Microelectromechanical Systems Structures Research Articles

Partial Gap Transduced MEMS Optoacoustic Oscillator Beyond Gigahertz

Electrostatically actuated microelectromechanical system (MEMS) oscillators are limited to few megahertz–gigahertz range on account of transduction inefficiency at higher frequencies. Piezoelectric transduction affords lower motional impedances at high frequencies, however mass-loading on account of metal electrodes imposes practical limits on the mechanical quality factor of piezoelectric resonators in the gigahertz frequency regime. In this paper, we present a silicon optoacoustic oscillator operating at 2.05 GHz with signal power 18 dBm and phase noise −80 dBc/Hz at 10-kHz offset from carrier. We employ displacement amplification and partial air gap capacitive transduction to enhance the transduction efficiency. An unconventional photolithography step is performed on a released MEMS structure, which greatly simplifies the fabrication process and allows electrical contact with the electrodes. Built-in nonlinear optomechanical modulation provides noiseless up-conversion of the oscillation signal all the way up to 16.4 GHz with −45-dBm signal power. We develop a phase noise model for the oscillator and identify the photodetector shot noise to be the dominant noise source. Using a high gain and low noise avalanche photodetector enables reduction of the far-from-carrier phase noise floor by 15dB. The phase noise model provides insights into understanding the influence of laser detuning on the oscillator noise performance, which has not been studied to date. [2013-0291]

(Invited) A Sub-1G MEMS Sensor

This paper presents our recent research progress of highly-sensitive micro-electro-mechanical systems (MEMS) capacitive accelerometers for monitoring micro motion and vibration at the acceleration scale of lower than 1 G (G = 9.8 m/s2 : gravitational acceleration). To reduce the Brownian noise (BN ) on the sensors, we have employed high-density metal for proof mass, and implemented the MEMS structures. Proof-of-concept inertial sensors have been developed by gold electroplating, and we have estimated the BN to be below 10 μG/Hz1/2, which is nearly one order of magnitude smaller than those of the conventional MEMS accelerometers with the same proof mass size. The experimental results indicate that the proposed sensor concept would be a promising way to further improve the sensitivity and miniaturization of micro-scale motion sensors, which could advance the emergence of new MEMS applications.

Vibration Damping Materials and Their Applications in Nano/Micro-Electro-Mechanical Systems: A Review.

The present review explores an overall view of the vibration damping materials ranging from traditionally used viscoelastic materials for macroscale damping to hybrid thin film heterostructures for micro-electro-mechanical systems (MEMS). Vibration damping materials like rubbers, polymers, metals, metal-matrix composites and smart materials are reviewed in terms of damping capacity, stiffness, mechanical strength and figure of merit. Nanoscale shape memory alloys, piezoelectric materials, carbon nanotubes, their composites and thin films are promising materials for future nanoscale damping devices. The main focus of this article is on our development of new vibration damping approach for MEMS structures comprising of ferroelastic/ferroelastic thin film heterostructures. For the first time, nanoindentation has been explored as an alternative tool to evaluate the damping capability of actual components (e.g., thin films for MEMS) where production of dynamic mechanical analyzer (DMA) test samples is not feasible. A comprehensive insight on the existing vibration damping materials and our new approach would definitely trigger some important applications in nano- and micro-electro-mechanical systems.

In situmeasurement of MEMS topography based on phase-shifting interferometry

Currently, phase-shifting interferometry is widely used in MEMS (micro-electro-mechanical system) microsurface topography measurements, and an expensive and high-precision piezoelectric transducer (PZT) is often necessary to realize phase-shift operation. Because of the feature of a MEMS structure which always has a flat substrate, a practical algorithm to calculate phase shifts by fast Fourier transformation (FFT) from gathered interference fringes of the substrate is presented, then microsurface topography can be reconstructed according to the obtained phase shifts. By means of the presented algorithm, an expensive and high-precision PZT is unnecessary and the phase-shift operation can even be carried out by rotating the fine focus adjustment knob. The accuracy and feasibility of the method have been verified by experiments. Experiments indicated that the presented method can satisfy the needs of in situ MEMS topography measurements and is very simple.

