Sensor application of Polyurethane Nanofiber Webs (PNW) has been explored in this study. PNW were prepared by electrospinning of the polyurethane solution in dimethylformamide. Silver nanoparticles with a size of 60 nm on PNW were produced by precipitation of AgNO3 in alkaline media. A micromachined capacitive pressure sensor has been designed using a special elastic and conducting thin membrane (PNW) as an active electrode sensing the pressure. With an additional inductor and a metal-semiconductor field effect transistor (MESFET), a microwave oscillator has also been designed as part of the system. The capacity variation due to changing pressure results in a frequency shift of this oscillator. This frequency shift is measured and corresponds to the pressure. The simulations performed on this structure show a very good linearity over a wide range of applied pressure which could be detected by change in oscillator frequency. These results indicate that an implementation of PNW as an active material in capacitative sensing applications of MicroElectroMechanical Systems (MEMS) could be feasible.

This study explores the effects of softbake parameters including temperature and duration on the homogeneity and toughness of a Benzocyclobutene (BCB) adhesive bond. Experiments were initially performed on silicon wafer pieces and then verified using quartered wafers bonded on a different machine. A four-point bend delamination test was employed to quantitatively measure interface toughness while uniformity was analyzed visually. A softbake temperature of 150 °C for 10 minutes yielded strong void-free bonds with an interface toughness of 23 J/m2.

Further to previous work, which demonstrated the ability to engineer the in- and out-of-plane stress components in PECVD silicon nitride, this paper reports on the control of stress in a metal-nitride-metal sandwich. The addition of a symmetric metallisation to the core nitride layer provides additional functionality by enabling electrical connectivity and electrostatic transduction. This represents a widely applicable structural layer for CMOS-compatible surface micromachining.

The use of advanced test structures coupled with wafer curvature measurements has allowed for detailed analysis of the stress components within the metal-nitride-metal sandwich and enables predictive engineering of the mechanical properties of the structural layer. Relationships between the in- and out-of-plane stress components of the metal-nitride-metal sandwich as a function of the nitride RF deposition power are reported and discussed. In comparison to values measured for nitride-only, a -30MPa/μm stress gradient offset is observed. A mechanism for the decrease in the stress gradient with the addition of the metallisation is proposed and compares well to modelling.

The optimised metal-nitride-metal sandwich can be repeatably engineered to realise low tensile in-plane stress (100MPa) and low out-of-plane stress gradient (0 ± 10MPa/μm). The effective Young's Modulus of the metal-nitride-metal sandwich was determined to be 150GPa; and a value of 195GPa was calculated for the nitride layer using analytical modelling. Work to further reduce the in-plane stress whilst maintaining low stress gradient is in progress by independently tuning the strain of the metal layers.

Piezoelectric materials can convert mechanical and electrical energy, a particularly useful tool in developing micro and nanoscale systems. Characterizing the electromechanical behavior is essential to the design and optimization of the material's and device's performance. This paper examines the influence of boundary (clamping) conditions, relative thickness variations between the active one to two micron thick piezoelectric membrane and underlying passive support structure, and the electrode coverage on the electromechanical behavior. Membranes were fabricated with silicon and lead zirconate titanate (PZT) with a ratio of Zr to Ti of 40:60 that provide thickness ratios between 1:2 and 2:1 by depositing the PZT using sequential solution deposition. PZT films contain a tensile stress that accumulates during processing, therefore a compressive stressed layer of tungsten was sputtered on bulk micromachined membranes to produce a near zero net residual stress. A nonlinear finite element numerical simulation technique is utilized for the analysis of the composite thin film. A comparison between the behavioral trends determined by simulation and experimental methods will be discussed.

Mechanical properties of micro-electro-mechanical systems (MEMS) materials at cryogenic temperatures are investigated to extend MEMS devices into space applications. A helium-cooled measurement setup mimicking the outer space environment is developed and installed inside a focused-ion-beam (FIB) system. T-shape, low-stress LPCVD silicon nitride cantilevers suspended on a silicon substrate are fabricated as the test structures using bulk micromachining technique. A lead-zirconate-titanate (PZT) translator and a silicon diode are utilized as an actuator and a temperature sensor in the measurement setup, respectively. The resonant frequencies of an identical cantilever with different “milling masses” are measured to obtain the thickness and the Young's modulus. Additionally, a bending test is performed to determine the fracture strength. From the experiments, the Young's modulus of LPCVD silicon nitride thin films varies from 260.5 GPa ± 5.4 GPa at room temperature (298 K) to 266.6 GPa ± 4.1 GPa at 30 K, while the fracture strength ranges from 6.9 GPa ± 0.6 GPa at room temperature to 7.9 GPa ± 0.7 GPa at 30 K.

