Moisture Effects on Copper Thin Film Adhesion

Abstract

The effects of moisture on copper thin film adhesion have been investigated using a modified version of the superlayer indentation test. Copper films up to 100 nm thick were sputter deposited on thermally oxidized silicon wafers. A 1 μm tungsten superlayer with 1 GPa compressive residual stress was sputter deposited on top of Cu films, providing extra energy for interfacial debonding upon indentation. The samples were first indented to a depth just greater than the thickness of the W/Cu bi-layer and then unloaded. Water was introduced to the indent, and a second higher load indent was made in the exact same location as the first one to drive crack propagation in the wet environment. For the indents made to the same maximum load, there was a dramatic increase in blister size in the wet environment compared to the blister size in the dry environment. Copper film adhesion in the presence of water was up to 20 times lower than in the dry environment (0.1 J/m2 vs. 2 J/m2). Chemical reactions at the crack tip along with the surface energy minimization by water are thought to be the causes for the adhesion reduction.