Sandrine Gentil

{1 0 0}-Textured, piezoelectric Pb(Zrx, Ti1−x)O3 thin films for MEMS: integration, deposition and properties

Pb(Zr-x, Ti1-x)O-3 (PZT) piezoelectric thin films are of major interest in MEMS technology for their ability to provide electro-mechanical coupling. In this work, the effective transverse piezoelectric coefficient e(31,f) of sol-gel processed films was investigated as a function of composition, film texture and film thickness. Dense, textured and crack-free PZT films have been obtained on silicon substrates up to a thickness of 4 mum. Crystallization anneals have been performed for every 0.25 mum. Nucleation on the previous perovskite layer combined with directional growth leads to a gradient of the compositional parameter x of +/-20% (at x = 0.53 average composition). Best properties have been achieved with {100}-textured film of x = 0.53 composition. Large remanent e(31,f) values of -11 to -12 C/m(2) have been obtained in the whole thickness range of 1-4 mum. These values are superior to values of undoped bulk ceramics, but smaller than in current, optimized (doped) bulk PZT

Shear mode bulk acoustic wave resonator based on c-axis oriented AlN thin film

A shear mode resonator based on bulk waves trapped in c-axis oriented AlN thin films was fabricated, simulated, and tested. The active 1.55 μm thick AlN layer was deposited on top of an acoustic Bragg reflector composed of SiO2/AlN λ/4 layer pairs. The resonance was excited by means of interdigitated electrodes consisting of 150 nm thick Al lines. Analytical and simulation calculations show that the in-plane electric field excites bulk acoustic wave shear modes that are trapped in such an AlN film slab. The experimental frequency corresponds well to the theoretical one. The evaluated resonance of the fundamental shear mode at 1.86 GHz revealed a coupling of 0.15% and Q-factor of 870 in air and 260 in silicon oil.

Downscaling of Pb(Zr,Ti)O3 film thickness for low-voltage ferroelectric capacitors: Effect of charge relaxation at the interfaces

In this communication we address two issues essential for low-voltage memory applications of ferroelectric thin films: the size effect on polarization switching, and polarization fatigue. According to the proposed concept, both of these phenomena are controlled by local injection of charge into the interfacial layers of the ferroelectric film. In the experimental part of this work, we show that the entrapped charge relaxation can be enhanced by introducing a thin RuO2 layer into the top interface of the Pt/PZT/Pt ferroelectric capacitor. Capacitors prepared in this way using PZT with a 45/55 Zr/Ti ratio show a substantial improvement of fatigue performance and withstand relatively well the thickness downscaling. As a result, these capacitors exhibited good ferroelectric properties for driving voltage amplitudes as low as 0.6–0.8 V. Our results suggest that control of charge relaxation at the interface is a key issue for development of low-voltage ferroelectric capacitors.

Growth and properties of transparent Pb(Zn-1/3, Nb-2/3)O-3-PbTiO3 single crystals with a cubic habitus

Transparent single crystals of Pb(Zn-1/3, Nb-2/3)O-3-PbTiO3 (PZN-PT) perovskite have been grown by the slow cooling method in a temperature gradient furnace. The optimization of growth techniques to crystallize PZN-PT with a cubic habitus is described in details. The growth temperature range and cooling rates was carried out in different lead fluxes, Crystal recovery from the residual flux was obtained by different techniques based on liquid flux elimination at the end of the growth process. The best transparent single crystals of PZN-PT, obtained by a cooling at 0.8 degrees C/h from 1150 degrees C to 930 degrees C, had a cubic habitus and a size of approx.: 4 x 4 x 2.5 mm(3). The chemical composition and the homogeneity of the crystals were tested by electron probe micro-analysis. Crystals characterization was performed using polarized light microscopy and dielectric and piezoelectric measurements. Phase transitions were determined by differential scanning calorimetry, optical and dielectric measurements.

