Abstract:
A semiconductor device includes: a foundation layer that is provided on a substrate and is electrically conductive; a nickel layer provided on the foundation layer; and a solder provided on the nickel layer, the nickel layer having a first region on a side of the foundation layer and a second region on a side of the solder, the second region being harder than the first region.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-212873, filed on Sep. 28, 2011, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to a semiconductor device and a method for manufacturing a semiconductor device. 
     (ii) Related Art 
     Recently, a CSP (Chip Size Package) is being used in order to downsize a semiconductor device. The CSP is flip-chip mounted on a printed circuit or the like with use of a solder ball. When a current is applied to the semiconductor device via the solder ball, the solder may diffuse into a foundation layer acting as an interconnection line. In this case, an electrical open or an electrical short may occur, and the semiconductor device may be broken. In order to restrain the diffusion of the solder, an anti-diffusion layer may be used. Japanese Patent Application Publication No. 2000-353763 discloses a method in which a layer having a thickness of approximately 70 μm and formed with a copper is formed on a resin, and a solder ball is formed on the Cu layer. The Cu layer acts as the anti-diffusion layer. 
     SUMMARY 
     It is an object to provide a semiconductor device and a method for manufacturing a semiconductor device restraining diffusion of a solder and restraining stress with low cost. 
     According to an aspect of the present invention, there is provided a semiconductor device including: a foundation layer that is provided on a substrate and is electrically conductive; a nickel layer provided on the foundation layer; and a solder provided on the nickel layer, the nickel layer having a first region on a side of the foundation layer and a second region on a side of the solder, the second region being harder than the first region. 
     According to an aspect of the present invention, there is provided a method for manufacturing a semiconductor device including: forming a foundation layer on a substrate; providing a nickel layer, of which upper face side is harder than lower face side, on the foundation layer; and providing a solder on the nickel layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a cross sectional view of a semiconductor device in accordance with a first embodiment; 
         FIG. 1B  illustrates an enlarged view of a region around an electrode; 
         FIG. 2A  illustrates a schematic view of an electrode of the first embodiment; 
         FIG. 2B  illustrates a graph illustrating hardness of a nickel layer; 
         FIG. 3A  through  FIG. 3C  illustrate a cross sectional view of a method for manufacturing the semiconductor device of the first embodiment; 
         FIG. 4  illustrates a graph illustrating a relation between a P concentration and hardness; 
         FIG. 5A  illustrates a schematic view of an electrode of a second embodiment; 
         FIG. 5B  illustrates a graph illustrating hardness of a nickel layer; 
         FIG. 6A  through  FIG. 6C  illustrate a cross sectional view illustrating a method for manufacturing a semiconductor device in accordance with a third embodiment; 
         FIG. 7A  illustrates an enlarged view around an electrode of the fourth embodiment; 
         FIG. 7B  illustrates a plane view of the electrode; 
         FIG. 8A  through  FIG. 8C  illustrate a cross sectional view of a method for manufacturing a semiconductor device of the fourth embodiment; and 
         FIG. 9  illustrates a plane view of an electrode of a modified embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     When the anti-diffusion layer formed with Cu is thick, the number of processes may increase and a cost may increase. Further, variability of in-plane evenness may increase. When the anti-diffusion layer is thin, it is difficult to sufficiently restrain the diffusion of the solder. 
     A description will be given of a best mode for carrying the present invention. 
       FIG. 1A  illustrates a cross sectional view of a semiconductor device in accordance with a first embodiment.  FIG. 1B  illustrates an enlarged view around an electrode. A thickness of layers in the cross sectional view is schematically illustrated. 
     In  FIG. 1A , a wafer-shaped semiconductor device before dividing is illustrated. As illustrated in  FIG. 1A , the semiconductor device of the first embodiment has a substrate  10  and an electrode  11 . An insulating layer may be provided between the electrodes  11  adjacent to each other. The substrate  10  is a semiconductor substrate having an insulating substrate such as silicon (Si), silicon carbide (SiC) or a sapphire and nitride semiconductor that is epitaxially grown on the insulating substrate. 
