Investigation Into The Effect Of Substrate On Pb(Zr0.52Ti0.48)O3 Films

Abstract

During the past several decades, PZT (Pb(Zr,Ti)O3) thin films were given extensive attention for their potential as sensors and actuators in microelectromechanical systems (MEMS) due to their excellent piezoelectric properties. However, for thin films deposited on much thicker substrate, the piezoelectric response is influenced by the substrate clamping, which deviates PZT properties compared with bulk materials. Therefore, to optimize the performance of PZT thin film for a MEMS device, a comprehensive understanding of the clamping effect of substrate on PZT thin films is extremely pertinent. This study investigated the effects of substrate on Pb(Zr0.52Ti0.48)O3films. To begin with the effects of structural layer on the mechanical and electrical properties of PZT film were investigated by varying film thickness and changing types of substrate. The PZT films were deposited by a sol-gel method on platinized silicon substrates, where silicon nitride and silicon oxide were used as a structural layer. The mechanical properties of PZT films were characterized by nanoindentation. Electrical properties as a function of film thickness and layer material were investigated. Residual stresses in PZT films were also characterized by Raman spectroscopy. The measured mechanical properties of PZT films on two types of substrate indicate that the structural layer has a significant influence on the obtained Young's moduli of PZT films. The PZT on SiNxbased substrates presented higher values than PZT on SiO2-based substrate throughout the whole indentation depth. The substrate effect on film hardness, however, was iii negligible since a hardness value of around 8.8GPa was measured for both PZT on SiNxand SiO2-based substrates. Significant influences of film thickness and substrate type on electrical properties were not observed for the investigated thickness range of the PZT films. To study the true orientation effect on the mechanical properties of oriented PZT films, a new model of thin film indentation was developed and used to deconvolute the effects of film orientation and structural layer on the Young’s modulus. The results indicated a clear substrate effect and the new model was able to extract the true film modulus from the measured flat region values. The moduli for (001) and (111)-oriented PZT film are about 159 GPa and 149 GPa, respectively. The electrical and piezoelectric properties of fabricated PZT cantilevers with varying Si thickness were also measured and analyzed. As expected, a smaller thickness of Si substrate resulted in high polarization, dielectric and piezoelectric constants, probably due to the change of the residual stress condition. A model was developed to predict the residual stress in PZT cantilevers. It was found that varying Si thickness resulted in different residual stress conditions in the released PZT cantilever, thus leading to different electrical and piezoelectric properties of the devices.