Abstract:
A laser package includes a submount, a laser die mounted on the submount, a lid mounted on the submount over the laser die, and a soft metal disposed between the laser die and the lid, wherein the soft metal conducts heat between the laser die and the lid. The soft metal is able to creep or cold flow under pressure to accommodate for varying manufacturing tolerances and varying thermal expansion rates of the elements in the laser package.

Description:
FIELD OF INVENTION 
     This invention relates to heat dissipation for laser transmitters, receivers, and transceivers. 
     DESCRIPTION OF RELATED ART 
     The performance and life of a laser are often affected by temperature. Some devices, such as lasers in dense wavelength division multiplexers (DWDM) or lasers with external electro-absorption modulators, require fine temperature control often within 1 degree Celsius. The heat generated by a laser must be dissipated away from the laser. 
     A semiconductor laser, such as an edge-emitting laser or a vertical cavity surface-emitting laser (VCSEL), generates a large amount of heat in a small area (i.e., high heat density). Thus, the conduction path away from the laser must be able to transfer the heat away at high density. 
     In additional to a low resistance thermal path, a semiconductor laser also requires short electrical connections (i.e., low inductance and capacitance connections) and very accurate alignment of the optical components. Flip chip assembly is often considered for short and high performance electrical connection. Flip chip connections, such as solder balls, can also be self-aligning using the surface tension of the solder. 
     However, flip chip mounting of a laser die has poor heat transfer characteristics. The contact area of the solder ball joint is very small and thus thermally resistive. Therefore, another heat path must be added to dissipate heat away from the laser. The back of the laser die is usually used as a path for beat transfer. However, the exact location of the laser and the thickness of the laser die have manufacturing variations that create a variation in the location of the back of the laser die. A conventional solid connection between the back of the laser die and a heat sink often creates an over-constrained design that overstresses and breaks the solder ball joints. Non-rigid connections, such as crushable gold wire mesh, carbon or metal loaded elastomer pads, and spring contacts, usually have poor heat transfer characteristics. Thermal and electrical connections must survive large temperature variations, such as those that occur when the finished package is soldered to a printed circuit board (PCB). These temperature variations can often be the stress that breaks connections in over-constrained designs. 
     Thus, what is needed is a laser package with a thermal connection that provides high heat transfer away from the laser for a large temperature variation while remaining adaptable to varying manufacturing tolerance. 
     SUMMARY 
     In one embodiment of the invention, a laser package includes a submount, a laser die mounted on the submount, a lid mounted on the submount over the laser die, and a soft metal disposed between the laser die and the lid, wherein the soft metal conducts heat between the laser die and the lid. In one embodiment, the soft metal is able to creep or cold flow under pressure to accommodate for varying manufacturing tolerances and varying thermal expansion rates of the elements in the laser package. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  illustrate perspective and side views of a laser package in one embodiment of the invention. 
         FIGS. 3 ,  4 ,  5 , and  6  illustrate the assembly of a laser package in one embodiment of the invention. 
         FIG. 7  illustrates a side view of a laser package where the temperature of a laser die is controlled independently from the other components in one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  illustrate perspective and side views of a laser package  10  in one embodiment of the invention. Package  10  may be part of a laser transmitter, receiver, or transceiver. 
     Package  10  includes a laser die  12 , a photodetector die  14 , and a laser driver die  16  mounted on the top surface of a submount  18 . In one embodiment, dies  12 ,  14 , and  16  are flip chip mounted by solder balls  20  on their top surface onto submount  18 . For clarity, only one solder ball  20  has been labeled in  FIG. 2 . 
     Laser die  12  includes one laser or an array of lasers, such as edge-emitting lasers or VCSELs. Photodetector die  14  includes one feedback photodetector or an array of feedback photodetectors, such as positive-intrinsic-negative (PIN) photodiodes. Laser driver die  16  includes integrated circuits (ICs) that drive laser die  12  and process the feedback signals from photodetector die  14 . Laser driver die  16  can include additional ICs depending on the application. Although shown as discrete components, dies  12 ,  14 , and  16  can be integrated into a single die. 
     Submount  18  includes interconnects  22  that couple dies  12 ,  14 , and  16 . For clarity, only two interconnects  22  have been labeled in  FIG. 2 . Submount  18  can include additional ICs depending on the application. A lid  24  is mounted on the top surface of submount  18  to at least partially enclose dies  12 ,  14 , and  16 . In one embodiment, lid  24  hermetically seals dies  12 ,  14 , and  16 . In one embodiment, lid  24  is silicon. 
     Alignment pins  26  are mounted on the back surface of submount  18 . Alignment pins  26  align package  10  to a connector containing optical fibers. A lens  28  is mounted on the back surface of submount  18  opposite of laser die  12 . Lens  28  focuses light emitted by laser die  12  into the optical fibers. Although shown as a discrete component, lens  28  can be integrated into submount  18 . 
