Microelectromechanical Systems Scale Research Articles

Modeling and Reliability Analysis of MEMS Gyroscope Rotor Parameters under Vibrational Stress.

Vibrational environments can cause drift or changes in Micro-Electro-Mechanical System (MEMS) gyroscope rotor parameters, potentially impacting their performance. To improve the effective use of MEMS gyroscopes, this study introduced a method for evaluating the reliability of parameter degradation under vibration. We analyzed the working principle of MEMS gyroscope rotors and investigated how vibration affects their parameters. Focusing on zero bias and scale factor as key performance indicators, we developed an accelerated degradation model using the distributional assumption method. We then collected degradation data for these parameters under various vibration conditions. Using the Copula function, we established a reliability assessment approach to evaluate the degradation of the MEMS gyroscope rotor's zero bias and scale factor under vibration, enabling the determination of reliability for these parameters. Experimental findings confirmed that increasing stress levels lead to reduced failure times and increased failure rates for MEMS gyroscope rotors, with significant changes observed in the zero bias parameter. Our evaluation method effectively characterizes changes in the reliability of the MEMS gyroscope rotor's scale factor and zero bias over time, providing valuable information for practical applications of MEMS gyroscopes.

A miniaturized statically balanced compliant mechanism for on-chip ultralow wide-bandwidth vibrational energy harvesting

Abstract. This research demonstrates a miniaturized statically balanced compliant mechanism (SBCM) at the micro-electromechanical systems (MEMS) scale. The primary objective is to integrate the MEMS-scale SBCM on chip as the fundamental structure of vibrational energy harvesters for powering low-energy-cost sensors and circuits. The static and dynamic characteristics of the micro-scale SBCM are investigated based on a 2D finite element analysis (FEA) model in COMSOL Multiphysics®. Static balancing is achieved by finely tuning the geometric parameters of the FEA SBCM model. The analytical, numerical, and FEA results confirm that the MEMS-scale SBCM is sensitive to ultralow wide-bandwidth excitation frequencies with weak accelerations. This micro-scale SBCM structure provides a structural solution to effectively lower the working frequencies of MEMS vibrational energy harvesters to ultralow ranges within a wide bandwidth. It overcomes the working frequency limit imposed by the size effect. This would significantly improve the dynamic performance of vibrational energy harvesters at the MEMS scale. In addition, a conceptual structure of the MEMS-scale SBCM is preliminary proposed for the integration of piezoelectric materials by MEMS technologies for vibrational energy harvesting.

Capture the early stage of shear banding for bulk metallic glass sheet for MEMS applications

Bulk metallic glasses (BMGs) are a class of promising candidate materials which applied in Microelectromechanical Systems (MEMS). However, a suitable mechanical parameter and an effective measurement approach are still lacked to characterize the onset of plastic deformation exactly for BMG sheets in bending. In this work, bending proof strength with permanent strain at the level of 0.005% (σp,0.005%) was proposed to represent the exactly onset of yielding for BMG sheets. This mechanical parameter was measured by a series of stepwise cyclic loading–unloading tests, which are particular suitable for timely capture the earlier stage of shear banding for BMG sheets within MEMS scale, thus showing the significant superiorities to monotonic loading test and conventional cyclic tests which needed to be equipped with the deflectometers. The σp,0.005% of Zr61Ti2Cu25Al12 (ZT1) BMG sheet with thickness of 100 μm was determined to be 1900±10 MPa, and physically correlates to the propagation stress of embryonic shear band instead of the nucleation stress. In addition, a quantitative correlation between permanent strain and shear offsets on BMG sheet surface has established, thus the permanent strain of thinner BMG sheets can be attained by a simple approach, and without relying on the conventional contact-type deflectometers with high accuracy. The σp,0.005% can be used as a more reliable and rational design parameter for thinner BMG sheets which applied in MEMS fields.

Acoustic projection by electro-osmosis.

