1. Field of the Invention
This invention relates generally to the directed self-assembly (DSA) of block copolymers (BCPs) using guiding lines to make an etch mask for pattern transfer into a substrate, and more particularly to a method for making the guiding lines.
2. Description of the Related Art
Directed self-assembly (DSA) of block copolymers (BCPs) has been proposed for making imprint templates for patterned media (also called bit-patterned media) magnetic recording disks and for patterning semiconductor devices, for example, for patterning parallel generally straight lines in MPU, DRAM and NAND flash devices. DSA of BCPs by use of a patterned sublayer that provides a chemical contrast pattern for the BCP film is well-known. After the BCP components self-assemble on the patterned sublayer, one of the components is selectively removed, leaving the other component with the desired pattern, which can then be used as an etch mask to transfer the pattern into an underlying substrate. The etched substrate can be used as an imprint template.
In conventional DSA by use of a chemical contrast pattern, an array of sparse guiding lines, usually made of a cross-linkable polymer mat, is lithographically patterned on the substrate. The pitch of these guiding lines (Ls) needs to be an integer multiple of the natural pitch (L0) formed by the block copolymer to be directed, i.e., Ls=nL0, where n is an integer equal to or greater than 1. However, the width of the guiding lines generally needs to be about 0.5 L0, or nearly equal to the width of one of the block copolymer components. While lithographic applications benefit from the fact that the guiding lines can be sparse (at a pitch of nL0) and that the high density comes from the density multiplication afforded by the block copolymers, the resolution requirement to pattern the width of the guiding lines keeps scaling with the final block copolymer dimensions. As lithographic applications advance to smaller dimensions, especially below 20 nm full pitch, the fabrication of guiding lines with a width below 10 nm while maintaining adequate roughness and width uniformity becomes ever more difficult and beyond what is possible with current resist materials for electron-beam (e-beam) or optical lithography.
Pending application Ser. No. 14/532,240 filed Nov. 4, 2014 and assigned to the same assignee as this application describes a pattern of guiding lines for DSA of a BCP that is an array of spaced guiding lines with oxidized sidewalls. A silicon-containing BCP self-assembles with the silicon-free BCP component wetting the oxidized sidewalls. The silicon-free BCP component is removed. The other BCP component then has a line density double that of the original spaced guiding lines and can be used as an etch mask to etch the underlying substrate. In the '240 application the spaced guiding lines are formed by electron-beam lithography. However, for small-pitch BCPs, it can be difficult to pattern guiding lines by e-beam lithography since it requires operating at the resolution limit of current e-beam resists. At the resolution limit of the e-beam resist, guiding lines are typically rough, broken, and/or wavy and cause defects in the BCP pattern that they guide. This effect is particularly severe for radial lines, which are required for making patterned media magnetic recording disks. Radial lines are more difficult to create by the e-beam tool due to the circumferential path of the beam, which results in an increased width roughness of the exposed lines. Also, radial lines require a smaller pitch than circumferential lines and thus a greater guiding line resolution to make patterned media bit structures with a high bit aspect ratio.
What is needed is an improved method for making guiding lines that results in a BCP pattern after DSA with better quality and lower roughness.