Alkylperfluorosilane Self-Assembled Monolayers on Aluminum:  A Comparison with Alkylphosphonate Self-Assembled Monolayers

The performance of micro- and nanoelectromechanical systems depends on the surface properties of the substrate material, such as chemical composition, roughness, friction, adhesion, and wear. Substrates of aluminum deposited onto Si (Al/Si) have been chemically reacted with perfluorodecyldimethylchlorosilane (PFMS), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), octylphosphonic acid (OP), and perfluorodecylphosphonic acid (PFDP) and then characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). PFMS/Al self-assembled monolayers (SAMs) were studied by friction force microscopy, a derivative of AFM, to better understand their micro- and nanotribological properties. The adhesion forces for PFMS/Al SAMs were found to be lower when compared to those of bare Al/Si; however, the coefficient of friction for both was comparable. XPS analysis revealed the presence of the corresponding alkyl chain molecules on PFMS/Al, ODP/Al, DP/Al, OP/Al, and PFDP/Al SAMs. The sessile drop static contact angle of pure water demonstrates that all the SAMs are extremely hydrophobic, giving contact angles typically >130° on PFDP/Al, ODP/Al, and PFMS/Al SAMs and >125° on DP/Al and OP/Al SAMs. The surface energy of PFMS/Al SAMs determined by the Zisman plot method is 16.5 ± 2 mJ/m2 (mN/m). The rms surface roughness of ODP/Al, DP/Al, OP/Al, PFMS/Al, and PFDP/Al SAMs, before exposure to warm nitric acid (pH 1.8, 30 min, 60−95 °C), as well as bare Al, is less than 40 nm as determined by AFM. The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid or silane molecule and the oxidized Al/Si surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60−95 °C) show ODP/Al SAMs to be the most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs.