Micromechanical Actuators Research Articles

Rise time reduction of thermal actuators operated in air and water through optimized pre-shaped open-loop driving

Electrothermal actuators have many advantages compared to other actuators used in micro-electro-mechanical systems (MEMS). They are simple to design, easy to fabricate and provide large displacements at low voltages. Low voltages enable less stringent passivation requirements for operation in liquid. Despite these advantages, thermal actuation is typically limited to a few kHz bandwidth when using step inputs due to its intrinsic thermal time constant. However, the use of pre-shaped input signals offers a route for reducing the rise time of these actuators by orders of magnitude. We started with an electrothermally actuated cantilever having an initial 10–90% rise time of 85 μs in air and 234 μs in water for a standard open-loop step input. We experimentally characterized the linearity and frequency response of the cantilever when operated in air and water, allowing us to obtain transfer functions for the two cases. We used these transfer functions, along with functions describing desired reduced rise-time system responses, to numerically simulate the required input signals. Using these pre-shaped input signals, we improved the open-loop 10–90% rise time from 85 μs to 3 μs in air and from 234 μs to 5 μs in water, an improvement by a factor of 28 and 47, respectively. Using this simple control strategy for MEMS electrothermal actuators makes them an attractive alternative to other high speed micromechanical actuators such as piezoelectric stacks or electrostatic comb structures which are more complex to design, fabricate, or operate.

Thermofluidic characteristics of combined electroosmotic and pressure driven flows in narrow confinements in presence of spatially non-uniform magnetic field

We investigate thermofluidic transport phenomena and entropy generation for combined electroosmotic and pressure-driven flows through narrow confinements, subjected to spatially varying non-uniform magnetic field. Going beyond the Debye–Hückel limit, we consider the size effects of the ionic species (steric effect) to analyse magnetohydrodynamic flow and heat transfer characteristics. We demonstrate that a confluence of the steric interactions with the degree of wall charging (zeta potential) may result in heat transfer enhancement, and overall reduction in entropy generation of the system under appropriate conditions. In particular, it is revealed that a judicious selection of spatially varying magnetic field strength may lead to an augmentation in the heat transfer rate. It is also inferred that incorporating non-uniformity in distribution of the applied magnetic field translates the system to be dominated by the heat transfer irreversibility. The novel scope of the current research lies in the state-of-art design of advanced micromechanical industrial smart-sensors, actuators, and biomedical devices.

Piezoelectric domains in the AlGaN hexagonal microrods: Effect of crystal orientations

Presently, the piezoelectric materials are finding tremendous applications in the micro-mechanical actuators, sensors, and self-powered devices. In this context, the studies pertaining to piezoelectric properties of materials in the different size ranges are very important for the scientific community. The III-nitrides are exceptionally important, not only for optoelectronic but also for their piezoelectric applications. In the present study, we synthesized AlGaN via self-catalytic vapor-solid mechanism by atmospheric pressure chemical vapor deposition technique on AlN base layer over intrinsic Si(100) substrate. The growth process is substantiated using X-ray diffraction and X-ray photoelectron spectroscopy. The Raman and photoluminescence studies reveal the formation of AlGaN microrods in the wurtzite phase and ensure the high optical quality of the crystalline material. The single crystalline, direct wide band gap and hexagonally shaped AlGaN microrods are studied for understanding the behavior of the crystallites under the application of constant external electric field using the piezoresponse force microscopy. The present study is mainly focused on understanding the behavior of induced polarization for the determination of piezoelectric coefficient of AlGaN microrod along the c-axis and imaging of piezoelectric domains in the sample originating because of the angular inclination of AlGaN microrods with respect to its AlN base layers.

Dynamic nozzles for drop generators.

