Marius Pustan

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.

Nanomechanical studies of MEMS structures

Abstract The stiffness is a fundamental qualifier of elastically deformable mechanical microcomponents and micromechanisms whose static, modal or dynamic responses need to be evaluated. This paper presents the results of theoretical studies of microcantilevers and microbridges and also a method for experimental stiffness testing using atomic force microscopy. Castiglianos displacement theorem is used herein to derive the stiffness equations. The results of the theoretical and experimental studies are compared and discussed.

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.

Deflection Determination of V-Beam Thermal Sensors Using Digital Image Correlation

The purpose of this paper is to present some results and benefits for using the 2D Digital Image Correlation (DIC) method on the deflection determination of the V-beam thermal sensor. Therefore, the influence of the main geometrical parameters of the V-beam type thermal sensors on its deflection is evaluated. An experimental set-up was designed is order to acquire high resolution images of the deformed sensor at different temperatures. The analytical and the finite element (FE) studies that were performed on theoretical models of the V-beam sensor led to the validation of the experimental results and thus to the validation of the method.

Nanomechanical studies and materials characterization of metal/polymer bilayer MEMS cantilevers

Abstract Bilayer microelectromechanical components such as microcantilevers, microbridges or micromembranes are usually used in microtransduction for actuation and sensing. One layer achieves the structural and elastic recovery function and the other layer acts as the active part by deforming under actuations. This paper describes the studies of mechanical characteristics of flexible bilayer microcantilevers fabricated from the polymer SU8 with a reflective nano-metallic layer on the top. The mechanical characteristics investigated are stiffness, modulus of elasticity, resonant frequency, bending strain and stress. The analytical relationships for bending stiffness of bilayer microcantilevers are determined by using Castiglianos second theorem. The first bending resonant frequency is computed based on the lumped-parameter model. Experimental tests of mechanical characteristics and materials characterization are developed using atomic force microscopy and nanoidentation. Finite element analysis is used to determine the maximum stress in the sample layers and their mechanical responses.

Experimental investigation by atomic force microscopy on mechanical and tribological properties of thin films

Abstract For this paper, a two part approach was taken to develop a fundamental understanding of the surface properties of four different hard thin films. On one front, atomic force microscopy was used to quantitatively measure both the adhesion and friction forces between the tip and sample surfaces. On the other front, the indentation technique was used to determine the mechanical properties of these materials (Youngs modulus and hardness). The main purpose of this study was to investigate different thin films deposited on silicon wafer substrate for improving the wear life and reducing the coefficient of friction. Nanomechanical and nanotribological characterization of thin films of chromium, nickel, platinum and titanium deposited on silicon were performed.

Design and fabrication of a MEMS chevron-type thermal actuator

This paper presents the design and fabrication of a MEMS chevron-type thermal actuator. The device was designed for fabrication in the standard MEMS technology, where the topography of the upper layers depends on the patterns of structural and sacrificial layers underneath. The proposed actuator presents some advantages over usual thermal vertical chevron actuators by means of low operating voltages, high output force and linear movement without deformation of the shaft. The device simulations were done using COVENTOR software. The movement obtained by simulation was 12 μm, for a voltage of 0.2 V and the current intensity of 257 mA. The design optimizes the in-plane displacement by fixed anchors and beam inclination angle. Heating is provided by Joule dissipation. The material used for manufacture of chevron-based actuator was aluminum due to its thermal and mechanical properties. The release of the movable part was performed using isotropic dry etching by Reactive Ion Etching (RIE). A first inspection wa...

Design, fabrication and characterization of RF MEMS switches with robust contact

This paper presents the fabrication and characterization of a mechanical microswitch structures for the out-of-the plane displacements. The mobile electrode is connected to the anchors through several rectangular hinges fabricated in different geometrical dimensions. The mechanical response of investigated switch structures depends on the number of hinges and their geometrical dimensions. The geometrical dimensions of hinges have a strong influence on the stiffness of mobile electrode. The scope of research work is to analyze the mechanical behavior microswitch structures in order to improve the accuracy in response and to increase the microdevice lifetime. The mechanical switch structures analyzed in this paper are fabricated by electron beam evaporation from aluminum, due to its adequate mechanical and electrical properties. Experimental tests are performed using an atomic force microscope. A mechanical force given by the bending deflection of an atomic force microscope probe and its stiffness is applied in the mid-position of the mobile electrode. The mobile electrode is deflected toward the substrate. The obtained experimental curve provides information about the stiffness of investigated structures. As the number of hinges increases, the stiffness of structures is increased. The stiffness has influence on the restoring force of mobile electrode from substrate after the acting signal is removed. The out-of-the plane switches can be monolithically integrated in radio frequency devices.

V-Beam Thermal Actuator’s Performance Analysis Using Digital Image Correlation

This paper analyses the influence of the V-beam thermal actuator’s geometrical parameters on its mechanical behavior. Experimental measurements were performed using Digital Image Correlation (DIC), which is a gray scale value tracking algorithm that evaluates the position shift of a pixel between at least two images taken at different deformation stages. These results are compared to multi-physics simulation data obtained using Finite Element Analysis (FEA) as well as to analytical values based on the electro-thermal and thermo-mechanical presented models.

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.