Fabrication of MEMS tonpilz transducers

Tonpilz transducers consist of a heavy tail mass, a piezoelectric spring, and a light head mass. Miniaturized tonpilz transducers are potentially interesting for the 10 to 100 MHz frequency range in imaging transducers due to their high efficiency and output capabilities. Piezoelectric thin films can be used as the active element in the construction of miniaturized tonpilz structures. A 4-10 μm film is necessary for the mass-spring-mass system to resonate at these high frequencies. In this work, fabrication and characterization of lead zirconate titanate (PZT) thick films on conductive oxide LaNiO₃ (LNO) coated silicon on insulator (SOI) substrates will be reported for this application. First, conductive LNO thin films, approximately 300 nm in thickness, were grown on SOI substrates by a metal-organic decomposition (MOD) method. The room temperature resistivity of the LNO was 6.5 × 10^-4 Ω·cm. Randomly oriented PZT (52/48) films up to 7 μm thick were then deposited using a sol-gel process on the LNO coated SOI substrates. 20 mol.% excess lead was added to the solutions to compensate for lead volatilization during film heat treatments. PZT films with LNO bottom electrodes showed good dielectric and ferroelectric properties. The dielectric permittivity (at 1 kHz) was over 1000. The remanent polarization of PZT films was larger than 26 μC/cm². The e_31,f coefficient of PZT thick films was larger than -6.5 C/m² when poled at -75 kV/cm for 15 minutes. A silver layer approximately 40 μm thick was screen-printed onto the PZT film to form the tail mass of the tonpilz structure. Elements were diced and the bulk silicon was removed by dry and wet-etching methods to leave the p-type silicon layer as tonpilz head mass. Fabrication of MEMS tonpilz microstructures will also be presented.