Reactive Phase Formation in Sputter-Deposited Ni/Al Thin Films

Abstract

We have investigated reactive phase formation in magnetron sputter-deposited Ni/Al multilayer thin films with a 3:1 molar ratio and periodicities ranging from 2.5-320 nm. In addition, we studied the transformation of a codeposited film of the same composition. We find that an amorphous phase has already formed during deposition, and that the extentof formation of this phase increases with decreasing periodicity. The first crystalline phase then nucleates from this amorphous phase upon annealing. The formation of the amorphous phase considerably reduces the driving force and explains why during subsequent reactions nucleation kinetics become important. We obtain Ni₂Al₉ as the first product phase during heat treatment in some cases before NiAl₃ occurs. For films with modulation periods larger than 40 nm, formation of NiAI₃ is a two stage process as reported earlier, with the first stage being due to nucleation and growth to coalescence of NiAl₃ grains, and the second stage being the growth of NiA1₃ normal to the initial interface until the reactant phases are consumed.