PZT thin films for MEMS devices

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.

Internal electric fields and electrode effects in ferroelectric thin films for piezoelectric energy harvesting

Robin Nigon

The continuing decrease of power requirements of electronic circuits offers the potential to deploy wireless systems as embedded sensors for cars or industrial tools, and implanted medical devices. Harvesting ambient vibrations by an appropriate energy harvesting (EH) device allows to avoid an undesirable battery replacement. At the scale of micro-electromechanical systems, where severe size constraints must be met, microfabricated EH devices with piezoelectric thin films offer the best energy density. Ferroelectric lead zirconate titanate (PZT) thin films with interdigitated electrodes (IDE) appear as the most promising device design for this purpose. Accurate characterization of the thin film response is necessary to determine the PZT composition and doping, the electrode geometry, and the stack design for maximum EH efficiency. Unfortunately, there is no rigorous description of the physical behavior of the IDE system to date. One goal of this thesis was thus to provide a better understanding of the experimental observations made by previous researchers. In addition, only a limited PZT composition and doping range has been investigated in this configuration. It was the second goal of this thesis to widen this range in order to determine the combination that yields the best EH efficiency. Finally, the risk of partial or total depoling over the device lifetime is always present in ferroelectric materials. The phenomena of aging and self-poling are of great interest to ensure proper retention of the poled state and, thus, the reliability of the harvesting device. Neither of the two are well understood. It was the third goal of this thesis to investigate the aging behavior and methods to promote self-poling. In this thesis work, we have have proposed a description of the physical behavior of the IDE system, and we have developed an analytical model for extracting the effective material properties from standard characterization measurements, which is well supported by both finite element (FE) simulations and experimental data. We found that if the substrate is conductive enough, a parasitic capacitance is present in parallel to the material response. We have provided a method to subtract the contribution of the parasitic capacitance, which has an accuracy of better than 4% in a wide range of IDE geometries as determined by FE simulations. We have investigated the performances of doped PZT thin films with IDE for several combinations of dopant and composition. We have improved an existing fabrication route to obtain textured PZT films on an insulating MgO layer. We found that dopants systematically reduced the piezoelectric response and retention capability, and increased the dielectric constant. All three are detrimental for EH. Undoped compositions should be chosen. We have studied methods to improve the stability of the poled state through aging and self-poling. Introducing the latter into the IDE configuration did not provide sufficiently strong effects to be of practical interest. On the contrary, the aging process may allow to tune the extrinsic contributions to the dielectric and piezoelectric response. It is likely caused by polarization discontinuities at grain boundaries. Further work is needed for fully optimizing this phenomenon. Finally, from the previous investigations, we could deduce and propose golden rules for the design of IDE structures, and discuss typical applications where they are advantageous.

EPFL2017

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

High coupling materials for thin film bulk acoustic wave resonators

Janine Conde

Radio frequency (RF) filters based on bulk acoustic wave resonances in piezoelectric thin films have become indispensable components in mobile communications. The currently used material, AlN, exhibits many excellent properties for this purpose. However, its bandwidth is often a limiting factor. In addition, no tuning is possible with AlN. Ferroelectrics would offer both larger coupling to achieve larger bandwidths, and tunability. However, their acoustic properties are not well known, especially in the thin film case. The goal of this thesis is to investigate the potential and identify the limitations of ferroelectric thin films for thickness mode resonators in the 0.5 - 2 GHz range. The Pb(Zrx,Ti1-x)O3 (PZT) solid solution system is the main candidate, since it is known for its large piezoelectric constants and its growth is already well studied. As a main test vehicle, free standing thin film bulk acoustic resonator (TFBAR) structures with Pt/PZT/Pt/SiO2 membranes were successfully fabricated using silicon micro-machining techniques. The main drawback of ferroelectrics is the damping of acoustic waves by domain wall motion both in the RF electric field and in the pressure wave. For this reason films with varying orientations and compositions were investigated. From the device structures the electro-mechanical coupling constants kt2, the quality factors (Q-factors) and several materials parameters have been obtained. High coupling constants have been found for sol-gel Pb(Zr0.53,Ti0.47)O3 films with a {100} texture, kt2 is found to be 0.4 for a 1 µm thick film and 0.8 for a 3.8 µm thick film. However, the Q-factors of these films are low, 18 for the first film and 3 for the second film. The increase of kt2 and the decrease of the Q-factor with frequency indicates that the domains present in these films contribute to these characteristic parameters. It was generally observed that high coupling constant are associated to low Q-factors. This became evident when comparing films with 53/47 composition, where both tetragonal and rhombohedral phases are present, to tetragonal films as well as when comparing {100} textures with (111) textures. Both for the 53/47 composition and for the {100} texture, ferroelastic domain walls are thought to play a bigger role than for tetragonal compositions and (111) textures. The highest figure of merit (FOM) of about 15 was found when combining the composition leading to a high coupling constant (53/47) and the orientation leading to lower losses (111). However the losses even in this film are too high for RF-filter applications. On the other hand, films with low Q-factors but high coupling could prove very useful as transducers for ultrasonic imaging applications, where low Q-factors are desired. The stiffness coefficients of the studied PZT films were shown to be higher than expected from ceramics data. Most likely the stiffness of ceramics always contains domain contributions leading to softening. In contrast, in textured films the variety of domain orientation is very much reduced. In order to reduce losses due the presence of ferroelastic domains three different potential solutions were explored. The first idea was to manipulate the domain populations of the films deposited on silicon by using heat and vacuum treatments. Silicon substrates are known from previous works to be unfavourable for high c-domain fractions. It was discovered that an anneal in vacuum at 550 °C lead to a significant reduction of c-domains in tetragonal 30/70 PZT ({100}). On the contrary, if the sample was subjected to a compressive stress during cooling, the c-domain fraction could be increased. Analysis of the film stress versus temperature curves revealed a trend consistent with theoretical predictions, i.e. a phase boundary between the c/a/c/a and the a1/a2/a1/a2 domain patterns between room temperature and the Curie temperature θC. However, even though this method reveals interesting results, it can not be exploited as a method to achieve a sufficient c-domain population. The second idea explored was the implementation of a high thermal expansion material as a substrate. PZT films deposited on MgO are known to be compressive due to the difference in thermal expansion of the two materials. The compressive stress leads to highly c-axis oriented PZT films. Devices using MgO substrates were fabricated, however difficulties in the micro-machining of the MgO substrate inhibited a complete liberation of the membrane. Nevertheless, preliminary measurements indicate these devices could lead to both high coupling and high Q-factors, suggesting that further detailed study of this method is worthwhile. As a third method for avoiding ferroelastic domains, the uniaxial ferroelectric potassium lithium niobate (KLN) was explored. The unique ferroelectric axis in this material means that only 180° domain walls are present, which can theoretically be removed by poling. This material has been deposited in thin film form using pulsed laser deposition (PLD). KLN thin films with a {001} texture were deposited successfully on Pt/Si substrates. The films were piezoelectric with a d33,f value of around 10 pm/V and a dielectric constant of 250. This is the first time that piezoelectric properties were measured on KLN thin films. A columnar structure has been observed, however the small grain size and the rough surface currently make it difficult to apply this material to TFBAR's.