Abstract:
A thermo-optic device may be formed with trenches that undercut the substrate beneath the thermo-optic device. Through the removal of the underlying substrate, the heat dissipation of the thermo-optic device may be reduced. This may reduce the thermal budget of the device, reducing the power requirements for operating the device in some embodiments.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/465,210, filed on Jun. 19, 2003. 
     
    
     BACKGROUND  
       [0002]     This invention relates generally to optical components including those used in optical communication networks.  
         [0003]     In optical communication networks, a waveguide core may extend across a semiconductor substrate. The core may be covered by an upper cladding and may be positioned over a lower cladding. The core may define an optical signal path. The cladding may have a lower refractive index than the core.  
         [0004]     In some cases the optical characteristics of the core may be thermally modified. For example, thermo-optic devices may be operated through the application of heat. The refractive index of an optical device may be changed by heating. Thermo-optic switches may be used in Mach-Zehnder interferometers and directional couplers, as two examples.  
         [0005]     Generally, the more heat that is dissipated by the thermo-optic device, the more the power requirements of the overall component. It is desirable to reduce the heat transfer to only that needed to achieve the thermo-optic effect.  
         [0006]     Thus, there is a need for ways to reduce the amount of heat loss in thermo-optic devices. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is an enlarged cross-sectional view of one embodiment of the present invention at an early stage of manufacture;  
         [0008]      FIG. 2  is an enlarged cross-sectional view of one embodiment of the present invention at a subsequent stage of manufacture;  
         [0009]      FIG. 3  is an enlarged cross-sectional view of one embodiment of the present invention at a subsequent stage of manufacture;  
         [0010]      FIG. 4  is an enlarged cross-sectional view of one embodiment of the present invention at a subsequent stage of manufacture;  
         [0011]      FIG. 5  is an enlarged cross-sectional view of one embodiment of the present invention at a subsequent stage of manufacture; and  
         [0012]      FIG. 6  is an enlarged cross-sectional view of one embodiment of the present invention at a subsequent stage of manufacture. 
     
    
     DETAILED DESCRIPTION  
       [0013]     Referring to  FIG. 1 , a waveguide core  12  may be defined on a lower cladding  11  over a semiconductor substrate  10 . In one embodiment, the core  12  may be part of a planar lightwave circuit. The core  12  and lower cladding  11  may, in turn, be covered by an upper cladding  14  as shown in  FIG. 2 .  
         [0014]     Referring to  FIG. 3 , an electric resistance heater  16  may be defined over the upper cladding  14  atop the core  12 . The heater  16  may be a more resistive material coupled to a source of power by a less resistive material. The electrical resistance heater  16  is selectively operable to change the optical properties of the core  12  in the vicinity of the heater  16 . For example, in one embodiment, a thermo-optic switch may be formed.  
         [0015]     Referring to  FIG. 4 , a pair of trenches  18  may be formed on either side of the heater  16  and core  12 . The trenches  18  may be spaced from the core  12  to leave protective upper cladding  14  around the core  12 , in one embodiment. The trenches  18  may extend through the upper cladding  14  and the lower cladding  11  down to the semiconductor substrate  10  in one embodiment of the present invention. A thermo-optic device  26  is defined between the trenches  18 , in one embodiment.  
         [0016]     Using the thermo-optic device  26  as a mask, an isotropic etch may be implemented into the substrate  10  through the trenches  18  to form the undercut regions  20 , in one embodiment of the present invention, shown in  FIG. 5 . The etchant is more selective of the substrate  10  material and is less selective of the cladding material  11  and  14 . Because of the isotropic nature of the etching, the etching extends under the lower cladding  11  on opposed sides of each trench  18 . By the term isotropic, it is intended to refer to an etchant that etches outwardly under a mask that defines an opening for the etchant to etch an underlying material.  
         [0017]     The resulting regions  20  extend under the structure that includes the core  12  and the heater  16 . One result of this under-etching is to reduce the amount of substrate  10  material underneath the core  12  and the heater  16 .  
         [0018]     Referring to  FIG. 6 , the trenches  18  may guide the anisotropic etching from the bottoms of the regions  20 . The etchant is more selective of the substrate  10  than of the cladding  11  or  14 . As a result, an anisotropically etched trench  22  extends below the regions  20  formed by isotropic etching. A substantial portion of the substrate  10  material underneath the core  12  and the heater  16  is removed, leaving a relatively thin pillar  24  of substrate  10 .  
         [0019]     The inventors of the present invention have determined that a substantial portion of the heat loss from heater  16  occurs through the semiconductor substrate  10 . By reducing the amount of available substrate  10  underneath the heater  16 , this heat loss may be reduced. The heat loss may increase the power needs of the device and dispersed heat may adversely affect the optical properties of surrounding components.  
         [0020]     In some embodiments, the regions  20  and the trenches  22  may be filled with a thermally isolating material. Also, in some embodiments, the trenches  18  may also be filled or covered with a thermally isolating material.  
         [0021]     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.  
         [0022]     What is claimed is: