Many Microelectromechanical Systems Research Articles

Residual stress tuning in UV- LIGA fabricated microstructures using deposition temperature and reverse pulse plating

This paper utilizes pulse reverse plating to control the residual stress in electroplated copper MEMS structures. The residual stress plays a significant role in many micro electro mechanical system (MEMS) devices and control (not just minimization) over it is desirable. We have tuned the stress either compressive or tensile for different plating parameters, without utilizing any additives in the bath or annealing in the process. We demonstrate the use of this technique using a UV-LIGA fabricated microstrain gauge, that is also utilized to measure the stress. We show that the stress can be tuned from a value of 364 MPa to –194 MPa by varying the pulse plating parameters like ratio of forward/reverse time and temperature.

Optimization of Quality, Reliability, and Warranty Policies for Micromachines under Wear Degradation.

This work presents an optimization technique to determine the inspection, warranty period, and preventive maintenance policies for micromachines suffering from degradation. Specifically, wear degradation is considered, which is a common failure process for many Micro-Electro-Mechanical Systems (MEMS). The proposed mathematical model examines the impact of quality control on reliability and the duration of the warranty period given by the manufacturer or the supplier to the customer. Each of the above processes creates implementation costs. All the individual costs are integrated into a single measure, which is used to build the model and derive the optimal parameters of the quality and maintenance policies. The implementation of various levels of the quality, warranty, and maintenance policies are compared with their optimum level options to highlight their contribution to the assurance and improving product quality. To the authors' best knowledge, the introduction of a warranty period is implemented for the first time in the open literature concerning this type of optimization model for MEMs and surely can bring additional advantages to their quality promotion strategy. The proposed optimization tool provides a comprehensive simultaneous answer to the optimal selection of all the values of the design variables determining the overall maintenance and quality management approach.

Improved anchor design for flat MEMS structure by suppressing deformation due to buried-oxide stress on silicon-on-insulator wafer

Silicon-on-insulator (SOI) wafers are used in many micro-electromechanical system (MEMS) devices because of their stable mechanical properties. However, one of the disadvantages of using the SOI configuration is residual stress deformation; strong residual stresses in the buried-oxide layer deform the device and deteriorate its performance. This could prevent the development of high-precision sensors and actuators. In recent years, a new deformation mode caused by this residual stress has been reported, involving tilting of the suspended MEMS structure owing to local deformation of the anchor. However, methods to solve this problem have not yet been proposed. In this study, we propose a simple T-shaped support as a solution to the tilt deformation. We verified its effects by finite element analysis and experiments; the proposed structure was observed to successfully reduce the tip displacement of a SOI-based cantilever by one-fourth. In addition, we applied the proposed method to a parallel-plate accelerometer as a demonstration to confirm that the sensitivity variations of the device reduced from 10.8% to 6.8% using the proposed support structure. Thus, we believe that the proposed design can contribute to the development of extremely accurate MEMS sensors and optical devices.

Thermal stress analysis for a CMOS-MEMS microphone with various metallization and materials

Many micro-electromechanical systems (MEMS) microphone structures are developing along with the growing market of portable and smart devices. Therein, the impact of thermal loading is one of the inevitable concerns that need to be considered from the viewpoint of microphone's reliability. This study focused on the thermal stress analysis for a complementary metal oxide semiconductor (CMOS)-MEMS microphone with various metallization and materials in diaphragm and back-plate under thermal loading; meanwhile, the weak points in the microphone structure under thermal loading were identified through simulation and experiment. In simulation, three-dimensional finite element (FE) models of the MEMS microphone with different metallization and materials in the diaphragm, including aluminum and copper, were built by commercial software ANSYS. The FE models were subjected to the thermal loading from −40 °C to 125 °C. In experiment, the failure mode of a CMOS-MEMS microphone chip by TSMC (Taiwan Semiconductor Manufacturing Company Limited) 0.18 μm CMOS process after 500 cycles of thermal loading from −40 °C to 125 °C was analyzed using the images obtained from scanning electron microscope (SEM). The results from simulation are consistent with those from experiment. The stress at the interface of the metal located at the bottom of diaphragm is higher than that at its top according to FE analysis and the stress concentration was found at the interface between the diaphragm and Si substrate in the microphone. The SEM images showed the delamination and crack locations of the microphone after thermal cycling test agreed with those from FE analysis. Moreover, diverse metallization and materials were studied by FE analysis for structural enhancement of the microphone. The results indicated that adopting Cu layer instead of Al layer in the diaphragm could decrease the thermal stress around 33% when under thermal loading. The weak spots of the CMOS MEMS microphone chip were found in this study and an effective enhanced approach was proposed for this microphone structure with alternative metallization and material.

