Real-time monitoring of atomic layer etching in Cl₂/Ar pulsed gas, pulsed power plasmas by optical emission spectroscopy

Optical emission spectroscopy was used as a real-time monitor of the atomic layer etching (ALE) of Si in an Ar inductively coupled plasma (ICP). Pulses of Cl2 gas were repetitively injected into a continuous flow of Ar, followed by the ignition of the ICP and the application of substrate rf bias power (either continuous or rapidly modulated). Optical emissions from Si, SiCl, SiCl2, Ar, and Cl were monitored along a line parallel and close to the substrate surface as a function of time during the bias period, as well as in the ICP without bias power. From an analysis of the time dependencies of the decays of emissions during the modulated bias periods, it is argued that emissions at high Ar carrier gas flow rates are mainly from the primary products sputtered by the energetic Ar ions. Products decay with different, non-exponential time signatures. Cl and SiCl2 emissions decay to nearly undetectable levels toward the end of the bias period. SiCl emission follows a decay profile between that of Si and SiCl2. The time-integrated SiCl2 emission intensity scales linearly with time and correlates with etching rates measured by laser interferometry. The amount of Si etched per ALE cycle and the degree of self-limiting evolution of etching products is a sensitive function of the timing between the initiation and termination of Cl2 flow into the reactor and the application of ICP power. Spatially resolved optical emission is shown to be a useful in situ diagnosis, providing mechanistic insights, as well as process optimization for plasma-assisted atomic layer etching. It is also shown that the emission bands between 360 and 400 nm that are commonly observed during Si etching in a chlorine-containing plasma and are often ascribed to SiCl3 or SiCl3+ are instead most likely the A2Σ → X2Πr system of SiCl.