Nickel-tin microbump structures and method of making same

Abstract

Techniques and mechanisms for providing effective connectivity with surface level microbumps on an integrated circuit package substrate. In an embodiment, different metals are variously electroplated to form a microbump which extends through a surface-level dielectric of a substrate to a seed layer including copper. The microbump includes nickel and tin, wherein the nickel aids in mitigating an absorption of seed layer copper. In another embodiment, the microbump has a mass fraction of tin, or a mass fraction of nickel, that is different in various regions along a height of the microbump.

1 . A method for forming microbumps on a substrate, the method comprising:
 patterning a dielectric layer, wherein a copper contact is exposed by an opening formed in the dielectric layer;   performing a deposition of a seed layer on the copper contact, the seed layer comprising copper;   electroplating nickel of a first microbump directly on the seed layer; and   electroplating tin of the first microbump directly on the nickel.   
     
     
         2 . The method of  claim 1 , wherein performing the deposition of the seed layer includes performing an electroless plating of the seed layer directly on the copper contact. 
     
     
         3 . The method of  claim 1 , wherein a bottom 10% of a volume of the first microbump has a first tin mass fraction, wherein a top 10% of a volume of the first microbump has a second tin mass fraction and wherein the second tin mass fraction differs from the first tin mass fraction by at least 5% of the first tin mass fraction. 
     
     
         4 . The method of  claim 3 , wherein the second tin mass fraction differs from the first tin mass fraction by at least 10% of the first tin mass fraction. 
     
     
         5 . The method of  claim 1 , wherein a total volume of tin of the first microbump is equal to at least 75% of a total volume of nickel of the first microbump. 
     
     
         6 . The method of  claim 1 , wherein a total tin mass fraction of the first microbump is in a range of 50% to 90%. 
     
     
         7 . The method of  claim 1 , further comprising forming a surface finish on the copper contact, wherein the seed layer is deposited on the surface finish. 
     
     
         8 . A method of fabricating an integrated circuit (IC) package substrate, the method comprising:
 forming a dielectric layer above a sub-surface-level metal layer, the sub-surface-level metal layer including copper contacts;   forming via holes in the dielectric layer, the via holes exposing the copper contacts;   forming a seed layer in the via holes and above the copper contacts, the seed layer including copper; and   forming microbumps on the seed layer, the microbumps including tin and nickel, wherein a bottom ten percent of a volume of each of the microbumps has a first tin mass fraction, wherein a top ten percent of the volume of each of the microbumps has a second tin mass, and wherein the second tin mass fraction differs from the first tin mass fraction by at least five percent of the first tin mass fraction.   
     
     
         9 . The method of  claim 8 , further comprising:
 prior to forming the seed layer, forming a surface finish on the copper contacts, wherein the seed layer is formed on the surface finish.   
     
     
         10 . The method of  claim 8 , wherein the seed layer is formed directly on the copper contacts. 
     
     
         11 . The method of  claim 8 , wherein nickel of each of the microbumps is directly on the seed layer. 
     
     
         12 . The method of  claim 8 , wherein the second tin mass fraction differs from the first tin mass fraction by at least ten percent of the first tin mass fraction. 
     
     
         13 . The method of  claim 8 , wherein a total tin mass fraction of each of the microbumps is in a range of 50% to 90%. 
     
     
         14 . A method of fabricating an integrated circuit (IC) package substrate, the method comprising:
 forming a dielectric layer above a sub-surface-level metal layer, the sub-surface-level metal layer including copper contacts;   forming via holes in the dielectric layer, the via holes exposing the copper contacts;   forming a seed layer in the via holes and above the copper contacts, the seed layer including copper; and   forming microbumps on the seed layer, the microbumps including tin and nickel, wherein a total volume of tin of each of the microbumps is equal to at least 75% of a total volume of nickel of each of the microbumps.   
     
     
         15 . The method of  claim 14 , further comprising:
 prior to forming the seed layer, forming a surface finish on the copper contacts, wherein the seed layer is formed on the surface finish.   
     
     
         16 . The method of  claim 14 , wherein the seed layer is formed directly on the copper contacts. 
     
     
         17 . The method of  claim 14 , wherein nickel of each of the microbumps is directly on the seed layer. 
     
     
         18 . The method of  claim 14 , wherein a total tin mass fraction of each of the microbumps is in a range of 50% to 90%. 
     
     
         19 . The method of  claim 14 , wherein a bottom ten percent of a volume of each of the microbumps has a first tin mass fraction, wherein a top ten percent of the volume of each of the microbumps has a second tin mass, and wherein the second tin mass fraction differs from the first tin mass fraction by at least five percent of the first tin mass fraction. 
     
     
         20 . The method of  claim 19 , wherein the second tin mass fraction differs from the first tin mass fraction by at least ten percent of the first tin mass fraction.