The second virial coefficient-based statistical thermodynamics of a diluted fluid of rigid particles of regular polygonal shape is discussed. Analytical expressions for the second virial coefficient, surface tension, isothermal compressibility and isothermal speed of sound are derived in this approximation. The obtained results provide analytical guidelines for numerical simulations and modeling studies of two-dimensional (2D) particulate fluids (thin films) widely used in processing of model (nano)composite materials and other technological processes.

Nanostructured thin films were assembled on interdigited microelectrode (IME) arrays as sensitive interfacial materials of an electrochemical detector, which can be integrated into microfluidic sensor devices. The goal is to produce sensor devices at extremes of miniaturization. The IME were created on glass wafers using conventional lithographic techniques. Open channels were etched on quartz or glass, and covered by PDMS materials, which were created using soft-lithography. The capability of chemical recognition was provided by the ligand framework structures of the nanostructured thin films on the electrode surface. A model system for such nanostructures involved the use of monolayer-capped gold nanoparticles of ∼2 nm core sizes which were assembled by carboxylic acid functionalized alkyl thiol linkers. The detection of dopamine was studied as a redox probe to test the feasibility of the microfluidic device. Results of cyclic voltammetric and chronoamperometric experiments are presented. Implications of the findings to the development of sensitive, selective, rapid and portable microanalytical devices for chemical/biological sensing are also discussed.

We report measurements of the tunneling current – distance (I-d) dependence above alkanethiol self-assembled monolayers (SAMs) on Au (111) substrates with high probe voltage. From the semilogarithmic I-d plots of hexanethiol and octanethiol SAMs, a kink in the tunneling current slopes is clearly observed, which shows the point where the STM tip contacts the end of the SAMs. The conductance decay constants of the vacuum layer and the alkanethiol SAMs are estimated from the tunneling current slopes. We also discuss the contact conductance of alkanethiol SAMs.

We simulate directional solidification using the phase field method solved with adaptive mesh refinement. We examine length scale selection for two cases. For small surface tension anisotropy directed at forty five degrees relative to the pulling direction, we observe a transition from a seaweed to dendrite morphology as the thermal gradient is lowered, consistent with recent experimental findings. We show that the morphology of crystal structures can be unambiguously characterized through the local interface velocity distribution. We derive semi-empirically a phase diagram for the transition from seaweed to dendrites as a function of thermal gradient and pulling speed. As surface tension anisotropy is increased and aligned with the pulling direction we observe cellular and dendritic arrays directed in the pulling direction. We characterize wavelength selection and obtain a new universal scaling of the wavelength that differs from previous theories.

This paper describes a bi-layer deposition technique to reduce the strain gradient of polycrystalline silicon-germanium (poly-SiGe) thin films without the use of any post-deposition annealing. By adjusting deposition conditions such as temperature, pressure, and/or flow rates of reactants, poly-SiGe films with required low average stresses can be obtained. Using the bi-layer technique, a strain gradient of 1.1×10-5 μm-1 (equivalent to 88 mm radius-of-curvature) has been achieved in 3.9 μm-thick poly-SiGe. This strain gradient would cause only 0.055 μm tip deflection for a 100 μm-long cantilever. The thermal budget was ∼10 hours at 425 °C, and no post-deposition annealing was required. The bi-layer film also exhibits low compressive average stress (-36 MPa) and low resistivity (0.55 mΩ-cm).

The characteristics and mechanisms of damage and failure in microfluidic joints consisting of Kovar metal tubes attached to silicon using borosilicate glass seals have been investigated. These joints are representative of seals for the MIT microrocket which is a silicon-based MEMS device. A key concern in such joints is the occurrence of cracks in silicon and glass due to residual stresses caused by a large thermal excursion during processing and the dissimilar coefficients of thermal expansion of the constituent materials. Joints with two types of glass compositions and joint configurations were fabricated, tested, and inspected. Axial tension tests were performed to investigate load carrying capability and the effect of thermally-induced cracks. Finite element models were used to obtain residual stresses due to the fabrication, and the location of the cracks from the experiments were found to coincide with the locations of the maximum principal stresses. The current work shows that the certain types of thermally-induced cracks are more detrimental to joint strength than others and a good bond between the Kovar tube and the silicon sidewall can help increase joint strength via shear load transfer.

