Christophe Malhaire

All-organic electrostrictive polymer composites with low driving electrical voltages for micro-fluidic pump applications

This paper focuses on the improvement of a relaxor ferroelectric terpolymer, i.e., poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)], filled with a bis(2-ethylhexyl) phthalate (DEHP). The developed material gave rise to a significantly increased longitudinal electrostrictive strain, as well as an increased mechanical energy density under a relatively low electric field. These features were attributed to the considerably enhanced dielectric permittivity and a decreased Young modulus as a result of the introduction of only small DEHP plasticizer molecules. In addition, the plasticizer-filled terpolymer only exhibited a slight decrease of the dielectric breakdown strength, which was a great advantage with respect to the traditional polymer-based electrostrictive composites. More importantly, the approach proposed herein is promising for the future development and scale-up of new high-performance electrostrictive dielectrics under low applied electrical fields through modification simply by blending with a low-cost plasticizer. An experimental demonstration based on a flexible micro-fluidic application is described at the end of this paper, confirming the attractive characteristics of the proposed materials as well as the feasibility of integrating them as micro-actuators in small-scale devices.

Experimental setup and realization of thin film specimens for microtensile tests

This paper describes a microtensile test system and the design as the realization of the samples dedicated to the tensile experiments. Two different technologies for the development of miniaturized specimens are detailed: self-standing tensile samples sustained by silicon frames and metal on polymer specimens obtained by laser cutting. The design of the samples has been optimized by means of finite element simulations. Aluminum beams with very large length on thickness ratio have been released from their silicon substrate using a standard etching process. Stress/strain curves are derived from experimental force/displacement values and discussed in terms of Youngs modulus values and critical parameters (flow and rupture stresses).

Electromechanical study of polyurethane films with carbon black nanoparticles for MEMS actuators

Pure polyurethane and nanocomposite carbon black (CB) polyurethane solutions were deposited by spin-coating on a silicon substrate using gold as the adhesion layer and electrode. Different test structures were achieved for electrical and mechanical characterizations. The incorporation of CB nanoparticles in the polyurethane matrix has a significant influence on the dielectric permittivity of the material with an increase of about one third of its value. The Youngs modulus of PU and nanocomposite PU films was determined by different characterization methods. Nanoindentation experiments have pointed out a Youngs modulus gradient through the film thickness. By performing mechanical tests (tensile, bulge, point deflection) on freestanding films, an average Youngs modulus value of about 30 MPa was found as well as a residual stress value of about 0.4 MPa. However, no influence of the presence of the nanoparticles was found. Finally, several MEMS actuators were realized and characterized. At their fundamental resonance frequency, the actuation of the nanocomposite membranes is more efficient than that of pure polyurethane. However, the time constant of the material seems to provide a major barrier for the development of high-frequency PU-based micro-actuators.

Long-Term Stability of Metal Lines, Polysilicon Gauges, and Ohmic Contacts for Harsh-Environment Pressure Sensors

This paper presents a study on the long-term stability of AlTi (with a TiW diffusion barrier) metal lines, polysilicon gauges, and metal on polysilicon contacts resistances for piezoresistive pressure sensors operating in harsh environments. Test structures have been exposed at 150 degC for a cumulated time of almost five months. Only the polysilicon resistivity proved to be stable over time, metal lines, and ohmic-contacts resistances showing irreversible drifts. Finally, the influence of such drifts over the long-term offset stability of piezoresistive pressure sensors has been discussed

Metallic nanoparticle-based strain sensors elaborated by atomic layer deposition

Platinum nanoparticle-based strain gauges are elaborated by means of atomic layer deposition on flexible polyimide substrates. Their electro-mechanical response is tested under mechanical bending in both buckling and conformational contact configurations. A maximum gauge factor of 70 is reached at a strain level of 0.5%. Although the exponential dependence of the gauge resistance on strain is attributed to the tunneling effect, it is shown that the majority of the junctions between adjacent Pt nanoparticles are in a short circuit state. Finally, we demonstrate the feasibility of an all-plastic pressure sensor integrating Pt nanoparticle-based strain gauges in a Wheatstone bridge configuration.

