Highly selective etching of silicon nitride to physical-vapor-deposited a-C mask in dual-frequency capacitively coupled C H₂ F₂ / H₂ plasmas

A multilevel resist (MLR) structure can be fabricated based on a very thin amorphous carbon (a-C) layer (≅80 nm) and Si₃ N₄ hard-mask layer (≅300 nm). The authors investigated the selective etching of the Si₃ N₄ layer using a physical-vapor-deposited (PVD) a-C mask in a dual-frequency superimposed capacitively coupled plasma etcher by varying the process parameters in the C H₂ F₂ / H ₂ /Ar plasmas, viz., the etch gas flow ratio, high-frequency source power (P_HF), and low-frequency source power (P_LF). They found that under certain etch conditions they obtain infinitely high etch selectivities of the Si₃ N₄ layers to the PVD a-C on both the blanket and patterned wafers. The etch gas flow ratio played a critical role in determining the process window for infinitely high Si₃ N ₄ /PVD a-C etch selectivity because of the change in the degree of polymerization. The etch results of a patterned ArF photoresisit/bottom antireflective coating/ Si O_x /PVD a-C/ Si₃ N₄ MLR structure supported the idea of using a very thin PVD a-C layer as an etch-mask layer for the Si₃ N₄ hard-mask pattern with a pattern width of 80 nm and high aspect ratio of 5.