Patent Publication Number: US-2018040580-A1

Title: Power electronics module with a support with a palladium/oxygen diffusion barrier layer and a semiconductor element connected thereto by means of sintering, and method for producing same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a power electronics module, having at least one semiconductor element, in particular a power semiconductor element, and a support having at least one functional surface for connection to the semiconductor element. The invention also relates to a method for producing such a power electronics module. 
     2. Discussion of the Related Art 
     In power electronics, the requirements made of corresponding power electronics modules are constantly increasing. This concerns, inter alia, the continuous use temperatures or peak temperatures to which a power electronics module should be reliably exposed. The connection of semiconductor elements, in particular of power semiconductors, to substrates and/or leadframes and/or DCB is increasingly being carried out with the aid of so-called silver sintering pastes. At elevated temperatures of, for example, above 200° C., the permeability of such silver layers for oxygen is so high that oxidation of the base metallization of the substrates may occur. Substrates are usually produced from copper. 
     As a consequence of oxidation of the base metallization, the coating and, thus, the semiconductor element may detach from the substrate. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of specifying a power electronics module developed further in which such undesired detachments of coatings and/or semiconductor elements cannot occur. The invention is furthermore based on the object of specifying a method for producing such a power electronics module. 
     The invention is based on the concept of specifying a power electronics module comprising at least one semiconductor element, in particular a power semiconductor element. Furthermore, the power electronics module comprises a support having at least one functional surface for indirect connection to the semiconductor element. 
     According to the invention, a barrier layer composed of palladium is formed directly or indirectly on the functional surface of the support at least in sections. The semiconductor element is connected, by means of a layer composed of a silver sintering paste, directly or indirectly to the side of the barrier layer facing away from the functional surface of the support. 
     On account of the formation of a barrier layer or intermediate layer composed of palladium, a barrier for oxygen diffusion is formed for the silver. The oxidation of the underlying material of the support is thus prevented. The barrier layer or intermediate layer constitutes a barrier for oxygen permeation or oxygen diffusion through the silver to the metallization of the support. Consequently, the adhesion of the silver sintering paste or silver sintering layer on the support does not decrease during the ageing of the power electronics module. Consequently, detachment of the silver sintering paste or silver sintering layer and thus also detachment of the semiconductor element from the support are prevented. 
     The barrier layer composed of palladium may have a layer thickness of 0.1 μm-1.0 μm, in particular of 0.3 μm-0.7 μm, in particular of 0.4 μm-0.6 μm. 
     In one embodiment of the invention, a silver layer, in particular applied electrolytically, may be formed at least in sections between the barrier layer, namely the barrier layer composed of palladium, and the layer composed of a silver sintering paste. 
     Said silver layer may have a layer thickness of 0.1 μm-5.0 μm, in particular of 0.5 μm-2.0 μm. Preferably, the silver layer with the layer thicknesses mentioned is deposited electrolytically on the barrier layer composed of palladium. The barrier layer composed of palladium is also preferably deposited electrolytically directly or indirectly on the functional surface of the support. 
     Furthermore, a nickel layer may be formed at least in sections between the functional surface of the support and the barrier layer. The nickel layer may have a layer thickness of 0.025 μm-3.0 μm, in particular of 0.1 μm-2.0 μm. 
     The support of the power electronics module may also be referred to as a substrate. A so-called leadframe may be involved here, for example. 
     The support may be formed for example from copper and/or a copper alloy and/or from nickel. 
     In one particularly preferred embodiment of the invention, a diffusion layer is formed by application of pressure and/or application of heat during the connection of the semiconductor element to the support. A palladium-silver diffusion layer is involved here. 
     The semiconductor element of the power electronics module according to the invention is applied by means of a silver sintering paste directly or indirectly to the side of the barrier layer facing away from the functional surface of the support. The application or connection of the semiconductor element to the support is carried out in the context of a sintering process. On account of the application of pressure and/or application of heat in the sintering process, a palladium-silver diffusion layer is formed in the power electronics module. In particular, the palladium of the barrier layer diffuses into the silver layer and/or into the layer composed of a silver sintering paste. 
     The palladium-silver diffusion layer may have a layer thickness of 5 nm-300 nm, in particular of 10 nm-200 nm. 
     The silver sintering paste is a standard silver sintering paste. 
     The invention is furthermore based on the concept of specifying a method for producing a power electronics module, in particular for producing a power electronics module according to the invention. The power electronics module to be produced comprises at least one semiconductor element, in particular a power semiconductor element, and a support having at least one functional surface for connection to the semiconductor element. 
     The method is characterized by the following method steps according to the invention:
         a) at least in sections directly or indirectly, in particular electrolytically, coating the functional surface of the support with a barrier layer composed of palladium;   b) applying, at least in sections, a layer composed of a silver sintering paste to a side of the semiconductor element or directly to the barrier layer of the support or indirectly to the barrier layer of the support;   c) connecting the semiconductor element to the support by means of the layer composed of silver sintering paste by application of heat.       

