Microelectromechanical Systems and Devices

The advances of microelectromechanical systems (MEMS) and devices have been instrumental in the demonstration of new devices and applications, and even in the creation of new fields of research and development: bioMEMS, actuators, microfluidic devices, RF and optical MEMS. Experience indicates a need for MEMS book covering these materials as well as the most important process steps in bulk micro-machining and modeling. We are very pleased to present this book that contains 18 chapters, written by the experts in the field of MEMS. These chapters are groups into four broad sections of BioMEMS Devices, MEMS characterization and micromachining, RF and Optical MEMS, and MEMS based Actuators. The book starts with the emerging field of bioMEMS, including MEMS coil for retinal prostheses, DNA extraction by micro/bio-fluidics devices and acoustic biosensors. MEMS characterization, micromachining, macromodels, RF and Optical MEMS switches are discussed in next sections. The book concludes with the emphasis on MEMS based actuators

Experimental methods for the characterization of fatigue in microstructures

The mechanical fatigue behavior of gold microbeams is analyzed. Dedicated devices have been designed and built able to produce alternate loading on gold specimens; the electrostatic actuation is used as driving force. Gold beams are tested under both bending and tensile alternate loadings. Results were used to plot S-N curves and fatigue Goodman-Smith diagram in order to estimate the fatigue limit of the material in presence of mean and alternate stress conditions. The surface topography evolution is studied and failure modes are discussed

Experimental methods for the characterization of fatigue in microstructures

The mechanical fatigue behavior of gold microbeams is analyzed. Dedicated devices have been designed and built able to produce alternate loading on gold specimens; the electrostatic actuation is used as driving force. Gold beams are tested under both bending and tensile alternate loadings. Results were used to plot S-N curves and fatigue Goodman-Smith diagram in order to estimate the fatigue limit of the material in presence of mean and alternate stress conditions. The surface topography evolution is studied and failure modes are discussed

Design and fabrication of suspended-gate MOSFETs for MEMS resonator, switch and memory applications

Wireless communication systems and handset devices are showing a rapid growth in consumer and military applications. Applications using wireless communication standards such as personal connectivity devices (Bluetooth), mobile systems (GSM, UMTS, WCDMA) and wireless sensor network are the opportunities and challenges for the semi-conductor industry. The trend towards size and weight reduction, low power consumption and increased functionalities induces major technological issues. Today, the wireless circuit size is limited by the use of lots of external or "off-chip" components. Among them, quartz crystal, used as the time reference in any wireless systems, is the bottleneck of the miniaturization. Microelectromechanical systems (MEMS) is an emerging technology which has the capability of replacing the quartz. Based on similar technology than the Integrated Circuit (IC), MEMS are referred as electrostatically, thermally or piezoelectrically actuated mechanical structures. In this thesis, a new MEMS device based on the hybridization of a mechanical vibrating structure and a solid-sate MOS transistor has been developed. The Resonant Suspended-Gate MOSFET (RSG-MOSFET) device combines both advantages of a high mechanical quality factor and the transistor intrinsic gain. The physical mechanisms behind the actuation and the behavior of this device were deeply investigated and a quasi-static model was developed and validated, based on measured characteristics. Furthermore, the dynamic model of the RSG-MOSFET was created, taking into account the non-linear mechanical vibrations of the gate and the EKV model, used for MOSFET modeling. Two fabrication processes were successfully developed to demonstrate the proof of concept of such a device and to optimize the performances respectively. Aluminum-silicon (Al-Si1%) and pure silicon-based RSG-MOSFETs were successfully fabricated. DC and AC characterizations on both devices enabled to understand, extract and evaluate the mechanical and MOSFET effects. A specifically developed RF characterization methodology was used to measure the linear and non-linear behaviors of the resonator and to evaluate the influence of each polarization voltages on the signal response. RSG-MOSFET with resonant frequencies ranging from 5MHz to 90MHz and quality factor up to 1200 were measured. Since MEMS resonator quality factor is strongly degraded by air damping, a 0-level thin film vacuum packaging (10-7 mBar) process was developed, compatible with both AlSi-based and silicon-based RSG-MOSFET. The technology has the unique advantage of being done on already released structure and the room temperature process makes it suitable for above-IC integration. In parallel, a front-end compatible process was defined and major build blocks were developed in industrial environment at STMicroelectronics. This technology is based on the Silicon-On-Nothing technology, originally developed for advanced transistor, and therefore making the MEMS resonator process compatible with CMOS co-integration. DC characterizations of SG-MOSFET had shown interesting performances of this device for current switch and memory applications. Mechanical contact of the gate with the MOSFET channel induces a current switching slope greater than 0.8mV/decade, much better than the theoretical MOSFET limit of 60mV/decade. Maximum switch isolations of -37dB at 2 GHz and -27dB at 10GHz were measured on these devices. A novel MEMS-memory has been demonstrated, based on the direct charge injection to the storage media by the mechanical contact of the metal gate. Charge injection and retention mechanisms were investigated based on measured devices. Cycling study of up to 105 cycles were performed without noticing major degradations of the electrical behavior neither mechanical fatigue of the suspended gate. The measured retention time places this memory in between the DRAM and the FLASH memories. A scaling study has shown integration and compatibilities capabilities with existing CMOS

