Improving SiO2 to SiNx etch selectivity during atomic layer etching with multiple selective organic pre-functionalization steps

Selective thermal pre-functionalization of plasma-deposited SiNx surface with benzaldehyde has been shown to facilitate the formation of a carbon-rich hydrofluorocarbon layer, which retards the etch of SiNx, thereby increasing the etch selectivity of SiO2 to SiNx during atomic layer etching (ALE). However, in technological applications in device manufacturing, the SiNx layer is often buried under the SiO2 layer or has been exposed to the atmosphere. This leads to low uptake of benzaldehyde on the SiNx surface either due to a residual CFx layer or due to the formation of surface SiOxNy. Using in situ attenuated total reflection Fourier transform infrared spectroscopy, we show that the selectivity for benzaldehyde attachment to SiNx over SiO2 is reduced with the slow accumulation of a fluorocarbon residue on both surfaces with an increasing number of ALE cycles. Using in situ ellipsometry, we show that if the two surfaces are dosed with benzaldehyde after the first and fifth ALE cycles, there is a nearly 60% increase in etch selectivity compared to dosing benzaldehyde once after the first ALE cycle. A nearly 100% improvement in etch selectivity was observed compared to the scenario without benzaldehyde pre-functionalization for a target SiO2 etch of ∼20 nm in 20 ALE cycles.