Challenges and opportunities of KrF photoresist development for 3D NAND application

Further increasing integrated circuit storage capacities while reducing the cost has led to the development of stacked 3D structures for NAND application. The 3D NAND structures can be enabled by using KrF photoresist coated at high thickness, and a staircase pattern can be generated through multiple etch steps. Pushing KrF lithography to high coating thickness creates several challenges for the materials and formulation design, such as lack of film transparency, as well as film cracking and delamination. The photoresist used in KrF lithography is based on poly(hydroxystyrene) (PHS) type polymers, which is associated with unique technical challenges for printing 10s microns scale features due to its high absorbance at 248 nm and its high glass transition temperature, Tg. Here we report the development of novel KrF photoresist materials for 3D NAND application with sufficient film transmittance, which forms desired straight profile with no footing. Our results also indicate that the film cracking could be mitigated by additive and process condition optimization, and film delamination could be addressed by adding an adhesion promotion layer.