Strategies to enhance etch selectivity during plasma-assisted atomic layer etching of silicon-based dielectrics

Abstract

Stringent processing windows are required for the fabrication of sub-7-nm semiconductor devices, which in turn place severe constraints on conventional plasma-assisted etching. Atomic layer etching (ALE) is a promising etching technique that can provide high etch fidelity, directionality, atomic-scale control, and selectivity to meet and even exceed the process constraints. In this work, we primarily focused on two research objectives: identification of the underlying surface phenomena during ALE of both SiO2 and SiNx using in situ attenuated total reflection Fourier transform infrared spectroscopy combined with in situ four wavelength ellipsometry; using selective gas-phase surface functionalization to enhance the overall etch selectivity of SiO2 to SiNx and vice versa. We first studied plasma-assisted ALE of SiO2 using two sequential half-cycles consisting of fluorocarbon (CFx) deposition and an activation step. Contrary to conventional etching, the interface between the CFx and SiO2 films remains atomically abrupt throughout the ALE process. This is attributed to the complete removal of the CFx film from the surface in every activation half-cycle. We also showed that the etch of SiO2 increases with increasing ALE cycle number due to the accumulation of a CFx film on the reactor walls which releases excess etchant into the chamber during the activation half-cycle. Typically, selectivity is achieved through manipulating the plasma and processing parameters in both half-cycles. To further increase overall etch selectivity, we developed a technique in which the SiO2 or SiNx surface is selectively functionalized through the gas-phase with a hydrocarbon prior to the start of or during ALE. This abundance of hydrocarbon on the functionalized surface promotes the formation of an etch inhibiting graphitic hydrofluorocarbon film after just a few ALE cycles. We demonstrated the facile selective gas-phase surface functionalization of SiO2 over SiNx with cyclic azasilanes and the selective gas-phase surface functionalization of SiNx over SiO2 with aldehydes. The overall etch of a cyclic azasilane functionalized SiO2 film is reduced to ~23% of the original etch of the bare SiO2 surface and etching ceased after ~4 ALE cycles due to the formation of an etch stop layer. Thus, this selective functionalization of SiO2 can be used to increase SiNx to SiO2 etch selectivity. ALE on an aldehyde functionalized SiNx film led to a decrease in the SiNx etch which ultimately translated to an increase in SiO2 to SiNx etch selectivity from ~2.1 to ~4.5. Lastly, we evaluated this functionalization method on a partially etched SiNx which mimics that found in various etch approaches. We show that an aldehyde will react with the damaged and etched SiNx surface. That functionalization also leads to an etch reduction. Therefore, we can use this selective functionalization methodology on a wide variety of etch processes.