Self-Assembled Monolayer-Assisted Negative Lithography

Abstract

Self-assembled monolayers (SAMs) have been widely employed as etching resists in wet lithography systems to form patterns in which the ordered molecular packing of the SAM regions significantly delays the etchant attack. A generally accepted recognition is that the SAMs ability to resist etching is positively correlated to the quality of the surface-assembled structures, and a more ordered molecular packing would correspond to a better etching resistance. Such a classical belief is debated in the present work by providing an alternative SAM-assisted negative lithography where ordered SAM regions are etched more quickly than their disordered counterparts. This method features a unique photoirradiation-imprinted patterning process that simply consists of two steps: (1) UV irradiation on an OH-terminated SAM-modified gold surface through a photomask and (2) the subsequent immersion of the exposed substrate in an aqueous etching solution of N-bromosuccinimide/pyridine to develop a wet lithographic pattern. The entire experimental process reveals a finding from previous work that the etching rate on the UV-exposed regions with disordered molecular packing could be modulated to be slower than that in the unexposed well-defined SAM regions. Longer irradiation times would also revert the patterns from negative to positive. Thus, by merely using one kind of SAM-modified surface to provide both positive and negative micropatterns on gold layers, one could obtain flexible opportunities for high-resolution micro/nanofabrication resembling photolithography.

Robust Photopatterning of Gold-Thiol Self-Assembled Monolayers

“…1,2,6,7,9,12,15,16,18,19,22 Patterning techniques have included micro-contact printing (µ-CP), subtractive transfer printing, scanning probe-based lithography, energetic beam lithography, and many others. 1,2,[6][7][8][9][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] These techniques though adaptable, are often limited in scalability and ease of adaptation to new systems. 16,19 Alternatively, photolithography, specifically UV photolithography (UVPL), has been shown to be a robust and scalable technique for patterning nanoscale soft materials.…”

“…1,2,[6][7][8][9][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] These techniques though adaptable, are often limited in scalability and ease of adaptation to new systems. 16,19 Alternatively, photolithography, specifically UV photolithography (UVPL), has been shown to be a robust and scalable technique for patterning nanoscale soft materials. 1,3,6,12,18,19 Photolithography, as a non-contact method, minimizes potential for contamination and enables the production of arbitrarily complex architectures.…”

“…The exposed area of the SAM is photooxidized (e.g., thiols to sulfoxides) [28][29][30] and can be removed or replaced by a secondary SAM either through sonication or an exchange mechanism. 1,3,4,7,11,12,16,18,22,26,27,[29][30][31][32][33] In this work, we will quantify the kinetics controlling the photooxidation of Au-thiol SAMs serving as photoresists and gain insight into the stability of the resulting patterned SAM for future device implementation. These aspects will be probed using scanning Kelvin probe force microscopy (SKPFM) and x-ray photoelectron spectroscopy (XPS).…”

“…Previous work has indicated that a minimum threshold of intermolecular forces must be achieved for SAMs to remain stable during the photooxidation process. 3,7,16,26,27,29,30,32 This threshold can be reached via long backbones or substituents that generate increased intermolecular interactions such as hydrogen bonding. 16 Additional, research has probed the mechanism for photooxidation with some results showing that the presence of ozone is not required for the oxidation of the Au-thiol bond but is a likely cataylist.…”

“…3,7,16,26,27,29,30,32 This threshold can be reached via long backbones or substituents that generate increased intermolecular interactions such as hydrogen bonding. 16 Additional, research has probed the mechanism for photooxidation with some results showing that the presence of ozone is not required for the oxidation of the Au-thiol bond but is a likely cataylist. 1,3,12,29,31,32 Although previous work has generated insights into the reactions occurring at the Authiol interface, it has yet to quantify the kinetics required for the photooxidation process to occur.…”