Key plasma parameters for nanometric precision etching of Si films in chlorine discharges

Abstract

Ultrathin layered films in new transistors architectures (FinFET and fully depleted SOI) require damage-free plasma etching techniques with unprecedented selectivity between materials. To assist the development of advanced processes, molecular dynamics simulations are performed to quantify modifications (plasma-induced damage, etch rate) of Si films after exposition to various Cl2 plasma conditions, simulated by bombarding the substrate with both ion (Cl+, Cl2+) and neutral (Cl, Cl2) species. All simulations show the formation of a stable SiClx reactive layer and a constant etch yield at steady state. The key plasma parameter to control the etching of ultrathin Si layers is the ion energy (Ei), which lowers significantly both the damaged layer thickness (from 1.8 nm at 100 eV to 0.8 nm at 5 eV when Γ = 100) and the etch yield when it is decreased. The neutral-to-ion flux ratio (Γ) is the second key parameter: its increase reduces the damaged layer thickness (from 1.8 nm for Γ = 100 to 1.1 nm for Γ = 1000 at 100 eV) while the etch rate grows. While maintaining Γ constant, the neutral dissociation rate and the ion composition do not influence significantly the etching process. Quantitatively, simulations suggest that plasmas with low ion energies (<15 eV) and high Γ ratios (>1000) should induce sub-nm thick reactive layers, confirming an interest in low-Te or pulsed plasmas (operating at low duty cycle) to achieve nanometric precision etching.