Several microactuator technologies have been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion represent the most common modes of microactuator operation. This investigation optimized the geometry of the asymmetrical electrothermal actuator to maximize its in-plane deflection characteristics. The MEMS polysilicon surface micromachined electrothermal actuator uses resistive (Joule) heating to generate differential thermal expansion and movement. In this investigation, a 3-D model of the electrothermal actuator was designed, and its geometry was optimized using the finite-element analysis (FEA) capabilities of the ANSYS computer program. The electrothermal actuator's geometry was systematically varied to establish optimum values of several critical geometrical ratios that maximize tip deflection. The value of the ratio of the length of the flexure component relative to the length of the hot arm was discovered to be the most sensitive geometrical parameter ratio that maximizes tip deflection.

The selection of actuators at the micro-scale requires an understanding of the performance limits of different actuation mechanisms governed by the optimal selection of materials. This paper presents the results of analyses for elastic bi-material actuators based on simple beam theory and lumped parameter thermal models. Comparisons are made among commonly employed actuation schemes (electro-thermal, piezoelectric and shape memory) at micro scales and promising candidate materials are identified. Polymeric films on Si subjected to electro-thermal heating are optimal candidates for high displacement, low frequency devices while ferroelectric thin films of Pb-based ceramics on Si/ DLC are optimal for high force, high frequency devices. The ability to achieve ∼10 kHz at scales < 100μm make electro-thermal actuators competitive with piezoelectric actuators considering the low work/volume obtained in piezoelectric actuation (∼ 10−8J.m−3.mV−2). Although shape memory alloy (SMA) actuators such as Ni-Ti on Si deliver larger work (∼ 1 J.m−3K−2) than electro-thermal actuators at relatively low frequencies (∼ 1 kHz), the critical scale associated with the cessation of the shape memory effect forms the bounding limit for the actuator design. The built-in compressive stress levels (∼ 1GPa) in thin films of Si and DLC could be exploited for realizing a high performance actuator by electro-thermal buckling.

Metals with one or more dimensions in the submicron regime are widely used in MEMS devices. Device stresses often exceed the strength of the corresponding bulk material by an order of magnitude and can lead to a variety of mechanical failures. At moderate temperatures, high stresses occur because dislocations are unable to move to relax the stress. This is partly because of an elastic dimensional constraint, but complex dislocation behavior, such as junction formation, annihilation, and nucleation, are also observed. In this report, we present results from analytical models, cellular automata simulations, and large-scale dislocation dynamics simulations of submicron films to examine the relationship between dislocation interactions and material strength. Our results reveal a complex relationship between dislocation interactions and stress inhomogeneity that arises from the stress fields of the dislocations. We show that the stress inhomogeneity increases both the likelihood of interactions and acts to increase the strain hardening rate.

The compressive magnetostriction values of Fe2.4Sm alloy thin film prepared on silicon, copper and titanium substrates (300μm thickness) by direct current magnetron sputtering process were investigated. When the residual gas pressure before argon sputtering and the sputtering pressure of argon gas (5 N) were below 3.2×10−4 Pa and 2.0×10−1 Pa, respectively, the thickness of the Fe2.4Sm films deposited was about 3 μm. Compressive magnetostriction of Fe2.4Sm alloy film on titanium sheet generates the large bending motion, rather than those on silicon wafer and copper substrates. The high magnetostrictive susceptibility of the films was observed at low magnetic field.

Thermoelastic damping (TED) represents the lower limit of material damping in flexural mode micro- and nanoresonators. Current predictive models of TED calculate damping due to thermoelastic temperature gradients along the beam thickness only. In this work, we develop a two dimensional (2D) model by considering temperature gradients along the thickness and the length of the beam. The Green's function approach is shown to be a robust means of solving the coupled heat conduction equation in one and two dimensions. In the 1D model, curvature information is lost and, hence, the effects of structural boundary conditions and mode shapes on TED are not captured. In contrast, the 2D model retains curvature information in the expression for TED and can account for beam end conditions and higher order modes. The differences between the 1D and 2D models are systematically explored over a range of beam aspect ratios, frequencies, boundary conditions, and flexural mode shapes.

In microelectromechanical systems (MEMS) device design and fabrication there exists a need for quick determination of fracture strength. This report describes several passive devices that use the residual stresses contained within structural MEMS materials to determine fracture strength. Stress concentrations of varying degrees are generated at micromachined notch roots, and the critical stress required for failure indicates the fracture strength. A variety of devices have been fabricated from materials, such as polysilicon, silicon nitride, and aluminum, with widely varying residual stresses, including both tensile and compressive.

