Infoscience

Thesis

PZT thin film growth and chemical composition control on flat and novel three-dimensional micromachined structures for MEMS devices

Piezoelectric micro-electro-mechanical systems (MEMS) are finding an increased interest for applications requiring high frequency operation and high mechanical quality. The aim of this work was to improve piezoelectric MEMS along two main research directions. The first one was devoted to the strongest piezoelectric thin film material so far established, namely to the solid solution system lead zirconate titanate (PZT), whose properties are peaking at the morphotropic phase boundary at 53/47 Zr to Ti ratio. Properties of such films were lagging behind the ones of bulk ceramics. In sol-gel deposited films, one of deteriorating factors was earlier identified in the compositional gradient preventing to hit the 53/47 composition throughout the whole film volume. In this work, this gradient could be lowered by the preparation of a new set of solutions, reducing the Zr concentration fluctuations from ±12 to ±2.5at%. In combination with a new sol-gel PbTiO3(100) seed layer, and an optimized lead excess and heating scheme, the transverse piezoelectric coefficient |e31.f| could be increased from 12 to 17 C/m2 in 2µm {100}-textured PZT thin films deposited on plane 100mm wafers. In parallel the dielectric constant increased from 1200 to 1500 with a decreasing loss tangent. It could be shown that the dielectric loss must be entirely due to domain wall motions. The second research direction was devoted to the introduction of new piezoelectric shapes by means of deposition into cavities. A free calotte membrane, consisting of a sputtered PZT thin film and its electrodes, has been successfully developed and fabricated as demonstrator of a novel three-dimensional transducer. Calotte profile holes in silicon were prepared by wet etching, and were used as micromould for the PZT membrane. The quasi-static vibration amplitude was measured by means of atomic force microscopy (AFM) yielding a responsivity of 3.5 nm/V, i.e. about 20 to 50 times the d31 value. The second device was a piezoelectric PZT coated active AFM cantilever. A PZT layer was sputter deposited on patterned (111) platinum bottom electrodes into deep cavities (>300µm) prepared by anisotropic wet chemical etching. The photolithography was made possible using spray coating techniques and correcting masks for aberration errors. The study of the basic properties of such a structure has shown promising results. The maximum deflection at the end of the beam was measured as 5.3 nm/V in quasi-static operation. The so derived e31,f value in the cavity corresponded well to the value obtained on flat wafers. Micro-structural analysis showed that PZT grains grow always perpendicular to the local surface inclination, showing no effect of directional sputter flux. The texturing scheme worked as well on inclined surfaces, i.e. the Pt(111) planes and the PZT {100} planes always stay parallel to the local surface.

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