In this paper, a novel mechanism allowing greater control over the formation of droplets is presented. This is achieved via the use of a dynamic nozzle of adjustable diameter. It is demonstrated that, by using such a nozzle, it is possible to greatly modify the formation and breakup of the ligament behind the main drop, leading to an overall reduction in the number of satellite droplets. Furthermore, by adjusting the delay between the beginning of the forming of the drop and the start of the nozzle constriction, a greater control over both the number of satellites and the size of the main drop can be achieved. It is also shown that only a minimal reduction of the nozzle's effective diameter is required in order to exploit the positive effects of the technique presented here. This opens the possibility of incorporating the technique into current droplet generator systems, e.g., via the use of piezoelectric driven nozzles or other micro-mechanical actuation technology.

Data-driven stochastic models for spatial uncertainties in micromechanical systems

Accurate uncertainty quantification in engineering systems requires the use of proper data-driven stochastic models that bear a good fidelity with respect to experimentally observed variations. This paper looks at a variety of modeling techniques to represent spatially varying uncertainties in a form that can be incorporated into numerical simulations. In the context of microelectromechanical systems, we consider spatial uncertainties at the device level in the form of surface roughness and at the wafer level in the form of non-uniformities that arise as a result of various microfabrication steps. We discuss methods to obtain roughness characterization data ranging from the use of a simple profilometer probe to imaging-based techniques for the extraction of digitized data from images. We model spatial uncertainties as second-order stochastic process and use Bayesian inference to estimate the model parameters from the input data. We apply the data-driven stochastic models generated from this process to micromechanical actuators and sensors in which these spatial uncertainties are likely to cause significant variation. These include an electrostatically-actuated torsion-spring micromirror, an electromechanical comb-drive actuator and a pressure sensor with a piezoresistive strain gauge. We show that the performance of these devices is sensitive to the presence of spatial uncertainties and a proper modeling of these uncertainties helps us make reliable predictions about the variation in device performance. Where data is available, we even show that the predicted variation can be validated against experimental observations, highlighting the significance of proper stochastic modeling in the analysis of such devices.

Chromatic confocal matrix sensor with actuated pinhole arrays.

We present two versions of a chromatic confocal matrix sensor for the snapshot acquisition of three-dimensional objects. The first version contains separate illumination and detection pinhole arrays, while the second version uses a single pinhole array in double pass. The discrete lateral measurement points defined by the illumination and detection pinhole arrays are evaluated in parallel with a hyperspectral detection module. As this approach enables the spectrometric evaluation of all lateral channels, multilayer objects can be analyzed. To increase the lateral resolution the pinhole arrays are moved by micromechanical actuators. The paper includes a quantitative evaluation of the chromatic confocal module and proof-of-principle experiments with the full sensor system.

Graphene quantum dots interfaced with single bacterial spore for bio-electromechanical devices: a graphene cytobot.

The nanoarchitecture and micromachinery of a cell can be leveraged to fabricate sophisticated cell-driven devices. This requires a coherent strategy to derive cell's mechanistic abilities, microconstruct, and chemical-texture towards such microtechnologies. For example, a microorganism's hydrophobic membrane encapsulating hygroscopic constituents allows it to sustainably withhold a high aquatic pressure. Further, it provides a rich surface chemistry available for nano-interfacing and a strong mechanical response to humidity. Here we demonstrate a route to incorporate a complex cellular structure into microelectromechanics by interfacing compatible graphene quantum dots (GQDs) with a highly responsive single spore microstructure. A sensitive and reproducible electron-tunneling width modulation of 1.63 nm within a network of GQDs chemically-secured on a spore was achieved via sporal hydraulics with a driving force of 299.75 Torrs (21.7% water at GQD junctions). The electron-transport activation energy and the Coulomb blockade threshold for the GQD network were 35 meV and 31 meV, respectively; while the inter-GQD capacitance increased by 1.12 folds at maximum hydraulic force. This is the first example of nano/bio interfacing with spores and will lead to the evolution of next-generation bio-derived microarchitectures, probes for cellular/biochemical processes, biomicrorobotic-mechanisms, and membranes for micromechanical actuation.