       FIG. 1B  illustrates an enlarged view of a region surrounded by a broken line of  FIG. 1A . The electrode  11  includes a foundation layer  12 , a nickel (Ni) layer  16 , a cover layer  18  and a solder ball  20 . The foundation layer  12  is, for example, formed with gold (Au) having a thickness of 5 μm to 10 μm and is provided on the substrate  10 . An insulating layer  14  is, for example, a lamination layer in which a silicon nitride (SiN) layer, a silicon oxide (SiO 2 ) layer, or a polyimide layer is laminated, and is provided on the substrate  10  and the foundation layer  12 . The foundation layer  12  is exposed through an opening formed in the insulating layer  14 . The Ni layer  16  is provided on the exposed region of the foundation layer  12  and on the insulating layer  14 . As sectioned by a broken line in  FIG. 1B , one region of the Ni layer  16  on the side of the foundation layer  12  is referred to as a first region  16   a , and the other region of the Ni layer  16  on the side of the solder ball  20  is referred to as a second region  16   b . A thickness of the first region  16   a  and the second region  16   b  is, for example, 0.1 μm or more to 5 μm or less. The cover layer  18  is, for example, formed with a metal such as Au having a thickness of 30 nm, is provided between the Ni layer  16  and the solder ball  20 , and covers an upper face and a side face of the Ni layer  16 . The solder ball  20  is, for example, formed with a solder (Sn—Ag—Cu based solder) mainly including tin, silver and copper, and is provided on the cover layer  18 . The foundation layer  12  is contacting with the upper face of a substrate layer of the substrate  10 . The Ni layer  16  is contacting with the upper face of the foundation layer  12 . The cover layer  18  is contacting with the upper face and the side face of the Ni layer  16 . The solder ball  20  is contacting with the upper face of the cover layer  18 . The Ni layer  16  acts as an UBM (Under Bump Metal) restraining diffusion of the solder from the solder ball  20 . The foundation layer  12  acts as a foundation of the electrode  11  and an interconnection layer. The cover layer  18  acts as a protection layer restraining oxidation of the Ni layer  16 . The solder ball  20  acts as an outer connection terminal coupling the semiconductor device with an outer mount substrate. 
     A description will be given of details of a structure of the Ni layer  16 .  FIG. 2A  illustrates a schematic view of the electrode of the first embodiment.  FIG. 2B  is a graph illustrating hardness of the Ni layer  16 . A horizontal axis of  FIG. 2B  indicates the hardness. A vertical axis of  FIG. 2B  indicates a depth from the upper face of the cover layer  18 . 
     As illustrated in  FIG. 2A  and  FIG. 2B , the Ni layer  16  is formed by an electroless plating method and includes the first region  16   a  and the second region  16   b  having different hardness. The second region  16   b  is harder than the first region  16   a . The hardness is changed in stages between the soft first region  16   a  and the hard second region  16   b . The hardness X 1  of the first region  16   a  is, for example, 150 Hv or more and less than 500 Hv. The hardness X 2  of the second region  16   b  is, for example, 500 Hv or more to 1000 Hv or less. The hardness of pure Ni is approximately 150 Hv. Generally, the hardness of the Ni formed by the electroless plating method is approximately 500 Hv. 
     The second region  16   b  is hardened in order to restrain the diffusion reaction of the solder from the solder ball  20  to the foundation layer  12 . However, when a layer gets harder, stress gets larger. When the first region  16   a  is softer than the second region  16   b , the stress of the second region  16   b  is suppressed. In the first embodiment, the second region  16   b  restrains the open or the short caused by the diffusion of the solder, and the first region  16   a  restrains a crack of the electrode  11  and the semiconductor substrate  10  caused by the stress of the second region  16   b . And, in the first embodiment, the Ni layer  16  restrains the crack of the solder ball  20  caused by thermal expansion coefficients of a chip and a mount substrate, compared to a case where a single layer having the same thickness as the Ni layer  16  and formed with hard Ni is provided. This is because it is difficult for the single hard Ni layer to suppress the stress of the solder ball  20 . On the other hand, the stress of the solder ball  20  is suppressed when the single Ni layer is soft. However, in this case, it is difficult to restrain the diffusion of the solder. The upper limit of the hardness of the first region  16   a  may be 450 Hv, 400 Hv, 300 Hv or the like. The hardness of the second region  16   b  may be 550 Hv to 950 Hv, 600 Hv to 900 Hv or the like. Preferably, difference of the hardness between the first region  16   a  and the second region  16   b  may be 100 Hv or more. In the first embodiment, the Ni layer  16  is denser than another Ni layer formed by an electrolytic plating method, a vapor deposition method or the like, because the Ni layer  16  is formed by the electroless plating method. Therefore, the diffusion of the solder is effectively restrained. 
