Abstract:
Energy is applied to a portion of a conducting body. In preferred embodiments, relative motion between the conducting body and the energy source is created such that the energy source moves along a thermal diffusion front, thereby enhancing the thermal diffusion front in the direction of the relative movement. The energy is preferably applied in a portion of the conducting body with higher thermal mass and the enhanced thermal diffusion front is directed toward a portion with lower thermal mass. The lower thermal mass portion expands, thereby creating fissures in surrounding material, then melts, flows through the fissures and contacts another conductor, thereby forming a conductive link.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to the field of forming connections and conducting links, and more particularly to forming connections and conductive links by directed thermal diffusion.  
           [0003]    2. Discussion of the Background  
           [0004]    It is known in the art to form interconnecting links between adjacent or closely situated conductive materials. In particular, in the semiconductor industry, it is necessary to form conductive links after an integrated circuit is formed in a number of applications. For example, because DRAM (dynamic random access memory) circuits sometimes contain manufacturing errors, they are often manufactured with extra rows and/or columns. When one row or column is found to be defective, a fuse is blown to disconnect the defective row or column and one of the extra rows or columns is connected in its place. The connection of the extra row or column is one example of an application wherein a conductive link must be formed. Another example of an application in which such links must be formed is certain types of programmable logic devices.  
           [0005]    Several technologies are known for forming such conductive links. For example, anti-fuses are well known in the art. Other examples of forming such links are discussed in U.S. Pat. Nos. 5,940,727, 5,861,325, 5,585,602, 5,920,789 (all to Bernstein) and U.S. Pat. No. 4,810,663 (to Raffel et al.). In the Bernstein technique, the links are formed by heating a conductor separated from another conductor by a dielectric material such that the dielectric material fractures and melted metal flows in fissures formed by the fractured dielectric material to connect the conductors. In the Raffel technique, conductive layers are separated by a material which forms an alloy with the conductive materials when heated by a laser.  
           [0006]    There are drawbacks to all of the foregoing links, including, but not limited to, use of non-standard processing steps, low conductivity, potential of damage to surrounding area, and/or the requirement of directing a laser beam at the exact location where an interconnecting link is desired.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method for forming a conductive link using a process referred to herein as directed thermal diffusion wherein energy is applied to a conducting body that includes portions with differing thermal mass, with the energy being applied in a portion of the conducting body with relatively higher thermal mass, resulting in a thermal diffusion front directed toward a portion of the conducting body with lower thermal mass. The lower thermal mass portion is surrounded by non-conducting material, while the higher thermal mass portion may or may not be surrounded by non-conducting material. The lower thermal mass portion then expands, thereby creating fissures in the surrounding non-conducting material. Next, the lower thermal mass portion melts and flows through the fissures until contact with another conductor is made, thereby forming a conductive link between the conducting body and the other conductor. In preferred embodiments, relative motion between the conducting body and the energy source is created such that the energy source moves toward the lower thermal mass portion, thereby creating an enhanced thermal diffusion front in the direction of the lower thermal mass portion.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    A more complete appreciation of the invention and many of the attendant advantages and features thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0009]    [0009]FIG. 1 is a perspective view of a conducting body including portions with higher and lower thermal masses having the portion with the lower thermal mass positioned over another conductor according to one embodiment of the present invention.  
         [0010]    [0010]FIG. 2 is a top view of the conducting body of FIG. 1.  
         [0011]    [0011]FIG. 3 is a cross sectional view of the conducting body of FIG. 1 taken along the line III-III.  
         [0012]    [0012]FIG. 4 is a perspective view of a conducting body including portions with higher and lower thermal masses having the portion with the lower thermal mass positioned over two other conductors according to a second embodiment of the present invention.  
         [0013]    [0013]FIG. 5 is a top view of the conducting body of FIG. 4.  
         [0014]    [0014]FIG. 6 is a cross sectional view of the conducting body of FIG. 4 taken along the line VI-VI. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0015]    The present invention will be discussed with reference to preferred embodiments of methods for forming conductive links using directed thermal diffusion. Specific details, such as dimensions of conducting bodies and materials, are set forth in order to provide a thorough understanding of the present invention. The preferred embodiments discussed herein should not be understood to limit the invention. Furthermore, for ease of understanding, certain method steps are delineated as separate steps; however, these steps should not be construed as necessarily distinct nor order dependent in their performance.  
         [0016]    [0016]FIG. 1 illustrates a conducting body  100  positioned over a target conductor  200 . In the embodiment of FIG. 1, it is desired to form a conductive link between the conducting body  100  and the target conductor  200 . The conducting body  100  may be formed from any number of conducting materials, but is preferably formed by a metal such as copper, aluminum, or an alloy thereof. The conducting body  100  includes a first portion  110  and a second, tapered portion  120 . The first portion  110  is physically larger and of higher thermal mass than the tapered portion  120 . In exemplary embodiments, the width of the tapered portion  120  is less than one micron, while the width of the first portion  110  is approximately 10 microns or more. The thickness of both the tapered portion  120  and the first portion  110  may be approximately 1 micron.  
