Laurie Valbin

Piezoelectric aluminum nitride thin films for ultrasonic transducers

Piezoelectric aluminum nitride (AlN) thin films have been developed to realize ultrasonic transducers. AlN up to 1.5m is deposited at low temperature (140 degree(s)C) by reactive DC magnetron sputtering of an Al target in argon and nitrogen on Si, Si/SiO2/Al, and Si/Al substrates, and is wet etched (rates from 0.1 micrometers /min to 0.2 micrometers /min and selectivity of 1:10 with Al, and no etching with Si). SiO2/Al/AlN/Al, Al/AlN/Al and Si/AlN/Al square and circular membranes, from 10 micrometers to 1.5 mm size are fabricated using silicon deep reactive ion etching (DRIE), which gives etch profiles about 90, which allows larger integration density than wet anisotropic etching for ultrasonic transducers arrays. By varying size and thickness of membranes, resonance frequencies from 10 kHz to 20 MHz are expected, acoustic and electrical measurements are in progress. Ultrasonic transducers using this technology will be used to measure flows velocity by Doppler method. Other potential applications for ultrasonic transducers include medical ultrasounds and sonar. Other structures are also in progress such as Thin Film Bulk Acoustic Resonator (TFBAR), and Lamb wave devices using this technology.

Detecting response of square and circular Si/AlN/Al membranes transducers by Laser Vibrometry Doppler and Impedancemeter

This paper investigates the vibration analysis of square and circular piezoelectric micro-transducers in the hundred kHz range as a function of experimental tools. These micro-transducers have been actuated by a piezoelectric Aluminum Nitride (AlN) 0.9 μm-thin layer sandwiched between N-doped Silicon (Si) 20μm-thick layer and aluminum (Al) 0.4μm-thin layer. Si/AlN/Al square and circular membranes, with size fixed to 1.5 mm were fabricated using micromachining technology. Analytical and 3D finite element method (3D-FEM) analysis using Comsol software has been performed to model static, modal and vibration behavior of these membranes. Typical numerical simulations and experimental results are presented and discussed. Comparison with standard impedancemeter measurement is shown to assess the performance of Laser Doppler Vibrometry (LDV) system. This works main contribution is mechanical and electrical characterization and comparison with a model.

Low-temperature piezoelectric aluminum nitride thin film

ABSTRACT Polycrystalline Aluminum Nitride (AIN) films were deposited on Si(100), A1/Si(100), and A1/SiO,/Si substrates by reactiveDirect Current (DC) magnetron sputtering of an Al target, under different conditions of substrate temperature, pressure, N7 I N7 + Ar ratio. The film properties were investigated by X-Ray Diffraction (XRD), scanning electron microscopy (SEM)and atomic force microscopy (AFM). Deposition rates in the range of 1 .2 to 1 .8 trn/h were obtained, the film grain sizewas around 4Onrn. To fabricate test structures, wet chemical etching was developed to etch A1N with a good selectivityrespect to Al and Si.Visual aspect and surface roughness show that the maximum temperature must be less than 300°C. X-Ray Diffractiontogether with dielectric constant measurement show that films are better oriented on Si(lOO) than on Al/Si(100).First results give relative dielectric constant S1 (T: free displacement) = 9.5 to I 1 , Eb, = 200 to 5OOV/trn, piezoelectriceffect, good crystal orientation.Membranes were also fabricated using deep reactive ion etching plasma (DRIE), to characterize the film and to fabricateMEMS sensors and actuators, and acoustic wave devices.Keywords: A1N, piezoelectric thin film, reactive DC sputtering, MEMS, acoustic wave devices

Continuous Intra Ocular Pressure Measurement Sensor for Glaucoma Diagnostic

Glaucoma is an ocular pathology usually associated with an increase in Intra Ocular Pressure (IOP). In this study, we are developing disposable eye lenses including a specific micro fabricated pressure sensor to measure IOP all day long. The information data will be wireless transferred via magnetic coupling to an external receiver. Our first work deals with the sensor design and fabrication. Simulation results based on classical electronic circuit tools will be presented and lead to several sensor solutions working at different RF frequencies. The fabrication process of the first sensors will also be described and an early IOP characterisation set-up will be presented to try to later quantify the sensor sensitivity, with IOP variations measured in the range 20 to 70mmHg.

Micromachined piezoelectric resonator at MHz application

This work investigates the Thin Film Bulk Acoustic Resonator operating at low frequencies. This study aims to substitute quartz resonators in the 4-27 MHz band and to fabricate selective filter for frequencies lower than 1GHz with quality factor higher than 10000. In this paper, we present the design, fabrication and testing of two different types of resonators. It consists of aluminum nitride film (0.8 μm) sandwiched between two aluminum electrodes (0.2 μm each). The first resonator is made by clamped edge beam and the second one is a free-free beam construction anchored in the middle of the cantilever. A demonstrator was achieved and the resonators are manufactured on a silicon substrate; AlN and Al layers were deposited on silicon using standard cathode sputtering technique. The resonators operate in extensional mode and the thicknesses of each of the materials are lower than 1μm. ANSYS, a Finite Element Analysis, has been performed to simulate the static, modal and harmonic behaviour. The simulation has been used, on the one hand, to determine the thickness of each material so as to reach the desired frequency range, on the other hand, to compare theoretical and experimental frequency values. First resonant frequencies between 2 and 10MHz were measured for resonators with dimensions of 20-40μm wide and 200-1000μm long and were found close to theory. Quality factor under 10000 operating in air has been achieved. These results confirm that such an integrated solution will replace Quartz oscillators and/or Surface Acoustic Wave filters in very compact applications.

Detection of out-of-plane and in-plane (XYZ) motions of piezoelectric microcantilever by 3D-Laser Doppler Vibrometry

This paper investigates the vibration analysis of microcantilever in the hundred kHz to MHz range using 3D Vibrometry experimental tool. These microdevices have been actuated by a piezoelectric Aluminum Nitride (AIN) 1.5 μm-thin layer. Analytical and 3D finite element method (3D-FEM) analysis using Comsol software has been performed to model vibration behavior. Typical numerical simulations and experimental results are presented and discussed. We particularly emphasize on the high detection in-plane and out-of plane motions of the microcantilevers actuated piezoelectrically in air environment. The bending, torsion and lateral modes have been successfully demonstrated. The purpose of this article is to review these latest developments tools in the microcantilever dynamic measurement and present an outlook of the future of these characterization in the next generation microsystems.

Mechanical behavior of T-shaped A1N membrane based on thin film elongation acoustic resonator

Microelectromechanical systems (MEMS) move rapidly towards commercialization, the issue of mechanical characterization has emerged as a major consideration in device design and fabrication. This paper investigates the vibration analysis of T shaped membrane which is fully adapted for oscillator circuit miniaturization. This shape has been actuated by a piezoelectric Aluminum Nitride (AlN) 1 μm-thin layer sandwiched between two aluminum electrodes and fabricated using micromachining technology. Analytical and 3D finite element method (3D-FEM) analysis using Comsol software have been performed to model the modal and mechanical vibration behavior, and to estimate the material properties which minimize the difference between the measured and simulated profiles. Typical numerical simulations and experimental results are presented and discussed. The mechanical characterization uses Laser Doppler Vibrometry (LDV) system which is quick, non-destructive, non-contact, with a high accurate detection of resonance frequency. This work main contribution is mechanical characterization and comparison with its model. The performances of the TFEAR presents a Q factor up to 1200 and a motional resistance close to 400 Ω at 12.78 MHz.