Florina Rusu

Relative humidity effect on pull-off forces in MEMS flexible structures measured by AFM

An atomic force microscope operating in contact mode is used in this paper for determining the pull-off forces as a function of relative humidity. The paper reports the measurements and the modeling of adhesion forces versus humidity in controlled ranges between 20 to 90%RH. In the low RH range (<20%) where the adsorbed water layer is insignificant, due to the absence of water meniscus, adhesion force increases slowly. However, in relative higher RH range (>20%), adhesion force increases very sharply once ‘liquid-like’ adsorbed water layer forms, because it increases the capillary force. After the relative humidity reaches about 70–80% the drop in the experimental values of the adhesive force is a result of desorption of water molecules and the corresponding decrease of water menisci contribution. To investigate the effect of relative humidity on adhesion for flexible structures such as aluminum microbridges with different lengths against silicon substrate we used an analytical method which encompasses the effect of capillarity as well as the solid-to-solid interaction. The capillary force is expressed as the sum between the Laplace force and the surface tension, while the solid-to-solid interaction is estimated using the DMT model. The analytical results are in good agreement with the experimental ones.

Investigation on the contact behaviour of MEMS micromembrane with serpentine hinges

This paper presents the study of micromembranes supported by serpentine hinges. Widely used in microelectromechanical systems as switches or optical micromirrors, these micromembranes are deflected to the substrate in order to close a circuit or to process a signal. The investigated micromembranes are electroplated from gold in different geometrical dimensions. Furthermore, the central plate of micromembrane is suspended by two or four serpentine hinges. The stiffness of micromembranes is given by the geometry of hinges. One of the failure causes of micromembranes, which are directly deflected to substrate, is the adhesion effect between the flexible plate and the substrate. The adhesive force depends on the mechanical restoring force given by the hinges stiffness. In the case of micromembranes for optical applications, one additional stress is provided by temperature. A temperature gradient applied on micromembranes changes the stiffness with influence on the adhesion force. The study of temperature effect on stiffness and adhesion force is performed using an atomic force microscope and a thermal controlled stage. Experimental results of stiffness as a function of temperature are compared to numerical data.

Effect of geometrical dimensions on the tribomechanical response of a gold micromembrane with bent beam hinges

The scope of this paper is experimental and numerical analysis of micromembranes supported by bent beam hinges fabricated from gold in different geometrical dimensions. The experimental tests are performed using an atomic force microscope in order to determine the micromembrane behaviour under a mechanical force. One of the main application of movable microcomponents is MEMS switching application where the flexible plate is directly deflected to substrate in order to close a circuit. Adhesion between the mobile plate and substrate depends on the roughness of the contact surfaces and is influenced by the micromembrane stiffness based on the restoring force. As the dimensions of hinges increases, the stiffness of micromembrane increases and the adhesion between mobile plate and substrate decreases, respectively.

Dynamic response of a polysilicon microbridge driven by different electrostatic forces

The paper presents the effect of operating parameters on the dynamic behavior of a polysilicon microbridge tested in ambient conditions and in vacuum. During experimental investigations, an AC harmonic load of an amplitude equal by 5 V is constantly applied on vibrating structure, and the DC offset signal is increased. The DC voltage has influence on the prestress position of vibrating structure with effect on the velocity and amplitude of oscillation. As the DC voltage is increased, the prestress position of microbridge is modified and the velocity of oscillation increases, respectively. The quality factor and the loss of energy during oscillation are determined for a microbridge oscillated in air and vacuum under different electrostatic forces. As the DC voltage increases, the microbridge oscillates to a new deflected position and the quality factor is changed. A significant influence of DC voltage on quality factor is determined of sample oscillated in vacuum based on internal dissipation energy. The experimental values are validated by the numerical and analytical results.

Demolding Moment Calculation for Injected Parts with Internal Trapezoidal Thread

At the final step of the injection process (of a plastic product with cavities) when the part is on the point to be ejected the adhesion phenomena occurs between the core and the plastic part. The mold adhesion effect has a significant influence on the mold design ejection system and over the whole process. This paper presents a computation methodology of the demolding moment for two cases of plastic injected parts with internal trapezoidal thread. Knowing the value of this moment offer us the possibility to adopt the right design solution of the ejector system and of the entire mold. As further work the author will try to validate this method through a set of practical experiments.

New Formulations on Motion Equations in Analytical Dynamics

Using the main authors researches on the energies of acceleration and higher order equations of motion, this paper is devoted to new formulations in analytical dynamics of mechanical multibody systems (MBS). Integral parts of these systems are the mechanical robot structures, serial, parallel or mobile on which an application will be presented in order to highlight the importance of the differential motion equations in dynamics behavior. When the components of multibody mechanical systems or in its entirety presents rapid movements or is in transitory motion, are developed higher order variations in respect to time of linear and angular accelerations. According to research of the main author, they are integrated into higher order energies and these in differential equations of motion in higher order, which will lead to variations in time of generalized forces which dominate these types of mechanical systems. The establishing of these differential equations of motion, it is based on a generalization of a principle of analytical differential mechanics, known as the D`Alembert – Lagrange Principle.

New Formulations on Acceleration Energies in Analytical Dynamics

This paper presents new formulations on the higher order motion energies that are applied in the dynamic study of multibody mechanical systems in keeping with the researches of the main author. The analysis performed in this paper highlights the importance of motion energies of higher order in the study of dynamic behavior of fast moving mechanical systems, as well as in transient phase of motion. In these situations, are developed higher order time variations of the linear and angular accelerations. As a result, in the final part of this paper is presented an application that emphasizes this essential dynamic aspect regarding the higher order acceleration energies.

Dynamic Behavior of MEMS Resonators

MEMS resonator represents currently one of the important research areas of Microelectromechanical Systems (MEMS). The usual applications of MEMS resonators are the radio-frequency electromechanical devices, MEMS gyroscopes and resonant sensors. The main part of a MEMS resonator is the mechanical vibrating structure that can be fabricated as microcantilevers, microbridges or in a more complex configuration as micromembranes. The scope of this paper is to investigate the dynamic behavior of an electrostatically actuated MEMS cantilever under different oscillating modes in order to determine the resonant frequency, amplitude and velocity of oscillations. Moreover, based on the resonant frequency experimental curves, the quality factor for different oscillating modes is determined. The effect of operating conditions on the frequency response of investigated microcantilever is monitored. As a consequence, the experimental tests are performed both in ambient conditions and in vacuum. The dynamic response of microcantilever in vacuum is influenced by the intrinsic dissipation energy and the sample behavior in air depends on the intrinsic losses as well as the extrinsic dissipation energy.