Patent Publication Number: US-2006017171-A1

Title: Formation method and structure of conductive bumps

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a formation method and structure of conductive bumps, and more particularly to a method and structure of conductive bumps with wetting layer of nickel-based post.  
      2. Description of the Prior Art  
      With the development of IC technology, the package of the IC is strictly required for the function of a product subjects to the technology of the package. The qualities of package devices are tightly related to the conductive bumps between IC and print circuit board.  
      For example,  FIG. 1  is schematically cross-viewed diagram illustrating the structure of a solder bump in accordance with a prior art. Shown on  FIG. 1 , a silicon wafer  10  has sequentially a bonding pad  12 , a passivation layer  14 , a conductive layer  22  and a solder bump  24  thereon. The bonding pad  12 , such as aluminum or copper pad, provides the silicon wafer  10  with a conductive surface for the electrical connection. Furthermore, the passivation layer  14  exposes the partial surface of the conductive bonding pad  12  and is configured for protecting and planarizing the surface of the silicon wafer  10 . The conductive layer  22 , such as an under bump metallurgy layer formed by electroplating, contacts and electrically connects the partial surface of the bonding pad  12 . Generally, the conductive layer  22  consists of an adhesive layer  16 , a barrier diffusion layer  18  and a wetting layer  20  for contacting a solder bump  24  with the bonding pad  12 . The wetting layer  20  may have a stud structure intruding into the bulk body of the solder bump  24  for strengthening the vertical support to prevent the solder bump  24  from collapsing.  
      However, during the process of reflowing, stannum (Sn) in the solder bump  24  aforementioned may diffuse downwards to form inter-metallic compound (IMC) of copper-stannum alloy (Cu 3 Sn) with the copper-based wetting layer  20 . The formation of the inter-metallic compound can not hinder stannum (Sn) in the solder bump  24  from successively diffusing toward the wetting layer  20 . Thus, the excessive consumption of stannum in the solder bump  24  causes the formation of the inter-metallic compound with an unwanted thickness. The thicker the inter-metallic compound is, the more possibly the fracture in thermal-fatigue test happens. Moreover, the excessive consumption of stannum in the solder bump  24  results in the poor connection between the solder bump  24  and a print circuit board during coming soldering and further poor quality of soldering. Furthermore, that copper stud successively reacts with the solder bump  24  may cause the copper stud failing in sustaining. Accordingly, it is important to prevent the formation of the inter-metallic compound to improve the quality of soldering.  
     SUMMARY OF THE INVENTION  
      The method of the present invention addresses many of the shortcomings of the prior art. It is one of objects of the present invention to provide a method of forming conductive bumps to resolve the downward diffusion issue of stannum (Sn) for general conductive bumps. The use of a nickel-based post can prevent stannum in a solder bump from diffusing downward a wetting layer.  
      It is another object of the present invention to provide a formation method and structure of lead-free conductive bumps to resolve the formation of excessive inter-metallic compounds. A nickel post is used as a wetting layer for preventing the formation of the excessive inter-metallic compounds and the collapse of the lead-free conductive bumps.  
      In accordance with an exemplary embodiment of the present invention, formation method and structure of a conductive bump are provided. A conductive bonding pad is on a wafer. A passivation layer covers the wafer and exposes a portion of the conductive bonding pad. A conductive barrier layer contacts and is positioned on the exposed conductive bonding pad. A wetting layer of nickel-based post contacts and is positioned on the conductive barrier layer. A conductive bump contacts and is positioned on the wetting layer of nickel-based post. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      The foregoing and other objects, features, and advantages of the invention will become more readily apparent upon reference to the following detailed description of a presently preferred embodiment, when taken in conjunction with the accompanying drawings in which like numbers refer to like parts, and in which:  
       FIG. 1  is a schematic cross-sectional diagram illustrating the formation of lead solder bump by deposition of thin film in accordance with a prior art;  
       FIGS. 2A through 2C  are schematic cross-sectional diagrams illustrating the method of forming conductive bumps in accordance with the present invention; and  
       FIG. 3  is a schematic cross-sectional diagram illustrating another embodiment in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      An appropriate and preferred embodiment will now be described in the formation of conductive bumps. It should be noted, however, that this embodiment is merely an example and can be variously modified without departing from the scope of the present invention.  
