Design and construction of new plasma sources at low and atmospheric pressure for chemical and morphological modification of polymeric materials

The present thesis entitled "Design and construction of new plasma sources at low and atmospheric pressure for chemical and morphological modification of polymeric materials", is positioned in the field of plasma sources, as well as the field of plasma surface engineering. The goals of this thesis are a) the design, construction, and study of new prototype plasma systems in vacuum and atmospheric pressure with innovative characteristics. b) the control of the surface morphology and nanotexture created during plasma etching, c) the empowering of new surfaces properties after plasma treatment towards smart, multi-functional materials and devices.In this thesis, an oxygen plasma process for anisotropic etching is used (material removal process) mostly for polymeric surfaces. The etching process is the synergetic effect of oxygen ions and neutrals for the conversion of the polymer to volatile species, and normally results in smooth or almost smooth surfaces with reduced thickness (compared to the initial material). Here we demonstrate that the main mechanism for the surface roughness formation during anisotropic plasma etching is the presence of etching inhibitors, a process that we coin as the “nonoinhibit” process: The nanoinhibit process is the simultaneous anisotropic plasma etching and inhibitor co-deposition (inhibitors are metals, oxides etc. coming from the reactor walls) on the material being etched. Inhibitors create local nanomasks, which are resistant to etching and result in surface roughness due to the anisotropy of the plasma process. This mechanism is confirmed by the plasma group as explained in chapter 5 through Secondary Ion Mass Spectroscopy (SIMS). The basic question for roughness control can be stated as: where do these nanoinhibitors come from? It is known that inhibitors are the result of ion bombardment and sputtering of the reactor walls, but which reactor part is having the main contribution in the creation of inhibitors? If the answer is known, then is it possible to control roughness. Thus, the design and construction of the novel plasma reactors in this thesis, focuses on the control of the inhibitor flux arriving on the surface.The thesis is separated in four parts. The first one involves the development of a new inductive plasma antenna and its electrostatic shielding, their incorporation into a high-density plasma reactor and the comparison with the existing plasma antenna for the creation of roughness. This section is presented in Chapters 3 and 4. This study describes the design and construction of the new inductive plasma antenna which can change its mode of operation, between inductive and helicon wave antenna depending on its wiring. An increased etching rate was observed in the inductive mode antenna compared to the existing commercial one which is a helicon wave antenna (in the Alcatel-Adixen MET tool), and approximately the same rate was observed when the new antenna is in helicon wave mode. Moreover, to control the roughness generated by the dielectric cylinder, a variable electrostatic shield was added between the antenna and the dielectric cylinder. The operation of the shield did not reduce the roughness formed during etching. This result indicated to us that the dielectric cylinder is not the main source of inhibitor flux and thus roughness formation.The second part, presented in Chapter 5, promotes a new method for the control of roughness, that is the construction of a variable shield of the clamping ring for controlling the ion flux heating the surface of the ring. A detailed analysis of the samples etched in different positions of the shield, with respect to their visual and morphological features was performed. This method led to the control of roughness evolution, without having to change any other plasma process parameter. In addition, the shield allowed us to move from self-organized structures to random hierarchical structures. Nanostructured surfaces produced using the variable shield showed a correlation between their optical properties and the distance between roughness aggregates as revealed through Fourier transform of the surface morphology. Our results clearly showed the evolution of specular reflection and transparency at various shielding positions.The third part (Chapter 6) concerns the construction of a prototype high-density plasma reactor and the development of plasma processes for applications that our group is working on, such us the development of surface roughness for biological applications, but also applications in MOS technology. A new plasma reactor has been designed and built for material processing and roughening. We present how the new prototype antennas and sources (presented in previous chapters) were embedded in an automated system that can be delivered to our laboratory and our Institute. Two processes were developed in the new system: a) Roughening of polystyrene plates for cell adhesion and proliferation, and b) development of an oxygen plasma process for the reoxidation of the Al2O3 thin film in GeOx / Ge structures for the formation of new enhanced MOS capacitors.High vacuum plasma processes are not always technically or economically viable for large area continuous processing of low-cost products. This was the incentive to push towards the atmospheric pressure plasma sources, a technology that currently is mainly implemented through plasma jets with complicated mechanical parts. Therefore, in this fourth part of the thesis presented in Chapters 7 and 8 we design, perform electromagnetic simulations, and construct an atmospheric pressure plasma reactor for the chemical modification and etching of polymer materials and liquids. Specifically, a prototype plasma reactor was built at atmospheric pressure capable of processing large surface areas without the use of vacuum systems. This source was characterized in terms of its electrical and optical properties as well as its efficiency in polymer processing by measurements of etching rates and water contact angles in PMMA films. The new source allows the creation of a uniform glow plasma over a wide area. Vacuum and atmospheric pressure plasma reactors are the tools for modifying and processing most materials used in applications in the field of microelectronics, photonics, biologically active surfaces, etc. The chemical modification as well as the controlled roughness formation that is empowered with the innovative plasma reactors developed in this thesis allows the formation of smart, passive, multi-functional surfaces that mimick nature. Our work adds a third parameter for controlling roughness creation (apart from etch time and etch bias), which is the control of the ion energy flux bombarding the sample holder and clamping ring and thus controlling the inhibitors flux. The present study is one of rare ones, which deal with the control of roughness evolution via the control of the ion flux. In addition, this study makes the transition from proof of concept demonstration to a fully automated prototype reactor spanning a large Technology Readiness Level. Finally, we conclude with a summary of the most important results and perspectives of the present study. This work has resulted in several publications and patent applications which are listed below.Η παρούσα διδακτορική διατριβή με τίτλο “Σχεδιασμός και κατασκευή νέων πηγών πλάσματος σε χαμηλή και ατμοσφαιρική πίεση για χημική και μορφολογική τροποποίηση πολυμερικών υλικών”, εντάσσεται στον τομέα της τεχνολογίας ηλεκτρικών εκκενώσεων πλάσματος, καθώς και της τεχνολογίας επιφανειακής κατεργασίας υλικών. Οι στόχοι της παρούσας διδακτορικής διατριβής είναι: α) o σχεδιασμός, η ανάπτυξη, κατασκευή, και μελέτη πρωτότυπων καινοτόμων αντιδραστήρων πλάσματος για την χημική και μορφολογική τροποποίηση υλικών σε χαμηλή (