RésuméThe deposition of DLC films from a low-pressure acetylene gas mixture in magnetron reactors has continuously driven the interest of scientists for decades. It is used widely by industrials in large batch coaters to produce high added value protective coatings, or by experimentalists to develop processes that include amorphous hydrogenated carbon as a key component in novel and exciting applications for e.g. in electronics, energy storage, or even medicine. It is still only possible to resolve ana-lytically the complex equations of the dynamics of cold temperature reactive plasmas for very simple cases, and gaining insight on the reac-tions and particle behaviour usually requires the use of numerical simu-lations. For very low-pressure discharges (below 1 Pa), the individual particle trajectory must be resolved, which implies the use of statistical Monte-Carlo approaches like PIC-MC. The high computational cost of such simulation limits the attainable powers, the simulation size and length, and the number of considered species. The object of the present study is therefore to find out if a realistic PIC-MC simulation of a PECVD discharge in the case of the deposition of DLC from acetylene can be developed, and to see if it is possible to compare simulations’ results in a constraining way with experiments.
We showed, that it is indeed possible to simulate this type of acetylenic discharge with a simple but self-coherent plasma chemistry model in a small 3D simulation box. With simulation times of up to 1.2 ms, an equilibrium could be attained for the densities and fluxes of all species. Even if ions are the main species created within the plasma, radicals accumulated to densities comparable to that of ions due to their slower diffusion speed. Differences in concentrations were observed across the simulation chamber between the thermally diffusing neutrals and the accelerated ions. Moreover, some species were created within the chamber from the reaction between species generated in the plasma bulk interacting with the background acetylene.
Those predictions were tested against mass spectrometric measurements in experiments with various acetylene ratios and discharge powers. The variations of species flux towards the substrate, with ratio were similar in both experiments and simulations. It was possible to match the linear evolution of the ions’ flux with power between experiments and simula-tions. Moreover, the simulations showed a correlation between the elec-tron density and all plasma products ones. This suggests that the power scaling of simulation’s predictions should be possible until the point where the reactive species concentrations would become non-neglectable compared to the acetylene one.
Since the concentrations and fluxes varied throughout the chamber in this configuration, variation of the film characteristics and of the deposi-tion rate were expected across the substrate. To test the simulations film deposition predictions, a dynamic surface chemistry model was set-up. It includes the creation of dangling bonds via the chemical sputtering due to the joint ion bombardment and hydrogen flux and the preferential deposition of radicals on dangling bonds. Since the equilibrium of the dangling bonds coverage was not obtained extrapolating the absorption of radicals was necessary. Despite this and the power extrapolation, simulations were able to give quantitatively accurate deposition rates and deposition profiles when compared to experiments. However, the hydrogen stoichiometry was higher in the simulations than the one measured with ERDA measurements. In order to improve the film dep-osition prediction several refining of the surface model would have to be included in future works. Some perspectives are given regarding the possible improvements of the current model, which could already be used for more complex configurations to reduce the need for the costly trial-and-error search for optimal deposition parameters.
|la date de réponse||30 sept. 2021|
|Sponsors||FEDER Région Wallonne|
|Superviseur||Stephane Lucas (Promoteur), Yoann Olivier (Président), Andreas Pflug (Jury), Erik C. NEYTS (Jury) & Stefan Grosse (Jury)|