Nanotribology-based novel characterization techniques for the dielectric charging failure mechanism in electrostatically actuated NEMS/MEMS devices using force-distance curve measurements

The work presents a comprehensive package of novel nanoscale characterization techniques to study dielectric charging in electrostatic nano- and microelectromechanical systems (NEMS and MEMS). The proposed assessment methodologies are based on the force-distance curve (FDC) measurements performed using an atomic force microscope (AFM) to measure, for the first time, the induced surface potential and adhesive force over charged dielectric films. They were employed to study plasma enhanced chemical vapor deposition (PECVD) silicon nitride films for application in electrostatic capacitive RF MEMS switches. Three different techniques were introduced including the application of FDC measurements to study charging in bare SiN(x) films, metal-insulator-metal (MIM) capacitors, and MEMS switches. The results from the three methods were correlated and compared with the published data from other characterization techniques, mainly charge/discharge current transient (C/DCT) and Kelvin probe force microscopy (KPFM). The unique advantages of the proposed FDC-based characterization techniques are twofold. First, they can measure the multiphysics coupling between the dielectric charging phenomenon and tribological issues at the interface between the switch bridge and the dielectric surface. Second, the FDC-based techniques can measure larger levels of induced surface potential over charged dielectric films which results from the high electric field normally used to actuate MEMS switches. Based on the proposed FDC techniques, the influence of several parameters on dielectric charging/discharging processes was investigated: the dielectric film thickness, deposition conditions, substrate, and electrical stress conditions.     

Dynamic transitions in molecularly thin liquid films under frictional sliding

The friction properties of the molecularly thin films of an asymmetric ether, 1,3-dimethylbutyl hexadecyl ether (DBHE), confined between mica surfaces were investigated using the surface forces apparatus. Kinetic friction was measured as a function of normal load and sliding velocity, and the static friction (stiction) was measured as a function of normal load and surface stopping time. Kinetic friction measurements exhibited unstable sliding dynamics: the friction force exhibited cyclic bumps and valleys in the sliding velocity range from about 10(-2) to 1 microm/s, but above and below the velocity range, smooth sliding was observed. Stop-start experiments showed a stiction spike when surface stopping time exceeded a characteristic nucleation time, indicative of the static friction state at very low sliding velocity. These results imply that the friction of the confined DBHE film has at least three responsible friction states--static friction and two different kinetic friction states--depending on the sliding velocity. The unstable sliding (bumps and valleys of the friction force) reflects the dynamic transition between two different kinetic states. The different friction states and their transitions are discussed on the basis of the recent experiments and theories of "inverted" stick-slip friction.     

Interfacial properties for real rough MEMS/NEMS surfaces by incorporating the electrostatic and Casimir effects–a theoretical study

This paper studied the adhesive properties of real rough micro/nano‐electromechanical systems (MEMS/NEMS) surfaces by considering the electrostatic force and the Casimir force theoretically, and an improved model has been proposed. A statistical approach for characterizing surface topography was used by taking the surface standard deviation, the asperity density and the radius of curvature into account. The effects of surface roughness on the electrostatic force and the Casimir force were analysed individually, and a comparison between the proposed model and existing models has been conducted. The whole adhesive force increases with the surface standard deviation, and the prediction by the proposed model becomes more in agreement with the one by existing models when the surface standard deviation is increased. The contribution of the Casimir force to the total adhesive force tends to vanish when the surface standard deviation is relatively large. The electrostatic force and the Casimir force contribute more to the total adhesive forces calculated based on the proposed model with the increase of the asperity density and the radius of curvature. Copyright © 2009 John Wiley & Sons, Ltd.     

Ionic liquid nanotribology: stiction suppression and surface induced shear thinning

The friction and adhesion between pairs of materials (silica, alumina, and polytetrafluoroethylene) have been studied and interpreted in terms of the long-ranged interactions present. In ambient laboratory air, the interactions are dominated by van der Waals attraction and strong adhesion leading to significant frictional forces. In the presence of the ionic liquid (IL) ethylammonium nitrate (EAN) the van der Waals interaction is suppressed and the attractive/adhesive interactions which lead to "stiction" are removed, resulting in an at least a 10-fold reduction in the friction force at large applied loads. The friction coefficient for each system was determined; coefficients obtained in air were significantly larger than those obtained in the presence of EAN (which ranged between 0.1 and 0.25), and variation in the friction coefficients between systems was correlated with changes in surface roughness. As the viscosity of ILs can be relatively high, which has implications for the lubricating properties, the hydrodynamic forces between the surfaces have therefore also been studied. The linear increase in repulsive force with speed, expected from hydrodynamic interactions, is clearly observed, and these forces further inhibit the potential for stiction. Remarkably, the viscosity extracted from the data is dramatically reduced compared to the bulk value, indicative of a surface ordering effect which significantly reduces viscous losses.     

