Abstract:
A semiconductor device having a highly resistant metal film and a method of producing such a semiconductor device, which includes a metal film formed on an electrode pad, and a protection film formed in an area where he metal film does not exist. The metal film has a greater thickness on its peripheral end portion in contact with the protection film. The semiconductor device can be produced by a semiconductor production method including the steps of activating the surface o the electrode pad with a chelating solution containing glycine and a compound having a metallic element as nuclei, and forming a metal film by electroless metal plating.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to semiconductor devices, and, more particularly, to a semiconductor device provided with a durable metal film formed on an electrode pad. The present invention also relates to a method of producing such a semiconductor device. 
     2. Description of the Related Art 
     Conventionally, a semiconductor device has a metal film called a barrier metal layer formed on an electrode pad. Such a barrier metal layer may be formed by electroless metal plating. For instance, FIG. 1A is an enlarged schematic view of a part including a barrier metal layer of a conventional semiconductor device. A semiconductor device  100  has an electrode pad  103  on a chip  101  made mainly of silicon. A barrier metal layer  105  is disposed on the electrode pad  103 . A passivation film  106  serving as a protection film is further disposed in an area where the barrier metal layer  105  does not exist. 
     As shown in FIG. 1A, the peripheral end portion  105 A of the barrier metal layer  105  is in contact with the passivation film  106 . Here, the peripheral end portion  105 A has substantially the same thickness as the passivation film  106 . Also, the barrier metal layer  105  is entirely flat. 
     FIG. 1B is an enlarged schematic view of a part including a barrier metal layer of another conventional semiconductor device  200 . In FIG. 1B, the same components as in FIG. 1A are denoted by the same reference numerals. The barrier metal layer of FIG. 1B is slightly thickener than the barrier metal layer of FIG.  1 A. Accordingly, the semiconductor device  200  differs from the semiconductor device  100  in having a slightly thicker barrier metal layer  115  and its peripheral end portion  115 A. In FIG. 1B, the peripheral end portion  115 A covers an end portion of the passivation film  106 . However, the rest of the barrier metal layer  115  has a uniform thickness and an entirely flat shape. 
     The semiconductor devices  100  and  200  both have a solder bump  107  on the respective barrier metal layers  105  and  115  to be attached to external electrodes. 
     As described above, the semiconductor devices  100  and  200  are connected to external electrodes via solder bumps. Conventional solder bumps are made mainly of lead. In recent years, however, solder bumps made mainly of tin are preferred in consideration of the environment. 
     However, when solder bumps made of tin as a main component and silver are used, the strength of the peripheral end portions  105 A and  115 A in contact with the solder bump  107  decreases, thereby causing incomplete bonding. Because of such incomplete bonding, there is a risk of the electrode pad  103  and the solder bump  107  being brought into contact with each other and nullifying the function of the barrier metal layer  105 . Also, there is another problem that the solder bump  107  and the barrier metal layer  105  might become separated from the electrode pad  103 . 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide semiconductor devices in which the above disadvantages are eliminated. 
     A more specific object of the present invention is to provide a semiconductor device having a highly resistant metal film formed on an electrode pad, and a method of producing such a semiconductor device. 
     The reasons of the above problems are not known in detail. However, it is assumed that digestion and diffusion of a metallic element constituting a solder bump progresses preferentially at the peripheral end portion of a metal film. The inventors of the present invention have studied the above problems, and have discovered that the metal film should have a certain shape to reduce the occurrence of the problems. 
     The objects of the present invention are achieved by a semiconductor device comprising: an electrode pad; a metal film formed on the electrode pad; a protection film formed in an area where the metal film does not exist; and a bump disposed on the metal film. The metal film has a greater thickness at its peripheral end portion which is in contact with the protection film. 
     As the result of intensive studies made by the inventors, it was found that, even if the solder bump contains a component having digestion and diffusion properties, the thicker peripheral end portion of the metal film in contact with the protection film can improve the durability of the metal film. In the semiconductor device of the present invention, the peripheral end portion of the metal film is thicker than the flat portion of the metal film. When bonding the semiconductor to an external terminal via solder bumps, the peripheral end portion can presumably disperse the diffusive element, thereby improving the durability of the metal film. 
     The peripheral end portion of the metal film may be 1.3 to 2 times thicker than the flat portion of the metal film. 
     The metal film of the semiconductor device of the present invention may have a peripheral end portion covering the inner peripheral end portion of the protection film. 
     The objects of the present invention are also achieved by a semiconductor device production method including the steps of activating the surface of the electrode pad with a chelating solution containing glycine and a compound having a metallic element as nuclei, and forming a metal film by electroless metal plating. 
     By activating the surface of the electrode pad, the metallic element is precipitated on the surface of the electrode pad, which is a suitable condition for metal plating. Electroless metal plating is then performed on the surface of the electrode pad to form a metal film with the metallic element as nuclei. The peripheral end portion of the metal film in contact with the protection film is thicker than the central portion of the metal film. 