Electro-osmosis (EO) is a non-traditional pumping and transduction mechanism with the ability to project acoustic energy in fluids. This investigation experimentally validates the influence of zeta potential, a well-studied physical characteristic used for quantifying the efficacy of an EO pump, on generation of sound pressure level. Acoustic signals of discrete frequencies were observed from 130 Hz to over 150 kHz. EO-type projectors are an attractive technology in that it does not contain moving parts, can be fabricated using a variety of materials, is intrinsically resilient to effects of hydrostatic pressure, and may be designed on the micro-electromechanical system scale.

The Research on Actuation Performance of MEMS Safety-and-Arming Device with Interlock Mechanism.

Micro-electromechanical systems (MEMS) safety-and-arming (S&A) device shows great potential in munition miniaturization, and it can be seen as the symbol of the fourth generation of weapons systems. In this paper, the design, fabrication, and actuation performance of a silicon based S&A device is presented. It is a multilayer stacked device, which is composed of the cover plate, the actuation chip, and the barrel plate. The electro-thermal principle is investigated in MEMS scale. With 11 V driving voltages, the structure of V-shape actuator, pawl, and slider can generate 100 μm and 45 μm displacement, and realize pulling, disengaging, and reengaging to change the device from the safety position into armed position smoothly (550 μm displacement). The rack and interlock mechanism formed by the pawl and slider gives the device the features of linear output displacement, low power consumption, input signal recognition, and sustained displacement. The 20,000 g setback acceleration is applied, and no structure damage can be found after the impact, which indicates the good anti-load ability of the MEMS S&A device. In order to solve the contradiction between the functional structure and the fabrication process, different structures are designed separately on different wafers. Both silicon and SOI wafers are used in the fabrication process, and the S&A device has been minimized into 8.5 mm × 8.5 mm × 0.8 mm successfully.

High Figure of Merit Nonlinear Microelectromagnetic Energy Harvesters for Wideband Applications

We report a new approach for designing high-performance microelectromechanical system (MEMS) electromagnetic energy harvesting devices, which can operate at low frequency (<;1 kHz) over the ultrawide bandwidth of 60-80 Hz. The output from the devices is increased significantly at a low optimized load and this overall enhancement in performances is benchmarked using a power integral (Pf) figure-of-merit. The experimental results show that the efficient nonlinear designs produce large Pf values, giving rise to one of the highest normalized Pf densities among the reported MEMS scale nonlinear energy harvesting devices. This improvement is achieved by suitably designing the nonlinear spring architectures, where the nonlinearity arises from the stretching strain of the specifically designed fixed-fixed configured spring arms under large deflections and gives rise to wideband output response. Different fundamental modes of the mechanical structures are brought relatively close, which further widens the power-frequency response by topologically varying the spring architectures and by realizing the same using the thin silicon-on-insulator substrate using MEMS processing technology. In addition, we have used the magnet as proof mass to increase the output in contrary to conventional approach of using the coil as the proof mass in micro-electromagnetic energy harvesters. The high performance obtained from the MEMS energy harvesters with integrated double layer micro-coil is compared with the same using wire wound copper coil. The experimentally obtained results are qualitatively explained by using a finite-element analysis of the designed structures.

Wafer-Level Vacuum Packaging by Thermocompression Bonding Using Silver after Fly-Cut Planarization

With the development of micro-electromechanical systems (MEMS), the integration of MEMS and large scale integrated (LSI) circuits together with hermetic packaging is getting more important for the reduction of device footprint and the improvement of performance. Wafer bonding technology is a good choice for such integration because it allows separate design and fabrication of MEMS and LSI. As a kind of wafer bonding, thermocompression bonding can realize electric interconnection as well as hermetic sealing with a smaller bond frame and still keeps sufficient bonding strength. The thermocompression bonding based on metals, Au, Al and Cu is well known. However, Cu needs pretreatments for oxide removal; Al requires high temperature and pressure which may cause damage to temperature–sensitive devices such as LSI. Au can be bonded at lower temperature with minimal surface pretreatments. However, thermocompression bonding using thin Au film cannot achieve vacuum sealing for micro-structured or non-flat wafers such as MEMS. Our previous work solved most of these problems using electroplated Au bonding frames after fly-cut planarization. However, for lower cost, Ag is proposed which is much cheaper and shows superior thermal and electrical conductivity. Whereas, the oxidation affinity of Ag may cause bonding problems related to a surface oxide layer. The diffusion of Ag into Si is also concerned for deteriorating device characteristics. In this study, we developed a novel inexpensive hermetic thermocompression bonding method using electroplated Ag bonding frames planarized by fly cutting. A barrier layer was used to block the Ag diffusion into a Si substrate during the bonding process. Extremely high shear strength and vacuum sealing were demonstrated at relatively low bonding temperature and bonding pressure. Usually minority carrier lifetime in LSI would be severely reduced due to the contamination of metal atoms. In order to avoid the diffusion of Ag atoms from the bonding areas into a Si substrate during the bonding process, a titanium nitride (TiN) barrier layer was adopted because of its excellent barrier property in conjunction with good thermal stability and low contact resistance. A layered structure (20nm Ti/ 100nm TiN/ 20nmTi/ 100nmAg) was deposited on Si substrate by sputtering at room temperature. This test sample was heated up to the bonding temperature, held for 1 h. Secondary ion mass spectroscopy (SIMS) was carried out through stacked layers. No detectable Ag atom concentration was found in the Si substrate (Fig 1), suggesting that the diffusion was stopped by TiN. Next, we verified the possibility of Ag for thermocompression bonding. A Si substrate was coated with the stacked layer mentioned above. Then the Ag frames of 39μm width and 10μm height were electroplated using photoresist mold. On one substrate, 64 dies were fabricated. Subsequently the bump frames were cut to 4.3μm in height by fly cutting with a diamond bit. The planarization step played a decisive role in our method. Generally the electroplated surface was rough. However after planarization, even on non-flat wafers, Ag bonding frame surface became smooth to establish enough contact surface for sufficient atom diffusion. Additionally under the shear strain of the diamond bit, small grain size was obtained which improved grain boundary diffusion. Moreover, native surface oxide or any other surface contamination was removed. The substrate with the planarized electroplated Ag frames and another Si substrate with sputter deposited Ag thin film pads were treated by Ar plasma and bonded in vacuum at 350~400℃ with a loading pressure of 54MPa. After the bonded wafer was diced into dies, die-shear tests were conducted to evaluate the shear strength. The achieved mean shear strength was up to 200 MPa. The samples fractured across the bonding material and in no case at the bonding interface. The lateral expansion of bonding frames due to pressure was less than 5%. From the cross-sectional scanning electron microscopy (SEM) of the bonded area, there was no visible bonding interface anymore (Fig 2). This suggested the newly formed oxide after planarization didn’t affect the bonding significantly. The thin oxide layer probably diffused into the bulk through grain boundaries. Additionally, vacuum sealing was successfully demonstrated by the deflection of Si thin diaphragms fabricated through an SOI wafer due to pressure difference across the diaphragms. After bonding process, the membrane’s clear deflection was observed at atmosphere and confirmed by topography measurement system (Fig 3). The proposed process using the planarized electroplated Ag frames was useful for both wafer-level heterogeneous integration and hermetic packaging and versatile for various microsystems. Figure 1

Multiscale modelling framework for the fracture of thin brittle polycrystalline films: application to polysilicon

Micro-electro-mechanical systems (MEMS) made of polycrystalline silicon are widely used in several engineering fields. The fracture properties of polycrystalline silicon directly affect their reliability. The effect of the orientation of grains on the fracture behaviour of polycrystalline silicon is investigated out of the several factors. This is achieved, firstly, by identifying the statistical variation of the fracture strength and critical strain energy release rate, at the nanoscopic scale, over a thin freestanding polycrystalline silicon film having mesoscopic scale dimensions. The fracture stress and strain at the mesoscopic level are found to be closely matching with uniaxial tension experimental results. Secondly, the polycrystalline silicon film is considered at the continuum MEMS scale, and its fracture behaviour is studied by incorporating the nanoscopic scale effect of grain orientation. The entire modelling and simulation of the thin film is achieved by combining the discontinuous Galerkin method and extrinsic cohesive law describing the fracture process.

MEMS-Scale Turbomachinery Based Vacuum Roughing Pump

This study forms part of a program to develop a micro-electro-mechanical systems (MEMS) scale turbomachinery based vacuum pump and investigates the roughing portion of such a system. Such a machine would have many radial stages with the exhaust stages operating near atmospheric conditions while the inlet stages operate at near vacuum conditions. In low vacuum such as those to the inlet of a roughing pump, the flow can still be treated as a continuum; however, the no-slip boundary condition is not accurate. The Knudsen number becomes a dominant nondimensional parameter in these machines due to their small size and low pressures. As the Knudsen number increases, slip-flow becomes present at the walls. The study begins with a basic overview on implementing the slip wall boundary condition in a commercial code by specifying the wall shear stress based on the mean-free-path of the gas molecules. This is validated against an available micro-Poiseuille classical solution at Knudsen numbers between 0.001 and 0.1 with reasonable agreement found. The method of specifying the wall shear stress is then applied to a generic MEMS scale roughing pump stage that consists of two stators and a rotor operating at a nominal absolute pressure of 500 Pa. The zero flow case was simulated in all cases as the pump down time for these machines is small due to the small volume being evacuated. Initial transient two-dimensional (2D) simulations are used to evaluate three boundary conditions, classical no-slip, specified-shear, and slip-flow. It is found that the stage pressure rise increased as the flow began to slip at the walls. In addition, it was found that at lower pressures the pure slip boundary condition resulted in very similar predictions to the specified-shear simulations. As the specified-shear simulations are computationally expensive it is reasonable to use slip-flow boundary conditions. This approach was used to perform three-dimensional (3D) simulations of the stage. Again the stage pressure increased when slip-flow was present compared with the classical no-slip boundaries. A characteristic of MEMS scale turbomachinery are the large relative tip gaps requiring 3D simulations. A tip gap sensitivity study was performed and it was found that when no-slip boundaries were present the pressure ratio increased significantly with decreasing tip gap. When slip-flow boundaries were present, this relationship was far weaker.

Single-crystal-silicon-based microinstrument to study friction and wear at MEMS sidewall interfaces

Since the advent of microelectromechanical systems (MEMS) technology, friction and wear are considered as key factors that determine the lifetime and reliability of MEMS devices that contain contacting interfaces. However, to date, our knowledge of the mechanisms that govern friction and wear in MEMS is insufficient. Therefore, systematically investigating friction and wear at MEMS scale is critical for the commercial success of many potential MEMS devices. Specifically, since many emerging MEMS devices contain more sidewall interfaces, which are topographically and chemically different from in-plane interfaces, studying the friction and wear characteristics of MEMS sidewall surfaces is important. The microinstruments that have been used to date to investigate the friction and wear characteristics of MEMS sidewall surfaces possess several limitations induced either by their design or the structural film used to fabricate them. Therefore, in this paper, we report on a single-crystal-silicon-based microinstrument to study the frictional and wear behavior of MEMS sidewalls, which not only addresses some of the limitations of other microinstruments but is also easy to fabricate. The design, modeling and fabrication of the microinstrument are described in this paper. Additionally, the coefficients of static and dynamic friction of octadecyltrichlorosilane-coated sidewall surfaces as well as sidewall surfaces with only native oxide on them are also reported in this paper.

Macro Machined Photoacoustic Non-Resonant Cell

Photoacoustic spectroscopy (PAS), which is widely used in gas sensing applications, is capable of a wide dynamic range, simplicity of use, high selectivity and sensitivity and also possesses novel detection characteristics when the cell are scaled into a micro system design. In a PAS system which is based on the photoacoustic effect, if the parameters of light power, chopper frequency, and absorption coefficient of the target analyte were decided, the level of the acoustic signal is inverse proportional to the radius square of the cell in which the sample is laid. So, miniaturizing the size of the cell can improve the system’s sensitivity. In our experiment, we initially designed a macro photoacoustic non-resonant cell (PNC) using stainless steel. As the macro cell is scaled to a dimension approaching the microelectromechanical systems (MEMS) level, the basic information gained from these initial tests is a special reference to fabricating a MEMS scale device. The non-resonance test conducted at 100 Hz demonstrated large signals (15.89mV) for CO2concentrations at 600 ppm and high signal-to-noise (S/N) values (~ 85:1). Then a structure of monolithic MEMS photoacoustic cavity was proposed which incorporate a three-wafer.

