Damage Analysis of Reactive Ion and Quasi-Atomic Layer Etched Silicon

Atomic layer etching (ALE) is a cyclic technique based on self-limiting processes, such as reactive gas adsorption and material removal by low-energy ion bombardment 1,2. In a typical ALE process Ar+ ions with energies of 20-60 eV are used to desorb the reaction products, e.g. SiClx for the Si ALE. Compared to a corresponding continuous reactive ion etching (RIE), where the ion energies often exceed 100 eV, the ALE should yield less surface damage due to low ion energy and its cyclic nature. However, there are few publications only dealing with studies of the surface damage in ALE 3–5.

For Si etching experiments with Cl2 and Ar as etch gases, we used a commercial Inductively Coupled Plasma RIE (ICP-RIE) Takachi™ tool from Plasma-Therm LLC, USA. The system was operating in a quasi-ALE (Q-ALE) regime with some RIE contribution during the removal step due to residual Cl2. In order to avoid surface contamination by lithographic masks, a custom-made metal shadow masks were used to protect some Si areas from the Ar+ ion bombardment. The Kelvin Probe Force Microscopy (KPFM) measurements were then performed both on the reference and the etched places to calculate the contact potential difference (CPD) values for the RIE and Q-ALE samples.
In the current report we present our results on application of KPFM to evaluate the surface damage of Si for both Cl2/Ar-based RIE and Q-ALE processes. Here we employed two methods for chlorinating the Si surface: a) molecular chlorination where plasma was only ignited during the Ar+ etching step and b) plasma chlorination where plasma was ignited during the entire process and pulsed during etching step. The KPFM is used to measure the CPD between the etched Si surface and the tip and this potential difference reflects the surface damage. At the same time, a surface morphology was also characterized in an atomic force microscopy mode. Both the CPD and the surface roughness are used to evaluate the damage after cyclic ALE processes at different RF-bias power. The results were then compared with a sample that had undergone RIE in order to provide a comprehensive evaluation of the impact of the etching process on the surface morphology of the samples.
We present and discuss the CPD and surface roughness data as a function of bias voltages for both RIE and Q-ALE. The experimental results in this study show that the CPD of Si after the Q-ALE processes are in close proximity to the theoretically calculated value. However, if the samples are subjected to continuous RIE with the same parameters, the surface potential deviates significantly from the theoretical value. This may indicate that the Q-ALE process gives a significantly lower damage of Si compared to a standard RIE.