3D laser scanner based on surface silicon micromachining techniques for shape and size reconstruction of the human ear canal

2005/2006As technology advances, hearing aids can be packaged into increasingly smaller housings. Devices that fit entirely within the deeper portion of the external auditory canal have been developed, called completely-in-the-canal (CIC). These aids are custom moulded and have high cosmetic appeal because they are virtually undetectable. They also have several acoustic advantages: reduced occlusion effect, reduced gain requirements, and preservation of the natural acoustic properties of the pinna and external ear. However, CIC hearing aids require proper fitting of the hearing aid shell to the subject ear canal to achieve satisfactory wearing comfort, reduction in acoustic feedback, and unwanted changes in the electro-acoustic characteristics of the aid. To date, the hearing aid shell manufacturing process is fully manual: the shell is fabricated as a replica of the impression of the subject ear canal. Conventional impression acquisition method is very invasive and imprecise, moreover the typical post-impression processes made on the ear impression leaves room for error and may not accurately represent the structural anatomy of patient’s ear canal. There are some laser approaches able to perform a 3D laser scanning of the original ear impression but, the entire shell-making process is completely dependent on the ear impression and often is the sole cause of poor fitting shell. Therefore, direct ear canal scanning is the only way to perform accurate and repeatable measurements without the use of physical ear impression. The conventional optical elements are not able to enter in the inner part of the ear and perform a scanning of the cavity. This work is devoted to the direct scanning of human external auditory canal by using electromagnetically actuated torsion micromirror fabricated by micromachining technique as scanner. This is the first ever demonstration of actual scanning of human external auditory canal by a single integral Micro-Electro-Mechanical System (MEMS). A novel prototype 3D scanning system is developed together with surface reconstruction algorithm to obtain an explicit 3D reconstruction of actual human auditory canal. The system is based on acquisition of optical range data by conoscopic holographic laser interferometer using electromagnetically actuated scanning MEMS micromirror. An innovative fabrication process based on poly(methylmethacrylate) (PMMA) sacrificial layer for fabrication of free standing micromirror is used. Micromirror actuation is achieved by using magnetic field generated with an electromagnetic coil stick. Micromirror and electromagnet coil assembly composes the opto-mechanical scanning probe used for entering in ear auditory canal. Based on actual scan map, a 3D reconstructed digital model of the ear canal was built using a surface point distribution approach. The proposed system allows noninvasive 3D imaging of ear canal with spatial resolution in the 10 μm range. Fabrication of actual shell from in-vivo ear canal scanning is also accomplished. The actual human ear canal measurement techniques presented provide a characterization of the ear canal shape, which help in the design and refining of hearing aids fabrication approaches to patient personalized based.XIX Ciclo197

