Comb-drive actuators for large displacements

The design, fabrication and experimental results of lateral-comb-drive actuators for large displacements at low driving voltages is presented. A comparison of several suspension designs is given, and the lateral large deflection behaviour of clamped - clamped beams and a folded flexure design is modelled. An expression for the axial spring constant of folded flexure designs including bending effects from lateral displacements, which reduce the axial stiffness, is also derived. The maximum deflection that can be obtained by comb-drive actuators is bounded by electromechanical side instability. Expressions for the side-instability voltage and the resulting displacement at side instability are given. The electromechanical behaviour around the resonance frequency is described by an equivalent electric circuit. Devices are fabricated by polysilicon surface micromachining techniques using a one-mask fabrication process. Static and dynamic properties are determined experimentally and are compared with theory. Static properties are determined by displacement-to-voltage, capacitance-to-voltage and pull-in voltage measurements. Using a one-port approach, dynamic properties are extracted from measured admittance plots. Typical actuator characteristics are deflections of about at driving voltages around 20 V, a resonance frequency around 1.6 kHz and a quality factor of approximately 3

Quasi-buckling of micromachined beams

Buckling of structures with imperfections, quasi-buckling (QB), is studied. At the bifurcation load, these structures show a smooth transition into one of the stable postbuckling equilibrium states, instead of the traditional sudden change in deflection characteristics. QB structures can show the classical snap-through buckling behaviour, i.e., a sudden change of postbuckling equilibrium state. The QB is described with a generalized temperature, Tg, representing the compression of the structure. Imperfections and distributed deflection loads are represented by a generalized pressure, pg. Experiments on micromachined beams, exposed to heating (Tg) and to a Lorentz force (pg), verify that the QB phenomena can efficiently transfer a longitudinal stress into a transversal deflection, with a scale-factor depending on both Tg and pg

Fabrication and electrical integration of robust carbon nanotube micropillars by self-directed elastocapillary densification

Vertically-aligned carbon nanotube (CNT) "forest" microstructures fabricated by chemical vapor deposition (CVD) using patterned catalyst films typically have a low CNT density per unit area. As a result, CNT forests have poor bulk properties and are too fragile for integration with microfabrication processing. We introduce a new self-directed capillary densification method where a liquid is controllably condensed onto and evaporated from CNT forests. Compared to prior approaches, where the substrate with CNTs is immersed in a liquid, our condensation approach gives significantly more uniform structures and enables precise control of the CNT packing density and pillar cross-sectional shape. We present a set of design rules and parametric studies of CNT micropillar densification by this method, and show that self-directed capillary densification enhances the Young's modulus and electrical conductivity of CNT micropillars by more than three orders of magnitude. Owing to the outstanding properties of CNTs, this scalable process will be useful for the integration of CNTs as functional material in microfabricated devices for mechanical, electrical, thermal, and biomedical applications

Using Micro-Raman Spectroscopy to Assess MEMS Si/SiO2 Membranes Exhibiting Negative Spring Constant Behavior

We introduce a novel micro-mechanical structure that exhibits two regions of stable linear positive and negative stiffness. Springs, cantilevers, beams and any other geometry that display an increasing return force that is proportional to the displacement can be considered to have a “Hookean” positive spring constant, or stiffness. Less well known is the opposite characteristic of a reducing return force for a given deflection, or negative stiffness. Unfortunately many simple negative stiffness structures exhibit unstable buckling and require additional moving components during deflection to avoid deforming out of its useful shape. In Micro-Electro-Mechanical Systems (MEMS) devices, buckling caused by stress at the interface of silicon and thermally grown SiO2 causes tensile and compressive forces that will warp structures if the silicon layer is thin enough. The 1 mm2 membrane structures presented here utilizes this effect but overcome this limitation and empirically demonstrates linearity in both regions. The Si/SiO2 membranes presented deflect ~17 μm from their pre-released position. The load deflection curves produced exhibit positive linear stiffness with an inflection point holding nearly constant with a slight negative stiffness. Depositing a 0.05 μm titanium and 0.3 μm layer of gold on top of the Si/SiO2 membrane reduces the initial deflection to ~13.5 μm. However, the load deflection curve produced illustrates both a linear positive and negative spring constant with a fairly sharp inflection point. These results are potentially useful to selectively tune the spring constant of mechanical structures used in MEMS. The structures presented are manufactured using typical micromachining techniques and can be fabricated in-situ with other MEMS devices

A fabrication process for electrostatic microactuators with integrated gear linkages

A surface micromachining process is presented which has been used to fabricate electrostatic microactuators. These microactuators are interconnected with each other and linked to other movable microstructures by integrated gear linkages. The gear linkages consist of rotational and linear gear structures, and the electrostatic microactuators include curved electrode actuators, comb-drive actuators, and axial-gap wobble motors. The micromechanical structures are constructed from polysilicon. Silicon dioxide was used as a sacrificial layer, and silicon nitride was used for electrical insulation. A cyclohexane freeze drying technique was used to prevent problems with stiction. The actuators, loaded with various mechanisms, were successfully driven by electrostatic actuation. The work is a first step toward mechanical power transmission in micromechanical system

Inductive machine learning of optimal modular structures: Estimating solutions using support vector machines