     Generally, when the thickness of the UBM gets larger, the stress gets larger. The Ni layer  16  is, for example, a thin layer having a thickness of 0.2 μm to 10 μm. Therefore, the stress of the Ni layer  16  may be reduced. And, the cost gets lower. The thickness of the first region  16   a  and the second region  16   b  may be 0.2 μm or more to 4.8 μm or less, and may be more than 0.1 μm and less than 5 μm. The thickness of the first region  16   a  may be the same as that of the second region  16   b , and may be different from that of the second region  16   b.    
     Next, a description will be given of a method for manufacturing the semiconductor device of the first embodiment.  FIG. 3A  through  FIG. 3C  illustrate a cross sectional view illustrating the method for manufacturing the semiconductor device in accordance with the first embodiment. 
     As illustrated in  FIG. 3A , the foundation layer  12  is formed on the substrate  10  by the electroless plating method or the like. Then, the insulating layer  14  is formed, and the opening is formed in the insulating layer  14 . As illustrated in  FIG. 3B , the Ni layer  16  is formed on the foundation layer  12  by the electroless plating method. Nickel electroless plating solution (hereinafter referred to as plating solution) includes a Ni ion and hypophosphorous or hypophosphite. Two types of plating equipment having different types of the plating solution are used, and the first region  16   a  and the second region  16   b  are formed. The details are described later. 
     As illustrated in  FIG. 3C , the cover layer  18  is formed by the electroless plating method or the like. The solder ball  20  is formed on the cover layer  18  by a printing method or a reflow method. With the processes, the semiconductor device illustrated in  FIG. 1A  is manufactured. Further, the substrate  10  is cut off and is divided into chips. Next, a description will be given of the composition of the plating solution. 
       FIG. 4  illustrates a graph of a relation between a P concentration and the hardness. A horizontal axis indicates the P (phosphorous) concentration of the plating solution used in the electroless plating method. A vertical axis indicates the hardness of the Ni layer  16 . As illustrated in  FIG. 4 , when the P concentration is 0 wt %, the hardness is approximately 200 Hv. When the P concentration is 2 wt %, the hardness is approximately 700 Hv. When the P concentration is 12 wt %, the hardness is approximately 500 Hv. In order to form the first region  16   a  and the second region  16   b  in the Ni layer, the P concentration has only to be changed in a range of 0 wt % to 15 wt %. In order to form the first region  16   a  having the hardness of 150 Hv or more and less than 500 Hv, the P concentration of the plating solution has only to be 0 wt % or more to 1 wt % or less, or more than 12 wt %. The P concentration may be less than 1 wt % or more than 12 wt %. In order to form the second region  16   b  having the hardness of 500 Hv or more, the P concentration of the plating solution has only to be 1 wt % or more to 12 wt % or less. In order to enhance the hardness of the second region  16   b , the P concentration may be 1.5 wt % or more to 10 wt % or less. 
     The solder of the solder ball  20  may be Sn—Cu based solder, Sn—Ag based solder, tin silver bismuth (Sn—Ag—Bi) based solder, or tin zinc (Sn—Zn) based solder or the like. It is preferable that the solder does not include lead (Pb) in view of environmental protection. The foundation layer  12  may be formed with a metal such as Cu or aluminum (Al) other than Au. The foundation layer  12  may be formed with an alloy including at least one of Au, Cu and Al. A material of the cover layer  18  may be a metal such as Ag other than Au. 
     The nitride semiconductor of the semiconductor layer is semiconductor including nitrogen such as gallium nitride (GaN), aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium aluminum nitride (InAlN), indium gallium nitride (InGaN), indium nitride (InN), or aluminum indium gallium nitride (AlInGaN). The material of the semiconductor layer may be gallium arsenic (GaAs) or the like other than the nitride semiconductor. The substrate  10  may be a Si substrate, a substrate in which silicon germanium (SiGe) substrate is laminated on a Si substrate, a substrate in which a GaAs-based semiconductor is laminated on a GaAs substrate, a substrate in which an InP-based semiconductor is laminated on an indium phosphide (InP) substrate, or the like. The semiconductor device may be a CSP acting as a FET (Field Effect Transistor), a HBT (Hetero junction Bipolar Transistor), a MMIC (Monolithic Microwave Integrated Circuit), a LED (Light Emitting Diode), a LD (Laser Diode), a TFT (Thin Film Transistor) used for a liquid crystal or the like. 
