Piezoelectric PZT films for MEMS and their characterization by interferometry

Piezoelectric films can be used in micro-electro-mechanical system (MEMS) devices because the piezoelectric effect can provide high forces with relatively low energy losses. The energy output by a piezoelectric film per unit area is proportional to the film thickness, so it is desirable to have relatively thick films. Chemical solution deposition (CSD) techniques were used to prepare lead zirconate titanate (PZT) thin films with Zr/Ti ratios of 30/70 and 52/48. Usually CSD processing is restricted to making crack-free single layer films of ca 70 nm thick, but modifications to the sol-gel process have permitted the fabrication of dense, crack-free single layers up to 200-300 nm thick, which can be built-up into layers up to 3 mu m thick. Thicker PZT films (> 2 mu m single layer) can be produced by using a composite sol-gel/ceramic process. Knowledge of the electro-active properties of these materials is essential for modeling and design of novel MEMS devices and accurate measurement of these properties is by no means straightforward. A novel double beam common path laser interferometer has been developed to measure the piezoelectric coefficient in films and the results were compared with the values obtained by Berlincourt method. A laser scanning vibrometer was also used to measuring the longitudinal (d(33)) and transverse (d(31)) piezoelectric coefficients for PZT films and ceramics and the results were compared to those obtained by the other methods. It was found that for thin film samples, the d(33,f) values obtained from the Belincourt method is usually larger than those obtained from the interferometer method but smaller than those from the vibrometer method and the reasons for this are discussed.