Mechanisms of Pb(zr0.53Ti0.47)O3 thin film etching with ECR/RF reactor

Abstract Ferroelectric capacitive devices for memory and MEMS applications require patterned ferroelectric thin films with high anisotropic etched features. In this paper, physical and chemical parameters during etching of Pb(Zr0.53Ti0.47)O3 (PZT) by a dual frequency ECR/RF reactor have been investigated. The removal characteristics of blanket films and films with a patterned mask were investigated as a function of gas chemistry (Ar, halogen gases), substrate bias RF power and working pressure (from 5 × 10−4 Pa to 1Pa). The etch processes were characterized in terms of etch rate, selectivity and mask stability. High etching rate processes (up to 70 nm/min with removable photoresist mask) were obtained and micron scale patterns were demonstrated. The impact of the etch process on the PZT surface layer modification was characterized by AFM, SEM, TEM and XPS. A strong influence of process chemistry and RF bias power on etching selectivity and surface topography (roughness, involatile residues) was observed. No surface damage layer was detected by Transmission Electron Microscopy. However, XPS revealed fluorine (up to 34%) and chlorine radicals (below 10%) in a 10nm thick surface layer.

Direct observation of the B-site cationic order in the ferroelectric relaxor Pb(Mg1∕3Ta2∕3)O3

By means of transmission electron microscopy (TEM) the structure of chemically ordered regions in Zr-doped Pb(Mg1∕3Ta2∕3)O3 has been studied. Large coherent ordered domains were obtained by the addition of 5mol%PbZrO3. The results obtained by conventional, analytical, and high-resolution TEM are consistent with the random-site description of the B-site order, the so-called random-layer model. Image simulations showed that a careful selection of the imaging conditions for high resolution transmission electron microscopy is necessary in order to determine the different B-site occupancies. By comparison of simulated with experimental images it was possible to directly distinguish between the different ordering models.

Local Growth of Sol-Gel Films by Means of Microhotplates

PZT (PbZr x Ti1 − x O3) thin films have been locally grown by means of sol-gel deposition and local annealing on microhotplates. The microhotplates were based on a stress compensated Si3N4/SiO2 membranes as structural elements and contained tantalum silicide filaments (Ta5Si3) for resistive heating. Above the filaments, a passivation layer of SiO2, an adhesion layer of Ti/TiO2 and a bottom electrode of Pt were deposited. Due to the low heat conductivity of the membrane, crystallization of the PZT film occurs exactly on the resistor filament. The formation of crystalline phase, i.e. perovskite PZT was verified by means of X-ray diffraction. A random PZT texture has been observed.

Flux growth and characterization of quasi cubic PZN-PT single crystals

Transparent single crystals of the perovskite Pb(Zn-1/3,Nb-2/3)O-3-PbTiO3 (PZN-PT) were grown from different lead fluxes in sealed platinum crucibles. A temperature gradient furnace was used (temperature gradient on crucible: 30-50 degreesC). Optimization of growth temperature range and cooling rates was carried out in fluxes of PbO, Pb3O4 with and without an addition of B2O3. The growth nature range was typically 1150 degreesC --> 930 degreesC (on crucible top) with cooling rates of 0.8 - 1 degreesC/h for the Pb3O4 flux. In these conditions, the best PZN-PT transparent single crystal was 4x4x2.5mm large and it showed a cubic habitus. The problem of crystal recovery from the residual flux Was solved by different methods based on liquid flux elimination at the end of the growth process.:The chemical composition and the homogeneity within the crystals were determined by electron probe micro-analysis and X-rays. Optical (polarized light microscope) and dielectric measurements were used for the characterization and determination of phase transitions, in single crystal with and without an electric field.

Micromachining of Piezoelectric MEMS

This paper reports on the fabrication of piezoelectric thin film devices using PZT (Pb(Zr,Ti)O3) in planar structures such as cantilevers and partially unclamped membranes. Compensation of mechanical film stresses and the use of a compatible dry etching sequence are key issues. PZT as well as platinum have been patterned in high density plasma reactors using standard photoresist masks. The platinum bottom electrode below PZT turned out to be a useful mask for deep silicon etching. The complete process has successfully been used to cut 3 µm wide slits into such stacks to liberate flat cantilevers with 10 µm silicon, 1.2 µm SiO2, 100 nm Pt and 1 to 2 µm PZT.

Integration of Piezoelectric Pb(ZrxTi1-x)O3 (PZT) Thin Films into Micromachined Sensors and Actuators

This chapter presents an overview on integration processes that have been developed for the fabrication of planar silicon structures coated by textured piezoelectric Pb(ZrxTi1−x)O3 (PZT) thin films. Key issues are the textured growth to achieve high piezoelectric coefficients and the stress compensation to control the bending of cantilevers as well as the stretching forces at membranes. Advanced dry etching techniques are needed for patterning the electrode films without damage to PZT, and without leaving residues. Some recent results on cantilever-microphone and piezoelectric micromachined ultrasonic transducer (pMUT) are presented.