Patent Publication Number: US-2018047697-A1

Title: Chip arrangement and method for forming a contact connection

Description:
The invention relates to a chip arrangement and to a method for forming a contact connection between a chip, in particular a power transistor or the like, and a conductor material track, the conductor material track being formed on a non-conductive substrate and the chip being arranged on the substrate or on a conductor material track. 
     The chips known from the state of the art can be realized with a housing and also as a naked semiconductor component. These chips are regularly called die, bare die or bare chip. Chips of this kind are processed without a housing and are applied directly to a substrate or to a printed circuit board. In doing so, a direct contact can be formed between the chip contact surfaces of the chip and conductor material tracks. Other chip contact surfaces of the chip can be connected to other conductor material tracks via contact conductors or so-called bond wires. Contact connections of this kind are also regularly formed on power transistors. In that context, for example, a lower chip contact surface of a power transistor or chip is arranged on a conductor material track and connected to it substantially across its entire surface. An upper chip contact surface of the chip opposite of the contacted lower chip contact surface is connected to another conductor material track running next to the chip via a plurality of bond wires by ultrasonic bonding or soldering, for example. This chip arrangement ensures the availability of a sufficiently large conductor cross-section for the transmission of high currents. Also, comparatively thick bond wires made of aluminum can be used for applications of this kind. 
     In particular in case of power transistors, such as MOSFETs, IGBTs and diodes for power modules, high thermal and electrical stresses occur, for example in a power range of 15 to 150 kW. These stresses can easily lead to component failure of the contact connection and of the chip arrangement. Moreover, the use of the chip arrangement in vehicles or wind energy plants, for example, may further reduce a service life of the chip arrangement because of high temperature fluctuations. For instance, a contact connection may break or be disconnected because of these stresses. Furthermore, a solder connection between a contact conductor and a conductor material track or a chip contact surface can only be heated to a maximum of 175° C. without permanently damaging the contact connection. Furthermore, a chip arrangement of this kind is comparatively expensive to produce because a plurality of contact conductors has to be arranged between the chip contact surface and the conductor material track in order to transmit high currents. 
     Therefore, it is the object of the present invention to propose a chip arrangement having a contact connection and a method for forming a contact connection in which the contact connection exhibits improved durability while its production is simplified. 
     This object is attained by a method for forming a contact connection having the features of claim  1  and by a chip arrangement having the features of claim  14 . 
     In the method according to the invention for forming a contact connection between a chip, in particular a power transistor or the like, and a conductor material track, the conductor material track is formed on a non-conductive substrate, the chip being arranged on the substrate or on a conductor material track, a silver paste or a copper paste being applied to each of a chip contact surface of the chip and the conductor material track, a contact conductor being immersed into the silver paste or the copper paste on the chip contact surface and into the silver paste or the copper paste on the conductor material track, a solvent contained in the silver paste or the copper paste being at least partially vaporized by heating and the contact connection being formed by sintering the silver paste or the copper paste by means of laser energy. 
     The substrate can be made of a plastic material or a ceramic material, conductor material tracks for connecting electronic components or semiconductor components being formed on the substrate at first. This may take place by way of a method well understood in the state of the art. The chip is subsequently arranged directly on the non-conductive substrate or on a conductor material track. If the chip is arranged on the conductor material track, it is electrically connected to the latter. On an upper side of the chip facing away from the substrate or the conductor material track, at least one upper chip contact surface is formed for forming a contact with the chip. By forming the contact connection, the chip contact surface will now be connected to a conductor material track adjacent relative to the chip or to another conductor material track via a contact conductor. To this end, a liquid or pasty silver paste or copper paste is applied to the conductor material track adjacent relative to the chip. The paste or metal paste is also applied to the upper chip contact surface. The paste can be applied automatically by means of an applicator. Then, the contact conductor is immersed into the paste on the chip contact surface and into the paste on the conductor material track so that the contact conductor is at least partially surrounded by the paste in each case. The silver paste or copper paste contains a solvent, which is at least partially vaporized by heating, which may lead to a reduction of the volume of the applied paste. Depending on the extent of heating, the solvent can be vaporized by evaporation or boiling. The ultimate formation of the contact connection takes place by sintering of the silver paste or copper paste by means of laser energy or laser sintering, a laser beam being aimed directly or indirectly at an area of the contact connection. Silver powder or silver particles contained in the silver paste will at least partially melt or sinter together, thus forming an electrical connection between the contact conductor and the chip contact surface and between the contact conductor and the conductor material track. The copper paste is sintered in the same way. An application of the silver paste or the copper paste to the chip contact surface, the conductor material track and, if applicable, the contact conductor and the subsequent sintering can take place in parallel or in sequence, wherein it is immaterial then whether the chip contact surface or the conductor material track is the first to be provided with paste or sintered. It is also possible to first finish forming a contact on the chip contact surface or on the conductor material track and to subsequently apply and sinter the paste for forming the second contact. Regarding the order of the method steps, the contact conductor can also first be arranged on the chip contact surface or the conductor material track before the paste is applied. Then, the paste is applied to the chip contact surface or the conductor material track with the contact conductor in such a manner that the contact conductor is simultaneously immersed in the paste. 