Structural optimization is usually handled by iterative methods requiring repeated samples of a physics-based model, but this process can be computationally demanding. Given a set of previously optimized structures of the same topology, this paper uses inductive learning to replace this optimization process entirely by deriving a function that directly maps any given load to an optimal geometry. A support vector machine is trained to determine the optimal geometry of individual modules of a space frame structure given a specified load condition. Structures produced by learning are compared against those found by a standard gradient descent optimization, both as individual modules and then as a composite structure. The primary motivation for this is speed, and results show the process is highly efficient for cases in which similar optimizations must be performed repeatedly. The function learned by the algorithm can approximate the result of optimization very closely after sufficient training, and has also been found effective at generalizing the underlying optima to produce structures that perform better than those found by standard iterative methods

Mode I and Mode II Measurements For Stiction Failed Micro-Electro-Mechanical Systems

Among MicroElectroMechanical Systems (MEMS), the most common type of failure is stiction. Stiction is the unintended adhesion between two surfaces when they are in close proximity to each other. Various studies have been conducted in recent years to study stiction. Our research group has shown the in-service repair of the stiction failed MEMS devices is possible with structural vibrations. In order to further understand this phenomenon and better predict, theoretically, the onset of repair we have constructed an apparatus to determine the Mode I, II, and III interfacial adhesion energies of MEMS devices failed on a substrate. Though our method is general, we are specifically focused on devices created using the SUMMiT V process. An apparatus has been constructed that has 8 degrees-of-freedom between the MEMS device, the surface on which the device is failed, and a scanning interferometric microscope. Deflection profiles of stiction failed MEMS (micro-cantilevered beams 1000 microns long, 30 microns wide, and 2.3 microns thick) have their deflection profiles measured with nanometer resolution by a scanning interferometric microscope. Using the experimental apparatus that is constructed, we determine the Mode I and Mode II interfacial adhesion energies using two methodologies. The first method utilizes the peel test scheme to determine pure Mode-I and Mixed Mode (Mode I and II) interfacial adhesion energies. In order to determine the values for the interfacial adhesion energies a nonlinear model was developed for the deflection of a beam that accounts for its stretching. Energy methods are then utilized to determine interfacial adhesion energies. Using the same experimental apparatus Mode II interfacial adhesion energies are measured directly with a novel technique developed in this work. This experimental method for measuring the Mode II interfacial adhesion energies for stiction failed MEMS devices uses a microcantilever beam (1500 μm long, 30 μm wide and 2.3 μm thick) attached to MEMS actuator with fix-fix beam flexure. Deflection of the spring is measured with the vernier scale of the actuator. Then a nonlinear elastic model for the fix—fix beam flexure is used to determine the interfacial adhesion energy between the failed microcantilever beam and the surface. A theory is developed to measure the strain energy release rates with finite crack growth, which gives the upper bounds of interfacial adhesion energy for Mode II fracture problem. A separate theory is developed for infinitesimal crack growth, which gives the exact interfacial adhesion energy of the Mode II fracture problem. Because the surface roughness plays an important role in the adhesion of MEMS structures, the surfaces of all structures have been characterized with an Atomic Force Microscope (AFM)

The effect of near-surface metallurgy on the machinability of cast iron

The increasing performance and durability of cutting tool inserts have created metallurgical challenges for production foundries to produce near-net shaped castings within strict dimensional tolerances. In order for foundries to take full advantage of the increased cutting speed capabilities, it becomes necessary to reduce machining allowances and produce much more stable casting surfaces. To accomplish this, a better understanding of the complex microstructures formed within the first 0.120 in. (3 mm) of the mold/metal interface (as-cast surface) is necessary. The goal of the work presented here was to examine the microstructures formed in the near-surface region of gray iron castings, determine what was responsible for formation, and how these microstructures behaved during the machining process. A series of experiments were performed to evaluate the effect of graphite flake morphology, matrix microstructure, and alloying elements on near-surface machinability. Three-dimensional cutting forces, quantitative metallography, and high-speed photographic measurements were used to evaluate the behavior of flake graphite, ferrite, coarse/dense pearlite, steadite, and carbides during the machining process. Data from the experiments also indentified the importance of inoculation practice, cooling rate, and mold sand properties on the final near-surface microstructure/machinability behavior. A case study was then performed for industrial brake rotor castings produced from class 35 gray cast iron, in which diagnosis of a machinability problem proved to be near-surface microstructure related. It was found that a combination of mold sand properties and inoculation practice were responsible for surface free-ferrite/graphite morphology microstructural defects --Abstract, page iii

Microfabricated structures with electrical isolation and interconnections

The invention is directed to a microfabricated device. The device includes a substrate that is etched to define mechanical structures at least some of which are anchored laterally to the remainder of the substrate. Electrical isolation at points where mechanical structures are attached to the substrate is provided by filled isolation trenches. Filled trenches may also be used to electrically isolate structure elements from each other at points where mechanical attachment of structure elements is desired. The performance of microelectromechanical devices is improved by 1) having a high-aspect-ratio between vertical and lateral dimensions of the mechanical elements, 2) integrating electronics on the same substrate as the mechanical elements, 3) good electrical isolation among mechanical elements and circuits except where electrical interconnection is desired