Selective Etching of SiO2 over Si3N4 through Varying the Concentration of Fluorine Species
Authors: KIM Bumsik , Lee Wonje , Lim Sangwoo 
Published: December 2024

Silicon dioxide (SiO2) and silicon nitride (Si3N4) are widely used as an insulating layer to separate interconnection, due to the characteristic of their wide band gap. As the feature size of semiconductors has been continuously decreasing and their structures have become more complex recent years, SiO2 as an insulating layer is reaching its limit. One of the ways to overcome this limitation, next generation 3D structure semiconductors are developed. In order to manufacture these kinds of devices, a removal of SiO2 is required, however, as both SiO2 and Si3N4 layers are revealed, the highly selective SiO2 etching process over Si3N4 is needed. If the Si3N4 layers were etched during the SiO2 layer etching process, the Si3N4 layer could not be function as an insulating layer and an etch stop layer. As an etchant for the SiO2 layer, hydrofluoric acid (HF) and ammonium fluoride (NH4F) combining solution is mainly used. However, it is known that HF solution also causes the material loss of the Si3N4 layers. Therefore, the study of etching behaviors of SiO2 and Si3N4 in HF solution is needed. In HF solutions, main etching species of SiO2 and Si3N4 exists as fluorine species. The concentration of these fluorine species depends on various conditions, such as pH of the solution or the composition of HF and NH4F. Therefore, the dependence of SiO2 and Si3N4 etching rates on the conditions of HF solutions were investigated.
To investigate the etching rates of SiO2 and Si3N4, the blanket SiO2 and Si3N4 wafer in which deposited on Si wafer by the low-pressured chemical vapor deposition method were used, respectively. A patterned SiO2/Si3N4 multi-stack structures were fabricated by plasma-enhanced chemical vapor deposition, to verify whether SiO2 was selectively etched over Si3N4 in HF solution. The HF solutions were prepared by adding various concentrations of HF to deionized water and NH4F was added in some cases. The pH of the HF solution was controlled by adding hydrochloric acid or ammonium hydroxide. The temperature maintained at 25 °C during the etching processes using a water bath. The blanket SiO2 and Si3N4 wafers were immersed in these solutions for 3 and 60 mins, respectively. The etching depth of the SiO2 and Si3N4 films was measured by spectroscopic ellipsometry. The morphology of patterned SiO2/Si3N4 multi-stack structures after etching process was observed using field-emission scanning electron microscopy (FE-SEM). The pH and the concentrations of each fluorine species of the prepared HF solutions were calculated based on the equilibrium constants, molar balance, and charge balance.
To obtain the HF solution with the high SiO2 etching selectivity over Si3N4, blanket SiO2 and Si3N4 wafers were etched under various pH and initial concentrations of HF solution. As the pH of the HF solution was increased, at the same initial HF concentration, the SiO2/Si3N4 etching selectivity was increased. Meanwhile, the etching selectivity of SiO2 over Si3N4 was increased with the initial concentrations of HF increased, at a fixed pH of the solution. To investigate the reason for the dependencies of the SiO2 and Si3N4 etching rates on pH and initial concentrations of HF, the concentrations of each fluorine species were calculated. As a result, the etching mechanisms and the etching behaviors of SiO2 and Si3N4 in the HF solutions were suggested. Based on the etching kinetics of SiO2 and Si3N4, a patterned SiO2/Si3N4 multi-stack structure etching conditions were controlled to investigate the SiO2/Si3N4 selective etching ability of the HF solution. The cross-sectional FE-SEM images visually showed that high SiO2 etching selectivity compared to Si3N4 was achieved. As a result, highly selective SiO2 etching without loss of Si3N4 was obtained by adjusting only the concentrations of HF and NH4F, without any special additives through the suggested SiO2 and Si3N4 etching kinetics.