process yield. 

   
     DRAWINGS 
       FIG. 1  illustrates a bulb type photonic device. 
       FIGS. 2–7  illustrate steps in the process of fabricating photonic devices such as the one shown in  FIG. 1 . 
       FIG. 8  is a flow chart illustrating a process for fabricating photonic devices such as the one shown in  FIG. 1 . 
   

   DESCRIPTION 
   Embodiments of the present invention were developed in an effort to minimize the handling of individual photonic elements in the fabrication of light emitting photonic devices. Some embodiments of the invention, therefore, will be described with reference to light emitting photonic devices. Embodiments of the invention, however, are not limited to such devices. Rather, embodiments of the invention may be used in any application or environment which might benefit from the new processes and devices. The exemplary embodiments shown in the figures and described below illustrate but do not limit the invention. Other forms, details, and embodiments may be made and implemented. Hence, the following description should not be construed to limit the scope of the invention, which is defined in the claims that follow the description. 
     FIG. 1  illustrates a bulb type photonic device  10  that includes a die  12  suspended in a dome shaped housing  14 . Die  12  is mounted on a pair of conductive posts  16  and  18  that extend through a base  20 . Post  16  and  18  serve as the contact points for die  12  to external electrical signals. Die  12  includes a light emitting photonic element  22  affixed to a transparent substrate  24 . A “photonic element” as used in this document means any material or structure that exhibits changes in the index of material on a length scale comparable to optical wavelengths, including but not limited to photonic crystals and other photonic bandgap materials. “Transparent” as used in this document means the property of transmitting one or more of infrared light, visible light or ultra-violet light. For example, in a photonic device  10  for use in a projector or display, substrate  24  will be transparent at least to visible light but need not be transparent to infrared and ultraviolet light. In another example, in a photonic device  10  for use in an infrared laser, substrate  24  will be transparent at least to infrared light but need not be transparent to visible and ultraviolet light. Photonic element  22  is suspended on conductive leads  26  and  28  across a recess  30  formed in substrate  24 . Base  20  is made from glass or another suitable insulating material. Housing  14  is made from glass or another suitable transparent material and sealed against base  20  using, for example, glass frit. The interior of housing  14  may be filled with an inert or halogen gas to help protect the interior components from environmental degradation and to help achieve the desired performance characteristics for device  10 . 
   A process for fabricating photonic devices such as photonic device  10  in  FIG. 1  will now be described with reference to  FIGS. 2–7 .  FIGS. 2 and 3  illustrate a transparent wafer  32  that has been partially processed using conventional wafer processing techniques.  FIG. 2  is a top down plan view of a portion of wafer  32 .  FIG. 3  is a partial section view taken along the line  3 — 3  in  FIG. 2 . In the embodiment shown, wafer  32  forms the substrate for the finished dies and, therefore, wafer  32  is also sometimes referred to as substrate  32 . Substrate  32 , for example, may be made of glass, sapphire or silicon. In other embodiments, the substrate on which the other features are formed could itself be formed on an underlying wafer or other structural feature. Where, as here, a light emitting device is being fabricated, a transparent wafer is the desired starting material 
   Referring to  FIGS. 2 and 3 , a sacrificial layer of spacer material  34  is deposited in recesses  36  formed along the surface of substrate  32 . Recesses  36  are etched into or otherwise formed in substrate  32  in the desired layout for photonic elements  38  and conductive leads  40 . Spacers  34  provide a temporary base on which photonic elements  38  are formed. Spacers  34  are removed later in the process to expose cavities around one side of photonic elements  38 . It is desirable, therefore, to form spacers  34  from a material that is selectively etch able with respect to substrate  32 , photonic elements  38 , and leads  40 . In the layout shown in  FIG. 2 , each recess  36  is generally rectangular so that the photonic element  38  will be completely surrounded by the transparent substrate  32  (in a plane along the surface of the substrate). In devices in which it is not necessary or desirable that substrate  32  surround photonic element  38 , then some processing efficiency may be realized by laying out recesses  36  as a series of trenches etched into the surface of substrate  32 . For such trench recesses, each photonic element  38  is bordered by transparent substrate  32  on only two sides, rather than on all four sides as in the layout shown in  FIG. 2 . 
