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ABSTRACT
This paper describes the resonant behaviour of PZT (lead zirconate titanate) micro electromechanical systems (MEMS) based thickness mode piezoelectric transducers fabricated using an innovative low temperature method. The results obtained from transducers with different thicknesses (23 to 42 μ m) on silicon and alumina substrates, were compared to the results obtained from an analytical MASON model.
The composite sol gel and wet etching process used to fabricate the MEMS transducers has been described in previous work [Citation1]. It involves producing a PZT powder/sol composite slurry which, when spun down, yields films a few micrometers thick. Repeated layering and infiltration produces PZT films up to 30 μ m thick. The low firing temperature used in this process (< 720°C) allows the use of silicon as a substrate, which offers the possibility to incorporate piezoelectric ceramics into MEMS. The PZT thick films are shaped using a wet etching technique, using a HF/HCl solution diluted in water.
The stress dependency of the thickness and porosity of the film have been demonstrated to be the major issue regarding the cracking behavior of such film thicknesses. An investigation into the curvature of the wafer study was performed using a Veeco Dektak stylus profiler.
Results obtained for a resonating device with a diameter of 635 μ m on two different substrates, such as silicon and alumina, are presented. After removal of the silicon substrate support devices were tested to determine the resonance and antiresonance frequencies. Theses were found to occur at approximately 69 and 73 MHz respectively for the 22 μ m thick PZT device. Resonance and antiresonace peaks can be observed at 70 and 77 MHz respectively for the 23 μ m thick PZT device. Estimated values of keff calculated from the positions of resonance and anti-resonance peaks show the keff to be approximately between 0.2 and 0.42 as a fonction of the film density.
A 30 μ m thick film resonates at 50 MHz. This device has direct applications for near surface medical imaging. The spin coating technique used for the fabrication opens a new field of possibilities regarding the 3D structure of such transducers.