**1. Introduction**

108 Microelectromechanical Systems and Devices

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The characteristic size of MEMS is ranging from atomic and molecular scales to micrometer and several millimeters scales. Components that reach micro-scale size have a high surface to volume ratio, which leaves them be highly subjected to micro-scale effect and susceptible to surface forces. Devices that utilize MEMS technology will often having mating surfaces. Adhesion force can arise from any number of phenomenon such as van der Waals, capillary, ionic and molecular forces. The components used in MEMS structures are very light (on the order of a few micrograms) and operate under very light loads (on the order of a few micrograms to a few milligrams). Surface forces between the adjacent surfaces are becoming dominant over the inertial force in MEMS devices. Because of the micro-sized component, the adhesion forces can pull the adjacent compliant structure into contact and result the interfacial adhesion, which may cause the device-malfunction to a great extent. The operation and performance of lightly loaded micro/nano components in MEMS are highly dependent on the adhesive interactions between mating surfaces. In a word, it is important that the mechanisms of interfacial adhesion should be explained, and separating techniques should be added to the design of MEMS scale components to ensure there is no unwanted contact. Furthermore, the interfacial adhesion between two adjacent mating surfaces is determined by the interaction of rough surfaces. The surface is all rough though in different range. The interactions among different asperities are complicated because the surface topography is consisting of so many asperities. It stands to reason that the proper surface characterization is necessary to elucidate the interfacial adhesion.

The interfacial adhesion is the science and technology of two interacting surfaces in relative motion and of related subjects and practices. It is also valuable in the fundamental understanding of interfacial phenomena to provide a bridge between science and engineering in MEMS. The differences between the conventional or macro-contact and micro/nano-adhesion are contrasted in Table 1. In macro-contact mechanics, tests are conducted on components with relatively large mass under heavily loaded conditions. In these tests, contacting between mating surfaces is inevitable and the bulk properties of mating components dominate the contacting performance. In micro/nano-adhesion, measurements are made on components, at least one of the mating components, with relatively small mass under lightly loaded conditions. The interaction is not limited only to the contacting condition. In this situation, though without contact, the attractive interaction

Surface Characterization and Interfacial Adhesion in MEMS Devices 111

Because of the complexities of surface microstructure, the measuring techniques and instruments are important to achieve the surface information to characterize and model the surface microstructure in MEMS devices. The accuracy of traditional contact (probing) as well as noncontact techniques has been perfected to a level allowing measurement of roughness in the nanometer range (Fig.2). The most accurate profilometer probes allow measurement of summit heights of several Angstroms (Bennet & Dancy, 1981; Bhushan et al., 1988). Yet, the comparatively poor lateral (horizontal) resolution significantly limits application of these techniques to the nanometer topographies when the distance between asperities is much less than the solution or 0.1-1μm. The development of techniques using probes smaller than the radius of the probing needle or the light wavelength makes it possible to extend the spectrum of surfaces studied (Myshkin et al., 2003). The scanning electron microscope (SEM) technique can be used to gauge topography with a comparable resolution both vertically and laterally by interpreting the emission intensity of the secondary electrons the topographic pattern (Myshkin et al., 1992). The scanning tunneling microscope (STM) has a still finer probe, which is the electron flux tunneled between the target surface and the needle tip. In this case the surface topography resolution is 0.01 and 0.1 nm in the vertical and lateral directions, respectively (Binnig & Rohrer, 1982). Hence, the STM technique and others resulting from its progress make it possible to use this approach for more accurate topographic investigations of solids on the nanoscale. Significant prospects are connected with the application of atomic force microscope (AFM) (Sarid, 1991)

**2.2 Surface measurement techniques** 

in which atomic-molecular surface effects are registered.

Fig. 2. Diagram of the height and spacing parameters and ranges of vertical-lateral resolution for different methods of roughness measurement (Myshkin et al., 2003).

The scanning tunneling microscope (STM) developed by Dr. Gerd Binnig and Heinrich Rohrer has revolutionized the study of surfaces and is rapidly becoming a required tool in

between mating opposite surfaces at small approaching distance can't be neglected. Some of the smaller asperities on the micro-sized surface will be stretched, while some of the taller ones will be compressed through contact. The classical contact mechanics is no longer valid in analyzing the interaction of mating micro/nano-sized surface. It is necessary to explore methods to solve the interfacial adhesion problems in MEMS devices.


Table 1. Comparison between macro-contact and micro/nano-adhesion

In this chapter, we will take a close look at surface geometric structure, or surface topography, and surface forces to elucidate the adhesion problems between mating microsized MEMS surfaces. Firstly, the complexities of the surface microstructure devices are discussed. Secondly, several typical surface-measurement instruments are introduced. Thirdly, the techniques to characterize the complex micro-scale surfaces are presented. Finally, the surface forces are described in a summary form, and then the adhesion models are given to interpret the adhesive interaction of MEMS devices.
