ALLOYING OF A LESS NOBLE-METAL IN ELECTRODEPOSITED CU THROUGH UNDERPOTENTIAL DEPOSITION

Underpotential deposition of Pb or Sn on Cu can be used to produce ele ctroplated Cu-Pb and Cu-Sn alloys, with small amounts of alloyed Pb an d Sn, from acid solutions that do not contain complexants. Such alloys are of interest as possible on-chip wiring for very large scale integ ration; the content of alloying agent must be kept small in order to m aintain a low resistivity. The primary requirement for the formation o f the alloys is that the deposition process occur in the range of unde rpotential deposition (UPD); this requirement can be met in solutions of methane sulfonic acid (MSA). Both Pb(II) and Sn(II) depolarize Cu d eposition from MSA; the Sn(TT) is the weaker depolarizer. Thus, in the absence of other additions to the Cu(II)/MSA solution, electrodeposit ion proceeds in the UPD range in the presence of Pb(II), producing a C u-Pb alloy, but positive to the UPD range in the presence of Sn(II), p roducing pure Cu. The deposition potential can be manipulated by addit ion agents in solution. A direct relationship exists between the poten tial and the amount of incorporated Pb or Sn for all solution composit ions and plating conditions, provided that the solution does not conta in strongly adsorbing species that interfere with the UPD process. The minor component has been shown to be incorporated in the metallic sta te, even though the deposition potential is positive to its reversible potential.