MEMS and Nanotechnology, Volume 6

Strain sensing at high temperatures, greater than 700°F, is often difficult. Traditional strain sensing uses the piezoresistive effect, which is temperature dependent. To reduce the temperature dependence of the strain sensor one could be built from a robust material such as silicon carbide, SiC. Making measurements using capacitive effects eliminates the effects of temperature within the sensing element. Using the more traditional MEMS material silicon is only an option at lower temperatures. Silicon has good reliability as a mechanical structure to around 900°F, and good electrical properties to 300°F. Having good properties above 700°F, silicon carbide is a robust material that has the ability to be used in high temperature MEMS applications. Using the capacitive effect for measuring strain was the original way to perform this task until the piezoresistive effect was harnessed. MEMS based capacitive strain sensors that have been built previously are known as resonant strain sensors, or the double ended tuning fork resonator. One step further from the double ended tuning fork is a novel capacitive strain sensor device. An examination of the novel approach to measure strain is performed. Modeling and simulation is presented using L-Edit and Coventorware. This asserts the device’s characteristics and gives the novel design merit to be used as a strain sensor.

Microelectromechanical systems (MEMS) switches offer much lower power consumption, much better isolation, and lower insertion loss compared to conventional field-effect transistors and PIN diodes however, the MEMS switch reliability is a major obstacle for large-volume commercial applications [1]. To enhance reliability, circuit designers need simple and accurate behavioral models of embedded switches in CAD tools to enable system-level simulations [2]. Where Macro-switch researchers assess electric contact performance based on the type of load that is being switched, in MEMS literature, micro-switch performance and reliability is characterized by testing the devices under “hot-switched” or “cold-switched” load conditions; simple models are developed from the “hot” and “cold” characterizations. By applying macro-switch performance characterization techniques, i.e. examining reliability based on the type of load that is being switched, clear characterizations of “hot” switching and “cold” switching external resistive, capacitive, and inductive loads are produced. External resistive loads were found to act as current limiters and should be suitable under certain criteria for reducing current density through the contact area and thus limiting device failure to mechanical failure modes. Alternatively, external capacitive loads increased current density under “hot” switching conditions at the moment the micro-switch closes; which increases the risk for material transfer and device failure. Under DC conditions, the inductive loads had little effect in either “hot” or “cold” switching environments.

Single-molecule manipulation techniques provide a unique tool for a close-up investigation of the complex biological properties and interactions. During the force measurement, a single molecule is pulled while its force response is monitored. However, quantifying these non-equilibrium data and using them to understand the structure-function relationship of biological systems have been challenging. We describe the mechanics of nanoscale biomolecules and the use of these force measurements for the free energy reconstruction using the recently derived non-equilibrium work theorem, i.e., Jarzynski’s equality. We also compare the results with those from other phenomenological approaches. Finally, mechanical characterization of systems such as overstretching transitions of DNA are presented, and the implications and challenges of these single-molecule force studies are discussed.

We present instrumentation and methodology for simultaneously measuring force and displacement at the atomic scale at 4 K. The technique, which uses a macroscopic cantilever as a force sensor and high-resolution, high-stability fiber-optic interferometers for displacement measurement, is particularly well-suited to making accurate, traceable measurements of force and displacement in nanometer- and atomic-scale mechanical deformation experiments. The technique emphasizes accurate co-location of force and displacement measurement and measures cantilever stiffness at the contact point in situ at 4 K using photon momentum. We present preliminary results of measurements made of the force required to rupture a single atomic bond in a gold single-atom chain formed between a gold flat and a gold tip. Finally, we discuss the possible use of the gold-gold bond rupture force as an intrinsic force calibration value for forces near 1 nN.

Mechanical drift between an atomic force microscope (AFM) tip and sample is a longstanding problem that limits tip-sample stability, registration, and the signal-to-noise ratio during imaging. We demonstrate a robust solution to drift that enables novel precision measurements, especially of biological macromolecules in physiologically relevant conditions. Our strategy – inspired by precision optical trapping microscopy – is to actively stabilize both the tip and the sample using locally generated optical signals. In particular, we scatter a laser off the apex of commercial AFM tips and use the scattered light to locally measure and thereby actively control the tip’s three-dimensional position above a sample surface with atomic precision in ambient conditions. With this enhanced stability, we overcome the traditional need to scan rapidly while imaging and achieve a fivefold increase in the image signal-to-noise ratio. Finally, we demonstrate atomic-scale (∼100 pm) tip-sample stability and registration over tens of minutes with a series of AFM images. The stabilization technique requires low laser power (<1 mW), imparts a minimal perturbation upon the cantilever, and is independent of the tip-sample interaction. This work extends atomic-scale tip-sample control, previously restricted to cryogenic temperatures and ultrahigh vacuum, to a wide range of perturbative operating environments.

