Formation and stability of ammonium fluorosilicate during etching of SiN <i>x</i> in CH2F2/Ar and SF6/H2 plasmas

During reactive ion etching of SiNx in fluorine-based plasmas where HF is produced through a combination of gas-phase and surface reactions, ammonium fluorosilicate (AFS) has been reported to form on the SiNx surface. However, the underlying mechanism for AFS formation and its role in SiNx etching is still unclear, particularly during etching under energetic ion bombardment. In this work, using in situ attenuated total reflection Fourier-transform infrared spectroscopy, we show that during SiNx etching with a CH2F2/Ar plasma, when the substrate was at room temperature at a bias voltage of −240 V, an etch stop occurred with the simultaneous accumulation of graphitic hydrofluorocarbon and AFS on the SiNx surface. Under nominally similar conditions in a CH2F2/Ar plasma, but with the substrate at 70 or 120 °C, no etch stop was observed along with the absence of hydrofluorocarbon and AFS. By eliminating the formation of a graphitic hydrofluorocarbon layer in a SF6/H2 plasma, also at a bias voltage of −240 V, we reveal that AFS accumulation does not necessarily lead to an etch stop, and a steady-state thickness of AFS appears on surface as a result of a balance between AFS formation due to SiNx etching and AFS decomposition due to the impingement of energetic ions and H radicals. The effect of the substrate bias voltage on the etch behavior and surface composition was also studied in CH2F2/Ar and SF6/H2 plasmas. Our results show that the AFS is likely mixed with the underlying SiNx film under energetic ion bombardment instead of being present as a well-defined uniform layer. As a result, the presence of AFS does not necessarily lead to an etch stop. AFS formed on the SiNx surface decomposed when exposed to H radicals generated in H2 plasma under self-bias. Therefore, we conclude that the H radicals can play a dual role where they facilitate AFS formation by scavenging F radicals to form HF, but can also directly remove AFS formed on the surface. Finally, the slow removal of AFS by increasing the temperature to 70 °C suggests that AFS accumulation can be controlled with the substrate temperature.