Abstract:
A switching element includes a bubble chamber, a heater and a heat conductor. The bubble chamber holds fluid. The bubble chamber includes a trench within a planar light circuit and includes a trench within an integrated circuit attached to the planar light circuit. The heater is located under the trench within the integrated circuit. The heat conductor is attached to the integrated circuit. The heat conductor is located within the trench within the integrated circuit. A portion of the heat conductor is in close proximity to the heater. The heat conductor is more heat conductive than the fluid within the bubble chamber.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This is a Divisional of application Ser. No. 10/702,015 filed on Nov. 5, 2003 now U.S. Pat. No. 7,031,564, the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND 
   The present invention relates to components useful in optical switching devices and pertains particularly to heat transfer structures. 
   Optical fibers provide significantly higher data rates than electronic paths. However, effective utilization of the greater bandwidth inherent in optical signal paths requires optical cross-connect switches. 
   One type of optical cross-connect switch utilizes total internal reflection (TIR) switching elements. A TIR element consists of a waveguide with a switchable boundary. Light strikes the boundary at an angle. In the first state, the boundary separates two regions having substantially different indices of refraction. In this state the light is reflected off of the boundary and thus changes direction. In the second state, the two regions separated by the boundary have the same index of refraction and the light continues in a straight line through the boundary. The magnitude of the change of direction depends on the difference in the index of refraction of the two regions. To obtain a large change in direction, the region behind the boundary must be switchable between an index of refraction equal to that of the waveguide and an index of refraction that differs markedly from that of the waveguide. 
   One type of TIR element is taught in U.S. Pat. No. 5,699,462 which is hereby incorporated by reference. The TIR element taught in this patent utilizes thermal activation to displace liquid from a gap at the intersection of a first optical waveguide and a second optical waveguide. In this type of TIR, a trench is cut through a waveguide. The trench is filled with an index-matching liquid. A bubble is generated at the cross-point by heating the index matching liquid with a localized heater. The bubble must be removed from the crosspoint to switch the cross-point from the reflecting to the transmitting state and thus change the direction of the output optical signal. Efficient operation of such a TIR element requires effective placement and operation of heating devices within and around the TIR elements. 
   SUMMARY OF THE INVENTION 
   A switching element includes a bubble chamber, a heater and a heat conductor. The bubble chamber holds fluid. The bubble chamber includes a trench within a planar light circuit and includes a trench within an integrated circuit attached to the planar light circuit. The heater is located under the trench within the integrated circuit. The heat conductor is attached to the integrated circuit. The heat conductor is located within the trench within the integrated circuit. A portion of the heat conductor is in close proximity to the heater. The heat conductor is more heat conductive than the fluid within the bubble chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross section of a bubble chamber area of a total internal reflection switching element in accordance with an embodiment of the present invention. 
       FIG. 2  shows a cross section of a bubble chamber area of a total internal reflection switching element in accordance with another embodiment of the present invention. 
       FIG. 3  shows a cross section of a bubble chamber area of a total internal reflection switching element in accordance with another embodiment of the present invention. 
       FIG. 4  shows a top view of pillars within a bubble chamber of a total internal reflection switching element in accordance with an embodiment of the present invention. 
       FIG. 5  shows a top view of pillars within a bubble chamber of a total internal reflection switching element in accordance with another embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a cross section of an area of a total internal switching element that includes a bubble chamber  14 . Bubble chamber  14  includes a trench  15  within a planar light circuit (PLC)  11 . Planar light circuit  11  includes waveguides that are switched at trench areas. Each trench is filled with an index-matching liquid (fluid). A bubble is generated at the cross-point by heating the index matching liquid with a localized heater. The bubble must be removed from the cross-point to switch the cross-point from the reflecting to the transmitting state and thus change the direction of the output optical signal. 
