Patterning via surface monolayer initiated polymerization: A study of surface initiator photoreaction kinetics

Abstract

Surface monolayer initiated polymerization (SMIP) is a potential method for achieving high resolution patterning of surfaces and materials that could be used as an alternative to conventional lithographic methods based on photoresist thin films. This article reports on the photochemical kinetic rate constants of two candidate azo-type surface bound photoradical initiator molecules. X-ray photoelectron spectroscopy was utilized to monitor the relative concentration of azo initiator on a silicon surface as a function of exposure dose to 248nm radiation. This photochemical decomposition data showed that the photoreaction for both initiators followed first order kinetics with photoreaction rate constants in the range of 4.5×10−3cm2∕mJ to 9.7×10−3cm2∕mJ. The difference in the observed rate constants for the two azo initiators was attributed primarily to differences in their quantum efficiencies. These differences in quantum efficiency were attributed to the size of the nontethered fragment that would be produced from the initiator, with larger fragments producing slower photochemical decomposition kinetics. Thus, photoradical initiators with small non-surface bound fragments are desirable in terms of increasing the photosensitivity of such SMIP processes. For successful positive tone imaging using a SMIP process with photoradical initiators, it was estimated that approximately 99% of the monolayer must be decomposed in order to produce images in the resulting polymer layers. Using this information in conjunction with the photochemical reaction rate constants for the azo initiators, exposure doses on the order of 400mJ∕cm2 and larger would be required for the present initiators.