Plasma diagnostics for the low-pressure plasma

Thin Solid Films 606 (2016) 19–44

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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Critical review

Plasma diagnostics for the low-pressure plasma polymerization process: A critical review Damien Thiry a,b, Stephanos Konstantinidis a, Jérôme Cornil c, Rony Snyders a,d a

Chimie des Interactions Plasma-Surface, CIRMAP, University of Mons, Place du Parc 20, B-7000 Mons, Belgium Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière B.P. 32229, 44322 Nantes Cedex 3, France Service de Chimie des Matériaux Nouveaux (CMN), CIRMAP, University of Mons, 20 Place du Parc B-7000 Mons, Belgium d Materia Nova Research Center, Parc Initialis, Avenue Nicolas Copernic 1, B-7000 Mons, Belgium b c

a r t i c l e

i n f o

Available online 28 February 2016 Keywords: Plasma polymerization Plasma polymer films Plasma diagnostic Growth mechanism

a b s t r a c t Since the 1980s, functionalized plasma polymer films have attracted a considerable attention owing to their promising utilization in a wide range of modern applications. For such materials, controlling the chemistry of the coatings by a clever choice of the process parameters represents the main challenge. And yet, it became quickly obvious that in view of the complexity of the growth mechanism, fine control of the layers properties can only be reached by understanding at a fundamental level the mechanistic formation of the layers. In this context, a detailed comprehensive study of plasma chemistry is therefore of crucial importance as the numerous interlinked chemical reactions occurring in the discharge govern the film properties. In this paper, the most common plasma diagnostics methods employed in the context of plasma polymerization process, namely Mass Spectrometry, insitu Fourier Transform Infrared Spectroscopy, Optical Emission Spectroscopy, Langmuir and Ionic probes are reviewed. After a light description of each technique, the main achievements for improving the mechanistic understanding of the layer formation are exposed. Moreover, the use of theoretical calculations based on Density Functional Theory (DFT) to support the understanding of the acquired data is highlighted. In view of the better control of the process allowed by the plasma phase investigation, some general conclusions and perspectives describing future developments in the field of plasma polymerization are finally discussed. © 2016 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Plasma polymerization . . . . . . . . . . . . . . . . . . . . . 2.1. Plasma fundamentals . . . . . . . . . . . . . . . . . . 2.2. Growth mechanism . . . . . . . . . . . . . . . . . . . 2.3. Control of PPF properties through deposition parameters . . 3. Which tools can we use to probe the plasma polymerization process? 3.1. Diagnostic techniques . . . . . . . . . . . . . . . . . . 3.2. Theoretical support for a better description of the data . . . 4. Overview of the diagnostics methods . . . . . . . . . . . . . . 4.1. Electrostatic probe . . . . . . . . . . . . . . . . . . . . 4.2. Optical emission spectroscopy . . . . . . . . . . . . . . 4.3. Mass spectrometry . . . . . . . . . . . . . . . . . . . 4.3.1. Neutral species analysis . . . . . . . . . . . . . 4.3.2. Ion analysis . . . . . . . . . . . . . . . . . . . 4.4. Ion probes . . . . . . . . . . . . . . . . . . . . . . . 4.5. Gas-phase Fourier transform spectroscopy . . . . . . . . . 5. Conclusions and perspectives . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E-mail address: [email protected] (D. Thiry).

http://dx.doi.org/10.1016/j.tsf.2016.02.058 0040-6090/© 2016 Elsevier B.V. All rights reserved.

