Preparation and characterization of ferroelectric ceramic/ceramic composite films for MEMS applications

Abstract

A new sol-gel based method has been developed for fabricating thick, dense and crack-free ferroelectric ceramic/ceramic composite films for micro-electro-mechanical system (MEMS) applications. The new method simply combines the modified sol-gel method together with the infiltration technique. A slurry comprising lead zirconate titanate (PZT) ceramic powder dispersed in a PZT precursor solution was first prepared and deposited on a silicon substrate by using the spin-coating technique to form a composite film. The large amount of pores generated in the film was then infiltrated by the subsequent deposition of a diluted PZT precursor solution. This additional infiltration process has been shown to be successful and effective in reducing the porosity and hence retaining the good dielectric ferroelectric and pyroelectric properties of the PZT/PZT composite films. Our results also reveal that the relative permittivity and pyroelectric coefficient of the composite films increase as the PZT powder volume fraction ? increases, following similar dependences predicted by the theoretical models. For a PZT/PZT composite film with ?= 0.37 and a thickness of 2.5 um, the observed relative premittivity, dielectric loss, remanent polarization and pyroelectric coefficient are 1150, 0.037, 21 uC/cm2 and 188 uC/m2K, respectively. A PZT/PZT composite film with a thickness of 16 um has also been successfully fabricated. Although there are a small amount of pores in the film, it exhibits good dielectric, ferroelectric and pyroelectric properties. A 2.5 um-thick PZT/PZT composite film has been successfully fabricated into a square membrane with a length of 3 mm and a thickness of 0.045 mm. The membrane can be driven to vibrate effectively by a small ac signal (0.5V), showing a resonance frequency of 63 kHz for the fundamental mode. The vibrating displacement at the center of the membrane is reasonably high, having a value of 3 nm/V at off-resonance frequencies and a value of 180 nm/V at resonance frequency. Likewise, the voltage response of the membrane at off-resonance frequencies (measured at a c-shaped electrode located around the center and normalized for a unit displacement at the center of the membrane) is also good, about 97 mV/nm. Without using any preamplifier and filter, the composite membrane can distinctly detect a moderate acoustic thud, giving a signal of amplitude as large as 40 mV. Ferroelectric composite films of lanthanum-doped lead zirconate titanate (PLZT) ceramic powder dispersed in a PZT sol-gel matrix have also been fabricated. In addition to good dielectric and pyroelectric properties, self-biased phenomenon has been observed indirectly in the film. It is suggested that the internal field generated by the large remanent polarization in the PZT matrix can "re-polarize" the PLZT ceramic powder at temperatures near the transition temperature of PLZT (120C). As a result, the large contribution to the pyroelectric effect by the PLZT powder at high temperatures is reversible and the high-temperature pyroelectric coefficient of the PLZT/PZT composite film should be reasonably high.