Plasma pretreatment to increase benzaldehyde attachment to the SiN <i>x</i> surface before atomic layer etching

During atomic layer etching, the selective surface prefunctionalization of plasma-deposited SiNx over SiO2 with benzaldehyde has been shown to effectively retard the etching of SiNx by accelerating the formation of an etch-stop layer: this, in turn, enhances the etch selectivity for SiO2 over SiNx. The uptake of benzaldehyde is lower if the SiNx surface has been pre-exposed to the atmosphere or an etching plasma. We demonstrate a H2/Ar plasma pretreatment strategy to improve the uptake of benzaldehyde on a pre-etched SiNx surface. Using in situ attenuated total reflection Fourier transform infrared spectroscopy, we first show that surface treatment of as-deposited SiNx with Ar or H2 plasmas reduces the surface reactivity to benzaldehyde most likely due to a loss of surface —NHx (x = 1, 2) groups. In contrast, surface pretreatment of the as-deposited SiNx surface with an Ar-diluted H2 plasma largely retains the surface reactivity to benzaldehyde under optimal conditions. We then translate this H2/Ar plasma pretreatment step to practical applications. We first remove a thin surface layer of SiNx with one ALE cycle, which consists of a C4F6/Ar plasma modification step followed by an Ar plasma activation step: this step mimics a SiNx surface that has been exposed to an etching plasma, which leaves a surface CFx residue. Using in situ infrared spectroscopy, we show that surface pretreatment with a H2/Ar plasma under these optimal conditions can partially remove the CFx residue on the SiNx surface. We further show that H2/Ar plasma treatment can restore benzaldehyde uptake to ∼80% of that on an as-deposited surface. Finally, we show that after treating the pre-etched SiO2 surface with an H2/Ar plasma, the SiO2 surface reactivity with benzaldehyde remains low, leading to an increase in the aldehyde attachment selectivity to pre-etched SiNx over SiO2 surfaces by ∼50%, and a recovery of ∼80% of that on as-deposited surfaces.