     The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is an enlarged view of a part including a barrier metal layer of a semiconductor device of the prior art; 
     FIG. 1B is an enlarged view of a part including a barrier metal layer of another semiconductor device of the prior art; 
     FIG. 2A is an enlarged view of a part of a semiconductor device of one embodiment of the present invention; 
     FIG. 2B is an enlarged view of a part of a semiconductor device of another embodiment of the present invention; 
     FIG. 3 is a flowchart showing a process of forming a metal film on an electrode pad when producing a semiconductor device of the present invention; 
     FIG. 4 shows an example activating solution used for a semiconductor device of the present invention; 
     FIG. 5 shows an example nickel plating solution used for a semiconductor device of the present invention; 
     FIG. 6 shows an AU layer formed on a surface of an electrode by electroplating; 
     FIG. 7 shows results of comparison tests of removal resistance between a semiconductor device of the prior art and a semiconductor device of the present invention; and 
     FIG. 8 shows a semiconductor device of the present invention fixed to a substrate via solder bumps. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is a description of embodiments of the present invention, with reference to the accompanying drawings. 
     FIG. 2A is an enlarged view of a part including a metal film of a semiconductor device of one embodiment of the present invention. FIG. 2B is an enlarged view of a part including a thicker metal film of a semiconductor device of another embodiment of the present invention. 
     As shown in FIG. 2A, a semiconductor device  1  has an electrode pad  13  formed on a chip  10  made mainly of silicon. The electrode pad  13  is made of aluminum, for instance. A barrier metal layer  15  made of metal is disposed on the electrode pad  13 . A passivation film  16  as a protection film is disposed in an area where the barrier metal layer  15  does not exist. The barrier metal layer  15  is made of nickel formed by electroless plating. The passivation film  16  is made of silicon oxide, for instance. 
     The peripheral end portion  15 A of the barrier metal layer  15  is in contact with the passivation film  16 . The peripheral end portion  15 A is thicker than the central portion of the barrier metal layer  15 , which is entirely flat. The flat portion has a thickness of about 1 μm, which is substantially the same as the passivation film  16 , while the peripheral end portion  15 A is about 2 μm in thickness, for instance. 
     A solder bump  17  is disposed on the barrier metal layer  15 . The bump  17  contains no lead, but may be made mainly of tin and also containing silver (Sn:Ag=97:3), for instance. 
     Another semiconductor device  2  shown in FIG. 2B differs from the semiconductor device  1  shown in FIG. 2A in that a barrier metal layer  25  of the semiconductor device  2  is slightly thicker than the barrier metal layer  15 , and has a peripheral end portion  25 A. In FIG. 2B, the same components as in FIG. 2A are denoted by the same reference numerals. 
     As shown in FIG. 2B, the peripheral end portion  25 A of the barrier metal layer  25  covers the inner peripheral end portion of the passivation film  16 . This is the effect of having the thicker barrier metal layer  25 , which is achieved by prolonging the time of the electroless plating process. Even with the thicker barrier metal layer  25 , the peripheral end portion  25 A is thicker than the central portion. The flat central portion of the barrier metal layer  25  has a thickness of about 2 μm, while the peripheral end portion  25 A is about 3 μm in thickness, for instance. 
     As described above, the barrier metal layers  15  and  25  have the thicker peripheral end portions  15 A and  25 A, respectively. Since tin has digestion and diffusion properties, it invades the peripheral end portions  15 A and  25 A. When this happens, the thick peripheral end portions  15 A and  25 A cause the tin to diffuse inside them, thereby maintaining the bonding state. Thus, the prior art problem of incomplete connection and removal of a bump and a barrier metal layer from an electrode pad can be effectively avoided. 
     FIG. 3 is a flowchart of a process of forming the barrier metal layer  15  as a metal film on the electrode pad  13 . 
     In this flowchart, the process is carried out for the semiconductor device  1  having the electrode pad  13  made of aluminum on the chip  10 , and the passivation film  16  formed outside the electrode pad  13 . In step S 10 , etching is performed on the aluminum electrode pad  13  of the semiconductor device  1 . More specifically, the semiconductor device  1  is immersed in 500 ml/l of sulfuric acid solution at 70° C. 
     In step S 20 , the semiconductor device  1  is thoroughly washed. 
     In step S 30 , the surface of the electrode pad  13  of the semiconductor device  1  is activated. This is a preparation step for the following step of obtaining the barrier metal layer  15  having the thicker peripheral end portion by electroless metal plating. An activation solution used here is shown in FIG.  4 . 
     The activation solution shown in FIG. 4 is a chelating solution containing 0.6 mmol/l of palladium chloride and 0.1 mol/l of glycine. Palladium serves to coordinate amino groups. In FIG.  4 , the activation solution used for producing the conventional semiconductor devices shown in FIGS. 1A and 1B is also shown for reference. The activation solution of the prior art contains ammonia, while the activation solution of the present invention contains glycine. 
