Abstract:
One embodiment is directed to a welding pad capable of receiving a ball-shaped copper wire at its end, including a first copper pad coated with a protection layer and topped with a second pad containing aluminum having dimensions smaller than those of the first pad and smaller than the ball diameter once said ball has been welded to the welding pad.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to the electric connection between an integrated circuit pad and a copper wire. 
     2. Description of the Related Art 
     A connection mode between an integrated circuit pad and a connection wire is shown in  FIGS. 1A and 1B . 
       FIG. 1A  shows a portion of an integrated circuit  1  comprising a connection pad  2  to which a copper wire  3 , comprising, at its end intended to be attached to the pad, a ball  4 , is desired to be welded or bonded. Pad  2  is formed of a stack of three layers, that is, a copper layer  5 , a titanium nitride barrier layer  6 , and an aluminum layer  7 . A passivation dielectric layer  8  covers the periphery of the welding pad and defines a window  9  which delimits surface  10  of the welding pad. 
     The welding is performed between the end of the wire comprising the copper ball and aluminum layer  7  heated between 150° and 180° C. The copper ball is pressed against the pad and ultrasounds are applied thereto. 
       FIG. 1B  shows the structure after welding. Ball  4  is crushed against surface  10  of pad  2 . Ball  4  penetrates into the aluminum layer. The aluminum pushed back by the ball forms extrusions  21  and a space  20  forms between the copper ball periphery and surface  10 . Contact areas  22  between the copper ball periphery and surface  10  are exposed to the ambient air which flows in space  20 . 
       FIG. 2A  illustrates the copper aluminum compounds present between layer  7  and ball  4 . From aluminum layer  7 , aluminum-rich compounds  23  such as CuAl 2 , followed by components comprising less aluminum  24 , such as CuAl, and finally, at the interface with copper ball  4 , copper-rich compounds  25  such as Cu 9 Al 4  or Cu 3 Al 2 , can be found. 
     By reaching areas  22 , the humidity of the ambient air and of the chlorine present in the materials or brought from the outside may modify the chemical composition of copper aluminum compounds. 
       FIG. 2B  illustrates the aluminum-oxygen compounds resulting from this transformation, such as Al 2 O 3  or aluminum-chlorine, such as AlCl 2 , forming layer  26  located between layer  23  and layer  4 . The copper aggregates in islands in layer  26 . 
     Stress resulting from the integrated circuit operation, like vibrations or temperature variations, weakens the interface layers located in area  22 , and a separation between the pad and the copper wire can sometimes be observed during the circuit lifetime, despite the optimizations of welding techniques. 
     A reliability issue linked to the welding of the copper wire on aluminum is thus raised, this issue becoming more critical during the use of the product. 
     BRIEF SUMMARY 
     One or more embodiments of the present disclosure is directed to performing a robust, time-reliable welding while keeping known methods of copper wire welding on aluminum. 
     Thus, an embodiment provides a specific pad structure having a shape enabling to prevent the penetration of ambient air into an area comprising copper aluminum compounds. 
     More specifically, an embodiment provides a welding pad capable of receiving a ball-shaped copper wire at its end, comprising a first copper pad coated with a protection layer and topped with a second pad containing aluminum, having dimensions smaller than those of the first pad and smaller than the ball diameter once said ball has been welded to the welding pad. 
     According to an embodiment, the second pad is made of pure aluminum. 
     According to an embodiment, the second pad is an alloy containing copper, preferably 0.8% of copper. 
     An embodiment provides a pad further comprising a dielectric covering the periphery of the first pad. 
     According to an embodiment, the protection structure is made of titanium nitride. 
     According to an embodiment, the second pad is divided into a plurality of protrusions. 
     An embodiment provides a method for manufacturing a welding pad comprising the manufacturing steps of: 
     forming a first copper pad coated with a protection layer; 
     forming at least one second aluminum pad partially covering the first copper pad and having a size smaller than the diameter of the copper ball that it is intended to receive; and 
     forming a window above the welding pad in a passivation layer exposing the second pad and covering the periphery of the first pad. 
     An embodiment of the present disclosure provides a method further comprising the steps of:
         taking the surface of the pad to a temperature ranging between 50° and 100° C.;   bringing the copper ball closer to said pad; and   pressing the copper ball and having it vibrate on the welding pad to entirely cover the second pad and a ring of the first pad adjacent to the second pad.       

