It is necessary to selectively etch Si₃N₄ in Si₃N₄/SiO₂ multi-stack structure for the fabrication of 3D NAND [1]. The etching reaction of Si₃N₄ in phosphoric acid is known as 3Si₃N₄ + 4H₃PO₄ + 27H₂O → 4(NH₄)₃PO₄ + 9H₂SiO₃ [2]. However, as the number of Si₃N₄ etching batch process increases, the Si₃N₄ etching performance of H₃PO₄ decreases. Therefore, a fresh H₃PO₄ solution should be used for each process. As a result, the consumption of H₃PO₄ and process cost increase. In addition, the etching by-products could be reattached to SiO₂ layers, which causes problems in the subsequent processes. In this study, the etching of Si₃N₄ and SiO₂ in superheated water was investigated. In addition, by adding proper additives to superheated water, the Si₃N₄ etching rate and Si₃N₄/SiO₂ etch selectivity were controlled.

Blanket Si₃N₄ and SiO₂ films on Si wafer were used. A patterned Si₃N₄/SiO₂ pair-layered structure was also used. HCl, NH₄OH, H₂SiO₃ and HF were used as additives. The thicknesses of the Si₃N₄ and SiO₂ films were measured using spectroscopic ellipsometry and FE-SEM. The etching rates of Si₃N₄ and SiO₂ were shown in Figure 1. The etch rate of Si₃N₄ and SiO₂ in superheated water (pH 7) was 23 and 1.5 Å/min, respectively. When HCl was added, the etching rate of Si₃N₄ was decreased. On the other hand, when NH₄OH was added, the etching rate of Si₃N₄ was increased. Therefore, it is believed that OH^- generated from the self-ionization of water in the superheater water plays an important role in the etching of Si₃N₄. In addition, proper additives were added to superheated water to improve the etching performance. The result of patterned Si₃N₄/SiO₂ structure using superheated water with additive was shown in Fig. 2(a). For comparison, the result of the 85 wt% H₃PO₄ process was shown in Fig. 2(b). In the superheated water with additive and the H₃PO₄ process, the lateral etched depths of the Si₃N₄ were similar. However, in the superheated water process, the thickness of the remaining SiO₂ layers was maintained without thinning. Therefore, optimized superheated water without H₃PO₄ show a very promising selective etch performance at the 3D Si₃N₄/SiO₂ pair-layer structure.

References

[1] S. Aritome, NAND flash memory technologies, p. 273, John Wiley & Sons, Hoboken, NJ (2015).

[2] K.B. Sundaram, R.E. Sah, H. Baumann, K. Balachandran, R.M. Todi, Microelectron. Eng., 70, 109 (2003).