   Once spacers  34  have been formed, photonic elements  38  are formed over substrate  32  at the spacer locations. A layer of metal or other conductive material is then applied over substrate  32  and elements  38 , and patterned to achieve the in-process wafer structure  42  shown in  FIGS. 2 and 3 . Alternatively, the conductive layer may be formed first on substrate  32 , patterned to form leads  40 , and then the photonic elements  38  formed over substrate  32 . In the embodiment shown, each spacer  34  is itself recessed so that each photonic element  38  sits down into spacer  34 , partially surrounded by substrate  32 . The recess may be formed in spacer  34  by etching away spacer material or by applying a conforming layer of spacer material that does not completely fill a recess  36  in substrate  32 . The extent to which the photonic elements  38  are recessed into substrate  32 , if at all, will depend on the desired performance characteristics of the device. If spacers  34  are not recessed, then photonic elements  38  may be formed layer by layer over substrate  32  and a conductive layer applied to communicate with photonic elements  38 . As noted above, each photonic element  38  represents generally any material or structure that exhibits changes in the index of material on a length scale comparable to optical wavelengths, including but not limited to one dimensional, two dimensional and three dimensional photonic crystals and other photonic bandgap materials. Photonic elements  38  may be formed on substrate  32  using any suitable technique. 
   The in-process wafer structure  42  shown in  FIGS. 2 and 3  is then cut or otherwise separated into individual dies. One such die, designated by part number  44 , is shown in  FIG. 4 . Die  44  is referred to as an in-process die  44  because sacrificial spacer  34  has not yet been removed to form the final die structure. Spacer  34  supports the small, fragile photonic elements  38  and leads  40  during the die cut and mounting operations. Referring now to  FIG. 5 , each in-process die  44  is mounted to a pre-fabricated base assembly  46 . Base assembly  46  includes conductive posts  48  protruding from base  50 . While any suitable process may be used to mount the dies  44 , an automated “pick and place” process in which the base assemblies  46  are fed to the work station on a tape  51  is illustrated. A robotic vacuum tool  52  may be used to “pick” each die  44  from a supply of dies and “place” the die  44  on posts  48 . Solder printed onto the tip of each post  48  is then flowed to mechanically secure and electrically connect die  44  to posts  48 . Glass frit (not shown) may also be applied to the connection for added mechanical strength. A solder connection  54  between die  44  and posts  48  is shown in  FIG. 7 . 
   Referring to  FIG. 6 , once dies  44  have been mounted on posts  48  to form in-process assemblies  56 , spacers  34  may be removed in a processing step that is sometimes referred to as a “filament release.” Spacers are removed by, for example, exposing dies  44  to the atmosphere in a plasma etcher  58 . Although other removal techniques may be used, plasma etching is desirable because it allows a continuation of the “assembly line” fabrication process utilized to mount dies  44  and the plasma etching equipment widely used in semiconductor wafer processing is easily adapted to the removal of spacers  34 . The resulting structure  59  is shown in  FIG. 7 . A dome shaped housing is then mounted and sealed to the base to complete the photonic device  10 , as depicted in  FIG. 1 . 
   The process flow described above for forming a photonic device is illustrated in the flow diagram of  FIG. 8 . Referring to  FIG. 8 , recesses are formed in the surface of a transparent wafer substrate in a desired layout for an array of photonic elements (step  60 ). A sacrificial layer of spacer material is deposited in the recesses (step  62 ). An array of photonic elements is then formed on the spacers (step  64 ). A layer of metal or other conductive material is formed on the substrate and then patterned to form conductive leads (step  66 ). The wafer is cut into individual dies (step  68 ) and each die is mounted on a pair of conductive posts protruding from a base (step  70 ). The spacers are then removed (step  72 ) and a transparent housing in mounted to the base over the dies (step  74 ). 
   The photonic device  10  shown in  FIG. 1 , in which a light emitting photonic element  22  is used to generate light, is just one example of a device that may be fabricated using the new processing techniques. Other configurations are possible. Photonic waveguides, for example, may be fabricated using processing techniques according to various embodiments of the invention. Embodiments of the invention also are not limited to fabricating bulb type photonic devices, but may be used to fabricate photonic devices in, for example, flat window packages and molded plastic packages. The fabrication process illustrated in  FIGS. 2–8  minimizes the handling of individual photonic elements, and the spacer supporting the photonic element is not removed until after the die has been securely mounted in its final position. In addition, conventional semiconductor wafer processing and mounting techniques and equipment may be readily adapted for use in the process. 
   As noted at the beginning of this Description, the exemplary embodiments shown in the figures and described above illustrate but do not limit the invention. Other forms, details, and embodiments may be made and implemented. Therefore, the foregoing description should not be construed to limit the scope of the invention, which is defined in the following claims.