Elisabeth Dufour-Gergam

Permalloy electroplating through photoresist molds

Abstract Electrodeposited Ni 80 Fe 20 magnetic films are used in storage devices and are usually used for MEMS. In this paper, we present some fundamental results concerning the influence of electrodeposition conditions on the films characteristics. In the first step, permalloy electrodeposition has been studied on copper evaporated films without resist patterns. The growth rate varies linearly as function of the current density (typically 120 nm min −1 for 10 mA cm −2 ). The variation of the alloy composition with deposition parameters is consistent with the so-called anomalous deposition effect, i.e., a preferential deposition of iron. The composition is the same for films with thickness in the range 0.6–20 μm and the Fe concentration decreases with current density. An alloy Ni 80 Fe 20 with a density close to the bulk value is obtained for a current density of 14.5 mA cm −2 . Characterization of magnetic properties with a vibrating magnetometer shows that good properties can be obtained (coercivity=0.35 Oe for a thickness of 600 nm). These results are applied to the patterning of permalloy by electrodeposition through photoresist molds.

An active piezoelectric energy extraction method for pressure energy harvesting

This paper presents an energy harvesting technique to power autonomous systems and more particularly active implantable medical devices. We employ a piezoelectric diaphragm placed in a fluidic environment such as blood subjected to very low frequency (2 Hz) pressure variations that is deflected in a quasi-static manner and transduces mechanical energy into electrical energy. In order to maximize energy generation and to get the most out of a given piezoelectric device, we propose to apply an optimized method to extract the piezoelectrically generated charge through the application of a controlled voltage. We believe that this method could be one of the improvement levers to achieve self-powered miniaturized implants. An analytical model is presented and shows that within its validity domain, the extracted energy is proportional to the desired applied voltage. Taking power electronics losses into account can yield a theoretical increase in the extracted energy of several thousand per cent. Experimental measurements in a pressure chamber have been carried out whose results corroborate the proposed model. For the tested setup, the application of a 10 V peak amplitude square-wave voltage increased the extracted energy by a factor of nine compared to a classical rectifier-based energy harvesting method.

Towards high fidelity high efficiency MEMS microspeakers

This paper presents simulations and microfabrication of different parts of high fidelity electrodynamic MEMS loudspeakers with high electroacoustic conversion efficiency. The originality of the structure lies on the use of rigid silicon membranes suspended by a whole set of silicon beams instead of flexible polymer membranes usually found in MEMS loudspeakers. The microspeaker structure includes a planar copper microcoil electroplated on the silicon membrane and permanent magnets bonded on the substrate. Microstructure of the silicon membrane was optimized using FEM simulations for providing both rigidity and lightness of the mobile part. The results presented on a deep RIE etched 15 mm diameter silicon membrane structured with 40 stiffening ribs and on a 30 µm thick microcoil with 35 turns experimentally show the feasibility of key stages required for manufacturing of MEMS microspeakers with outstanding properties.

Effects of direct and pulse current on copper electrodeposition through photoresist molds

In this paper, electrodeposition of copper films and copper lines with direct and pulse currents is compared. In the first step, electrical and physicochemical characterizations of the copper films are realized, and the optimized electrodeposition parameters are so determined. The use of pulse reverse current is also investigated. In the second step, these parameters are applied to micromolding. Copper lines are electro-deposited and compared in order to determine the more suitable current mode for micromolding.

Fabrication of free-standing porous silicon microstructures

The high specific surface of porous silicon and its high reactivity makes this material a good candidate for chemical sensors based on electrical or electromechanical devices. In this paper, several processes are presented to realize free-standing porous silicon microstructures (membranes and cantilever beams). Good resonance quality factors were measured (Q = 110 and 760 respectively) demonstrating that porous silicon is a suitable material for resonant chemical sensors.

Implanted, inductively-coupled, radiofrequency coils fabricated on flexible polymeric material: application to in vivo rat brain MRI at 7 T.

