P. Gerber

Short-time piezoelectric measurements in ferroelectric thin films using a double-beam laser interferometer

An evolution of the double-beam laser interferometer used for piezoelectric measurements in ferroelectric thin films is reported. Measuring the d33 hysteresis of a ferroelectric material using lock-in technique with large time constants requires a varying bias field to be applied to the sample over a long period of time. This long-term application leads to electrical stress during the measurement. We present a measurement technique using a different source for the applied field and a varied method for averaging the interferometric response. The measurement time for a complete d33 hysteresis will be shortened down to several seconds. Also, the cycle frequency becomes comparable to electrical hysteresis measurements. Experimental results on quartz and Pb(Zr(X),Ti(1−X))O3 are given to demonstrate the capabilities of the interferometer and the new measurement method.

Effects of the top-electrode size on the piezoelectric properties (d33 and S) of lead zirconate titanate thin films

The effects of a decreasing top electrode size on the electric and piezoelectric properties of tetragonal Pb(ZrX,Ti1−X)O3 thin films are investigated. The effective piezoelectric small-signal coefficient d33,eff and the piezoelectric large signal-strain S are measured using a double-beam laser interferometer. Both properties are found to decrease rapidly with decreasing size of the used Pt top electrode for the investigated dimensions of 5mmto100μm edge length (square pads). While the loss of d33,eff is as high as 75%, the influence on the relative permittivity is only small. The source of the pad size effect on the measured piezoelectric properties is found to be the mechanics of the layered structure commonly used for piezoelectric measurements (Pt∕PZT∕Pt∕TiO∕SiO2∕Si), [PZT,Pb(Zrx,Ti1−x)O3] which is verified by finite element simulations.The effects of a decreasing top electrode size on the electric and piezoelectric properties of tetragonal Pb(ZrX,Ti1−X)O3 thin films are investigated. The effective piezoelectric small-signal coefficient d33,eff and the piezoelectric large signal-strain S are measured using a double-beam laser interferometer. Both properties are found to decrease rapidly with decreasing size of the used Pt top electrode for the investigated dimensions of 5mmto100μm edge length (square pads). While the loss of d33,eff is as high as 75%, the influence on the relative permittivity is only small. The source of the pad size effect on the measured piezoelectric properties is found to be the mechanics of the layered structure commonly used for piezoelectric measurements (Pt∕PZT∕Pt∕TiO∕SiO2∕Si), [PZT,Pb(Zrx,Ti1−x)O3] which is verified by finite element simulations.

Effects of ferroelectric switching on the piezoelectric small-signal response (d33) and electrostriction (M33) of lead zirconate titanate thin films

The effects of an increasing small signal amplitude on the piezoelectric small-signal response and electrostriction of tetragonal Pb(Zrx, Ti1−x)O3 thin films are investigated. The piezoelectric small-signal coefficient d33, piezoelectric large signal-strain S, and electrostriction coefficient M33 are measured using a double-beam laser interferometer. A continuously increasing influence of the small signal amplitude is found starting at very low values. In particular, the impact on the measured coercive field Ec is found to be stronger than the impact of the cycling frequency of the applied bias field. Also, unexpected electrostrictive behavior is investigated and explained by the influence of ferroelectric switching on the intrinsic piezoelectric lattice strain. Furthermore, the influence of an applied small-signal on the piezoelectric large-signal response is investigated.

Composition influences on the electrical and electromechanical properties of lead zirconate titanate thin films

The influences of the Zr content on the structural, electrical, and electromechanical properties of Pb[Zr(x),Ti(1−x)]O3 [PZT(x∕1−x)] thin films are investigated in detail. Additionally to measuring all major characteristics of the samples, the electromechanical large-signal behavior is modeled. Raising the Zr content increases the unit cell size and forces the preferred phase to become rhombohedral above the morphotropic phase boundary (MPB). The increased unit cell size changes the switching behavior and increases the intrinsic behavior of the unit cells. The intrinsic behavior is reduced by the phase change, which also introduces non-180° domain wall motion, improving the large-signal strain. Additionally, the domain configuration in saturation is more stable further away from the MPB. Finally, the most suitable materials will be selected for different applications.

Thickness Dependence of Piezoelectric Properties for PZT Thin Films with Regard to MEMS Applications

In this work we report on the investigation of the piezoelectric and ferroelectric properties of Pb(Zr,Ti)O3 (PZT) thin films with a composition of 45% zirconium and 55% titanium prepared by chemical solution deposition (CSD) regarding the major properties required in MEMS applications. Therefore we have measured the polarization and the piezoelectric coefficient d 33 and strain using a high-resolution double beam laser interferometer. We found that d 33 alike the strain is nearly independent of the sample thickness. To estimate the lifetime of MEMS we stressed the samples by applying switching and non-switching voltage pulses. In the non-switching case, no change of the material properties was observed whereas the use of switching voltage pulses resulted in a strong decrease of the piezoelectric properties and the polarization. Here we found a linear behavior between the material properties in the fatigued state and the sample thickness.

Effects of ferroelectric fatigue on the piezoelectric properties (d33) of tetragonal lead zirconate titanate thin films

The fatigue of the electromechanical properties of tetragonal PbZrx, Ti1−xO3 thin films is investigated. The decrease of electromechanical small-signal response is compared to the fatigue of the electric properties and examined in detail. Property fatigue is attributed mainly to switching-failure of the unit cells.

Effects of reversible and irreversible ferroelectric switchings on the piezoelectric large-signal response of lead zirconate titanate thin films

The effects of reversible and irreversible switching processes on the electromechanical large-signal strain S of tetragonal Pb(Zrx,Ti1−x)O3 thin films are investigated and discussed. Starting from electric small- and large-signal measurements, the percentage of switched unit cells cesw is calculated. The result is then used in combination with the measured electromechanical field-induced small-signal response to calculate the field-induced large-signal strain S. Enhanced models for this calculation are developed improving a known model. Differences between the calculated and measured large-signal strains are discussed in respect to parameter influences and irreversible contributions. In addition, detailed insight on the switching processes in respect to the electromechanical properties is given.

NEW RESULTS ON FATIGUE AND IMPRINT EFFECT

ABSTRACT A ferroelectric thin film capacitor is the key element in non-volatile ferroelectric Random Access Memories (FeRAMs). There are three important failure-mechanisms influencing the performance of these capacitors: fatigue, retention, and imprint. In this contribution the fatigue and imprint effect are investigated in detail. The material under investigation is lead zirconate titanate (PZT) which was deposited as ferroelectric thin film. Several fatigue measurements were performed with different cycling frequencies, different applied voltage amplitudes, and at varying hysteresis frequencies to investigate dependencies of these parameters on the fatigue effect. An equation is introduced in order to fit the measured behavior of polarization over time and to assign physical parameters to the numerical fitting values. This equation is used to fit the single fatigue curves and the obtained parameters are discussed. Finally, various imprint measurements were performed. Here, the imprint effect was investigated with respect to varying doping concentrations of Nd and Fe and to the initial hysteresis state.