Evaluation of MEMS Structures with Directional Characteristics Based on FRAT and Lifting Wavelet

Steps and grooves, which have typical directional characteristic, are two main functional structures of MEMS (Micro-Electro-Mechanical Systems). This paper proposes a method for analysis and evaluation of MEMS steps and grooves based on finite radon transform (FRAT) and lifting wavelet. The method consists of three steps. Firstly, FRAT is adopted to detect the directional characteristic of a MEMS structure. Secondly, on the basis of the directional characteristic obtained, the profiles of the MEMS structure are analyzed by lifting wavelet. Finally, Histogram-fitting is employed for areal evaluation of a MEMS structure. Simulated and experimental results show that MEMS structures with directional characteristic can be extracted and evaluated by the method effectively.

Thermal MEMS actuator operation in aqueous media/seawater: Performance enhancement through atomic layer deposition post processing of PolyMUMPs devices

A method to enhance thermal microelectromechanical systems (MEMS) actuators in aqueous media by using dielectric encapsulation layers is presented. Aqueous media reduces the available mechanical energy of the thermal actuator through an electrical short between actuator structures. Al2O3 and TiO2 laminates with various thicknesses were deposited on packaged PolyMUMPs devices to electrically separate the actuator from the aqueous media. Atomic layer deposition was used to form an encapsulation layer around released MEMS structures and the package. The enhancement was assessed by the increase of the elastic energy, which is proportional to the mechanical stiffness of the actuator and the displacement squared. The mechanical stiffness of the encapsulated actuators compared with the noncoated actuators was increased by factors ranging from 1.45 (for 45 nm Al2O3 + 20 nm TiO2) to 1.87 (for 90 nm Al2O3 + 40 nm TiO2). Displacement measurements were made for all laminate combinations in filtered tap water and seawater by using FFT based displacement measurement technique with a repeatability of ∼10 nm. For all laminate structures, the elastic energy increased and enhanced the actuator performance: In seawater, the mechanical output energy increased by factors ranging from 5 (for 90 nm Al2O3) to 11 (for 90 nm Al2O3 + 40 nm TiO2). The authors also measured the long-term actuator stability/reliability in seawater. Samples were stored for 29 days in seawater and tested for 17 days in seawater. Laminates with TiO2 layers allowed constant operation over the entire measurement period.

Synthesis of Anodic Oxide Thin Films on Si{100} Wafers and Their Characterization in TMAH for MEMS

Silicon dioxide thin films are most widely used for different applications in microelectromechanical system (MEMS) fabrication such as etch mask, structural and sacrificial layers. Anodic oxidation method offers several advantages over other techniques, such as room temperature deposition, low cost, simple experiment setup, etc. In the present work, anodic oxidation of silicon is employed to grow oxide thin films at wafer-scale at room temperature. As-grown oxide films are characterized using ellipsometry, scanning electron microscope (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, optical microscope and 3D measuring laser microscope. Thickness uniformity and refractive index are measured using ellipsometry. FTIR spectra are used to conform the absence of hydroxyl and water absorption in the films. In order to exploit the application of anodic oxide in MEMS, the etch rates and the corner undercutting are studied in different concentration TMAH solutions at various temperatures. The as-grown oxide thin films with a thickness of about 155 nm are successfully demonstrated for the realization of various shapes freestanding MEMS structures such as rectangular cantilever beams, diaphragms, etc. using TMAH solution. Furthermore, as-grown oxide is used as mask to fabricate cavities with different shape and sizes in silicon substrate using wet anisotropic etching.

