Role of SiCl4 addition in CH3F/O2 based chemistry for Si3N4 etching selectively to SiO2, SiCO, and Si

Dry etching of amorphous silicon nitride (Si3N4) selectively toward silicon dioxide (SiO2), silicon oxicarbide (SiCO), and crystalline silicon (c-Si) in an inductive coupled plasma reactor using CHF3/O2/He chemistry with SiCl4 addition is studied. Plasma exposure of c-Si, SiO2, and SiCO leads to an oxifluoride deposition. The deposition rate is the same for all these materials and increases linearly with the amount of SiCl4 added. On the other hand, Si3N4 etching is observed at very small amount of SiCl4 added (2 SCCM), while oxide deposition takes place at higher SiCl4 flow (10 SCCM). Quasi-in situ angle resolved x-ray photoelectron spectroscopy investigation shows oxifluoride SiOxFy deposition on c-Si and SiCO, while a thin F-rich reactive layer is observed on Si3N4. The oxidation of the Si3N4 surface with O2 plasma prior to CHF3/O2/He with small SiCl4 addition plasma treatment showed that the oxidation state plays a significant role in the etching/deposition equilibrium. In addition, it is found that oxifluoride deposition on Si3N4 is driven by ion energy, with deposition observed at 0 V substrate bias voltage, while etching is observed for values higher than 150 V. All these results show that a competition takes place between silicon oxifluoride deposition and etching, depending on the substrate material, surface oxidation, and ion energy. Based on the additional optical emission spectroscopy data, we proposed insights to explain the different etching and deposition behaviors observed. It is focused on the crucial role of ion energy and the nitrogen presence in Si3N4 leading to the formation of CN and HCN, helping to get a thinner reactive layer and to evacuate etch by-products on Si3N4 while an oxifluoride buildup on the other materials takes place.