Abstract:
Flexible ground connectors are adapted to withstand temperature-induced stresses. The connectors may be formed of low thermal conductivity materials. The connectors may be used within a semiconductor package that also encloses a thermoelectric cooling device, a conductive submount, and a semiconductor light source. The submount may be grounded to the package wall by locating a pair of the flexible ground connectors across a gap to a ledge in the wall. The ground connectors may be formed of stainless steel, and they may be gold plated for improved electrical conductivity.

Description:
FIELD OF THE INVENTION 
     The invention generally relates to semiconductor packages, and more particularly to optoelectric semiconductor packages with greater mechanical compliance and reduced thermal loss at the ground connections. 
     BACKGROUND 
     Known optoelectric semiconductor packages typically include an optical subassembly which contains a laser chip and a conductive platform. The chip delivers an optical signal to a lens in the optical subassembly, and the signal is then launched from the lens into a high speed connector, such as a metallic wire or pin. The high speed connector leads from the platform to connect the chip with a semiconductor device that utilizes optical signals. Examples of known laser packages may be found in U.S. Pat. Nos. 6,106,161 (Basavanhally et al.) and 5,881,193 (Anigbo et al.). The chip is generally grounded through the platform to the package body. Typically, the ground connection is accomplished through a solder bridge or a conductive epoxy bridge. 
     The temperature at which the chip operates may be governed by a thermoelectric cooling (TEC) device, which serves to control and/or stabilize the wavelength of the light emitted by the chip. An example of the use of TEC devices in an optoelectric semiconductor package may be found in U.S. Pat. No. 6,055,815 (Peterson). The chip, conductive platform and TEC device may be located within a package. The package provides physical protection to the assembled components as well as attendant connectors. 
     As the TEC device heats and cools, thermal stresses are created and the ground connection between the conductive platform and the package body flexes due to differing thermal expansions of the relevant materials making up the conductive platform and the package body. Over time, the flexing of the ground connection, i.e., the solder or epoxy bridge, may cause the bridge to break. 
     SUMMARY 
     The invention provides a semiconductor package which includes a chip, a conductive support structure, a heat transfer device, and a package body having a cavity. The chip, conductive support structure and heat transfer device are positioned within the cavity. An optoelectric connector extends from the cavity out of the package body. At least one ground connector connects the conductive support structure with the package body. The ground connector is adapted to flex with thermal changes. 
     The invention also provides a method for reducing thermal loss through ground connections in an optoelectric semiconductor package. The method includes providing a chip and a conductive support structure within a cavity of a package body. The cavity may have a wall with a ledge. At least one ground connector may be positioned between the conductive support structure and the ledge. According to one aspect of the invention, the ground connector is formed of stainless steel foil. 
     According to another aspect of the invention, a semiconductor package having ground connectors is adapted to be compliant or flexible under thermal stress. The method includes providing a cavity including an interior wall spaced apart from a conductive support structure, and attaching at least one ground connector to the conductive support structure and the cavity interior wall. If desired, the ground connector may be formed of a conductive foil which flexes with thermal stress. 
     These and other advantages and features of the invention will be more readily understood from the following detailed description which is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a semiconductor package constructed in accordance with an embodiment of the invention. 
     FIG. 2 is a cross-sectional view taken along line II—II of the semiconductor package of FIG.  1 . 
     FIG. 3 is a perspective view of semiconductor components within the semiconductor package of FIG.  2 . 
     FIG. 4 is a partial cross-sectional view of a ground connector of the semiconductor package of FIG.  2 . 
     FIG. 5 is a partial cross-sectional view of a ground connector constructed in accordance with another embodiment of the invention. 
     FIG. 6 is an enlarged view taken within circle VI of the ground connector of FIG.  4 . 
     FIG. 7 is an enlarged view of a ground connector constructed in accordance with another embodiment of the invention. 
     FIG. 8 is an enlarged view of a ground connector constructed in accordance with another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1-3, there is illustrated a semiconductor package  10  which includes a package body  12 . The package body  12  includes an interior body wall  14  (FIG.  2 ), which acts as a ground for the package body  12 , and a ledge  16  extending transversely from the wall  14 . Held within a cavity  18  of the package body  12  are a conductive support structure  24 , a semiconductor chip  26 , an electrical connector  28 , an assembly including an optical subassembly and a heat transfer device, such as a thermoelectric cooling (TEC) device, denoted generally as  22 . Preferably, the chip  26  is a laser chip capable of emitting optical signals. A lid  20  closes the cavity  18 . 
