Corina Birleanu

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...

The influence of titanium dioxide on the tribological characteristics of a Fe-based friction composite material:

New Fe-Cu-graphite-Ni-TiO2 (2 to 8 wt% TiO2) composite materials are studied for friction applications. The tribological characteristics of these materials were monitored by a pin-on-disc method against a cast iron plate. An increase of the friction coefficient when the titanium dioxide content was increased up to 6 wt% was noticed. Instead, adding 8 wt% of titanium dioxide determined the decrease of friction coefficient. A gradual decrease of the wear rate was marked out when titanium dioxide content was increased. The best behavior determined by the optimal ratio friction coefficient/wear rate was the Fe-Cu-graphite-Ni-TiO2 composite containing 6 wt% titanium dioxide.

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.

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.

Design, simulation and testing of polymeric microgrippers with V-shaped electrothermal actuators and encapsulated heaters

This paper presents the design, simulation, fabrication, and characterisation of polymeric microgrippers based on V-shaped electrothermal actuators for bio-micromanipulation and microassembly applications. The microgrippers were simulated using FEM in order to determine the electro-thermo-mechanical behavior. The microgrippers were fabricated using the SU-8 biocompatible polymer as structural material. The metallic microheaters were encapsulated in the polymeric structural layers of the microgrippers to improve thermal efficiency and to reduce the undesirable out-of-plane displacement of the gripper. Electro-mechanical testing and characterisation have been performed to determine the openings of the microgripper tips as function of electrical current. For the evaluation of the microgripper displacement, a measurement based on an optical image approach was used. A displacement of 40 μm can be obtained for an electrical current of around 25 mA. Over 26 mA the heaters are still working but a softening and a damaging status in the polymer were observed. A comparison between the simulation results and the displacement measurements is presented.

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 behavior of smart MEMS in industrial applications

Abstract: Many of the microelectromechanical systems (MEMS) industrial applications require vibrating components that operate to a high quality factor and small energy dissipation during oscillations. To improve the reliability design of MEMS resonators, the effect of operating conditions on the dynamical response of vibrating components has to be accurately determined. As a function of the operating conditions, the dynamical response and the loss of energy in vibrating MEMS components are influenced by the damping of the surrounding medium and depend on the intrinsic effects of mechanical structure. In order differentiate between the extrinsic damping and the intrinsic effects, experiments have to be performed both in ambient conditions and in a vacuum. In this chapter, some analytical models accompanied by experimental tests are presented to estimate the dynamical response and the loss of energy on samples fabricated from polysilicon with different geometrical configurations.