     The foregoing and other objects, features, and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIGS. 1A and 1B  are cross-section views illustrating the welding between a copper wire and a pad containing aluminum according to the state of the art; 
         FIGS. 2A and 2B  show the chemical components capable of forming at the copper aluminum interface; 
         FIG. 3  is a cross-section view of a connection between a copper wire and a pad to which it is welded according to an embodiment; 
         FIGS. 4A and 4B  are respective cross-section and top views according to an alternative embodiment; and 
         FIGS. 5A to 5D  illustrate steps of the method for manufacturing a welding pad according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
       FIG. 3  shows a connection wire connected to a welding pad. The welding pad comprises on a copper pad  30  a barrier layer  31 , for example, made of TiN, and an aluminum pad  32 . The aluminum pad has a smaller surface area than the copper pad and the barrier layer topping this pad, whereby aluminum pad  32  is fully contained within the surface corresponding to the surface area of the copper pad. Once a copper wire  35  having a ball  36  formed at its end has been welded by ultrasounds on the pad surface, ball  36  is crushed. The dimension of the aluminum pad is such that after crushing, ball  36  comprises, on the one hand, a portion  38  in contact with the upper surface of aluminum pad  32 , and on the other hand, a peripheral pad  39  which contacts barrier layer  31 . In practice, after the crushing and friction operation, the barrier layer may have disappeared so that the portion of ball  36  at the periphery of aluminum pad  32  comes in direct contact with copper pad  30 . 
     Thus, the peripheral ring of ball  36  which, after welding, comes into contact with barrier layer  31  and/or copper layer  30 , forms a protective ring which, in some embodiments, avoids or at least limits the progress of humidity infiltrations towards the periphery of region  38  of contact between the copper ball and the aluminum pad. Contact region  38  is thus protected from any contamination. Tests carried out by the present inventors have shown that due to this arrangement, the reliability of a connection between a copper wire and a pad is considerably increased. In particular, lifetime tests have shown a reduction or elimination of the separation phenomena over time. 
     Further, conventionally,  FIG. 3  shows a protection layer  40  which defines a window delimiting the useful portion of the welding pad. 
     As an example of dimensions, the lateral dimensions of copper pad  30  may be on the order of 100×100 μm 2 , the window defined in layer  40  substantially having the same dimensions. Aluminum pad  32  may have dimensions on the order of 50×50 μm 2 . Ball  36  initially formed at the end of copper wire  35  may have a diameter on the order of 50 μm, to have a lateral extension on the order of 75×75 μm 2  after crushing. Further, copper pad  30  may have a thickness approximately ranging from 2 to 3 μm, barrier layer  31  may have a thickness approximately ranging from 20 to 30 nm, and aluminum pad  32  may have a thickness approximately ranging from 1 to 2.5 μm. Of course, these values are provided as an example only and the dimensions of the pad, and more specifically of aluminum pad  32 , will be adapted to the dimensions of wire  35  and of ball  36  formed at the end of this wire, and to the dimensions taken by this ball after crushing and vibrating to weld the wire to the pad. 
       FIGS. 4A and 4B  are a cross-section view and a top view of an alternative embodiment in which aluminum pad  32 , instead of being continuous, is divided into protrusion or spaced apart pads  42 . Four spaced apart pads  42  have been shown herein. In practice, according to the pad dimensions and to the technologies used, any number of protrusions or pads may be used. 
     It is considered that this variation may be useful in certain cases to further improve the bonding between the copper wire and the pad. 
       FIGS. 5A to 5D  shows an example method of forming a pad of the type illustrated in  FIG. 3 . 
     As illustrated in  FIG. 5A , on the upper portion of a substrate  1 , currently on the last metal layer formed from among the metallization levels covering an integrated circuit, a copper pad  30  surrounded with an insulating layer  50  has been formed. In practice, insulating layer  50  may first have been formed, after which a copper layer, previously planarized by chemical-mechanical polishing, may have been deposited, to obtain, as shown, the copper pad fully surrounded with insulating layer  50 . A barrier layer  48 , such as TiN, is formed on the upper surface of the copper pad. Other types of barrier layers may be used, for example, Ti, Ta, TaN. 
     As illustrated in  FIG. 5B , an aluminum layer  51  is then deposited on the structure. 
     As illustrated in  FIG. 5C , this aluminum layer is then etched to define an aluminum pad  32  having dimensions selected as indicated previously. 
     At the step illustrated in  FIG. 5D , a passivation layer  52 , which is opened to have a slightly smaller opening than the surface of copper pad  30 , so that there exists a free area at the periphery of aluminum pad  32 , is redeposited. 
     Specific embodiments of the present disclosure have been described. The specific embodiments described above can be combined to provide further embodiments. Additionally, various alterations, modifications, and improvements will readily occur to those skilled in the art. In particular, the dimensions of the various elements and the thicknesses of the various layers will be selected according to the technologies used. Further, although an aluminum pad has been previously mentioned, this pad, instead of being a pure aluminum pad, may be an alloy of aluminum and copper or of any other selected material. In particular, technologies in which the various depositions may be performed at temperatures lower than 300° C., and even at 150° C., will preferably be selected. It is to be appreciated that the steps of the method described above may be performed sequentially, in parallel, omitted, or in a different order than is described. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present disclosure. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled.