Micro-Resonators: The Quest for Superior Performance

Microelectromechanical resonators are no longer solely a subject of research in university and government labs; they have found a variety of applications at industrial scale, where their market is predicted to grow steadily. Nevertheless, many barriers to enhance their performance and further spread their application remain to be overcome. In this Special Issue, we will focus our attention to some of the persistent challenges of micro-/nano-resonators such as nonlinearity, temperature stability, acceleration sensitivity, limits of quality factor, and failure modes that require a more in-depth understanding of the physics of vibration at small scale. The goal is to seek innovative solutions that take advantage of unique material properties and original designs to push the performance of micro-resonators beyond what is conventionally achievable. Contributions from academia discussing less-known characteristics of micro-resonators and from industry depicting the challenges of large-scale implementation of resonators are encouraged with the hopes of further stimulating the growth of this field, which is rich with fascinating physics and challenging problems

Selectively Tuning a Buckled Si/SiO\u3csub\u3e2\u3c/sub\u3e Membrane MEMS through Joule Heating Actuation and Mechanical Restriction

This research followed previous work and attempted to modify the spring in two ways. First, a Ti/Au meander resistor was deposited atop the membrane in an effort to actuate the membrane and change the spring constant. Secondly, a series of overhanging cantilevers were attached to the bulk substrate surrounding the membrane in an effort to constrain the membrane buckling deflection to the negative stiffness region. Membrane buckling was investigated through Finite Element analysis (FEA) and analytical equations. Deflections were measured using an interferometric microscope (IFM) and force/deflection measurements were captured using a unique measurement scheme. The results concluded that by introducing a thermal stress, the membrane could be actuated with a corresponding 3x increase in spring constant. Additionally, the overhanging beams restricted the membrane deflection by up to 30%, but, because of a lack in beam stiffness, failed to restrict the membrane to the negative stiffness region. This research laid the ground work for future work in this area

Micro-opto-mechanical switching and tuning for integrated optical systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (p. 243-260).Integrated optical circuits have the potential to lower manufacturing and operating costs and enhance the functionality of optical systems in a manner similar to what has been achieved by integrating electronic circuits. One of the basic optical elements required to enable integrated optical circuits is an integrated optical switch, analogous to transistor switches used in integrated electronic circuits. An ideal switch for integrated optical circuits would provide wavelength-selective switching. Wavelength- selective behavior is an important characteristic for devices intended for networking applications as wavelength division multiplexing (WDM) of optical signals has become the accepted standard. A major contribution of this thesis is the design, fabrication, and experimental demonstration of a wavelength-selective, integrated optical switch. This switch operates by combining a microring resonator filter with a microelectromechanical system (MEMS) device that allows the normally static ring resonator filter to be switched on and off. This represents the first demonstration of a wavelength-selective integrated optical MEMS switch. Additional contributions of this work include a new study of dielectric charging, analysis of the use of titanium nitride as structural material for MEMS, two new MEMS actuation techniques that lead to higher speed and/or lower actuation volt- age, and a feasibility analysis for wavelength tuning using a generalized version of the switch design. A model for the evolution of dielectric charging during the actuation of MEMS devices was developed to address a deviation of the experimentally fabricated devices from the theoretical predictions according to older models.(cont.) The new model predicts the experimental voltage versus displacement behavior of the wave-length selective switch accurately, and offers new insights into the physics of dielectric charging. The use of titanium nitride as a MEMS material was conceived as a solution to residual stress problems that are common in cantilever-type of actuators in general, including the wavelength-selective switch. Specific details on MEMS implementation using titanium nitride are discussed in the thesis. To address CMOS compatibility and speed challenges, two new complementary MEMS switch actuation techniques were developed. The new methods require less voltage and energy for actuation while at the same time reducing the switching time of the device to levels unachievable with current MEMS actuation techniques. Preliminary theoretical and experimental results are presented and discussed. Finally, the thesis covers the feasibility analysis of a version of the switch design where the motion is analog and, hence, can be used for tuning of resonant integrated optical structures. The analysis shows that the required positional accuracy is achievable with on-chip capacitive position sensing and feedback control, and points to a promising new direction for mechanically tunable integrated photonics. While these contributions are all outgrowths of work directed towards realizing an integrated optical circuit, they are also significant for applications such as radio- frequency (RF) MEMS switching and free-space optical MEMS devices (i.e. micro- mirror arrays for projection displays).by Gregory Nolan Nielson.Ph.D

Modelling the electrostatic actuation of MEMS: state of the art 2005.

