Patent Publication Number: US-2009218230-A1

Title: Method of producing electronic component

Description:
CROSS-REFERENCE TO THE RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2008-047950 filed on Feb. 28, 2008; the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method of producing an electronic component having a connection terminal composed of a solder bump. 
     There is a conventionally known technology to perform flip-chip connection of a semiconductor chip and an interposer substrate such as a BGA substrate via a solder bump formed on the semiconductor chip. Heretofore, a Pb-based solder such as a Sn—Pb solder has been used for the solder bumps, but it is now being replaced by a lead-free solder (for example, Sn—Cu, Sn—Ag—Cu, etc.) not containing Pb in view of environmental protection measures in these years. 
     As a method of forming such a lead-free solder bump, there is proposed a method of forming a Sn—Cu solder bump or a Sn—Ag—Cu solder bump by forming a layer of a solder-composing element Cu as an under bump metal (UBM) layer on a substrate by sputtering or the like, depositing a Sn solder or a Sn—Ag solder thereon by electrolytic plating, and reflowing to dissolve Cu in the UBM layer into the solder when the solder is reflowed. This method takes in the second or third element Cu of the solder from the UBM layer to decrease the number of plating elements, thereby simplifying the plating process. Specifically, the plating process becomes complex in terms of the maintenance of the chemical properties of a plating solution and the precise control of the compositions as the number of plating elements increases. It is considered that such complex management and control can be simplified by decreasing the number of plating elements. 
     The above method needs to remove the UBM layer from the portion other than the bump forming portion after plating the Sn solder or Sn—Ag solder. To remove the UBM layer, wet etching, dry etching or the like is used, but the wet etching is suitable in terms of productivity and economic efficiency because it has a fast processing speed. However, when the wet etching is used, etching proceeds isotropically, and relatively large undercuts are produced in the UBM layer (Cu layer) below the plating layer. As a result, the final composition of solder bumps is varied to change the melting point or to lower the mechanical strength, possibly causing a decrease in mounting yield. The solder bumps are arranged in a narrow pitch pattern (e.g., 50 μm or below) in these years and therefore an effect of the undercuts on the solder composition is particularly large. Besides, if an amount of an undercut becomes large, a diameter of the bump after the solder reflow becomes small, its control becomes difficult, adhesiveness to the substrate lowers, and peeling becomes easy to occur. Thus, the above-described method using wet etching cannot be applied to the formation of narrow pitch-patterned solder bumps, and therefore, there remains a problem that productivity and economic efficiency cannot be improved. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a method of producing an electronic component, comprising forming a film of a first metal above a substrate; converting partly the film of the first metal into a film containing a second metal by replacement of at least part of the first metal with the second metal; forming a film of a third metal above the film containing the second metal; and removing the film of the first metal other than the film containing the second metal by wet etching using the film of the third metal as a mask. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  to  FIG. 1G  are sectional views showing a process of a method of producing an electronic component according to a first embodiment. 
         FIG. 2  is a sectional view showing an example of generation of an undercut in comparison with the first embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention are described with reference to the drawings, which are provided for illustration only, and the present invention is not limited to the drawings. 
     First Embodiment  
     A first embodiment is described below.  FIG. 1A  to  FIG. 1G  are sectional views showing an process for an electronic component production according to this embodiment. In this embodiment, formation of bumps of a Sn—Ag—Cu ternary lead-free solder or a Sn—Ag binary lead-free solder on an electronic component is described. 
     As shown in  FIG. 1A , a Ti film  13  with a thickness of e.g. 5000-angstrom and a Cu film  14  with a thickness of e.g. 1 μm are sequentially formed as an UBM layer  12  by methods other than electrolytic plating such as electroless plating, vapor deposition or sputtering on a substrate  11  of an electronic component such as a semiconductor chip, on which unshown electrode pad and passivation film are arranged. A first layer of the UBM layer  12  may be composed of Ti, Ta, W, Cr, V, Zr, Ni, or alloys or compounds resulting from combining any two or more thereof and may also have a laminated structure having such films laminated in plural. 
