Asymmetric surface roughness measurements and meniscus modeling of polysilicon surface micromachined flaps

Polycrystalline silicon (polysilicon) films are primary structural materials for microelectromechanical systems (MEMS). Due to relatively high compliance, large surface-to-volume ratio, and small separation distances, micromachined polysilicon structures are susceptible to surface forces which can result in adhesive failures. Since these forces depend on surface properties especially surface roughness, three types of microhinged flaps were fabricated to characterize their roughness and adhesive meniscus properties. The flaps enabled access to both the top and bottom surfaces of the structural polysilicon layers. Roughness measurements using an atomic force microscope revealed that MEMS surfaces primarily exhibit non-Gaussian surface height distributions, and for the release procedures studied, the bottom surface of the structural layers was significantly smoother and prone to higher adhesion compared to the top surface. A non-symmetric surface roughness model using the Pearson system of frequency curves was coupled with a capillary meniscus adhesion model to analyze the effects of surface roughness parameters (root-mean-square, skewness, and kurtosis), relative humidity, and surface contact angle on the interfacial adhesion energy. Using the measured roughness properties of the flaps, four different surface pairs were simulated and compared to investigate their effects on capillary adhesion. It was found that since the base polysilicon layer (poly0) was rougher than the base silicon nitride and the structural layer on poly0 was also rougher than that on silicon nitride, depositing MEMS devices on poly0 layer rather than directly on silicon nitride will reduce the adhesion energy.     

纳米级动态粘着接触的有限元模型与仿真

对纳米级动态粘着接触过程进行仿真,是为了研究微纳米尺度的机电系统(如MEMS)中所存在的纳米级表面接触和摩擦,对系统进行减粘附设计.利用商用有限元分析软件ANSYS建立的纳米尺度的动态粘着接触模型,采用原子间Len-nard-Jones作用势函数来描述表面力,用数值求解的方法来描述基本球-盘模型的接触-分离行为,最终得到接触过程的力-位移曲线和枯着接触的分离点和粘着力的大小.将有限元粘着接触模型仿真所得结果与常用的接触模型(DMT模型、JKR模型)解析解对比,证明了有限元粘着接触模型的适用性.由于有限元模型不受接触副几何形状和材料性质的限制,可以进一步对表面形貌修饰后的粘性接触过程和多峰粘着接触过程进行仿真.     

Development of Realistic Pressure Distribution and Friction Limit Surface for Soft-Finger Contact Interface of Robotic Hands

Various models have been presented for pressures distribution in the contact interface of a soft finger and object in the literature. These models have been proposed without considering the effect of the tangential forces which are usually exerted in the contact interface of a soft finger and object during grasping and manipulation. Having an accurate pressures distribution model across the contact interface is important for designing tactile sensors and improving the modeling of the friction limit surface (LS). In this paper, a new and more accurate model is proposed to describe the asymmetry of the pressure distribution in the contact interface of a hemispherical soft finger under both normal and tangential forces. This model is derived based upon observations in the previous literature stating that the contact interface would move and skew toward the direction of the tangential force. According to the proposed pressure distribution model in this study, an improved and more accurate LS is presented. The LS profile obtained by this model is compared with the corresponding results based on the previous models. The new results show that the consideration of the skewness or asymmetry in the pressure distribution (due to the tangential force) causes the LS profile to shrink compared with that constructed with symmetric pressure distribution assumption. This shrinkage, as a result of the skewness and asymmetry of the pressure distribution, makes the contact interface more vulnerable. Furthermore, this new model can also provide a more accurate tool for the analysis of grasping and manipulation involving soft contact interface.     

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.     

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.     

Si基Cu/NiFe薄膜的生长及其粘附特性研究

微机械(MEMS)工艺和集成电路(IC)工艺中,在硅(Si)片上电铸高深宽比坡莫(NiFe)合金材料常出现脱落现象.提出了一种电铸NiFe合金材料的新方法,这种方法制作的合金薄膜厚度达200 μm时不脱落.此方法即对等离子刻蚀后的硅片溅射种子层铜(Cu),然后对种子层进行电镀,当其厚度达到约15 μm时,再进行NiFe合金的电铸.本文用扫描电镜、x射线衍射仪和剥离实验研究了薄膜粘附特性.研究结果表明当对种子层电镀后,随着Cu种子层厚度的增加,Cu/NiFe薄膜与基体的粘附强度增加,而薄膜的残余应力降低;同时Cu膜表面粗糙度增加,也增加了NiFe膜与Cu膜的粘附强度.     