     The Glycine in the solution can restrict homogeneous precipitation of the palladium, so that the palladium can be prevented from being precipitated uniformly from the solution. The glycine serves to precipitate a large amount of palladium on the peripheral end portion of the aluminum electrode pad and an appropriate amount of palladium in the remaining area. 
     In step S 40 , the semiconductor device  1  is thoroughly washed. 
     In step S 50 , electroless metal plating is performed on the surface of the aluminum electrode pad  13  to form a metal film. Here, the electroless metal plating may be electroless nickel plating, for instance. A nickel plating solution used in the nickel plating is shown in FIG.  5 . In the step S 50 , the surface of the aluminum electrode pad  13  is immersed in the nickel plating solution for about 3 minutes. A nickel metal film having a thickness of about 1 μm is thus uniformly formed on the surface of the aluminum electrode pad  13 , except the peripheral end portion  15 A has a thickness of about 2 μm. 
     The peripheral end portion  15 A is thicker than the rest of the barrier metal layer  15 , because a large amount of palladium is precipitated on the peripheral end portion of the aluminum electrode pad  13  in the activation step. The nickel is precipitated with the palladium as nuclei to form the metal film. Accordingly, the barrier metal layer  15 , which is a nickel film, has a greater thickness on its peripheral end portion. 
     In step S 60 , the semiconductor device  1  is again thoroughly washed. In step S 70 , the semiconductor device  1  is then dried, and the process of forming the barrier metal layer  15  comes to an end. 
     It should be understood that the barrier metal layer  15  of the semiconductor device  1  shown in FIG. 2A is formed in the flowchart of FIG.  3 . In the case of the semiconductor device  2  having the barrier metal layer  25 , which is thicker than the barrier metal layer  15 , the time for immersing the aluminum electrode pad  13  in the nickel plating solution should be about  6  minutes to form a nickel metal film having a thickness of about 2 μm uniformly on the surface of the aluminum electrode pad  13  in the step S 50 . Here, the peripheral end portion  25 A covers the inner peripheral end portion of the passivation film  16 , having a thickness of about 3 μm. 
     The bump  17  is placed on the barrier metal layer  15  or  25  of the respective semiconductor device  1  or  2 . The bump  17  may be a solder bump made of tin and silver at a ratio of 97:3 (Sn:Ag). After being placed on the barrier metal layer  15  or  25 , the solder bump is heated at 270° C. As a result, the solder bump melts and adheres to the entire surface of the barrier metal layer  15  or  25 . 
     It should be understood that a solder bump is not necessarily placed on the barrier metal layer in advance. It is possible to produce a solder bump separately and attach the solder bump to the barrier metal layer when the semiconductor device is bonded to an external electrode. In such a case, the semiconductor device has an oxidation resistant film, such as a gold-plated metal film, placed on the barrier metal layer having the thicker peripheral end portion. As shown in FIG. 6, an Au film  18 , for instance, can be formed on the surface of the barrier metal layer  15  by electroplating. 
     FIG. 7 shows the results of comparison tests of barrier metal layer removal resistance between semiconductor devices of the prior art and semiconductor devices of the present invention. The test conditions are also shown in FIG.  7 . 
     The semiconductor device  200  shown in FIG. 1B was used as the semiconductor device of the prior art, and the semiconductor device  2  shown in FIG. 2B was used as the semiconductor device of the present invention. Accordingly, the central portion of each barrier metal layer formed on the electrode pad was 2 μm in thickness. The semiconductor devices of the present invention each had the peripheral end portion having a thickness of about 3 μm. 
     A solder bump containing Sn and Ag at a ratio of 97:3 was fixed onto each barrier metal layer, and was subjected to a heating process under the conditions shown in FIG. 7 up to 5 times. A probe was driven 10 μm above the surface of each semiconductor device at 30 μm/s to apply a shearing force to the side surface of the solder bump. 
     As shown in FIG. 7, 30% of the semiconductor devices of the prior art had detachment of the respective aluminum electrode pads from the barrier metal layers. On the other hand, no detachment or exfoliation occurred on the semiconductor devices of the present invention until the fourth heating process. However, the detachment occurred on 60% of the semiconductor devices in the fifth heating process. 
     As can be seen from the results, the bond between the electrode pad and the barrier metal layer is stronger in the present invention than in the prior art. Accordingly, the semiconductor devices of the present invention have higher durability. 
     FIG. 8 shows a semiconductor device of the present invention integrally fixed to a substrate via solder bumps. In this figure, a flip-chip semiconductor device  3  is fixed to a substrate  4  via solder bumps  37 . 
     The present invention is not limited to the specifically disclosed embodiments, but variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 11-189280, filed on Jul. 2, 1999, the entire contents of which are hereby incorporated by reference.