     [Second Embodiment] 
     A second embodiment is an embodiment in which the hardness of the Ni layer  16  changes continuously.  FIG. 5A  illustrates a schematic view of an electrode of the second embodiment.  FIG. 5B  illustrates a graph of the hardness of the Ni layer. In  FIG. 5A , hatchings of the Ni layer  16  are omitted, and the first region  16   a  and the second region  16   b  are schematically illustrated by dotted ellipses. 
     As illustrated in  FIG. 5A  and  FIG. 5B , the hardness of the Ni layer  16  continuously gets higher from the foundation layer  12  side to the solder ball  20  side. The hardness X 1  of the lower face of the Ni layer  16  is, for example, 150 Hv and less than 500 Hv. The hardness X 2  of the upper face of the Ni layer  16  is, for example, 500 Hv or more to 1000 Hv or less. In accordance with the second embodiment, the low cost is achieved, the diffusion of the solder is restrained, and the stress is restrained, as well as the first embodiment. There is no region of which hardness drastically changes in the Ni layer  16 . Therefore, the peeling of the Ni layer  16  at an interface between regions having different hardness is restrained. 
     Next, a description will be given of a method for manufacturing the semiconductor device in accordance with the second embodiment.  FIG. 3A  through  FIG. 3C  are also in common with the second embodiment. The plating solution is the same as the first embodiment. The P concentration is constant. The temperature of the plating solution is continuously changed from 30 degrees C. to 80 degrees C. in the electroless plating method for forming the Ni layer  16 . At the starting point of the electroless plating method, the temperature of the plating solution is low about 30 degrees C. Thus, the soft first region  16   a  is formed. After that, the temperature is gradually increased. The temperature of the plating solution is high about 80 degrees C. in the latter half of the electroless plating method. Thus, doping of P into the Ni layer  16  is promoted. Thus, the hard second region  16   b  is formed. The rest processes are the same as the first embodiment. The temperature of the plating solution may be 20 to 70 degrees C., 40 to 90 degrees C. or the like other than 30 to 80 degrees C. 
     [Third Embodiment] 
     A third embodiment is an embodiment in which a heat treatment is performed in the process for forming the Ni layer.  FIG. 6A  through  FIG. 6C  illustrate a cross sectional view of a method for manufacturing a semiconductor device in accordance with the third embodiment. 
     As illustrated in  FIG. 6A , the Ni layer  16  is formed on the foundation layer  12 . The plating solution of the third embodiment is the same as the first embodiment. The P concentration is constant. Therefore, the hardness of the Ni layer  16  after the electroless plating is even regardless of regions of the Ni layer  16 , and is, for example, approximately 100 Hv to 500 Hv. As illustrated in  FIG. 6B , the cover layer  18  is formed on the Ni layer  16 . An upper face of a wafer is subjected to a laser light scanning, or the laser light is radiated to the upper face of the wafer as a whole. Thus, as illustrated with meshed lines in  FIG. 6C , a region of the Ni layer  16  near the upper face thereof and the cover layer  18  are heated to 350 degrees C. An excimer laser, a YAG (Yttrium Aluminum garnet) laser or the like is used as the laser light source. A laser annealing is performed in an atmosphere including an inert gas such as a hydrogen (H 2 ) gas, a nitrogen (N 2 ) gas or an argon (Ar) gas and air. The hard second region  16   b  is formed in the Ni layer  16  through the laser annealing. The heat is continuously conducted from the upper face to the lower face of the Ni layer  16 . And, the hardness of the Ni layer  16  continuously changes as illustrated in  FIG. 5B . The processes after the laser annealing are the same as the first embodiment. 
     When the heat treatment temperature is 300 degrees C. to 500 degrees C., the hardness of the second region  16   b  is over 800 Hv. When the heat treatment temperature is around 400 degrees C., the hardness increases to 900 Hv to 1000 Hv. In order to achieve preferable hardness, the heat treatment temperature is adjusted between 100 degrees C. to 600 degrees C. In order to harden the second region  16   b  more, it is preferable that the heat treatment temperature is 200 degrees C. to 500 degrees C., 300 degrees C. to 450 degrees C., or the like. In order to keep the hardness of the first region  16   a  low and harden the second region  16   b  more, it is preferable that the region of the Ni layer  16  near the upper face thereof is intensively subjected to the heat treatment for a short time such as one second. The laser annealing is preferable as the heat treatment method. And the degradation of the semiconductor is restrained and the efficiency of the manufacturing processes is improved, because the treatment time is short. In the case of the laser annealing, the heat treatment temperature and the radiation time may be changed when the output of the laser, the beam scanning speed, and the pulse width are adjusted. 
