Process optimisation of a MEMS based PZT actuated microswitch

In this work the fabrication of a piezoelectrically actuated microswitch for high current applications is proposed. The device was obtained by assembling two silicon substrates: one containing the signal lines and the other enclosing a silicon nitride (Si3N4) microcantilever. This latter operates as a switch by closing the circuit with a metal electrode on its tip. A layer of lead zirconate titanate (PZT), deposited by sol gel method, constitutes the actuation element of the microcantilever. The bottom and top electrodes of the PZT were respectively made of thin Ta/Pt and Au layers while the signal lines and the contact electrode on the microcantilever tip were made of Ti/Al. A comparison between microcantilevers characterised by different bottom electrodes is presented: (i) Ta/Pt deposited at 300 degrees C and patterned by wet etching, (ii) Ta/Pt deposited at 100 degrees C and patterned by lift-off. The microcantilevers fabricated with the latter metallisation show a considerable symmetry of the piezoelectric response and a wide microcantilever displacement. Thus, in this device the cantilever microstructures are able to close the gap between the signal lines with a resulting contact resistance of about 2 Omega. (C) 2014 Elsevier B.V. All rights reserved.

Measurement of effective piezoelectric coefficients of PZT thin films for energy harvesting application with interdigitated electrodes

Nachiappan Chidambaram, Andrea Mazzalai, Paul Muralt

Interdigitated electrode (IDE) systems with lead zirconate titanate (PZT) thin films play an increasingly important role for two reasons: first, such a configuration generates higher voltages than parallel plate capacitor-type electrode (PPE) structures, and second, the application of an electric field leads to a compressive stress component in addition to the overall stress state, unlike a PPE structure, which results in tensile stress component. Because ceramics tend to crack at relatively moderate tensile stresses, this means that IDEs have a lower risk of cracking than PPEs. For these reasons, IDE systems are ideal for energy harvesting of vibration energy, and for actuators. Systematic investigations of PZT films with IDE systems have not yet been undertaken. In this work, we present results on the evaluation of the in-plane piezoelectric coefficients with IDE systems. Additionally, we also propose a simple and measurable figure of merit (FOM) to analyze and evaluate the relevant piezoelectric parameter for harvesting efficiency without the need to fabricate the energy harvesting device. Idealized effective coefficients e(IDE) and h(IDE) are derived, showing its composite nature with about one-third contribution of the transverse effect, and about two-thirds contribution of the longitudinal effect in the case of a PZT film deposited on a (100)-oriented silicon wafer with the in-plane electric field along one of the < 011 > Si directions. Randomly oriented 1-mu m-thick PZT 53/47 film deposited by a sol-gel technique, was evaluated and yielded an effective coefficient e(IDE) of 15 C.m(-2). Our FOM is the product between effective e and h coefficient representing twice the electrical energy density stored in the piezoelectric film per unit strain deformation (both for IDE and PPE systems). Assuming homogeneous fields between the fingers, and neglecting the contribution from below the electrode fingers, the FOM for IDE structures with larger electrode gap is derived to be twice as large as for PPE structures, for PZT-5H properties. The experiments yielded an FOM of the IDE structures of 1.25 x 10(10) J/m(3) and 14 mV/mu strain.

Institute of Electrical and Electronics Engineers2012

Polarisation reversal in ferroelectric PVDF and PZT films

Roman Gysel

Ferroelectrics are characterised by a spontaneous polarisation that can be reversed by an external electric field. The stability of the polarisation states and the possibility for controlled switching between the states render ferroelectric materials very attractive for memory applications, in which polarisation states are associated to a binary information "0" or "1". Fast, dense, and reliable data storage has gained enormous importance in our information technology based society. In this context, the fundamental physics of ferroelectric materials attracted great interest with the ultimate goal of reaching memory elements with shortest access times and highest densities. Two model systems of ferroelectric materials were studied in this thesis on the polarisation reversal: an inorganic, classical perovskite and an organic polymer. Despite the fundamental structural differences between lead zirconate titanate [Pb(Zr,Ti)O3] and the investigated polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] copolymer, the switching kinetics in these materials can be described by a similar formalism. Our approach of a combination of macroscopic and microscopic techniques enables an insight into the mechanisms of the polarisation reversal and allows for a better understanding of macroscopic effects governed by mechanisms occurring on the nanoscale. The experimental techniques developed for the ferroelectric capacitor structures have been successfully applied for studying a first hybrid PVDF/silicon ferroelectric transistor for non-volatile information storage. The major accomplishments of this thesis are the following: We demonstrated for the first time a piezoelectric scanning probe study on the cross section of ferroelectric thin films. In combination with switching pulses, this allowed us to directly image the polarisation reversal in ferroelectric films quasi-dynamically. The utility of cross-sectional domain imaging as a powerful tool for the study of the polarisation reversal in ferroelectrics has been corroborated. The apparent contradiction between observations interpreted as a fast sideways growth and the classical scenario of a fast forward growth could be explained by an oblique polarisation direction with respect to the film plane. A surface-stimulated nucleation of reverse domains, with preference on the electrode with a negative potential was observed. It was interpreted in terms of a built-in field due to a depletion layer at the ferroelectric-electrode interface. A combination of microscopic and macroscopic methods used for the first time in organic ferroelectric thin films enabled a better insight into polarisation reversal of the copolymer of vinylidene fluoride and trifluoroethylene P(VDF-TrFE). The polarisation reversal was found to be impeded by a restricted domain growth. This limits the applicability of the conventional Kolmogorov-Avrami model as well as the nucleation-limited switching model with a broad distribution of nucleation times. We demonstrated that an interface-adjacent passive layer may impact on the switching properties and gives rise to a retardation of the polarisation reversal. Furthermore, an extraordinary dielectric constant increase was observed in the films with a passive layer due to an additional domain wall contribution. We fabricated for the first time a hybrid silicon/ferroelectric polymer ferroelectric single transistor memory element with non-volatile operation. The observed difference of the polarisation reversal kinetics of the ferroelectric in the simple capacitor and in the transistor structure was explained by the gate oxide buffer layer in the transistor. Our previously developed tools and approaches enabled a better understanding of retention loss mechanism. A semi-quantitative retention model was suggested that explains the observed exponential retention loss. The investigation of relevant fundamental aspects of ferroelectric materials and the development of novel characterisation techniques in this work can be of interest for further development of non-volatile memory devices.

EPFL2008

PZT thick films by diol chemical solution deposition

Andreas Seifert

Process optimization and properties of lead zirconate titanate (PZT) films for piezoelectric micromachined ultrasonic transducers (pMUTs) for scanning probe devices will be presented. The goal of the work was a replacement of the tetragenic and mutagenic solvent and a decrease of time-consuming PZT 2-methoxy ethanol (2MOE) route. An alternative diol-based solution synthesis process was developed and "Design Of Experiment" (DOE) was used to achieve processing optimization for thick and crack free films. Tight parameter control allowed to develop a highly reproducible PZT diol process. The crystallization behaviour of crack-free PbZr0.53Ti0.47O3 films (1-5 mu m) with oriented perovskite structure was examined by X-ray diffraction and surface analysis using scanning electron microscopy. Piezoelectric and dielectric properties were examined. The effective transverse piezoelectric coefficient e(31,f) of sol-gel processed films was investigated for 4 mu m thick layers. Best properties were achieved with {1 0 0}-textured films, where a remanent e(31,f) value of -7.3 C/m(2) was measured for 4.1 mu m thick films.