Eniko T. Enikov

A bulk microfabricated multi-axis capacitive cellular force sensor using transverse comb drives

This paper presents design, fabrication and calibration results for a novel 2-DOF capacitive force sensor capable of resolving forces up to 490 µN with a resolution of 0.01 µN in x, and up to 900 µN with a resolution of 0.24 µN in y. A simple fabrication process using deep reactive ion etching (DRIE) on silicon-on-insulator (SOI) wafers forms the 3D high aspect ratio structure. A transverse mode comb drive movement is used to greatly improve device sensitivity. Among other advantages of the developed process is a dice-free release of wafer structures, allowing fragile structures to be individually packaged. Notching or footing effects and bowing effects are well-known problems in DRIE on SOI wafers. Techniques to overcome notching and bowing effects using a PlasmaTherm SLR-770 etcher are presented that do not require hardware modifications. The application of the force sensor is for providing real-time force feedback during individual cell manipulation tasks.

Analytical model for analysis and design of V-shaped thermal microactuators

An analytical solution of the thermoelastic bending/buckling problem of thermal microactuators is presented. V-shaped beam actuators are modeled using the theory of beam-column buckling. Axial (longitudinal) deformations including first-order nonlinear strain-displacement relations and thermal strains are included. The resulting nonlinear transcendental equations for the reaction forces are solved numerically and the solutions are compared with a nonlinear finite element (FE) model. A test actuator has also been fabricated and characterized. The obtained accuracy of the prediction is within 1.1% of the nonlinear FE solution and agrees well with the experimental data. A corresponding one-dimensional (1-D) heat transfer model has also been developed and validated against experimental i-V measurements at various temperatures. The developed analytical models are then used to analyze maximum stress and the heat transfer paths. It has been confirmed that the heat flux toward the substrate is a dominant heat dissipation route in sacrificially released devices.

Microchaotic Motion of Digitally Controlled Machines

Without control, the desired motions of machines do not occur, and the desired equilibria and stationary motions are often unstable. Human operator or computer control may be needed to control and stabilize these machines. An important common feature of both analog and digital controllers is the time delay that is introduced into the system. Even when these delayed systems should be stable, the experiments show small stochastic oscillations around the desired motion, as are often experienced in robotics. In case of the stabilization of an inverted pendulum, the analysis of the equation of motion shows that chaotic vibrations occur around the equilibrium even when stochastic effects related to human control are not present. In advanced design work of digitally controlled machines, it is vital to know the characteristics of this chaotic behavior. The estimation of the distribution of vibration amplitudes and the frequency range should be available at the design stage. This initiates the analysis of the so-called microchaos or μ-chaos.

Three-dimensional microfabrication for a multi-degree-of-freedom capacitive force sensor using fibre-chip coupling

The design and fabrication of a novel multi-degree-of-freedom force sensor is described. The three-dimensional structure of the sensor is a result of combining several microfabrication techniques: wet bulk micromachining, fusion bonding, chemical mechanical polishing, deep RIE, LPCVD, PECVD and thermally evaporated thin films. The sensor is designed to operate in the 0-500 µN force range and the 0-10 µNm torque range. The flexibility of the process to create overhanging structures with arbitrary lengths and heights is illustrated by the integration of micro-tweezers directly onto the force sensor. Among other advantages of the developed process is a dicing-free self-release of wafer structures. This allows very fragile structures, such as micromirrors and other optical components, to be individually packaged.

PCB-integrated metallic thermal micro-actuators

The development of thermal micro-actuators on printed circuit boards is described. The fabricated metal actuators are shown to have similar displacement characteristics when compared with silicon-based devices described in the literature. The actuators are benchmarked with respect to power consumption, stroke, and response time. It is further demonstrated that simple analytical estimates for the response time are in good agreement with the experimental measurements and finite element analysis. The thermal cooling transient times are captured using a two-step constant-current excitation method. The fabrication process and potential application areas of the developed device are also provided.

