Piezoelectric acoustic sensors and ultrasonic transducers based on textured PZT thin films

This work dealt with the establishment of a reproducible and industrially exploitable fabrication technology for piezoelectric MEMS based on PZT thin films. {100}-textured piezoelectric 1-4 μm thick films were found to be the most suited materials for MEMS applications where a strong transverse e31,f coefficient is required. Values of 11.5 to 12.5 C/m2 throughout the investigated thickness range have been obtained. PZT MEMS requires some special microfabrication processes. The patterning of platinum electrodes in ICP reactor with an Ar/Cl2 gas mixture gave satisfying results. For 1 μm PZT films, excellent results were achieved with dry etching in an ECR/RF low-pressure reactor. Above 1 μm thickness, wet etching technique was used. The chosen process flow was such that all layers up the top electrode were first deposited, and then sequentially patterned. In this way, the functional properties were completely maintained. Finally, the process flow related to PZT was successfully combined with deep silicon etching techniques to produce free standing structures. The applicability of the processes has been demonstrated by means of two piezoelectric micro-electro-mechanical devices. First, a cantilever acoustic sensor for low frequency applications (< 50 Hz) has been developed for integration in photoacoustic gas detectors. A high device sensitivity of up to 150 mV/Pa has been obtained with noise level corresponding to a few mPa, mainly due to the preamplifier. The second device was based on a silicon/PZT diaphragm to produce a vibrating structure in view of fabricating ultrasonic transducers for position sensors and non-destructive testing probes. The study of the basic properties of such structures has shown excellent results as coupling factors k2 up to 5% for PZT 2 μm / Si 4 μm SOI membranes held with four small bridges. Ultrasonic wave transmission between two devices has been achieved up to a distance of 200 mm. The microfabrication of piezoelectric MEMS was found to be a complex task where all aspects from materials, microfabrication, and device design to assessment of performance are closely interconnected. Piezoelectric MEMS can offer exciting opportunities to fabricate innovative and performing devices.

Epitaxial Piezoelectric MEMS on Silicon

Don Isarakorn

Integration of new functional materials into silicon microsystems is a key factor to enable technology for a wide range of innovative MEMS devices. Piezoelectric materials are of primary interest for integrating sensing and actuation functions in MEMS due to their high forces and high energy densities. The use of PZT thin films in MEMS applications offers the possibility of increasing the sensitivity or actuation capabilities of the devices compared to alternatives such as AlN and ZnO. In general, PZT thin films exhibit smaller piezoelectric coefficients and polarizations than PZT bulk materials due to grain size, composition, crystallographic orientation, non-defined stoichiometry and mechanical boundary conditions. Moreover, PZT thin films are typically grown onto amorphous surfaces resulting in polycrystalline structures, which often lead to degraded performance due to fatigue and aging characteristics. Since epitaxial PZT films exhibit properties, including piezoelectric coefficients, polarizations, and dielectric constants, generally superior to polycrystalline films, it is of high interest to consider their application in MEMS devices, however, there are some real challenges to be solved. Once these issues are overcome, epitaxial PZT thin films could offer interesting potentials in the realization of high performance piezoelectric MEMS. In this thesis, several aspects related to the development of epitaxial piezoelectric MEMS on silicon are investigated, which cover the following topics: the deposition and integration of high quality epitaxial PZT thin films on silicon wafers; the establishment of microfabrication techniques with associated process flows; and the FEM supported design, and characterization of epitaxial piezoelectric MEMS. A short overview is first given on the current state-of-the-art of piezoelectric MEMS. The integration of epitaxial oxide films on silicon wafers and their properties is then briefly described. The epitaxial oxide thin film heterostructures are based on a piezoelectric Pb(Zr0.2Ti0.8)O3 layer grown on 2'' silicon wafers through two oxide layers: SrTiO3 used as buffer and metallic SrRuO3 used as bottom electrode. The optimized microfabrication process for these oxide layers with specific attention in maintaining the piezoelectric properties of the epitaxial PZT films is presented. The polarization was measured to optimize their processing with at the end no degradation of the piezoelectric properties throughout the process. The epitaxial PZT thin films exhibit a large piezoelectric coefficient d31 of 130-140 pm V-1, allowing the realization of MEMS devices with enhanced actuation/detection properties, such as large amplitude actuation with lower driving voltage, high sensitivity, and high efficiency in energy conversion. The superior properties of the epitaxial PZT thin film and the effectiveness of the optimized microfabrication techniques have been demonstrated by two examples of epitaxial PZT MEMS devices. First, different epitaxial PZT cantilevers with and without a Si proof mass have been developed for vibration energy harvesting applications. A high power density of up to 14 µW g-2 was obtained with high current generation and usable voltage, while maintaining lower optimal resistive load. The second application is based on an epitaxial PZT membrane to produce a resonating device. The study of basic characteristics of such device has shown excellent results as it shows a strong harmonic oscillation response with a high quality factor at atmospheric pressure. The finite element model of the epitaxial PZT membrane has then been developed for localized-mass sensing application to determine the resonant frequency, and the effect of the position of the mass and of the resonant mode on the mass sensitivity. The mass sensitivity is of the order 10-12 g Hz-1, which is in excellent agreement with the simulated value. The minimum detectable mass of ∼5 ng can be achieved. These results indicate that the integration of epitaxial thin films on silicon is a promising technology, which improves the performances of piezoelectric MEMS devices. Finally, the concept of charge integration technique for static measurement in piezoelectric sensors is proposed. This technique improves the detection sensitivity of piezoelectric sensors in low frequency measurements, which makes them suitable for chemical and biological detection in a liquid environment.

