We have constructed a humidity-controlled chamber in which deflections of polysilicon cantilever beams are observed by interferometry, resulting in in-situ adhesion measurements within a fracture mechanics framework. From adhesion energy measurements for uncoated hydrophilic beams, we demonstrate an exponential dependence of adhesion on relative humidity (RH). We can explain this trend with a single-asperity model for capillary condensation. For coated hydrophobic beams, adhesion is independent of RH up to a threshold value which depends on the coating used. However, we have found that exposure to very high RH (greater than or equal to 90%) ambients can cause a dramatic increase in adhesion, surprisingly with a stronger effect for perfluorodecyltrichlorosilane (FDTS, C₁₀H₄F₁₇SiCl₃) than octadeycltrichlorosilane (ODTS, C₁₈H₃₇SiCl₃). Newly developed computational mechanics to measure adhesion in the presence of an applied load allow us to explore how the adhesion increase develops. We believe that water adsorption at silanol sites at the FDTS/substrate interface, possibly exacerbated by coupling agent migration, leads to water islanding and the subsequent adhesion increase at very high RH levels.

Date Published: 15 September 1999
PDF: 14 pages
Proc. SPIE 3825, Microsystems Metrology and Inspection, (15 September 1999); doi: 10.1117/12.364289

Published in SPIE Proceedings Vol. 3825:
Microsystems Metrology and Inspection
Christophe Gorecki, Editor(s)