Modelling and Study of a Microwave Plasma Source for High-rate Etching

The aim of the study is to optimize an existing microwave powered remote plasma source (RPS) with respect to the etching rate and gas temperature and to simplify the setup to save production costs. The RPS, which is shown in figure 1, is a low-pressure plasma source where the plasma is generated and exists mainly in the chamber of the source. Only radicals migrate out of the RPS. This is one important feature, that the plasma source is used for etching processes when ion bombardment and high thermal strain of the substrate must be prevented. The etching process is a chemical process, where the radicals react with the substrate surface atoms forming gaseous molecules. The benefit is a damage-free, dry and clean substrate surface. To achieve these goals, a FEM-based model of the RPS has been developed to investigate the microwave distribution and the microwave coupling into the plasma chamber, as well as the plasma itself. In this paper different examples of FEM based microwave simulations by different conditions and their experimental validations will be presented. To compare the calculated electric field distribution in the RPS with the real field distribution, PMMA-substrates were placed inside the plasma chamber of the source. They are heated up by the electric field and evaluated with an infrared camera and liquid crystal sheets. Both the measured and the calculated field distribution show a very good conformity. When the electric field is high enough in the plasma chamber the plasma ignites, the electron density and thus the permittivity and the conductivity increase, which changes again the electric field distribution. For this purpose, the FEM-model has been extended by the Drude model1. The model considers the equation of motion with a damping term for the electrons, leading to an expression for the conductivity. Results for various electron densities as well as their corresponding electric field distributions are presented and compared with optical measurements.     Fig. 1. The figure shows the scheme of the RPS with its main components and functions.