Demolding of hot embossed polymer microstructures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from PDF version of thesis.Includes bibliographical references (p. 219-229).Polymer-based microfluidic "lab on a chip" technology promises to reduce cost and extend access to medical diagnostic tests that formerly required expensive and labor-intensive lab work. The predominant methods for manufacturing these devices are miniaturized molding processes including casting, injection molding, and hot embossing. These techniques have in common the use of a mold to define the shape of functional features (fluidic channels), the separation of the part from the mold as a process step (demolding), and the intended re-use of the mold to produce additional parts. The demolding step in particular poses significant challenges for mass production. Demolding affects several issues including production rate, part quality, and mold lifetime, and demolding-related defects are frequently observed. Despite its importance, there has been no comprehensive effort to analyze demolding theoretically or experimentally. This thesis aims to deepen the understanding of demolding of polymer microstructures in order to facilitate mass manufacturing of polymer-based devices with micro-scale functional features, such as microfluidic chips. A theory of demolding mechanics has been proposed that combines the effects of thermal stress, friction, and adhesion in a unified framework. A metric by which demolding can be characterized experimentally--the demolding work--has been proposed by analogy with interfacial fracture and has been related to underlying physical mechanisms. Finite element simulations based on this theory of demolding have been performed to investigate the effects of important parameters, including demolding temperature and feature geometry. A test method for characterizing demolding by directly measuring the demolding work for individual microstructures has been developed and applied to hot embossing to study the effects of process parameters such as demolding temperature, the effects of feature geometry and layout, and the impacts of mitigation strategies such as low-adhesion mold coatings. The results of these demolding experiments broadly agree with expected trends based on the theory of demolding mechanics proposed herein. A dimensionless parameter aggregating the effects of feature geometry and layout has been identified and related to the occurrence of demolding-related defects, the demolding process window, and the demolding temperature that minimizes the demolding work. These findings have been generalized to provide processing and design guidance for industrial application of polymer micro-molding.by Matthew E. Dirckx.Ph.D

Carbon nanotube surfaces for low force contact application

This thesis focuses on developing a testing method to estimate the mechanical and electrical characteristics of CNT-based contact surfaces. In particular the use of gold thin films deposited on a CNT forest has potential to offer a very effective contact surface. Two different pieces of experimental apparatus were used in this research to determine the mechanical and electrical properties of gold/multi-walled carbon nanotube (Au/MWCNT) composites: 1) a modified nano-indentation; and 2) a PZT actuator test rig. These apparatuses were used to mimic force-displacement and contact behaviour of the MEMS relay?s contact at a maximum load of 1 mN with dry-circuit and hot-switched conditions. The surfaces were compared with reference Au-Au and Au-MWCNT contact pairs studied under the same experimental conditions. In the modified nano-indentation experiment, tests of up to 10 cycles were performed. The results showed that the Au-MWCNT pair electrical contact resistance improved when the Au-Au/MWCNT pair was used. Additionally the Au-Au/MWCNT pair electrical contact resistance (Rc) was comparable with the Au-Au contact pair. When the Au-Au/MWCNT composite surface is in contact with the Au hemispherical probe it provides a compliant surface. It conforms to the shape of the Au hemispherical probe. For a higher number of cycles, a PZT actuator was used to support Au/MWCNT planar coated surfaces. This surface makes electrical contact with a gold coated hemispherical probe to mimic the actuation of a MEMS relay?s contact at higher actuation frequencies. This apparatus allows the performance of the contact materials to be investigated over large numbers of switching cycles. Different current loads were used in this experiment, 1mA, 10mA, 20mA and 50mA at 4V supply. The Rc of these surfaces was investigated as a function of the applied force under repeated cycles. Under current loads of 1mA and 10mA the Au/MWCNT composite surface provides a stable contact resistance of up to more than a million cycles and no degradation was observed on the surface. Compared with Au-Au contact pair, degradation occurred after 220 cycles. The Au-Au contact pair shows delamination of the Au surface on the probe. The possible reason is the softening or melting of the Au surface. Furthermore, under higher current loads of 20mA to 50mA, degradation had occurred after 50 million cycles (at 20mA) and degradation had occurred at around 45 to 150 cycles for 50mA to 30mA respectively. This is because of the softening or melting of the Au and Au fatigue after a large number of cycles. This study is the first step to show the potential of CNT surfaces as an interface in low force electrical contact applications. With this research, current trends in materials used on contacts and fabrication methods can be explored and even modified or adopted. The use of CNT?s and their composites for contacts can be tested using the available apparatus to look at their performance and reliability in terms of mechanical and electrical properties. This is useful for MEMS contacts that form part of MEMS relay devices