Micromechanical Actuators Driven by Ultrasonic Power Transfer

Advances in miniature devices for biomedical applications are creating ever-increasing requirements for their continuous, long lasting, and reliable energy supply, particularly for implanted devices. As an alternative to batteries, energy harvesting and wireless power delivery are receiving increased attention. Although the former is generally only suited for lowpower diagnostic microdevices, the latter has greater potential to extend the functionality to include more energy demanding actuation such as drug release or mechanical adjustment of prosthetic devices. With this aim, we present an ultrasonic method of wireless power delivery for actuation, the novelty being that the actuation is powered by ultrasound directly, rather than via piezoelectric conversion. This paper describes a coupled mechanical system remotely excited by ultrasound at 200 kHz and providing conversion of acoustic energy into motion of a micromechanical mechanism using a receiving membrane coupled to a discrete oscillator. We address the problem of acoustic and mechanical impedance mismatch and simulate harmonic and frequency response of the system. The fabrication method is described, and the resulting devices are characterized, revealing hysteresis in their frequency response that is related to nonlinear behavior in the membrane–oscillator coupling. The experimental samples were successfully driven by an ultrasonic source, at various separations, and demonstrated maximum oscillator vibration amplitudes in the range of 9.6–9.9 $\mu{\rm m}$ . This provided a mechanical amplification with respect to the membrane in the range of 240–250.

Sensitivity of micromechanical actuation on amorphous to crystalline phase transformations under the influence of Casimir forces

Amorphous to crystalline phase transitions in phase change materials (PCM) can have strong influence on the actuation of microelectromechanical systems under the influence of Casimir forces. Indeed, the bifurcation curves of the stationary equilibrium points and the corresponding phase portraits of the actuation dynamics between gold and AIST (Ag${}_{5}$In${}_{5}$Sb${}_{60}$Te${}_{30}$) PCM, where an increase of the Casimir force of up \ensuremath{\sim}25$%$ has been measured upon crystallization, show strong sensitivity to changes of the Casimir force as the stiffness of the actuating component decreases and/or the effective interaction area of the Casimir force increases, which can also lead to stiction. However, introduction of intrinsic energy dissipation (associated with a finite quality factor of the actuating system) can prevent stiction by driving the system to attenuated motion towards stable equilibrium depending on the PCM state and the system quality factor.

Robust PID control design for an electrostatic micromechanical actuator with structured uncertainty

A robust proportional integral derivative (PID) controller coupled to a feedforward compensator is designed for set-point regulation manoeuvres of an electrostatic micromechanical system. The system is linearised at multiple operating points, and the feedforward compensator provides the nominal voltage. Perturbations around these points are handled from the PID controller, whose gains are tuned via the utilisation of a linear matrix inequality (LMI) approach, which guarantees robustness against the switching nature of the linearised system dynamics. The maximum microspring-stiffness parametric uncertainty that can be tolerated within this scheme, is computed through the use of the small gain theorem. Simulation studies are presented that proves the efficacy of the suggested scheme.

Micromechanical and microfluidic devices incorporating resonant metallic gratings fabricated using nanoimprint lithography

Optical filters based on resonant gratings have spectral characteristics that are lithographically defined. Nanoimprint lithography is a relatively new method for producing large area gratings with sub-micron features. Computational modeling using rigorous coupled-wave analysis allows gratings to be designed to yield sharp reflectance maxima and minima. Combining these gratings with microfluidic channels and micromechanical actuators produced using micro electromechanical systems (MEMS) technology forms the basis for producing tunable filters and other wavelength selective elements. These devices achieve tunable optical characteristics by varying the index of refraction on the surface of the grating. Coating the grating surface with water creates a 33% change in the resonant wavelength whereas bringing a grating into contact with a quartz surface shifts the resonant wavelength from 558 nm to 879 nm, a fractional change of 58%. The reflectivity at a single wavelength can be varied by approximately a factor of three. Future applications of these devices may include tunable filters or optical modulators.

