Andrea Mazzalai

Piezoelectric Al1−xScxN thin films: A semiconductor compatible solution for mechanical energy harvesting and sensors

The transverse piezoelectric coefficient e31,f of Al1-xScxN thin films was investigated as a function of composition. It increased nearly 50% from x = 0 to x = 0.17. As the increase of the dielectric constant was only moderate, these films are very suitable for energy harvesting, giving a 60% higher transformation yield (x = 0.17) as compared to pure AlN. A higher doping might even lead to a 100% augmentation. The thickness strain response (d33,f) was found to increase proportionally to the ionic part of the dielectric constant. The e-type coefficients (stress response), however, did not augment so much as the structure becomes softer. As a result, the transverse voltage/strain response (h31,f-coefficient) was raised only slightly with Sc doping. The low dielectric loss obtained at all compositions suggests also the use of Al1−xScxN thin films in sensors.

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

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 eIDE and hIDE 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-μm-thick PZT 53/47 film deposited by a sol-gel technique, was evaluated and yielded an effective coefficient eIDE 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 × 1010 J/m3 and 14 mV/μ strain.

Characterization and Fatigue of the Converse Piezoelectric Effect in PZT Films for MEMS Applications

A measurement setup for the detailed study of the transverse piezoelectric coefficient e31,f in the converse (actuator) mode was developed. It allows the assessment of the piezoelectric stress in thin films on silicon cantilevers and provides for a correlation of this stress with large and small signal responses to ferroelectric polarization and dielectric response, both as a function of slowly sweeping electric field. This test is important for the understanding of piezoelectric thin films in microelectromechanical systems. The method is illustrated at hand of sol-gel lead-zirconate-titanate (PZT) thin films, and verified also with AlN and AlN-ScN alloy thin films. A 1-μm thick, sol-gel derived PZT(53/47) gradient-free sample showed a response of -18.3 C/m2 at 100-kV/cm electric field. Reliability tests of PZT thin films were carried out with the same setup in an accelerated manner. The piezoelectric activity did not degrade significantly up to 109 unipolar pulses at 100 kHz with an amplitude of -150 kV/cm. The increase in leakage toward the end of the cycles was explained by a thermal runaway effect.

Comparison of lead Zirconate Titanate thin films for microelectromechanical energy harvester with interdigitated and parallel plate electrodes

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/μ strain).

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.

Comparison of lead zirconate titanate (PZT) thin films for MEMS energy harvester with interdigitated and parallel plate electrodes

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 IDE structures exploit the longitudinal piezoelectric effect, which is about twice as high as the transverse piezoelectric effect exploited with PPE structures. There are only few studies on PZT IDE structures and their piezoelectric properties, and even no studies with a direct experimental comparison of the two options. The biggest challenge in using PZT with IDE structures are texture control on insulating buffer layers, and efficient poling at higher electrode gaps. Still, IDE structures are better suited for energy harvesting application as they can generate higher voltages with the same strain. We have proposed a figure of merit (FOM) for easy comparison with similar parallel plate electrode (PPE) structures. Our FOM corresponds to twice the energy density stored per unit strain deformation. For 1 μm random PZT on titania buffered silicon, a FOM of 1.26 × 1010 J/m3 and 12.9 mV/μ strain was achieved when compared to 1.03 × 1010 J/m3 and 1.14 mV/μ strain for highly oriented, well poled 1 μm PZT with PPE system.

Simultaneous piezoelectric and ferroelectric characterization of thin films for MEMS actuators

The progress in lead zirconate titanate Pb(Zrx,Ti1-x)O3 (PZT) thin film deposition and integration technology has led to an exponential growth of piezoelectric micro-electromechanical systems (piezo-MEMS), particularly for pure actuator devices such as inkjet print-heads and autofocus lenses. These devices rely on the transverse effective piezoelectric coefficient e31,f in the converse mode. Thin film material development as well as quality monitoring during production require the measurement of dielectric, ferroelectric, and piezoelectric responses in a relevant way. We conceived and characterized a cheap, versatile, and easy to use setup, based on cantilever tip displacement detection and a charge amplifier allowing for simultaneous measurements of polarization and in-plane piezoelectric stress. The derivative of the obtained stress function gives directly e31,f as a function of the electric field. In this work, data on unipolar excitation of sol-gel deposited PZT thin films are presented. The so derived, “active” e31,f was found to be 40% larger than the value obtained from measurements of the direct effect (sensor mode).

Al (1−x) Sc (x) N thin films as promising non-ferroelectric materials for energy harvesting

We report on microstructure evolution and enhancement of longitudinal and transverse piezoelectric responses of AlN thin film with partial substitution of Al3+ ions by Sc3+ ions. Piezoelectric AlScN alloys are very promising for energy harvesting and sensor applications. AlScN 83/87 alloys exhibit a figure of merit that allows for a 60% efficiency increase as compared to pure AlN.

Dynamic and long-time tests of the transverse piezoelectric coefficient in PZT thin films

A set up originally designed for e31f measurements of piezoelectric thin films was operated in a way to assess life time issues with simultaneous recording of polarization change, PV-loop integral, and piezoelectric beam deflection. A 1μm thick PZT 53/47 thin film was subjected to 109 unipolar pulses with 150 kV/cm driving voltage at 100 kHz and 50 % duty cycle. The observed fatigue was restricted to polarization change and leakage, and did not show up in the deflection. The ferroelectric loop recovered in a few minutes after stopping the voltage pulses. Further analysis suggests a heating of the element by the dissipated energy of the PV loop integral, becoming worse when leakage increases at higher temperatures. The critical piezoelectric stress was reasonably close to values derived from toughness coefficients of bulk PZT ceramics and for crack lengths corresponding to the film thickness.

Niobium doped lead zirconate titanate thin films grown by chemical solution deposition

Niobium (Nb) doping is known to have a beneficial effect on many properties of lead zirconate titanate (PZT) ceramics. Substituting titanium (Ti) or zirconium (Zr) on a B-site, Nb ions form positive point defects that repel oxygen vacancies, even though they are compensated by negative lead (Pb) vacancies of half their concentration. As a consequence, PZT domains are known to move more easily. Nb doped ceramics excel in high piezoelectric coefficients dij and eij, and high permittivities. In this work, we investigated concentration gradient issues, dielectric, ferroelectric, and piezoelectric properties of Nb doped, {100}-textured PZT thin films. The {100}-texture could be maintained throughout the investigated compositional range. As it is known that sol-gel processing tends to form Zr/Ti gradients, it was of interest to know whether Nb forms gradients, and if yes, in which direction. We observed a behaviour similar to one of Zr, an enrichment away from where nucleation happens, thus the top part of the layer. The transverse piezoelectric coefficient e31,f was measured in the direct mode at zero electric field, and in the converse mode as a function of the electric field. The Nb doped films exhibited higher dielectric constants and higher break-down fields, but lower remnant polarizations because of enhanced backs-switching. As compared to “standard” sol-gel PZT films, they show an increased piezoelectric performance at high fields and improved reliability.