Cold Atmospheric Plasma System - Simulation, Fabrication, Diagnosis and Thinfilm deposition

Abstract

In this thesis, we report the various aspects of fabricating a Cold Atmospheric Plasma system, which can be used for Plasma Enhanced Chemical Vapour Deposition. The greatest advantage of this system is its vacuum free operation, which provides a cost e effctive alternative over conventional high vacuum systems. We have designed a reactor geometry for such a plasma system, in which, the contamination due to ambient air is kept at a minimum value using a low flow of Ar (500 sccm). Towards this end, we have modeled and simulated the flow pattern of Ar gas entering the reactor geometry and have studied its e effectiveness in removing air from the plasma zone. We have fabricated such a geometry and studied the contamination at different flow rates of Ar by observing the plasma optical emission. Further, the aspect of lamentation in atmospheric pressure plasma has been studied and we have identified a few process parameters which can convert a filamentary discharge to a diffused glow. Subsequently, a complete system was developed, including an in-house built high voltage power supply, to generate a plasma with low contamination and less number of laments. We have also carried out plasma diagnostics, specifically to estimate the Electron Energy Distribution Function (EEDF) of the plasma, by analysing the radiation emitted from an Ar plasma, acquired using an Optical Emission Spectroscope. The peaks in the spectrum were curve flatted with Voigt pro les and their widths and intensities were mapped to the electron number density and the EEDF of the plasma, using the mathematical models for Stark broadening and Corona population respectively. An optimization routine based on Nelder-Mead simplex algorithm was run to estimate the optimal values of these plasma parameters that produced a good match between the simulated spectrum and the experimentally acquired one. This analysis estimated that the value of electron number density in our plasma was in the range 0:82 1017 cm 3 to 3:56 1017 cm 3 and the electron temperature was in the range 0.36 eV -0.39 eV . It also predicted that the EEDF closely approximated a Maxwellian distribution. As a proof of concept, the fabricated reactor was used to deposit thin films of Polyacetylene over microscopic cover glass slides by polymerizing Acetylene gas in the cold plasma. Deposition rates as high as 1 m=min, were obtained during thin lm deposition of the polymer. The polymeric structure of the lm was studied using NMR and FT-IR. XPS measurement revealed 5% O2 inclusion in the samples. XRD showed no distinguishable peak, indicating the amorphous state of the films. The surface morphology investigated using SEM revealed highly porous broid kind of structures, which appeared to be agglomeration of particles with sizes in the order of few micrometers. P-type Polyacetylene lms were fabricated by doping them with 5.3% by atomic concentration of I2 vapours. The UV-Visible spectroscopy study revealed a bandgap of 2.05 eV for undoped and 1.49 eV for the doped Polyacetylene samples. The lms exhibited an increase in conductivity by two orders of magnitude; from 3:6 10 13 1cm 1 to 3:5 10 11 1cm 1 for un-doped and doped Polyacetylene samples respectively.