Kinetic Effect of Additives in High Temperature Phosphoric Acid on the Etching of Si3N4/SiO2
Authors: Park Taegun , Kim Taehyeon , Son Changjin , Lim Sangwoo 
Published: February 2020

For the integration of 3D NAND, which has multiple Si3N4 and SiO­­2 pair-layer stack structure, highly selective etching of Si3N4 to SiO­­2 is required without decrease in the etching rate of Si3N4. While phosphoric acid is widely used as an etchant of Si3N4 in actual processes [1], chemical additives are added to the phosphoric acid as accelerators or inhibitors to control the kinetics of the reaction. In general, etching of Si3N4 is done at around 160 °C, but in the meantime, the process temperature may increase with the introduction of single wafer tool. In this study, we investigated the effects of various additives in H3PO4 on the Si3N4 etch rate and Si3N4/SiO2 etch selectivity at a higher temperature.
LPCVD Si3N4 and SiO2 blanker wafers were etched in H3PO4 with the addition of various etching accelerators and inhibitors such as HF, NH4F, Si(OH)4, Si(OC2H5)4, and H2SiF6 at a temperature range from 160 to 200 °C. The thickness of the wafer before and after etching was measured by ellipsometry to measure etching rates of the films.
When experiments were carried out with additives containing F, the etching rates of Si3N4 and SiO2 were increased by more than 30%. On the other hand, the Si3N4 and SiO2 etching rates decreased with the addition of the Si-based additives, but the decrease was remarkable in SiO2. In addition, the Arrhenius plots of Si3N4 and SiO2 were plotted to obtain activation energies of the reactions. As shown in Fig. 1(a), the F-containing additives act as catalyst in the Si3N4 etching reaction, reducing activation energy than the conventional phosphoric acid process (54 kJ/mol·K). On the other hand, Si-based additive show no significant change in the activation energy for the etching of Si3N4 (Fig. 1(b)). This is because addition of Si-based material to H3PO4 does not change reaction pathway, but suppresses the rates of etching reaction by Le-Chateliers principle [2]. On the other hand, SiO2 shows slightly different etching tendency. As shown in Fig. 2(a), the activation energy of the SiO2 etching reaction decreased with the addition of F-containing additives than that of phosphoric acid process (81 kJ/mol·K). However, Si-containing additives increased activation energy as inactivating agents.
In addition, fluorosilicic acid (H2SiF6) was selected as an additive because it was expected that it reacts with water in aqueous solution to produce hydrates of Si and HF, giving effects of both F-containing and Si-containing additives. It was observed that the etching rate of Si3N4 was increased, but the etching rate of SiO2 was decreased (Data not shown here). In addition, the activation energy of the etching of Si3N4 was lower than that of phosphoric acid. On the other hand, the activation energy of the SiO2 etching increased. Therefore, it is suggested that the Si-inactivating effect is greater than the catalytic etching effect of fluorine. Based on the current study, it is concluded that a higher etch selectivity with an increased Si3N4 etch rate is achievable with the addition of proper additives by controlling the activation energy.
References
[1] S. Aritome, NAND flash memory technologies, p. 273, John Wiley & Sons, Hoboken, NJ (2015).
[2] D. Seo, J. S. Bae, E. Oh, S. Kim, S. Lim, Microelectron. Eng., 118, 66 (2014).

Figure 1