     In a first step, accordingly, the support, which may also be referred to as a substrate or a leadframe, is coated at least in sections with a barrier layer composed of palladium. The support or the substrate or the leadframe is produced from copper, for example. The barrier layer composed of palladium is applied in particular electrolytically to the support at least in sections. In a further step, a layer composed of silver sintering paste is applied at least in sections either to the semiconductor element or to the support. The layer composed of a silver sintering paste may be applied either to the semiconductor element or to the support for example by means of a stencil printing method or a spraying method or a dispensing method. If the layer composed of a silver sintering paste is applied on the semiconductor element, the layer composed of a silver sintering paste should be applied on that side of the semiconductor element which later is connected to the support. 
     If the layer composed of a silver sintering paste is applied on the support, the layer composed of a silver sintering paste should be applied directly or indirectly to the barrier layer of the support. The semiconductor element and the support must be positioned with respect to one another in such a way that the semiconductor element can be connected to the barrier layer of the support directly or indirectly by means of the layer composed of silver sintering paste. The semiconductor element is connected to the support by means of the layer composed of silver sintering paste with application of heat being carried out. The connection process may also be referred to as joining the semiconductor element to the support. 
     In a further embodiment of the method according to the invention, it is conceivable for the barrier layer composed of palladium to be coated with a silver layer at least in sections before connection to the semiconductor element. Preferably, the coating of the barrier layer with a silver layer at least in sections is carried out by electrolytic deposition. 
     The functional surface of the support may be coated with a nickel layer at least in sections before step a), that is to say before the, in particular electrolytic, coating of the functional surface of the support with a barrier layer composed of palladium. A nickel layer is applied to the support, in particular to the functional surface of the support, primarily if the support consists of copper or a copper alloy. 
     Application of pressure may be carried out in step c), that is to say during the connection of the semiconductor element to the support. 
     A palladium-silver diffusion layer is formed by application of pressure and/or application of heat during the connection of the semiconductor element to the support. In other words, during the connection or joining of the semiconductor element to the support on account of the application of pressure and/or application of heat in the power electronics module a palladium-silver diffusion layer is formed. This is effected by the palladium of the barrier layer diffusing into the silver layer or into the layer composed of a silver sintering paste. 
     On account of such a palladium-silver diffusion layer, this forms an effective barrier for the oxygen permeation or oxygen diffusion through the silver layer or layer of a silver sintering paste to the base metallization of the support. The oxidation of the layers is thereby prevented. The silver sintering layer and hence the semiconductor element adhere on the support during the entire lifetime. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
       The invention is explained more specifically below with further details on the basis of exemplary embodiments with reference to the accompanying schematic drawings. 
       In said drawings: 
         FIG. 1  shows a power electronics module according to the invention in accordance with a first embodiment; and 
         FIG. 2  shows a power electronics module according to the invention with a palladium-silver diffusion layer having been formed. 
     
