Low Damage Etching with Atomic Layer Precision

In this presentation, we describe a Low Damage Etch Chamber (LoDEC) for atomic layer etching (ALE) comprising: (1) an electron beam source (1--2 keV) for generating radical-poor, low electron temperature ($T_{e}$ $\sim$ 0.3 eV) plasma, (2) a remote plasma source (RPS) for supplying radicals to the substrate, and (3) a bias generator for creating the voltage drop (0--50 V) between the substrate and the plasma to accelerate ions over etch-threshold energies. In LoDEC, we reproduced the conventional \textit{Si}-etch ALE cycling scheme: in 1$^{\mathrm{st}}$ part of the cycle, \textit{Cl} atoms are injected by RPS to passivate the surface for $\Delta t=\tau_{p}$, and in 2$^{\mathrm{nd}}$ part, RPS is turned off and etching is done in e-beam \textit{Ar}/$N_{2}$ plasma at low bias power for $\Delta t=\tau_{b}$. As $\tau_{b}$ is increased, we observe saturation in the etch depth per cycle, $\Delta_{c}$, signifying that the entire passivation layer is being removed each cycle, resulting in layer-by-layer etching. In LoDEC, this scheme can be implemented at much lower ion energies, $E_{i}$, than in conventional tools, potentially resulting in lower damage to advanced materials. We also obtained the dependence of $\Delta_{c}$ on ion energy and $\tau_{p}$ for a given \quad $\tau_{b}$. Finally, using LoDEC we developed a novel technique for etching \textit{a-Si} in \textit{Cl} below known threshold energy, at $E_{i}$ $\sim$ 5 -- 7 eV (TEM shows etch rates of $\sim$ 3--4 nm/min).