Thermocompression Bonding with Passivated Copper-Based Contacting Metal

Abstract

Methods and systems for low-force, low-temperature thermocompression bonding. The present application teaches new methods and structures for three-dimensional integrated circuits, in which cold thermocompression bonding is used to provide reliable bonding. To achieve this, reduction and passivation steps are preferably both used to reduce native oxide on the contact metals and to prevent reformation of native oxide, preferably using atmospheric plasma treatments. Preferably the physical compression height of the elements is set to be only enough to reliably achieve at least some compression of each bonding element pair, compensating for any lack of flatness. Preferably the thermocompression bonding is performed well below the melting point. This not only avoids the deformation of lower levels which is induced by reflow techniques, but also provides a steep relation of force versus z-axis travel, so that a drastically-increasing resistance to compression helps to regulate the degree of thermocompression.

What is claimed is: 
     
         1 . A method for bonding microelectronic elements, comprising the steps of:
 a) using plasma-activated radical-enriched gas flow at substantially atmospheric pressure: to reduce native oxides from the surfaces of first copper-based contacting metallizations on a first side of a first element; and to passivate the surfaces of said first copper-based contact metallizations against re-oxidation;   b) using plasma-activated radical-enriched gas flow at substantially atmospheric pressure: to reduce native oxides from the surfaces of second copper-based contacting metallizations on a second element; and to passivate the surfaces of said second copper-based contacting metallizations against re-oxidation;   c) compressing said first and second copper-based contacting metallizations together, without any conductive liquid phase material, to thereby bond said second element to said first element;   d) repeating said steps a), b), and c), to thereby bond copper-based contacting metallizations on subsequent elements to copper-based contacting metallizations on the previous element.   
     
     
         2 . The method of  claim 1 , wherein said copper-based contacting metallizations are composed of pure copper. 
     
     
         3 . The method of  claim 1 , wherein said reducing steps are performed in a reduction-only atmosphere. 
     
     
         4 . The method of  claim 1 , further comprising the step of bonding an additional element to the previous elements by said steps a) and b), wherein only one side of said additional element has copper-based contacting metallizations. 
     
     
         5 . The method of  claim 1 , wherein said compressing step is performed at a temperature of approximately 300° C. 
     
     
         6 . The method of  claim 1 , wherein said second element has copper-based contacting metallizations both on a first side and also on a second side. 
     
     
         7 . The method of  claim 1 , wherein said copper-based contacting metallizations are copper-based contacting metallization bumps. 
     
     
         8 . The method of  claim 1 , wherein said copper-based contacting metallizations are copper-based contacting metallization pads. 
     
     
         9 . The method of  claim 1 , wherein said copper-based contacting metallizations are copper-based contacting metallization pillars. 
     
     
         10 . The method of  claim 1 , wherein said compressing step compresses said copper-based contacting metallizations by no more than 40% of the initial heights of said copper-based contacting metallizations. 
     
     
         11 . The method of  claim 1 , wherein said compressing step compresses said copper-based contacting metallizations by no more than 30% of the initial heights of said copper-based contacting metallizations. 
     
     
         12 . The method of  claim 1 , further comprising heating said elements during said compressing step.