Responsive organometallic polymer grafts: electrochemical switching of surface properties and current mediation behavior

Quantitative adherence and friction measurements between atomic force microscopy (AFM) tips and reversibly oxidized and reduced poly(ferrocenyl dimethylsilane) (PFDMS) molecular layers grafted to Au are reported. Poly(ferrocenylsilanes) (PFSs) such as PFDMS owe their redox responsiveness to the presence of ferrocene units, bridged by substituted silicon units, in the main chain. Polymers were obtained by anionic polymerization, which allowed us to copolymerize sulfur containing end groups that facilitated grafting to Au surfaces. Electrochemical atomic force microscopy (ECAFM) was used to study adherence and friction as a function of the oxidation state of the polymer. Measurements of interfacial friction as a function of applied load on the nanoscale using Si(3)N(4) AFM tips revealed a reversible increase of the friction coefficient and adherence strength of the PFDMS layers with increasing oxidation state in NaClO(4) electrolytes. The variation of the electrolyte salts (NaClO(4) or NaNO(3)) allowed an assessment of surface counterion adsorption effects. Issues related to the interpretation of observed friction and adherence changes such as electrolyte anion-ferrocenium ion pair effects, and electrostatic forces due to tip surface charges are discussed. Unidirectional current flow was detected in cyclic voltammograms of the PFDMS layers in NaClO(4). This electrode rectification behavior could in principle be utilized for applications in thin film devices based on PFS films.     

Adhesion and friction studies of Au nanoparticle‐textured surfaces with colloidal tips

Surface roughness plays an important role in affecting the adhesive force and friction force in microelectromechanical systems (MEMS)/nanoelectromechanical systems (NEMS). One effective approach of reducing adhesion and friction of contacting interfaces is to create textured surface, which is especially beneficial for MEMS'/NEMS' production yield and product reliability. In this article, we present a convenient method to fabricate the nano‐textured surfaces by self‐assembling Au nanoparticles (NPs) on the silicon (100) surfaces. The nanoparticle‐textured surfaces (NPTS) with different packing density and texture height were prepared by controlling the assembling time and the size of Au NPs. The morphologies and chemical states of NPTS were characterized by atomic force microscope (AFM), field emission scanning electron microscope, and XPS. The adhesion and friction on the NPTS were studied by AFM with colloidal tip. The results show that the nano‐textured surfaces have effectively reduced adhesive force and friction force compared with the 3‐aminopropyl trimethoxysilane self‐assembled monolayer surfaces. The lowered adhesion and friction were attributed to the reduced real area of contact between NPTS and colloidal tip. The adhesion and friction of the NPTS are varying with the texture packing density and dependent on both the texture height and asperities spacing, which are related to the size and coverage ratio of NPs on surfaces. Copyright © 2011 John Wiley & Sons, Ltd.     

Forces and friction between hydrophilic and hydrophobic surfaces: influence of oleate species

The atomic force microscope has been used to investigate normal surface forces and lateral friction forces at different concentrations of sodium oleate, a frequently used fatty acid in the deinking process. The measurements have been performed using the colloidal probe technique with bead materials consisting of cellulose and silica. Cellulose was used together with a printing ink alkyd resin and mica, whereas silica was used with a hydrophobized silica wafer. The cellulose-alkyd resin system showed stronger double layer repulsion and the friction was reduced with increasing surfactant concentration. The adhesive interaction disappeared immediately on addition of sodium oleate. The normal surface forces for cellulose-mica indicated no apparent adsorption of the sodium oleate however, the friction coefficient increased on addition of sodium oleate, which we ascribe to some limited adsorption increasing the effective surface roughness. The silica-hydrophobic silica system showed a completely different surface force behavior at the different concentrations. An attractive hydrophobic interaction was evident since the surfaces jumped into adhesive contact at a longer distance than the van der Waals forces would predict. The strong adhesion was reflected in the friction forces as a nonlinear relationship between load and friction and a large friction response at zero applied load. Indirect evidence of adsorption to the hydrophilic silica surface was also observed in this case, and QCM studies were performed to confirm the adsorption of material to both surfaces.     