Wafer-Level Vacuum Sealing by Transfer Bonding of Silicon Caps for Small Footprint and Ultra-Thin MEMS Packages

Vacuum and hermetic packaging is a critical requirement for optimal performance of many micro-electro-mechanical systems (MEMS), vacuum electronics, and quantum devices. However, existing packaging solutions are either elaborate to implement or rely on bulky caps and footprint-consuming seals. Here, we address this problem by demonstrating a wafer-level vacuum packaging method featuring transfer bonding of 25- $\mu \text{m}$ -thin silicon (Si) caps that are transferred from a 100-mm-diameter silicon-on-insulator (SOI) wafer to a cavity wafer to seal the cavities by gold–aluminum (Au–Al) thermo-compression bonding at a low temperature of 250 °C. The resulting wafer-scale sealing yields after wafer dicing are 98% and 100% with sealing rings as narrow as 6 and 9 $\mu \text{m}$ , respectively. Despite the small sealing footprint, the Si caps with 9- $\mu \text{m}$ -wide sealing rings demonstrate a high mean shear strength of 127 MPa. The vacuum levels in the getter-free sealed cavities are measured by residual gas analysis to be as low as 1.3 mbar, based on which a leak rate smaller than ${2.8}\times {10}^{-14}$ mbarL/s is derived. We also show that the thickness of the Si caps can be reduced to 6 $\mu \text{m}$ by post-transfer etching while still maintaining excellent hermeticity. The demonstrated ultra-thin packages can potentially be placed in between the solder bumps in flip–chip interfaces, thereby avoiding the need of through-cap-vias in conventional MEMS packages. [2018-0257]

Monostable Dynamic Analysis of Microbeam-Based Resonators via an Improved One Degree of Freedom Model.

Monostable vibration can eliminate dynamic bifurcation and improve system stability, which is required in many microelectromechanical systems (MEMS) applications, such as microbeam-based and comb-driven resonators. This article aims to theoretically investigate the monostable vibration in size-effected MEMS via a low dimensional model. An improved single degree of freedom model to describe electrically actuated microbeam-based resonators is obtained by using modified couple stress theory and Nonlinear Galerkin method. Static displacement, pull-in voltage, resonant frequency and especially the monostable dynamic behaviors of the resonators are investigated in detail. Through perturbation analysis, an approximate average equation is derived by the application of the method of Multiple Scales. Theoretical expressions about parameter space and maximum amplitude of monostable vibration are then deduced. Results show that this improved model can describe the static behavior more accurately than that of single degree of freedom model via traditional Galerkin Method. This desired monostable large amplitude vibration is significantly affected by the ratio of the gap width to mircobeam thickness. The optimization design results show that reasonable decrease of this ratio can be beneficial to monostable vibration. All these analytical results are verified by numerical results via Differential Quadrature method, which show excellent agreement with each other. This analysis has the potential of improving dynamic performance in MEMS.

Wafer-Level Vacuum Packaging Enabled by Plastic Deformation and Low-Temperature Welding of Copper Sealing Rings With a Small Footprint

