Temperature Dependence of Internal Stress and Crystal Growth of Dilute Cu Alloy Films

Annealing behavior of dilute Cu-X alloys (adding element X = transition metal and rare-earth metal with less than 3 at %) was investigated in terms of resistivity, internal stress, grain growth and hillock formation. The resistivity increases with addition of impurities regardless of kinds of adding elements. Generally, resistivity starts to decrease on annealing above 200 °C. Among present Cu dilute alloys, Sn addition shows the lowest resistivity 2.5 μΩcm on annealing at 400 °C. However, compared with a pure Cu film, salient grain growth of present dilute alloys does not takes place even at temperatures above 300 °C , where the grain size is nearly the same as that of as-deposited films. In-situ surface observation using an atomic force microscope (AFM ) revealed that hillocks did not grow on cooling stage (under tension), but started to form on heating stage (under compression). The scanning electron microscopy (SEM) observation of hillocks thus formed in present dilute alloy films shows that the external appearance of these defects was quite different from those observed in Al and Al alloy films. They most likely grow with a preferential crystal plane, not irregular growth like Al and Al alloy films. The internal stresses in most of the present as-deposited dilute Cu alloy films were nearly zero or compression of –25 to –100MPa, and upon annealing, they started to increase in tensile manner due to thermal stresses induced by the mismatch of the thermal expansion between substrates and deposited films. A large stress relaxation started to occur above 250°C, associating with a large number of hillock formation.