Dielectric properties under corona charging of thermoplastics for holographic optical switching

Photothermoplastics are materials which have been proposed for optical switching devices: a grating is recorded by surface deformation of the thermoplastic layer, coated on an organic photoconductive layer used, like in xerography, for an electrostatic image recording material. But first of all an electrical field is applied through a corona charging sequence and obviously dielectric properties of the abietic ester thermoplastic material are of importance. This paper discusses the behaviour of such a layered thermoplastic material in a corona set-up, by measuring the corona current and the surface voltage following thickness and temperature of the sample and charging or discharging times.     

Sliding-friction electrification of polymers and charge decay

Linear low-density polyethylene (LLDPE) and polypropylene (PP) were charged by sliding under a cylindrical aluminium contact. The surface charge accumulation on the polymer films was time dependent, and a function of the sliding velocity, metal contact force, and film type. The surface potential increased linearly with velocity in the range 0.33–0.75 m/s, and showed a square-root dependence on the contact force up to 6.5 N. Thermally stimulated charge decay (TSCD) studies showed longer charge lifetimes in samples friction-charged on one side than in corona-charged samples. Friction charging creates deeper near-surface traps than are normally present in the polymers. Charge stability is further imporved by simultaneously friction charging both surfaces of the films, increasing the half-value charge decay temperature (T_1/2) by 27°C and 37°C over that in corona-charged samples (i.e., to 98°C and 120°C for LLDPE and PP, respectively). These monopolarity electrets should prove useful in charge storage devices.     

Photocontrol of cell adhesion and proliferation by a photoinduced cationic polymer surface

In this study photoinduced cation generation, based on the photochemical properties of malachite green (MG), was used for the surface design and in vitro photochemical control of cell adhesion and proliferation. The MG-derivatized surface was prepared by coating a photoreactive polymer as a substrate onto a poly(ethylene terephthalate) (PET) sheet. The photoreactive polymer was radical copolymer of styrene with the MG-derivatized monomer diphenyl(4-vinylphenyl)methane leucohydroxide (degree of substitution of MG unit: 12.4 mol%). Water contact angle measurements and X-ray photoelectron spectroscopy revealed high hydrophobicity and homogeneous distribution of the MG groups on the outermost surface of the coated film, respectively. When the coated film was exposed to ultraviolet light (UV) irradiation at wavelengths of 290-410 nm, a time-dependent color change of the film was observed from pale yellow, before irradiation, to green. These results indicated generation of cations on the film surface by photochemical cation generation of the MG groups, which was quantitatively characterized by force versus distance curves measurements in atomic force microscopic (AFM) observation using a carboxylated AFM tip. The seeding and culture of endothelial cells showed a marked reduction in adhesion on the nonirradiated coated film surface, whereas the UV-irradiated surface promoted cell adhesion and proliferation except for incubation in serum-free medium, which was similar to commercial tissue culture PET sheet. These observations may be due to adsorption of cell adhesive proteins, typified by fibronectin, in serum-containing medium onto the cationized photoreactive copolymer surface by electrostatic interactions.     

AFM-based nanotribological and electrical characterization of ultrathin wear-resistant ionic liquid films

Ionic liquids (ILs) are considered as lubricants for micro/nanoelectromechanical systems (MEMS/NEMS) due to their excellent thermal and electrical conductivity. So far, only macroscale friction and wear tests have been conducted on these materials. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a crucial step to understand how these novel materials can efficiently lubricate MEMS/NEMS devices. To this end, the adhesion, friction and wear properties of two ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) and 1-butyl-3-methylimidazolium octyl sulfate (BMIM-OctSO4), applied on Si(100), are investigated for the first time using atomic force microscopy (AFM). Data is compared to the perfluoropolyether lubricant Z-TETRAOL, which has high thermal stability and extremely low vapor pressure. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated using AFM-based surface potential and contact resistance techniques. Thermally treated coatings containing a mobile lubricant fraction (i.e., partially bonded) were better able to protect the Si substrate from wear compared to the fully bonded coatings, and this enhanced protection is attributed to lubricant replenishment.     

Two-dimensional Monte Carlo simulations of ionic and nonionic silane self-assembly on hydrophilic surfaces

Aqueous chemistries have recently been shown to be useful for the deposition of hydrophobic films of nonionic and cationic silanes on hydrophilic substrates for the prevention of stiction in MEMS. The Monte Carlo method is used to simulate in two dimensions the self-assembly of silane films on a hydrophilic surface. We investigate the impact of charged group in cationic silane on the overall structure of the films. We characterize the film structure with spatial pair correlations at each molecular layer of the deposited films. The simulations reveal long-range correlations for the film of cationic silanes. Based on our two-dimensional simulations, we report an average "most probable" structure for the films of nonionic and cationic silanes.     

