Passivation of poly-Si surface using vinyl and epoxy group additives for selective Si₃N₄ etching in H₃PO₄ solution

This work investigates the mechanism of polycrystalline silicon (poly-Si) dissolution in H₃PO₄. The etching rate of poly-Si was more rapid than that of silicon dioxide (SiO₂) in H₃PO₄ solution, because poly-Si surface possesses unstable Si¹⁺, Si²⁺, and Si³⁺ states – rather than Si⁴⁺, which is more stable. Additives such as epichlorohydrin, 1,2-epoxy-3-phenoxypropane, vinyl bromide, and phenyl vinyl ether with epoxy or vinyl groups were introduced into the H₃PO₄ to passivate the poly-Si surface. This significantly increased the presence of the Si⁴⁺ state and provided hydrophobic functional groups on the poly-Si surface to reduce the wettability of the poly-Si surface. The introduction of these additives significantly reduced the etching rate of poly-Si. 3-glycidyloxypropyl trimethoxysilane, 3-glycidyloxypropyl dimethoxymethylsilane, allyltrimethoxysilane, and vinyltrimethoxysilane with Si–O bonds, as well as epoxy or vinyl groups in their molecular structures, were demonstrated to suppress the etching rate of both SiO₂ and poly-Si. The introduction of these additives to H₃PO₄ promoted the selective removal of silicon nitride (Si₃N₄) layers from multi-layered Si₃N₄/SiO₂ trench structures without the loss of either poly-Si or SiO₂.