Optimization of GaAs ECR etching in chemically assisted ion beam process using Cl₂/Ar plasma

A study to characterize and optimize the GaAs etching by Cl₂/Ar electron cyclotron resonance (ECR) plasma is reported. Etching characteristics such as smooth surface morphology and vertical sidewall are the focus of this work. In the first group of experiment (group A), the temperature, process pressure, microwave power, and RF power were varied to observe the etch rate of the GaAs substrate. Though the surface was generally rough, starting conditions for the second group of experiments (group B), were inferred from group A experiments. In group B experiments, the DC self-bias (produced from the RF power) and the Cl₂ as well as Ar flow rates were varied. Crystallographic sidewall profile (orientation-dependent etching phenomenon) was obtained under conditions of <|40| V DC self-bias (75 W RF power), 120 °C, Cl₂ and Ar total flow rate of 50 sccm, 10 mTorr pressure and 600 W microwave power. The crystallographic sidewall has vertical planes and inward-sloped planes in perpendicular directions. The vertical plane was parallel to (0 1 1̄) while the inward-sloped plane was originally parallel to (0 1 1) before plasma etching. Vertical sidewall and smooth surface were achieved at a DC bias of |70| V, 120 °C, and Cl₂ and Ar flow rates of 10 and 40 sccm, respectively, and these were considered as optimized conditions under the present circumstances. The root-mean-square (RMS) surface roughness associated with the vertical sidewall profile was 3.6 nm, and the etch rate of GaAs was approximately 40 nm/s.