Microand Nanoelectromechanical Systems Research Articles

Molecular dynamics study of graphene-coated reinforced tribomechanical properties: Hard versus soft substrates

As a two-dimensional material, graphene film is a good solid lubricant candidate for improving the tribomechanical performance of micro-and nano-electromechanical systems. Graphene has a significant impact on protecting the top layer of a substrate from wear and reducing friction. However, the frictional behavior has been found to vary when coating graphene onto different substrates. In the present study, using molecular dynamics (MD) simulations, graphene was deposited as a monolayer coating on the substrate sliding against the diamond tip, which revealed not only significant improvement in tribological behavior by reducing both friction and contact deformation volume, but also notably demonstrated the difference between hard and soft substrate coating with graphene. Under 58.5nN loading, the optimal antifriction effect of graphene coating on Au substrate was 94.2%, with a higher friction indicating a greater indentation depth and greater contact deformation. The contact deformation volume can be used as a standard to measure the surface friction of graphene. Unlike hard Si substrates, monolayer graphene coatings are prone to rupture at 200 nN loads. At low loads, the single graphene coating has the best anti-friction effect on the soft gold substrate, and at higher loads, the graphene on the hard silicon substrate is not prone to rupture, providing continuous and stable lubrication. The study also revealed a tunable toughening mechanism of graphene layers. The mechanism entails a reduction in the volume of substrate contact deformation with the progressive addition of graphene layers. This finding has the potential to enhance the tribological performance of the graphene-coated substrate.

Electronic friction and tuning on atomically thin MoS2

Friction is an energy dissipation process. However, the electronic contribution to energy dissipation channels remains elusive during the sliding friction process. The friction and dissipation on atomically thin MoS2 with semiconductive characteristics are studied and tuned by the gate-modulated carrier concentration. The electronic contribution to energy dissipation of friction on atomically thin MoS2 was confirmed and regulated through tuning the strength of the electron-phonon coupling. The electron-phonon coupling can be strengthened and depressed to increase and decrease friction by the gate-modulation of the carrier concentration. The fitting of the friction on atomically thin MoS2 and carrier concentration is approximately linear which is in accordance with Langevin equation induced friction. Then the active, dynamical, and repeated tuning of friction on atomically thin MoS2 with semiconductive properties is achieved by the active modulation of carrier concentration with gate voltage. These observations help us to understand the electronic friction in essence, provide a utility approach to tune the friction intelligently on atomically thin two-dimensional materials with semiconductive properties and achieve superlubric properties for the application in various micro-and nanoelectromechanical systems.

An ultra-efficient recycling folded cascode OTA based on GAA-CNTFET technology for MEMS/NEMS capacitive readout applications

This paper demonstrates an ultra-efficient recycling folded cascode (RFC) operational transconductance amplifier (OTA) as one of the cardinal blocks in most analog microsystems. Two positive feedbacks are utilized in the proposed RFC OTA to enhance the input transconductance and the output impedance. All GAA-CNTFETs are biased in the near-threshold region (VT≈0.37 V and VDD=±0.2 V) to reduce power consumption. The simulation results indicate that with CLoad = 5pF, the proposed RFC OTA shows 102 dB open-loop gain and 17.2 MHz unit gain-bandwidth with an ultra-low power consumption of 111.5nW and a reasonable slew rate response (17.3 V/µs). A capacitive readout circuit for micro-and nano-electromechanical systems (MEMS/NEMS) applications is suggested to assess the proposed RFC OTA's performance in practical applications. By utilizing an RC sensing feedback structure, the proposed transimpedance amplifier presents 155dBΩ gain with a 400 MHz bandwidth. Compared to the other counterparts, the proposed GAA-CNTFET-based TIA presents fascinating power savings 86% total average improvement) and 1.6 times total average gain improvement. Our simulation results accentuate that the proposed GAA-CNTFET-based RFC OTA is a powerful candidate for designing ultra-efficient MEMS/NEMS capacitive accelerometers readout circuits.

Generic Rotating-Frame-Based Approach to Chaos Generation in Nonlinear Micro- and Nanoelectromechanical System Resonators.

This Letter provides a low-power method for chaos generation that is generally applicable to nonlinear micro- and nanoelectromechanical systems (MNEMS) resonators. The approach taken is independent of the material, scale, design, and actuation of the device in question; it simply assumes a good quality factor and a Duffing type nonlinearity, features that are commonplace to MNEMS resonators. The approach models the rotating-frame dynamics to analytically constrain the parameter space required for chaos generation. By leveraging these common properties of MNEMS devices, a period-doubling route to chaos is generated using smaller forcing than typically reported in the literature.

Cast Monocrystalline Silicon: New Alternative for Micro- and Nano-Electromechanical Systems?

