Selective Gas-Phase Functionalization of SiO2 and SiNx Surfaces with Hydrocarbons.

Selective functionalization of dielectric surfaces is required for area-selective atomic layer deposition and etching. We have identified precursors for the selective gas-phase functionalization of plasma-deposited SiO2 and SiNx surfaces with hydrocarbons. The corresponding reaction mechanism of the precursor molecules with the two surfaces was studied using in situ surface infrared spectroscopy. We show that at a substrate temperature of 70 °C, cyclic azasilanes preferentially react with an -OH-terminated SiO2 surface over a -NHx-terminated SiNx surface with an attachment selectivity of ∼5.4, which is limited by the partial oxidation of the SiNx surface. The cyclic azasilane undergoes a ring-opening reaction where the Si-N bond cleaves upon the reaction with surface -OH groups forming a Si-O-Si linkage. After ring opening, the backbone of the grafted hydrocarbon is terminated with a secondary amine, -NHCH3, which can react with water to form an -OH-terminated surface and release CH3NH2 as the product. The surface coverage of the grafted cyclic azasilane is calculated as ∼3.3 × 1014 cm-2, assuming that each reacted -OH group contributes to one hydrocarbon linkage. For selective attachment to SiNx over SiO2 surfaces, we determined the reaction selectivity of aldehydes. We demonstrate that aldehydes selectively attach to SiNx over SiO2 surfaces, and for the specific branched aliphatic aldehyde used in this work, almost no reaction was detected with the SiO2 surface. A fraction of the aldehyde molecules reacts with surface -NH2 groups to form an imine (Si-N═C) surface linker with H2O released as the byproduct. The other fraction of the aldehydes also reacts with surface -NH2 groups but do not undergo the water-elimination step and remains attached to the surface as an aminoalcohol (Si-NH-COH-). The surface coverage of the grafted aldehyde is calculated as ∼9.8 × 1014 cm-2 using a known infrared absorbance cross-section for the -C(CH3)3 groups.

Gas Phase Organic Functionalization of SiO2 with Propanoyl Chloride.

The reaction mechanism of propanoyl chloride (C2H5COCl) with -SiOH-terminated SiO2 films was studied using in situ surface infrared spectroscopy. We show that this surface functionalization reaction is temperature dependent. At 230 °C, C2H5COCl reacts with isolated surface -SiOH groups to form the expected ester linkage. Surprisingly, as the temperature is lowered to 70 °C, the ketone groups are transformed into the enol tautomer, but if the temperature is increased back to the starting exposure temperature of 230 °C, the ketone tautomer is not recovered, indicating that the enol form is thermally stable over a wide range of temperatures. Further, the enol form is directly formed after exposure of a SiO2 surface to C2H5COCl at 70 °C. We speculate that the enol form, which is energetically unfavorable, is stabilized because of hydrogen bonding with adjacent enol groups or through hydrogen bonding with unreacted surface -SiOH groups. The surface coverage of hydrocarbon molecules is calculated as ∼6 × 1012 cm-2, assuming each reacted -SiOH group contributes to one hydrocarbon linkage on the surface. At a substrate temperature of 70 °C, the enol form is unreactive with H2O, and H2O molecules simply physisorb on the surface. At higher temperatures, H2O converts the ketone to the enol tautomer and reacts with Si-O-Si bridges, forming more -SiOH reactive sites. The overall hydrocarbon coverage on the surface can then be further increased through cycling H2O and C2H5COCl doses.

Surface Phenomena During Plasma-Assisted Atomic Layer Etching of SiO2.

Surface phenomena during atomic layer etching (ALE) of SiO2 were studied during sequential half-cycles of plasma-assisted fluorocarbon (CFx) film deposition and Ar plasma activation of the CFx film using in situ surface infrared spectroscopy and ellipsometry. Infrared spectra of the surface after the CFx deposition half-cycle from a C4F8/Ar plasma show that an atomically thin mixing layer is formed between the deposited CFx layer and the underlying SiO2 film. Etching during the Ar plasma cycle is activated by Ar+ bombardment of the CFx layer, which results in the simultaneous removal of surface CFx and the underlying SiO2 film. The interfacial mixing layer in ALE is atomically thin due to the low ion energy during CFx deposition, which combined with an ultrathin CFx layer ensures an etch rate of a few monolayers per cycle. In situ ellipsometry shows that for a ∼4 Šthick CFx film, ∼3-4 Šof SiO2 was etched per cycle. However, during the Ar plasma half-cycle, etching proceeds beyond complete removal of the surface CFx layer as F-containing radicals are slowly released into the plasma from the reactor walls. Buildup of CFx on reactor walls leads to a gradual increase in the etch per cycle.