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.     

Intermolecular force and surface roughness models for air bearing simulations for sub-5 nm flying height sliders

When the spacing between the slider and the disk is less than 5 nm, the intermolecular forces between the two solid surfaces can no longer be assumed to be zero. The model proposed by Wu and Bogy (ASME J Trib 124:562–567, 2002) can be view as a flat slider–disk intermolecular force model. The contact distance between the slider and disk needs to be considered in this model when the slider-disk spacing is in the contact regime. To get more accurate intermolecular force effects on the head disk interface, the slider and disk surface roughness need to be considered, when the flying height is comparable to the surface RMS roughness value or when contact occurs. With the intermolecular force model and asperity model implemented in the CML air bearing program, the effect of intermolecular adhesion stress on the slider at low flying height is analyzed in the static flying simulation. It is found that the intermolecular adhesion stress between the slider and the disk has slight effect on the slider-disk interface for a flying slider.     

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.     

Tribological characteristics of novel branched perfluoropolyether lubricant films in hard disk drives

In this study, the basic tribological characteristics of novel branched perfluoropolyether (PFPE) lubricant films such as TA-30 and QA-40 were examined. Their surface free energy characteristics and adhesive and friction forces were investigated using an atomic force microscope. The interactions between the lubricant molecules and the water molecules were also examined by monitoring the changes in the contact angle of distilled water on the test lubricant films. The interactive forces such as the adhesive and friction forces of a film that is approximately one monolayer thick were found to be strongly dependent on the conformation of the lubricant molecules on diamond-like carbon thin films. In addition, the TA-30 and QA-40 lubricant molecules appeared to interact with the water molecules more actively than conventional Ztetraol2000 molecules. These results afforded fundamental insight into the tribological performance of novel branched PFPE lubricants in the head-disk interface.     

采用RIE沉积法研制微纳工艺中的防粘连层

摩擦和粘附问题已经成为影响微机电系统(MEMS)工艺性能和可靠性的主要因素.针对MEMS/NEMS中微构件表面改性问题,采用反应离子刻蚀(RIE)在硅片表面沉积氟化物薄膜,以达到降低硅片的表面能,减少其摩擦和粘附的目的.实验还将RIE沉积法制得的薄膜与自组装(SAMs)薄膜在浸润角、表面能、表面粗糙度等方面进行了对比.     

Nanoparticle Liquids for Surface Modification and Lubrication of MEMS Switch Contacts

Contact failures in microelectromechanical system (MEMS) switches, particularly during hot switching, prevent their widespread use. In this paper, a nanoparticle liquid (NPL) lubricant is synthesized and deposited on MEMS switch contacts as a nanotechnology-based lubricant. NPLs are monolithic hybrid materials comprised of an inorganic nanosized metallic core and an organic low viscosity corona. The NPL used here utilizes either Au or Pt nanoparticles as the core and a mercaptoethanesulfonate ionic liquid as the corona. Performance, reliability, and chemical/physical processes on hot-switched NPL-lubricated contact surfaces were investigated at high (1 mA) and low (10 ) currents using a micro/nanoadhesion apparatus as a MEMS switch simulator with in situ monitoring of contact resistance and adhesion force. This was coupled with ex-situ analyses of the contacts using scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and scanning Auger spectroscopy. NPLs exhibited improved electrical performance and durability (orders of magnitude improvement) as compared to uncoated and self-assembled monolayer-coated switch contacts. Improved performance and reliability results from the following: 1) controlled nanoscale surface roughness that spreads current through multiple nanocontacts; 2) restricted size melting regions and termination of nanowire growth (prevents shorting) during contact opening; 3) enhanced thermal and electrical conductivity that reduces lubricant degradation; and 4) lubricant self-healing (flow to damaged areas) as confirmed with physical and chemical analyses. Based on these results, NPLs show excellent potential as surface modifiers/lubricants for MEMS switch contacts.     

A suite of robust controllers for the manipulation of microscale objects.

