Pattern size and pitch are two major factors that directly affect the capacity of a data storage unit. Electron beam lithography, because it can be used for direct writing at the nanometer scale, is one of the potential candidates to fabricate ultra-fine and ultra-compact patterns for the next generation of data storage media.
In the paper, we present methods to reduce the pattern size and pitch written by electron beam on positive tone resist Poly(methylmethacrylate) (PMMA). The first method uses a writing dose which is much lower than the critical dose of the PMMA. However, in this technique, the distance between the adjacent writing spots (which are approximately Gaussian in spatial extent) is finely adjusted until the partially overlapping Gaussian functions form a contour with periodic dose peaks higher than the critical dose. This overlapping exposure results in a periodic exposure of the PMMA. Most importantly, this writing strategy, using the overlapping of low dose Gaussian beams, results in patterns with smaller size and pitch compared to the conventional methods that use a higher writing dose than the critical dose. To further improve the results, we use ultrasonic development in combination with cold development to greatly increase the resist contrast. The increased contrast results in smaller pattern sizes, and enables smaller pattern pitches, as well as improving pattern uniformity and reproducibility.
Finally, as a demonstration of the technique using a conventional beam voltage of 15kV, we achieve ultra-fine and high density patterns with ∼10nm linewidths and 50nm in pitch, which corresponds to a storage capacity near 200 billion bits/inch2. In comparison, standard techniques (using a dose slightly higher than the critical dose and normal development strategy), we can only obtain patterns with linewidths of about 20-30nm and pitch of about 100nm.