     As shown in  FIG. 1B , a resist film  15  with a thickness of, for example, 70 μm is laminated on the Cu film  14  by a spin coating technique, and then an octagonal opening  15 A having, for example, a circumscribing circle with a diameter of 100 μm is formed in the resist film  15 , at only a portion where a bump is formed, by a lithography technique. 
     As shown in  FIG. 1C , when electric current is passed through the UBM layer  12  (Cu layer  14 ) and an Ag plating solution is also supplied into the opening  15 A, a film  16  composed of Ag, a nobler metal than Cu, is formed by electrolytic plating on the Cu film  14  exposed at the bottom of the opening  15 A. At this time, before electric current is passed through the UBM layer  12 , displacement plating is performed to displace at least part of Cu with Ag, using electric potential difference between Cu and Ag, so that the portion exposed at the bottom of the opening  15 A of the Cu film  14  is selectively converted into a film  14 A composed of Cu—Ag (or Ag). Then, electric current is started to pass to form the Ag film  16  having a thickness of, for example, 1 μm. 
     A ratio of Cu replaced by Ag and a thickness of the Ag film  16  to be formed are appropriately determined according to the composition of the solder bumps to be formed finally. Therefore, it is also possible to perform only the displacement plating without forming the Ag film  16 , and a Sn film  17  described later is formed just above the Cu—Ag (or Ag) film  14 A. When Cu has been partly replaced by Ag, heat treatment may be subsequently performed to make alloying or compounding. Alloying or compounding provides an advantage that etching selectivity can be further enhanced for Cu wet etching as described later. 
     As shown in  FIG. 1D , when electric current is passed through the UBM layer  12  and a Sn plating solution is also supplied into the opening  15 A, Sn film  17  with a thickness of, for example, 50 μm is formed by electrolytic plating on the Ag film  16  or the Cu—Ag (or Ag) film  14 A exposed at the bottom of the opening  15 A. The thickness of the Sn film  17  is also determined appropriately according to the composition of the solder bumps to be formed finally. The Sn film  17  may be formed by a generally known method such as vapor deposition, sputtering or electroless plating other than electrolytic plating. 
     As shown in  FIG. 1E , after the resist film  15  is removed by a chemical agent such as a resist stripping solution, the Cu film  14  is removed by wet etching using the Sn film  17  or the Sn film  17  and the Ag film  16  as a mask. A solution which dissolves Cu but does not dissolve or is at least harder to dissolve Ag than Cu is used as an etching solution, which includes an ammonia-hydrogen peroxide mixture, a sulfuric acid-hydrogen peroxide mixture and a phosphoric acid-hydrogen peroxide mixture. Specifically, a WLC-C (trade name) manufactured by MITSUBISHI GAS CHEMICAL COMPANY, INC. or the like available on the market as an etching solution for Cu is preferably used. Use of such an etching solution selectively removes the Cu film  14  only and can suppress or prevent the generation of an undercut below the Sn film  17  and the Ag film  16  involved in wet etching. 
       FIG. 2  shows an example of subjecting the Cu film  14  to the wet etching process after forming a film in the same manner as in the present embodiment except that the Ag film  16  and the Sn film  17  are formed on the Cu film  14  without forming the Cu—Ag (or Ag) film  14 A, and it is seen that an undercut  18  of the Cu film  14  is generated below the Sn film  17  and the Ag film  16 . In this embodiment, a portion exposed at the bottom of the opening  15 A in the Cu film  14  is selectively converted into the film  14 A comprising Cu—Ag (or Ag), and an etching solution which dissolves Cu but does not dissolve or is hard to dissolve at least Ag is used, so that only the Cu film  14  can be removed selectively, and the generation of an undercut below the Sn film  17  and the Ag film  16  can be suppressed or prevented. 