Preparation and tribological studies of C60 thin film chemisorbed on a functional polymer surface

A novel self-assembled C60 film was prepared by chemical adsorption of C60 molecules onto an amino-group-containing polyethyleneimine-coated silicon substrate surface. The contact angle of distilled water on the C60 film was measured, the thickness was determined by means of ellipsometric analysis, and the morphology was observed with an atomic force microscope. The tribological properties of the films were investigated as well. It was found that the C60 thin film had a contact angle of about 72 degrees and thickness of 1.8 nm and exhibited a surface domain microstructure composed of fullerene clusters. Due to the hydrophobicity and low surface energy, the C60 film possessed good adhesive resistance and had an adhesive force of about 7.1 nN, which was about an order of magnitude lower than that of the silicon substrate surface. Moreover, the C60 film showed good friction reduction, load-carrying capacity, and antiwear ability, which were attributed to the higher mechanical stiffness and elastic modulus of C60 molecules. Besides, the friction coefficient decreased with increasing sliding velocity and normal loads, due to the rolling effect of the physisorbed C60 molecules.     

On the role of electrostatic forces in the adhesion of polymer particles to solid surfaces

Simultaneous measurements have been made of the adhesive force and double electric charge of particles after their removal from a metal surface. For the systems investigated, the adhesive force and charge on the particles increase with particle diameter according to a power law with an exponent close to 2. Such dependence can be explained on the basis of the electrostatic nature of the adhesive forces. A double electric layer exists at the interface between the particles and the metal surface. A calculation was made of the surface density of charge for the polyvinyl chloride particle-steel system.     

Bulk phase separation study by atomic force microscope friction imaging of natural rubber/poly(methyl methacrylate) film

The first observation of bulk phase separation in immiscible natural rubber (NR)/poly(methyl methacrylate) (PMMA) film using atomic force microscopy (AFM) is reported. Three different forms of AFM measurements: topographic, friction force imaging, and nanoindentation have been effectively used to investigate combined morphological and compositional mapping of the NR/PMMA system. The fracture temperature during sample microtoming and material physical properties could be responsible for the observed topographic contrast. The stronger contrast of friction imaging, relative to topographic imaging, is ascribed to local variations in mechanical properties of the phase-separated domains. Friction force imaging associated with nanoindentation response, performed under AFM force mode, highlights the AFM's ability for probing local friction, adhesion, and elastic properties, and for compositional mapping of heterogeneous polymer film. The resulting friction force imaging along with the response of the nanoindentation are in good agreement, indicating that PMMA exists mainly near the modified NR surface.     

烧结型NdFeB永磁体表面有机纳米复合膜的制备及性能

采用自组装分子膜技术在烧结型NdFeB永磁体表面制备了三嗪硫醇三乙基硅烷(TES)自组装分子膜(TES-SAMs), 在TES-SAMs的基础上利用自主开发的有机镀膜技术制备了具有含氟官能团的三嗪硫醇(ATP)有机纳米复合薄膜(TES-ATP). 通过X射线光电子能谱仪(XPS)、傅里叶变换红外(FTIR)光谱仪、椭圆偏振光谱仪、原子力显微镜(AFM)和接触角测量仪对薄膜的表面状况进行评价, 使用UMT-2型摩擦磨损试验机研究TES-SAMs和TES-ATP的微摩擦学性能. 研究结果表明: TES-SAMs和TES-ATP的膜厚分别是5.08和29.78nm; 表面自由能从基体的73.13 mJ·m^-2下降到TES-SAMs的63.69 mJ·m^-2和TES-ATP复合膜的10.19 mJ·m^-2, 且TES-ATP复合膜对蒸馏水的接触角为123.5°, 成功实现了NdFeB表面由亲水到疏水的转换.TES-SAMs和TES-ATP均能有效降低摩擦系数, TES-SAMs的摩擦系数为0.22, TES-ATP的摩擦系数为0.12, 而基体的摩擦系数为0.71; 同时, TES-ATP还表现出良好的抗磨性能. TES-ATP复合膜为微机电系统中的摩擦磨损问题的解决提供了一种新思路.     

