Stable Gold Bump Solder Connections

Abstract

A metallic interconnect structure ( 200 ) for connecting a gold bump ( 205 ) and a contact pad ( 212 ), as used for example in semiconductor flip-chip assembly. A first region ( 207 ) of binary AuSn 2 intermetallic is adjacent to the gold bump. A region ( 208 ) of binary AuSn 4 intermetallic is adjacent to the first AuSn 2 region. Then, a region ( 209 ) of binary gold-tin solid solution is adjacent to the AuSn 4 region, and a second region ( 210 ) of binary AuSn 2 intermetallic is adjacent to the solid solution region. The second AuSn 2 region is adjacent to a nickel layer ( 213 ) (preferred thickness about 0.08 μm), which covers the copper pad. The nickel layer insures that the gold/tin intermetallics and solutions remain substantially free of copper and thus avoid ternary compounds, providing stabilized gold bump/solder connections.

1 . A method for fabricating a device comprising the steps of:
 providing a semiconductor chip having a gold bump;   providing a substrate having a contact area;   covering the contact area with a layer of nickel;   depositing a body of tin-based solder on the nickel layer, the solder;   bringing the gold bump into contact with the solder body;   raising the temperature to reflow the solder body thereby forming gold/tin intermetallics and solutions adjacent to the gold bump:
 a first region of binary AuSn 2  intermetallic adjacent to the gold bump; 
 a region of binary AuSn 4  intermetallic adjacent to the first AuSn 2  region; 
 a region of binary gold/tin solid solution adjacent to the AuSn 4  region; 
 a second region of binary AuSn 2  intermetallic adjacent to the solid solution region and in contact with the nickel layer. 
   
     
     
         2 . The method according to  claim 1  wherein the steps of depositing a nickel layer and depositing a solder body are selected from a group of techniques including electrolytic plating and electroless plating. 
     
     
         3 . The method according to  claim 1  wherein the tin-based solder is deposited on the nickel layer, while the nickel surface is still wet. 
     
     
         4 . The method according to  claim 1  wherein the solder reflow temperature is between 217 and 280° C. 
     
     
         5 . The method according to  claim 1  wherein the reflow step includes staying nearing a peak temperature for less than 10 seconds. 
     
     
         6 . The method according to  claim 1  further including repeating the solder reflowing step. 
     
     
         7 . The method according to  claim 1 , further comprising depositing a layer of gold between the nickel layer and the tin-based solder. 
     
     
         8 . The method of  claim 7  wherein the gold layer has a thickness between about 0.05 and 0.1 μm. 
     
     
         9 . The method of  claim 1 , wherein the solder body is free of copper.