Fabrication of Large-Area Three-Dimensional Microstructures on Flexible Substrates by Microtransfer Printing Methods

Micro-electro-mechanical system (MEMS) technology has radically changed the scale, performance, and cost of a wide variety of sensors and actuators by taking advantage of batch fabrication. The multidisciplinary nature of MEMS employs knowledge of diverse technical areas to realize improved and novel transducer systems. This also brings various associated challenges that are otherwise being ignored in a simple macro-dimensional system. MEMS devices typically comprise several deposited thick and thin films as well as bonded of dissimilar materials (like silicon, metal, glass, etc.). Residual stress is one of the most common outcomes during this integration/stacking of distinctly different materials for the fabrication of novel MEMS structures. The residual stress may significantly affect the performance and reliability of the fabricated devices. Thus, the evaluation and regulation of residual stress are one of the crucial aspects to assess the functioning of modern-day MEMS devices. This paper reviewed the origins of residual stress in MEMS fabrication processes. Different techniques involved in testing and characterization of the residual stresses are reviewed. Few important case studies are discussed to highlight the effect of residual stress (generated during various fabrication processes) on characteristics of different MEMS structures. The brief overview of the possible route to minimize the residual stresses is also presented.

Design and development of an affordable, user‐friendly and portable proof‐of‐concept prototype for the detection of the physical stress of the muscles of different human body parts have been reported. The prototype is composed of four major components—sensor, signal processing unit (SPU), power supply and display units. The sensor consists of a soft and flexible substrate with high density array of micro/nano patters on polydimethylsiloxane (PDMS), which in turn is coated with ultra‐thin layers of electrically conducting aluminium (Al) and reduced graphene oxide (RGO) layers. The conductive layer coated on the flexible PDMS substrate helps in detecting the raw surface electromyogarphic (RSEMG) potential of fingertip, tip‐toe, wrist or tongue, once they come in contact with the sensor. The patterned Al/RGO coated PDMS substrate generates an electrical signal, which can be correlated to the stress level of the body parts at a very high precision. The electrical signal of the sensor is transmitted wirelessly to a mobile android application for the display of the results with the help of the SPU. The prototype can be useful for the early detection of stress disorders related to heart, nerves and muscles, among others.

Chemical warfare agent sensor using MEMS structure and thick film fabrication method

Replication of optical MEMS structures in sol–gel materials

Suspended aluminum nitride structures grown via metal organic vapor phase epitaxy

As a first step toward formulating guidelines for the design of dynamically reliable MEMS, we analyze the mechanical response of, and formulate failure criteria for, a large class of shock loaded microsystems. MEMS are modeled as microstructures attached to elastic substrates, and the shocks are modeled as pulses of acceleration applied to the substrate over a finite time duration. The relevant time scales in the analysis are the acoustic transit time, the time period of vibrations, and the duration of the applied shock load. For many MEMS structures and shock loads (with durations in the range 50-5000 /spl mu/s), the substrates respond as rigid bodies and are expected to be immune to stress-wave-induced damage. Time-domain criteria, obtained to distinguish between the impulse, resonant, and quasistatic responses of the microstructures, correlate well with the experimentally observed responses of different MEMS devices. The formulation of displacement-based and stress-based failure criteria is discussed, along with their sensitivity to the applied strain rate. A case study, in which these results are applied to evaluate the reliability of a packaged surface-micromachined device, is presented.

Manufacturing and design of micro-electro-mechanical systems (MEMS) often involves consideration of micro-rods or other beam structures that play an important role in MEMS performance. Many MEMS structures are made of either silicon or nickel, however, novel designs may also include more complex multifunctional components made of composite materials. This chapter reviews research in the area of composite micro-rods, which are composed of a coated fiber surrounded by an easily-deformable insulating polymer matrix. Such structural configuration allows for multifunctional design and formability of diverse shapes for micro-components in MEMS applications. Here, the influence of nanostructured fiber-matrix interphases on the global deformation response and local stress distributions in the composite micro-rods subjected to compressive loads is investigated. Finite element modeling of homogeneous and heterogeneous interphases is used to determine parameters that affect the deformation response of a composite micro-rod. Linear elastic and nonlinear elastic matrix material models are used in the simulations. Multi-scale effects in transverse deformation of micro-rods are examined. The ratio of the matrix and interphase Young’s moduli is found to affect the character of global deformation. A transition from barreling to bollarding type of deformation is seen as this ratio changes. This interphase controlled change in the deformation mode constitutes a multi-scale coupling between the micron-size interphase and the global response of a deforming rod. The radial, hoop and shear stress distributions in the interphase region are also seen to be highly dependent on the type of interphase present and its sub- micron property gradients.

Thermal cycling response of layered gold/polysilicon MEMS structures

An approach to low-cost, wafer-level packaging of microelectromechanical systems (MEMS), e.g., microresonators, is reported. The process does not require wafer-to-wafer bonding and can be applied to a wide range of MEMS devices. A sacrificial polymer-placeholder is first patterned on top of the MEMS component of interest, followed by overcoating with a low dielectric constant polymer overcoat. The sacrificial polymer decomposes at elevated temperature, and the volatile products from the sacrificial material permeate through the overcoat polymer leaving an embedded air-cavity around the MEMS structure. Thus, the device is released from the sacrificial polymeric material, housed in a protective overcoat. The protected MEMS device can then be handled and packaged like an integrated circuit. The electrical characteristics of the microresonators before and after packaging were essentially the same, showing the packaging scheme does not alter the device performance. This approach is applicable to both surface and bulk micromachined devices

