Thermocompression Bonding with Passivated Indium-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 contacting metallizations on a first element; and to passivate the surfaces of said contacting metallizations against re-oxidation;   b) using plasma-activated radical-enriched gas flow at substantially atmospheric pressure: to reduce native oxides from the surfaces of indium-based bumps on a second element; and to passivate the surfaces of said indium-based bumps against re-oxidation;   c) compressing said indium-based bumps and said 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 the indium-based bumps on subsequent elements to the contacting metallizations on the previous element;   wherein said indium-based bumps are comprised of at least 80% atomic indium.   
     
     
         2 . 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 one side of said additional element has neither contacting metallizations nor indium-based bumps. 
     
     
         3 . The method of  claim 1 , wherein said compressing step is performed at a temperature which is below the melting point of said indium-based bumps. 
     
     
         4 . The method of  claim 1 , wherein said compressing step is performed at room temperature. 
     
     
         5 . The method of  claim 1 , wherein said compressing step is performed at a temperature which is more than 100° C. below the melting point of said indium-based bumps. 
     
     
         6 . The method of  claim 1 , wherein said indium-based bumps are comprised of at most 10% atomic silver. 
     
     
         7 . The method of  claim 1 , wherein said indium-based bumps are comprised of at most 5% atomic silver. 
     
     
         8 . The method of  claim 1 , wherein said indium-based bumps are comprised of at least 95% atomic indium. 
     
     
         9 . The method of  claim 1 , wherein said contacting metallizations are comprised essentially of nickel. 
     
     
         10 . The method of  claim 1 , wherein said contacting metallizations are comprised essentially of silver. 
     
     
         11 . The method of  claim 1 , wherein said contacting metallizations are comprised of at least 70% atomic indium. 
     
     
         12 . The method of  claim 1 , wherein said second element has contacting metallizations on a first side and indium-based bumps on a second side. 
     
     
         13 . The method of  claim 1 , wherein said step a) reduces native oxides from said contacting metallizations and also simultaneously passivates said contacting metallizations against re-oxidation. 
     
     
         14 . The method of  claim 1 , wherein said step b) reduces native oxides from said indium-based bumps and simultaneously passivates said indium-based bumps against re-oxidation. 
     
     
         15 . The method of  claim 1 , wherein said compressing step does not substantially deform said contacting metallizations. 
     
     
         16 . The method of  claim 1 , wherein said contacting metallizations are contacting metallization bumps. 
     
     
         17 . The method of  claim 1 , wherein said contacting metallizations are contacting metallization pads. 
     
     
         18 . The method of  claim 1 , wherein said contacting metallizations are contacting metallization pillars. 
     
     
         19 . The method of  claim 1 , wherein said compressing step compresses said indium-based bumps by no more than 40% of the initial heights of said indium-based bumps. 
     
     
         20 . The method of  claim 1 , wherein said compressing step compresses said indium-based bumps by no more than 30% of the initial heights of said indium-based bumps. 
     
     
         21 . The method of  claim 1 , further comprising heating said elements during said compressing step.