Spatial structure of plasma density and electron temperature in capacitive RF discharges with a single ring-shaped narrow trench of various depths

Capacitive RF plasmas are used for a variety of technological applications, but suffer from low plasma densities and poor lateral uniformity. This limits the system throughput. Here, the effect of implementing a single narrow trench of 2 mm width and various depths (5 -- 15 mm) into the powered electrode on the spatial structure of the electron density and temperature is studied experimentally by probe measurements. The plasma is driven at 13.56 MHz in Argon at a fixed pressure (\textasciitilde 50 Pa) and power (20 W). The plasma density is found to increase in the presence of the trench and its radial profile shows a peak above the trench. The density becomes homogeneous further away from the electrode at all trench depths and the electron temperature distribution remains uniform. The measured radial density profiles are in good agreement with a diffusion model for all trench depths. Under the conditions investigated the trench of 10 mm depth is found to result in the highest density at various axial and radial positions. The results show that the radial uniformity of the plasma density at various axial positions can be improved by using structured electrodes of distinct depths.