New route for selective etching in remote plasma source: Application to the fabrication of horizontal stacked Si nanowires for gate all around devices

Abstract

This letter proposes a new route to achieve selective etching using remote plasma (RP) source processes that offer purely chemical etching by the action of plasma neutral species only. The proposed approach relies on cycling a two-step process comprising a surface treatment step and an etching step. The concept is based on the strong sensitivity of plasma neutrals' absorption to the chemistry of the substrate. By modifying the material surface state with a treatment, the etching can be delayed or even suppressed, converting a nonselective surface into one for which the etching process is area selective. This approach has been applied to the isotropic and selective removal of SiGe relative to Si in order to fabricate horizontal stacked-Si nanowires for gate all around devices. The RP process alternates an oxidation step using remote He/O2 plasma and a short etching step using remote NH3/NF3/O2 plasma. Real-time ellipsometry measurements indicate that in remote NH3/NF3/O2 plasma, there exists an incubation time before the etching starts, which is greater on Si than on SiGe films, allowing highly selective SiGe etching in the transient regime. Moreover, the Si surface modification through oxidation can inhibit reactive sites, and thus delay or even suppress the Si etching, allowing infinite SiGe over Si selectivity. By using this approach, 10 nm-thick horizontal stacked-Si nanowires could be obtained.