Fixture and method for uniform electroless metal deposition on integrated circuit bond pads

Abstract

A method and an apparatus for uniform electroless plating of layers onto exposed metallizations in integrated circuits such as bond pads. The apparatus provides means for holding a plurality of wafers, and rotating each wafer at constant speed and synchronous within the plurality. Immersed in a plating solution flowing in substantially laminar motion and at constant speed, the method creates periodic superposition of directions and speeds of the motion of the wafers and the motion of the plating solution. The invention creates periodically changing wafer portions where the directions and speeds are additive and where the directions and speeds are opposed and subtractive. Consequently, highly uniformly layers are electrolessly plated onto the exposed metallizations of bond pads. If the plated layers are bondable metals, the process transforms otherwise unbondable pad metallization into bondable pads.

We claim:  
     
         1 . A method for controlled electroless plating of uniform metal layers onto exposed metallizations in integrated circuits positioned on the active surface of semiconductor wafers, comprising the steps of: 
 maintaining a plurality of said wafers approximately parallel to each other at predetermined distances;    immersing said wafers into an electroless plating solution flowing in laminar motion at constant speed substantially parallel to said active surface of said wafers;    rotating each of said wafers at constant speed and synchronously with each other; and    creating periodic relative motion in changing directions between said plating solution and said wafers, thereby uniformly plating layers onto said exposed metallizations by controlled electroless deposition.    
     
     
         2 . The method according to    claim 1    wherein said exposed metallizations are non-oxidized copper metallizations of bond pads positioned in said integrated circuits having copper metallizations.  
     
     
         3 . The method according to    claim 1    wherein said plurality of said wafers comprises between 10 and 30 wafers.  
     
     
         4 . The method according to    claim 1    wherein said relative motion comprises a periodic superposition of directions and speeds of the motion of said wafers and the motion of said solution, thus creating periodically changing wafer portions where the directions and speeds are additive and where the directions and speeds are opposed and subtractive.  
     
     
         5 . The method according to    claim 1    further comprising the steps of: 
 inserting the wafers into a clean-up or presoak bath;  
 removing the wafers from the clean-up or presoak bath; and  
 inserting the wafers into the plating solution.  
 
     
     
         6 . An apparatus for controlled electroless plating of uniform layers onto exposed metallizations in integrated circuits positioned on the active surface of semiconductor wafers, comprising: 
 means for holding a plurality of said wafers approximately parallel to each other at predetermined distances;    means for rotating each wafer of said plurality;    means for electroless plating in a solution flowing substantially in laminar motion at constant speed substantially parallel to said active surface of said wafers; and    means for creating periodic relative motion in changing directions between said plating solution and said wafers, whereby uniformly plated layers are electrolessly deposited onto said exposed metallizations.    
     
     
         7 . The apparatus according to    claim 6    wherein said means for rotating wafers creates constant wafer speed and synchronous rotation between wafers.  
     
     
         8 . The apparatus according to    claim 6    wherein said holding means comprises a plurality of grooved rollers positioned parallel to each other, each of said rollers having grooves around said rollers, shaped to support said wafers, the respective grooves of each roller positioned in a plane suitable for holding one of said wafers.  
     
     
         9 . The apparatus according to    claim 8    wherein said plurality of rollers comprises three rollers.  
     
     
         10 . The apparatus according to    claim 6    wherein said rotating means comprises a central sun gear driving said grooved rollers positioned in parallel around said central gear.  
     
     
         11 . The apparatus according to    claim 6    further including a motor associated with the apparatus which rotates the apparatus in a plating solution.