PZT (Lead Zirconate Titanate (Pb(Zr,Ti)O3)) is currently the dominating piezoelectric material when it comes to achieving high strains and forces at a given voltage. It is more than a factor 10 better than AlN or ZnO in this respect. PZT thin film integration onto silicon wafers for MEMS applications started around 1990 and industry interest is growing rapidly.
Compared to AlN, PZT thin film integration, process and property optimization has been much more difficult. Extensive work has been done to develop procedures to nucleate the correct phase (perovskite) with a specific orientation. Process temperatures for PZT are high, typically 650 °C, making chemical interdiffusion and electrode instabilities a challenging problem. Typically, electrodes of a noble metal such at platinum must be used.
It is not possible to grow PZT directly on silicon because of interfacial reactions and Pb-Si interdiffusion at elevated temperatures. Platinum both exhibits the needed conductivity to act as an electrode, diffusion barrier properties and lattice matching to PZT. Secondly, another configuration is interdigitated electrodes. In this case PZT must be grown with an insulating diffusion barrier as ZrO2 or TiO2 between the film and the silicon wafer.
Much progress in improving PZT thin films and their properties has been made during the last 20 years. A good indicator of the progress is the transverse piezoelectric coefficient e31,f relevant to most of the lower frequency (< 10 MHz) applications, and being proportional to stiffness coefficients, indicating as well the film density.
The quality of integrated PZT has been improved very much, and surpasses today’s epitaxial films. This can only be explained by domain contributions that seem to be stronger in polycrystalline thin film on platinum, than in epitaxial films on perovskite substrates.
The improvement of piezoelectricity in PZT and related thin films over the years. The numbers aside of the dots indicate the film thickness in microns. By courtesy of Paul Muralt.
Published April 22, 2010