   An integrated circuit  12  is attached to PLC  11 , for example by wafer bonding or soldering. Within integrated circuit  12 , trench  16  is aligned with trench  15  to fully form bubble chamber  14 . A single resistor with more than one leg (unshaped) or a resistor  21  and a resistor  22  are the localized heater used to generate bubbles within bubble chamber  14 . A pillar  19  and a pillar  20  are coated with heat conductor  17  in order to conduct heat from resistor  21  and resistor  22  within walls of trench  16 . For example, pillars  19  and pillar  20  are composed of oxide. Heat conductor  17  is composed of, for example, tantalum, tungsten, aluminum, silicon or another heat conducting material or stack of heat conducting material. Resistor  21  and resistor  22  are composed of Tantalum Aluminum (TaAl) or some other resistive material. 
   Sections  18  are part of a third metal layer on integrated circuit  12 . An arrow  24  represents a solder gap height for solder used to attach integrated circuit  12  to PLC  11 . For example, the solder gap is five to seven microns. 
   In order to form pillar  19  and pillar  20 , an oxide layer is formed on top of integrated circuit  12 . For example, this is done by a Teos process or by a spin on glass process. A patterned etch is performed down to the oxide immediately over resistor  21  and resistor  22 . Heat conductor  17  is formed within the resulting trench. This is done, for example, by depositing heat conductive material and then removing the heat conductive material everywhere except for the desired locations of heat conductor  17 . 
   The oxide layer formed on top of integrated circuit  12  is also removed at locations where it is desired to have a gap between PLC  11  and integrated circuit  12  in order to store and/or transport fluid for use within total internal switching elements. 
   Heat conductor  17  is used to radiate heat from resistors  21  and  22 . This is used, for example to warm walls of trench  16 . This results in a larger operating range and more stable bubbles since surface tension effect to imperfections in local gaps are reduced or removed. The viscosity, density and surface tension of the fluid within bubble chamber  14  change with heat. The heat is used to maintain the liquid near the reflecting wall above the evaporation point for a large range of applied power. The physical structures are used to create varied thermal maps and increased local pressure and recondense liquid. The physical heated bubble chamber can also be used to control flow fields to prevent impurities within the fluid from being drawn in and deposited onto resistors either by physical absorption or by chemical reaction. 
   Oxide pillars  19  and  20  also act as barriers for thermal and fluidic cross talk. The use of heat conductor  17  and pillars  19  and  20  reduce the need for metal or solder film designs that are sensitive to chemical and thermal ranges. 
   The shape and location of the heat conductors depend on the specific application. The heat conductors are more heat conductive than fluid used in the bubble chambers. The best heat conductors are orders of magnitude more heat conductive than fluid used in the bubble chambers. 
     FIG. 2  shows another example of a cross section of an area of a total internal switching element that includes a bubble chamber  34 . Bubble chamber  34  includes a trench  35  within a planar light circuit (PLC)  31 . Planar light circuit  31  includes waveguides that are switched at trench areas. Each trench is filled with an index-matching liquid (fluid). A bubble is generated at the cross-point by heating the index matching liquid with a localized heater. The bubble must be removed from the cross-point to switch the cross-point from the reflecting to the transmitting state and thus change the direction of the output optical signal. 
   An integrated circuit  32  is attached to PLC  31 , for example, by wafer bonding or soldering. Within integrated circuit  32 , trench  36  is aligned with trench  35  to fully form bubble chamber  34 . A resistor  41  and a resistor  42  are the localized heater used to generate bubbles within bubble chamber  34 . A pillar  39  and a pillar  40  are used to fill in a material  37  as shown. Pillar  39  and pillar  40  are used to conduct heat from resistor  41  and resistor  42  within walls of trench  36 . For example, pillars  39  and pillar  40  are composed of tantalum, tungsten, aluminum, silicon or another heat conducting material or stack of heat conducting material. Material  37  is composed of, for example, oxide. Resistor  41  and resistor  42  are composed of Tantalum Aluminum (TaAl) or some other resistive material. Sections  38  are part of a third metal layer on integrated circuit  32 . 
