Selective Si3N4 Etching in Si3N4/SiO2 Pair-Layer Stack Using Non-H3PO4-Based Superheated Water

Abstract

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.