Nanotribology and nanomechanics of MEMS/NEMS and BioMEMS/BioNEMS materials and devices

The micro/nanoelectromechanical systems (MEMS/NEMS) need to be designed to perform expected functions typically in millisecond to picosecond range. Expected life of the devices for high speed contacts can vary from few hundred thousand to many billions of cycles, e.g., over a hundred billion cycles for digital micromirror devices (DMDs), which puts serious requirements on materials. For BioMEMS/BioNEMS, adhesion between biological molecular layers and the substrate, and friction and wear of biological layers may be important. There is a need for development of a fundamental understanding of adhesion, friction/stiction, wear, and the role of surface contamination, and environment. Most mechanical properties are known to be scale dependent. Therefore, the properties of nanoscale structures need to be measured. MEMS/NEMS materials need to exhibit good mechanical and tribological properties on the micro/nanoscale. There is a need to develop lubricants and identify lubrication methods that are suitable for MEMS/NEMS. Methods need to be developed to enhance adhesion between biomolecules and the device substrate. Component-level studies are required to provide a better understanding of the tribological phenomena occurring in MEMS/NEMS. The emergence of micro/nanotribology and atomic force microscopy-based techniques has provided researchers a viable approach to address these problems. This paper presents a review of micro/nanoscale adhesion, friction, and wear studies of materials and lubrication studies for MEMS/NEMS and BioMEMS/BioNEMS, and component-level studies of stiction phenomena in MEMS/NEMS devices.     

Assessment of dielectric charging in electrostatically driven MEMS devices: A comparison of available characterization techniques

The present work investigates the results of different characterization methods for the dielectric charging phenomenon applicable to metal–insulator–metal (MIM) capacitors and electrostatically actuated micro-electro-mechanical-systems (MEMS). The discharge current transients (DCT), thermally stimulated depolarization current (TSDC) and Kelvin probe force microscopy (KPFM) assessment methods have been applied to either MIM capacitors or electrostatic capacitive MEMS switches or both. For the first time, the KPFM methodology has been used to create a link between the results obtained from the DCT and TSDC techniques applicable for MIM and the results from MEMS switches. The comparison shows that the application of KPFM method to MIM and MEMS leads to the same results on the electrical properties of the dielectric material. This provides a novel powerful tool for the assessment of dielectric charging for MEMS switches using MIM capacitors which have much simpler layer structure. On the other hand the TSDC method reveals a continuous distribution of relaxation time constants, which supports the dependence of relaxation time constant calculated for MEMS on the duration of the observation time window.     

MEMS reliability from a failure mechanisms perspective

Over the last few years, considerable effort has gone into the study of the failure mechanisms and reliability of micro-electromechanical systems (MEMS). Although still very incomplete, our knowledge of the reliability issues relevant to MEMS is growing. This paper provides an overview of MEMS failure mechanisms that are commonly encountered. It focuses on the reliability issues of micro-scale devices, but, for some issues, the field of their macroscopic counterparts is also briefly touched. The paper discusses generic structures used in MEMS, stiction, creep, fatigue, brittle fatigue in silicon, wear, dielectric charging, breakdown, contamination and packaging.     

Reliability of MEMS shallow arches based actuator under the combined effect of mechanical shock and electric loads

Critical issues for the development of MEMS devices are their performance, reliability and survivability when subjected to unwanted loads, such as when dropped on a hard surface. These active forces can lead to tremendous destruction in these tiny mechanisms, such as stiction and all related short circuit problems in MEMS devices. Investigating the reliability of micro-structures under mechanical shock loads is a challenging job, driven, in part, by the large deflections that exacerbate system nonlinearities, such as those due to geometric (such as mid-plane stretching) and also the actuating nonlinear electric load. The proposed work aims to establish computationally efficient approaches that are capable of analyzing the transient dynamics of bi-stable MEMS devices, such as shallow arches, to mechanical shock and electric loadings. This investigation aims to improve the understanding of how mechanical shock loads can deteriorate the bi-stability of MEMS shallow arches. To this end, a Galerkin expansion reduced-order modeling (ROM) will be exploited. The capability of the ROM in simulating the bi-stable dynamical response of such devices to the combined effect of electrostatic force and shock load is thoroughly studied and analyzed. The ROM is utilized to explore the effect of several design parameters on the dynamic response of initially curved microbeams to shock loads: such as the shock amplitude, the shock duration, the beam initial curvature, and the DC voltage. Universal curves for the snap-through and pull-in voltages thresholds versus shock amplitude for various values of the nondimensional design constraints of the ROM are generated. These curves will present valuable information about the interaction between the shock and electrostatic forces and how to utilize this interaction to build new devices and propose new technologies.     