   In order to form pillar  39  and pillar  40 , an oxide layer is formed on top of integrated circuit  32 . For example, this is done by a Teos process or by a spin on glass process. A patterned etch is performed down to the oxide immediately over the locations of pillar  39  and pillar  40 . Pillar  39 , pillar  40  and bottom area  48  are formed of heat conducting material. A chemical mechanical polish (CMP) process is used to reduce pillar  39  and pillar  40  to a desired height. The oxide layer formed on top of integrated circuit  32  is also removed at locations over resistor  41  and resistor  42  and where it is desired to have a gap between PLC  31  and integrated circuit  32  in order to store and/or transport fluid for use within total internal switching elements and to allow electrical interconnect. Material  37  remains. 
   Pillar  39  and pillar  40  are used to radiate heat from resistors  41  and  42 . This is used, for example to warm walls of trench  36 . This results in a larger operating range and more stable bubbles since surface tension effect to imperfections in local gaps are reduced or removed. Material  37  acts to insulate and protect pillar  39  and pillar  40 . The use of material  37  and pillars  39  and  40  reduce the need for metal or solder film designs that are sensitive to chemical and thermal ranges. 
     FIG. 3  shows another example of a cross section of an area of a total internal switching element that includes a bubble chamber  54 . Bubble chamber  54  includes a trench  55  within a planar light circuit (PLC)  51 . Planar light circuit  51  includes waveguides that are switched at trench areas. Each trench is filled with an index-matching liquid (fluid). A bubble is generated at the cross-point by heating the index matching liquid with a localized heater. The bubble must be removed from the cross-point to switch the cross-point from the reflecting to the transmitting state and thus change the direction of the output optical signal. 
   An integrated circuit  52  is attached to PLC  51 , for example by wafer bonding or soldering. Within integrated circuit  52 , trench  56  is aligned with trench  55  to fully form bubble chamber  54 . A resistor  61  and a resistor  62  are the localized heater used to generate bubbles within bubble chamber  54 . A pillar  59  and a pillar  60  are used to conduct heat from resistor  61  and resistor  62  within walls of trench  56 . For example, pillars  59  and pillar  60  are composed of single crystal silicon or SiC, or a liquid settable ceramic such as Ceraset or Lanxide liquid settable ceramics available from KiON Corporation. A bottom area  68  is formed of heat conducting material. Resistor  61  and resistor  62  are composed of Tantalum Aluminum (TaAl) or some other resistive material. Sections  58  are part of a third metal layer on integrated circuit  52 . 
   Pillar  59  and pillar  60  can be attached to PLC  12  via micro molding. Pillar  59  and pillar  60  are used to radiate heat from resistors  61  and  62 . This is used, for example to warm walls of trench  56 . This results in a larger operating range and more stable bubbles since surface tension effect to imperfections in local gaps are reduced or removed. 
   Dependent on a particular application, pillars can be shaped and arranged in a variety of ways to form a heat conductor. For example,  FIG. 4  shows a top view of one arrangement where a pillar  71 , a pillar  72 , a pillar  73  and a pillar  74  reside on an oxide region  70  of an integrated circuit. Pillar  71  and pillar  72  are end pillars that can optionally be added to increase fluidic impedance and surface tension. 
     FIG. 5  shows a top view of another arrangement of pillars to form a heat conductor. The heat conductor includes a pillar  81 , a pillar  82 , a pillar  83 , a pillar  84 , a pillar  85  and a pillar  86  residing on an oxide region  80  of an integrated circuit. 
   In addition to use in TIRs, heat conductors can be used in other microfluidic devices that contain a single drop of fluid. For example, a pillar in the shape of a round tube can be used within a chamber in an inkjet printhead. The pillar can be used to heat the whole surface area of the chamber with heat supplied by one or more resistors at the base or at different heights along the pillar. 
   For example, such pillars can be used to allow precise inkjet depositions for DNA micro/nano arrays used for expression profiling. Using one or more pillars to direct heat in an inkjet tube allows reduction in the surface tension of ink allowing for smaller drops (pico to zepto liter) to be formed. 
   The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.