     Owing to the fact that a silver paste or a copper paste is used instead of a solder to produce the contact connection, the contact connection lasts substantially longer. The sintered paste is substantially more resistant to high temperature fluctuations and high operating temperatures. For instance, operating temperatures of the chip of up to 300° C. can be achieved with sintered silver paste. Moreover, it is no longer necessary to form a large number of contact connections between the chip terminal surface and the conductor material track in order to be able to transmit high currents. The method according to the invention is largely independent from a cross-section of the contact conductor, making it possible to substantially reduce the number of contact conductors and contact connections, which makes the production of the chip arrangement more cost-effective on the whole. 
     A stranded wire, preferably a flat litz wire, particularly preferably a stranded wire or a flat litz wire made of copper or a copper alloy can be used as a contact conductor. A stranded wire is particularly flexible, which means that in contrast to a contact conductor in the form of a solid wire, no shear or tensile stresses such as those potentially caused by large temperature differences can occur in the respective contacts on the chip contact surface or the conductor material track. As opposed to a wire or bond wire, a flat litz wire has a relatively large cross-section, which is why it is able to transmit high currents. 
     Also, the stranded wire can be at least partially infiltrated by the silver paste or copper paste. By immersing the stranded wire into the paste, the paste, that is silver particles or copper particles, can penetrate the stranded wire so that a particularly tight connection between the silver particles or copper particles and the stranded wire can be formed during sintering. The paste can also be absorbed by the stranded wire owing to its capillary effect so that the stranded wire is filled with silver particles or copper particles in the area of the chip contact surface and of the conductor material track. 
     Furthermore, it may be envisaged that only one stranded wire is used per chip contact surface. This is made possible by the fact that the stranded wire or the flat litz wire can have a comparatively large width that is approximated to the dimensions of the chip contact surface. In this case, it will no longer be necessary to use a plurality of contact conductors. If only one stranded wire is used to connect the chip contact surface and the conductor material track, the method can be implemented in a particularly quick and thus cost-effective manner. 
     Heating of the silver paste or the copper paste can be achieved by arranging the substrate on or on top of a heating element. The substrate and thus the conductor material track as well as the chip contact surface can then be heated until the solvent of the paste vaporizes. Thus, the use of an oven or of a similar device for external heating of the paste can be entirely omitted. 
     Moreover, it may be envisaged that the substrate is supported by means of a clamping means during heating. On the one hand, the substrate can thus be exactly positioned, and on the other hand, the clamping means can have a heating element or form the heating element itself. In this way, heating of the silver paste or the copper paste to vaporize the solvent is simplified even further. 
     In the course of the method, the silver paste or the copper paste can be sintered prior to a complete vaporization of the solvent. Thus, the silver paste or copper paste can be prevented from fully drying before it is sintered. Completely solvent-free or dried paste is prone to fractures due to the drying process or due to movement, which may be conducive to failure of the contact connection. 
     The contact conductor can be pressed onto the chip contact surface and/or onto the conductor material surface by means of a pressing device during sintering. In this way, a particularly tight contact between the contact conductor and the chip contact surface or the conductor material track can be formed. Furthermore, a formation of undesired cracks during cooling of the sintered silver paste or copper paste can be avoided. 
     In another embodiment of the method, the contact conductor can be severed after sintering. For example, the contact conductor can be stored on a coil or reel and can be fed to the chip contact surface and to the conductor material track in an automated manner. In doing so, first an end of the contact conductor can be placed on the chip contact surface. A section of the contact conductor can be arranged on the conductor material track or vice-versa. After sintering of the silver paste or the copper paste and formation of the contact connection, the contact conductor can be severed or cut at the section so that a free end of the contact conductor is available again for forming another contact connection. 
     The chip contact surface can be formed by applying a copper strip to a chip surface. In that case, the chip surface can be made of a semiconductor material, such as silicon. By applying the copper strip, metallization of the chip surface is particularly simple. The copper strip can also facilitate a formation of the electrical contact with the contact conductor. 
     Moreover, a contact metallization can be applied to the conductor material track and/or to the chip contact surface. The contact metallization can be comparatively thin and substantially improve wettability of the chip contact surface and of the conductor material track with silver paste or copper paste or molten silver paste or copper paste. 
     The contact metallization can be formed by physical vapor deposition (PVD), sputter deposition, galvanization or electroless plating. 
     The contact metallization can be made of silver, nickel, copper, gold, palladium, aluminum or another alloy of one of said metals. A contact metallization made of silver, for example, can be formed to be comparatively thin and also allows a comparatively better current density distribution and a particularly favorable thermal dissipation of heat energy, such as for cooling the chip. Moreover, the silver paste or the copper paste can merge particularly well with the silver of the contact metallization during sintering. 
     The chip arrangement according to the invention, in particular for power transistors or the like, comprises a chip, a non-conductive substrate having a conductor material track formed thereon and a contact conductor, the chip being arranged on the substrate or on a conductor material track, wherein a silver paste or a copper paste is applied to each of a chip contact surface of the chip and the conductor material track, the contact conductor being immersed into the silver paste or the copper paste on the chip contact surface and into the silver paste or the copper paste on the material conductor track, a solvent contained in the silver paste or the copper paste being vaporized by heating and a contact connection being formed by sintering of the silver paste or the copper paste by means of laser energy. 
     Regarding the advantageous effects of the chip arrangement according to the invention, reference is made to the description of advantages of the method according to the invention. Other advantageous embodiments of the chip arrangement become apparent from the dependent claims back-referenced to claim  1 . 
     In the following description, preferred embodiments of the invention will be explained in more detail with reference to the accompanying drawing. 
    