We propose embedded microsensors to investigate the mechanical stress in copper damascene lines in a standard CMOS microelectronic technology. Those sensors are based on silicon piezoresistive effect where strain in the active silicon is induced by orientated copper lines. The challenge is to correlate the electrical sensors signal directly to stress variation in lines.

We have performed electrical measurements of the structures as a function of temperature. A coupled analytical and Finite Element thermomechanical Model of the structure was developed and a good agreement with measurements was obtained.

A novel micromolding approach was developed to process liquid biopolymers with high aqueous solvent contents (>90% water). Specifically silk fibroin was cast into a well-defined scaffold-like structure for potential tissue engineering applications. A method was developed to pattern the hydrophilicity and hydrophobicity of the polydimethylsiloxane (PDMS) mold surfaces. The water based biopolymer solution could then be directly applied to the desired regions on the cast surface. The variations in degree of hydrophilicity and hydrophobicity on the PDMS surfaces were quantified through contact angle measurements and compared to the outcome of the molded silk structures. Through this method free-standing structures (vs. relief surface-patterning) could be fabricated.

Bottom – up techniques were used to obtain gas sensors based on individual SnO2 nanowires placed over microhotplates with integrated heaters. These nanowires were electrically contacted to pre-patterned microelectrodes by means of Focused Ion Beam (FIB) nanofabrication methodologies. The performance of these sensors, which exhibit reproducible and stable responses, was evaluated as function of different gas atmospheres and dissipated power by the heater, demonstrating that this technological approach could be used to develop functional devices based on nanomaterials.

We present a study of the influence on Young's modulus, stress and electrical conductivity of poly-dimethylsiloxane membranes implanted with titanium ions at energies from 5 to 35 keV, with doses up to 8×1016 ions/cm2. The motivation for this study was to find the optimum implantation conditions to create electrodes for microfabricated dielectric electroactive polymer actuators, which must combine low resistivity with low stiffness. Two implantation techniques are used, Filtered Cathodic Vacuum Arc (FCVA) and the more conventional Low Energy broad beam Implanter (LEI). Of the two, it is found that the FCVA implanter is much better suited to create compliant electrodes.

Electrowetting is a promising method for manipulating small volumes of liquid on a solid surface. This complex phenomenon couples electrical and fluid properties and offers many potential surprises. The complex electrical and capillary interactions in electrowetting are illustrated by an analysis of an electrowetting configuration that produces an oscillating droplet motion from a steady DC voltage input. The paper presents an analysis of the electrowetting forces to explain the oscillation and presents a new method for measuring electrowetting forces using a Hysitron Triboindenter. Initial results are compared with predictions from numerical models and simplified analytical solutions.

The patterning of strontium-doped lead zirconium titanate (PSZT) and lanthanum-doped lead zirconium titanate (PLZT) films has been examined using excimer laser radiation. Both types of film were deposited by rf magnetron sputtering using in-situ heating and a controlled cooling rate in order to obtain the perovskite-oriented phase. The depth of laser ablation in both PSZT and PLZT films showed a logarithmic dependence on fluence. The threshold fluence required to initiate ablation was ∼1.25 mJ/cm2 for PLZT and ∼1.87 mJ/cm2 for PSZT films. The ablation rate of PLZT films was slightly higher than that of PSZT films over the range of fluence (10-150 J/cm2) and increased linearly with number of pulses. The higher ablation rate of PLZT films has been attributed to the finer grainsize (160-200 nm) than in the PSZT films (1.0-1.2 µm).

Test methods for mechanical property measurement of the thin films used in MEMS have been developed and refined over the last decade. This brief review considers only tensile testing since that is the preferred method for measuring Young's modulus, strength, etc. for macroscale structural materials. There are basically two kinds of tensile specimens – framed specimens whose support strips are cut after mounting and semidetached specimens fastened to the substrate at one end. The loading and force measurement systems are similar and use commercial transducers. Strain is measured either by overall grip displacement or by digital imaging in most cases. Initial works are described and followed by descriptions of some recent applications. The paper concludes with a suggested test method that could be amenable to standardization.

The adhesive pressure needed to separate two ocatdecyltrichlorosilane (ODTS) coated surfaces both with and without the addition of tricresyl phosphate (TCP) as a function of separation rate is characterized using molecular dynamics simulation. The simulations predict that when TCP is added between surfaces the adhesive pressure needed for separation is reduced compared to the system containing ODTS only. Both the adhesive pressure and the break up of the TCP layers exhibit a separation rate dependence that appears unrelated to the rate of diffusion of TCP on the ODTS. The ability of the TCP to remain localized to defected areas of the ODTS layer upon normal separation of the contact is also characterized. It is found that the TCP remains localized to defect sites, which effectively coats the damaged area.

