Construction and Evaluation of Gas Feed-in System, Electrodes and Gas Exhaustion System applied to Low Pressure Plasma Coating Processes

Abstract

Plasma polymerization is a technique used to deposit functional thin film coatings (nanometer until some micrometer thickness) on different materials surface. The thin film coating is deposited directly from a partially ionized precursor gas subjected to the effect of the plasma onto the surface of the substrate, which is placed in a low-pressure plasma chamber. This coating process regarding the achievement of the desired properties is in general well developed. The challenge is based on the scaling up process in respect to a large volume coater. Concerning this aspect only very few and rough information are available for constructors in respect to lay out of low-pressure plasma coater with the aim to achieve homogeneous coating deposition rate as well as homogeneous coating properties over all the products surfaces in contact with the plasma. The aim of the present research work is mainly to investigate the potential of guiding the gas flow inside a large volume plasma polymerization-system and to investigate how this gas guiding directly influences the coating homogeneity as well as the coating properties. Thereby the creation of construction guidelines to support future developments under the frame of low-pressure plasma technology is part of the task. To achieve this purpose, gas feed-in system, gas exhaustion system and electrode geometry were investigated. Taking into consideration that experimentally the local gas flow behavior cannot be directly measured inside a PECVD system computational fluid dynamics (CFD) simulations were carried out using the commercial software Ansys Fluent 16.0A to investigate the gas flow behavior. Additionally, to get more knowledge about the electric field created by the RF-electrode in combination with the powered electrode, electrostatic simulations were done using the commercial software Ansys Maxwell 16.0A . Gas flow simulations showed clearly that the gas exhaustion system influences the local gas flow much more (in intensity and range) in comparison to the gas feed-in system. In this regard, a perforated polymeric plate was developed to reduce the influence of the gas exhaustion system and it was named under the frame of this work as a Baffle Platea . Coating experiments exhibited a clear improvement in the coating deposition homogeneity due to the installation of the Baffle Plate. The construction guidelines for gas feed-in systems and gas exhaustion systems were defined using gas flow simulations. To evaluate the simulation results, a non-uniformity coefficient was defined to define a construction border. For this work, the construction border was established at non-uniformity coefficient equals 5%. It means that construction layout which leads to non-uniformity coefficient higher than 5% will be not recommended for this application. From the analyzed parameters of the gas feed-in system, the pipe section area was the most relevant influencing the homogeneity of gas distribution. Regarding the gas exhaustion system, the simulation results showed that the distance between the Baffle Plate and the top of the chamber has a huge influence in the homogeneity of the gas exhaustion. Coating experiments were performed to correlate the distribution of the coating deposition rate in addition to the coating properties with the local gas velocity and the local electric field. Therefore, the construction guidelines must be supplemented with hints how to handle the influence of the walls (chamber, electrodes and product holder systems). The selected construction guidelines revealed to be useful for the development and construction of a new low-pressure plasma chamber of about 3 mA . Following the knowledge acquired from this research work the realized coating showed on an area of 2 mA a standard deviation of the deposition rate less than 17%. With this it had been demonstrated, that especially for large volume PECVD-systems following the rules of the construction guidelines is essential.