Dielectric charging induced drift in micro device reliability-a review

The movement or migration of charges in dielectric materials like silicon oxide, silicon nitride and glass, is recognized as one of the most significant causes of drift instability of MEMS devices which utilize electrostatic capacitive methods for sensing and driving. This paper reviews the current researches on the characteristics of drift phenomenon of three micro capacitive devices, micro switches, micro resonators and micro mirrors. The dielectric charging forms including polarization, ion injection and charge migration are presented in detail to explain the process and mechanism of how the charging effects gives rise to the drift of performance and influence the reliability of micro systems, and then the corresponding solutions to overcome specific drift issues are proposed based on the essential conditions needed to cause dielectric charging.     

Smart MEMS concept for high secure RF and millimeterwave communications

This paper presents a fully suspended MEMS based technology to assess the smart microsystem concept. The demonstrator is a redundancy ring for millimeterwave space communication. Reliability investigations are presented showing that the membrane is withstanding thermal stress, vibration and shocks. Concerning the switch, the main reliability behavior deals with the dielectric charging. We present a devoted DC life test stress coupled with a microwave characterization that allows to identify the screening and the stiction effects. MEMS based building blocks are presented featuring low insertion loss and high isolation in the millimeterwave range. Finally, it is proposed a dedicated topology of circuit that includes their reliability behavior.     

静电式RF MEMS开关的可靠性

MEMS开关是最常见的RF MEMS控制元件,是RF结构中一个关键的MEMS器件。长期可靠性是目前制约MEMS开关商业化进程中的一个主要问题。主要综述了静电式RF MEMS开关可靠性的新进展。欧姆式开关通常由于黏附或接触电阻的增大而失效,电容式开关的主要失效机理则与电介质层的充电有关。接触材料的选择是决定欧姆开关可靠性最重要的一个因素,"主动断开/被动接触"MEMS开关适用于软金属材料欧姆接触的可靠性要求。改善电容式开关可靠性的途径是改善介电层、优化驱动电压波形等以减小介质层的充电。     

ESD failure signature in capacitive RF MEMS switches

RF MEMS are commonly known as electrostatic devices using high electric field for their actuation. They can be exposed to transient voltages in any environment, and are very sensitive. According to this point of view, it is necessary to understand and analyze the degradations and failure criteria that can make them useless or reduce their lifetime. This paper deals with the investigation of ESD failure signature in capacitive RF MEMS. ESD experiments were carried out using a transmission line pulsing technique. It has been observed that electrical discharges give rise to sparks or electrical arcing and induced DC parameter shift, which can directly lead to changes in RF metrics. The contact-less dielectric charging effects of ESD pulses have been reported in this paper. It has been found that induced charges are predominant compared to injected ones through the trend of slope of the shift in the voltage corresponding to the minimum of capacitance.     

Science-based MEMS reliability methodology

In cases where device numbers are limited, large statistical studies to verify reliability are impractical. Instead, an approach incorporating a solid base of modelling, simulation, and material science into a standard reliability methodology makes more sense and leads to a science-based reliability methodology. The basic reliability method is (a) design, model and fabricate, (b) test structures and devices, (c) identify failure modes and mechanisms, (d) develop predictive reliability models (accelerated aging), and (e) develop qualification methods. At various points in these steps technical data is required on MEMS material properties (residual stress, fracture strength, fatigue, etc.), MEMS surface characterization (stiction, friction, adhesion, role of coatings, etc.) or MEMS modelling and simulation (finite element, analysis, uncertainty analysis, etc.). This methodology is discussed as it relates to reliability testing of a micro-mirror array consisting of 144-piston mirrors. In this case, 140 mirrors were cycled full stroke (1.5 μm) 26 billion times with no failure. Using our technical science base, fatigue of the springs was eliminated as a mechanism of concern. Eliminating this wear-out mechanism allowed use of the exponential statistical model to predict lower bound confidence levels for failure rate in a “no-fail” condition.     