     A high speed optoelectric connector  30 —such as a metallic (for example, gold) wire or pin, or a V connector, or a K connector—extends from within the cavity  18  out of the package body  12  through a channel  36  (FIG. 3) of a conduit  34  (FIG. 2) toward a telecommunications device (not shown). The conduit  34  is positioned on an external surface  17  of the package wall  14 . 
     A waveguide  32  (FIG. 3) is located on an upper surface of the conductive support structure  24 . The waveguide  32  has a signal plane  31  between a pair of ground planes  33 . The waveguide  32  serves to transmit signals from the chip  26  to the high speed connector  30 . The waveguide  32  may take any suitable waveguide form, but is preferably a coplanar waveguide, as illustrated. The conductive support structure  24  is preferably a submount formed of a conductive material, such as, for example, beryllium oxide. 
     The TEC device of the assembly  22  provides active temperature control based upon the dynamic characteristics of the chip  26 . The optical performance output of an optoelectric chip, such as the chip  26 , changes over time and with temperature variations. As an output signal of the chip  26  changes with time and temperature, the TEC device of the assembly  22  is able to place the optical output signal within desired specifications. 
     A gap  40  exists between the semiconductor components within the cavity  18 , i.e., the structure  24  and the assembly  22 , and the ledge  16  of the wall  14 . At least one, and preferably two ground connectors  38  are positioned across the gap  40 . The connectors  38  provide a physical connection with the ground connectors  33  of the structure  24 , so as to provide a ground connection between the structure  24  and the package body  12 . In a preferred embodiment of the invention, the ground connectors  38  are on opposite sides of the signal plane  31 . The invention should not be limited, however, to the preferred embodiments shown and described in detail herein. 
     In the embodiment shown in FIG. 4, the gap  40  has a width W 1  which is less than half as long as the length L 1  of the ground connectors  38 . In the embodiment shown in FIG. 5, the gap  40  has a width W 2  which is at least half as long as the length L 1  of the the ground connectors  38 . Most preferably, the gap  40  is less than 0.010 inches. 
     The ground connectors  38  are formed of a conductive material, preferably a metallic material. Most preferably, the ground connectors  38  are formed of a material which conducts electricity but is a poor thermal conductor, such as, for example, stainless steel. Stainless steel also inhibits thermal loss since it is a poor conductor of heat. Stainless steel foil also allows the ground connectors  38  to be more compliant, allowing greater flex due to thermal changes caused by the TEC device of the assembly  22 . The TEC device of the assembly  22  causes thermal changes to the conductive support structure  24 . Specifically, the TEC device of the assembly  22  causes the structure  24  to expand and shrink, thereby shortening and lengthening, respectively, the width W 1  of the gap  40 . 
     In high speed applications, such as in optoelectric semiconductor devices, the electrical current travels close to the surfaces of the connection structures. In one aspect of the invention, the ground connectors  38  are plated with a highly electrically conductive material, such as a layer of gold  39 . The plating  39  enhances the electrical conductivity of the ground connectors  38 . Since the electrical current travels close to the surfaces of the connection structures, the plating  39  may be as thin as about 10 microns. Furthermore, since the plating  39  is relatively thin compared to the connector  38 , thermal loss at the ground connectors  38  remains inhibited since the connectors  38  are primarily formed of stainless steel. The plating  39  may surround the ground connectors  38 , or it may be on opposing sides of the ground connectors, on one side of the ground connectors, or it may be in selected portions, such as stripes, on the ground connectors. 
     The ground connectors  38  may be attached to the structure  24 . In one aspect of the invention, the ground connectors  38  are attached to the structure  24  with a soft solder  50  (FIG.  6 ). In another aspect of the invention, the ground connectors  38  are attached to the structure  24  with a conductive epoxy  150  (FIG.  7 ). 
     As noted above, the ground connectors  38  are formed of a material, such as stainless steel, which allows for flex due to thermal changes caused by the TEC device of the assembly  22 . In one aspect of the invention, as illustrated in FIG. 8, ground connectors  138  may be mounted across the gap  40  onto the package body  12  and the structure  24 . The ground connectors  138  include a crease  139 , which allows for flex of the ground connectors  138  in response to thermal changes caused by the TEC device of the assembly  22 . 
     While the invention has been described in detail in connection with preferred embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.