An experimental study of sidewall adhesion in microelectromechanical systems

A surface micromachine was designed specifically for studying sidewall adhesion in microelectromechanical systems (MEMS). The dependence of surface adhesion on contact load and ambient conditions was investigated under quasistatic normal loading conditions. Insight was obtained into the relative contributions of van der Waals and capillary forces to the measured adhesion force. Several shortcomings in previous adhesion studies of MEMS were overcome, and measurement of the true adhesion force was achieved under different testing conditions. The present experimental procedure enables the isolation of the van der Waals component of the adhesion force and the determination of the contributions of both contacting and noncontacting asperities to the total adhesion force at the inception of surface separation. The major benefits of the developed experimental methodology and surface micromachine are discussed in the context of adhesion results obtained for different values of apparent contact pressure, ambient pressure, and relative humidity.     

范德华力作用下粗糙表面静态粘附的研究

通过在硅微接触表面上涂覆低表面能的憎水性OTS膜以除去接触面间的表面张力,把两表面均接地以除去接触面间的静电力,研究了仅有范德华力作用时硅微结构接触表面的粘附.根据实际粗糙表面凸峰自相似的高度分布,计算了发生粘附后,微观接触表面产生弹性和塑性变形的两种情况下的范德华粘附能,分析了表面形貌对其影响.     

Lagrangian particle model for multiphase flows

A Lagrangian particle model for multiphase multicomponent fluid flow, based on smoothed particle hydrodynamics (SPH), was developed and used to simulate the flow of an emulsion consisting of bubbles of a non-wetting liquid surrounded by a wetting liquid. In SPH simulations, fluids are represented by sets of particles that are used as discretization points to solve the Navier-Stokes fluid dynamics equations. In the multiphase multicomponent SPH model, a modified van der Waals equation of state is used to close the system of flow equations. The combination of the momentum conservation equation with the van der Waals equation of state results in a particle equation of motion in which the total force acting on each particle consists of many-body repulsive and viscous forces, two-body (particle-particle) attractive forces, and body forces such as gravitational forces. Similar to molecular dynamics, for a given fluid component the combination of repulsive and attractive forces causes phase separation. The surface tension at liquid-liquid interfaces is imposed through component dependent attractive forces. The wetting behavior of the fluids is controlled by phase dependent attractive interactions between the fluid particles and stationary particles that represent the solid phase. The dynamics of fluids away from the interface is governed by purely hydrodynamic forces. Comparison with analytical solutions for static conditions and relatively simple flows demonstrates the accuracy of the SPH model.     

玻纤增强聚乙烯界面行为的分子模拟研究

为研究复合材料界面与力学性能的关系,本文运用分子力学(MM)和分子动力学(MD)方法,从玻纤(GF)与聚乙烯(PE)界面结构和能量的微观角度分别探讨了分子静态和动态下界面的吸附、粘结行为,并从分子微观角度提出玻纤增强聚合物复合材料力学性能的观点。结果表明:体系中静电力、键合力和范德华力的作用使得GF／PE混合体系比纯PE体系更加稳定;当GF加入量过大时,体系中非键合原子之间近程排斥力增大,色散力减小,影响到基体PE与增强物GF的粘接力,也不能起到良好的粘结作用。     

Analysis of the demolding forces during hot embossing

Hot embossing is one of the main processing techniques for polymer microfabrication, which helps the LIGA (UV-LIGA) technology to achieve low cost mass production. When hot embossing of high aspect ratio microstructures, the deformation of microstructures usually occurs due to the demolding forces between the sidewall of mold inserts and the thermoplastic (PMMA). The study of the demolding process plays a key role in commercial manufacturing of polymer replicas. In this paper, the demolding behavior was analyzed by Finite element method using ABAQUS/Standard. Simulation identified the friction force caused by interface adhesion and thermal stress due to shrinkage between the mold and the polymer as the main sources of the demolding forces. Simulation also showed that the friction force made a greater contribution to the deformation than thermal stress, which is explained in the accompanying theoretical analysis. To minimize the friction force the optimized experiment was performed using PTFE (Teflon) as anti-adhesive films and using Ni-PTFE compound material mold inserts. Both lowered the surface adhesion energy and friction coefficient. Typical defects like pull-up and damaged edges can be reduced.     