A suite of novel robust controllers is introduced for the pickup operation of microscale objects in a microelectromechanical system (MEMS). In MEMS, adhesive, surface tension, friction, and van der Waals forces are dominant. Moreover, these forces are typically unknown. The proposed robust controller overcomes the unknown contact dynamics and ensures its performance in the presence of actuator constraints by assuming that the upper bounds on these forces are known. On the other hand, for the robust adaptive critic-based neural network (NN) controller, the unknown dynamic forces are estimated online. It consists of an action NN for compensating the unknown system dynamics and a critic NN for approximating a certain strategic utility function and tuning the action NN weights. By using the Lyapunov approach, the uniform ultimate boundedness of the closed-loop manipulation error is shown for all the controllers for the pickup task. To imitate a practical system, a few system states are considered to be unavailable due to the presence of measurement noise. An output feedback version of the adaptive NN controller is proposed by exploiting the separation principle through a high-gain observer design. The problem of measurement noise is also overcome by constructing a reference system. Simulation results are presented and compared to substantiate the theoretical conclusions.     

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.     

Adhesion in micromechanical systems

In this paper adhesion is considered as a factor that to a large extent defines the reliability of technological processes in microelectromechanical systems (MEMS) manufacturing using superficial technology. The physical and chemical conditions of its emergence are investigated, as well as the short-range and superficial forces causing adhesive collapse in MEMS are assessed in quantitative terms. Some suggestions on adhesion prevention at the final stages of the technological process are formulated. The existing and prospective methods used to prevent adhesion in electronic industry are also discussed.     

Determination of the Pull-Off Forces and Pull-Off Dynamics of an Electrostatically Actuated Silicon Disk

To increase the mechanical sensitivity of micromachined yaw rate sensors, the proof mass can be designed as an electrostatically levitated flywheel. Therefore, the flywheel is kept in its levitated position by an electrostatic field exerted by an array of electrodes above and underneath the flywheel. Once the flywheel is in contact with either the bottom electrodes or the top electrodes, adhesion forces arise which might prohibit the flywheel from being lifted back to its levitated operation position. The subject of this paper is the investigation of the pull-off forces emerging during the start-up phase of an electrostatically actuated silicon disk, which is a test structure for such a levitated flywheel gyroscope. For the experimental determination of the pull-off forces, a test device based on a levitated flywheel gyroscope design is fabricated featuring soft polymeric and thus insulating springs for the inhibition of the lateral movement of the disk. Furthermore, the surfaces of the silicon disk and the electrode contact areas are modified to reduce the amount of the adhesion forces. For such textured surfaces, a pull-off force of 0.25 plusmn 0.05 mN is measured for a circular disk with a diameter of 3 mm. This corresponds to an adhesion energy per area of 3.5 ldr 10^-6 mJ/m². Thus, a repeatable pull-off of the flywheel at moderate actuation voltages is achieved.     

Six-DOF vibrations of partial contact sliders in hard disk drives

A six-degree-of-freedom slider dynamic simulator is developed to analyze the slider’s motion in the vertical, pitch, roll, yaw, length and width directions. The modified time-dependent Reynolds equation is used to model the air bearing and a new second order slip model is used for a bounded contact air bearing pressure. The simulator considers the air bearing shear acting on the air bearing surface and the slider–disk contact and adhesion. Simulation results are analyzed for the effects of the disk surface micro-waviness and roughness, skew angle, slider–disk friction and micro-trailing pad width on the vertical bouncing, down-track and off-track vibrations of a micro-trailing pad partial contact slider.     

Friction and wear study of the hemispherical rotor bushing in a variable capacitance micromotor

Friction and wear are the most serious problems for micromotors in microelectromechanical systems (MEMS). In the paper, a linear-sliding wear model of the contact between the rotor bushing and the ground plane is presented to describe the wear and a corresponding simplified method is proposed to simulate the wearing process. The effects of geometry parameters, material properties and applied operating conditions on the evolution of dimensional and volumetric wear rates and frictional torques are explored for normally loaded rotating rotor bushing sliding on the ground plane. A hemispherical-bushing-on-ground-plane configuration finite element model (FEM) is set up and the implementation of the contact task based on ANSYS and the contact element approach is introduced to provide the numerical simulations acted as a guide to solution of the contact problems in micromotors. Numerical simulations and results of the wear rates, frictional torques, contact stresses and contact pressure are studied and the effects of roughness, material properties, geometry parameters and FEM mesh of the bushing and the ground plane are discussed. It is indicated that the nonlinear effects cannot be ignored and the results should not be used to predict the absolute wear lifetime whereas surface engineering, lower wear materials and rational designs for micromotors in MEMS should be applied to bring the friction and wear behaviors into the acceptable regimes.     

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.