Reducing hole-size variation and defect ratio after pattern transfer by using self-assembled polymer with spherical structure

Abstract

This study presents a method to reduce hole-diameter variation and defect ratio in patterning of a self-assembled block copolymer (BCP) for imprint-mold fabrication. The BCP material used is PMMA-b-poly(methyl acrylate) polyhedral oligomeric silsesquioxane (PMAPOSS) in which PMMA spheres with 18.3-nm-pitch are aligned in the hexagonal close-packed positions in the PMAPOSS matrix. When the self-assembled BCP film was etched in the conventional dry-development process, the hole-diameter variation and the amount of hole defects (defect ratio), defined as “no-opening defects” or “connecting holes,” increased. Variation of PMMA sphere diameter and/or position in the perpendicular direction to the substrate plane was assumed to be the main cause of the increase in hole-diameter variation and defect ratio after BCP development. To optimize the etching conditions for BCP development, a new model representing the relationship between defect ratio and relative standard deviation of PMMA sphere diameter and/or position under various etching conditions (selectivity and anisotropy) was developed. The model indicates that to reduce defect ratio, higher etching selectivity of PMMA to PMAPOSS and etching anisotropy are required. It also indicates that optimizing etching anisotropy is more effective than optimizing etching selectivity. On the basis of these modeling results, the dry-development conditions were optimized; namely, selectivity (etching ratio of PMMA to PMAPOSS) was increased from 4 to 6 and anisotropy (etching ratio of anisotropy to isotropy) was increased from 1.6 to 2.1. As a result, the defect ratio decreased from 31% to 5%. A 1.7 tera-dot/in.2 imprint-mold was fabricated by applying the directed self-assembly process.