Electron energy distribution function measurements in a low pressure expanding jet plasma

Abstract

Summary form only given. Streaming plasmas operating at low pressures can be viewed as high-current density, fully space-charge-compensated and charged particles (electrons, ions) sources. Because of these features, some applications as thin-film deposition, etching, ion nitriding and spectroscopic light sources are based on this discharge type. For a given application, it is desirable to control both the absolute and relative concentration of the active species (neutral particles and ions) in order to establish the best process conditions. The electron energy distribution function (EEDF) is closely associated with the rates of the major elementary processes, particle and power balance in the discharge, and it determines the global stability and operational properties of the plasma. Control of the EEDF is, thus, one of the major challenges in the application of chemically active plasmas in the microelectronic and other industries. Prediction of the evolution of the EEDF is crucial for fine-tuning the complex plasma. In this study, we report on the existence of two electron temperature populations in a low pressure plasma jet through the investigation of the spatial evolution of EEDF measured by a single Langmuir probe. Experiments have been done in a constrictive plasma source where the plasma jet is generated and operated with argon, oxygen and its mixtures. The discharge was generated by a dc power supply (3500V, 0.5A) and the applied power was varied in the range of 10 to 50 W. In all experiments the total pressure was fixed at 4 mTorr. A mobile single Langmuir probe was mounted on the discharge axis in order to study the spatial evolution of EEDF. The EEDF was determined using the second derivative of the Langmuir probe current-voltage characteristic on the Druyvesteyn method. The second derivative was filtered using the least-squares fitting method. The results show that inert plus reactive gas mixing can change the EEDF from a Maxwellian to a `two-temperature' structure in low pressure plasma jet discharge. This result suggests the existence of two groups of electrons: low and high energy. Moreover, a detailed experimental investigation of the spatial evolution of EEDF with different gases indicated the deflection of the non-Maxwellian plasma in the beginning of the plasma jet for Maxwellian plasma when the probe is moved to longer distances.