Combined with high-field MRI scanners, small implanted coils allow for high resolution imaging with locally improved SNR, as compared to external coils. Small flexible implantable coils dedicated to in vivo MRI of the rat brain at 7 T were developed. Based on the Multi-turn Transmission Line Resonator design, they were fabricated with a Teflon substrate using copper micromolding process and a specific metal-polymer adhesion treatment. The implanted coils were made biocompatible by PolyDimethylSiloxane (PDMS) encapsulation. The use of low loss tangent material achieves low dielectric losses within the substrate and the use of the PDMS layer reduces the parasitic coupling with the surrounding media. An implanted coil was implemented in a 7 T MRI system using inductive coupling and a dedicated external pick-up coil for signal transmission. In vivo images of the rat brain acquired with in plane resolution of (150 μm)(2) thanks to the implanted coil revealed high SNR near the coil, allowing for the visualization of fine cerebral structures.

Modeling and Characterization of MicroPirani Vacuum Gauges Manufactured by a Low-Temperature Film Transfer Process

The novelty of this paper is the proof of functional microdevice fabrication using a recently developed low-temperature transfer process. The process is based on adhesion control of molded Ni microstructures on a donor wafer by using plasma-deposited fluorocarbon films. Low-temperature adhesive bonding of the microstructures on the target wafer using benzocyclobutene sealing enables mechanical tearing off from the donor wafer. Interest of this process for manufacturing microsensors is demonstrated here in the case of microbeams used as pressure sensors based on the Pirani principle. A simple analytical model is used to estimate the electrothermal behavior of the suspended microwires as a function of the ambient gas pressure. Estimations are compared to experimental measurements performed on Ni electroplated microwires of 550-1200-μm length, 10-μm width, and 0.7-7- μm thickness characterized into a vacuum chamber. These microsensors present a maximum of sensitivity in the range of 0.1-100 mbar, which is in line with standard performances of Pirani gauges. The presented results thus demonstrate the interest of a simple film transfer process for the elaboration of 3-D functional microstructures.

How medium osmolarity influences dielectrophoretically assisted on-chip electrofusion

Cells submitted to an electric field gradient experience dielectrophoresis. Such a force is useful for pairing cells prior to electrofusion. The latter event is induced by the application of electric field pulses leading to membrane fusion while cells are in physical contact. Nevertheless, the efficiency of dielectrophoretic pairing and electrofusion of cells are highly dependent on medium properties (osmolarity and conductivity). In this paper, we examine the effect of medium osmolarity on volume, viability and electrical properties of cells. Then we characterize in real time the impact of electropermeabilization of cells on their dielectrophoretic response. To do so, a microfluidic device, inducing particular field topologies is used. These real time observations are correlated to numerical analysis of the Clausius-Mossotti factor. Taking into account the identified changes, an electrofusion protocol adequate to the optimal medium (100 mOsm, 0.03 S/m) is defined. Up to 75% simultaneous binuclear rapid electrofusions were achieved and monitored with average membrane fusion duration lower than 12s.

Composition and structure of NiTiCu shape memory thin films

Preliminary results on NiTiCu shape memory alloy thin-film deposition on silicon are reported. Characteristics of these films (composition, density and electrical resistivity) were correlated with working gas pressure and rf power during sputtering. The films were then annealed in order to crystallize them. The properties of these films were examined as functions of annealing and measurement temperatures in order to evidence shape memory behaviour.

WSi Schottky diodes: effect of sputtering deposition conditions on the barrier height

Tungsten thin films were deposited on silicon substrates by DC magnetron sputtering in argon and xenon. The effects of the sputter deposition conditions were studied. X-ray diffraction was used to examine the structure and the lattice parameters while the stress was determined from the measurement of the substrate curvature after metallization by using a profilometer. The resistivity was measured by using a four-point probe. The barrier heights (Φ B ) for W/Si Schottky diodes were determined from I-V and C-V measurements. A compressive-to-tensile stress transition is observed as the working gas pressure is increased. The transition occurs at higher pressure for the lighter gas (argon) and coincides with a significant increase of the W-film resistivity. We show that the change in stress and resistivity, which is frequently observed, is only associated with the transformation of the α-W-phase into the β-W-phase for films prepared with argon. The films deposited in xenon always exhibit the α-W-structure. In addition, a change (ΔΦ B 50 meV) in the Schottky barrier height on n-type is observed at the critical pressure. On the other hand, the barrier height on the p-type remains constant under all the experimental conditions investigated. These last results indicate that the Fermi level at the interface is pinned with respect to the valence band edge.