The development towards smaller devices with more functions integrated calls for new and improved manufacturing processes. The screen-printing process is quite well suited for miniaturised and integrated devices, since thick films can be produced in this manner without the need for further machining. On the other hand, the process of screen printing thick films involves potential problems of thermal matching and chemical compatibility at the processing temperatures between the functional film, the substrate and the electrodes. As an example of such a miniaturised device, a MEMS accelerometer based on PZT thick film will be presented. The design and process flow of this accelerometer has been optimised by means of finite element modelling (FEMLAB©). Consequently it has proved possible to eliminate post-processing steps after the screen printing of the PZT thick film.

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A transducer for medical imaging was fabricated using a pad-printing process to deposit a curved lead-free thick film (KNN-based) on an electroded backing (porous KNN). First, the electromechanical properties of the pad-printed thick film were measured. An effective thickness coupling factor of 34% (similar to those obtained with the same composition by a screen-printing process) was obtained. The transducer has a f-number of 1.3 and a focal distance around 5 mm. The center frequency is at 12 MHz and the -dB bandwidth is 93% and a high sensitivity is observed. For the first time, images of human skin in vivo were obtained using a lead-free film. They are compared to those obtained with a PZT-based transducer.

The transient analysis of piezoelectric transducers is often performed using finite-element or finite-difference time-domain methods, which efficiently calculate the vibration of the structure but whose numerical dispersion prevents the modeling of waves propagating over large distances. A second analytical or numerical simulation is therefore often required to calculate the pressure field in the propagating medium (typically water) to deduce many important characteristics of the transducer, such as spatial resolutions and side lobe levels. This is why a hybrid algorithm was developed, combining finite- difference and pseudo-spectral methods in the case of 2-D configurations to simulate accurately both the generation of acoustic waves by the piezoelectric transducer and their propagation in the surrounding media using a single model. The algorithm was redefined in this study to take all three dimensions into account and to model single-element transducers, which usually present axisymmetrical geometry. This method was validated through comparison of its results with those of finite-element software, and was used to simulate the behavior of planar and lens-focused transducers. A high-frequency (30 MHz) transducer based on a screen-printed piezoelectric thick film was fabricated and characterized. The numerical results of the hybrid algorithm were found to be in good agreement with the experimental measurements of displacements at the surface of the transducer and of pressure radiated in water in front of the transducer.

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