Structure and method for contact pads having a recessed bondable metal plug over of copper-metallized integrated circuits

Abstract

A metal structure for an integrated circuit, which has copper interconnecting metallization ( 311 ) protected by an overcoat layer ( 320 ). A portion of the metallization is exposed in a window ( 301 ) opened through the thickness of the overcoat layer. The metal structure comprises a patterned conductive barrier layer ( 330 ) positioned on the copper metallization, wherein this barrier layer forms a trough with walls ( 331 ) conformal with the overcoat window. The height ( 331 a ) of the wall is less (between 3 and 20%) than the overcoat thickness ( 320 a ), forming a step ( 340 ). A plug ( 350 ) of bondable metal, preferably aluminum, is positioned in the trough and has a thickness equal to the trough wall height ( 331 a ).

1 . A metal structure for an integrated circuit having copper interconnecting metallization protected by an overcoat layer, portions of said metallization exposed in a window opened through the thickness of said overcoat layer, comprising: 
 a patterned conductive barrier layer positioned on said copper metallization in said window, said barrier layer forming a trough having walls conformal with said window and a trough height less than said overcoat thickness; and    a plug of bondable metal positioned in said trough, said plug having a thickness substantially equal to said trough height so that said window is a pad suitable for wire bonding.    
     
     
         2 . The metal structure according to  claim 1  wherein said overcoat thickness ranges from about 0.6 to 1.5 μm.  
     
     
         3 . The metal structure according to  claim 1  wherein said overcoat comprises one or more layers of silicon nitride, silicon oxy-nitride, silicon dioxide, silicon carbide, or other moisture-retaining compounds.  
     
     
         4 . The metal structure according to  claim 1  wherein said wall height is between 6 and 30% less than said overcoat thickness, creating a step height of 0.1 to 0.2 μm.  
     
     
         5 . The metal structure according to  claim 1  wherein said bondable metal is aluminum or an aluminum alloy.  
     
     
         6 . The metal structure according to  claim 1  wherein said plug has a thickness between about 0.4 and 1.4 μm.  
     
     
         7 . The metal structure according to  claim 1  wherein said plug has a surface on a flat level with said trough walls.  
     
     
         8 . The metal structure according to  claim 1  further comprising a ball bond attached to said plug.  
     
     
         9 . The metal structure according to  claim 1  wherein said barrier layer comprises tantalum nitride.  
     
     
         10 . The metal structure according to  claim 1  wherein said barrier layer is selected from a group consisting of tantalum, titanium, tungsten, molybdenum, chromium, vanadium, alloys thereof, stacks thereof, and chemical compounds thereof.  
     
     
         11 . The metal structure according to  claim 1  wherein said barrier layer has a thickness between about 0.02 and 0.03 μm.  
     
     
         12 . A metal structure for an integrated circuit having copper interconnecting metallization protected by an overcoat layer, portions of said metallization exposed in a window opened through the thickness of said overcoat layer, comprising: 
 a patterned conductive barrier layer positioned on said copper metallization in said window, said barrier layer forming a trough having walls conformal with said window and a trough height substantially equal to said overcoat thickness; and    a plug of bondable metal positioned in said trough, said plug having a thickness substantially equal to said trough height so that said window is a pad suitable for wire bonding.    
     
     
         13 . A wafer-level method of fabricating a metal structure for a contact pad of an integrated circuit having copper interconnecting metallization protected by an overcoat layer including silicon nitride, comprising the steps of: 
 opening a window in said overcoat layer to expose said copper metallization, said window having walls reaching through the thickness of said overcoat layer;    depositing a barrier metal layer over said wafer to cover said exposed copper metallization, window walls, and overcoat surface;    depositing a bondable metal layer over said barrier layer in a thickness sufficient to fill said overcoat window; and    chemically-mechanically polishing said wafer so that said layers of bondable metal and barrier metal are removed over said overcoat outside said window.    
     
     
         14 . The method according to  claim 13  further comprising the step of controlling the continued chemical-mechanical polishing step so that a pre-determined amount of metal height is selectively removed from said filled window, whereby a structural step is formed from said overcoat surface to the remaining metal.  
     
     
         15 . The method according to  claim 13  wherein said step of chemically-mechanically polishing comprises a step of coarse polishing followed by a step of fine polishing.  
     
     
         16 . The method according to  claim 15  wherein said step of chemically-mechanically coarse polishing comprises a removal rate of approximately 400 nm/min.  
     
     
         17 . The method according to  claim 15  wherein said step of chemically-mechanically fine polishing is selective and comprises a removal rate of approximately 100 nm/min.  
     
     
         18 . The method according to  claim 13  wherein said step of chemically-mechanically polishing comprises a step of coarse polishing followed by a step of etching.  
     
     
         19 . The method according to  claim 13  wherein said controls include polishing speed, time, and temperature.  
     
     
         20 . The method according to  claim 13  wherein said step comprises between 3 and 20% of said overcoat thickness.