Reduced electromigration and stressed induced migration of cu wires by surface coating

Abstract

The idea of the invention is to coat the free surface of patterned Cu conducting lines in on-chip interconnections (BEOL) wiring by a 1-20 nm thick metal layer prior to deposition of the interlevel dielectric. This coating is sufficiently thin so as to obviate the need for additional planarization by polishing, while providing protection against oxidation and surface, or interface, diffusion of Cu which has been identified by the inventors as the leading contributor to metal line failure by electromigration and thermal stress voiding. Also, the metal layer increases the adhesion strength between the Cu and dielectric so as to further increase lifetime and facilitate process yield. The free surface is a direct result of the CMP (chemical mechanical polishing) in a damascene process or in a dry etching process by which Cu wiring is patterned. It is proposed that the metal capping layer be deposited by a selective process onto the Cu to minimize further processing. We have used electroless metal coatings, such as CoWP, CoSnP and Pd, to illustrate significant reliability benefits, although chemical vapor deposition (CVD) of metals or metal forming compounds can be employed.

1 . A method for forming conductors with high electromigration resistance comprising the steps of forming a patterned conductor on a substrate, forming a conductive film over said surface of said conductor, said conductive film forming a metal to metal metallurgical bond. 
   
   
       2 . The method of  claim 1 , wherein said step of forming a conductive film includes the step of forming said conductive film by electroless deposition whereby said surface of said conductor is protected from oxidation and corrosion and provides high electromigration resistance and high resistance to thermal stress voiding. 
   
   
       3 . The method of  claim 1 , wherein said electroless deposited film has a thickness in the range of 1 to 20 nanometers. 
   
   
       4 . The method of  claim 1 , wherein said electroless deposited film has a thickness in the range of 1 to 10 nanometers. 
   
   
       5 . The method of  claim 1 , wherein said step of electroless deposition includes the steps of first immersing said substrate in a solution of metal ions whereby a layer of nanoparticles of metal are formed on said surface of said conductor, second immersing said substrate in an electroless complexed solution of metal ions and hypophosphite ions whereby a metal-phosphide conductive film is formed on said surface of said conductor, and annealing said substrate in one of an inert and reducing atmosphere at a temperature of at least 300° C. for at least 2 hours whereby excellent adhesion is obtained between said conductor and said metal phosphide conductive film. 
   
   
       6 . The method of  claim 5 , wherein said step of second immersing is omitted. 
   
   
       7 . The method of  claim 5 , wherein said conductive film is selected from the group consisting of CoWP, CoSnP, CoP, Pd, In and W and is in the range from 1 to 20 nm thick. 
   
   
       8 . The method of  claim 2 , wherein said step of electroless deposition includes the steps of first immersing said substrate in a solution of metal ions whereby a layer of nanoparticles of metal are formed on the surface of said conductor, second immersing said substrate in an electroless complexed solution of metal ions and dimethylamino borane whereby a layer of metal-boron conductive film is formed on said surface of said conductor, and annealing said substrate in one of an inert and reducing atmosphere at a temperature of at least 300.degree. C. for at least 2 hours whereby excellent adhesion is obtained between said conductor and said metal boron conductive film. 
   
   
       9 . The method of  claim 1 , wherein said conductive film is applied on the surface of said conductor by physical methods such as Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), evaporation, sputtering and thermal metal interdifffision. 
   
   
       10 . The method of  claim 9 , wherein said conductive film is selected from the group consisting of Pd, In, W and mixtures thereof. 
   
   
       11 . The method of  claim 8 , wherein said conductive film is selected from the group consisting of CoB, CoSnB, CoWB and NiB.