A Model for Adhesive Forces in Miniature Systems

Abstract

Miniature devices including MEMS and the head disk interface (HDI) in magnetic storage often include very smooth surfaces, typically having root-mean-square roughness, Rq of the order of 10 nm or less. When such smooth surfaces contact, or come into proximity of each other, either in dry or wet environments, then strong intermolecular (adhesive) forces may arise. Such strong intermolecular forces may result in unacceptable and possibly catastrophic adhesion, stiction, friction and wear. In the present work, an adhesion model termed sub-boundary lubrication (SBL) model is used to calculate the adhesion forces at typical MEMS interfaces. The model uses the Lennard-Jones attractive potential to characterize the intermolecular forces, and a statistical surface roughness model. The normal separation between the MEMS surfaces that are investigated is of the order of 100 nm down to fully contacting interfaces. Several levels of surface roughness are investigated. The first case is composed of rougher interfaces with combined Rq = 15.8 nm. The second case is an intermediate rough case with Rq = 6.8 nm, and the last case is a super smooth interface with combined Rq = 1.4 nm. The latter is a typical roughness of polished polysilicon films, whereas the rougher interfaces are typical of as deposited polysilicon films. The model reveals the significance of the surface roughness on the adhesion forces as the surfaces become smoother, and suggests that a link between macrotribology and micro/nanotribology is adhesion.