     A soft metal thermal conductor  30  is disposed between the inner surface of lid  24  and the bottom surface of flip chip mounted laser die  12 . Soft metal  30  can creep or cold flow. A soft metal, such as indium, can cold flow at a pressure of  500  pounds per square inch. A soft metal, again such as indium, can cold flow much faster than package  10  (specifically laser die  12  and lid  24 ) can thermally cycle (i.e., expand and contract). Soft metal  30  can be indium, gallium, mercury, or tin/lead solder Soft metal  30  may be liquid or solid depending on the temperatures of laser die  12  and lid  24 . The inner surface of lid  24  and the bottom surface of laser die  12  have corresponding metal pads  32  and  34  ( FIG. 2  only) that control the location of the soft metal by not letting soft metal  30  flow out of the joint. The soft or liquid metal  30  is held in place by surface tension. The design of pads  32  and  34  are such that these pads do not dissolve into soft metal  30 . 
     Similar to soft metal  30 , a soft metal thermal conductor  36  can be disposed between the inner surface of lid  24  and the bottom surface of flip chip mounted photodetector die  14 . The inner surface of lid  24  and the bottom surface of photodetector die  14  have corresponding metal pads  38  and  40  ( FIG. 2  only) that control the wetting of soft metal  36  when it is liquid. 
     Furthermore, a soft metal thermal conductor  42  can be disposed between the inner surface of lid  24  and the bottom surface of flip chip mounted laser driver die  16 . The inner surface of lid  24  and the bottom surface of laser driver die  16  have corresponding metal pads  44  and  46  ( FIG. 2  only) that control the wetting of soft metal  42  when it is in the liquid state. 
     In one embodiment where the soft metal is indium and the metal pads are gold, a gold-indium intermetallic is formed at their interface. The gold-indium intermetallic does not melt and maintains the connection under tension at temperatures when the indium is solid. Gold is particularly well suited as the material for the metal pads because it can be easily plated and the gold intermetallic at the surfaces resists melting even at high temperature. If the metal pads are made of another material, an over-constrained design may result when the innermatalic continues to diffuse through the whole joint and significantly stiffens the joint. 
     The soft metal thermal conductor yields or flows under a low force even when it is solid. Thus, the soft metal thermal conductor stays in constant contact between the lid and the dies when the soft metal is liquid or solid, thereby providing high heat transfer between the lid and the dies. Furthermore, the soft metal thermal conductor accommodates changes in the manufacturing tolerances and the small movement between parts caused by coefficient of thermal expansion (CTE) mismatch between the materials of package  10 . 
       FIGS. 3 ,  4 ,  5 , and  6  illustrate the assembly of laser package  10  in one embodiment of the invention. 
     In a first step shown in  FIG. 3 , laser die  12 , photodetector die  14 , and laser driver die  16  are flip chip mounted on solder balls onto substrate  18 . Dies  14 ,  12  and  16  can have their backsides metallized at the wafer level to form metal pads  34 ,  40 , and  46  before being sawn into individual dies. 
     In a second step shown in  FIG. 4 , metal pads  32 ,  38 , and  44  are formed on the inner surface of lid  24 . In one embodiment, metal pads  32 ,  38 , and  44  are formed by vacuum metallization. 
     In a third step shown in  FIG. 5 , soft metals  30 ,  36 ; and  42  are disposed on metal pads  32 ,  38 , and  44  of lid  24 . Soft metals  30 ,  36 , and  42  can be metal performs (stamped soft metal pieces) placed, or pressed, on metal pads  32 ,  38 , and  44 . Preforms  30 ,  36 , and  42  can be loose or held in place by the slight force of interlocking when they are pressed down into metal pads  32 ,  38  and  44 . 
     In a fourth step shown in  FIG. 6 , lid  24  is mounted on the top surface of submount  18  to at least partially enclose dies  12 ,  14 , and  16 . At the same time, soft metals  30 ,  36 , and  42  are sandwiched between lid  24  and dies  12 ,  14 , and  16 . Note that the thickness of soft metals  30 ,  36 , and  42  are selected to be greater than the gap between lid  24  and dies  12 ,  14 , and  16 . By making the preforms thicker than required for final assembly, they will cold flow to the correct thickness during assembly. 
       FIG. 7  illustrates a side view of a laser package where the temperature of a laser die is controlled independently from the other components in one embodiment of the invention. In this embodiment, the solder ball joints used to flip chip mount dies  12 ,  14 , and  16  are also used to thermally isolate laser die  12  away from the other components. A thermal electric cooler  82  is rigidly mounted to the inner surface of lid  24 . In one embodiment, lid  24  includes a metal feed through (e.g., like a PCB via) to power thermal electric cooler  82 . Metal pad  32  is formed on the bottom surface of thermal electric cooler  82 . Soft metal  30  is disposed between laser die  12  and thermal electric cooler  82  to provide a conductive path. Soft metal  30  allows thermal electric cooler  82  to be rigidly mounted to lid  24 , thereby allowing good heat conduction while permitting relative movement between the components along this conductive path. Thermal electric cooler  82  cools laser die  12  without affecting dies  14  and  16 . In this embodiment, lid  24  can be metal or ceramic, such as aluminum nitride. 
     Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Numerous embodiments are encompassed by the following claims.