Rolanas Dauksevicius

Design, fabrication, and simulation of cantilever-type electrostatic micromechanical switch

A cantilever-type electrostatically actuated microelectromechanical (MEMS) switch and its fabrication technology have been developed for the first time in Lithuania, in Kaunas University of Technology. The microdevices were fabricated using nickel surface micromachining technology on substrates made of semiconductor (silicon) and insulator materials (quarts and ceramics). The microswitch consists of cantilevered nickel structure suspended over actuation and contact electrodes. The width of the cantilever contacting element is 30 μm, thickness is about 2.0 μm and length ranges from 67 to 150 μm. Implementation of microswitches as a substitute for present switching devices poses many problems. In particular lower switching speed and reduced lifetime are considered to be among the most significant ones. These characteristics are determined both by design and dynamic phenomena that are taking place during its operation. Specifically, when the microswitch closes, it bounces several times before making permanent contact. These impact interactions greatly influence microswitch durability and switching speed. With the aim of improving these parameters a comprehensive finite element model is being developed that takes into account not only electrostatic actuation and squeeze-film damping effects but also describes important dynamic phenomena - impact interactions that take place during switching. Experimental research of electrical and dynamic characteristics is also carried out with the purpose of device model validation and correction. The paper presents design and fabrication process of the developed microswitch as well as initial simulation and measurement results.

Enhanced pressure response in ZnO nanorods due to spontaneous polarization charge

We present measurements of the induced charge flow when a compressive force is applied by contacting to a ZnO nanowire (NW). The measured charge transfer from the NW is over 104 times larger than expected from the strain-induced piezoelectric response, and is comparable in magnitude to the spontaneous polarization charge, associated with an ideal ZnO NW. A model is presented that compares the total energy of an isolated NW and external capacitor with the total energy when the capacitor and NW form a closed circuit. The analysis shows that it is possible, for realistic values of surface defect creation energy, to have spontaneous polarization charge transferred from a NW to an external capacitor when a circuit is completed between them. We propose that it is possible to use spontaneous polarization charge to get a significantly enhanced response in ZnO-based NW pressure sensors.

Modeling and Simulation of Contact-Type Electrostatic Microactuator

This chapter presents 3-D FE modeling and simulation of dynamics of microcantilever operating in ambient air near fixed surface. The phenomenon of squeeze-film damping is further analyzed numerically. Frequency response and transient analyses are carried out in order to determine influence of squeeze-film damping on free and forced vibrations of the microcantilever under different ambient air and vibration excitation conditions. Subsequently numerical analysis of the 3-D microcantilever under the effect of electrostatic field is provided. Static and dynamic simulations are performed in order to study important operational characteristics. Finally, the chapter is concluded with FE modeling of the microcantilever with incorporated adhesive-repulsive contact model, which uses a “classical” linear elastic link element combined with the van der Waals force-based term that accounts for the influence of dominant intermolecular interactions in the contact zone. This model is then used in conjunction with squeeze-film damping formulation in order to predict behavior of contact bouncing under different air damping and contact conditions.

Theoretical Analysis of a Micromotor

This chapter is dedicated to analytical and numerical analysis of electrostatic micromotor with focus on key aspects of its design and control. FE modeling and simulation is reported providing results of vibration analysis of the rotor in vacuum and viscous medium. Analytical dynamic micromotor model is presented together with results of analysis of electrostatic forces between a pair of poles together with simulations of different motor configurations and investigation of torque curves. This chapter also describes the developed control methodology of pole switching sequences of MEMS motors.

Technological Realization of MEMS Structures and Their Experimental Investigation

The last chapter presents the developed surface-micromachining technology that was patented by the authors and is suitable for fabrication of various MEMS actuators and sensors. Peculiarities of the microtechnology are highlighted such as utilization of special fractal microstructures, which enhance bonding strength of the microdevice to a substrate resulting in increased reliability. The chapter also describes design issues of electrostatic microswitches that were fabricated using the developed process and may be successfully implemented into various “system-on-chip” applications. Results of electrical and dynamic testing of the fabricated microswitches are provided including measurements of actuation voltages and natural frequencies with mode shapes. Microscope-based probe station and laser Doppler vibrometry system used for experiments are described as well. This chapter also provides overview of the developed micromotor fabrication technology that is based on application of standard UV lithography and plating. Produced prototypes of the electrostatic micromotors are demonstrated including their specifications. Lastly, a device developed for the micromotor electric control is presented that is able to adjust voltage, frequency, number of phases and other parameters.

Dynamics of Elastic Vibro-Impact Microsystems

In this chapter the main ideas of the application of the finite element method for analysis of elastic microsystems are related to the theories of nonlinear vibro-impact oscillations, optimal control and design. The results obtained during simulations indicate that structural parameters of microsystem have significant influence on the dynamical process. Small variations of the parameters, which are common during MEMS fabrication, could be crucial for device performance. From another point of view, the presented herein new discovered phenomena of nonlinear mechanics are related to the possibilities to excite higher modes of vibrations of elastic links due to the local changes of cross-section, location and stiffness of contacting surfaces as well as by means of optimal configurations. This new approach helps designers to create stable and dynamically-reliable microstructures.

Finite Element Modeling and Simulation of Squeezed-Film Effects in a Vibrating MEMS Structure

This paper presents results of numerical analysis of dynamics of a microstructure under the influence of air damping, which is modeled by linearized compressible Reynolds equation. Developed finite element model accounts for air rarefaction and is valid in a wide pressure range. Simulation results indicate that the influence of air damping may result not only in a pronounced dissipative effect but also in natural frequency shift due to system stiffening caused by air compression. This phenomenon is observed under combination of particular values of system and excitation parameters

Investigation of Electrostatic Cantilever-Type Micromechanical Actuator

Electrostatic microswitch and its fabrication technology have been developed for the first time in Lithuania, at Kaunas University of Technology (KTU). The microdevices were produced by using a nickel surface micromachining process. The microswitch consists of a cantilevered nickel structure suspended over the bottom electrodes. The width of the structure is 30 μm, thickness - 2 μm and length ranges from 67 to 150 μm. Implementation of microswitches as a substitute for the present solid-state switching devices poses many problems. In particular higher actuation voltages, lower switching speed and a reduced lifetime are considered to be among the most significant ones. With the aim of improving these parameters a finite element model is currently developed that takes into account not only microscale-specific electrostatic actuation and air damping effects but also includes the bouncing phenomena. Experimental studies of electrical and dynamic characteristics were also carried out with the purpose of model validation and correction. The paper presents initial results of theoretical modal and air damping analysis as well it shows the first attempts to measure vibration modes of the cantilever structure of the microswitch using Laser Doppler vibrometer.

MEMS measurement by optical holography method

Investigation of dynamics of microelectromechanical systems (MEMS) is an important problem from the point view of engineering, technology and metrology. Due to high surface to volume ratio of micro-electromechanical systems (MEMS), more attention must be paid to control their surface characteristics. Time average optical holography - has found many industrial applications and still is a promising method (like others: laser interferometry in small object displacement analysis.) This technique can reveal the shape, direction, and magnitude of the stress induced displacement in the structure under study. In this way, time average holography is a powerful tool for analysis of micro scale vibrations. The threshold of sensitivity of this measurement technique is defined by the magnitude of the wavelength of the illuminating laser beam. Also, this is a full field non-destructive technique capable to register the motion of the whole surface instead of a single point. The time average holography method is proposed to control kinetics of oscillations of the micro scale object, operating at the different amplitudes of periodical excitation. Theoretical calculations as well as experimental verification are described.