Direct surface force measurements of polyelectrolyte multilayer films containing nanocrystalline cellulose

Polyelectrolyte multilayer films containing nanocrystalline cellulose (NCC) and poly(allylamine hydrochloride) (PAH) make up a new class of nanostructured composite with applications ranging from coatings to biomedical devices. Moreover, these materials are amenable to surface force studies using colloid-probe atomic force microscopy (CP-AFM). For electrostatically assembled films with either NCC or PAH as the outermost layer, surface morphology was investigated by AFM and wettability was examined by contact angle measurements. By varying the surrounding ionic strength and pH, the relative contributions from electrostatic, van der Waals, steric, and polymer bridging interactions were evaluated. The ionic cross-linking in these films rendered them stable under all solution conditions studied although swelling at low pH and high ionic strength was inferred. The underlying polymer layer in the multilayered film was found to dictate the dominant surface forces when polymer migration and chain extension were facilitated. The precontact normal forces between a silica probe and an NCC-capped multilayer film were monotonically repulsive at pH values where the material surfaces were similarly and fully charged. In contrast, at pH 3.5, the anionic surfaces were weakly charged but the underlying layer of cationic PAH was fully charged and attractive forces dominated due to polymer bridging from extended PAH chains. The interaction with an anionic carboxylic acid probe showed similar behavior to the silica probe; however, for a cationic amine probe with an anionic NCC-capped film, electrostatic double-layer attraction at low pH, and electrostatic double-layer repulsion at high pH, were observed. Finally, the effect of the capping layer was studied with an anionic probe, which indicated that NCC-capped films exhibited purely repulsive forces which were larger in magnitude than the combination of electrostatic double-layer attraction and steric repulsion, measured for PAH-capped films. Wherever possible, DLVO theory was used to fit the measured surface forces and apparent surface potentials and surface charge densities were calculated.     

Surface modification of plasma‐pretreated expanded poly (tetrafluroethylene) films by graft copolymerization

A two‐step process based on a low‐pressure helium plasma treatment followed by acrylic acid‐grafting copolymerization was used for the surface modification of expanded polytetrafluoroethylene (ePTFE) films. The effects of plasma treatment power and treatment time on the hydrophilicity of the film surface were investigated. The wettability of the ePTFE film surface was characterized by water contact angle, and the surface compositions of the untreated and treated ePTFE samples were evaluated by atomic force microscopy and XPS. Contact angle measurements revealed that the hydrophilicity of the ePTFE film surface was greatly enhanced by the combined actions of the plasma treatment and acrylic acid grafting, and the contact angle decreased from 145° to 66°. Atomic force microscopy analyses showed that the surface roughness increased after the plasma treatment. XPS analyses showed substantial increase in the concentration of carbon and oxygen atoms and a decrease in the concentration of fluorine atoms at the film surface. T‐peel strength showed an improved bonding strength between the film and an adhesive tape after the treatment. Copyright © 2012 John Wiley & Sons, Ltd.     

Influence of Ce addition on the catalytic behavior of alumina-supported Cu-Co catalysts in NO reduction with CO

Polyvinylidene difluoride (PVDF) is one of the most widely used piezoelectric materials in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS) due to its excellent properties. Its applications range from biological to electric devices, such as an artificial hip joint, a microgripper, and a force sensor. It is critical to understand friction, adhesion, and wear mechanisms of this material. In this study, effect of piezoelectricity and lubricant with electric field on tribological properties was investigated, using poled and unpoled PVDF. To understand the tribological properties at nano- and macroscales, scale effect was also studied using an AFM and a tribometer. Relevant mechanisms are discussed.     

An empirically validated analytical model of droplet dynamics in electrowetting on dielectric devices

Explicit analytical models that describe the capillary force on confined droplets actuated in electrowetting on dielectric devices and the reduction in that force by contact angle hysteresis as a function of the three-dimensional shape of the droplet interface are presented. These models are used to develop an analytical model for the transient position and velocity of the droplet. An order of magnitude analysis showed that droplet motion could be modeled using the driving capillary force opposed by contact angle hysteresis, wall shear, and contact line friction. Droplet dynamics were found to be a function of gap height, droplet radius, surface tension, fluid density, the initial and deformed contact angles, contact angle hysteresis, and friction coefficients pertaining to viscous wall friction and contact line friction. The first four parameters describe the device geometry and fluid properties; the remaining parameters were determined experimentally. Images of the droplet during motion were used to determine the evolution of the shape, position, and velocity of the droplet with time. Comparisons between the measured and predicted results show that the proposed model provides good accuracy over a range of practical voltages and droplet aspect ratios.