Mechanism of UV Photoreactivity of Alkylsiloxane Self-Assembled Monolayers

Abstract

A molecular level understanding of the photoreactivity of self-assembled monolayers (SAMs) becomes increasingly important as the spatial resolution starts to be limited by the size of the resist and the spatial extent of the photochemical reactions in photoresist micropatterning. To this end, a number of surface characterization techniques were combined to understand the reactive agents, reactive sites, kinetics, and reaction pathways in the UV photoreactivity of octadecylsiloxane (ODS) SAMs. Quantitative analysis of our results provides evidence that ground state atomic oxygen is the primary reactive agent for the UV degradation of ODS SAMs. UV degradation, which follows zero-order kinetics, results in the scission of alkyl chains instead of the siloxane headgroups. Our results suggest that the top of the ODS SAMs is the preferential reactive site. Using a novel, highly surface sensitive technique, fluorescence labeling of surface species, we identified the presence of submonolayer quantities chemical functional groups formed by the UV degradation. These groups are intermediates in a proposed mechanism based on hydrogen abstraction.

Clean and reactive nanostructured cellulose surface

“…Ozone absorbs the 254 nm UV 127 radiation and dissociates into molecular oxygen and atomic oxygen (Zhang et al 2006). UV light 128 excites ambient oxygen molecules from their ground spin-triplet state into a spin-singlet state thus 129 creating a more reactive environment in which the immediate cleavage of organic bonds like C-C, C-H7 and O-H into volatile organics takes place (Ye et al 2005). This procedure has been found to be more 131 efficient than UV radiation or ozone treatment separately.…”

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“…Fluorescent labeling, on the other hand, is a promising method because of its high sensitivity, ease of operation and in situ applicability. It has been widely used in biological applications [46,47], polymer chemistry [48][49][50][51], and study of self assembled monolayers [52][53][54][55].…”

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“…The 185 nm photons induce the dissociation of O 2 by generating singlet oxygen (O( 1 D)), which collides with inert N 2 to generate triplet oxygen (O( 3 P)) and reacts with O 2 to produce ozone (O 3 ). The UV also breaks O 3 to O( 1 D), which cleaves organic bonds such C-C, C-H and O-H [16,17].…”