Véronique Rochus

A Micro–Macroapproach to Predict Stiction due to Surface Contact in Microelectromechanical Systems

Stiction, which results from contact between surfaces, is a major failure mode in microelectromechanical systems (MEMS). Indeed, microscopic structures tend to adhere to each other when their surfaces come into contact and when the restoring forces are unable to overcome the interfacial forces. Since incidental contacts cannot be completely excluded and since contacts between moving parts can be part of the normal operation of some types of MEMS, stiction prediction is an important consideration when designing micro- and nanodevices. In this paper, a micro-macro multiscale approach is developed in order to predict possible stiction. At the lower scale, the unloading adhesive contact-distance curves of two interacting rough surfaces are established based on a previously presented model [L. Wu , J. Appl. Phys. 106, 113502, 2009]. In this model, dry conditions are assumed, and only the van der Waals forces as adhesion source are accounted for. The resulting unloading adhesive contact-distance curves are dependent on the material, surface properties such as elastic modulus and surface energy, and rough surface topography parameters (the standard deviation of asperity heights and the asperity density). At the higher scale, a finite element analysis is considered to determine the residual cantilever beam configuration due to the adhesive forces once contact happens. Toward this end, the adhesive contact-distance curve computed previously is integrated on the surface of the finite elements as a contact law. The effects of the design parameters can then be studied for the given material and surface properties.

Influence of adhesive rough surface contact on microswitches

Stiction is a major failure mode in microelectromechanical systems (MEMS). Undesirable stiction, which results from contact between surfaces, threatens the reliability of MEMS severely as it breaks the actuation function of MEMS switches, for example. Although it may be possible to avoid stiction by increasing restoring forces using high spring constants, it follows that the actuation voltage has also to be increased significantly, which reduces the efficiency. In our research, an electrostatic-structural analysis is performed to estimate the proper design range of the equivalent spring constant, which is the main factor of restoring force in MEMS switches. The upper limit of equivalent spring constant is evaluated based on the initial gap width, the dielectric thickness, and the expected actuation voltage. The lower limit is assessed on the value of adhesive forces between the two contacting rough surfaces. The MEMS devices studied here are assumed to work in a dry environment. In these operating conditi...

Submicron three-terminal SiGe-based electromechanical ohmic relay

This paper demonstrates functional NEM cantilever relays fabricated in a CMOS-compatible low-T (400°C) CVD SiGe process flow. Devices with a length in the micrometer range (<;3μm), a width in the range 0.2-1μm, a thickness and a gap of below 100nm were successfully fabricated and characterized. A high on/off current ratio (of better than 10⁸:1), a subthreshold swing (S) better than 150μV/decade and “essentially zero” off-state leakage current were experimentally observed. A life time of minimum 10³ switching cycles was demonstrated. A maximum current density of around 10μA/μm² without causing stiction due to Joule-heating was found.

Modeling of electromechanical coupling problem using the finite element formulation

A modeling procedure is proposed to handle the strong electro-mechanical coupling appearing in micro-electro-mechanical systems (MEMS). The finite element method is used to discretize simultaneously the electrostatic and mechanical fields. The formulation is consistently derived from variational principles based on the electro-mechanical free energy. In classical weakly coupled formulations staggered iteration is used between the electro-static and the mechanical domain. Therefore, in those approaches, linear stiffness is evaluated by finite differences and equilibrium is reached typically by relaxation techniques. The strong coupling formulation presented here allows to derive exact tangent matrices of the electro-mechanical system. Thus it allows to compute non-linear equilibrium positions using Newton-Raphson type of iterations combined with adaptive meshing in case of large displacements. Furthermore, the tangent matrix obtained in the method exposed in this paper greatly simplifies the computation of vibration modes and frequencies of the coupled system around equilibrium configurations. The non-linear variation of frequencies with respect to voltage and stiffness can then be investigated until pull-in appears. In order to illustrate the effectiveness of the proposed formulation numerical results are shown first for the reference problem of a simple flexible capacitor, then for the model of a micro-bridge.

Improvement of Pull-in Voltage of Electromechanical Microbeams Using Topology Optimization

The electrostatic actuation devices used in MEMS are generally based on capacitive systems in which one electrode is mobile and the other one is fixed. Applying voltage between the electrodes generates an electrostatic force which tends to reduce the gap between the electrodes. Due to the non-linearity of the electrostatic force in function of the distance between electrodes, there exists a limit voltage from which there is no equilibrium between the electrostatic and mechanical forces leading to the pull-in phenomenon. In some applications, the pull-in instability is undesirable and maximizing pullin voltage is searched. The pull-in behavior involves a strong coupling between mechanical and electrostatic phenomena. Therefore the computation of the pull-in voltage for a given system requires multiphysics finite element simulations [1]. In addition, to compute efficiently the pull-in conditions, the multiphysics finite elements method is combined with a Riks-Crisfield algorithm [2]. The considered design problem consists in maximizing the pull-in voltage of a microbeam. Indeed, microbeam is the simplest example of electrostatically actuated MEMS exhibiting pull-in and consequently it is suited to serve as test to develop topology optimization of similar devices. Topology optimization is formulated as the research of the optimal distribution of a fixed volume of material. To avoid important modification of the electric field by the optimization process, this first study considers a non design electrode and uses topology optimization to design an optimal suspension structure. In this way, the structural optimization domain is separated from the electrical domain. The solution procedure of the optimization problem is based on CONLIN optimizer using a sequential convex linear programming. On each step of the optimization process, the sensitivity analysis is performed with the formulation of eigenvalue topology optimization problem on the basis of the computed pull-in conditions [3]. Two applications of this new method are finally proposed.