EPFL2011

PZT thin film growth and chemical composition control on flat and novel three-dimensional micromachined structures for MEMS devices

Florian Calame

Piezoelectric micro-electro-mechanical systems (MEMS) are finding an increased interest for applications requiring high frequency operation and high mechanical quality. The aim of this work was to improve piezoelectric MEMS along two main research directions. The first one was devoted to the strongest piezoelectric thin film material so far established, namely to the solid solution system lead zirconate titanate (PZT), whose properties are peaking at the morphotropic phase boundary at 53/47 Zr to Ti ratio. Properties of such films were lagging behind the ones of bulk ceramics. In sol-gel deposited films, one of deteriorating factors was earlier identified in the compositional gradient preventing to hit the 53/47 composition throughout the whole film volume. In this work, this gradient could be lowered by the preparation of a new set of solutions, reducing the Zr concentration fluctuations from ±12 to ±2.5at%. In combination with a new sol-gel PbTiO3(100) seed layer, and an optimized lead excess and heating scheme, the transverse piezoelectric coefficient |e31.f| could be increased from 12 to 17 C/m2 in 2µm {100}-textured PZT thin films deposited on plane 100mm wafers. In parallel the dielectric constant increased from 1200 to 1500 with a decreasing loss tangent. It could be shown that the dielectric loss must be entirely due to domain wall motions. The second research direction was devoted to the introduction of new piezoelectric shapes by means of deposition into cavities. A free calotte membrane, consisting of a sputtered PZT thin film and its electrodes, has been successfully developed and fabricated as demonstrator of a novel three-dimensional transducer. Calotte profile holes in silicon were prepared by wet etching, and were used as micromould for the PZT membrane. The quasi-static vibration amplitude was measured by means of atomic force microscopy (AFM) yielding a responsivity of 3.5 nm/V, i.e. about 20 to 50 times the d31 value. The second device was a piezoelectric PZT coated active AFM cantilever. A PZT layer was sputter deposited on patterned (111) platinum bottom electrodes into deep cavities (>300µm) prepared by anisotropic wet chemical etching. The photolithography was made possible using spray coating techniques and correcting masks for aberration errors. The study of the basic properties of such a structure has shown promising results. The maximum deflection at the end of the beam was measured as 5.3 nm/V in quasi-static operation. The so derived e31,f value in the cavity corresponded well to the value obtained on flat wafers. Micro-structural analysis showed that PZT grains grow always perpendicular to the local surface inclination, showing no effect of directional sputter flux. The texturing scheme worked as well on inclined surfaces, i.e. the Pt(111) planes and the PZT {100} planes always stay parallel to the local surface.

EPFL2007

Piezoelectric Pb(Zr-x,Ti1-x)O-3 thin film cantilever and bridge acoustic sensors for miniaturized photoacoustic gas detectors

Nicolas Ledermann, Paul Muralt

Novel, highly sensitive piezoelectric acoustic sensors based on partially unclamped Pb(Zr-x, Ti1-x)O-3 (PZT) coated cantilever and bridge have been fabricated by silicon micromachining. High sensitivity at low frequencies (5-100 Hz) has been achieved by patterning very narrow slits (3 to 5 mum) around the structures. A typical response of 100 mV Pa-1 and a noise equivalent pressure of 1.6 mPa Hz(1/2) at 20 Hz have been measured using a 10 pF charge amplifier. Stress compensation, dry etching and integration of high performance piezoelectric thin films were the key issues. PZT/Pt/SiO2 stacks have been patterned by reactive ion etching and stress compensation has been achieved by compensating the PZT film's tensile stress by adjusting the thickness of a thermal SiO2 layer. The integration of sol-gel PZT films with a transverse piezoelectric coefficient e(31,f) of -12.8 C m(-2) has been realized without any degradation of the properties. The microphones were successfully integrated into a miniature photoacoustic detector and tested for CO2 detection. Concentrations down to 330 ppm could be measured with significant signals.