The potential of ellipsometry in unraveling the plasma sheath potential

Abstract

The electric field in the sheath of a low pressure plasma plays an important role in many plasma material processing methods. To further optimize these methods - and even more importantly to understand fundamental plasma processes - a good understanding of this electric field profile is crucial. However, determining the absolute values and the profile of the sheath electric field experimentally has appeared to be extremely difficult and therefore all available methods lack spatial resolution or are invasive. For instance laser-based techniques have poor spatial resolution and their usage is limited to only those regions where the electric field is relatively high. Probe techniques disturb the electric field and are therefore not feasible to use in the plasma sheath. In this report, a novel approach to electric field measurements in the plasma sheath based on plasma levitated micro-particles is proposed. The first steps that are taken to realize this idea are described here. In the framework of this project a rotating compensator ellipsometer has been developed, including the microcontroller system and the associated source code. To verify the feasibility of the proposed method and the correct functioning of the built diagnostics, the following essential milestones have been achieved. The correct working of the developed ellipsometer has been verified by an investigation of different oxidized Si wafers. The results obtained by the self-built ellipsometer and those obtained by a commercial ellipsometer are compared. The results are in good agreement thanks to the significant effort which is put in to the calibration of the in-house built ellipsometer. To convert the measured ellipsometric angles into physical parameters a model based on Mie theory is derived. Calculations are performed to create a lookup table for the ellipsometric angles psi and delta for different particle radii. The first experiments on confined clouds of particles are conducted and the first ellipsometric angles are determined. Analyzing the obtained data has not been possible yet as the model does not take into account the size distribution of the particles in the cloud. Insufficient light is detected during single particle experiments hence a better and more stable light source will be installed to increase the signal. A crucial part of the proposed diagnostics is the imaging setup. This setup is fully functional and will be used to determine the position of the plasma levitated dust particle during the electric field measurements. Two test measurements to determine the change of position of a dust particle due to mass loss in a reactive oxygen plasma are performed successfully. The work described above demonstrates the feasibility of the proposed diagnostics to develop into a mature measurement technique with superior specifications. To that end, the next steps to further improve the ellipsometry setup and challenges have been defined and elaborated towards measurements on a single particle and the proverbial Holy Grail for a plasma sheath physicist: determining the electric field profile at the boundary of a plasma.

Date of Award	31 Aug 2015
Original language	English
Supervisor	L.P.T. Schepers (Supervisor 1), Job Beckers (Supervisor 2) & Gerrit M.W. Kroesen (Supervisor 2)