PMMA removal options by wet development in PS-b-PMMA block copolymer for nanolithographic mask fabrication

In the integration flow of directed self-assembly with block copolymers (BCP), the selective removal of one phase of the polymer with respect to the other one is an important step. Different strategies can be implemented such as wet only, exposure, and wet or dry only, each one presenting its highlights and limitations. In this paper, the authors report a wet etching technique allowing an efficient removal of polymethylmethacrylate (PMMA) in PMMA cylinder-forming polystyrene-b-polymethylmethacrylate (PS-b-PMMA) BCP in order to generate contact holes. They confirm that exposure with ultraviolet (UV) light or electron beam is required for PMMA degradation before its complete removal by wet development in acetic acid. A dose-to-clear of 300 mJ/cm2 and 200 μC/cm2 using UV exposure and e-beam exposure, respectively, is demonstrated for a thin BCP film coated on a silicon substrate. Complementary characterizations are employed: by ellipsometry to monitor the BCP thickness, by scanning electron and atomic force microscopies to monitor the surface topography and precisely determine the dose-to-clear. It is also shown that this UV exposure dose is dependent on the materials stack used underneath the BCP film due to the radiation absorption. Furthermore, the authors have investigated the ion implantation in the BCP film with oxygen and hydrogen ions and it seems to be another alternative for PMMA degradation in the wet development approach.