(Invited) In Situ Plasma and Surface Diagnostics to Understand the Etching Mechanism of SiN x in a HF Plasma

During SiN x etching with HF plasma, ammonium fluorosilicate (AFS) forms as a transient byproduct significantly influencing the etch behavior. While AFS formation during SiN x etching has been known for several decades, the underlying mechanism for its formation and removal remains unclear. In this study, we have used in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy to study the changes in the chemical boning on the SiN x surface during reactive ion etching, along with the AFS formation and removal dynamics. We have also identified the reaction products generated in the plasma using in situ quadrupole mass spectrometry. For applications such as etching of alternating stacks of SiO 2 and SiN x for the fabrication of 3-D NAND memory devices, it is desirable to etch SiN x and SiO 2 at approximately the same rate. Our initial hypothesis is that AFS formation accelerates the etching of SiN x compared to SiO 2 . Therefore, to promote the etching of SiO 2 we nitrided the surface with an NH₃ plasma to create surface Si-NH x ( x = 1, 2) species (see Fig. 2a). Subsequent HF plasma exposure revealed AFS formation on the SiO 2 surface (Fig. 2b), but it lowered the SiO 2 etch rate compared to the untreated surface. Therefore, to accelerate the etch rate of SiO 2 in a HF plasma, we will report on other process parameters including the average ion energy during etching and surface nitridation, effect of diluents in the gas phase, and the substrate temperature.