    
     
       In the drawing: 
         FIG. 1  shows a schematic sectional view of a first embodiment of a chip arrangement during a method step; 
         FIG. 2  shows a schematic sectional view of the chip arrangement of  FIG. 1  after completion of a contact connection; 
         FIG. 3  shows a schematic sectional view of a second embodiment of a chip arrangement; and 
         FIG. 4  shows a schematic sectional view of a third embodiment of a chip arrangement. 
     
    
    
       FIG. 1  shows a chip arrangement  10  in a schematic sectional view during the production of a contact connection  11 . The chip arrangement  10  comprises a non-conductive substrate  12 , on whose surface  13  a plurality of different conductor material tracks  14  and  15  are formed. The conductor material tracks  14  and  15  are separated from each other via an isolating gap  16  and are thus electrically isolated from each other. A contact metallization  17  made of silver or a silver alloy is formed on or applied to each of the conductor material tracks  14  and  15 . Alternatively, the contact metallization  17  can also be made of nickel, copper, gold, palladium, aluminum or another alloy of one of said metals. A chip  18 , which is made of a semiconductor material, is formed on conductor material track  15 . A contact metallization  23  made of silver or a silver alloy is applied to a rear side  19  of the chip  18  facing away from conductor material track  15 , and contact bumps  21  are applied to a front side  20  of the chip  18  facing toward conductor material track  15 . The contact bumps  21  connect the chip  18  to the conductor material track  15 . Accordingly, a rear chip contact surface  25  and a front chip contact surface  26  are formed on the chip  18 . The chip contact surface  26  of the chip  18  is contacted, that is electrically connected, with a conductor surface  27  of conductor material track  15 . A defined amount of silver paste  29  has been applied to each of a conductor surface  28  of conductor material track  14  and to the chip contact surface  25  of the chip  18 . A contact conductor  30  is realized as a flat litz wire  31  made of copper and is immersed into the silver paste  29  with each of its ends  32  and  33 , the silver paste  29  substantially surrounding the ends  32  and  33 . 
     As can be taken from  FIG. 2 , the silver paste  29  was sintered after the solvent contained therein had largely vaporized. Sintering took place in that laser energy was introduced to the silver paste  29  and each of the ends  32  and  33 . In this way, a first contact  34  of the contact connection  11  was formed with the silver paste  29  and the end  32  on the conductor surface  28  and a second contact  35  of the contact connection  11  was formed with the silver paste  29  and the end  33  on the chip contact surface  25 . The silver particles (not visible) of the silver paste  29  penetrated the ends  32  and  33  and were sintered or partially melted together by the sintering, a particularly tight connection having been formed with the filaments (not visible) of the flat litz wire  31  and the conductor surface  28  and the chip contact surface  25 , respectively. Thus, the sintered silver paste  29  is solidified in contact areas  36  and  37 . 
       FIG. 3  shows a second embodiment of a chip arrangement  38 , which differs from the chip arrangement shown in  FIG. 2  in that it comprises a flat litz wire  39  that was severed only after sintering of the silver paste  29 . An end  40  of the flat litz wire  39  thus protrudes from a contact area  41 , a section  42  of the flat litz wire  39  serving to form a contact  43  with the chip  18 . 
       FIG. 4  shows a third embodiment of a chip arrangement  44 , which differs from the chip arrangement shown in  FIG. 2  in that it comprises conductor material tracks  45  and  46 , between which the chip  47  is arranged. The chip  47  forms two front chip contact surfaces  48  and  49  on a front side  50  of the chip  47 , a rear side  51  of the chip  47  being arranged directly on a substrate  52 . Each of the chip contact surfaces  48  and  49  is connected to conductor surfaces  57  and  58  of conductor material tracks  45  and  46 , respectively, via contact conductors  53  and  54 , which are realized as flat litz wires  55  and  56 , respectively. 
     In the embodiment examples described above, the silver paste can also be replaced with copper paste.