Plasma Processes and Polymers Special Issue on: A State of the Art of Analytical Techniques for Plasma Processing and Deposition of Organic Coatings

Plasma-based processes involving polymers and plasma-deposited organic coatings (plasma polymers) have been developed over several decades, and they have found numerous implemented and potential applications in diverse areas such as adhesion promotion, biomaterials, microelectronics and barrier coatings. However, the combined complexity of non-equilibrium plasmas and of organic materials has limited our understanding of the processes, the products of those processes, and successful application. Processing parameters are numerous and sometimes difficult to quantify; several critical parameters like reactor geometry, real power density at the processing location, etc. are therefore frequently misjudged. Moreover, the modified surfaces or coatings produced can be inhomogeneous and subject to time-dependent alteration, commonly referred to as ageing. While an individual organic plasma process can no longer be considered as a “black box”, and we now possess sufficient knowledge about mechanisms involved and the effect of various experimental parameters, inter-laboratory comparisons still yield results with very high standard deviations, especially when the analysis of surface chemistry is taken into account. This serves as a constant reminder that our fundamental theoretical understanding of real processes is still limited and that there is plenty of room for creative research in this field. With limited predictive theory, scientific development must rely on proven plasma and sample analysis in order to move forward. However, most analytical methods were developed for standard, relatively well-defined materials; there again, the unique and complex features of plasma processes provided a significant challenge to reliable measurements. Rather than being a limitation, this state of fact has stimulated researchers into developing and improving highly innovative analytical solutions tailored to plasma processes; this has been to the great benefit of researchers both inside and outside the plasma field. Examples of such developments are numerous and ongoing, as evidenced by the wide array of techniques presented in this special issue, which regroups analysis of both the processing plasma as well as the coatings and modifications produced by those plasmas. This last category can be further subdivided into methods studying the surface and ones for studying the bulk (for example, mechanical) properties. The latter are gaining more and more interest as plasma-deposited organic coatings are moving ever-closer to applications. Those novel techniques often share an important part of the complexity of both the plasmas and the materials they analyse. This special issue has therefore been assembled with a twofold objective: i) to present an overview of state-of-the-art analytical techniques available to researchers working with plasma deposition and modification of organic coatings; and ii) to provide basic guidelines for successful application of these innovative techniques, which are frequently accompanied by limitations and pitfalls. The authors were encouraged to provide extensive experimental details and thorough evaluations of the limits and specific requirements of their analysis techniques. This Special Issue begins with an excellent review of surface wettability measurements for three-dimensional porous polymeric materials treated by plasma, by Fisher et al. One feature article and two full papers deal with mechanical properties of plasma polymers. Cech offers an overview of the nanoindentation techniques and their application to measuring elastic modulus and hardness of organosilicon plasma polymers. Lerouge et al. present the stability, hardness and adhesion of amine-rich plasma polymers using quartz crystal microbalance measurements, nanoindentation and peel tests. Bahre et al. report stress measurements of amorphous hydrogenated carbon films deposited on polymeric substrates, by evaluating the radii of curvature. The Special Issue also includes a feature article and a full paper on the use of mass spectrometry for characterizing plasma polymers: Delcorte et al. present a state-of-the-art review on chemical analysis of plasma-treated surfaces and of plasma polymers by secondary ion mass spectrometry (SIMS), while Cristaudo et al. provide an application of SIMS for ultra-shallow molecular depth profiling of the topmost layer of a plasma-treated polymer surface. A set of four feature articles offers in-depth discussion of several major analytical tools for characterizing plasma processes and plasma polymers. Fourier transform infrared spectroscopy (FTIR) fundamentals and applications are reviewed by Grundmeier et al. Chu reports current applications of plasmon-based techniques (surface plasmon resonance - SPR - and optical waveguide spectroscopy - OWS) for the analysis of plasma-deposited functional films. A complete guide for the surface analysis of nitrogen and oxygen-rich plasma polymers by X-ray photoelectron spectroscopy is proposed by Girard-Lauriault et al. Perrotta et al. discuss the evaluation of water permeation through barrier layers by an innovative combination of ellipsometry, porosimetry, and electrochemical impedance spectroscopy. In the same vein of combining analytical tools for deeper understanding of molecular features, Fouquet et al. report on the use of thermal analysis combined with mass spectrometry to evaluate molecular weights of fluorinated plasma polymers. In their full paper, Nisol et al. present an original use of epi-fluorescence microscopy as an alternative tool for quantifying anti-biofouling characteristics of plasma polymers. Mitschker et al. study degradation processes of a polymer surface during plasma polymerization using polarization modulation infrared reflection-absorption spectroscopy (IRRAS). Bridging the gap between plasma polymers and the corresponding plasma discharges, they also report on the use of optical emission spectroscopy (OES) to analyze oxygen atom densities and -fluences in their microwave plasmas. As a concluding chapter of this Special Issue, Britun et al. offer an overview on time-resolved optical analysis of high-power impulse magnetron sputtering (HiPIMS) discharges. We, guest-editors, are grateful to all authors for their excellent contributions that make up this Special Issue; we are convinced that it will trigger much interest both within and outside the plasma community. Our sincere thanks also go to the reviewers for ensuring the quality of the articles, and to the editorial board of Plasma Processes and Polymers for their enthusiasm towards this Special Issue on analytical methods. We are indebted to Dr. Renate Förch whose insightful coordination made this process a smooth ride. In summary, reliable analytical methods are the cornerstone of successful plasma-related research. We hope that readers will find in this special issue the expert insight they expect and that this will lead to increased confidence in results obtained with complex analytical tools. Guest editors: Thierry Fouquet Luxembourg Institute of Science and Technology Pierre-Luc Girard-Lauriault McGill University

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