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.