A plasma processing combined with trench isolation technology for large opening of parylene based high-aspect-ratio microstructures

This paper presents a parylene-based trench isolation method with which to create high-aspect-ratio microelectromechanical system structures. The silicon-based structures were electrically isolated by supported parylene beams, and the movements of suspended silicon structures used to sense or actuate were not confined by trench openings. The proposed process is a simple, low-temperature, and dry-etching fabrication process for structure releasing and electrical isolation, and does not involve the LPCVD, PECVD, ion implantation, sputtering processes, or sandwiched oxide/polysilicon/metal isolation traditional methods require. The parylene-based electrical isolated beams can be created through multiple steps of parylene deposition/remove inside a silicon mold. By enhancing the microtrenching effect, the suspended structure does not thoroughly remove the floor polymer inside the trenches when the trench aspect ratio is relatively small. The steps of trench etching, sidewall protection, structure release, and photoresist stripping can be finished by modifying the etching or passivation steps in the BOSCH process, and it can be integrated as the macro commands of ICP etcher. The single-run of the ICP-RIE process can automatically finish the suspended silicon structure creation. By using the proposed process, a test device 50μm thick and with a maximal trench aspect ratio of 10 and a maximal suspended structure width of 40μm was created. The proposed process can be used to fabricate devices for large in-plane displacement, increasing the sensitivity of the sensors and actuators.

Micro-masonry for 3D additive micromanufacturing.

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called 'inks') from a substrate where they are generated to a different substrate by utilizing elastomeric stamps. Transfer printing enables the integration of heterogeneous materials to fabricate unexampled structures or functional systems that are found in recent advanced devices such as flexible and stretchable solar cells and LED arrays. While transfer printing exhibits unique features in material assembly capability, the use of adhesive layers or the surface modification such as deposition of self-assembled monolayer (SAM) on substrates for enhancing printing processes hinders its wide adaptation in microassembly of microelectromechanical system (MEMS) structures and devices. To overcome this shortcoming, we developed an advanced mode of transfer printing which deterministically assembles individual microscale objects solely through controlling surface contact area without any surface alteration. The absence of an adhesive layer or other modification and the subsequent material bonding processes ensure not only mechanical bonding, but also thermal and electrical connection between assembled materials, which further opens various applications in adaptation in building unusual MEMS devices.

New laser excitation method for modal analysis of microstructure

A novel impulse laser excitation technique to determine the dynamic response of micro-electro-mechanical systems (MEMS) has been investigated. During the laser excitation experiments, MEMS structures are excited by the wide-band impact force created by the laser–target interaction, and Laser Doppler Vibrometer (LDV) is introduced to measure the vibration velocity of MEMS structures. A distinguishing characteristic of the methodology is that both the excitation and measurement are non-contact, which is especially suitable for the testing of MEMS microstructures that are not easily accessible. This novel excitation method and MEMS modal analysis system are verified by experiments on various cantilever beams. The results show that the laser excitation is capable of exciting the first three modes of cantilevers.

Development of topologically structured membranes of aluminum oxide

In recent years, nanomembranes have become one of the most widely used construction material for ultrasensitive and ultrathin applications in micro-electromechanical systems (MEMS) and other sensor structures due to their remarkable mechanical properties. Among these, the mechanical stability is of particular importance. We present an approach to the analysis of the stability of nanostructured anodic aluminum oxide free membranes subjected to mechanical bending. The membranes tested were with a thickness of 500 nm to 15 urn in various topological shapes; we describe the technological schemes of their preparation. Bends were applied to membranes prepared by using a selective process of etching and anodizing. The results of the preparation of the membranes are discussed, together with the influence of the angle of deflection, and the number of bendings. The results obtained can be used in designing MEMS structures and sensors which use nanostructured anodic aluminum oxide.

Directional acoustic response of a silicon disc‐based microelectromechanical systems structure

A microelectromechanical systems (MEMS)-based structure capable of operating mechanically as a directional acoustical sensor is presented. The structure, fabricated through the commercially available SOIMUMPS foundry process, consists of two circular discs attached to a central suspension beam, fixed at both ends. The design of the structure resembles other directional MEMS microphones that mimic the directional hearing organ of the parasitoid fly, Ormia ochracea. Modal analysis and mechanical acoustic directionality analysis using both laser Doppler vibrometry and finite element modelling have been implemented. It is demonstrated that this coupled MEMS structure exhibits an acoustic directional response, with a one-to-one relationship between the relative vibration amplitudes of the two coupled discs and the angle of sound, from −75° to +60°.

