Focused ion beam lithography using ladder silicone spin-on glass (LS-SOG) as a negative-tone resist was investigated. The use of 200 keV Be2+ ion and tetramethylammoniumhydroxide developer resulted an in 80-nm-width line pattern with a vertical profile. The pattern collapse during the development was avoided by the use of fluorocarbon-based solvent. The development mechanism of LS-SOG was investigated by using nuclear magnetic resonance and gel permeation chromatography analysis.