Contact-mode polycrystalline silicon microdevices were tested in reciprocating sliding in high vacuum. Changes in the adhesion and friction properties of the microdevice sidewall surfaces were evaluated by tracking the development of the adhesion force and static friction coefficient with the accumulation of sliding cycles. After a run-in period of no discernible changes, the adhesion force increased monotonically with the number of sliding cycles, while the static coefficient of friction decreased continuously. Scanning electron microscopy revealed the existence of two sliding regimes where asperity deformation and adhesion were the dominant wear processes. The evolution of adhesion and static friction at sidewall contact interfaces is interpreted in the context of physicochemical phenomena intrinsic of poly-crystalline silicon microdevices.

This paper investigates the effect of bearing clearance on the impact behavior of microfabricated silicon journal bearings. The design of a novel test apparatus to assess microbearing wear behavior is presented. Microbearing designs, microfabrication processes, and metrology characterization techniques are discussed. A dynamic impact model of the bearing system based on classical impulse-momentum relations is formulated in order to assess the effect of clearance on rotor speed. Coefficient of restitution values obtained over the range of kinematically allowable radial clearance specifications are found to agree well with previously published results for polysilicon microstructures.

In order to alleviate or eliminate the occurrence of stiction during the actuation of microstructures, the real contact area available for contact must be reduced. Au nanoparticles were intentionally deposited using gas-expanded liquids onto polysilicon cantilever beam arrays to increase surface roughness. The nanoparticle-coated beams were subjected to an actuation voltage of 120 V. Following actuation, the adhesion of beams was quantified by estimating the apparent work of adhesion. Au nanoparticles deposited onto these microstructures were shown to drastically reduce the effects of in-use stiction. Capillary adhesion due to condensation of ambient moisture was effectively eliminated.

The stiction forces which exists in micro-electro-mechanical systems are characterized by surface energy and surface roughness. To simulate this contact condition, a three dimensional fractal surface geometry and an adhesive contact model for a single asperity are used together to create a numerical adhesive rough surface solution methodology. This novel method of solution determines the characteristic adhesive contact type for each individual asperity uniquely at the time of load and area integration. Such a simulation more accurately represents the physics of the asperity based contact. Numerical results for the adherence force are presented as a function of surface topography, interface compliance, and the work of adhesion for a MEMS interface. The magnitude of the force required to separate an adhesive rough surface interface is given in relation to a polysilicon system.

We have measured pre-sliding tangential deflections (PSTD) between micromachined surfaces of up to 200 nanometers in length before the static friction event using a polysilicon nanotractor actuator [1,2]. The detailed PSTD structure is resolved by a one-nanometer-resolution in-plane optical metrology we have developed, and may be a manifestation of discrete asperity-asperity interactions leading to an effective spatial distribution of friction coefficients. Results indicate a dependence on surface treatment, with a perfluorinated eight-carbon chain monolayer coating showing substantially different PSTD than an eighteen-carbon chain hydrocrabon monolayer. This behavior may qualitatively be related to variations in dynamic versus static friction. We present a simple phenomenological model that captures some of the behavior of PSTD, and suggest some possible microscopic interpretations.