Abstract:
A method for electrically coupling thermoelectric cooling (TEC) elements together is described. The TEC elements are encased within an encapsulating material, such as epoxy, and a resist layer is laid on either end of the encapsulating material, covering the ends of the TEC elements. The resist layers are selectively developed to open locations in the resist layers in between adjacent elements. Conductive material, such as gold, is sputter deposited into the locations to provide electrical coupling of the elements.

Description:
FIELD OF THE INVENTION 
     The invention generally relates to the fabrication of semiconductor devices and associated thermoelectric cooling elements. 
     BACKGROUND 
     Heat transfer devices, such as thermoelectric coolers (TECs), are used in some high speed semiconductor devices, such as optoelectric semiconductor devices. TECs incorporate discrete elements which are electrically coupled together. These elements are generally formed from fragile materials commonly used in semiconductor fabrication, such as bismuth telluride. 
     Known methods of electrically coupling the elements to each other include aligning the elements with each other and soldering individual TEC elements in a matrix to a metallized support structure, such as a submount formed of beryllium oxide. 
     For example, a conventionally fabricated semiconductor device  10 , as shown in FIG. 1, includes an optoelectronic device  12  physically situated on and electrically coupled to a thermoelectric cooling (TEC) device  25 . The TEC device  25  includes a metallized ceramic plate  14 , a plurality of fragile TEC elements  18 , and a heat sink  20 . Each of the elements  18  is positively-doped at one end and negatively-doped at the opposite end. The elements  18  are electrically coupled to each other through soldered connections  23 . The elements  18  are further electrically coupled to the plate  14  and the heat sink  20  via solder balls  22 . 
     The known methods of electrically connecting conventional TEC devices  25 , i.e., soldering connections, to semiconductor devices present a disadvantage in that tolerances in the Z-axis direction (FIG. 1) of the semiconductor devices are large, generally no smaller than one mil, or one times ten to the minus three (1×10 −3 ) of an inch. Further, the standard deviation between semiconductor devices so manufactured is also large, often resulting in large numbers of a batch of such semiconductor devices failing to meet production standards. 
     SUMMARY 
     In one aspect, the invention provides an apparatus with a heat transfer structure that includes a plurality of heat transfer elements each having a positively-doped region and a negatively-doped region, an encapsulating material encapsulating the heat transfer elements in a block, and conductive connectors formed on the encapsulating material and electrically connecting the heat transfer elements together. The apparatus further includes an optoelectronic device electrically connected to the heat transfer structure. 
     In another aspect, the invention further provides a thermoelectric cooling device including a plurality of heat transfer elements each having a positively-doped region and a negatively-doped region, an encapsulating material encapsulating the heat transfer elements in a block, and conductive connectors electrically connecting the heat transfer elements together, the conductive connectors being formed on the encapsulating material. 
     In another aspect, the invention also provides a method for fabricating a semiconductor device. The method includes encapsulating a plurality of heat transfer elements within an encapsulating material to form a block of encapsulated heat transfer elements, each element having a positively-doped and a negatively-doped region, providing at least one resist layer covering an end of the heat transfer elements, selectively preparing locations in the resist layer, each of the locations extending to the encapsulating material and between one heat transfer element and an adjacent heat transfer element, and forming conductive material in the locations, wherein the conductive material electrically connects the heat transfer elements together. 
     These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a conventionally fabricated semiconductor device including a thermoelectric cooler device. 
     FIG. 2 is a perspective view showing thermoelectric cooling elements encased within an encapsulating material in accordance with an embodiment of the invention. 
     FIG. 3 is a side view of the encased elements of FIG. 2 being patterned in accordance with an embodiment of the invention. 
     FIG. 4 is a top view of resist material etched in a pattern in accordance with an embodiment of the invention. 
     FIG. 5 is a perspective view showing the thermoelectric cooling elements of FIG. 2 electrically coupled together. 
     FIG. 6 is a side view of a semiconductor device constructed in accordance with an embodiment of the invention. 
     FIG. 7 illustrates process steps for forming a thermoelectric cooling device in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 2-6 provide various views illustrating the fabrication of a thermoelectric cooling (TEC) device and a semiconductor device utilizing such a TEC device in accordance with an embodiment of the invention. FIG. 7 provides process steps for the fabrication of the TEC device. TEC devices constructed in accordance with an embodiment of the invention reduce the tolerances in the electrical coupling of the individual TEC elements to less than two microns. This reduced tolerance in turn lessens the size of, and diminishes the standard deviation the semiconductor devices utilizing such a TEC device in the Z direction, thereby saving in fabrication costs. 
     With specific reference to FIG. 2, there is shown a plurality of individual TEC elements  118  embedded within an encapsulating material  130 . The encapsulating material  130  is preferably a viscous fluidic material which hardens over a short time, either with no outside stimulus or in response to a temperature change or other curing condition. Most preferably, the material  130  is a dielectric material such as epoxy, or an elastomer such as rubber. The encapsulating material  130  extends between the opposite ends of the TEC elements  118  such that the encapsulating material  130  is generally flush with the ends of the TEC elements  118 . Specifically, the encapsulating material  130  may be even with the ends of the TEC elements  118  or it may be offset to one or both of the ends of the TEC elements  118 . 
