Abstract:
A well may be formed for access to an optical waveguide core by a process that results in an L-shaped well. The L-shaped well may then be filled with a polymer. By controlling the size of each portion of well, the occurrence of bubbles within the well and cuts to the core may be reduced.

Description:
BACKGROUND  
         [0001]    This invention relates generally to well structures for hybrid optical waveguides.  
           [0002]    An optical waveguide is a structure that conveys an optical light signal. Conventionally, an optical waveguide may include a substrate and a core encased within an upper and lower cladding. The bulk of the light signal typically travels within the core along the axis of the waveguide structure.  
           [0003]    In order to access the core, a well may be formed through the upper cladding. This well may be formed by etching the upper cladding. Sometimes such etching does not completely expose the core of the waveguide, resulting in reduced access to the light signal. In other cases, the core of the waveguide might not only be exposed but it may be damaged by the etching process, resulting in reduced transmission of the light signal.  
           [0004]    After the well is formed, the well may be filled with a polymer. If the well is not appropriately formed, voids may be formed within the well where the polymer cannot completely fill the well, also resulting in reduced transmission of the light signal.  
           [0005]    Thus, there is a need to form wells for accessing optical cores in hybrid optical circuits that are less prone to forming voids or bubbles or cuts or other damage to the core itself. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is an enlarged, cross-sectional view of one embodiment of the present invention at an early stage of manufacture;  
         [0007]    [0007]FIG. 2 is an enlarged, cross-sectional view of the embodiment shown FIG. 1 at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0008]    [0008]FIG. 3 is an enlarged, cross-sectional view at a subsequent stage in accordance with one embodiment of the present invention;  
         [0009]    [0009]FIG. 4 is an enlarged, cross-sectional view at a subsequent stage in accordance with one embodiment of the present invention;  
         [0010]    [0010]FIG. 5 is an enlarged, cross-sectional view at a subsequent stage in accordance with one embodiment of the present invention;  
         [0011]    [0011]FIG. 6 is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0012]    [0012]FIG. 7 is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; and  
         [0013]    [0013]FIG. 8 is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    Referring to the transverse cross-sectional view of FIG. 1, a substrate  14  may be covered with a lower cladding  10 . The lower cladding may be, for example, a silica material. A core  12  may be formed as a ridge or line over the lower cladding  10 . The axis of the ridge is oriented perpendicular to the transverse plane shown in FIG. 1. The core may, for example, be a germanium doped silica or an oxynitride material.  
         [0015]    As shown in FIG. 2, a thin seed layer  16  of a metallic material, such as nickel, may be formed across the core  12  and the lower cladding  10 . In one embodiment, the metal  16  may be deposited. Using standard photo-lithographic processes, a first mask is defined on the seed layer  16 . The seed layer  16  is then etched to the desired pattern as shown in transverse cross section in FIG. 3. A wet etch may be advantageous for this etching process since the seed layer  16  can be fully removed from the non-patterned areas, reducing optical losses from residual etch-stop material. The pattern of the seed layer  16  may have a width as shown in FIG. 3 transverse of the axis of the core  12 , and it may have a length (not shown) along the axial direction of the core  12  around a selected point on the core  12 . The cross section of the figures is taken through the selected point.  
         [0016]    After removal of any masking materials, the thickness of the etch stop layer  16   a  may be increased by plating additional metal, such as nickel, up to a thickness of 2 microns in one embodiment of the present invention shown in FIG. 4. The etch stop layer  16   a  retains a pattern of the seed layer  16  reproducibly.  
         [0017]    Referring next to FIG. 5, a top cladding  18  may be applied by any of a number of well known processes, such as plasma enhanced chemical vapor deposition (PECVD), followed by annealing. A second mask layer is applied and patterned and the upper portion of a well  24  is formed, for example by dry etching through the top cladding  18  down to the etch stop layer  16   a , as shown in FIG. 6.  
         [0018]    The etch stop layer  16   a  is then completely removed as shown in FIG. 7. In one embodiment, the etch stop layer  16   a  may be removed by a wet etch using a differential etch formulation that thoroughly removes the etch stop layer  16   a  without significantly affecting the cladding layers  10  and  18  or the core  12 . Removing the etch stop layer  16   a  exposes the substantially unaffected core  12  within the well  24 .  
         [0019]    Provided that the opening produced by the second mask is smaller than the etch stop  16   a  pattern produced by the first mask, the dimensions of the well  24  are determined, in its upper region  26 , by the dimensions of the second mask and its process parameters and, in the lower region  20 , by the first mask and its process parameters.  
         [0020]    As shown in FIG. 7, the lower region  20  of the well  24  has a depth determined by the thickness of the etch stop layer  16   a . The upper region  26  of the well  24  is narrower than the long region  20 , forming an overhang  28  in one embodiment. Depending on the alignment of the second mask relative to the first mask, this overhang  28  may be asymmetric.  
         [0021]    A polymer  22  may be dispensed into the well  24  as shown in FIG. 8. The polymer spreads outwardly from its initial point of contact until it reaches the furthest most edges of the well  24 . If the lower portion  20  of the well  24  is deep enough, the polymer  22  may fully wet the furthest edge of the well  24  along the lower surface of the overhang  28 , resulting in a complete fill as shown in FIG. 8. If, on the other hand, the etch stop layer  16   a  is too thin, the spreading polymer  22  bridges up from the floor of the well  24  to the inside of the overhang  28 , leaving behind a trapped air pocket or bubble under the overhang.  
         [0022]    The effect of such a bubble can be very negative on the performance of the finished waveguide. The index of refraction difference between the inside and the outside of the bubble is typically 0.5, resulting in strong optical scattering and refraction. Any bubbles within the optical path of the waveguide may degrade the insertion loss.  
         [0023]    By enlarging the size of the well  24  until any bubbles trapped at its edge lay outside the optical path, the problem may be reduced. However, such a method may leave the problem unchanged at the ends of the waveguide where bubbles may be trapped directly between the core  12  and the upper cladding  18 . By making the etch stop layer  16   a  thick enough so that the dynamics of the polymer  22  spreading produces full wetting under the overhang  28 , bubble formation may be reduced.  
         [0024]    A cut is the opposite type of problem. If the second mask is displaced along the direction of the waveguide so that the overhang  28  becomes negative, the etch of the top cladding  18  produces a cut in the core  12 . The depth of this cut will depend on the process parameters. This imperfection may produce scatter and refraction loss just as a bubble does, although with a refractive index difference corresponding to the core  12  to polymer  22  refractive index difference. Since the cut is located within the core, its optical effects can be severe.  
         [0025]    By using a two-step fabrication for the etch stop layer  16   a , both bubbles and cuts may be reduced. Cuts may be reduced by making the second mask pattern smaller than the first so that the alignment errors in the process do not permit etching of the core. Unfortunately, this approach creates the possibility of bubbles trapped into the resultant overhang  28 . Bubbles may be reduced by using an etch stop layer  16   a  that is thick enough to raise the overhang  28  above a threshold level where complete wetting occurs at the edge of the well and under the overhang  28 .  
         [0026]    The dual layer metal etch stop  16   a  is used because the seed layer  16  may be very thin and is reproducibly patterned with a wet etch step that removes traces of the metal (and thereby optical loss) from the waveguide. The dual metal etch stop  16   a  may also be advantageous because the thickness of the patterned etch stop layer can be reproducibly manufactured by controlling the deposition process and time.  
         [0027]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.