Role of SiCl$_4$ addition in CH$_3$F/O$_2$ based chemistry for Si$_3$N$_4$ etching selectively to SiO$_2$, 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 (ICP) reactor using CHF3/O2/He chemistry with SiCl4 addition is studied. Plasma exposure of c-Si, SiO2 and SiCO leads to an oxide deposition. The deposition rate is the same for all these materials and increases linearly with the amount of SiCl4 added. On the other side, Si3N4 etching is observed at very small amount of SiCl4 added (2sccm) while oxide deposition takes place at higher SiCl4 flow (10sccm). Quasi-in situ Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS) investigation shows oxifluoride SiOxFy deposition on c-Si and SiCO while a thin F-rich reactive layer is observed on Si3N4. The oxidation of Si3N4 surface followed by CHF3/O2/He chemistry with small SiCl4 addition showed that oxidation state plays a significant role on 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 superior to 150 V. All these results show that a competition takes place between silicon oxifluoride deposition and etching, depending on substrate material, surface oxidation and ion energy. Based on some additional Optical Emission Spectroscopy (OES) data, we proposed some 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 formation of CN and HCN, helping to get a thinner reactive layer and to evacuate etch by-products on Si3N4 while an oxifluoride build-up on the other materials.