Mechanics of adhesion in MEMS
#1

Abstract
A review is presented of the mechanics of microscale adhesion in microelectromechanical
systems (MEMS). Some governing dimensionlessnumbers such as Tabor number, adhesionparameter
and peel number for microscale elastic adhesion contact are discussed in detail. The peel number is
modi. ed for the elastic contact between a rough surface in contact with a smooth plane. Roughness
ratio is introduced to characterize the relative importance of surface roughness for microscale
adhesion contact, and three kinds of asperity height distributions are discussed: Gaussian, fractal, and
exponentialdistributions. Both Gaussian and exponentialdistributionsare found to be special cases of
fractal distribution. Casimir force induced adhesion in MEMS, and adhesion of carbon nanotubes to
a substrate are also discussed. Finally, microscale plastic adhesion contact theory is brie y reviewed,
and it is found that the dimensionless number, plasticity index of various forms, can be expressed by
the roughness ratio.
Keywords: Microscale adhesion contact; Tabor number; adhesion parameter; peel number; surface
roughness; fractal distribution; surface energy; surface force.
1. INTRODUCTION
Scale effect is one of the fundamental issues in building MEMS [1]. The most
challenging issues lie in the fact that the surface-to-volume ratio increases when
the MEMS dimensions decrease. The types of forces that in uence microscale
devices are different from those that in uence devices with conventional scale.
This is because the size of a physical system bears a signi. cant in uence on the physical phenomena that dictate the dynamic behavior of that system. For example,
larger-scale systems are in uenced by inertia effects to a much greater extent than
smaller-scale systems, while smaller systems are more in uenced by surface effects.
Therefore, surface effect induced strong adhesion, friction and wear are major problems limiting both the fabrication yield and operation lifetime of many MEMS devices [2, 3]. Strong adhesion is generally caused by capillary, electrostatic, vander Waals forces, and other kinds of ‘chemical’ forces [4, 5]. Stiction is a term that has been applied to the unintentional adhesion of compliant microstructure surfaces when restoring forces are unable to overcome interfacial forces. The stiction problem of MEMS can be divided into two categories: release-related stiction and in-use stiction. Release-related stiction occurs during the sacri. cial layer removal process in fabrication of microstructures, and such stiction is caused primarily by capillary forces. In-use stiction usually occurs upon exposure of successfully released microstructures to a humid environment.


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