A monodisperse negative Si‐containing photoresist (MAS) has been developed for use as the top imaging layer for a bilayer system. MAS is a formulation of poly(allyldimethylsilyl‐α‐methylstyrene) (PMA Si) and a bisazide. PMA Si was synthesized by anionic polymerization of the monomer with n‐butyl lithium in THF at −78 °C. The polymer has a high softening point (Ts=164–166 °C) and a narrow molecular weight distribution (Mw=41 400, Mw/Mn=1.05). Near‐UV‐exposure has been carried out using a Kasper 2001 P contact printer. The Dig of MAS (0.2 μm in thickness) is 4.5 mJ/cm2 on a Si wafer and the γ value is over 2.0. The thickness reductions of MAS and MP‐1300 have been measured under various O2‐RIE conditions. The etching ratio of MP‐1300/MAS ranges from 3:1 to 10:1 depending on conditions and is decreased with an increase in rf power and with a decrease in chamber pressure. Under the optimized development and O2‐RIE condition, 0.75 μm line and space (L/S) patterns were accurately fabricated on the Si wafer having 0.5 μm high step. It is also noteworthy that 2.0 μm L/S patterns with a steep profile were obtained even in the MP‐1300 layer of 4.0 or 6.0 μm in thickness.