    
    
     The same reference numerals are used hereinafter for identical and identically acting parts. 
       FIG. 1  illustrates the structure of a power electronics module  10  according to the invention. 
     The module comprises a semiconductor element  11 . Said semiconductor element  11  may be in particular a power semiconductor element. Furthermore, the power electronics module  10  comprises a support  12 . The support  12  is for example a leadframe formed from a copper material. The support  12  has a functional surface  13 , which serves for indirect connection to the semiconductor element  11 . 
     A nickel layer  14  is applied on the functional surface of the support  12 . The layer thickness d 1  of the nickel layer  14  may be 0.05 μm-3.0 μm, for example. 
     A barrier layer  15  composed of palladium is formed at least in sections on the nickel layer  14 . In other words, the barrier layer  15  composed of palladium is formed indirectly on the functional surface  13  of the support  12 . The barrier layer  15  composed of palladium may have a layer thickness d 2  of 0.1 μm-0.5 μm. A silver layer  17 , in particular applied electrolytically, is formed at least in sections on the barrier layer  15 , namely on the side  16  of the barrier layer  15  facing away from the functional surface  13  of the support  12 . The silver layer  17  may have a layer thickness d 3  of 0.5 μm-2.0 μm. 
     A layer  19  composed of a silver sintering paste is formed at least in sections on the side  18  of the silver layer  17  facing away from the functional surface  13  of the support  12  or from the barrier layer  15 . The layer  19  composed of a silver sintering paste serves for connecting the semiconductor element  11  to the support  12 . 
       FIG. 1  illustrates the state in which the semiconductor element  11  has already been joined together with the support  12 . The layer  19  composed of a silver sintering paste may either be applied on the side  18  of the silver layer  17  before the joining together process, such that the semiconductor element  11  is attached to the layer  19  composed of silver sintering paste by the side  20  facing the support  12 . 
     Alternatively, it may be provided that the layer  19  composed of a silver sintering paste is firstly applied on the side  20  of the semiconductor element  11 . This is followed by the semiconductor element  11  provided with a silver sintering paste being joined together indirectly with the support  12 . 
     As can be gathered from  FIG. 1 , the layer  19  composed of a silver sintering paste is indirectly connected to the side  16  of the barrier layer  15  facing away from the functional surface  13  of the support  12 . 
     A sintering process is carried out in order to connect the semiconductor element  11  to the support  12  by means of the layer  19  composed of silver sintering paste. This is accompanied by application of pressure and/or application of heat. That is to say that while at least slight pressure is applied to the semiconductor element  11  and to the support  12  or to the layers  14 ,  15 ,  17  and  19  situated between the support material  12  and the semiconductor element  11 , said pressure serving for adhering the semiconductor element  11  on the underlying layers, application of heat is carried out at the same time. 
     As is illustrated in  FIG. 2 , the palladium of the barrier layer  15  diffuses into the silver of the silver layer  17 , such that a palladium-silver diffusion layer  21  is formed. Said palladium-silver diffusion layer  21  forms a barrier for the oxygen permeation or oxygen diffusion through the silver sintering paste layer  19  and silver layer  17  to the metal of the support  12 . Oxidation of the support metallization is thus prevented. The silver layer  17  or the layer  19  composed of a silver sintering paste consequently does not detach from the support  12  or the nickel layer  14  even at relatively high temperatures to which the power electronics module  10  is exposed. 
     The layer thickness d 4  of the palladium-silver diffusion layer  21  may be 5 nm-300 nm, in particular 10 nm-200 nm. 
     Tests have shown, for example, that the silver layer  17  or the layer composed of silver sintering paste  19  is not detached from the barrier layer  15  even after being subjected to a temperature of 245° C. over 690 hours. 
     It should be pointed out at this juncture that all elements and components described above in association with the embodiments in accordance with  FIGS. 1 and 2 , as considered by themselves or in any combination, in particular the details illustrated in the drawings, are claimed as essential to the invention. 
     LIST OF REFERENCE SIGNS 
     
         
         
           
               10  Power electronics module 
               11  Semiconductor element 
               12  Support 
               13  Functional surface 
               14  Nickel layer 
               15  Barrier layer 
               16  Side of barrier layer 
               17  Silver layer 
               18  Side of silver layer 
               19  Layer composed of a silver sintering paste 
               20  Side of semiconductor element 
               21  Palladium-silver diffusion layer 
             d 1  Layer thickness of nickel layer 
             d 2  Layer thickness of barrier layer 
             d 3  Layer thickness of silver 
             d 4  Layer thickness of palladium-silver diffusion layer