Chemical Modification of Self-Assembled Monolayers by Exposure to Soft X-rays in Air

Abstract

Methyl-, vinyl-, and trifluoroacetoxy-terminated self-assembled monolayers (SAMs) of alkylsiloxanes on SiOx/Si substrates were exposed to soft X-rays (0−4000 mJ/cm2) at air pressures from 2 × 10-2 to 2 Torr. The exposed and unexposed monolayers were characterized by using advancing-contact-angle measurements of water, ellipsometry, and X-ray photoelectron spectroscopy (XPS). No significant differences in the thicknesses of the monolayers were observed under any exposure conditions. Advancing-contact angles of water (θa) on all of the monolayers did not change with increasing dose up to 2000 mJ/cm2 for exposures performed at 2 × 10-2 Torr. A 15% loss of fluorine was observed from the CF3COO-terminated SAMs at this pressure at a dose of 4000 mJ/cm2. The θa decreased monotonically with dose for all monolayers exposed at 0.5, 1, and 2 Torr of air pressure. The rate of decrease of θa increased with increasing air pressure. A simple kinetic model based on competing oxidation and cross-linking reactions of reactive surface species fit the data well. The model adequately described the asymptotic value of the contact angle at high doses for the three exposure pressures and was insightful for the analysis of the role of oxygen in surface-modification reactions. Loss of fluorine from the CF3COO-terminated SAMs followed the same trends as the contact-angle data. XPS data showed that hydroxyl (C−OH) and aldehyde (CHO) groups were incorporated onto the surface of the SAMs upon irradiation at 0.5, 1, and 2 Torr of air pressure, irrespective of the initial terminal groups of the SAMs. The hydroxyl groups were shown to be reactive for the formation of bilayer structures. These results are relevant for the optimization of chemical contrast and sensitivity in imaging layers based on SAMs for nanolithographic techniques using ionizing radiation.