Analytical Modeling and Experimental Verification of the Vibrations of the Zigzag Microstructure for Energy Harvesting

This paper addresses an issue in energy harvesting that has plagued the potential use of harvesting through the piezoelectric effect at the micro-electro-mechanical systems (MEMS) scale. Effective energy harvesting devices typically consist of a cantilever beam substrate coated with a thin layer of piezoceramic material and fixed with a tip mass tuned to resonant at the dominant frequency of the ambient vibration. The fundamental natural frequency of a beam increases as its length decreases, so that at the MEMS scale the resonance condition occurs orders of magnitude higher than ambient vibration frequencies, rendering the harvester ineffective. Here, we propose a new geometry for MEMS scale cantilever harvesters with low fundamental frequencies. A “zigzag” geometry is proposed, modeled, and solved to show that such a structure would be able to vibrate near resonance at the MEMS scale. An analytical solution is presented and verified against Rayleigh’s method and is validated against a macroscale experiment. The analysis is used to provide design guidelines and parametric studies for constructing an effective MEMS scale energy harvesting device in the frequency range common to low frequency ambient vibrations, removing a current barrier.

Hints of atomic scale phenomena in adhesion and friction measurements with MEMS devices

Friction and wear on the nanometre scale currently hamper the use of microelectromechanical system (MEMS) devices with sliding surfaces. Based on the friction force microscope principle, we have developed MEMS devices that can be used to probe the tribological phenomena taking place in MEMS. We find that several measurements hint towards atomic scale processes that are relevant on the MEMS scale, even though MEMS themselves are much larger than atomic dimensions. The reason is that the tribological properties of MEMS devices originate from the phenomena taking place on the nanometre scale between the contacting/sliding rough MEMS surfaces. We have observed stochastic variation in the adhesion between oxygen plasma cleaned MEMS surfaces, and slow creep in MEMS contacts, both hinting at atomic scale processes that need to be understood to further complete our picture of MEMS adhesion, friction and wear.

English

The development of microelectromechanical systems (MEMS) technology and the suitability and compatibility of sizes of microactuators with the boundary layer thickness fueled the active flow separation control to gain the air flow momentum for the last few years. The present paper deals with the development of a robust, largedeflection, and large-force MEMS-based microballoon actuator for aerodynamic control of flight vehicles such as projectiles, micro air vehicles, aircrafts, etc. Experiments were carried out on the scaled-up models for different input pressure conditions to study the response of microballoon actuator. To evaluate the performance of the microballoon actuators, simulation studies on MEMS scale models were conducted in the CoventorWare environment. Simulation studies involving static and dynamic analyses have been carried-out on the microballoon actuator models. Various geometric and input parameters influencing the behaviour of the microballoon actuator were investigated. It has been observed that a maximum deflection of 1.2 mm to 1.5 mm can be achieved using microballoon actuators and the maximum operational frequency of 60 Hz to 80 Hz can be used for the operation of microballoon actuators. Also, the sizes of the microballoon actuators designed are compatible and suitable to be used in turbulent boundary layer of aerodynamic flight vehicles. Defence Science Journal, 2009, 59(6), pp.642-649 , DOI:http://dx.doi.org/10.14429/dsj.59.1570

Fracture Toughness and Subcritical Crack Growth in Polycrystalline Silicon

The fracture behavior of polycrystalline silicon in the presence of atomically sharp cracks is important in the determination of the mechanical reliability of microelectromechanical system (MEMS) components. The mode-I critical stress intensity factor and crack tip displacements in the vicinity of atomically sharp edge cracks in polycrystalline silicon MEMS scale specimens were measured via an in situ atomic force microscopy/digital image correlation method. The effective (macroscopic) mode-I critical stress intensity factor for specimens from different fabrication runs was 1.00±0.1MPa√m, where 0.1MPa√m is the standard deviation that was attributed to local cleavage anisotropy and grain boundary effects. The experimental near crack tip displacements were in good agreement with the linearly elastic fracture mechanics solution, which supports K dominance in polysilicon at the scale of a few microns. The mechanical characterization method implemented in this work allowed for direct experimental evidence of incremental (subcritical) crack growth in polycrystalline silicon that occurred with crack increments of 1-2μm. The variation in experimental effective critical stress intensity factors and the incremental crack growth in brittle polysilicon were attributed to local cleavage anisotropy in individual silicon grains where the crack tip resided and whose fracture characteristics controlled the overall fracture process resulting in different local and macroscopic stress intensity factors.

