Processing of rf-sputtered lead zirconate titanate thin films on copper foil substrates

Abstract

Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films on copper foils were fabricated via rf magnetron sputtering and ex situ crystallized in a forming gas atmosphere. The PZT/Cu system is interesting due to the low oxygen partial pressure (pO₂) required during crystallization to prevent Cu oxidation, as well as the mismatch of substrate and film coefficients of thermal expansion. The formation of a Cu₂O interlayer at pO₂ levels not thermodynamically predicted suggests that the film and substrate cannot be thought of as being in equilibrium. It was expected that thicker films would provide a stronger barrier to oxygen diffusion from the ambient to the substrate, but instead Cu₂O formation was found to increase with PZT film thickness. Therefore, the PZT film likely plays an active role in substrate oxidation, perhaps as a source of oxygen. Cu oxidation was significantly minimized by wrapping the films in a sacrificial copper envelope during crystallization. This likely resulted in a gettering of oxygen, which buffered the local pO₂ inside the envelope near the—Cu–Cu₂O thermodynamic equilibrium curve during heating. Two distinct mechanisms may be responsible for the oxidation of the Cu substrate. These can explain the film thickness and crystallization temperature dependence of the Cu₂O interlayer formation, as well as justify the use of a Cu envelope. The dielectric response and hysteresis are shown for samples with varying amounts of the Cu₂O interlayer. A dielectric relaxation near 1 kHz was correlated to the presence of the interlayer.