Wafer-level vacuum packaging is vital in the fabrication of many microelectromechanical systems (MEMS) devices and enables significant cost reduction in high-volume MEMS production. In this paper, we propose a low-temperature wafer-level vacuum packaging method based on plastic deformation and low-temperature welding of copper sealing rings with a small footprint. A device wafer with copper ring structures and a cap wafer with corresponding metalized grooves are placed inside a vacuum chamber and pressed together at a temperature of 250 °C, resulting in low-temperature welding of the copper, and thus, hermetic sealing of the cavities enclosed by the sealing rings. The vacuum pressure inside the fabricated cavities 146 days after bonding was measured using residual gas analysis to be as low as $2.6\times 10^{-2}$ mbar. Based on this value, the leak rate is calculated to be smaller than $ {3.6}\times {10}^{-16}$ mbarL/s using the most conservative assumptions, demonstrating the excellent hermeticity of the seals. Shear testing was used to demonstrate that the seals are mechanically stable with over 90 MPa in shear strength for $5.2~\mu \text{m}$ -high Cu sealing rings with widths down to $8~\mu \text{m}$ . The reported method is potentially compatible with complementary metal-oxide-semiconductor (CMOS) substrates and may be applied to vacuum packaging of 3-D heterogeneously integrated MEMS on state-of-the-art CMOS substrates. [2016-0252]

Low-Temperature Aluminum-Aluminum Wafer Bonding

Thermo-compression wafer bonding is a key technology for the wafer-level production of hermetically sealed cavities, which are essential for the functioning of many microelectromechanical systems (MEMS). Aluminum, with its low material price, high thermal and electrical conductivities and its complementary metal oxide semiconductor (CMOS) compatibility, is a promising candidate for the fabrication of CMOS-MEMS, in which the sensor/actuator part is bonded to the electrical circuit. The chemically highly stable oxide layer on the Al surface cannot be removed by conventional methods. It acts as a thin diffusion barrier layer between the two aluminum metallization layers, and therefore inhibits successful low temperature Al-Al wafer bonding. So far, effective Al-Al wafer bonding has required extreme processing temperatures of >300°C and high contact pressures. Typically, 450°C-550°C led to bonded Al-Al wafers with incorporated alumina (Al2O2) precipitates and reasonable bonding quality. By using the EVG®580 ComBond®system, in which the aluminum oxide is first removed physically, followed by bonding of the two metallization layers – both performed in a high vacuum cluster – for the first time successful Al-Al wafer bonding was feasible at a temperature of 100°C. In the present work, the microstructure of structured and unstructured aluminum films on silicon substrates was characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). C-mode scanning acoustic microscopy (C-SAM), TEM and energy dispersive x-ray spectroscopy (EDXS) interface studies of Al-Al wafer pairs bonded with this novel method revealed oxide-free, atomic contact and grain growth across the original interface. Further, the bond strength was characterized based on dicing yield and pull tests.

Design of MEMS vision tracking system based on a micro fiducial marker

Many microelectromechanical systems (MEMS) devices require considerable design effort to embed their own sensors to monitor themselves.In this study, a MEMS-based vision tracking system is developed based on micro fiducial markers. The vision tracking system recognizes the predetermined patterns of the micro-scale fiducial markers and calculates the position and rotation of the MEMS elements. Due to its good accessibility, the presented system can be applied to MEMS devices without significant effort or modification. This tracking system and three micro vision markers are applied to a MEMS nanopositioner as a linear displacement sensor. With three fiducial markers printed on a nanopositioner, the presented system can monitor the linear displacement of the nanopositioner with the error less than 1% of an intended motion and the jitter error less than 1μm. The presented MEMS vision tracking system also demonstrated its capabilities to track multiple MEMS elements simultaneously in MEMS-based micro-manipulation.

Deformation of varifocal mirror with narrow frame by SOI wafer residual stress

AbstractSilicon‐on‐insulator (SOI) wafers are used for many micro‐electro‐mechanical systems (MEMS). The influences of residual stress of SOI wafer on the fabricated structures are important especially for micro‐mirror because the optical performances are often affected by them. In this study, we propose an analytical calculation for the deformation of mirror and frame of a varifocal mirror. The deformations of the fabricated structures were measured using a white light interferometer. The frame was tilted at 2.8 mrad which was explained by the calculated value of 3.9 mrad. The mirror was deflected by 100 nm with the top‐flat shape, which also agreed well with the proposed analysis. © 2012 Wiley Periodicals, Inc. Electron Comm Jpn, 95(8): 26‐33, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11381

Degradation-based burn-in with preventive maintenance

As many products are becoming increasingly more reliable, traditional lifetime-based burn-in approaches that try to fail defective units during the test require a long burn-in duration, and thus are not effective. Therefore, we promote the degradation-based burn-in approach that bases the screening decision on the degradation level of a burnt-in unit. Motivated by the infant mortality faced by many Micro-Electro-Mechanical Systems (MEMSs), this study develops two degradation-based joint burn-in and maintenance models under the age and the block based maintenances, respectively. We assume that the product population comprises a weak and a normal subpopulations. Degradation of the product follows Wiener processes with linear drift, while the weak and the normal subpopulations possess distinct drift parameters. The objective of joint burn-in and maintenance decisions is to minimize the long run average cost per unit time during field use by properly choosing the burn-in settings and the preventive replacement intervals. An example using the MEMS devices demonstrates effectiveness of these two models.