Time Multiplexed Active Neural Probe with 1356 Parallel Recording Sites

We present a high electrode density and high channel count CMOS (complementary metal-oxide-semiconductor) active neural probe containing 1344 neuron sized recording pixels (20 µm × 20 µm) and 12 reference pixels (20 µm × 80 µm), densely packed on a 50 µm thick, 100 µm wide, and 8 mm long shank. The active electrodes or pixels consist of dedicated in-situ circuits for signal source amplification, which are directly located under each electrode. The probe supports the simultaneous recording of all 1356 electrodes with sufficient signal to noise ratio for typical neuroscience applications. For enhanced performance, further noise reduction can be achieved while using half of the electrodes (678). Both of these numbers considerably surpass the state-of-the art active neural probes in both electrode count and number of recording channels. The measured input referred noise in the action potential band is 12.4 µVrms, while using 678 electrodes, with just 3 µW power dissipation per pixel and 45 µW per read-out channel (including data transmission).

Effects of the electrode positions on the dynamical behaviour of electrostatically actuated MEMS resonators

The influence of the lower electrode positions on the dynamic response of polysilicon MEMS resonators is studied and presented in this paper. The change in the frequency response of investigated MEMS resonators as function of the lower electrode positions is measured using a vibrometer analyzer. The decrease in the amplitude and velocity of oscillations if the lower electrode is moved from the beam free-end toward to the beam anchor is experimental monitored. The measurements are performed in ambient conditions in order to characterize the forced-response Q-factor of samples. A decrease of the Q- factor if the lower electrode is moved toward to the beam anchor is experimental determined. Different responses of MEMS resonators may be obtained if the position of the lower electrode is modified. Indeed the resonator stiffness, velocity and amplitude of oscillations are changed.

Poly-SiGe-based MEMS Xylophone Bar Magnetometer

This paper presents the design, fabrication and preliminary characterization of highly sensitive MEMS-based Xylophone Bar Magnetometers (XBMs) realized in imecs poly-SiGe MEMS technology. Key for our Lorentz force driven capacitively sensed resonant sensor are the combination of reasonably high Q-factor and conductivity of imecs poly-SiGe, our optimized multiphysics sensor design targeting the maximization of the Q-factor in a wide temperature range as well as our proprietary monolithic above-CMOS integration and packaging schemes. Prototypes 3-axis devices were fabricated and characterized. We present optical vibrometer and electrical S-parameter measurements of XBMs performed in vacuum with a reference magnet at increasing sensor separation. The optical oscillation amplitude is well correlated with the magnetic field amplitude. The electrical 2-port measurements, 1st port as Lorentz force actuator and 2nd port as capacitive sensor, also reproduces the designed magnetic field dependence. This opens the way towards the on-chip integration of small footprint extremely sensitive magnetometers.

Coupled electro-mechanics simulation methodology of the dynamic pull-in in micro-systems

The aim of this paper is to deal with multi-physics simulation of micro-electro-mechanical systems (MEMS) based on an advanced numerical methodology. MEMS are very small devices in which electric as well as mechanical and fluid phenomena appear and interact. Because of their microscopic scale, strong coupling effects arise between the different physical fields, and some forces, which were negligible at macroscopic scale, have to be taken into account. In order to accurately design such micro-electro-mechanical systems, it is of primary importance to be able to handle the strong coupling between the electric and the mechanical fields. In this paper, the finite element method (FEM) is used to model the electro-mechanical interactions and to perform static and transient analyses. The application example considered here is a micro-bridge consisting in a clamped-clamped beam suspended over a substrate (the lower electrode). When a voltage is applied between the beam and the substrate, electrostatic forces appear which force the beam to bend. When the applied voltage increases, the electrostatic force becomes dominant and the plates stick together. The corresponding critical voltage is called the pull-in voltage. When the dynamic behavior of the system is taken into account, it is shown that two new parameters have to be defined: the dynamic pull-in displacement and the dynamic pull-in time.

Effects of the geometrical dimensions on stress and strain of electrostatically actuated MEMS resonators at pull-in and stiction positions

The aims of this study are to estimate the effect of geometrical dimensions on stiffness, stress and strain of flexible MEMS structures at pull-in and stiction positions. Microcantilevers and microbridges are often used as flexible mechanical elements in microsystems for sensing and actuation functions. Analytical models for stiffness, stress and strain of flexible microcomponents as microcantilevers are developed and presented in this paper. Experimental investigations are performed on samples fabricated from gold with different geometrical dimensions using Atomic Force Microscope. The distribution of stress in deformable MEMS components is changed as function of the contact areas between the flexible plate and substrate.