One of the approaches to fabrication of MEMS involves surface micromachining to define dies on single crystal silicon wafers, dicing of the wafers to separate the dies, and electronic packaging of the individual dies. Dicing and packaging of MEMS accounts for a large fraction of the fabrication costs, therefore, nondestructive evaluation at the wafer level, before dicing, can have significant implications on improving production yield and costs. In this paper, advances in development of optoelectronic holography (OEH) techniques for nondestructive, noninvasive, full-field of view evaluation of MEMS at the wafer level are described. With OEH techniques, quantitative measurements of shape and deformation of MEMS, as related to their performance and integrity, are obtained with sub-micrometer spatial resolution and nanometer measuring accuracy. To inspect an entire wafer with OEH techniques, measurements of overlapping regions of interest (ROI) on a wafer are recorded and adjacent ROIs are stitched together through efficient 3D correlation analysis algorithms.

Sol-Gel, dip-coated titaniumIV and zirconiumIV dioxide monolayered films were deposited on soda-lime glass using titanium and zirconium acetylacetonates as precursors, respectively, and their structural and morphological characterization carried out. The films were heat-treated at different temperatures, 25°C, 150°C, 300°C, and 500°C and, according to Low-angle X-Ray Diffraction analyses (XRD), it was found that TiO2 (anatase) and tetragonal zirconia were present on the substrate, when heated at 500°C. Yoshida and Yajima's method, based on optical information given by the films transmittance UV-Vis spectra, was used to estimate film thickness, refractive index, and, using the Lorentz-Lorentz relationship, their volume fraction. Film thinning and phase formation with increasing temperature was observed, and both titania and zirconia films showed similar behavior during firing. Smooth films with a roughness value of around 2 nm are obtained for the two cases as indicated by Atomic Force Microscopy (AFM) of the surface. Also, formation temperature at around 500°C resulted in the optimum condition to obtain clean stoichiometric TiO2 and ZrO2 thin films on glass substrates as confirmed by X-Ray Photoelectron Spectroscopy (XPS) measurements.

Laser micromachining has proven to be a very powerful and successful tool for precision machining and microfabrication with applications in electronics, MEMS, medical, and biomedical fields. For MEMS fabrication and packaging, several approaches based on localized heating and bonding have been proposed such as localized eutectic bonding, fusion bonding, solder bonding, and chemical vapor deposition (CVD) bonding. However, these approaches are based on resistive heating which requires intimate contact at the electrodes, and this is not preferred for MEMS packaging applications. As a good alternative, laser microwelding has advantages of non contact, low heat distortion, high speed, high precision, and consistent weld integrity. Therefore, it is finding increasing use in MEMS fabrication and packaging applications. In laser microwelding, the material in the HAZ of the workpiece experiences heating, melting, and re-solidifying stages, and the final characterizations of the laser microwelds can be influenced by various factors such as the laser beam properties, the system cooling condition, and the surface roughness/reflectivity of the component. This paper presents characterization of laser microwelds, with emphasis on study of deformation and strength of the microwelds made by a Nd:YAG pulsed laser. Optical interferometry is used to nondestructively record fringe patterns of the shape and deformation of the laser microwelded workpiece before, during, and after the microwelding process, and tensile tests are performed to study strength of the laser microwelds as a function of different welding parameters. Results indicate that quality of the laser microwelds depends on selection of an appropriate set of parameters controlling the microwelding process.

The tribological issues associated with silicon-based microelectromechanical systems (MEMS) are well known. A popular solution to improve the tribological behavior is to apply different kinds of thin films. One film material, shown to have favorable properties in specialty applications, and which may also be suited for MEMS, is titanium carbide (TiC).

This paper examines the microfrictional properties of titanium carbide surfaces with two surface roughnesses sliding against polished 2 mm diameter TiC counterbodies. A comparison of the microfrictional behavior is made with various other surfaces sliding against the same material. Results indicate that the microfriction of smooth TiC sliding against a smooth TiC surface is low and similar to silicon or TiC sliding against a hydrophobic self-assembled monolayer. However, friction increases when the polished TiC ball slides against a rough TiC surface. Experiments at various relative humidities show that friction increases with increase in the relative humidity for two smooth TiC surfaces sliding against one another, but is reduced at higher relative humidity if the surface of one of the sliding partners is considerably rough.

