Uilibestm isereached buttheypreviothsly

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Unformatted text preview: eedgbvinhefthetescan ooftfuhleurfougherasee, gbvbnhefthevesluantooo fuhleurfougherasee, gbvbnhe qheveslualtooo hte eurfeiceht sses, gSvbngeqhevalum oo the height sets, Sab , equivalent to orc e c l b he y c ace, ct t i y t g ma erual bfe htel s c eicntt sctsd Syi t gequate ri m bfefthtel s c eicntt sctsd Syi t ge t uat ari m f ft hs h a g , et t i a i , t ui s ent t f orc i m y h a e, et t i a , m i ace n t orc l y h a e, et t i a , m i en t equilibrium is reached, but by then the uilibestm isereached, buttheypreviothsly ilibestmistance ded, buttheypfuenothslyhgbhestmistance ded, buttheypfuenothslyhgbhest fdistance distributhenpfuncouslyhgbvz) fdistance distribution function hab (z) for eq highriu asp rities are b then uequ igihriu d isereach istribution th nctioe uilibriu fd risereach istribution th nctioe higivz) asperities are tio revi tion a i( en or e h g ven asp rities are n a ( en o b revi ueq higivz) asp rities are b revi u n a ( en or not the only ones governing the surnace thnternlyion eenergy rnithg Gaussian ce thnternistion eenergy rnithg Gaussian ce thnternistion eenergy rnithg Gaussian ce interaistion energy the Gaussian case. It is f ot i e o act on s gove n e the surna case. Italy on s gove n e the surna case. Italy on s gove n e the surfa case. It c f ot i e o c f ot i e o c What is Stiction? At the micro-scale, the forces associated with the stiction effect are often greater than other micro-forces. As a result, if stiction is present in a micro-system, it can dominate the system. This usually has negative effects, such as microobjects sticking together with no way to separate them, or high amounts of dissipative loss between two objects in sliding contact. © N. Dechev, University of Victoria 25 Stiction vs. Friction Friction occurs between two objects in contact with each other and a normal force Fn, between them. The frictional force Ff depends on the coefficient of friction, and is independent of contact area, as Fn shown: Ff Fn In this sense, if the normal force is removed, the force of friction will be gone. Stiction is an adhesion force that will occur ‘upon contact’ between objects, and will keep the two objects in contact indefinitely, even if the initial force that brought the two objects together is gone. Stiction is directly proportional to surface area of contact. © N. Dechev, University of Victoria 26 The Causes of Stiction for MEMS Stiction will occur upon contact between micro-objects, so a good question may be: Why did that contact occur in the first place? Electrostatic forces Fluid surface tension effects due to the drying process after the HF release of sacrificial layers Shock loading or rapid acceleration that can bring two surfaces together Inadequate stiffness of supporting micro-beams against gravity or normal operation Desired contact by design © N. Dechev, University of Victoria 27 The Causes of Stiction for MEMS Once contact has occurred, there are four major phenomena that individually contribute to the ‘overall effect’...
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