Davide Balma

Resonant PZT MEMS Scanner for High-Resolution Displays

A resonant piezoelectric scanner is developed for high-resolution laser-scanning displays. A novel actuation scheme combines the principle of mechanical amplification with lead zirconate titanate (PZT) thin-film actuation. Sinusoidal actuation with 24 V at the mechanical resonance frequency of 40 kHz provides an optical scan angle of 38.5° for the 1.4-mm-wide mirror. This scanner is a significant step toward achieving full-high-definition resolution (1920 × 1080 pixels) in mobile laser projectors without the use of vacuum packaging. The reported piezoscanner requires no bulky components and consumes <; 30-mW power at maximum deflection, thus providing significant power and size advantages, compared with reported electromagnetic and electrostatic scanners. Interferometry measurements show that the dynamic deformation is at acceptable levels for a large fraction of the mirror and can be improved further for diffraction-limited performance at full resolution. A design variation with a segmented electrode pair illustrated that reliable angle sensing can be achieved with PZT for closed-loop control of the scanner.

Magnetoelastic Clock System for Nanomagnet Logic

In recent years, magnetic-based technologies, like nanomagnet logic (NML), are gaining increasing interest as possible substitutes of CMOS transistors. The possibility to mix logic and memory in the same device, coupled with a potential low power consumption, opens up completely new ways of developing circuits. The major issue of this technology is the necessity to use an external magnetic field as clock signal to drive the information through the circuit. The power losses due to the magnetic field generation potentially wipe out any advantages of NML. To solve this problem, new clock mechanisms were developed, based on spin transfer torque current and on voltage-controlled multiferroic structures that use magnetoelastic properties of magnetic materials, i.e., exploiting the possibility of influencing magnetization dynamics by means of the elastic tensor. In particular, the latter shows an extremely low power consumption. In this paper, we propose an innovative voltage-controlled magnetoelastic clock system aware of the technological constraints risen by modern fabrication processes. We show how circuits can be fabricated taking into account technological limitations, and we evaluate the performance of the proposed system. Results show that the proposed solution promises remarkable improvements over other NML approaches, even though state-of-the-art ideal multiferroic logic has in theory better performance. Moreover, since the proposed approach is technology-friendly, it gives a substantial contribution toward the fabrication of a full magnetic circuit and represents an optimal tradeoff between performance and feasibility.

High frequency torsional MEMS scanner for displays

A high frequency resonant torsional microscanner actuated with thin film PZT is modeled, fabricated, and characterized. Sinusoidal actuation with 24 V at a mechanical resonance frequency of 39870 Hz provides a total optical scan angle of 38.5 deg. for the 1.4 mm wide mirror. It provides significant power and size advantages compared to electromagnetically and electrostatically actuated scanners. This scanner is a significant step towards achieving full HD resolution with mobile laser projectors.

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).

Electric Clock for NanoMagnet Logic Circuits

Among Field-Coupled technologies, NanoMagnet Logic (NML) is one of the most promising. Low dynamic power consumption, total absence of static power, remarkable heat and radiations resistance, in association with the possibility of combining memory and logic in the same device, make this technology the ideal candidate for low power, portable applications. However, the necessity of using an external magnetic field to locally control the circuit represents, currently, the weakest point of this technology. The high power losses in the clock generation system adopted up to now wipes out the most important advantages of this technology.

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.

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).

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.