A MEMS Device for Studying the Friction Behavior of Micromachined Sidewall Surfaces

A microelectromechanical system device fabricated by deep reactive ion etching for friction characterization was developed with single-crystal silicon in this paper. Two orthogonally placed electrostatic comb-drive actuators were adopted to apply the normal load and generate the tangential motion. A sensing plate for sliding contact and a driving plate with two bumps designed for the Hertzian contact are included in the testing device. With an image processing technique developed, experimental displacement data were extracted from the captured video frames. A quasi-static stick–slip model was developed to predict the transitions between static and kinetic friction at the contacting sidewall surfaces. Both static and kinetic friction coefficients can be determined by using this model, and these measured results are shown to be insensitive to errors in the calculation of the electrostatic forces. The measured displacements of the driving and sensing plates are in good agreement with the trend predicted by the model. Based on the Hertz theory, the contact silicon interface has been found to be in an elastic regime at the scale of the designed bumps. With the aid of the quasi-static stick–slip model, a saturation phenomenon of the kinetic friction at the sidewall surfaces was observed while the normal load was increased. $hfill$[2007-0302]      

Dichlorodimethylsilane as an anti-stiction monolayer for MEMS: acomparison to the octadecyltrichlorosilane self-assembled monolayer

This paper presents a quantitative comparison of the dichlorodimethylsilane (DDMS) monolayer to the octade-cyltrichlorosilane (OTS) self-assembled monolayer (SAM) with respect to the film properties and their effectiveness as anti-stiction coatings for micromechanical structures. Both coatings have been evaluated in several ways, including atomic force microscopy (AFM), contact angle analysis (CAA), work of adhesion by cantilever beam array (CBA) technique and coefficient of static friction using a sidewall testing device. While water and hexadecane contact angles are comparable, the DDMS coated microstructures exhibit higher adhesion than OTS coated ones. Furthermore, coefficient of static friction data indicate that the DDMS films are not as effective at lubrication as the OTS SAMs are, although both exhibit much improvement over chemical oxide. However, AFM data show that the samples which receive DDMS treatment accumulate fewer particles during processing than those which receive the OTS SAM treatment. The thermal stability of the DDMS film in air far exceeds the OTS SAM, as the DDMS remains very hydrophobic to temperatures upwards of 400°C     

Static Friction in Polysilicon Surface Micromachines

Surface micromachines of polycrystalline silicon were used to investigate the dependence of static friction in microelectromechanical systems on the external load, apparent contact area, and environmental conditions. An analytical model of the micromachine at the inception of sliding was used to determine the normal load consisting of the restoring and levitation forces exerted by the micromachine's comb-drive actuators. The apparent shear strength at the contact interface(s) exhibited a nonlinear dependence on the apparent contact pressure. Relatively higher static coefficient of friction and interfacial shear strength were obtained in room air than vacuum ambient. The static coefficient of friction was found to depend on the normal load, apparent contact area, and ambient conditions (i.e., relative humidity). Electrical contact resistance measurements indicated that sliding in room air promoted thickening of the native oxide film at asperity contacts. The experimental evidence suggests that modification of the surface topography occurred at the asperity level. However, these submicroscopic changes in the surface topography did not affect the overall static friction behavior, for the test cycles simulated in the friction experiments.$hfillhbox[1225]$     

Theoretical study of van der Waals dispersion force between macroscopic bodies with a periodic material distribution

A method has been developed for calculating the van der Waals dispersion force between a macroscopic body consisting of a uniform material and a macroscopic body with a spatially periodic material distribution. The periodic material distribution is one dimensional in the x-direction. The periodically distributed material property function is expanded as a Fourier series. The van der Waals forces for a distribution of two materials were then calculated as a typical example of a periodic material distribution. The effects of parameters such as the duty ratio of the material distribution, the refractive index ratio of the two materials, and the length of the uniform body on the van der Waals force were shown.     

Investigation of anti-stiction coating for ohmic contact MEMS switches with thiophenol and 2-naphthalenethiol self-assembled monolayer

Anti-stiction coating with a conductive self-assembled monolayer (SAM) formed by π-conjugated bonds was investigated for ohmic contact microelectromechanical system (MEMS) switches with low-load contacts. SAMs of thiophenol (C₆H₅SH, TP) or 2-naphthalenethiol (C₁₀H₇SH, 2NT) were coated on Au samples with different surface roughness to investigate the effects of the surface asperities on the adhesion force and contact resistance. The adhesion force was measured using a silicon tipless cantilever in the relative humidity range of 10-85% and the contact resistance was measured in the contact force range of 0-70 μN using a conductive tipless cantilever coated with Au for the SAM coated samples and compared with those for a Au sample surface. The adhesion force measurements indicate that the TP and 2NT coatings can prevent a liquid meniscus from forming on device surfaces due to their hydrophobic character caused by the protruding aromatic group. In addition, it was confirmed that these coatings could reduce van der Waals forces more than Au coating. Contact resistance measurements revealed that an electric current begins to flow with smaller contact force for TP and 2NT coated samples than for Au coated samples. The measured contact resistances of the SAM and Au coated samples were comparable. Based on these results, SAMs of TP and 2NT have excellent potential as anti-stiction coating for MEMS switch contacts.     