Shock Impact Reliability and Failure Analysis of a Three-Axis MEMS Gyroscope

This paper presents the reliability assessment of a three-axis microelectromechanical systems (MEMS) gyroscope subjected to various shock loading conditions. The reliability tests include three different impact orientations and several acceleration levels of shock impact, ranging from 1500 to 15 000 g. The package failure and functional failure of the MEMS devices are studied separately. The failure analysis shows package failures of the MEMS device at a shock level above 8000 g and the functional failures of the device caused by stiction or fractures in the comb structure at a moderate shock level around 4000 g. To have a comprehensive understanding of the failure modes and predict the failure modes, dynamic finite element analyzes with direct integration are employed to investigate the nonlinear responses of the MEMS device under shock impact loadings. Internal collisions between the movable elements and the stationary parts are modeled by contact definitions. The simulation results, such as the calculated structural deformation and stress distributions, can be used to predict potential failure sites and offer explanations to the observed package failures and comb structure fractures. Furthermore, the locations of possible stiction inside the MEMS structure are also predicted by the simulation results.

Lapping assisted dissolved wafer process of silicon for MEMS structures

Dissolved wafer process (DWP) is being extensively used to fabricate complex micro-electro-mechanical system (MEMS) structures. Etching non-uniformity, increased surface roughness and duration of DWP is often influence MEMS devices yields. This paper presents a modified DWP involving lapping and polishing followed by chemical etching of silicon to release MEMS based structure. The lapping experiments are performed using silicon-carbide (SiC) and alumina (Al2O3) abrasive. The polishing of the silicon samples is also done. The lapped and polished surfaces are compared with etched silicon surfaces in KOH and EDP solutions. The lapping-polishing process is found to be 2.5 (Al2O3)–3 (SiC) times faster than a standard etching processes based on KOH and EDP solutions. The average roughness (Ra) of the lapped–polished silicon surfaces are found to be 19.2 and 32.9 nm corresponding to SiC and Al2O3 abrasive respectively. The Ra value of EDP and KOH etched silicon surfaces are found to be 16.2 and 238.3 nm respectively. Based on the lapping—polishing results, SiC based lapping followed by polishing of silicon surface can be used as an alternate of etching of silicon during DWP. In this paper, a two-step DWP, involving lapping-polishing followed by EDP chemical etching of silicon, is used to fabricate suspended comb-type microaccelerometer structure.

Wearable, Human‐Interactive, Health‐Monitoring, Wireless Devices Fabricated by Macroscale Printing Techniques

Wearable human‐interactive devices are advanced technologies that will improve the comfort, convenience, and security of humans, and have a wide range of applications from robotics to clinical health monitoring. In this study, a fully printed wearable human‐interactive device called a “smart bandage” is proposed as the first proof of concept. The device incorporates touch and temperature sensors to monitor health, a drug‐delivery system to improve health, and a wireless coil to detect touch. The sensors, microelectromechanical systems (MEMS) structure, and wireless coil are monolithically integrated onto flexible substrates. A smart bandage is demonstrated on a human arm. These types of wearable human‐interactive devices represent a promising platform not only for interactive devices, but also for flexible MEMS technology.

Wavelength specific excitation of gold nanoparticle thin-films

Advances in microelectromechanical systems (MEMS) continue to empower researchers with the ability to sense and actuate at the micro scale. Thermally driven MEMS components are often used for their rapid response and ability to apply relatively high forces. However, thermally driven MEMS often have high power consumption and require physical wiring to the device. This work demonstrates a basis for designing light-powered MEMS with a wavelength specific response. This is accomplished by patterning surface regions with a thin film containing gold nanoparticles that are tuned to have an absorption peak at a particular wavelength. The heating behavior of these patterned surfaces is selected by the wavelength of laser directed at the sample. This method also eliminates the need for wires to power a device. The results demonstrate that gold nanoparticle films are effective wavelength-selective absorbers. This “hybrid” of infrared absorbent gold nanoparticles and MEMS fabrication technology has potential applications in light-actuated switches and other mechanical structures that must bend at specific regions. Deposition methods and surface chemistry will be integrated with three-dimensional MEMS structures in the next phase of this work. The long-term goal of this project is a system of light-powered microactuators for exploring cellular responses to mechanical stimuli, increasing our fundamental understanding of tissue response to everyday mechanical stresses at the molecular level.