Micro and nano structures of carbonised polymer through pyrolytic transformation from polymer structures

Micro and nano structures of carbonised polymers resulting from the pyrolytic transformation of polymer structures are presented. Polymers have become increasingly popular as materials for micro/nano electromechanical systems (MEMS/NEMS), especially for chemical or biological application. Focus is on the transformation of polymer structures into carbonised structures using a pyrolysis process. Combination of this pyrolysis process with conventional MEMS/NEMS fabrication technology could provide various fine structures of carbonised polymer. Carbonised polymers have advantages over conventional carbon materials, with respect to compatibility with MEMS, because they can be transformed directly from a polymer structure. Three-dimensional micro and nano free-standing structures of carbonised polymer as typical MEMS/NEMS structures are reported. Micro molding process is used to demonstrate a unique polymer structure to be pyrolysed. Furthermore, EB lithography technology is employed for the patterning of polymers in addition to UV photolithography which is used by previous researches. A 100 nm wide bridge structure is designed as nano structures. In addition, the presented structures of carbonised polymer are expected to be applied to micro and nano functional devices such as electrochemical sensors by making the best use of their carbon-like features     

Modeling of dielectric charging in electrostatic MEMS switches

The most important failure mechanism for electrostatic MEMS switches is dielectric charging, which contributes to a significant reduction of the device lifetime. In this study the correlation between the dielectric properties and the switch lifetime is evaluated. The conduction mechanism and trapping kinetics for two types of PECVD SiN_x are determined by I–V sweeps and constant-current injections from Metal–Insulator–Metal (MIM) capacitors. This type of procedure is used as a basis for modeling the charge build-up in a switch. Despite significant differences between the dielectrics, in terms of leakage current and trapping properties, the numerical model of charge build-up fits well with experimental data. We conclude that the switch lifetime can be correlated with the trapping properties of the dielectric itself.     

Center-Shift Method for the Characterization of Dielectric Charging in RF MEMS Capacitive Switches

Radio frequency (RF) micro-electro-mechanical systems (MEMS) capacitive switches show great promise for use in wireless communication devices such as mobile phones, but for the successful application of these switches their reliability needs to be demonstrated. One of the main factors that limits the reliability is charge injection in the dielectric layer (SiN) which can cause irreversible stiction of the moving part of the switch.      

自组装技术在MEMS中的应用

起源于生物化学领域的自组装技术正被广泛地应用到了化学、材料、生物、电子、机械等不同的学科中。在MEMS和NEMS中,自组装作为一种新型的“自下而上”的微(纳)结构制备和装配技术而得到积极的关注,并显示出良好的应用前景。在阐述自组装技术发展的基础上,介绍了该技术在MEMS中的典型应用,讨论一些待解决的关键问题,最后展望了自组装技术的发展趋势。     

从MEMS到NEMS进程中的技术思考

回顾和概括了MEMS机电器件进入μm/nm尺度后解决的几个关键性问题。结合微机电器件发展的典型事例进行分析,对我国科学工作者在其中的贡献给予肯定。分析目前机电器件走向nm尺度的一些相关概念理解和需要着重研究的纳米效应问题。对机电器件从微到纳的发展趋势进行了展望。     

Failure Analysis Issues in Microelectromechanical Systems (MEMS)

Failure analysis and device characterization of MEMS components are critical steps in understanding the root causes of failure and improving device performance. At the wafer and die level these tasks can be performed with little or no sample preparation. Larger challenges occur after fabrication when the device is packaged, capped, sealed, or otherwise obstructed from view. The challenges and issues of MEMS failure analysis lie in identifying the root cause of failure for these packaged, capped, and sealed devices without perturbing the device or its immediate environment. Novel methods of ainin access to the device or preparing the device for analysis are crucial to accurately determining the root cause of failure. This paper will discuss issues identified in performing root cause failure analysis of packaged MEMS devices, as well as the methods employed to analyze them.     

