Silicon-containing copolymers of MMA tested for a positive-tone high-resolution bi-layer e-beam resist system

Copolymers of methylmethacrylate (MMA) and trimethylsilyl-methylmethacrylate (SiMMA) in various molar ratios have been synthesised and characterised for use as a resist in electron beam lithography. The development procedure has been optimised in terms of contrast and surface roughness. Both the etch resistance during oxygen plasma etching and the 'dose window' (dose range between positive and negative tone behaviour) have been studied as a function of silicon content. In the trade-off between etch resistance and dose window, polymers with 15-50% SiMMA monomers show the most promising behaviour. A 1 μm thick copolymer layer shows a five times higher etch resistance in an oxygen plasma than Novolak and chemically amplified resists under the same conditions. However, the etch resistance of the copolymers is not constant in time, because a resistant silicon oxide top layer is built up gradually, as has been shown by interferometry and Rutherford Backscattering Spectrometry. For high-resolution electron beam writing, very thin layers are required. In a 100 nm thick layer spaces of 25 nm width have been made. However, at this thickness the etch rate is still comparable to that of Novolak. The application to a bi-layer system also appears to be hampered by island formation of the (insufficient number of) silicon oxide species during etching.