This microfrictional study shows that TiC is well suited for microtribological applications. However, for optimal performance, the surface characteristics need to be tailored to the operating conditions.

Carbon nanotubes offer a surface very similar to that of graphite, a reference substrate in physisorption experiments aimed at studying substrate-adsorbate interactions. The curvature, however, introduces new questions. What are the effects of this on condensation pressures or heats of adsorption? Can one experimentally distinguish between different adsorption sites? In this study, we compare adsorption isotherms of several simple gases (Kr, Xe, CCl4) on singlewall (SWNTs) and multi-wall carbon nanotubes (MWNTs), before and after opening. For mechanically opened SWNTs, the accessibility of the adsorption sites and the molecular arrangements of the adsorbed gases are discussed. With the much bigger, well-defined MWNTs, the “cutting method” called upon a nitric acid treatment followed by a CO2 oxidation. TEM investigations and physisorption studies clearly revealed tube opening and that the inner channels became accessible to Kr molecules.

The existing interpretations of indentation test data (either theoretical or numerical approaches) have been largely based on isotropic plasticity models of polycrystalline materials while most of the metallic thin films widely used in many microelectronic and MEMS applications are strongly textured with a few grains or only a single grain running through the thickness of the films. The multicrystalline nature of the thin films on correlating their indentation and tensile properties is the focus of our investigation. Using multicrystalline aluminum and copper alloy thin sheets as model material systems, both tensile tests and indentation tests were performed and the testing results were compared based on a 3D crystal plasticity finite element analysis. The correlation between the indentation data and the tensile test data (at an effective or equivalent strain) is critically examined for these two multicrystalline materials.

The controllable etching of InP semiconductor surfaces is desired in removing damaged layers for facilitating regrowth and obtaining good electronic properties. We have observed that organic acids (α-hydroxy acids: tartaric, lactic, citric and malic) used in conjunction with HCl to etch (100) InP result in smoother and defect free surfaces, in comparison to etches based on inorganic acids alone. The chelating action of the organic acids aids in efficiently removing In from the surface. Based on these results, it is hypothesized that a consideration of the energy levels of the semiconductor and adsorbed molecules could help in devising new semiconductor etchants.

The fabrication and characterization of thin-film silicon resonators processed at temperatures below 110°C on glass substrates is described. The microelectromechanical structures consist of surface micromachined bridges of phosphorus-doped hydrogenated amorphous silicon (n+-a-Si:H) deposited by plasma-enhanced chemical vapor deposition (PECVD) suspended over a metallic gate counterelectrode. The structures are electrostatically actuated. Resonance frequencies in the MHz range and quality factors as high as 5000 are observed in vacuum. The effect of the geometrical dimensions of the bridges and of the measurement pressure on the resonance amplitude and frequency is studied. The elementary energy dissipation processes in a-Si:H-based resonators are discussed. At atmospheric pressure, air damping dominates the energy dissipation. In vacuum, intrinsic mechanisms, such as clamping losses and surface losses, control the energy dissipation.

The formation and nature of living materials are fundamentally different from those of synthetic materials. Synthetic materials generally have static structures, and are not capable of adapting to changing environmental conditions or stimuli. In contrast, living systems utilize energy to assemble, reconfigure, and dismantle materials in a dynamic, highly non-equilibrium fashion. The overall goal of this work is to identify and explore key strategies used by living systems to develop new types of materials in which the assembly, configuration, and disassembly can be programmed or “self-regulated” in microfluidic environments. As a model system, kinesin motor proteins and microtubule fibers have been selected as a means of directing the transport of molecular cargo, and assembly of nanostructures at synthetic interfaces. Initial work has focused on characterizing and engineering the properties of these active biomolecules for robust performance in microfluidic systems. We also have developed several strategies for functionalizing microtubule fibers with metal and semiconductor nanoparticles, and demonstrated the assembly of composite nanoscale materials. Moreover, transport of these composite assemblies has been demonstrated using energy-driven actuation by kinesin motor proteins. Current work is focused on developing mechanisms for directing the linear transport of microtubule fibers, and controlling the loading/unloading of nanoparticle cargo in microfluidic systems.