Johan Moulin

Optimization and Microfabrication of High Performance Silicon-Based MEMS Microspeaker

A novel structure of electrodynamics microelectromechanical systems (MEMS) microspeaker designed for mobile electronics is proposed in this paper. The originality of the device lies on the use of a rigid silicon membrane suspended by highly flexible silicon springs, contrary to most MEMS and non-MEMS microspeakers, which use polymer diaphragms. Important rigidity of the membrane and high linearity of the magnetic actuation conferred outstanding sound quality. The design of the silicon springs enabled large out-of-plane displacement of the membrane, which improved the bass rendering and the acoustic intensity over the whole bandwidth. The low density of silicon material helped to reduce the mobile mass and thus improved the microspeaker efficiency. A prototype with a membrane diameter of 15 mm and a thickness of 20 μm is microfabricated and characterized. The silicon springs enabled out-of-plane displacement of more than 300 μm. Acoustic intensity of 80-dB SPL is measured at 10 cm with 500-mW input power. This sound pressure level is obtained at frequencies from 330 Hz up to 70 kHz. Thanks to the membrane backside microstructure, most of the membrane proper modes are shifted out of the audible bandwidth. The measured electroacoustic efficiency is almost three times better than that of conventional microspeakers.

Micro-strip ferromagnetic resonance study of strain-induced anisotropy in amorphous FeCuNbSiB film on flexible substrate

The magnetic anisotropy of a FeCuNbSiB (Finemet®) film deposited on Kapton® has been studied by micro-strip ferromagnetic resonance technique. We have shown that the flexibility of the substrate allows a good transmission of elastic strains generated by a piezoelectric actuator. Following the resonance field angular dependence, we also demonstrate the possibility of controlling the magnetic anisotropy of the film by applying relatively small voltages to the actuator. Moreover, a suitable model taking into account the effective elastic strains measured by digital image correlation and the effective elastic coefficients measured by Brillouin light scattering, allowed to deduce the magnetostrictive coefficient. This latter was found to be positive (λu2009=u200916u2009×u200910−6) and consistent with the usually reported values for bulk amorphous FeCuNbSiB.

Combining ferromagnetic resonator and digital image correlation to study the strain induced resonance tunability in magnetoelectric heterostructures

This paper reports the development of a methodology combining microstrip ferromagnetic resonance and digital image correlation in order to simultaneously measure the voltage-induced strains and the magnetic resonance in artificial magnetoelectric heterostructures (magnetic films/piezoelectric substrate or magnetic films/flexible substrate/piezoelectric actuator). The overall principle of the technique and the related analytical modelling are described. It is powerful to estimate the magnetostriction coefficient of ferromagnetic thin films and can be used to determine the effective magnetoelectric coefficient of the whole heterostructures in addition to the piezoelectric coefficient related to the in-plane voltage-induced strains. This methodology can be applied to system for which the strains are well transmitted at the different interfaces.

Magnetoelastic effect in soft amorphous and nanocrystalline FeCuNbSiB thin films

Thin films of FeCuNbSiB have been sputtered on SiO2/Si substrates with thickness varying from 50 nm to 1.8 microns, then annealed at temperature ranging from 200 to 500 °C. The coercivity globally decreases with the film thickness down to 100 A m−1 for as-deposited samples and 10 A m−1 for annealed samples. However, it appears a local maximum for thickness close to 1 micron (i.e. the exchange length), regardless the annealing temperature. This behavior is supposed to be related to a modification of the magnetic domain structure, as the RAM applied to thin films and the dependence of the transitions points (glass state temperature Tg and crystallization temperature Tx) on the film dimension are monotonous. In addition, the films have been annealed under magnetic field and strong anisotropy energy has been reached, in the range of 150 J m−3.

Electrodynamic MEMS: Application to Mobile Phone Loudspeakers

This paper presents an electrodynamic MEMS for mobile phone loudspeaker applications. The whole structure of the loudspeaker is a new conception to reach higher performances than in existing devices: a linear behavior to ensure a high acoustic fidelity and a high efficiency to increase the power autonomy. So, the motor is ironless, constituted of permanent magnet only. Several electrodynamic structures are presented and studied with analytical formulations of the magnetic field. The emissive part is a plane silicon surface, very rigid and light, the suspension is achieved by silicon beams, which are not sensitive to mechanical fatigue, the electroplated copper coil is thick and requires a specialist technique to be deposited. The moving part displacements are in a range far larger than in existing MEMS (600 μ m). The trends for dimensioning the structure are investigated and prototypes realized and tested, with NdFeB ring magnets. As a result, the 70 dB SPL at 10 cm bandwidth reaches up to 100 kHz, and the behavior is particularly linear.

Influence of Oxygen Contamination on Magnetic Properties of Amorphous and Nanocrystallized FeCuSiNbB Thin Films

Thin films of amorphous FeCuSiNbB alloy have been deposited by RF sputtering with various deposition rates. The bulk oxygen content has been characterized using EDS and XPS. Its dependence on deposition rate shows that water vapour in the sputtering chamber is at the origin of the contamination. It allows also estimating the adsorption coefficient of the oxygen on the sample to be around 15 % at 350 K. The magnetic hardness and the resistivity increase with the contamination in oxygen. In devitrified films, this increase is also related to an enrichment of the residual amorphous matrix in oxygen.

Magnetic domain-wall motion study under an electric field in a Finemet® thin film on flexible substrate

Abstract We study the influence of applied in-plane elastic strains on the static magnetic configuration of a 530xa0nm magnetostrictive FeCuNbSiB ( Finemet ® ) thin film. The in-plane strains are induced via the application of a voltage to a piezoelectric actuator on which the film/substrate system was glued. A quantitative characterization of the voltage dependence of the induced-strain at the surface of the film was performed using a digital image correlation technique. Magnetic Force Microscopy (MFM) images at remanence (H=0xa0Oe and U=0xa0V) clearly reveal the presence of weak stripe domains. The effect of the voltage-induced strain shows the existence of a voltage threshold value for the strike configuration break. For a maximum strain of e XX ~ 0.5 × 10 − 3 we succeed in destabilizing the stripes configuration helping the setting up of a complete homogeneous magnetic pattern.

Optimization of an MEMS Magnetic Thin Film Vibrating Magnetometer

This paper presents models developed through analytical or numerical computation and finite-element analysis to improve the resolution of a new kind of vibrating magnetometers. This peculiar magnetometer uses the piezoelectric transduction to actuate a quartz resonator at its resonance frequency taking advantage of the high

A miniature vibrating cantilever magnetometer

Q

High Aspect Ratio Magnetoimpedance Sensors Fabricated by Micromolding

factor of a quartz resonator to achieve high resolution. The magnetic sensitive element is a thin ferromagnetic film of nickel–cobalt, which is sputtered on the moving beams of the resonator. This magnetic thin film applies a periodic torque on the resonator, shifting its resonance frequency. This torque depends on the magnetic field applied; therefore, the value of the magnetic field can be deduced from the frequency shift measurement. The aim of this paper is to develop and improve sensitivity models, which will be useful tools in the future work to establish the optimal geometry for a resonator and the best position of the magnetic thin film on it in order to improve the sensitivity and resolution of the global sensor.