Casted silicon wafers dominate the current photovoltaic (PV) market due to much lower fabrication costs as compared to well-known Czochralski (Cz) –growth. Traditionally casted silicon ingots have been multicrystalline, but recent developments in casting technology have enabled also the growth of single crystalline (sc) silicon ingots. While the resulting sc-Si ingot quality is naturally high enough for PV, it is not sufficient for the integrated circuit (IC) industry, mainly due to the increased amount of intrinsic point defects and dislocations in comparison to Cz-Si. However, many applications that do not have such stringent requirements for substrates, such as micro-and nano-electromechanical systems (MEMS and NEMS), could potentially find this material beneficial. Indeed, here we take the first step in studying the applicability of cast mono-Si for such applications. More specifically, we focus on advanced focused ion beam lithography combined with deep reactive ion etching for NEMS and wet etching for MEMS. Our results show that the quality of cast mono-Si is high enough for successful patterning in both micro- and nanoscale. Sub-micron resolution is achieved and the Ga+ doses required for successful patterning are comparable to conventional Cz-Si. The preliminary results presented here thus show great promise for cast mono-Si as a low-cost alternative for micro- and nano-electromechanical systems. [2019-0081]

Substrate Influence on the Mechanical and Tribological Characteristics of Gold Thin Films for MEMS Applications

The development of micro-and nanoelectromechanical systems (MEMS/NEMS) makes use of different thin films such as aluminum, gold, silicon, silver, titanium nitride, silicon carbide etc. This study is a research concerning the influence of substrate nature on the tribological and mechanical characteristics of gold thin films elaborated by thermal evaporation method, for space applications. Three different substrates were employed, namely: C45 steel, plastic (polycarbonate) and glass. Atomic force microscopy investigations were performed in order to characterize the obtained thin films at nanoscale. The nanohardness, Young’s modulus, roughness and the friction force are some characteristics that were determined. A significant influence of substrate nature on both mechanical and tribological properties of researched gold thin films was marked out. Regarding the topography, the smallest roughness was determined on the gold thin films deposited on glass substrate.

A MEMS-based high frequency x-ray chopper

Time-resolved x-ray experiments require intensity modulation at high frequencies(advanced rotating choppers have nowadays reached the kHz range). We here demonstratethat a silicon microlever oscillating at 13 kHz with nanometric amplitude can be used as ahigh frequency x-ray chopper. We claim that using micro-and nanoelectromechanicalsystems (MEMS and NEMS), it will be possible to achieve higher frequenciesin excess of hundreds of megahertz. Working at such a frequency can open awealth of possibilities in chemistry, biology and physics time-resolved experiments.

Special issue on micro- and nano-electromechanical systems

This issue of Nonlinear Dynamics is devoted to recent advances in the dynamics of microand nanoelectromechanical systems (MEMS and NEMS). These systems are composed of micro-machines inherently coupled to electric circuits or embedded on silicon chips. Market research companies quote various numbers for the size of MEMS market and its growth rate. Considering the variation of these numbers among different sources and over time, one is probably well advised to take them with a grain of salt. But there is no doubt that MEMS is now an established, “dynamic,” and growing technology and that NEMS is its natural extension. They are today the dominant technology in many sensor and actuator applications; such as accelerometers, pressure sensors, and projection systems. The dynamics of MEMS and NEMS are rich in nonlinearity. Nonlinear phenomena appear in the properties of materials operating under large environmental variations, in the geometry of structures undergoing large deformations, and in the actuation forces rele-

Improved algorithm for material characterization by simulated indentation tests

The paper involves the establishment of a neural network model with improved algorithm for reverse analysis of simulated indentation tests considering the effects of friction on the contact surfaces. Extensive finite element analyses covering a wide practical range of materials obeying power law strain-hardening have been carried out to simulate the indentation tests. The results obtained from the simulated dual indentations using conical indenters with different geometries considering effects of friction are adopted in the training and verification of the least squares support vector machines involving structural risk optimization. The characteristics and performances of the neural network model for this class of problems are given and deliberated. The tuned networks are able to predict accurately the mechanical properties of a new set of materials. The approach has great potential for the applications on the characterization of a small volume of materials in micro-and nano-electromechanical systems (MEMS & NEMS).

Deformation of nanostructured materials: insights from experimental observations

Increased interest in nanoscale deformation has been spurred by the rapid miniaturization of features and components in microelectronic devices, magnetic storage media, and MEMS/NEMS, i.e., microand nanoelectromechanical systems, respectively. Materials for these applications are often chosen because of their electronic, optical, or magnetic properties. Frequently, however, the mechanical properties are the performance-limiting factor. Until recently, the techniques used to produce samples resulted in a high number of induced artefacts and, hence, irregular mechanical properties. Thanks to progress in processing methods, we can now make nanograin samples of high purity and high density that, under experimental conditions, show reproducible characteristics. Such fully dense materials are excellent candidates for fundamental mechanical property studies as well as for high-performance applications. Compared with their microcrystalline counterparts,nanostructured metals with grain size smaller than 100nm possess attractive properties such as high yield and fracture strength and improved wear resistance. But evidence also suggests that miniaturization may influence their microstructural and mechanical stability, resulting in undesirable shape change, mechanical response, thermal behavior, and electrical performance. These instability phenomena are the main focus of our research efforts. Small sample size generally rules out standardized tests, such as tensile and compression tests. But advances in methods such as nanoindentation accommodate very small amounts of material. Traditional nanoindentation testing measures the elastic modulus and hardness of engineering materials by pushing a probe into the material while monitoring load and penetration depth. Now, instrumentation offers new possibilities. For example, we perform cyclic nanoindentation tests with up to a million load Figure 1. Shown is the grain structure of nickel following 10 loading cycles using a pyramidal tip. The initial grain size was 30nm. Some areas of the indented region (along the edge as indicated in the insert) show significantly larger grains and increased surface roughness after cyclic contact loading, as marked by arrows.