     As shown in  FIG. 1F , the first layer of the UBM layer  12 , the Ti film  13 , which is exposed by removing the Cu film  14 , is removed by wet etching, using the Sn film  17 , the Ag film  16  and the Cu—Ag (or Ag) film  14 A as a mask. The etching solution is desired not to dissolve the Sn film  17 , the Ag film  16  and the Cu—Ag (or Ag) film  14 A. For example, a diluted hydrofluoric acid solution diluted to about 0.5 to 1 wt %, a KOH and hydrogen peroxide solution or a WLC-T (trade name) available on the market as an etching solution for Ti and manufactured by MITSUBISHI GAS CHEMICAL COMPANY, INC. is preferably used. Different from the Cu film  14  which has the Cu—Ag (or Ag) film  14 A formed, the Ti film  13  is entirely homogeneous, so that the undercut  18  is generated below the Cu—Ag (or Ag) film  14 A. But, its amount does not become a problem in practical use because, for example, a distance between an end of the Cu—Ag (or Ag) film  14 A and an end of the Ti film  13  after wet etching is as small as, for example, about 0.1 μm, having no effect on the final composition of the solder bumps. 
     Then, as shown in  FIG. 1G , an ordinary solder reflow process is conducted. This process produces a eutectic alloy of the Sn film  17 , the Ag film  16  and the Cu—Ag (or Ag) film  14 A to form a Sn—Ag—Cu ternary or Sn—Ag binary solder bump  19  corresponding to a component ratio of individual elements Sn, Ag and Cu. The reflow process may be performed before the Ti film  13  is removed. In this case, the Sn—Ag—Cu ternary or Sn—Ag binary solder bump  19  is used as a mask for wet etching of the Ti film  13 . 
     In this embodiment, since the solder bump formed portion of the Cu film  14  formed as the UBM layer  12  is converted into the Cu—Ag (or Ag) film  14 A, when the Cu film  14  of the portion other than the solder bump formed portion is removed by the wet etching method, only the Cu film  14  to be removed can be removed selectively, and the generation of undercuts can be suppressed or prevented. Thus, the final composition of the solder bump can be precisely controlled, and a decrease in mounting yield due to a change in melting point or a decrease in mechanical strength or adhesiveness can be prevented. Even if the solder bumps have a narrow pitch pattern, their final compositions and dimensions can be precisely controlled, so that it becomes possible to apply the wet etching to the Cu film etching, productivity of an electronic component including the narrow pitch-patterned solder bumps can be improved, and the production cost can be reduced. 
     Other Embodiments  
     In the first embodiment, it was described that the bumps of the Sn—Ag—Cu ternary lead-free solder or the Sn—Ag binary lead-free solder were formed. But, in a case where bumps are formed of a solder not having the above composition, the embodiment can also be applied extensively by appropriately selecting the materials and processes. 
     For example, when an Au film is formed instead of the Ag film  16  in the first embodiment, bumps can be formed of a Sn—Au—Cu ternary lead-free solder or a Sn—Au binary lead-free solder. When a Bi film is formed instead of the Cu film  14 , the bumps can be formed of a Sn—Ag—Bi ternary lead-free solder or a Sn—Ag binary lead-free solder. In this case, when an Au film is formed instead of the Ag film  16 , the bumps can be formed of a Sn—Au—Bi ternary lead-free solder or a Sn—Au binary lead-free solder. When an Ag film is formed instead of the Cu film  14  and an Au film is formed instead of the Ag film  16 , the bumps can be formed of a Sn—Au—Ag ternary lead-free solder or a Sn—Au binary lead-free solder. 
     It is also possible to form one or more films of other metals on/under the Sn film  17 . Thus, it is possible to form bumps of a quarternary or more solder (e.g., Sn—Ag—In—Bi). 
     In the first embodiment, the process of converting the Cu film  14  partly into the Cu—Ag (or Ag) film  14 A was performed by the displacement plating method. But, the above process is not exclusive if the conversion into the Cu—Ag (or Ag) film  14 A, namely the Ag-containing film can be performed. When the displacement plating method is used, it is necessary that the metal (Cu, Bi or the like) to be replaced is electrochemically less noble than the replacing metal, but if another method is applied, such limitation is eliminated, and flexibility of selection of metal species can be increased. 
     The present invention is not limited to the descriptions of the embodiments described above. The structure, material quality, arrangement of individual members and the like can be modified appropriately without departing from the spirit and scope of the invention.