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During the selective etch of SiO₂ and SiN_x in a hydrofluorocarbon plasma, the etch-stop layer on SiN_x was hypothesized to be a combination of graphitic carbon and ammonium fluorosilicate (AFS). However, the mechanism of formation of the AFS, its thermal stability, and stability under directional ion bombardment are not fully understood. 

In this work, we monitored the surface bonding changes of SiN_x film during RIE in a CH₂F₂/Ar plasma at room temperature and bias voltages ranging from 100 to 300 V by using in situ ATR-FTIR spectroscopy. As expected, the etch rate was lower at a lower bias voltage, and the etch stopped with rapid accumulation of AFS and graphitic hydrofluorocarbon layer. Interestingly, no salt or graphitic carbon layers formed when we decoupled the RIE process into a CH₂F₂/Ar plasma deposition step and an Ar plasma activation step with a bias voltage of –240 V. This implies that the formation of AFS and graphitic carbon layers requires the participation of etch byproducts and plasma species. We also observed a linear etch with no accumulation of AFS and graphitic carbon layers after adding H₂ to feed gases at a bias voltage of –240 V. We speculate that the graphitic carbon layer was efficiently removed by hydrogen radicals, making the AFS fragile under high-energy ion bombardment. We anticipate that SiN_x etching with HF plasma may provide further insights into the formation mechanism of AFS layer by eliminating the graphitic carbon layer that forms on the SiN_x surface prior to an etch stop.