Maskless Grayscale Lithography Using a Positive-Tone Photodefinable Polyimide for MEMS Applications

The novel use of a positive-tone photosensitive polyimide for the rapid production of grayscale features using a maskless lithography system is demonstrated. The removal rate of the polyimide, HD-8820, is characterized as a function of exposure dose. A broad contrast curve is found that is suitable for grayscale lithography. Three-dimensional polyimide structures up to 22 μm thick are demonstrated, and the surface roughness after the final cure is Ra = 4.4 nm, which is suitable for many microelectromechanical systems (MEMS) applications, including many optical applications. Tensile testing of 63 polyimide samples shows excellent mechanical properties for four different polyimide thicknesses produced with grayscale lithography. The modulus of elasticity is found to be 1.92 GPa, the yield strength to be 103 MPa, the fracture strength to be 133 MPa, and the percent elongation to be 51%. The test results show that the mechanical properties are consistent and do not change due to a partial exposure to UV light. The entire fabrication sequence, from computer-aided design file to cured structure, can be performed in less than 4 h, making this a fast low-cost method of producing polymer MEMS devices with excellent mechanical properties.

Electrical Noise in MEMS Capacitive Elements Resulting From Environmental Mechanical Vibrations in Harsh Environments

Many microelectromechanical systems (MEMS) devices possess charged capacitor structures where the suspension system allows relative electrode motion due to internal or external stimuli. When such a device is subjected to external mechanical vibrations present in a harsh operating environment, unwanted movement between the capacitor plates can generate a noise current which is injected into the connected circuitry. This paper analyzes this phenomenon and presents a model for the dynamics of a MEMS device with capacitive plates experiencing relative motion due to external stimuli. A Fourier series expansion of the current is developed to characterize the frequency content of the signal in closed form for a given vibration frequency, and simulation and experimental results are presented.

Deformation of Varifocal Mirror with Narrow Frame by SOI Wafer Residual Stress

Silicon-on-insulator (SOI) wafers are used for many micro-electro-mechanical systems (MEMS). The influences of residual stress of SOI wafer on the fabricated structures are important especially for micro-mirror because the optical performances are often affected by them. In this study, we propose an analytical calculation for the deformation of mirror and frame of a varifocal mirror. The deformations of the fabricated structures were measured using a white light interferometer. The frame was tilted at 2.8mrad which was explained by the calculated value of 3.9mrad. The mirror was deflected by 100nm with the top-flat shape, which also agreed well with the proposed analysis.

Novel Test Structures for Hermeticity Testing of Wafer Bonding Technologies

Many Micro-Electro-Mechanical System (MEMS) devices such as gyroscopes, accelerometers, infrared uncooled arrays, radio frequency (RF) resonators etc. require vacuum packaging. In this paper novel MEMS resonators designed as double ended tuning forks (DETF) with pressure sensitive quality factor are presented. The study of different DETF-designs clearly identified the strucutres' ability to monitor the inside cavity pressure of hermetically sealed micro packages. Therefore a new test vehicle for the development and long-time reliability of hermetic wafer bonding technologies can be provided.

Measurement of Leak Rate for MEMS Vacuum Packaging

Many Micro-Electro-Mechanical Systems (MEMS) devices such as accelerators, gyroscopes, uncooled infrared sensors, etc., require vacuum packaging. The vacuum maintaining lifetime directly determines the vacuum packaging reliability. This research presented a quantitative analysis of the relationship between the leak rate and the vacuum maintaining lifetime, and demonstrated that the leak rate measurement plays an important role. This paper also explored the application limitations in vacuum packaging using a helium spectrometer leak tester to measure the leak rate because the measured leak rate was nonlinear with respect to the actual leak size. According to the fact that the damping coefficient changes with the pressure, a tuning fork crystal chip as a pressure sensor was used to monitor the pressure changes in the package cavity. The leak conductance was also calculated from the pressure tracking data to analyze the leak modes; the molecular flow model and gas desorption model were found to fit the measurement results of leak conductance.