Modeling and Design of Microjet Actuators

A computational model is developed to aid the design of microelectromechanical systems (MEMS) for use in active turbulence control. The focus here is on micro-actuators and, in particular, a design employed by syntheticjet devices. This consists of a diaphragm within a cavity that, by its piezoinduced motion, creates an ejection of fluid through an orifice in the cavity’s lid. The diaphragm is modeled using classical thin-plate theory, with the stiffness of the attached piezodevice incorporated. For numerical economy, the fluid motion within the cavity is not modeled; instead, the pressure is calculated with the perfect gas law. However, in the orifice, where viscous forces are more dominant, one-dimensional Navier‐Stokes equations are solved. The actuator system is modeled in its entirety. All that is required to calculate the outlet jet velocity is the input voltage applied to the piezodevice. The numerical model is validated against experimental data for synthetic-jet devices and used to predict their optimal dimensions. An alternative mode of forcing the diaphragm is proposed that does not suffer from the drawbacks inherent in synthetic-jet operation at MEMS scale. This mode generates a jump in cavity pressure, creating a pufflike jet disturbance. This concept is explored with the aim of uncovering practical issues and simple design guidelines.

MEMS-based satellitemicropropulsion via catalyzed hydrogen peroxide decomposition

Microelectromechanical systems (MEMS) techniques offer greatpotential in satisfying the mission requirements for the nextgeneration of miniaturized spacecraft being designed by NASAand Department of Defense agencies. More commonly referred toas `nanosats', these spacecraft feature masses in the rangeof 10-100 kg and therefore have unique propulsion requirements.The propulsion systems must be capable of providing extremelylow levels of thrust and impulse while also satisfyingstringent demands on size, mass, power consumption and cost. Webegin with an overview of micropropulsion requirements and somecurrent MEMS-based strategies being developed to meet theseneeds. The remainder of the paper focuses on the progress beingmade at NASA Goddard Space Flight Center toward the developmentof a prototype monopropellant MEMS thruster which uses thecatalyzed chemical decomposition of high-concentration hydrogenperoxide as a propulsion mechanism. The products ofdecomposition are delivered to a microscale converging/diverging supersonic nozzle, which produces the thrustvector; the targeted thrust level is approximately 500 µN witha specific impulse of 140-180 s. Macroscale hydrogenperoxide thrusters have been used for satellite propulsion fordecades; however, the implementation of traditional thrusterdesigns on the MEMS scale has uncovered new challenges in fabrication, materials compatibility, and combustion andhydrodynamic modeling. A summary of the achievements of theproject to date is given, as is a discussion of remainingchallenges and future prospects.

Friction and wear of rotating pivots in MEMS and other small scale devices

Micro-electromechanical systems (MEMS) is a rapidly growing interdisciplinary technology dealing with the design and manufacture of miniaturised machines or moving (or micro) mechanical assemblies (MMAs) with major dimensions at the scale of tens, to perhaps hundreds, of microns. Because they depend on the cube of a representative dimension, component masses and inertia rapidly become small as size decreases whereas surface or tribological effects, which scale with the square of dimension, become increasingly important. In this paper, the effects of design, material properties and applied operating conditions on the evolution of both friction forces and dimensional and volumetric wear rates are explored for normally loaded rotating pivots of various geometries. It is demonstrated that if such MEMS–MMAs are to have acceptable lifetimes then loaded contacting surfaces will need to be of inherently low wear material pairings. The difference in scale between these small devices and conventional wear testing rigs also creates problems in matching conditions between micro- and macro-situations and suggests that wear data might usefully be obtained from suitably designed MEMS devices acting as miniature wear test machines.