Gap-fill type HSQ/ZEP520A bilayer resist process-(IV): HSQ-rod and HSQ-tip hardening processes

HSQ island formed by directly e-beam exposure (DE) and wet development is used as a dry etching mask material. However, the HSQ islands with high aspect ratio are susceptible to collapse during wet development process due to surface tension. To improve this, HSQ-rod and HSQ-Tip structures were achieved by dry stripping of ZEP520A after thermal reflow of ultra-thin HSQ (hydrogen silsesquioxane) gap-filled (GF) ZEP520A contact holes (C/H) in previous study. Aspect ratio of HSQ island formed by latter process is higher than that by the former since the latter is without wet develop procedure which tends to washout the HSQ island. In this paper, gap-fill processes followed by a hardening process to prevent bending of HSQ island are studied to form sub-50 nm HSQ islands (rod or tip) with high aspect ratio. Diluted HSQ is used to gap-fill the exposed ZEP520A C/H or C/H after thermal reflow. The hardening processes include high temperature baking and e-beam curing with high beam current. Experimental results are summarized below. Aspect ratio of GF type HSQ-rod larger than 7 is obtained. Bending of GF type HSQ island (rod or tip) with high aspect ratio is also observed. HSQ-rod hardened by high temperature baking tends to fracture. E-beam curing proves to be efficient for HSQ island (rod or tip) hardening and the required curing doses are dependent on HSQ-rod CD. Smallest HSQ-Tip CD hardened by e-beam curing is ~12.5nm. It is found that e-beam curing of GF type HSQ island and e-beam exposure of DE type HSQ island has the same effect and mechanism in cross-linking of HSQ molecules to increase mechanical strength.