Stiction effect

Figure 1. Evolution of the tangential force during the displacement of the sample at constant velocity (v = 400 nm. s ) Stiction effect.

The stick-slip process is a result of the discontinuous nature of the real friction as well as of the Coulomb friction discussed here. The numerical procedure suggested in the paper could be implemented for the solution of the first problem of dynamics including the stiction effect. However, the second problem of dynamics, if zero velocity in a pair occurs, cannot be solved uniquely. We suggest here a numerical procedure for the solution of the following problem in case of a moving open kinematic chain with n pairs and ven vector S of the driving forces in pairs, the explicit form dynamic equations to be deriv at the... 

A final approach to avoid stiction effects is to apply a self-assembled monolayer (SAM) that is hydrophobic as a part of the release process. This process forms a hydrophobic Teflon-like coating on the contacting surfaces that reduces the capillary forces decreasing stiction effects [2]. 

Figure 1.23. Molecular weight effect on the fly stiction for ZdolTX.

The surface of all inorganic materials exposed to ambient (humid) air is always covered with a thin layer of water adsorbed from the gas phase. The thickness of the adsorbed water layer varies with the humidity and surface chemistry. This water layer has been shown to reduce wear in MEMS operation. However, the high surface tension of the water film can cause an in-use stiction problem. The gas-phase lubrication concept discussed here employs the same equilibrium adsorption principle as the water adsorption in humid environments. The difference is that our approach utilizes a surfactant-like molecule that can provide low adhesion and good lubrication. The entry summarizes the advantages of gas-phase lubrication for MEMS devices and discusses the effect of alcohol adsorption on the adhesion and lubrication of silicon oxide surfaces. 

Owing to the extensive infrastructure related to the chosen materials base (silicon), and the relative ease in which silicon based microsystems can be produced, a convenient approach to overcoming some of these reliability issues is to apply coatings for stiction, friction, and wear control to the devices after they are produced. This is analogous to painting buildings or automobiles after they are constructed to make them more weather resistant. Therefore, facile and effective coating processes that satisfy the needs of the microsystems for reliable performance have been developed. 

In light of these constraints, an effective chemical modification for anti-stiction treatments involves the application of a molecular film to the micromachine surface. This is most often accomplished through a process known as self-assembled monolayer (SAM) deposition.f Self-assembled monolayers are molecular films that spontaneously form on a (usually pretreated) surface upon exposure to a reactive precursor molecule. SAM precursors generally consist of three main parts a terminal group, a backbone, and a head group. Fig. 3 shows these parts on a model SAM... 

Lastly, it should be noted that there have been techniques published to remove the effect of stiction after the event. Gogoi and Mastrangelo have published a technique for using Lorentz forces to lift the microstructure that is stuck [24]. 

Section 22.6 has accounted for one important nonlinearity, namely the velocity limit of the valve. Another nonlinearity that can cause control problems is static friction, sometimes referred to as stiction, although more properly known as Coulomb friction. Static friction acts to prevent or impede relative motion by opposing the force applied. Its effect on valve movement may be measured as the difference between the valve s demanded travel (equal to the normalized controller output) and the actual valve travel seen on the plant. This difference will lie normally in the normalized range 0.001 to 0.005 (0.1 to 0.5% of total valve travel) for valves fitted with a valve positioning system, although the author has had experience of an important control valve with a value measured at... 

The altimeter had a lighting input and 28 V DC for an internal vibrator to prevent the needle from sticking particularly when there are only small changes in altitude (an effect known as stiction ). 

Alley L, Cuan GJ, Howe RT, Komvopoulos K (1992) The effect of release-etch processing on surface microstructure stiction. In Proceedings of the technical digest IEEE solid-state sensor and actuator workshop, Hilton Head, June 1992, pp 202-207... 

This chapter provides a concise review of the current status of the self-assembled monolayers as molecular lubricants for microelectromechanical systems operating in harsh environments, including elevated temperatures and in fluids. In particular, we focus on the similarities and differences in the structure-property relationships of two SAMs commonly employed to prevent in-use stiction in MEMS, namely octadecyltrichlorosilane (OTS, CH3(CH2)i7SiCl3) and perfluorodecyl-trichlorosilane (FDTS, CF3(CF2)7(CH2)2SiCl3). We discuss the effect of harsh environments on these monolayers and how their degradation impacts their properties and the range of conditions for which these monolayers can be employed effectively for MEMS. 

Hydrophobic siloxane monolayers are effective at preventing capillary adhesion in MEMS operating in ambient air. However, the use of SAMs to prevent stiction in harsh environments (elevated temperature or fluids) has been investigated to a lesser extent. Packaging or specific application conditions can expose MEMS to elevated temperatures. Similarly, the advent of bioMEMS and microfluidic devices opens the door to a wide range of applications in liquid environments. 

Silane SAMs have been deposited onto MEMS components and micromotors, and their effect on the performance of the components has also been evaluated [38, 39]. They improved the performance of the components by reducing the stiction in micromachines [4]. 

Because of the very large surface-area-to-volume ratios of micro-devices, adhe-sion/stiction has been considered the most important failure mode and the major obstacle for the commercialization of micro-electromechanical systems (MEMS). In this chapter, most important surface forces are introduced. The physical origin and mathematical models of these surface forces are presented. Then, adhesion effects such as wetting and surface energy, which are related to these surface forces, are extensively discussed. Self-assembled monolayers (SAMs) have recently received considerable attention as molecular-level lubricants in MEMS. The structure and the surface characteristics of SAMs are introduced. Experiments, molecular dynamics (MD) simulations, and theoretical models on the adhesion force between the atomic force microscope (AFM) tip and sample are discussed in detail. Finally, the adhesion problems related to super-hydrophobic films are discussed. 

The effects of dwell time on the stiction curve are shown on Figure 2 in the static friction regime, at constant velocity and under zero applied normal load, the increasing rate of the amplitude of the friction force increases when the dwell time is longer. The position of the transition is also shifted towards the higher displacements. Several stiction curves corresponding to experiment realised at different velocity and... 

From the scaling properties noted above, the work necessary to cause the tip to slide can be defined per unit of contact area by integration of the stiction curve and normalisation. The dependence of this energy with the velocity is shown to follow a power law (Figure 10). This effective separation energy is larger than the expected thermodynamic work of adhesion (several tens of mJ m" ) by one or two order of magnitude. This is to be attributed to the dissipative character of the materials. The exponent is similar to the one which describes the frequency dependence of the complex modulus. 

The effect of Coulomb friction and stiction on force control with... 

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