The use of colloidal probes for mechanical testing of single molecules in an atomic force microscope (AFM) provides an attractive alternative to conventional microfabricated AFM cantilever force sensors, in that they have a much greater surface area available for specific binding to a target molecule. This is of particular importance for molecules have a low binding probability or are sterically inhibited from binding in some way. There are, however, several features unique to colloidal probe force measurements performed in a fluid environment, one of which is the presence of hydrodynamic forces acting on the sphere at the tip of the cantilever as it moves through the fluid. This force must be subtracted from the total measured force to isolate the molecular interaction of interest. Herein, a method is described to perform such a correction based on Brenner’s equation, and the method is demonstrated on data from the mechanical testing of a single DNA molecule.

This work builds involves leveraging our recent thin film mechanics model on the discontinuous transfer of strain from the film to the substrate. In applying this model with well-defined film and substrate properties we were able to decouple the effects of elastic modulus and Poisson’s ratio mismatch in the indentation process. In doing so we identified new insight in the processes of pile-up and strong evidence suggested a dependence on film thickness and ratios of film/substrate of elastic modulus and Poisson’s ratio. Atomic force microscopy was employed to characterize the degree of pile-up and correlate it with the above dependencies. We believe these efforts will enable the prediction of the degree of pile-up and subsequently the removal of its influence in measuring thin film behavior.

For materials which exhibit a power-law relationship between stress and strain rate, it is theoretically possible to evaluate the exponent (m) which governs the relationship by means of instrumented indentation. However, in practice, tests at small strain rates take so long that the results can easily be dominated by thermal drift. A new test method is developed in which several constant strain rates are examined within a single indentation test by switching strain rates as the indenter continues to move into the material. Switching strain rates within a single test overcomes the problem of long testing times by examining large strain rates first and transitioning to smaller strain rates as the test proceeds. The new method is used to test a sample of fine-grained nickel sold by NIST as a standard reference material for Vickers hardness. The strain-rate sensitivity of this sample is measured to be m = 0.021. This value is in good agreement with values obtained by others on fine-grained nickel using both instrumented indentation and uniaxial creep testing.

In his 1927 paper in Nature, B. van der Pol described experiments in which an electrical circuit forming a relaxation oscillator was externally forced with continuously varying frequency. The circuit’s response, he found, was entrained to be only at whole submultiples of the forcing frequency, i.e. f/2, f/3, up to f/40. We describe similar results found in an optically actuated MEMS limit cycle oscillator. Doubly supported beams are excited into self-oscillation in their first mode of vibration by illuminating them within an interference field which couples absorption to displacement. While in limit cycle oscillation, they are mechanically shaken out of plane with continuously varying frequency, and the limit cycle response is seen to be entrained to multiples or submultiples of the forcing frequency f/3, f/2, f, 2f, up to 7f.

Metal insulator transition (MIT) materials, or phase change materials (PCM) are material compounds that have the ability to be either conductors or insulators. Vanadium dioxide (VO₂) and germanium telluride (GeTe) exhibit such a transition property. These materials have ferroelectric properties as well as a variable resistivity. The ability to vary the resistance of a single material is useful when designing integrated circuits on the micro scale. By varying the temperature or the electric field across these materials, we are able to change the resistivity within a portion of a line. This can in turn be used to create a switch within a wire. In order to measure these changing properties, we developed novel surface micromachined test structures capable of using a variety of MIT materials. By varying the electric field or the thermal gradient across an area of the wire segment, we were able to adjust the resistivity of the material. Therefore, by tailoring the properties of specific portions of a conductor, we were able to control current flow in a circuit without needing a micro-mechanical or a microelectronic device.

Undesirable stiction, which results from the contact between surfaces, is a major failure mode in micro-switches. Indeed the adhesive forces can become so important that the two surfaces remain permanently glued, limiting the life-time of the MEMS. This is especially true when the contact happens between surfaces where elasto-plastic asperities deform permanently until the surfaces reach plastic accommodation, increasing the surface forces. To predict this behavior, a micro adhesive-contact model is developed, which accounts for the surfaces topography evolutions during elasto-plastic contacts. This model can be used at a higher scale to study the MEMS behavior, and thus its life-time. The MEMS devices studied here are assumed to work in a dry environment. In these operating conditions only the Van der Waals forces have to be considered for adhesion. For illustration purpose, an electrostatic-structural analysis is performed on a micro-switch. To determine the degree of plasticity involved, the impact energy of the movable electrode at pull-in is estimated. Thus the maximal adhesive force is predicted using the developed model.

