Contact angle measurements of lubricated silicon wafers and hydrogenated carbon overcoats

Abstract

We have investigated the dependence of contact angles on the thickness of perfluoropolyether lubricants on silicon wafers and on disks overcoated with amorphous hydrogenated carbon (CHx). Similar lubricants and carbon-overcoated disks are found in many of today's disk drives. From the contact angles, we observe that the work of adhesion for alkanes on these lubricated surfaces can be separated into a term that describes the direct interaction with the lubricant material and in to a term that describes how the interaction with the lubricant material and into a term that describes how the interaction of the alkane with the substrate surface is mediated by the intervening lubricant layer. ΔW = ao′ + bo′e∼L/L1, where L is the lubricant layer thickness. Lo is the screening length of the substrate interaction through the lubricant layer, and ao′ and bo′ are parameters that describe the strength of the interactions. The contact angles for alkanes are further analyzed using the Zisman method to determine the critical surface tension of these surfaces. Water contact angle measurements reveal that the surfaces become more hydrophobic with increasing thickness of the perfluorinated lubricants. Moreover, the contact angles decrease in time via a nonexponential process and relax at a slower rate for samples with thicker lubricant films. In general, we observe that adding a perfluoropolyether lubricant lowers the surface energy by screening out the interaction with the underlying substrate. The screening distance depends on the type of a substrate, lubricant material, annealing condition, and type of liquid in the contact angle droplet, but is always in the range of 2-10 Å for alkane droplets and 20-47 Å for water droplets.