Comparison of two experimental methods for the mechanical characterization of thin or thick films from the study of micromachined circular diaphragms

The purpose of this study was to compare two experimental methods and evaluate the effectiveness of a set of analytical models in order to measure the initial stress and the Youngs modulus value of thin and thick film materials. Two types of experiments were performed on micromachined circular diaphragms: bulge testing and vibrometry. The range of validity and accuracy of the analytical models with respect to the vibration of the diaphragms was discussed from the finite element simulations. It was shown that the a/t ratio should be considered carefully to determine the value of the Youngs modulus by vibrometry with an acceptable error. A relative error of approximately ±10% on E was obtained for a/t ≤ 750. For 750 ≤ a/t ≤ 1000, the value of the dimensionless parameter k must also be considered. It has been shown that the residual stress value can be obtained with an accuracy of 10% or less, given that k > 12. As an illustration, experimental methods and models were applied to the characterization of a thick electroplated gold film and a sputter-deposited Inconel thin film. Circular structures were defined by vertical sidewalls etched on the back of a Si wafer using the deep reactive ion etching technique. In addition to analytical models, parametric finite element simulations and a design optimization technique were used to determine the materials mechanical properties. The static deflections of the diaphragms were measured as a function of the applied pressure. The resonant frequencies and mode shapes of the vibrating structures were observed under vacuum by white-light interferometric microscopy. For gold, it was found that E = (53 ± 20) GPa and σ(0) = (180 ± 10) MPa. For Inconel, it was found that E = (157 ± 14) GPa and σ(0) = (172 ± 5) MPa.

Realization of Thin Film Specimens for Micro Tensile Tests

This paper is focused on specimens design and fabrication for micro tensile tests. The experimental approach is based on a new micro tensile testing system and the development of silicon frames sustaining submicron thick self-standing films. The beam design has been optimized using Finite Element Simulations. SiN and Al beams with very large length on thickness ratio have been released from silicon substrate using standard etching process. The experimental force-displacement curve that has been obtained on a 3 mm x 400 mum times 1 mum aluminum beam is shown and discussed.

Reliability study of AlTi/TiW, polysilicon and ohmic contacts for piezoresistive pressure sensors applications

This paper presents a study on the thermal drift and long term stability of AlTi (with a TiW diffusion barrier) metal lines, polysilicon gauges and metal on polysilicon contact resistances for piezoresistive pressure sensors operating in harsh environments. Test structures have been exposed at 150/spl deg/C for a cumulated time of almost six months. All metal lines structures show a relative resistance decrease of around 4/spl times/10/sup -3/. On the other hand, polysilicon resistivity variations were not observable except for a few samples that showed a relative increase up to 7/spl times/10/sup -3/. Polysilicon contact resistance showed no particular trend with aging time. The impact of these results on the overall sensor reliability has also been discussed.

Piezo-tunnel effect in Al/Al2O3/Al junctions elaborated by atomic layer deposition

In this work, the electrical transport in Al/Al2O3/Al junctions under mechanical stress is investigated in the perspective to use them as strain sensors. The metal/insulator/metal junctions are elaborated with a low temperature process (≤200u2009°C) fully compatible with CMOS back-end-of-line. The conduction mechanism in the structure is found to be Fowler-Nordheim tunneling, and efforts are made to extract the relevant physical parameters. Gauge factors up to −32.5 were found in the fabricated devices under tensile stress. Finally, theoretical mechanical considerations give strong evidence that strain sensitivity in Al/Al2O3/Al structures originates not only from geometrical deformations but also from the variation of interface barrier height and/or effective electronic mass in the tunneling oxide layer.

Study of piezo-tunnel effect in Metal/A 2 O 3 /metal junctions

In this work, the electrical transport in Al/A₂O₃/Pt and Al/A₂O₃/Al junctions under mechanical stress was investigated. The junctions were fabricated by evaporation for the metals (with shadow mask lithography) and Atomic Layer Deposition (ALD) for the A₂O₃. First, IV characteristics were extracted from the unstressed sample and these curves were fitted with the Fowler-Nordheim current expression to identify the conduction mechanism. Then, gauges were tested under mechanical strain using the beam deflection method. Gauge factors of -34 for Al/A₂O₃/Al and -70 for Al/A₂O₃/Pt junctions were measured. Finally, the geometrical gauge factor is discussed and a possible effect of the variation of the Metal/A₂O₃ interfaces barrier height under stress is suggested as a plausible cause for the difference in gauge factor observed between the two junction types.