Influence of van der Waals force on the pull-in parameters of cantilever type nanoscale electrostatic actuators

In this paper, the influence of the van der Waals force on two main parameters describing an instability point of cantilever type nanomechanical switches, which are the pull-in voltage and deflection are investigated by using a distributed parameter model. The fringing field effect is also taken into account. The nonlinear differential equation of the model is transformed into the integral form by using the Green's function of the cantilever beam. The integral equation is solved analytically by assuming an appropriate shape function for the beam deflection. The detachment length and the minimum initial gap of the cantilever type switches are given, which are the basic design parameters for NEMS switches. The pull-in parameters of micromechanical electrostatic actuators are also investigated as a special case of our study by neglecting the van der Waals force.

Method and apparatus for sorting biological cells with a MEMS device

A micromechanical actuator (92) for sorting hematopoietic stem cells for use in cancer therapies. The actuator (92) operates by diverting cells into one of a number of possible pathways (94, 100) fabricated in the fabrication substrate of the micromechanical actuator (92), when fluorescence is detected emanating from the cells. The fluorescence results from irradiating the cells with laser light, which excites a fluorescent tag attached to the cell. The micromechanical actuator (92) thereby sorts the cells individually, with an operation rate of 3.3kHz, however with the massively parallel 1024-fold device described herein, a throughput of 3.3 million events/second is achievable.

Electronic transduction in model enzyme sensors assisted by a photoisomerizable azo-polymer

We develop a stochastic model of electronic transduction by means of a rod-like azo-polymer (single peptide molecule doped with a given amount of azo-benzene structural units) in polymer-redox enzyme biosensor. We propose a configuration where the azo-polymer is anchored next to the enzyme reaction center and functions as a light-driven micromechanical actuator shuttling electrons toward the electrode. We show that the output catalytic current is exponentially sensitive to variations in geometrical size of the polymer (a ‘switch off’ effect) and suggest a scheme where the switching effect is triggered by polymer photoisomerization, resulting in its overall length change.

Frequency-shifting analysis of electrostatically tunable micro-mechanical actuator

A numerical approach for eigenvalue analysis of electrostatically tunable micro-mechanical actuators is proposed. An effi- cient algorithm for calculating the natural frequwncy shifting due to applied DC turning voltage is proposed. In the calculation of the coupled field problem, 3-D FEM and BEM are employed. The numerical examples are presented and the numerical analysis results are compared with experimental data. Previous works on the analysis of tunable electrostatic actuators used very simplified models in which micro- mechanical actuators are assumed as single mass-spring systems (3-5). However, actuator systems must be modeled to continuum structures because the actuators assumed as rigid body in previous works are generally deformable. Moreover, in real situations, the electrostatic field and the elastic deformation field of the system are 3-dimensionally coupled. An advanced numerical approach, which can treat continuous fields, is used only in the calculation of the electrostatic force applied on the device (6). In order to accurately predict the behavior of the real system, the analysis of the coupled fields in 3-D continuous system and an algorithm for evaluating the effects of electrostatic field on natural frequency are necessary. In this work, a numerical scheme for eigenvalue analysis of electrostatically tunable micro-mechanical actuators is proposed. We have used the boundary element method (BEM) and the finite element method (FEM) for the 3-D coupled analysis of electrostatic field and deformation of a device. In order to consider the effects of electrostatic field on natural frequency, an equivalent stiffness matrix for electrostatic tuning voltage is introduced. We can perturb the equilibrium structure using a concerned eigenvector of operating mode and then linearize the corresponding electrostatic force variation in order to determine the equivalent stiffness. For examples, a simple beam-shaped structure and a tunable micromirror are analyzed and the numerical analysis results are compared with experimental data.

