The Ohio State University

Micro/Nanoscale Tribology and Mechanics of Components and Coatings for MEMS

Sriram Sundararajan
Ph.D. Dissertation,
The Ohio State University,
Department of Mechanical Engineering, 2001

Abstract

‘Microelectromechanical systems’ (MEMS) is the collective term for microcomponents and microdevices that have been developed using lithography-based and other techniques with physical dimensions ranging from a couple to a few hundred microns.Several studies have shown that tribology (friction and wear) is an important factor affecting the performance and reliability of MEMS. Good mechanical properties are also critical for mechanical integrity of microstructures.There is a need to develop a fundamental understanding of tribological phenomena and to evaluate mechanical properties on the scale pertinent to MEMS.This research addresses this need using atomic force microscopy (AFM)-based experimental techniques.

To address the problem of friction, a study of the static friction of polysilicon micromotors was performed. A technique to measure the static friction forces in the devices was developed and forces measured indicated that the coefficient of static friction for unlubricated motors was far larger than one. A molecularly thin bonded layer of perflouropolyether lubricant appeared to reduce the static friction and rendered the contact interfaces insensitive to the environment. Meniscus effects and surface roughness characteristics of the contacting surfaces were identified as the mechanisms for high friction.

To address the problem of wear, ultra-thin hard amorphous carbon coatings for use as protective coatings were studied. Nanoscale scratch and wear studies were conducted to identify the optimum coating properties for the best scratch/wear resistance. Ploughing, associated with plastic deformation, was identified as the initial failure mechanism followed by brittle fracture and delamination. High hardness and matching of elastic modulus values of the coating and the substrate promoted better scratch/wear resistance

AFM-based techniques to evaluate mechanical properties of nanometer-sized silicon and silica (SiO₂) beams under static and dynamic loading were developed. Elastic modulus and fracture toughness appeared to be comparable to bulk values while bending strength values were on order of magnitude higher than values obtained from larger specimens. Cleavage fracture appeared to be the failure mechanism under both static and dynamic loading.

Surface topography is known to have a significant effect on localized friction on the nanoscale, which is pertinent to tribology of MEMS. The effect of surface topography on the friction forces measured using an AFM was studied to understand its origins and to clarify confusing interpretations in the literature. Topography-induced transitions in the friction signal always corresponded to transitions in surface slope even when friction signals from opposing scan directions are subtracted. The ratchet mechanism and the dynamic effect of an AFM tip colliding against a surface feature with a sudden increase in slope were found to be the reasons for this observation.