Wear of Polysilicon Surface Micromachines Operated in High Vacuum

The evolution of wear at sidewall surfaces of polysilicon microelectromechanical systems was investigated in high vacuum under controlled normal load and sliding speed conditions. The static adhesion force was used as an indicator of the changes in wear characteristics occurring during oscillatory sliding contact. Measurements of the static adhesion force as a function of sliding cycles and scanning electron microscopy observations of micromachines from the same batch process subjected to nominally identical testing conditions revealed two distinctly different tribological patterns, namely, low-adhesion/high-wear behavior and high-adhesion/low-wear behavior. The static adhesion force and wear behavior were found to be in direct correlation with the micromachine operational lifetime. Transmission electron microscopy, selected area diffraction, and energy dispersive X-ray spectroscopy yielded insight into the origin, microstructure, and composition of wear debris and agglomerates adhered onto the sliding surfaces. Results demonstrate a strong dependence of micromachine operational life on the removal of the native oxide film and the organic monolayer coating as well as the formation of agglomerates consisting of organic coating material and wear debris.     

An on-chip micro-friction tester for tribology research of silicon based MEMS devices

An on-chip micro-tribotester has been developed to investigate the friction and wear properties on side contacting surfaces of single crystal silicon that is most widely used in usual microelectromechanical systems actuators. The device is fabricated with standard bulk silicon process and bonding technology based on parameters that have been theoretically calculated to get the stiffness and friction forces. In this device, two comb shuttles are used. One comb shuttle is used to contact the friction surfaces under a certain normal load. The other comb shuttle moves back and forth to provide relative motion between the two friction surfaces. The tested two surfaces are the top surface of an anchor with rounded end and the lateral surface of a beam that has been connected to the two comb shuttles. Tribology experiments on the etched silicon surfaces that are side contacted have been carried out. Friction coefficients testing results suggest that dynamic friction coefficient is about 0.31–0.33 and the obtained static friction coefficient increases with the decrease of normal force. Wear experiment indicates that oxidation happens between the rubbing surfaces during the course of the testing. Wear debris is collected as agglomerates because of the function of adhesion force and water vapor and the agglomerates that are collected on top and lateral surfaces are of different shapes.     

Adhesive surface design using topology optimization

Tailoring adhesive properties between surfaces is of great importance for micro-scale systems, ranging from managing stiction in MEMS devices to designing wall-scaling gecko-like robots. A methodology is introduced for designing adhesive interfaces between structures using topology optimization. Structures subjected to external loads that lead to delamination are studied for situations where displacements and deformations are small. Only the effects of adhesive forces acting normal to the surfaces are considered. An interface finite element is presented that couples a penalty contact formulation and a Lennard–Jones model of van der Waals adhesive forces. Two- and three dimensional design optimization problems are presented in which adhesive force distributions are designed such that load-displacement curves of delaminating structures match target responses. The design variables describe the adhesive energy per area of the interface between the surfaces, as well as the geometry of the delaminating structure. A built-in length scale in the formulation of the adhesion forces eliminates the need for filtering to achieve comparable optimal adhesive designs over a range of mesh densities. The resulting design problem is solved by gradient based optimization algorithms evaluating the design sensitivities by the adjoint method. Results show that the delamination response can be effectively manipulated by the method presented. Varying simultaneously both adhesive and geometric parameters yields a wider range of reachable target load-displacement curves than in the case varying adhesive energy alone.     

Sidewall morphology of electroformed LIGA parts-implications for friction, adhesion, and wear control

LIGA fabricated parts are finding application in a wide variety of micro-mechanical systems. For these systems to operate reliably, friction between contacting sidewall surfaces must be understood and controlled. The roughness of the as-plated sidewall is an important determinate of friction forces at such contacts. LIGA sidewalls were characterized in order to provide a basis for predicting the friction, adhesion, and wear behavior of LIGA micromachines. A variety of unexpected sidewall morphologies were observed during this investigation. Three morphologies were identified: a fine scale roughness, a linear through thickness feature, and a group of larger high aspect ratio features. Each morphology has been associated with a specific aspect of the LIGA manufacturing process. Potential friction, adhesion, and wear management strategies suggested by these features have been discussed. In addition, the asperity behavior in a LIGA sidewall contact has been predicted based on the finest roughness observed.     