High Resolution Position Monitoring of Suspended MEMS towards Biological and Chemical Sensors

ABSTRACTWe present an integrated readout technique for interrogating the suspension height of micro-electro-mechanical systems (MEMS) structures. This readout technique is envisaged to be useful in applications such as MEMS-based biological and chemical sensing, where it is necessary to obtain the accurate position of a MEMS beam. The approach is based on the suspended MEMS structure modulating light transmission in an underlying optical waveguide via Fabry-Perrot phenomena. The performance of the technique is predicted via finite difference time domain (FDTD) simulations the results of which are confirmed by experimental measurements.

Evaluation of Model ARX For Elastic Masses MEMS Using the Indexes RMSE, AIC, and BIC

This paper proposes the use of gray box mathematical modeling, through Identification Systems theory, for obtaining the behavioral performance of micro electro mechanical systems (MEMS) device. The target structures are two elastic masses, called double bridge and hinge, installed in a comb-drive actuator with simple typology and unidirectional displacement. The model structure used is the autoregressive with exogenous inputs (ARX), in which the parameters are estimated using the least squares method. The ARX estimated models, for both elastic masses, are compared to the MEMS structures simulated in the ANSYS computational tool, considering the relative error percentage and cross-validation indicators, and the root mean square error, Akaike information criterion, and Bayesian information criterion indexes. From the simulation results, it was observed that the ARX estimated models, for both elastic masses, presented a satisfactory accuracy. Therefore, the use of mathematical modeling can be suggested for MEMS device designers in order to contribute to the behavioral performance and the quality of these microstructures, reducing the production costs and manufacturing time.

Control of stress in tantalum thin films for the fabrication of 3D MEMS structures

Sputtered tantalum (Ta) thin-films subjected to high compressive stress have been used for fabricating out-of-plane microelectromechanical systems (MEMS) clamped–clamped beams. The as-deposited film stress has been investigated as a function of sputtering conditions (pressure and power) in order to determine the optimal deposition parameters for achieving a compressive state. A variation of the as-deposited film stress has been observed as a function of time at atmospheric conditions and reduced by ∼85% using annealing in oxygen atmosphere at 100 °C. By surface-micromachining Ta thin-films deposited in compressive state (beyond the critical buckling load), buckled out-of-plane beams have been fabricated with lengths in the range of 500 μm−5.8 mm and central out-of-plane deflection up to ∼60 μm. A final stress of ∼−850 MPa has been calculated from the measured beams' deflection. The modal behavior of the buckled out-of-plane beams has been investigated with optical measurements and finite element simulations. The large curvature and high residual stress experienced by the beams have shown to affect the usual order of consecutive bending mode shapes. The devices presented can be utilized for implementing cutting-edge three-dimensional MEMS.

Preparation of sacrificial layer for MEMS devices by lift-off technology

To simplify the preparation of sacrificial layer in micro-electromechanical system structure, new processes have been developed. By using lift-off technology, sacrificial layer was selectively deposited into the pit that prepared for sacrificial layer releasing. Then a short time polishing process was used to remove the burrs around the pit. This method offers several advantages including reducing the difficulty of polishing and hence processing cost. Methods and apparatuses to provide a continuously smoothly sacrificial layer surface in the pit were disclosed. However, it is important to control the homogeneity of filling and avoid the shadow effect at the corner of the pit. The unpadded regions would form a steep step and cause the cracking of structure layer. This problem was discussed and solved by rotating the tilted wafer during sacrificial material deposition. The surface roughness after the preparation process of sacrificial layer was measured. The root mean square roughness of sacrificial layer was 1.247 nm and reduced to 0.861 nm after a short time polishing process. Finally, a film bulk acoustic wave resonator with stacked Mo/AlN/Mo films had been fabricated on the sacrificial layer based on this method.