Wide and continuous wavelength tuning of microcavity devices for optoelectronic applications

Ultra-widely tunable microcavity devices implemented by surface micromachining are studied. We model, fabricate, and characterize 1.55-μm vertical-resonator-based optical filters and vertical cavity surface emitting lasers (VCSELs) capable of wide, monotonic, and kink-free tuning by a single control parameter. Our devices are comprised of single or multiple horizontal air gaps in the dielectric and InP-based material system. Distributed Bragg mirrors with multiple air gaps are implemented. Due to the high refractive index contrast between air (n = 1) and InP (n = 3.17), only three periods are sufficient to guarantee a reflectivity exceeding 99.8% and offer an enormous stop-band width exceeding 500 nm. Unlike InGaAsP/InP or dielectric mirrors, they ensure short penetration depth of the optical intensity field in the mirrors and low absorption values. Stress control of the suspended membrane layers is of utmost importance for the fabrication of these devices. By controlling the stress, we are able to fabricate InP membranes that are extremely thin (357 nm thick) and at the same time flat (radius of curvature above 5 mm). Micromechanical single parametric actuation is achieved by both thermal and electrostatic actuation. Filter devices with a record tuning more than 142 nm with 3.2 V are presented.

Modeling of ultrawidely tunable vertical cavity air-gap filters and VCSELs

Tunable vertical cavity devices including an air-gap integrated in the cavity have been designed, fabricated, and investigated. The ultrawide wavelength tuning is realized by micromechanical actuation of Bragg mirror membranes. Based on optical and mechanical model calculations, the air-gap filters and vertical cavity surface emitting lasers (VCSELs) are designed for investigating mainly the optical tuning efficiency. In our research, we focus on two different mirror material systems, dielectric Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and InP/air-gap Bragg mirrors and on two tuning concepts, respectively. For the dielectric mirrors, continuous tuning is achieved by thermal actuation of the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mirror membranes, and for InP/air-gap mirrors, electrostatic actuation of the InP membranes is used. To verify the optical and mechanical simulations, InP/air-gap filters are characterized by measuring reflectance spectra and the tuning behavior. The measured results agree with the simulations used to optimize the micromechanical and optical characteristics of air-gap filters and VCSELs for optical communication applications.

Innovative concept for the fabrication of micromechanical sensor and actuator devices using selectively metallized polymers

We report on an innovative concept for the fabrication of micromechanical sensors and actuators, where polymer devices fabricated by injection molding are covered by metal layers of copper, nickel–phosphorus and gold using electroless plating. First we discuss the mechanical properties of miniaturized mechanical polymer beams covered with a metal layer. We find that at a sufficient metal layer thickness the mechanical properties of the beams are dominated by the metal cover surrounding the beam. Furthermore, we designed and fabricated a differential accelerometer from metallized liquid crystal polymer and present very first experimental results. We find a sensitivity and frequency response which is well suited to cover a broad band of applications. We conclude that this new technology can be used to fabricate diverse micromechanical sensor or actuator devices.

Innovative design and modelling of a micromechanical relaywith electrostatic actuation

The behaviour of a surface-micromachined electrostaticmicrorelay and the influence of the design parameters on the pull-involtage and the contact force are analysed by means of finite-elementmethod simulations. Three models have been used and compared - a fullythree-dimensional coupled electromechanical numerical simulation usingCoSolveEM, a two-dimensional coupled simulation, and a simplifiedsemi-analytical model based on the parallel-plate capacitorapproximation - and these have shown good conformity. As a new idea, anextension paddle attached to the contact bar is proposed and the positiveinfluence on the pull-in voltage and the contact force isdemonstrated.

A one-chip scanning retina with an integrated micromechanical scanning actuator

A microsensor that merges sensing and scanning functions on a single chip has been designed and fabricated, resulting in the first integrated scanning retina of its kind. A microfabrication technique has been developed to combine a one-dimensional array of photodiodes and electrostatically driven scanning slits on a single chip. The scanner actuates 12-/spl mu/m-wide microslits by up to 20 /spl mu/m on top of 30-/spl mu/m-wide photodiodes, and the motion generates an effect similar to that of the retinal scanning vergence found in the insects' compound eyes. The silicon retina is coupled with an array of 120-/spl mu/m-diameter microlenses to compose a microsized scanning compound eye, and the effect of retinal scanning in edge and position detection is demonstrated with the sensor. Each individual visual unit of the compound eye detects light contrast due to the scanning motion of the visual axis. The architecture of this scanning retina increases the resolution of a visual system with a relatively small number of receptors.