     The changing of the P concentration of the first embodiment, the changing of the plating temperature of the second embodiment, and the heat treatment of the third embodiment may be combined. For example, the P concentration of the plating solution may be changed, and the temperature may be changed continuously. For example, the second region  16   b  is hardened more, when the Ni layer  16  is formed by changing the P concentration continuously, and the Ni layer  16  is subjected to the laser annealing. In particular, it is preferable that the heat treatment is performed, in order to achieve the hardness of 1000 Hv. The plating solution may include a Ni ion, dimethylamine-borane and gluconic acid, or dimethylamine-borate and gluconate. In this case, it is possible to adjust the hardness of the Ni layer  16  by adjusting the concentration of boron (B), changing the temperature of the plating solution, or the heat treatment. 
     [Fourth Embodiment] 
     A fourth embodiment is an embodiment in which concavity and convexity is formed on the upper face of the Ni layer  16 .  FIG. 7A  illustrates an enlarged view around the electrode of the fourth embodiment, and illustrates a cross sectional view taken along a line A-A of  FIG. 7B .  FIG. 7B  illustrates a plane view of the electrode. In  FIG. 7B , the solder ball  20  is seen through. A region surrounded by a broken line circle is a region where the solder ball  20  is jointed. 
     As illustrated in  FIG. 7A , the concavity and convexity is formed on the upper face of the foundation layer  12 . And concavity and convexity is formed on a surface of the Ni layer  16  according to the concavity and convexity of the foundation layer  12 . The cover layer  18  is formed along the concavity and convexity of the Ni layer  16 . Concavity and convexity is formed on a surface of the cover layer  18  according to the concavity and convexity of the Ni layer  16 . The solder ball  20  is provided so as to contact with the concavity and convexity of the cover layer  18 . The depth of a concave portion  22  of the Ni layer  16  is, for example, 0.5 μm to 10 μm. As illustrated in  FIG. 7B , the surface of the Ni layer  16  has a waffle structure in which a plurality of the concave portions  22  are distributed. A surface area of the foundation layer  12  and a surface area of the Ni layer  16  are enlarged because of the concavity and convexity. Thus, stress concentration to a small region is restrained. Therefore, even if a diameter of the electrode  11  is reduced, the stress can be restrained. Further, anchor effect enhances jointing strength between the foundation layer  12  and the Ni layer  16 , between the Ni layer  16  and the cover layer  18 , and between the cover layer  18  and the solder ball  20 . Thus, reliability of mounting is improved. 
     Next, a description will be given of a method for manufacturing a semiconductor device in accordance with the fourth embodiment.  FIG. 8A  through  FIG. 8C  illustrate a cross sectional view of the method for manufacturing the semiconductor device of the fourth embodiment. An explanation of the same processes as the first embodiment is omitted. 
     As illustrated in  FIG. 8A , after forming the foundation layer  12 , the concavity and convexity are formed on the upper face of the foundation layer  12  by an etching method or the like. The concavity and convexity may be formed before or after forming the insulating layer  14 . As illustrated in  FIG. 8B , the Ni layer  16  is formed by the electroless plating method. Thus, the concavity and convexity are formed on the surface of the Ni layer  16  according to the concavity and convexity of the foundation layer  12 . Here, the hardness of the first region  16   a  and the second region  16   b  is adjusted by changing the P concentration as well as the first embodiment. As illustrated in  FIG. 8C , the cover layer  18  is formed along the concavity and convexity of the Ni layer  16  by a vapor deposition method or a sputtering method. The concavity and convexity are filled with molten solder through a reflow process, and the solder ball  20  is formed. The concavity and convexity may be directly formed on the surface of the Ni layer  16  and the surface of the cover layer  18  other than the upper face of the foundation layer  12 . 
     Next, a description will be given of a modified embodiment of the fourth embodiment.  FIG. 9  illustrates a plane view of an electrode of the modified embodiment. As illustrated in  FIG. 9 , the surface of the Ni layer  16  may have a structure in which the concave portions  22  are arrayed in a cross shape. The number and the alignment of the concave portions  22  may be changed. In the fourth embodiment, one of the changing of the P concentration, the changing of the temperature of the plating solution, and the heat treatment may be applied. And, two or more of the changing of the P concentration, the changing of the temperature of the plating solution, and the heat treatment may be combined. 
     The present invention is not limited to the specifically disclosed embodiments and variations but may include other embodiments and variations without departing from the scope of the present invention.