Microassembly experiments with transparent electrostatic gripper under optical and vision-based control

This paper describes the assembly experiments conducted with a novel miniature assembly cell for microelectromechanical systems. The cell utilizes a novel transparent electrostatic gripper and uses several disparate sensing modalities for position control: computer vision for part alignment with respect to the gripper, a fiber-coupled laser, and a position-sensitive detector for part to assembly alignment. The assembly experiments performed indicate that the gripping force and stage positioning accuracy of the gripper are sufficient for insertion of micromachined parts into slots etched in silicon substrates. Details of the cell operation, the control algorithm used, and their limitations are also provided. Potential applications of the developed assembly cell are assembly of miniature optical systems, integration of optoelectronics, such as laser diodes with CMOS, and epitaxial lift-off of thin films used in optoelectronic devices.

Optically transparent gripper for microassembly

Production of complex Micro-Opto Electro-Mechanical Systems (MOEMS) often requires assembly of a system from individual components built by mutually incompatible processes. This fabrication step also constitutes the largest portion of the total cost (about 80%), and is one of the major roadblocks to successfully implementing a complex microsystem. Our previous experience with such systems shows, that gripping and manipulation of microparts significantly differs from the assembly of macroscopic devices. The main difference stems from the increased role of the surface electrostatic forces and the reduced influence of body forces such as gravity. This paper describes one possible use of the surface forces in the development of a novel optically transparent electrostatic microgripper. The principle of operation, design and simulation of the new device are described. Several models describing the gripping force are also presented. The out-of-plane and in-plane holding (frictional) forces are measured as a function of the applied voltage for two common materials - silicon and nickel. The fabrication sequence and the materials used are discussed.

Charge writing in silicon?silicon dioxide for nano-assembly

Interest in using electrostatics for active nano-assembly has grown significantly over the last five years. One common electret structure for such electrostatic constructs is the silicon–silicon dioxide interface. In this paper, an experimental and mathematical analysis of the process of writing negative charge spots in Si–SiO2 is presented. It is demonstrated that controlling the spread of the charge can reduce the spot size and the drop in written potential. Simulation results of a one-dimensional charging model that assumes tunnelling of electrons through the oxide and trapping within SiO2 are presented and compared with the experimental data. The model assumes charge trapping at the Si–SiO2 interface and none at the oxide–air interface or within the oxide bulk. Conducted experiments also show that although the lateral spread of charge places a lower limit on the minimum spot size in silicon–silicon dioxide structures, the use of a hydrophobic hexamethyldisilazane layer can be effective in improving the size stability of the written electrical spots.

Microassembly of hybrid magnetic MEMS

A general assembly strategy for assembling hybrid magnetic MEMS devices is proposed. The scaling of MEMS devices leads to the dominance of surface-effect forces such as electrostatic, surface-tension and van der Waals forces. The contact phase of an assembly task is complicated by the presence of these surface-effect forces and magnetic forces. Assembly strategies must account for the presence of these forces in order to guarantee successful repeatable assemblies. A detailed model for the magnetic interaction of microparts is developed and experimentally verified. This model is used to synthesize assembly strategies for micromagnetic parts. A flexible automated assembly workcell has also been developed to validate and demonstrate the proposed microassembly strategies.

Microsystems Mechanical Design

Preface.- Enikov, E. T., Introduction to Micro-Systems and to the Techniques for Their Fabrication.- De Bona, F., Microstructures Under Electrostatic Loads: Discrete System Modelling.- Brusa, E., Dynamics of Mechatronic Systems at Microscale.- De Bona, F., Munteanu, M. Gh., Continuum Microstructures Loaded Electrostatically.- Lazarov, K. V., Enikov, E. T., Design of Electro-Thermal micro-Positioners: Mechanics and Electronic Position Detection.- De Bona, F., Zelenika, S., Design of Compliant Micromechanisms.- Enikov, E. T., Micro- and Nano-assemby and Manipulation Techniques for MEMS.- Lee, L. M., Cheung, L. S. L., Zohar, Y., Microfluidics: Device Science and Technology.