基于AFM的机器人化纳米操作中纳观力的初步研究

针对基于原子力显微镜(AFM)的机器人化纳米操作，对探针作用下探针—基片—微粒之间纳观力的作用规律进行了初步分析．指出起主要作用的纳观力为范德华力、接触斥力、纳米摩擦力、毛细作用力以及纳米静电力等五种，并初步推导了各种纳观力的表达形式．通过力—距离曲线仿真与实验验证了所进行分析的合理性；该分析有助于进行纳米操作的精确控制．     

Theoretical study of van der Waals dispersion pressures considering one-dimensional material distributions in the in-plane direction

The van der Waals dispersion pressures between a half-space consisting of a uniform material and a half-space with a one-dimensional material distribution in the in-plane direction have been theoretically derived. Two patterns of material distribution were considered: a periodic distribution of materials (Pattern 1) and a distribution of two materials with a single interface (Pattern 2). The van der Waals pressure for Pattern 1 was derived based on a Fourier series, while the van der Waals pressure for Pattern 2 was derived as elementary functions. Both of the van der Waals pressures derived consist of two terms: a conventional term between half-spaces made of uniform materials and a spatial fluctuation term due to the material distribution. The basic characteristics of these van der Waals pressures were quantitatively clarified. Furthermore, an approximate method for obtaining the van der Waals pressure of Pattern 1 from Pattern 2 was proposed.     

Touch-down induced contact and friction forces at the dimple/gimbal interface

A dynamic simulation is performed to investigate contact and friction forces at the dimple/gimbal interface. The time history of slider motion, the resultant force as well as the pitch and roll moments acting on the slider are determined. The time-dependent contact and friction forces at the dimple/gimbal interface are obtained. The effect of material properties on contact and friction forces at the dimple/gimbal interface is investigated. Experimental results for touch-down and take-off characteristics are presented.     

Micro-tribological interface model for friction-induced cold start-up running dynamics

The robustness and noise warranty costs of many automotive friction materials like transmission belts or brake pad are directly affected by the frictional properties in cold start-up running. This paper presents a friction model for start-up running under cold conditions. The absorbed water, phase changes and variable lubrication regimes in cold start-up running are taken into account. The model includes the breakage of ice adhesion, ice–ice friction between the ice films on friction pairs, melting water-mediated mixed lubrication and boundary lubrication, friction elevation due to capillary adhesive effect, as well as dry contact in the end of cold start-up running. Thermal analysis is applied in conjunction with the friction model to estimate the parameters in phase transitions. A meniscus model is also integrated into the friction model to address the friction elevation due to the variation of water film thickness. It is illustrated that if the thickness of surface ice film is larger than a critical value, the static friction coefficient could be close to 1; if the thickness of melting water film is higher than the average roughness of the surface, static friction could increase due to capillary effect, and kinetic friction could decrease due to mixed lubrication resulting in wide modulation of friction during intermittent start-up transitions. This paper also presents the application of the model to elucidate the friction mechanisms in cold brake noise where the cold wet coefficient of friction (cof) could be substantially higher than the dry cof. The effects of temperature, roughness and load on cof are also characterized.     

Effect of layer compliance on frictional behavior of soft robotic fingers

In this paper, the modeling and design for frictional loads applied by robotic fingertips through a soft contact interface are investigated. The dependence of the sustainable friction forces and moments due to the normal force applied on the contact area is studied. A model that considers the dependence of both the contact area and the coefficient of friction on the application of normal force is proposed. Specimens of simplified fingertips, made with different materials and shaped with different thickness of the covering soft layer, were tested in order to validate the proposed model via designed experiments. The results obtained have very important relevance to the design of robotic hands and fingers equipped with soft pads or skins, and can help in the choice of suitable features for the different layers of contact pads or skins.     

A study of hand grip pressure distribution and EMG of finger flexor muscles under dynamic loads

A matrix of miniature and flexible pressure sensors is proposed to measure the grip pressure distribution (GPD) at the hand-handle interface of a vibrating handle. The GPD was acquired under static and dynamic loads for various levels of grip forces and magnitudes of vibration at different discrete frequencies in the 20–1000 Hz range. The EMG of finger flexor muscles was acquired using the silver-silver chloride surface electrodes under different static and dynamic loads. The measured data was analysed to study the influence of grip force, and magnitude and frequency characteristics of handle vibration on: (i) the local concentration of forces at the hand-handle interface; and (ii) the electrical activity of the finger flexor muscles. The results of the study revealed high interface pressure near the tips of index and middle fingers, and base of the thumb under static grip conditions. This concentration of high pressure shifted towards the middle of the fingers under dynamic loads, irrespective of the grip force, excitation frequency, and acceleration levels. The electrical activity of the finger flexor muscles increased considerably with the grip force under static as well as dynamic loads. The electrical activity under dynamic loads was observed to be 1·5–6·0 times higher than that under the static loads.