Measurement of electronics and mechanics of single molecules provides a fundamental understanding of conductance as well as bonding at the atomic scale. To study the mechanics at these length scales, we have built a conducting atomic force microscope (AFM) optimized for high displacement and force resolution. Here, we simultaneously measure conductance and force across single Au-molecule-Au junctions in order to obtain complementary information about the electronics and structure in these systems. First we show that single-atom Au contacts, which have a conductance of G₀ (2e²/h), have a rupture force of about 1.4 nN, in excellent agreement with previous theoretical and experimental studies. For a series of amine and pyridine linked molecules which are bound to Au electrodes through an Au-N donor-acceptor bond, we observe that the rupture force depends on the backbone chemistry and can range from 0.5 to 0.8 nN. We also study junctions formed with molecules that bind through P-Au and S-Au interactions. We find that both the conductance signatures and junction evolution of covalent S-Au bond (thiolate) and a donor-acceptor S-Au bond (thiol) are dramatically different. Finally, we perform density functional theory based adiabatic molecular junction elongation and rupture calculations which give us an insight into the underlying mechanisms in these experiments.

The magnetic tweezer is a simple and stable single-molecule manipulation instrument. However, the standard probe-tracking methods have typically failed to reach the high resolution (∼0.3 nm) needed to measure motor protein stepping. In this paper we present a novel illumination geometry, based on an inverted microscope with Hg lamp illumination, that aims to push the resolution of magnetic tweezers to their ultimate thermal limits. Using a metal-coated coverslip and motorized magnets, we convert a standard inverted microscope into a high-resolution magnetic tweezers instrument. Our novel optical geometry reduces the restrictions on magnet design inherent to transmission-based illumination, and does not require fiber-optic coupling. We introduce a high-speed CMOS camera as the optical detector, and demonstrate how an improvement in temporal resolution directly impacts the spatial resolution.

Carbon nanotube (CNT) based electron field emission devices may have an advantage over metal Spindt tip style designs due to the ability to create a highly localized electric field at the extremely small diameter tip of the CNT. The primary objective for this work is to create a robust micro structure to support low voltage field emission from the CNTs in a gated device. This paper will discuss the micro fabrication techniques used to etch 2–4 μm thick thermal oxide layers on silicon substrates. A chrome layer is deposited by electron beam evaporation to make the gate layer of the triode device and act as an etch mask. The metal layer is then coated with photoresist, patterned with hole openings ranging from 8 to 12 μm in diameter and wet etched in acid through to the SiO₂ layer. Different dry etch chemistries combined with wet etching are used to study the effect on the SiO₂ sidewall. The shape and slope of the SiO₂ sidewall and gate opening play a vital role in fabricating a robust triode device that doesn’t easily short out when the CNTs are grown later in the process.

In this contribution experimental and theoretical investigations of sheet metal mesocrystals with coarse texture are performed. One focus of this work is on size effects due to a lack of statistical homogeneity. The overall mechanical response is then strongly influenced by the orientation of the individual grains. For this purpose a crystal-plasticity-based finite-element model is developed for each grain, the grain morphology, and the specimen as a whole. The crystal plasticity model itself is rate-dependent and accounts for local dissipative hardening effects. This model is applied to simulate the thin sheet metal specimens with coarse texture subjected to tensile loading at room temperature. Investigations are done for body-centered-cubic Fe-3%Si and face-centered-cubic Ni samples. Comparison of simulation results to experiment are given.

This work focuses on the synthesis of nano-structured Al6061 using machining under plane strain and evaluation of its mechanical properties. It discusses an unusual application of the machining process by using it as a severe plastic deformation (SPD) process to develop nano-structured or ultra-fine grained materials. Chips obtained from this process show higher hardness than the bulk material which is in agreement with results reported in existing literature. Chips with minimum curvature have been obtained using restricted contact tool and extrusion-machining processes. Hardness of the straight chips obtained by the stated methods, though higher than the bulk material, was less than the hardness of the curled chips obtained from conventional orthogonal machining. Furthermore, hardness of the chips obtained using tool with restricted contact length of 0.6mm showed lesser variation. Hence they were used to prepare samples for the tensile test. A novel method was used to prepare small test specimens from chips to measure tensile strength. Specimens made from the chips had higher ultimate tensile strength (53%) and yield strength (85%) than that of bulk material. Improvement in strength was accompanied by a reduction in ductility (58%) for chips as compared to bulk material. It was observed that for both the chip and the bulk material, the reduction in gauge length leads to lower values of Young s modulus showing size effect.

