Rodica Voicu

Testing and characterisation of electrothermal microgrippers with embedded microheaters

This paper presents our work on design, fabrication and characterisation of electrothermal microgrippers for micromanipulation and microassembly applications. The SU-8 based microgrippers were designed with embedded microheaters in the actuation structures of the grippers to improve thermal efficiency and to reduce the undesirable out of plane movement of the gripper tips. Electrothermal testing and characterisation have been conducted to determine the displacement between the gripper tips under an applied voltage. A measurement method based on an image tracking approach was used to measure gripper displacement. It has been shown that a displacement of 11 μm can be obtained at the actuation voltage of 0.65 V. The microheaters were also used as the sensors to determine the heater temperature by measurement of the resistance change at the applied voltage. The temperature coefficient of resistance (TCR) was determined independently and then used to calculate the heater temperature based on the measured resistance change. The values of TCR of the thin film chromium/gold microheaters were determined to be 0.00135/°C and 0.00147/°C which are significantly less than the value of 0.0034/°C for bulk gold material. Thus it is important to determine the TCR of the thin film microheaters for accurate calculation of the heater temperature at an applied actuation voltage.

Design optimization for an electro-thermally actuated polymeric microgripper

Thermal micro-actuators are a promising solution to the need for large-displacement, gentle handling force, low-power MEMS actuators. Potential applications of these devices are micro-relays, assembling and miniature medical instrumentation. In this paper the development of thermal microactuators based on SU-8 polymer is described The paper presents the development of a new microgripper which can realize a movement of the gripping arms with possibility for positioning and manipulating of the gripped object. Two models of polymeric microgripper electro-thermo-mechanical actuated, using low actuation voltages, designed for SU-8 polymer fabrication were presented. The electro-thermal microgrippers were designed and optimized using finite element simulations. Electro-thermo-mechanical simulations based on finite element method were performed for each of the model in order to compare the results. Preliminary experimental tests were carried out.

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.

A probabilistic model for predicting the uncertainties of the humid stiction phenomenon on hard materials

Stiction is a major failure in microelectromechanical system (MEMS) devices in which two contacting surfaces can remain stuck together because of the adhesive forces. Due to the difference between the surfaces roughness and the adhesive force range, the real contact areas are usually smaller than the apparent one, resulting in a scatter in the adhesive forces. Consequently, the stiction is an uncertain phenomenon. In this work, we develop a probabilistic model to predict the uncertainties of stiction due to the capillary forces acting on stiff materials. This model contains two levels: at the deterministic level, the model can predict the pull-out adhesive contact forces for a given surface topology; at the probabilistic level, the model generates independent identically distributed surfaces on which the deterministic solution can be applied to evaluate the uncertainties related to the stiction phenomenon. An alternative to the statistical method is proposed to evaluate the adhesive forces.Numerical surfaces are generated from a psd function defined from AFM measurements.An asperity approach is applied on these surfaces to compute the adhesive forces.The method is able to capture uncertainties and size effects in the adhesive forces.The method can be used to predict the stiction in MEMS devices.

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 and analysis of polysilicon thin layers and MEMS vibrating structures

Natural frequencies of an array of microcantilevers designed with different dimensions were analysed using computer simulations. Four different types of polysilicon were obtained in different processing conditions varying the deposition temperatures (580°C, 610°C, 630°C, 650°C). A topography scan, a comparative friction test and adhesive tests (pull off-force measurement) were carried out using Atomic Force Microscope (AFM) in order to characterize the material properties. Mechanical properties such as hardness and Youngs modulus have been investigated using the nanoindenter technique. The microcantilevers have been manufactured using polysilicon as structural material and surface micromachining technique.

Investigations of surface properties of SiO 2 and Si 3 N 4 thin layers, used for MEMS vibrating structures applications

To understand the surface topography of MEMS vibrating structures, different effects as contact phenomena, stiction, friction, adhesion, etc, on the lifetime and performance of these devices, require special consideration. We obtained results regarding the topographic analyses of 3 different experimental samples, using the same design, but different materials for manufacturing, specific to MEMS technology. Three different test methods were performed on three separate types of samples. The samples were fabricated as following: Silicon, Silicon Oxide layer on Silicon and Silicon Nitride layer on a Silicon Oxide layer on Silicon. In order to measure their properties a topography scan, a comparative friction test and a pull off-force measurement were carried out using an Atomic Force Microscope (AFM).

Analysis of Displacements of a Polymeric Microgripper

A polymeric, electro-thermally actuated microgripper has been designed. Preliminary experimental tests were carried out. Analyses of different displacements of the microgripper were performed. We present a study of different geometries of the metallic layers of the microgripper structure in order to optimise the model structure, by reducing the displacements (vertical and horizontal) which can make the structure instable or difficult to operate. The analyses of the displacements were studied using finite-element modelling and performing electro-thermo-mechanical simulations.

Nonlinear numerical analysis and experimental testing for an electrothermal SU-8 microgripper with reduced out-of-plane displacement

This paper reports the results of numerical nonlinear electro-thermo-mechanical analysis and experimental testing of a polymeric microgripper designed using electrothermal actuators. The simulation work was carried out using a finite element method (FEM) and a commercial software (Coventorware 2014). The biocompatible SU-8 polymer was used as structural material for the fabrication of the microgripper. The metallic micro-heater was encapsulated in the polymeric actuation structures of the microgripper to reduce the undesirable out-of-plane displacement of the microgripper tips, and to electrically isolate the micro-heater, and to reduce the mechanical stress as well as to improve the thermal efficiency. The electrothermo-mechanical analysis of the actuator considers the nonlinear temperature-dependent properties of the SU-8 polymer and the gold thin film layers used for the micro-heater fabrication. An optical characterisation of the microgripper based on an image tracking approach shows the thermal response and the good repeatability. The average deflection is ~11 μm for an actuation current of ~17 mA. The experimentally obtained tip deflection and the heater temperature at different currents are both shown to be in good agreement with the nonlinear electro-thermo-mechanical simulation results. Finally, we demonstrate the capability of the microgripper by capture and manipulation of cotton fibres.

SH-SAW sensor on langasite for mass detection in liquid media

This paper presents experimental realization of shear horizontal surface acoustic wave (SH-SAW) sensor on Langasite (LGS) for mass detection in liquid media. The presence of mycotoxins in food or pesticides in water supplies can produce grave implications on human health. Sensitive and selective sensors capable of quickly detecting the level of contamination with fungal toxins growing on grains in storage, are needed. Shear horizontal surface acoustic wave (SH-SAW) devices provide an attractive platform for the design of biosensors that operate in liquid media. The paper reports on a SH-SAW delay line designed and fabricated on langasite (LGS), Euler angles (0°, 22°, 90°). The SH-SAW devices have been fabricated using the microfabrication method by lift-off lithography process to define the patterns for the IDT structures. Frequency response of device was monitored using Anritsu vector network analyzer (VNA). A test setup was implemented for the characterization of LGS SH-SAW-based sensors.