Skeletal muscle tissues were constructed using an in vitro model, by differentiating skeletal myoblasts using an array of linear microstructures with the medium aspect ratios. The adaptation of skeletal myoblasts has been characterized with immunoflurescence microscopy during cell proliferation and differentiation. In particular, the dependence of the alignment efficiency on the dimensions of the microstructures was studied. The morphology difference of the myotubes in the three-dimensional tissues was reported. This paper holds the promise of efficient on-chip fabrication of skeletal muscle tissues and has an important implication in direct muscle repair and muscular mechanics.

Magnetically actuated micro-actuators are proposed to propel and manipulate fluid in micro-channels. As the fluid flows at low Reynolds number in such systems, the actuator should move in an asymmetric manner. The proposed actuators are polymer films with embedded magnetic particles, which are actuated by an external magnetic field. Based on the nature of the particles, the films can be either ferromagnetic or super-paramagnetic. We have identified four configurations in which the actuator exhibits an asymmetric motion.

Micromechanical structures were investigated nondestructively via laser-Doppler-vibrometry to determine defect structures. Therefore, silicon membrane structures were characterized by their measured resonant frequencies and mode shapes. The influence of defects on the micromechanical structures is shown on the measured dynamic properties. Defect samples were indentified on the basis of the ratios of measured resonant frequencies and the quantified comparison of mode shapes without an identification of unknown parameters. The investigations showed that a fast determination of defect structures is possible by measured dynamic properties.

Due to its low electrical resistivity and high thermal stability, C54-TiSi2 thin films can be used in some MEMS application, such as RF MEMS, to reduce RC delay and improve dynamic performance. In this paper, TiSi2 thin films have been prepared for the first time by using cathodic arc deposition to study the impact of energetic ion bombardment on the film microstructure and subsequent C49-C54 phase transformation during annealing. TiSi2 compound was used as the cathode and a substrate bias was varied to control kinetic energy of the ions during the film growth. Rutherford backscattering spectrometry and transmission electron microscopy were utilized to characterize the film composition and microstructure. The composition of the resultant TiSix thin films varies from x=2.4 to x=1.4 when the substrate bias was varied from floating to –200V. The as-deposited TiSix films are amorphous under no substrate heating and a phase separation at nano scale with inhomogeneous distribution of Ti and Si atoms was observed within the amorphous phase. Si atoms are seen to segregate on the boundary of Ti-rich domains and the domain size increased with the substrate bias. For a 90nm-thick TiSi2 film deposited on a SiO2/Si substrate, kinetics of the C49-C54 phase transformation was studied by measuring the change of film resistivity upon rapid thermal annealing. It was found that the C49-C54 phase transition temperature is higher (>900°C) for the arc-deposited TiSi2 thin films compared to evaporated and sputtered films and the activation energy for the transformation was calculated to be 6.1±0.2eV.

This paper reports on recent improvements of the bulge and microtensile techniques for the reliable extraction of material parameters such as the Young's modulus E, Poisson's ratio ν, plane strain modulus Eps = E/(1–ν2), prestress σ0, fracture strength μ, Weibull modulus m and strain hardening coefficients n, and on the direct comparison between the two methods. The bulge technique is extended to full wafer measurements enabling throughputs of data with statistical relevance whereas key improvements of a previous fabrication process of microtensile specimens lead now to much higher yields, approaching 100%. Both techniques are applied to an extensive set of materials, brittle and ductile, typically used in MEMS applications. These include thin films of silicon nitride, silicon oxide, polycrystalline silicon and aluminum deposited by techniques such as thermal oxidation, LPCVD, PECVD and PVD.

Fatigue fracture under cyclic loading of single crystal silicon (SCS) is concerned and fatigue properties and mechanism are investigated widely. Surface oxide is considered to have an important role on fatigue fractures. In this paper, the tensile testing of SCS whose surface was intentionally oxidized and the effect of the oxide thickness on the mechanical properties were reported. After fabrication of SCS specimens, they were oxidized in dry oxidization at 1100 ºC. Quasi-static tensile testing of SCS specimens with no, 100-nm-oxide, and 200-nm-oxide thick oxide was performed. As the results, the deviation in fracture strain was decreased by oxidation, and the fracture origin was observed to be the inner silicon part. These results might be caused by the decrease of surface roughness and defects formation by oxygen precipitation during thermal oxidation.