Characterization, modeling and reliability of RF MEMS Switches and Photovoltaic Silicon Solar Cells

The main goal of this thesis is the failure and reliability investigation of RF-MEMS switches and photovoltaic solar cells. For technical developer people the reliability issue is often consider a secondary problem in electronic devices since it is not considered an important factor in the production chain. This concept is changing is the last years because reliability studies are considered an important technological step to improve the production process. This fact is confirmed by the investments that companies adopt to test their products. In the particular case of this thesis, we can easily mention the solar cell production line where the cells are subjected to reliability tests that extrapolate the efficiency and the fill factor in order to study the performances and to consequently improve the production process. Concerning RF MEMS Wireless communication systems for space applications require electronic components with a high level of reliability, a low power consumption and they should be as small as possible in order to be better integrated in satellites. Radio Frequency Micro Electro Mechanical System (RF-MEMS) can be considered one of the best candidates to comply with previous requirements and, under certain conditions, they can completely replace an entire solid-state circuit. RF-MEM devices in general are characterized by a good miniaturization, an easily integration in a standard solid-state circuit, an almost zero power consumption, a good RF linearity and a high quality factor Q. Concerning RF-MEMS switches RF performances, they exhibit a very low insertion loss, lower than 0.1 dBm up to 60 GHz and, at the same time, a good isolation, more than 20 dBm. From an electrical and mechanical point of view the power consumption of these switches is close to zero because of an “on-state” current around pA and they are almost unaffected by high level of acceleration or deceleration because of their mass that is extremely small. The possibility to integrate the production of these devices in the standard foundry silicon processes and their integration with mature semiconductor technology are a great advantage for their spread making possible to produce them in an easy and cheap way. Over the last 10 years important developments on MEMS switches have been done all over the world. As a matter of fact, these switches are quite attractive since they combine excellent RF performances and low power consumption of mechanical switches with the small size and low weight of semiconductor devices. However, the appearance of MEMS switches on the market has been hindered by the need for specific packaging as well as by reliability issues. Reliability is a major issue for any satellite since it is almost impossible to envisage any repair work once the spacecraft has been launched. Hence, reliability is a key driver when designing any RF equipment. If we consider a RF-MEMS switch, we have to guarantee that his electromechanical performances will be the same after an intensive usage in harsh environment, for instance after millions or billions of cycles and after the exposure to different kind of radiations. In case of their application in a redundancy scheme, they have to be completely operative even after a long period of activity or inactivity. The aim of this thesis is to perform an electrical characterization and several reliability tests on different kind of RF-MEMS switches in order to analyze which are the weaknesses and the strengths of this new technology. Electrical characterizations have been done using two different measurement systems. The first, based on a vector network analyzer and a power supply, has been used to test the RF performances of the devices and to extract the actuation and deactuation voltages. The second set up, based on the internal RF signal generator of the VNA, an 8-GHz digital signal oscilloscope and a profilometer (polytec MSA 500), has been used to characterize the electrical performances like actuation time, release delay and dynamic performances. Cycling stress, one of the most common test used to understand the robustness of this kind of devices, has been performed on different topologies of switch in order to better understand how some parameters of the RF MEMS switch, such as the shape of the beams or the actuation voltage, impact on the reliability of the device. Furthermore, the influence of continuous actuation stress on the reliability of dielectric-less switches has been investigated, comparing different designs and studying the variation of the main electrical parameters induced by the stress and the successive recovery phase. Concerning PV solar cells A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. The operation of a photovoltaic (PV) cell requires 3 basic attributes: (i) the absorption of light, generating either electron-hole pairs or excitons, (ii) the separation of charge carriers of opposite types and (iii) the extraction of those carriers to an external circuit. Over the last decades, many research groups have tried to improve the conversion processes in order to increase the efficiency of solar cells and to reduce the parasitic effects that limit the energy conversion. This has generated a real challenge to the best conversion efficiency. The average efficiency of multicrystalline silicon solar cells at the beginning of 2014 was about 16% but in research labs different solar cells have exceeded the 20% with records over 24%. The continuous growth of the solar cells efficiency has been achieved thanks to the reliability study of the single cells and to the degradation analysis of the real photovoltaic systems. These studies have revealed the critical points of PV solar cells and have led to a constant improvement of the production processes. The aim of this thesis is the study of the reliability problems related to a single solar cell and to a string of solar cells subjected to different illumination conditions. Different characterization procedures have been developed in order to study the failure mechanisms and to study the weaknesses and the strengths of the technology. Four types of measurement set-ups have been utilized: (i) the first system is able to extract the IV curves in dark and light conditions. This simple measurement procedure has to be opportunely calibrated in order to obtain right results in term of efficiency and fill factor. (ii) The second system extracts the thermographic image of a single solar cell. It can be used to analyze hot spot and other failure mechanisms in the silicon structure. (iii) The third system extracts the electroluminescence and the photoluminescence of a single solar cell. It is able to extract and analyze the defects in the crystalline structure of the materials. (iv) The fourth is the LOANA system: a commercial tool able to extract the External Quantum Efficiency and the Internal Quantum Efficiency with the measurement of the reflectance. All these characterization procedures have been utilized to study the evolution of the failure mechanisms when a single solar cell is subjected to reverse biasing stresses. The study of the catastrophic degradation of solar cells submitted to reverse current stress is of crucial importance since the failure can lead to the rapid increase of the temperature with a consequent risk of fire and to the breaking of the entire PV system. This particular situation can occur when the PV system is not uniformly illuminated and the solar cells of the system present not uniform shunt resistance. Additional studies have been performed in the modelization of a solar cell with the two-diode model. The study and modeling of solar cells allow to obtain right results in term of efficiency and fill factor extrapolation. Moreover, the modelization allows the study of string of solar cells working in particular conditions in which the illumination level is not uniform in a whole panel. The simulations allow to predict the dangerous situations and to design appropriate prevention systems