Most of MEMS devices are actuated using electrostatic forces. Parallel or lateral plate actuators are the types commonly used. Nevertheless, electrostatic actuation has some limitations due to its non-linear nature. This work presents a methodic overview of the existing techniques applied to the Micro-Electro-Mechanical Systems (MEMS) electrostatic actuation modeling and their implications to the dynamic behavior of the electromechanical system

Surface micromachined MEMS variable capacitor with two-cavity for energy harvesting

In this research, a novel MEMS variable capacitor with two capacitive cavities for energy harvesting was developed that use the wasted energy associated with undesirable mechanical vibrations to power microelectronic sensors and actuators widely found in structures and systems surrounding us. The harvested power, though very small, can have a profound effect on the usage of microsensors. First, the self-powered sensors will no longer require regular battery maintenance. Second, the self-powered chip is a liberating technology. On a circuit board, it can simplify the connection. On a commercial jet, the sensors can greatly simplify cabling. The design, fabrication, modeling and complete set of characterization of MEMS variable capacitors with two-cavity are presented in details in this thesis. The MEMS variable capacitors are unique in its two-cavity design and use of electroplated nickel as the main structural material. The device consists of 2x2 mm² movable capacitive proof mass plates with a thickness of 30 [mu]m suspended between two fixed electrodes forming two vertical capacitors. When the capacitance increases for one cavity, it decreases for the other. This allows using both up and down directions to generate energy. The suspended movable plates are supported by four serpentine springs with a thickness of 3-5 [mu]m that are attached to the address lines on a silicon substrate only at the anchors' points which is made of electroplated nickel. The serpentine suspension beams are made with a width, thickness and total length (four serpentine turns) of 15 [mu]m, 5 [mu]m and 1485 [mu]m. Five gold stoppers with height of 2-4 [mu]m were electroplated on the fixed plates to prevent snap-down of the movable plates by overwhelming electrostatic force. SiO2 and Si3N4 thin layers were patterned on the fixed plates to insulate the stoppers and enhance the dielectric property of capacitive cavities. The MEMS variable capacitor with two-cavity has been designed and modeled using MEMS CAD tool and COMSOL Multi-PhysIncludes bibliographical references (pages 108-118)

Electrochemical to mechanical energy conversion

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references.Electrode materials for rechargeable lithium ion batteries are well-known to undergo significant dimensional changes during lithium-ion insertion and extraction. In the battery community, this has often been looked upon negatively as a degradation mechanism. However, the crystallographic strains are large enough to warrant investigation for use as actuators. Lithium battery electrode materials lend themselves to two separate types of actuators. On one hand, intercalation oxides and graphite provide moderate strains, on the order of a few percent, with moderate bandwidth (frequency). Lithium intercalation of graphite can achieve actuation energy densities of 6700 kJ m-3 with strains up to 6.7%. Intercalation oxides provide strains on the order of a couple percent, but allow for increased bandwidth. Using a conventional stacked electrode design, a cell consisting of lithium iron phosphate (LiFePO4) and carbon achieved 1.2% strain with a mechanical power output of 1000 W m 3 . Metals, on the other hand, provide colossal strains (hundreds of percent) upon lithium alloying, but do not cycle well. Instead, a self-amplifying device was designed to provide continuous, prolonged, one-way actuation over longer time scales. This was still able to achieve an energy density of 1700 kJ n 3, significantly greater than other actuation technologies such as shape-memory alloys and conducting polymers, with displacements approaching 10 mm from a 1 mm thick disc. Further, by using lithium metal as the counterelectrode in an electrochemical couple, these actuation devices can be selfpowered: mechanical energy and electrical energy can be extracted simultaneously.by Timothy Edward Chin.Ph.D

