Per- and polyfluoroalkyl substances (PFAS) have been identified by various regulatory bodies as substances of concern. In line with the objective of safer and sustainable by design, a comprehensive program has been initiated to address these concerns. Part of this program includes the development of non -fluorinated photoacid generators (PAGs) without introducing new chemicals with unintended consequences. Using computational chemistry and synthetic organic chemistry , several scaffolds amenable to PAG library design have been realized. These novel PAGs offer facile tunability and advantages in many critical design parameters such as pKa, diffusion, absorption, shelf-life stability, and scalability. These early generation non-fluorinated PAGs show competitive and similar lithographic performance compared to fluorinated PAGs in i-line, krypton fluoride (KrF) laser, argon fluoride (ArF) laser and extreme ultraviolet (EUV) lithography.
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.
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