Highly Efficient Si3N4/SiO2 Selective Etching Process with Non-Phosphoric Acid Solutions
Authors: Lee Jinhoon , Lim Sangwoo 
Published: December 2024

Silicon nitride (Si3N4) and silicon dioxide (SiO2) are widely used as insulator or sacrificial film materials in the fabrication of semiconductor devices. In recent years, many 3D structured semiconductor devices such as NAND Flash and CFETs have been manufactured to increase the integration of semiconductor devices. In this case, a substrate with alternating layers of Si3N4 and SiO2 is used, and the device is fabricated by selectively etching the Si3N4 layer and depositing a dielectric or metal in its place. Therefore, the technology for selectively etching Si3N4 from Si3N4/SiO2 stacked substrates is very important. To selectively etch Si3N4, 85 wt% of phosphoric acid (H3PO4) is commonly used. However, H3PO4 has economic and environmental problems due to its high consumption during the etching process and non-recyclability. Additionally, the deformation problem of SiO2 layer after etching, particularly oxide regrowth, remains as a critical issue. The regrown oxide fills the gaps in the nano-patterned structure, hindering the mass transfer of the etchant and resulting in uneven etching. Therefore, to improve production yield and enhance the quality of the final device products, the development of an etchant and etching process that can replace H3PO4 is necessary.

In this study, an etching process with high selectivity and good pattern geometry was developed using non-phosphoric acid-based etchants. It was also studied to improve the etching efficiency by adding various additives to the non-phosphoric acid based etching solution. The main etching solution was formulated based on nucleophilic chemicals. In addition, various additives, which are expected to improve selectivity by promoting the etching of Si3N4 and inhibiting the etching of SiO2, were added to the main etching solution to determine the effect of the additives on the etch rate. These additives, mainly surfactants (such as alkyl carboxylates, alkylamines, alkyl sulfonates, and alkylbenzene sulfonates) or antioxidants (cysteine, ascorbic acid, furan, pyrrole, imidazole, triazole, tetrazole, and piperazine, etc.), were combined in different configurations.

The newly developed non-phosphoric acid etch process resulted in a significantly higher etch rate of Si3N4 compared to the conventional phosphoric acid etch process, and it also increased the Si3N4/SiO2 selectivity. When additives were introduced into the main etching solution, the maximum etch rate of Si3N4 was further enhanced while maintaining a low etch rate of SiO2, leading to an increase in the Si3N4/SiO2 selectivity. This enhancement is attributed to the formation of a passivation barrier on the SiO2 surface by the additive, which inhibits its reaction with the etchant. Through analysis of Si3N4/SiO2 multi-stack pattern structures etched, it was observed that oxide regrowth did not occur on the SiO2 layer after etching, even with much higher etch selectivity and Si3N4 etch rate. As a result, by utilizing a non-phosphoric acid etch solution along with appropriate additives, it was possible to enhance both the etch rate and selectivity while preventing oxide regrowth in the residual SiO2 layer. The application of this process is expected to facilitate the development of a high-throughput, low-cost, and eco-friendly semiconductor etching process compared to H3PO4-based processes.