MICROFABRICATION AND MODELLING OF DIELECTRIC ELASTOMER ACTUATORS

Dielectric elastomer actuators (DEAs) are a class of polymeric actuators that have gained prominence over the last decade. A DEA is comprised of a polymer sandwiched between two compliant electrodes. When voltage is applied between the two electrodes, electrostatic attraction between the electrodes compresses the elastomer in that direction and stretches it in the other two directions. DEAs produce dimensional changes (strains) up to 300% upon application of an electric field. DEAs have tremendous potential for applications requiring large displacements and have been demonstrated for many macro-scale (centimeter and larger) applications such as robots, loudspeakers, and motors. There are potentially many useful applications for micro-scale DEAs (less than millimeter-sized devices with micron-sized actuators) in the fields of micro-robotics, micro-optics, and micro-fluidics. However, miniaturization of DEAs is challenging because many of the materials and DEA fabrication methods used on the macro-scale cannot be adapted for micro-scale fabrication of DEAs. This thesis explores the feasibility of developing fabrication strategies for micro-scale DEAs by adapting micro-electromechanical systems (MEMS) technology. In addition, fabrication protocols for micro-scale DEAs have been developed. The other aspect of this thesis is the design of bending DEAs. Benders are useful because for a given actuation strain, greater deflection can be observed by controlling the stiffnesses and thicknesses of different layers. A general guideline for designing bending DEA configurations such as unimorph, bimorph, and multilayer stacks was developed using a multilayer analytical model. The design optimization is based on the effect of thickness and stiffness of different layers on curvature, blocked force, and work. Complaint electrodes and their design are important for DEAs to enable the elastomer to stretch unrestricted. Thus, design criteria for the fabrication of crenellated electrodes and crenellated elastomers with electrodes were investigated. This guideline enabled design of structures with appropriate axial or bending stiffnesses based on the amplitude, angle, length, and thickness. Simple analytical equations for axial and bending stiffness for crenellated electrodes with different shapes were derived. In addition, numerical simulations of crenellated elastomer with stiff electrode were performe

Thin-film piezoelectric-on-substrate resonators and narrowband filters

A new class of micromachined devices called thin-film piezoelectric-on-substrate (TPoS) resonators is introduced, and the performance of these devices in RF and sensor applications is studied. TPoS resonators benefit from high electromechanical coupling of piezoelectric transduction mechanism and superior acoustic properties of a substrate such as single crystal silicon. Therefore, the motional impedance of these resonators are significantly smaller compared to typical capacitively-transduced counterparts while they exhibit relatively high quality factor and power handling and can be operated in air. The combination of all these features suggests TPoS resonators as a viable alternative for current acoustic devices. In this thesis, design and fabrication methods to realize dispersed-frequency lateral-extensional TPoS resonators are discussed. TPoS devices are fabricated on both silicon-on-insulator and thin-film nanocrystalline diamond substrates. The performance of these resonators in simple and low-power oscillators is measured and compared. Furthermore, a unique coupling technique for implementation of high frequency filters is introduced in which dual resonance modes of a single resonant structure are coupled. The measured results of this work show that these filters are suitable candidates for single-chip implementation of multiple-frequency narrow-band filters with high out-of-band rejection in a small footprint.Ph.D.Committee Chair: Farrokh Ayazi; Committee Member: James D. Meindl; Committee Member: John D. Cressler; Committee Member: Nazanin Bassiri-Gharb; Committee Member: Oliver Bran

Studies In Small Scale Thermal Convection

The effect of non-Fourier heat transfer and partial-slip boundary conditions in Rayleigh-Bénard are analyzed theoretically. Non-Fourier fluids possess a relaxation time that reflects the delay in the response of the heat flux to a change in the temperature gradient while the partial slip boundary condition assumes that the fluid velocity and temperature are not equal to that of the wall. Both non-Fourier and partial-slip effects become important when small-scale heat transfer applications are investigated such as convection around micro- and nano-devices as suggested by the extended heat transport equations from kinetic theory. Other applications are also known to exhibit one or both of these effects such as low-temperature liquids, nanofluids, granular flows, rarefied gases and polymer flows. Small scale effects are measured by the Knudsen number. From this, non-Fourier effects can be estimated, measured non-dimensionally by the Cattaneo number and modelled using the frame indifferent Cattaneo-Vernotte equation which is derived from Oldroyd’s upper-convected derivative. Linear stability of non-Fourier fluids shows that the neutral stability curve possesses a stationary Fourier branch and an oscillatory branch intersecting at some wave number, where for small relaxation time, no change in stability is expected from that of a Fourier fluid. As the relaxation time increases to a critical value, both stationary and oscillatory convection become equally probable. Past this value, oscillatory instability is expected to occur at a smaller Rayleigh number and larger wave number than for a Fourier fluid. Non-linear analysis of weakly non-Fourier fluids shows that near the onset of convection, the convective roll intensity is stronger than for a Fourier fluid. The bifurcation to convection changes from subcritical to supercritical as the Cattaneo number increases and the instability of the convection state for a non-Fourier fluid is shown to occur via a Hopf bifurcation at lower Rayleigh number and higher Nusselt number than for a Fourier fluid. When hydrodynamic slip and temperature jump boundary conditions are considered, a significant reduction in the minimum critical Rayleigh number and corresponding wave number are found. Depending on the sign used for second-order coefficients, critical conditions can be greater than or less than that for first-order boundary conditions