Wafer-level thin-film encapsulation for MEMS

The diversity and complexity of many microelectromechanical systems (MEMS), combined with the mechanical nature of the devices involved, means that the handling, dicing and packaging of these structures can pose many problems. So-called ‘zero-level’ packaging options are now often used to protect the devices at the wafer scale before the wafer is diced and sent for conventional packaging. This paper describes a novel process flow for the fabrication of integrated MEMS thin-film packages within a low-temperature, CMOS-compatible process. A double sacrificial layer is used, which encapsulates the device of interest within a shell of silicon oxide. The sacrificial layer is then removed through lateral etch channels and the shell is sealed. The technique requires minimal extra wafer space, allows the use of low-temperature materials within the process flow, and the novel channel design means that the shell may be easily sealed. Preliminary visual and electromechanical tests using simple fixed–fixed beam test structures indicate that the package is sealed, the device is undamaged and that encapsulation has little or no effect on device performance.

Influence of bending-extension coupling on the modal frequencies of nonsymmetrically laminated MEMS microcantilevers

Many microelectromechanical systems (MEMS) devices are based on some aspect of the modal vibration response of microcantilever beams, and these microcantilevers often have nonsymmetrically laminated construction. Such construction can lead to complex mechanical coupling effects which are not included in elementary beam theory, yet this theory is often used to estimate the modal frequencies of microcantilevers. In this paper, the bending-extension coupling effects on the modal frequencies of a typical two-layer microcantilever beam element in a MEMS chemical sensor are analyzed. Classical Lamination Theory is combined with a Rayleigh-Ritz analysis to develop a model for the modal frequencies of this microcantilever. Results are compared with those from the elementary beam theory and a finite element model. Bending-extension coupling effects are found to depend on the beam length-to-thickness ratio, on the coating thickness-to-beam thickness ratio, and on the mode number. [1369].

Self-aligned 0-level sealing of MEMS devices by a two layer thin film reflow process

Many micro electromechanical systems (MEMS) require a vacuum or controlled atmosphere encapsulation in order to ensure either a good performance or an acceptable lifetime of operation. Two approaches for wafer-scale zero-level packaging exist. The most popular approach is based on wafer bonding. Alternatively, encapsulation can be done by the fabrication and sealing of perforated surface micromachined membranes. In this paper, a sealing method is proposed for zero-level packaging using a thin film reflow technique. This sealing method can be done at arbitrary ambient and pressure. Also, it is self-aligned and it can be used for sealing openings directly above the MEMS device. It thus allows for a smaller die area for the sealing ring reducing in this way the device dimensions and costs.The sealing method has been demonstrated with re-flowed aluminium, germanium, and boron phosphorous silica glass. This allows for conducting as well as non-conducting sealing layers and for a variety of allowable thermal budgets. The proposed technique is therefore very versatile.

Bistable Configurations of Compliant Mechanisms Modeled Using Four Links and Translational Joints

Bistable mechanical devices remain stable in two distinct positions without power input. They find application in valves, switches, closures, and clasps. Mechanically bistable behavior results from the storage and release of energy, typically in springs, with stable positions occurring at local minima of stored energy. Compliant mechanisms offer an elegant way to achieve this behavior by incorporating both motion and energy storage into the same flexible element. Interest in compliant bistable mechanisms has also recently increased because of the advantages of bistable behavior in many micro-electro-mechanical systems (MEMS). Design of compliant or rigid-body bistable mechanisms typically requires simultaneous consideration of both energy storage and motion requirements. This paper simplifies this process by developing theory that provides prior knowledge of mechanism configurations that guarantee bistable behavior. Configurations which include one or more translational, or slider, joints are studied in this work. Several different mechanism types are analyzed to determine compliant segment placement that will ensure bistable mechanism operation. Examples demonstrate the power of the theory in design.