Infoscience

Thesis

Germination et croissance de films minces de Pb(Zr, Ti)O3 sur silicium passivé et substrats métalliques

We have studied deposition by in-situ reactive sputtering of ferroelectric Pb(ZrxTi1-x)O3 (PZT) thin films on Pt and RuO2 electrodes, using temperatures in the 490-620°C range. Nucleation on the electrode was found to be of prime importance for the formation, texture and hence quality of the PZT films. In consequence, an important part of this work lies in the elaboration of appropriate electrodes. A detailed study and stabilisation of the electrodes on passivated silicon (Si-SiO2) led to the identification and elimination in large part of ill controllable effects such as diffusion through Pt of Pb, oxygen and the reactive metals (Ti, Zr and Ta) used as adhesion layers. A strong link between diffusion of Pb and that of oxygen was shown. On a reactive material, Pt is not an efficient barrier, be it against the diffusion of oxygen, Pb or underlying material. However, RuO2, in combination with a passivating metal (Cr), constitutes a good electrode and barrier. RuO2 blocks the diffusion of Pb and acts as an oxygen buffer, trapping Cr by oxydoreduction and hence hindering its diffusion into the PZT. The insertion of a noble metal (Ru) between Cr and RuO2 lowers the contact resistance and allows direct contact between substrate and electrode up to 700°C. This solution was also adapted to deposition on unpassivated silicon. We have established the existence of several crystallographic orientation relationships between Pt (face centred cubic), RuO2 (tetragonal rutile) and PbTiO3 / PZT (cubic perovskite). On Pt (111), perovskite grows (111)-oriented if the initial layer is rich in TiO2 or (100) if it is rich in PbO. RuO2 grows (100)-oriented on Pt (111), and, with a TiO2 layer, allows nucleation of (111)-oriented perovskite. Using electrodes of varying degrees of texture, we have verified that these orientations are true epitaxial relationships. PZT deposited by reactive sputtering under oxidising conditions can contain an important amount of excess Pb which is incorporated into the perovskite, as is evidenced from the considerable increase of the lattice size. We have proposed that the excess Pb forms a solid solution between PZT and Pb2O3. This occurrence is specific to sputtering and due to the extremely oxidising environment created by the plasma. The properties are modified by the presence of excess Pb in the perovskite lattice. Permittivity is near the value obtained from data on cerarnics. However, the piezoelectric coefficients are about halved and the morphotropic phase boundary seems to be shifted towards compositions rich in Ti. The electrode – PZT structures show good reliability, allowing their integration in devices based on micromachined silicon. This allowed the fabrication of the first functional micromotor driven by a PZT thin film.

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