Alloying of a Less Noble Metal in Electrodeposited Cu Through Underpotential Deposition

Abstract

Underpotential deposition of Pb or Sn on Cu can be used to produce electroplated Cu‐Pb and Cu‐Sn alloys, with small amounts of alloyed Pb and Sn, from acid solutions that do not contain complexants. Such alloys are of interest as possible on‐chip wiring for very large scale integration; the content of alloying agent must be kept small in order to maintain a low resistivity. The primary requirement for the formation of the alloys is that the deposition process occur in the range of underpotential deposition (UPD); this requirement can be met in solutions of methane sulfonic acid (MSA). Both Pb(II) and Sn(II) depolarize Cu deposition from MSA; the Sn(II) is the weaker depolarizer. Thus, in the absence of other additions to the Cu(II)/MSA solution, electrodeposition proceeds in the UPD range in the presence of Pb(II), producing a Cu‐Pb alloy, but positive to the UPD range in the presence of Sn(II), producing pure Cu. The deposition potential can be manipulated by addition agents in solution. A direct relationship exists between the potential and the amount of incorporated Pb or Sn for all solution compositions and plating conditions, provided that the solution does not contain strongly adsorbing species that interfere with the UPD process. The minor component has been shown to be incorporated in the metallic state, even though the deposition potential is positive to its reversible potential.