Fabrication of trilayer resist using photocuring-imprint lithography

Abstract

We fabricated fine trilayer resist structures (photocurable polymer/spin-on-glass (SOG)/Novolak resist) with a smooth-top surface and low-edge roughness, which is useful for standard patterning for the evaluation of critical dimension-atomic-force microscopy using photocuring-imprint lithography. A very serious pattern shrinkage problem observed at the top of the photocurable polymer after etching at room temperature was greatly improved by using substrate cooling, where the temperature was decreased to −120 °C. Finally, we obtained improved pattern shrinkage of the photocurable polymer, ranging from a maximum of 23 nm to a minimum of 4 nm, at optimum etching conditions. The pattern reduction did not occur in the following etch process. From this result, we noticed that throughout the trilayer resist etch process, the pattern reduction was mainly generated in the photocurable polymer etching step. SOG was used as the hard mask. The SOG showed high etch selectivity against the bottom layer of the Novolak resist in O2-based plasmas. The SOG etched only in pure SF6 gas, but the SOG was not etched in O2-based plasmas. Novolak resist etching at room temperature showed very small pattern shrinkage, no residue, no undercut, and fine features, while the Novolak etching at −120 °C showed much residue and damaged etch profiles. It is probable that in contrast to −120 °C, at room temperature, the residue of the Novolak resist was clearly removed by the reactive species, which are activated by the increased temperature. After trilayer resist etching, removal of the SOG layer was needed for the measurement of the pattern shrinkage and the line-edge roughness (LER). In this experiment, the SOG layer was removed by using anhydrous hydrofluoric acid. This method did not cause the pattern collapse which is frequently found in wet etching removal. Furthermore, trilayer resist patterns after the removal of the SOG layer showed very fine etch profiles, and the LER variation was very small through the whole trilayer resist etching process.