Investigation of high performance Edge Lifted Capacitors reliability for GaAs and GaN MMIC technology

This paper reports extensive investigations of Edge Lifted Capacitors (ELC) and standard metal–insulator–metal (MIM) capacitors with different refractive index and thickness of Silicon Nitride (Si₃N₄) dielectric films. The wafer-level electrical measurements reveal size dependence of capacitances and breakdown voltages. Physical characterization was performed using Fourier transform infrared spectroscopy (FTIR) to understand intrinsic properties of the studied films and failure-related cross sections were used to predict possible leakage mechanisms. Reliability testing of Human Body Model (HBM) and Machine Model (MM) electrostatic discharge (ESD), time-dependent dielectric breakdown (TDDB), and biased high temperature accelerated stress testing (bHAST) were performed and will be reviewed for GaAs and GaN monolithic microwave integrated circuit (MMIC) applications.     

Packaging for microelectromechanical and nanoelectromechanical systems

Packaging is a core technology for the advancement of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS). We discuss MEMS packaging challenges in the context of functional interfaces, reliability, modeling and integration. These challenges are application-dependent; therefore, two case studies on accelerometers and BioMEMS are presented for an in-depth illustration. Presently, most NEMS are in the exploratory stage and hence a unique path to identify the relevant packaging issues for these devices has not been determined. We do, however, expect the self-assembly of nano-devices to play a key role in NEMS packaging. We demonstrate this point in two case studies, one on a silicon nanowire biosensor, and the other on self-assembly in molecular biology. MEMS/NEMS have the potential to have a tremendous impact on various sectors such as automotive, aerospace, heavy duty applications, and health care. Packaging engineers have an opportunity to make this impact a reality by developing low-cost, high-performance and high-reliability packaging solutions.     

Enhancement of RF-MEMS switch reliability through an active anti-stiction heat-based mechanism

MicroElectroMechanical Systems for Radio Frequency applications (i.e. RF-MEMS) show very good performance and characteristics. However, their employment within large-scale commercial applications is still limited by issues related to the reliability of such components. In this work we present the Finite Element Method (FEM) modelling and preliminary experimental results concerning an active restoring mechanism, embedded within conventional MEMS/RF-MEMS ohmic (and capacitive) relays, capable of retrieving the normal operation of the switch if stiction occurs (i.e. the missed release of an actuated switch when the controlling bias is removed). The mechanism exploits the heat generated by an electric current flowing through an high-resistivity poly-silicon serpentine (Joule effect), to induce deformations in the suspended MEMS structures. Such changes in the mechanical structure result in shear and vertical restoring forces, helping the membrane release. The FEM-based thermo-electromechanical simulations discussed in this work include the coupling between different physical domains, starting from the imposed current, to the MEMS deformation. The preliminary experimental data reported in this paper show a speed-up of the dielectric discharge time due to the generated heat, as well as a change in the S-parameters, due to the membrane expansion, compatible with an upward bending of the central contact (i.e. restoring force), useful to counteracting stiction due to micro-welding.     

Simultaneous Quality and Reliability Optimization for Microengines Subject to Degradation

Micro-Electro-Mechanical Systems (MEMS) represent an exciting new technology, but to achieve more widespread usage and wider adoption within more industrial applications, they must be highly reliable, and manufactured to stringent quality standards. Many challenging manufacturing issues are of concern during the fabrication of MEMS, such as precise dimensional inspection, reliability modeling, burn-in scheduling, avoiding stiction, and maintenance strategies. However, only limited mathematical tools for improving MEMS reliability, quality, and productivity are currently available. This paper proposes a mathematical model to jointly determine inspection & preventive replacement policies for surface-micromachined microengines subject to wear degradation, which is a major failure mechanism for certain MEMS devices. The optimal specification limits for inspection, and the replacement interval are determined by simultaneously optimizing MEMS quality and reliability. The proposed model can be used as a tool for decision-makers in MEMS manufacturing to make sound economical and operational decisions on reliability, quality, and productivity. While illustrated considering one specific microengine design, the proposed model can be applied to a broader range of MEMS devices that experience wear degradation between rubbing surfaces.      

Alpha particle radiation effects in RF MEMS capacitive switches

The paper investigates the effect of 5 MeV alpha particle irradiation in RF MEMS capacitive switches with silicon nitride dielectric film. The investigation included MIM capacitors in order to obtain a better insight on the irradiation introduced defects in the dielectric film. The assessment employed the thermally stimulated depolarization currents method for MIM capacitors and the capacitance–voltage characteristic for MEMS switches. Asymmetric charging was monitored in MIM capacitors due different contact electrodes and injected charge interactions.