       FIGS. 2A-2C  are schematic cross-sectional diagrams illustrating the manufacture of conductive bumps in accordance with one embodiment of the present invention. Depicted on  FIG. 2A , a wafer  110  has one or more conductive bonding pads  112 , a passivation layer  114 , an adhesive layer  116  and a barrier diffusion layer  118  thereon. In one embodiment, the wafer  110 , such as a silicon wafer, may have other semiconductor devices on an active surface. On the other hand, the active surface of the wafer  110  contacts the conductive bonding pad  112  and the passivation layer  14 . The conductive bonding pads  112  on the active surface, such as aluminum or copper pads, are formed by any suitable methods and configured for the electrical connection with other exterior circuits. Furthermore, the passivation layer  114  made of oxide, nitride or other organic materials covers the active surface and the parts of the conductive bonding pad  112  for the purposes of protecting and planarizing the active surface. It is noted that the passivation layer  114  also exposes the partial surface of the conductive bonding pad  112 .  
      Next, by any suitable methods, such as executing evaporation or sputtering after photolithography and etching, an adhesive layer  116  and a barrier diffusion layer  118  are sequentially formed on the conductive bonding pads  112 , in which the adhesive layer  116  is positioned on and contacts the exposed the conductive bonding pad  112  and the parts of the passivation layer  114 . In the embodiment, the adhesive layer  116  is a layer or layers of titanium, chromium, nickel-chromium alloy, aluminum or tantalum-based metal, but not limited aforementioned. Furthermore, the barrier diffusion layer  118  is a layer or layers of platinum, palladium, nickel, rhodium, wolfram or molybdenum-based metal, but not limited aforementioned. Alternatively, a conductive barrier layer for replacing both the adhesive layer  116  and the barrier diffusion layer  118  is applied on the conductive bonding pad  112 , in which the conductive barrier layer is a layer or layers of tantalum/tantalum nitride. It is understandable that such a conductive barrier layer is a complex layer of plating capable of adhesion and barrier diffusion.  
      Next, depicted in  FIG. 2B , a mask layer  130 , such as a dry film or a photoresist liquid layer, is applied on the barrier diffusion layer  118  and the passivation layer  114 . With photolithographical patterning and etching, the part of the mask layer  130  above the barrier diffusion layer  118  is removed to expose the partial surface of the barrier diffusion layer  118 . Next, one of the features of the present invention, a nickel-based wetting layer  120  is formed on the barrier diffusion layer  118  and contacts the exposed barrier diffusion layer  118 . The nickel-based wetting layer  120 , the barrier diffusion layer  118  and the adhesive layer  116  constitute an under bump metallurgy layer  122 . In the embodiment, the electroplating or sputtering method is applied to the formation of the nickel-based wetting layer  120 , as so to provide it with the thicker thickness. On the other hand, the sidewall of the nickel-based wetting layer  120  recesses onto the barrier diffusion layer  118  and the adhesive layer  116 . Accordingly, the nickel-based wetting layer  120  advantageously increases wettable area and strengthens a coming conductive bump with the intrusive post structure thereinto. Thus, the nickel-based wetting layer  120  advantageously prevents the coming conductive bump from collapsing and the conductive bonding pad  112  from damage. In the embodiment, the nickel-based wetting layer  120  is a layer or layers of nickel metal or nickel alloy.  
      Next, the conductive bump  124  is formed by screen printing or electroplating. Shown in  FIG. 2C , the mask layer  130  is removed and reflowing is then applied to the conductive bump  124 . In the embodiment, a nickel-stannum alloy (Ni x Sn) generates between the interface of the conductive bump  124 , such as a lead-free bump, and the nickel-based wetting layer  120 . The formation of nickel-stannum alloy, such as Ni 3 Sn, wholly prevents the formation of traditional copper-stannum alloy (Cu x Sn) so that the reliability of a product is improved. Moreover, the nickel-stannum alloy also hinders the interface between the conductive bump  124  and the nickel-based wetting layer  120  from the successive diffusion of elements contributive to unwanted alloy reaction. Thus, the consumption of the interface between the conductive bump  124  and the nickel-based wetting layer  120  can be reduced to prevent poor soldering and weak support provided by the under bump metallurgy layer.  
       FIG. 3  is a schematic cross-sectional diagram illustrating a conductive bump in accordance with another embodiment of the present invention. The disclosed structure different from the one in  FIG. 2C  is to have a wetting layer  119  that is constituted the under bump metallurgy layer  122  with the adhesive layer  116  and the barrier diffusion layer  118 . In the embodiment, electroplating or sputtering method is applied to the formation of the wetting layer  119 . The wetting layer  119  is a layer or layers of nickel metal or nickel alloy.  
      Accordingly, structure and formation method of conductive bump are provided. A conductive bonding pad is formed on a wafer. A passivation layer covers the wafer and exposes a portion of the conductive bonding pad. An under bump metallurgy layer contacts and is positioned on the exposed conductive bonding pad. A layer of post nickel metal contacts and is positioned on the under bump metallurgy layer. A conductive bump contacts and is positioned on the under bump metallurgy and further embeds the layer of post nickel metal.  
      While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.