Converse mode piezoelectric coefficient for lead zirconate titanate thin film with interdigitated electrode

The use of interdigitated electrodes (IDEs) in conjunction with ferroelectric thin films shows many attractive features for piezoelectric MEMS applications. In this work, growth of {1 0 0}-textured lead zirconate titanate (PZT) thin films was achieved on insulating MgO buffered, oxidized silicon substrates. IDEs were fabricated by lift-off techniques and cantilevers were formed by dicing. The deflection upon application of a sweeping voltage was measured as large signal response in parallel to the ferroelectric polarization (PV loop). Likewise, the small signal piezoelectric response was measured in parallel to the capacitance-voltage (CV) measurement. In this way, a complete picture of the ferroelectric-piezoelectric element was obtained. From the deflection, the in-plane piezoelectric stress in the PZT thin film was derived and, from this, the effective piezoelectric coefficients. For the latter, two types were defined: an engineering type corresponding to the average value along the IDE, which can directly be compared to coefficient of a parallel plate electrode (PPE) capacitor and a second one that approximately yields the idealized coefficient governing between the electrode fingers. The IDE structures were experimentally compared with PPE structures of identical film thickness. The resulting coefficients were of opposite sign, as expected. In spite of a much better polarization loop, the IDE device showed a lower average piezoelectric stress. The estimated peak value between the fingers was about the same as in the PPE device, corresponding to about 20 C m(-2). Nevertheless, the result is very promising for cases where compressive piezoelectric stresses are required and for preventing cracking due to large piezoelectric tensile stresses in PPE systems.

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

Comparison of Lead Zirconate Titanate Thin Films for Microelectromechanical Energy Harvester With Interdigitated and Parallel Plate Electrodes

Davide Balma, Nachiappan Chidambaram, Andrea Mazzalai, Paul Muralt

Lead zirconate titanate (PZT) thin films on insulator-buffered silicon substrates with interdigitated electrodes (IDEs) have the potential to harvest more energy than parallel plate electrode (PPE) structures because the former exploit the longitudinal piezoelectric effect, which is about twice as high as the transverse piezoelectric effect used by PPE structures. In this work, both options are compared with respect to dielectric, ferroelectric, and piezoelectric properties, leakage currents, and figure of merit (FOM) for energy harvesting. The test samples were silicon beams with {100} PZT thin films in the case of the PPE geometry, and random PZT thin films for the IDE geometry. Both films were obtained by an identical sol-gel route. Almost the same dielectric constants were derived when the conformal mapping method was applied for the IDE capacitor to correct for the IDE geometry. The dielectric loss was smaller in the IDE case. The ferroelectric loops showed a higher saturation polarization, a higher coercive field, and less back-switching for the IDE case. The leakage current density of the IDE structure was measured to be about 4 orders of magnitude lower than that of the PPE structure. The best FOM of the IDE structures was 20% superior to that of the PPE structures while also having a voltage response that was ten times higher (12.9 mV/mu strain).

Ieee-Inst Electrical Electronics Engineers Inc2013

PZT thin films for MEMS devices

Andrea Mazzalai

The recent progress in synthesis and integration of highly piezoelectric lead-zirconate-titanate (PZT) thin films onto silicon substrates are paving the way for new, more efficient and faster devices in micro electro-mechanical systems (MEMS). In comparison to competing principles, piezo-MEMS devices need less power and are faster than thermally actuated devices, and work with lower voltage than electrostatic actuators. Within the frame of the development of mass production routines for the microfabrication of piezo-MEMS devices, there are several bottlenecks to be solved. Among them, the most relevant one is represented by the need of a high-quality, high-throughput PZT thin film deposition route. The popular sol-gel method results often in satisfactory performance in terms of piezoelectric activity, but the lack of automation makes this deposition method unsuitable for a large-scale production. Magnetron sputtering is instead a high-throughput and reliable technology, but high-performing PZT thin films are not straightforward to achieve. In this work we defined a high-quality, high throughput, sputtering process, directly on an industrial machine suitable for large silicon substrates. We show how the film orientation can be tuned between highly-(100) textured and (111), simply by playing with the thickness of a sputtered seed layer. The flexibility offered by magnetron sputtering in terms of microstructural tuning and its impact on the piezoelectric properties is discussed as well. With an optimized process we measured a transverse effective piezoelectric coefficient e31,f = -23 C/m2 on samples showing very low dielectric losses and leakage currents. Energy harvesting (EH) from ambient vibrations can represent a relevant technological breakthrough as it allows self-powering of portable electronic devices for which wiring or battery replacement is either too costly or unpractical. Examples are wireless sensor nodes for structural health monitoring, drug delivery devices, and many others. The most promising route to achieve adequate power levels and acceptable unit costs is to implement piezoelectric energy conversion at the MEMS scale. Whereas devices excited through a frequency coupling with the resonance of the harvester have made significant progress, only few studies have been performed for the exploitation of motion sources characterized by low frequencies and high amplitudes, such as human motion. In this work we report about design, modeling, microfabrication and characterization of an energy harvester based on PZT thin films equipped with interdigitated electrodes (IDE). The advantages of this configuration with respect to the standard parallel-plate (PPE) capacitor one have been studied in detail, and experimentally compared with the PPE configuration realized with AlN-ScN alloys. The PZT-IDE version realizes a similar high output voltage range, however, with a much larger electromechanical coupling coefficient. This means that our new version can be used in combination with lower quality factor resonant structures. As a consequence, one can realize efficient harvesting also in presence of air damping and thus avoid vacuum packaging. The remarkable resulting energy conversion efficiency makes this geometry particularly suitable for vibration energy conversion from human motion.