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

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.

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

Nicolas Ledermann

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.

EPFL2003

Fabrication and characterisation of ultrasensitive monocrystalline silicon cantilevers with integrated magnetic tips

Alain Dysli

During this project, ultrasensitive cantilevers designed for nuclear magnetic resonance applications were fabricated and characterised. The cantilevers are 500, 340 or 200 μm long, 10 or 20 μm wide and 500 or 340 nm thick and integrate a thin film of cobalt at the free end. This thin-film can be magnetised. Their fabrication requires novel techniques such as silicon-on-insulator technology, Al protective layer deposition, high anisotropic dry etching or HF-vapour release. A dynamic and a static model of the cantilevers were elaborated. The models describe the behaviour of the cantilevers when actuated by an external magnetic field in the air and in vacuum.

2006

Characterization and Fatigue of the Converse Piezoelectric Effect in PZT Films for MEMS Applications

Davide Balma, Nachiappan Chidambaram, Ramin Matloub Aghdam, Andrea Mazzalai, Paul Muralt

A measurement setup for the detailed study of the transverse piezoelectric coefficient e(31,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-mu m thick, sol-gel derived PZT(53/47) gradient-free sample showed a response of -18.3 C/m(2) 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.

Institute of Electrical and Electronics Engineers2015

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

Nicolas Ledermann

EPFL2003