Editorial

Advances in the field of microelectromechanical systems (MEMS) together with the companion field of Nanotechnology pose some of the greatest challenges that are currently being solved by experimental mechanics. This Special Issue of Strain addresses the theme of current trends and future directions in MEMS and Nanotechnology. A number of authors have been invited to share with us their expertise relating to the theme and 10 papers were finally selected to appear in this volume. These papers concentrate on pertinent issues relating to design, fabrication including packaging, testing and characterisation of various devices enabled by recent advances in MEMS and Nanotechnology. In fact, according to one of the authors featured herein, as recent as 2005, value of products based on technologies discussed in this volume was about $8bn with 40% of that being in sensors and the balance was for medical products, personal communication and portable devices as well as other daily use devices. Growth projections indicate that value of the products will rise to $40bn in 2015 and $200bn by 2025. Undoubtedly, the projected 25-fold growth in products – during a time span of two decades – will be enabled by development of appropriate technologies for integrating multiple devices with electronics in a single package to achieve capabilities that are unthinkable today. As discussed in this volume, current trend in micro/nanosystems is to produce ever smaller, lighter and more capable devices at a lower cost than ever before. In addition, the finished products have to operate at very low power and in extremely adverse environments while assuring durable and reliable performance. Some of the new devices are being developed to function at high operational speeds, others to make accurate measurements of operating conditions of specific processes as well as to facilitate medical diagnosis. Continuing advances in MEMS and Nanotechnology allow monolithic integration of specific devices with driving, controlling and signal processing electronics. This integration improves performance of the devices as well as reduces the cost of manufacturing, packaging and interconnecting these devices by combining them with a complementary electronic subsystem(s) in the same manufacturing and packaging process. As such, MEMS and Nanotechnology have a potential to facilitate our forthcoming attempts to resolve challenges posed by recent evolution of events in economy, energy, transportation, communication, health care as well as a number of other contemporary issues. One of the systems, discussed herein, that were made possible by the theme technology, is a micromirror system for optical applications. This microsystem is a part of a group of micro-optoelectromechanical systems (MOEMS). The MOEMS, fabricated using silicon and polysilicon micromachining processes, have widespread applications including, but not limited to, optical beam steering, scanners, adaptive optical arrays, flat panel displays, optical interconnects, internet, HDTV, etc. Specific advantages that MOEMS have over their larger, conventional, counterparts are lower mass, faster response speeds, lower operating power, compact design, and a potential for large arrays of micro-optical elements. All in all, effective development of MEMS and Nanotechnology requires multidisciplinary knowledge that relates to various aspects of design, analysis/simulation, properties of materials, fabrication with special emphasis on packaging, and testing/characterisation of the finished products to be useful in contemporary applications, including health care. These aspects are enumerated within this volume. More specifically, Marinis commences this Special Issue with a discussion of the future of MEMS and related products by stating that the future is multifaceted, complex and subject to change, in response to investment by government and commerce; the author forecasts the future from a perspective of technology development, business growth and system integration. Furthermore, Marinis states that the key to enabling the projected 25-fold growth in MEMS products (from an estimated $8bn in 2005 to $200bn in 2025) is development of appropriate technologies for integrating multiple devices with electronics on a single chip. At present, either the MEMS device is fabricated in polysilicon, as part of the CMOS wafer fabrication sequence, or a discrete MEMS device is packaged with a separate ASIC chip. Neither of these approaches is satisfactory for building the high value, system-on-chip products that are imagined. Weng in his paper presents semiconductor process module integration and a solution to the problem of defects reduction on a wafer fabricated in a damascene process by considering BEOL (Back-End-Of-Line) structure. According to Cole et al., shape memory alloys (SMAs) are being noticed as microactuators capable of high-power and high-frequency responses. SMA films with compositional gradients have a unique feature of an intrinsic two-way shape memory effect that greatly amplifies their functionality. The authors propose a new method for fabricating and characterising compositionally graded SMA thin films for thermomechanically controlled self-assembly. The technique provides increased control over the gradient architecture to facilitate design of more functional actuators. Realising that mechanical properties of materials depend on the volume of material that is being deformed and that to date there are no known studies of how confined geometries affect the brittle-to-ductile transitions in BCC materials, Walter et al. perform bulge testing of thin films subjected to temperatures ranging from cryogenic to elevated levels in an attempt to generate new data on behaviour of thin molybdenum films. Rodriguez-Vera and his coauthors present the implementation of a structured light projection technique for analysis of 3D vibration modes of microsamples. They perform automated Fourier transform analysis of microcantilever-type samples at each frame