     Preferably, the TEC elements  118  are initially securely held in a constant position relative to one another by, for example, an adhesive material. For example, the adhesive material may be mounted upon a hoop or other supporting structure and then may contact one of the ends of the TEC elements  118 . Once securely held in constant position relative to one another, the TEC elements  118  are encapsulated by the encapsulating material  130  at an initial step  200  (FIG.  7 ). 
     Prior to electrically coupling the TEC elements  118 , the ends of the TEC elements  118  may be smoothed by a lapping machine or with another device capable of smoothing fragile elements formed of, for example, bismuth telluride. 
     Referring specifically to FIG. 3, a resist material  140  is then deposited in layers  142 ,  144  over the opposite ends of the TEC elements  118  and the encapsulating material  130  to form a block  135  at a step  205  (FIG.  7 ). As illustrated, some TEC elements  118  have a positively-doped region  117  closest to the resist layer  142  and a negatively-doped region  119  closest to the resist layer  144  and are adjacent to TEC elements  118  having the negatively-doped region  119  closest to the resist layer  142  and the positively-doped region  117  closest to the resist layer  144 . Then, at a step  210  (FIG.  7 ), a pattern  146  is formed into the resist material layers  142 ,  144 . The pattern  146  includes a plurality of selectively formed shallow trenches  148  which extend down to and overlap the ends of the TEC elements  118 . The trenches  148  may be formed by a method well known in the art, such as by transmitting light through a patterned mask  101  to develop portions of the resist layers  142 ,  144  which are removed to create the trenches  148 . 
     The pattern  146  of trenches  148  (FIGS. 4-5) efficiently connects the elements  118  serially by connecting the positively-doped region  117  of one TEC element  118  with the negatively-doped region  119  of another TEC element  118 . As an example of the pattern  146  illustrated in FIG. 4, a trench  148  is shown in the resist layer  142  between the negatively-doped region  119  of a TEC element  118   a  and the positively-doped region of a TEC element  118   b . A trench  148  is also formed in the resist layer  144  between the TEC element  118   b  and the negatively-doped region  119  of a TEC element  118   c . A further trench  148  is formed in the resist layer  142  between the TEC element  118   c  and the positively-doped region  117  of a TEC element  118   d . An additional trench  148  is formed in the resist layer  144  between the TEC element  118   d  and the negatively-doped region  119  of a TEC clement  118   e  and another trench  148  is formed in the resist layer  142  between the TEC element  118   e  and the positively-doped region  117  of a TEC element  118   f.    
     After forming the pattern  142 , the block  135  is placed in an evaporator, or similar deposition apparatus at a step  215  (FIG.  7 ). A conductive material, preferably gold, is deposited onto the resist layers  142 ,  144 , becoming deposited within the trenches  148 , creating connectors  150  (FIGS. 5,  6 ). After the deposition, the remaining resist layers  142 ,  144  and the overlying gold layer on the resist are removed at a step  220  (FIG.  7 ), leaving the conductive, e.g. gold, connectors  150  in the pattern  146  extending between TEC elements  118  and supported in part by the encapsulating material  130 . 
     The TEC elements  118  within the encapsulating material  130  are then attached to a submount  164  and electrically coupled to a heat sink  168  (FIG. 6) at a step  230  (FIG.  7 ), thereby creating the thermoelectric cooling device  165 . The coupling of the TEC elements  118  with the submount  164  may be with a non-conductive epoxy, such as the material forming the encapsulating material  130 , or it may be by soldering. If soldered, a dielectric material (not shown) is positioned between the TEC elements  118  and the conductive submount  164 , and leads extending off from one or more of the connectors  150  are connected to an optoelectronic device  162 , such as a transmitter or a receiver, through, for example, ribbon bonds. 
     FIG. 6 shows a completed semiconductor device  160  which incorporates the thermoelectric cooling device fabricated in accordance with an embodiment of the invention. Specifically, the TEC elements  118 , which are encapsulated in the encapsulating material  130  and electrically coupled via the connectors  150  partially supported by the material  130 , are electrically connected to the optoelectronic device  162  through the submount  164 . Further, The TEC elements  118  are thermally connected directly to the heat sink  168 . 
     The semiconductor device  160  can be made with the tolerances in the Z-axis direction which are considerably smaller than Z-axis tolerances experienced in conventional devices. Specifically, the Z-axis direction tolerances expected for the semiconductor devices  160  are a few microns, or between about 5×10 −4  inches and about 7.6×10 −4  inches, as compared with a generally no smaller than one mil (1×10 −3  of an inch) tolerance in the Z-axis direction experienced in conventional devices. Additionally, greater reliability in the tolerances in the Z direction are achieved, thereby reducing the number of semiconductor devices discarded for failing to meet production quality standards. 
     While the foregoing has described in detail exemplary embodiments of the invention, it should be readily understood that the invention is not limited to the 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. For example, while the connection of the optoelectronic device  162  with the TEC elements  118  has been described as being through ribbon bonds, some of the metallization of the connectors  150  may be formed along a side of the encapsulating material to allow for connection with the die by, for example, printed wiring. 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.