Characterization and stabilization of atmospheric pressure DC microplasmas and their application to thin film deposition.

摘要

A comprehensive study was carried out to examine the feasibility of depositing thin films using atmospheric pressure DC and RF microplasma discharges. Atmospheric pressure plasmas for material processing are desirable because of the lower costs, higher throughput, continuous processing, and potentially novel applications obtainable by not using vacuum systems. However, several major concerns exist because of the higher pressures. These are related to: (1) discharge stability, (2) non-thermal operation, (3) non-uniformities, and (4) particle formation. These concerns were investigated using DC and RF microplasmas for atmospheric pressure plasma enhanced chemical vapor deposition (AP-PECVD) of thin films. The DC discharges were fundamentally characterized in Air, helium, nitrogen, argon, hydrogen, methane and mixtures thereof by: (1) voltage-current measurements, (2) current density measurements, (3) microscopic visualization and manipulation of the discharges, and (4) extensive spectroscopic measurements. Measurements were made for breakdown, transitional and stable regimes in current ranges from 0 mA to 40 mA, and discharge gaps sizes from 20 mum to 10 mm. The main focus of the optical emission spectroscopy (OES) was the measurements of rotational and vibrational temperature measurement made by the fitting of experimental data to spectral models of the N2 2nd positive system (N2 C3piu→B3pi g), though the N+2 1st negative system, the NO beta, gamma, delta, and epsilon systems, and the atomic emissions lines were also used for additional temperature measurements, species identification, and measurements of relative concentrations. The structure, electric field, current density and voltage-current measurements indicated that the DC microplasmas operate as density scaled version of the low-pressure normal glow discharge regime with notable exceptions. Spectroscopic measurements show the gas temperatures to vary from ambient to 2000K depending on conditions and to be non-equilibrium discharges, T gas Tvib· An analysis of the thermodynamics and stability of these discharges revealed that these discharges are in thermal balance because of their small size and can be stabilized through careful design of the external circuit. These stability requirements were analyzed in detail both experimentally and through mathematical modeling using a linearized perturbation analysis. The tailored discharges were used in the AP-PECVD of hydrogenated amorphous carbon film (a-C:H) from H2, CH4 precursors. Regimes of particle formation and thin film deposition were found. The characteristics of the deposits were measured with profilometry, Raman spectroscopy and other techniques. The films deposited are uniform thin films though very localized by the small size of the discharge. More commonly used atmospheric pressure RF capacitively coupled plasmas (CCP) utilizing helium buffering for stabilization were also investigated for AP-PECVD. Characterization revealed that the high concentrations of helium were required to maintain low current densities and prevent alpha to gamma mode transition and instabilities. Comparisons were made between the DC, and RF atmospheric pressure deposition system and results from the literature. Properties in general are similar to those achieved by other techniques with the structure of the amorphous carbon films ranging from more polymeric-like-carbon to more diamond-like-carbon. In comparison to the RF deposition, the DC system could deposit more durable films, likely due to higher power densities and ion energies, but the discharge was more sensitive to the substrate properties. The DC microplasma system could also deposit thin films with significantly less helium buffering as helium was not necessary for stabilization.