delivered by a high-speed camera with micrometric resolution. Great sensitivity and full-field-of-view capabilities of Talbot imaging allow examination of differences between real and theoretical behaviours which is of utmost importance in designing and testing structural dynamic responses of MEMS and other microstructures. Furlong presents a combination of analytical, computational and experimental solutions methodology for evaluation of a micromirror device for interferometric applications. He develops analytical and computational nonlinear geometrical models in order to determine optimal loading-displacement operational characteristics of a micromirror and uses optoelectronic holographic microscopy methodologies, based on multiple wavelengths, to typify these characteristics as a function of the operational mode(s) of the micromirror. His results indicate that the micromirror considered is suitable for optical phase measurements and appropriate for the development of miniature optical instruments for characterisation of shapes and deformations. This will undoubtedly lead to advancement of new tools for experimental mechanics. In their paper, Lall et al. investigate a design-envelope approach based on optical feature extraction techniques for drop and shock survivability of electronic packaging using six-leaded systems employing several solder alloys. Their investigation utilises digital image correlation for transient dynamic measurements and optical feature extraction. During this study, board assemblies have been subjected to shock impact in various orientations including industry-standard zero-degree drop and a vertical free drop. In all cases, measurements have been taken on both the package and the board side of assemblies and accuracy of high-speed optical measurements has been compared with that of discrete strain gauges. A number of currently used package architectures were examined. Explicit finite-element models have been developed and correlated with experimental data. The potential for damage identification and tracking of various solder alloys has been investigated and the corresponding data, on identification of damage alternatives for competing failures, has been presented. On the basis of statistical pattern recognition, the authors have developed design envelops intended for component integration to assure survivability in shock and vibration environments at the user-specified confidence level. Sheehy et al. consider microscale mechanical devices subjected to shock and vibration stimuli. They investigate the use of experimental and simulation methods in quantifying behaviour of selected MEMS devices. Representative microcantilevers were analysed under vibration and shock in order to determine mechanical properties of single crystal silicon. The experimental approach allowed non-invasive in situ monitoring of microcantilevers upon impact using laser Doppler vibrometry and high-speed imaging. Their investigation of the shock response of representative microcantilevers indicates that orientation plays a significant role in the sensitivity of a microdevice to shock due to planarity of the fabrication technique used. Finite-element analysis in conjunction with in situ high-speed imaging proved to be a viable non-invasive inverse technique to determine the loci and amplitude of tensile stress within generic microscale devices. Vibrating microcantilevers were also investigated by Klempner et al. However, now the goal was to determine Quality factors of silicon microcantilevers coated with thin metal films. This paper recognises that MEMS are typically fabricated out of materials that are good structurally at the microscale but can be relatively poor electrical conductors. For this reason, selected areas of MEMS are coated with various thin metal films to provide electrical pathways. These coatings, however, can drastically alter mechanical properties of a device. One-side or two-side metalised microcantilevers enable a number of MEMS-based products, especially sensors, which – according to the first paper in this volume – contribute major portion of revenue derived from MEMS-based products. Regardless of their implementation, sensors are always designed to provide the most precise response to the signals they are developed to detect and/or monitor. One way to describe this sensitivity/precision is to use the Quality factor. In this paper, Klempner et al. investigate using ring-down method how metallisation of microcantilevers affects the Quality factors. Results show that the greater the thickness of the aluminium coating the lower the Quality factor will be. The authors also show a significant temperature dependency of the Quality factor on aluminium-coated microcantilevers, which has important ramifications for sensors used in harsh environments especially in the surroundings characterised by variable temperature. In the paper concluding this Special Issue, Furlong et al. use computer-aided optoelectronic holography to obtain preliminary measurements of the sound induced displacements of the tympanic membrane of cadaver cats and chinchillas. Holograms, processed at video rates, were used to characterise the frequency dependence of tympanic membranes as tone frequency was swept through a range of interest. Stroboscopic holography was used at selected frequencies to measure in full-field-of-view displacements of the tympanic membrane surface with very high resolution and nanometer measurement accuracy. Time-average holographic data suggest standing wave patterns on the cat’s tympanic membrane surface, which move from simple uni-mode to bi-modal patterns at low frequencies through complicated multimodal patterns above 3 kHz, to highly ordered arrangements of displacement waves with tone frequencies above 15 kHz. The frequency boundaries are lower in chinchilla than cat. I wish the readers to benefit from the papers in this Special Issue and to learn from them as much I have enjoyed working with the authors during the development of this volume.