Low-temperature etching of silicon oxide and silicon nitride with hydrogen fluoride

Etching of high aspect ratio features into alternating SiO₂ and SiN layers is an enabling technology for the manufacturing of 3D NAND flash memories. In this paper, we study a low-temperature or cryo plasma etch process, which utilizes HF gas together with other gas additives. Compared with a low-temperature process that uses separate fluorine and hydrogen gases, the etching rate of the SiO₂/SiN stack doubles. Both materials etch faster with this so-called second generation cryo etch process. Pure HF plasma enhances the SiN etching rate, while SiO₂ requires an additional fluorine source such as PF₃ to etch meaningfully. The insertion of H₂O plasma steps into the second generation cryo etch process boosts the SiN etching rate by a factor of 2.4, while SiO₂ etches only 1.3 times faster. We observe a rate enhancing effect of H₂O coadsorption in thermal etching experiments of SiN with HF. Ammonium fluorosilicate (AFS) plays a salient role in etching of SiN with HF with and without plasma. AFS appears weakened in the presence of H₂O. Density functional theory calculations confirm the reduction of the bonding energy when NH₄F in AFS is replaced by H₂O.