         [0017]    In preferred embodiments, the entire conducting body is formed of the same material. However, it is possible to form the first portion  110  from a different material than the second portion  120 . For example, the tapered portion  120  could comprise a portion of a line of conducting material and the first portion  110  could comprise a different material formed over the tapered portion  120  (such that the first and second portions  110 ,  120  are formed by different layers in contact with each other). The tapered portion  120  is positioned over a target conductor  200 . The tapered portion  120  is not in contact with the target conductor  200 ; rather, they are separated by a dielectric material such as BPSG (boro-phosphoro silicate glass, not shown in FIG. 1). The target conductor  200  includes a wide pad  210  to which is connected narrower conductors  220 . The pad  210  may be any conducting material, but is preferably chosen so as to form a good bond with the material in the tapered portion  120  of the conducting body  100 .  
         [0018]    Referring now to FIG. 2, the process of forming a conductive link between conducting body  100  and the target conductor  200  starts with an energy pulse  250  being applied to the first portion  110  of conducting body  100 . The energy pulse is preferably supplied by a pulsed laser (not shown in FIG. 2). The power and duration of the energy pulse  250  are chosen such that the total heat ΔQ=M 1 *C v *ΔT (where ΔQ is the change in heat, M 1  is the thermal mass of the first portion  110 , C v  is the thermal conductivity of material in the first portion  110 , and ΔT is the change in temperature) transferred to the first portion  110  does not cause the temperature of the first portion to rise above the melting point. Using the dimensions set forth above, the power of a typical laser pulse will be on the order of a milli-Joule and will last approximately 10 nanoseconds. A second energy pulse  251  is then applied to the first portion  110 . The second pulse  251  is spaced apart from the first pulse  250  in a direction D toward the tapered portion  120 . The spacing and time between the first and second pulses  250 ,  251  is chosen such that the thermal diffusion front created by the first pulse  250  is strengthened in a direction D by the second pulse  251 . Subsequent pulses  252 - 254  are then applied along the direction D so as to further strengthen the thermal diffusion front in the direction D toward the tapered portion  120 .  
         [0019]    When the thermal diffusion front reaches the tapered portion  120 , the energy ΔQ=M 2 *C v *ΔT applied to the smaller thermal mass M 2  of the tapered portion  120  raises the temperature T of the tapered portion  120 . The tapered portion  120  first expands as the temperature T rises. This expansion causes fissures, or cracks, to develop in the dielectric material (not shown in FIG. 2) surrounding the tapered portion  120  and the target  200 . The tapered portion  120  then melts. Melted material from the tapered portion  120  then flows in the fissures/cracks to create one or more conductive links  222  to the pad  210  of the target conductor  200  as shown in FIG. 3. The conductive links  222  cool and remain in place after the energy source is removed from the conducting body  100 .  
         [0020]    A second embodiment of the present invention is shown in FIGS.  4 - 6 . This embodiment is similar to the above embodiment with the exception that connections to two target conductors  400   a  and  400   b  must be made. The embodiment of FIGS.  4 - 6  may therefore be used when the conducting body  100  is provided solely for the purpose of forming the conducting link between the first target  400   a  and the second target  400   b  and the first portion  110  is not connected to any other portion of a circuit. This should be contrasted with the embodiment of FIGS.  1 - 3 , where the first portion  110  is connected to one portion of a circuit and the target  200  is connected to a second portion of a circuit.  
         [0021]    One aspect of the above-described method is that the energy is supplied to a portion of conducting body  100  at a position remote from the location where the actual link is formed. The ability to form links at locations remote from a location where energy is applied may be of critical importance in some semiconductor applications where the circuit layout is such that directing an energy pulse to the location where the link is required in impossible or impractical. Another aspect of the embodiment described above is that the relatively large size of the first portion facilitates application of the energy pulses. For example, the focus of the laser beam and the motion control of the laser beam or semiconductor wafer will not require as much precision where the relatively large first portion of the conducting body  100  is provided.  
         [0022]    In the embodiments described above, the first portion  110  may is preferably left exposed to facilitate application of the energy pulses  250 - 254 . It is also possible, however, to cover the first portion  110 . In such embodiments, the first portion  110  is preferably covered by a transparent substance such as the substances used in many passivation layers and/or dielectrics.  
         [0023]    In the preferred embodiments discussed above, the total heat transferred to the first portion  110  does not cause the first portion to melt. However, it is also possible to apply sufficient total heat to melt the first portion  110 . This may be particularly useful in embodiments wherein the first portion  110  is left exposed so that the expansion of the first portion  110  due to melting will not crack any surrounding material but rather will expand in the direction of the exposed areas.  
         [0024]    In the above-described embodiments, energy may be applied by a pulsed energy source such as a laser beam. It should be understood that it is also possible to apply the energy continuously. It is also possible to move the energy supply with respect to the conduction body, to move the conducting body with respect to the energy supply, or to move both the energy supply and the conducting body so as to create relative motion between them.  
         [0025]    Obviously, numerous other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.