Surface prefunctionalization of SiO2 to modify the etch per cycle during plasma-assisted atomic layer etching

Abstract

The authors show that organic functionalization of an SiO2 surface can be used to controllably alter the etch per cycle during plasma-assisted atomic layer etching (ALE). The ALE process consisted of two half-cycles: CFx film deposition from a C4F8/Ar plasma and an Ar plasma activation step. The surface reactions, composition, as well as film thickness were monitored using in situ surface infrared spectroscopy combined with in situ four-wavelength ellipsometry. The SiO2 surface was prefunctionalized with bis(dimethylamino)dimethylsilane, which reacted with surface —SiOH groups in a self-limiting heterogeneous gas-solid reaction. After this reaction, the SiO2 surface was functionalized with Si—CH3 groups. ALE was performed on both as-deposited and functionalized SiO2 surfaces. Analysis of the infrared spectra after seven sequential ALE cycles on a bare SiO2 sample showed no accumulation of graphitic carbon on the SiO2 surface, and thickness measurements showed a net etch of ∼1.8 nm of SiO2. On the other hand, infrared spectra after seven sequential ALE cycles on the organically functionalized SiO2 surface clearly showed a buildup of graphitic hydrofluorocarbon on the surface. This graphitic hydrofluorocarbon film accumulated during each cycle, and after four cycles drastically reduced etching of SiO2 in later cycles. The overall etching of SiO2 after seven cycles was ∼1.0 nm. The infrared spectrum of this accumulation layer also shows intense Si—O—C and SiFx (x = 1, 2, 3) stretching vibrations, but CFx deposition on bare SiO2 shows weak intensity for the same features. Detailed analysis shows that the interface between CFx and SiO2 is atomically abrupt during ALE of bare SiO2 but has a high degree of mixing and covalent bonding in the etch-inhibiting graphitic fluorocarbon film that forms on the —CH3 functionalized SiO2 surface.