Based on the work of Fleck et al., the influence of strain gradients on the deformation behaviour of metals in small dimensions has been studied intensively. However, since almost 20 years comparable torsion experiments have never been repeated. In this work, the deformation behaviour of Au and Al (containing 1 wt.-% Si) wires with diameters ranging from 15 to 40 m was investigated in both tension and torsion, using a self-developed test facility. Size effects were observed in the torsion tests as in the classical experiments, performed by Fleck and co-workers. However, thermal treatments of the wires in combination with grain structural analysis show clearly that the microstructure of the wires plays an important role when comparing the as-received and the annealed state with fine and coarse grains, respectively. Moreover, a variation of the deformation velocity within the tests on AlSi1 wires showed an additional influence on the strength level in tension and the size effect in torsion.

The tympanic membrane is one of the major structures of the ear that aids in the hearing process, giving humans one of the five major senses. It is hypothesized that sound induced displacements of the membrane, which allow humans to hear, are directly related to the membrane’s medial layer which is comprised of a network of collagen fibers. Limitations in available medical imaging techniques have thus far inhibited the further study of these fibers. In this paper we detail an imaging system that we developed with the capability to quantitatively and noninvasively image the internal structures of biological tissues in vitro through spatial domain optical coherence tomography (OCT). By utilizing spatial OCT, we can correlate the characteristics of internal collagen fibers to sound induced displacements in the tympanic membrane. This will eventually lead to improved modeling of the middle-ear and a better understanding of hearing mechanics.

MEMS technology has led to the development of new uncooled infrared imaging detectors. These MEMS detectors consist of arrays of bi-metallic cantilevered beams that deflect linearly as a function of temperature associated with infrared radiation from the scene. The main advantage of these detectors is the optical readout system that measures the tilt of the beams based on the intensity reflected light. This removes the need for electronic readout at each of the sensing elements and reduces the fabrication cost and complexity of sensor design, as well as eliminating the electronic noise at the detector. The optical readout accuracy is sensitive to the uniformity of individual pixels on the array. The hypothesis of the present research is that direct measurements of the change in deflection will reduce the need for high pixel uniformity. Measurements of deflection change for a vacuum packaged detector with responsivity of 2.4 nm/K are made with a Linnik interferometer employing the four phase step technique. The interferometer can measure real-time, full-field height variations across the array. In double-exposure mode, the current height map is subtracted from a reference image so that the change in deflection is measured. A software algorithm locates each mirror on the array, extracts the measured deflection at the tip of a mirror, and uses that measurement to form a pixel of a thermogram in real-time. A blackbody target projector with temperature controllable to 0.001 K is used to test the thermal resolution of the imaging system. The minimum temperature resolution is below 250 mK.

Bulge testing is known for its ability to quantify the mechanical behavior of homogeneous thin membranes. In this method the measured quantities are related to the averaged stress and strain using bulge equations that only exist for a very limited set of membrane geometries. A novel 3D Digital Image Correlation (DIC) method is proposed to directly measure the strain and curvature fields without using any closed form approximation of the deformation kinematics. Importantly, for membranes under pressure, the stress is directly related to the curvature.

A technique developed for studying the internal friction and energy loss of nano-scale thin metal films on substrate is presented. The test microstructure was designed on the triangular cantilever beam and fabricated by the standard C-MOS processes, which can improve stress distribution non-uniform problem of conventional cantilever beam. The thickness of deposited film on its surface could reduce to several nanometers. Nanocrystalline Al thin film with thickness of sub-micrometer and nanometer were performed to observe its internal friction and energy loss response under dynamic frequency response of the cantilever beam structure generated by electrostatic force within vacuum pressure. The results show the measurement system used here can accurately measures the energy loss of thin film. The internal friction measurement results provided evidence for the grain boundary motion and dislocation motion in the nanoscale thin films. Moreover, the length scale dependence on loss mechanism of tested films was observed.

A detailed experimental study is conducted to understand damage initiation and growth in epoxy particulate composites using a multi-wall carbon nanotube (MWCNTs) conductive network under two different loading conditions: (a) quasi-static shear and (b) fracture. Two different particulates (a) Cenospheres (aluminum silicate hollow spheres), and (b) carboxyl-terminated butadiene acrylonitrile copolymer (CTBN) rubber of three different volume fractions (10%, 20% and 30%) and mass fractions (10phr, 20phr and 30phr) respectively are used in thermoset epoxy resin composites. First, MWCNTs are well dispersed in an epoxy matrix using ultrasonication, and later the above particulates are added and shear-mixed into the solution to prepare composites. A v-notch rail shear specimen configuration for shear experiments, and single edge notch tension (SENT) configuration for fracture are considered in this experimental study. A four-point probe methodology along with high-resolution data acquisition is employed to capture electrical-resistance response of network changes associated with non-linear deformation, damage initiation and growth within composites under said loading conditions. It is identified from experiments that the electrical response associated with the above mechanisms is quite different with the addition of particulates compared to that of epoxy with no particulate.