Markus Kohli

Piezoelectric actuation of PZT thin-film diaphragms at static and resonant conditions

The piezoelectric response of silicon diaphragms covered with sputter-deposited PbZr0.45Ti0.55O3 (PZT) films has been investigated in view of their application in ultrasonic micro-actuators. The behaviour of resonance frequencies and quasistatic deflections has been studied as a function of membrane thickness and d.c. bias. The total stress in the films and the piezoelectric constant, d(31), have been derived by means of two different methods. The results are consistent with direct strain measurements by optical interferometry and with bulk ceramic values of identical composition.

Fabrication and Characterization of Pzt Thin-Film Vibrators for Micromotors

For the first time we have characterized a micromotor driven by a piezoelectric PZT (PbZrxTi1-xO3) thin film. Sputter and sol-gel techniques have been applied for the deposition of the PZT films onto a silicon stator membrane, which is 20-30 mu m thick and has a diameter of 4 mm. The amplitudes of the membrane deflections are measured by means of laser interferometry. They are as large as 800 nm V-1 at the first resonance (26 kHz) and 60 nm V-1 at 1 kHz. This is one order of magnitude larger than previously reported for a ZnO-activated device of similar geometry. The motor operates at 1-3 V-r.m.s., with speeds of up to 200 rpm at 1.1 V-r.m.s. and torques of 35 nN m at 2.5 V-r.m.s. and 1 mN force between rotor and stator. Compared with the conceptually identical ZnO version published by Racine et al., this is an improvement by a factor of three in speed per volt. Taking into account the linear increase of the torque with the stator vibration frequency, the torque per volt is a factor of two higher. A long-term test of 100 h showed no degradation of the motor performance.

Pyroelectric thin-film sensor array

Pyroelectric thin-film point detectors and 1 x 12 arrays have been fabricated and characterized. They consist of sol-gel-deposited PZT thin-film elements on micromachined Si3N4/SiO2 membranes. The measured current and voltage response as a function of modulation frequency of a 1 x 12 array element is compared with finite-element calculations. Voltage responsivities of almost 3000 V W-1 in vacuum and 800 V W-1 in air have been achieved for 0.4 mm x 0.9 mm elements. Some point detectors have been completely packaged and correct operation in a movement detection system has been demonstrated.

Removal of 90° domain pinning in (100) Pb(Zr0.15Ti0.85)O3 thin films by pulsed operation

X-ray diffraction showed that the volume fraction of 90° domains in tetragonal Pb(Zr,Ti)O3 thin films could be substantially reduced by either hot dc poling or by a bipolar pulsed-field process. In both cases, the (001) Bragg peak increased and the (100) peak decreased. However, ferroelectric hysteresis loops look quite different. Hot dc poling leads to large internal fields and high frozen-in polarisation, whereas pulse treatment removes the voltage shifts. This relaxation of the loop and x-ray diffraction results indicate a liberation of defect-pinned domain walls by removing, reorienting, or randomizing charged defects or defect dipoles. Alignment of defects during hot dc poling contributes to piezoelectric and pyroelectric activities.

Pb(Zr,Ti)O3 thin films on zirconium membranes for micromechanical applications

An efficient, electrically conductive, chemical barrier for the integration of piezoelectric Pb(Zr,Ti)O3 (PZT) films on reactive metal substrates has been developed, opening new possibilities for PZT integration on micromechanical and semiconductor devices. Very reactive zirconium films have been taken in order to test the quality of the specially designed RuO2/Cr buffer under the condition of in situ sputter deposition of PZT at 600 °C. The PZT/RuO2 interface was found to be free of intermediate phases. A PZT activated metallic micromechanical element was demonstrated with a thin film Zr membrane.

Interferometric study of piezoelectric degradation in ferroelectric thin films

Degradation of piezoelectric properties has been studied in ferroelectric PZT films by means of optical interferometry. The degradation under dc bias and aging of poled films have been observed. The decay of piezoelectric coefficient with time is described by a logarithmic law with aging rates comparable to those of switching polarization. The aging rates are shown to be sensitive to poling conditions and to orientation of the poling field with respect to the direction of preferred polarization. The fatigue measurements revealed a decrease of piezoelectric coefficient with a simultaneous shift of piezoelectric hysteresis loops. Degradation tests on thin membranes covered by PZT films have been reported.

Processing and properties of thin film pyroelectric devices

Pyroelectric PbTiO3 thin films devices with two temperature compensating elements on a SiO2/Si3N4 membranes have been fabricated and characterized. The measured voltage responsivity as a function of radiation modulation frequency has been compared to finite element model calculation. The relevant film properties have been compared for sputter and a sol-gel deposition techniques. The calculated responsivity amounts to 30 V/W. The measured pyroelectric signal is a few mu V.

Thin film pyroelectric array as a detector for an infrared gas spectrometer

We demonstrate the implementation of a thin film pyroelectric array as a detector in an infrared gas spectrometer. The 1 x 50 element array is integrated onto a silicon chip with a sol-gel deposited zirconium modified lead titanate film (PZT) as the pyroelectrically active layer and a submicron membrane for thermal insulation. The spectrometer fitted with this detector covers a spectral range of 1.6 mu m and is capable of simultaneously detecting 15 ppm of CO2 and 200 ppm CO. Limitations of the present setup stem from a not optimized thermal light source and the discrete readout electronics

Poling of ferroelectric thin films

Poling of lead zirconate titanate pyroelectric thin films has been studied. Poling temperature and electrical field were optimized to achieve high figures of merit. We have found a direct correlation between the onset of degradation current and the final pyroelectric coefficient showing that thermal activation for defect motion is imperative for poling. We have proposed that defects and defect dipole complexes play a major role to obtain stable and metastable domain configurations after dc poling at high temperature and field cycling, respectively.

Poling of pyroelectric thin films

Abstract Poling of calcium and zirconium modified lead titanate pyroelelectric thin films has been studied. Poling temperature and electrical field were optimized to achieve high figure of merits. It was found that thermal activation is important to obtain highly poled films. Defect-dipole complexes are supposed to play a major role to retain the polarization and also to enhance it. Porous calcium substituted films achieved an up to three times higer voltage response than zirconate substituted thin films, because porosity decreased the dielectric constant significantly.