Workshop on "Robotic assembly of 3D MEMS".

Proceedings of a workshop proposed in IEEE IROS'2007.The increase of MEMS' functionalities often requires the integration of various technologies used for mechanical, optical and electronic subsystems in order to achieve a unique system. These different technologies have usually process incompatibilities and the whole microsystem can not be obtained monolithically and then requires microassembly steps. Microassembly of MEMS based on micrometric components is one of the most promising approaches to achieve high-performance MEMS. Moreover, microassembly also permits to develop suitable MEMS packaging as well as 3D components although microfabrication technologies are usually able to create 2D and "2.5D" components. The study of microassembly methods is consequently a high stake for MEMS technologies growth. Two approaches are currently developped for microassembly: self-assembly and robotic microassembly. In the first one, the assembly is highly parallel but the efficiency and the flexibility still stay low. The robotic approach has the potential to reach precise and reliable assembly with high flexibility. The proposed workshop focuses on this second approach and will take a bearing of the corresponding microrobotic issues. Beyond the microfabrication technologies, performing MEMS microassembly requires, micromanipulation strategies, microworld dynamics and attachment technologies. The design and the fabrication of the microrobot end-effectors as well as the assembled micro-parts require the use of microfabrication technologies. Moreover new micromanipulation strategies are necessary to handle and position micro-parts with sufficiently high accuracy during assembly. The dynamic behaviour of micrometric objects has also to be studied and controlled. Finally, after positioning the micro-part, attachment technologies are necessary