Microelectromechanical systems (MEMS) have found several applications in various fields from homeland security to personalized health care. However, rendering MEMS into autonomous wireless systems operating in any given environment requires the integration of energy harvesters into the MEMS structures, ensuring thus the self-powering of the devices. In this work, we investigated the mechanical and magnetic properties of Samarium Cobalt (SmCo) thin films, with the goal to implement them into electromagnetic energy harvesters. The films were deposited by sputtering on suspended silicon cantilevers fabricated with a front-side micromachining process. The magnetic films, grown under various pressures and thermally annealed at several temperatures and ambient conditions, were studied in terms of their mechanical and magnetic properties. Depending on the fabrication parameters, the stresses that developed in the magnetic material, deposited on top of the cantilevers, are altered from compressive (downward deflection of the cantilevers) to tensile (upward deflection), indicating that it is possible to control not only the magnetic properties of the films, but also the mechanical properties of the complete structure. Our results suggest that SmCo magnetic films are suitable candidates for integration in suspended structures for the development of electromagnetic micro-generators.

A method to fabricate complex structures on non-planar surfaces by utilizing a transfer method is presented. 3-dimensional and high-aspect-ratio microelectromechanical systems (MEMS) structures are transferred to flexible membranes, and can be further transferred to any non-planar surfaces. Here, we used a structure formed on a silicon-on-insulator (SOI) wafer by deep reactive-ion etching (DRIE) as an example. Both photoresist SU8-2050 and AZ 4620 were used as the flexible membranes. The effect of the structure and density of pillars on the transferring procedure was studied. Potentially this method could be utilized to transfer any MEMS structures and devices to any non-planar surfaces, thus possessing great potential in MEMS devices.

This paper describes a fabrication process to integrate different types of energy-converting microelectromechanical system (MEMS) devices for a power generator. MEMS structures and a fabrication process for the monolithic integration are proposed for the purpose of accumulating small amounts of energy from various surrounding environments. For the MEMS power generator, thermocouples and movable plates are simultaneously fabricated in thick-film formation processes with deep reactive ion etching of silicon and gold electroplating. The fabricated thermoelectric devices of gold and silicon produced voltages in accordance with the temperature difference at the thermocouples. The vibrational devices with gold movable plates resonated with external vibrations. These results confirm that the monolithic integration process of MEMS devices can be established for harvesting thermal and vibrational energies from their surroundings.

Microelectromechanical Systems (MEMS) devices typically need to be designed against a very low failure probability, which is on the order of or lower. Experimental determination of the target strength for such a low failure probability requires testing of tens of thousands of specimens, which can be cost prohibitive for the design process. Therefore, understanding the probabilistic failure of MEMS devices is of paramount importance for design. Currently available probabilistic models for predicting the strength statistics of MEMS structures are based on classical Weibull statistics. Significant advances in experimental techniques for measuring the strength of MEMS devices have produced data that have unambiguously demonstrated that the strength distributions consistently deviate from the Weibull distribution. Such deviations can be explained by the fact that the Weibull distributions are derived based on extreme value statistics, which is inapplicable to MEMS devices where the dimensions of the material microstructure are not negligible compared to the characteristic structural dimensions. This paper presents a robust probabilistic model for strength distribution of polycrystalline silicon (poly‐Si) MEMS structures that could be extended to other brittle materials at the microscale. The overall failure probability of the structure is related to the failure probability of each material element along its sidewalls through a weakest‐link statistical model. The failure statistics of the material element is determined by both the intrinsic random material strength as well as the random stress field induced by the sidewall geometry. Different from the classical Weibull statistics, the present model is designed to account for structures consisting of a finite number of material elements, and it predicts a scale effect on their failure statistics. It is shown that the model agrees well with the measured strength distributions of poly‐Si MEMS specimens of different sizes, and the calibrated mean strength of the material element is consistent with the theoretical strength of silicon. Meanwhile, it is shown that the two‐parameter and three‐parameter Weibull distributions cannot provide optimum and consistent fits of the observed size‐dependent strength distributions, and thus have very limited prediction capability. The present model explicitly relates the strength statistics to the size effect on the mean structural strength, and therefore provides an efficient means of determining the failure statistics of MEMS structures.

Stress distribution is one of the main factors influencing the reliability of MEMS (Micro Electro Mechanical System) structure. In most cases, MEMS devices work in motion state, the dynamic stress often affects the performances of the MEMS devices. For the first time, dynamic streses were measured by means of Raman spectroscopy and high-frequency modulation technology, the measurement result shows that dynamic stress at a certain point in silicon micro-resonator is in agreement with the analysis of theory.

The most commonly employed tools for wafer thickness and topography metrology are based on capacitance method, which due to physical size of probes, and may not be suitable for direct measurement of multi-layer non-conductive wafers or Micro Electromechanical Systems (MEMS) structures. Recently developed that low coherence interferometry provides solution, which overcomes limitations of these methods. Selected MEMS applications including characterization of deep (high aspect) trenches and membrane structures have been also developed. The above listed applications were limited to measurements of relative distance between two optical interfaces in material transparent at the wavelength of probing radiation. Absolute distance gauging by fiber optic low coherence interferometer is difficult due to large thermal drift (of the order of 0.04 mm/K). We demonstrate that this drift is a result of thermal changes of refractive index of fiber optic glass. We present solution eliminating this drift is based on introduction of the additional reference plane in the signal arm of the Michelson interferometer. Use of this reference plane eliminates influence of changes of refractive index of glass fibers on result of measurement and improves thermal stability of low coherence interferometer by three orders of magnitude.