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1. Introduction The interactions of a solid with its surrounding are mainly defined by the physico-chemical properties of its surface. It is therefore not surprising that since many decades, continuous research and developments in materials science have contributed to the rapid growth of surface and coating technologies which nowadays still increasingly attract considerable attention. Surface technologies refer to the modification of the surface (e.g., chemical functionalization, etching, coating,…) of a material without changing its bulk properties. For instance, tailor-made coatings allow adjusting mechanical (wear, friction), chemical (corrosion, permeation, temperature insulation, biocompatibility, wettability), electrical (conductivity), and optical (transmission, reflection, absorption, color) properties of materials [1]. Through the years, numerous processes have been developed for the modification of surfaces via the synthesis of thin films. A nonexhaustive list includes chemical vapor deposition, pulsed laser deposition, spin coating, sol gel, spin casting, thermal evaporation and plasmabased technologies [1–4]. Among them, the plasma-based processes are of particular interest by combining significant advantages such as their low process temperature, enabling the treatment of a wide range of materials including polymers, and the absence of solvents making these techniques compatible with the modern quest for environmentally friendly technology. Another key advantage of these processes is their versatility enabling one to modulate the properties of a given surface over a wide range (e.g., crystallinity, morphology, chemical composition of the deposited material) by adjusting the synthesis conditions [5–9]. All these attractive properties justify the popularity gained by plasma technologies and their important development in numerous industrial fields such as automotive, aeronautics and microelectronics [1,10,11]. If in the past, research and applications have often focused on the development of inorganic thin films, the design of organic surfaces is nowadays more and more important with applications in the fabrication of antibacterial surfaces [12], protein biochips [13,14] or platforms for biomolecules immobilization [15,16]. These surfaces can also be synthesized using plasma-based technology, more specifically by means of the plasma polymerization method, allowing the formation of solid organic thin films referred to as plasma polymer films (PPF). Despite the use of the word “polymer”, PPF present little resemblance to the conventional polymers except for their organic nature [17]. Indeed, PPF are not characterized by the assembling of a repeating unit, but by a random network presenting a cross-linking density significantly higher than conventional polymers (see Fig. 1). In order to avoid confusion between plasma and conventional polymers, the term “precursor” is sometimes preferred instead of monomer to name the molecule from which the material is built. Nevertheless, both terms are accepted and currently employed in the plasma polymerization community. The formation of solid deposits from organic compounds using glow discharges is not new. It was indeed first reported by Dutch researchers in 1796 [18]. These materials adhered tightly to the walls of the glassmade reactors and were observed to be insoluble in most solvents.

Fig. 1. Schematic comparison of a plasma polymer film and a conventional polymer material obtained from the same precursor/monomer.

Nevertheless, they were considered as a nuisance until the work of Goodman who demonstrated that a 1 μm thick plasma-polymerized styrene film deposited on a titanium foil made a satisfactory dielectric for a nuclear battery [19]. Since that time, the potential of these organic coatings has been revealed and a systematic investigation of the plasma polymerization process has been carried out. More information about the history of the plasma polymer science can be found in Refs. [20–22]. It is now demonstrated and accepted that PPF exhibit interesting physico-chemical properties for organic materials such as high thermal, mechanical and chemical stabilities [4]. Moreover, due to their intrinsically good adhesion properties, numerous materials (e.g., glass, polymers, metals), even with complex geometry (e.g., carbon nanotubes [23–26], micro/nanoparticles [27–31]), can be homogeneously covered [4]. All these features justify their use in a wide range of applications. Historically, they were first developed in the search of physical barriers with applications in the field of corrosion protection [32,33] and food packaging [34,35]. In this context, highly cross-linked PPF were needed. Such PPF are obtained when a high level of precursor fragmentation occurs in the plasma. Thus, highly energetic conditions have usually been employed due to the precursor fragmentation dependence on the energy dissipated in the system [21]. As a result of these extensive fragmentation reactions, poor control of the PPF chemistry was achieved. Since the 1980s, with the rise of micro- and nano-technologies, plasma polymerization has been further developed in the search for PPF with controlled and tailored chemistry while keeping their other inherent properties. In this context, functionalized PPF containing/supporting \\COOH [36–41], \\OH [42,43], \\NH2 [44–51], \\COOR [52–56], \\COR [57–59], \\CFx [60–62], \\Br [20,63], \\SH [27,64–68], thiophene-based units [69,70] have been developed. The interest in this class of materials arises from their potential use in modern fields of applications including the development of supports for biomolecules immobilization [71–77] or cell growth [78–80], interlayers for promoting adhesion of metal coatings [81], biocompatible [82] or antibacterial coatings [12,83], controlled drug release coatings [84–87], superhydrophobic surface [62], conductive layers [70],etc. For these applications, the chemical composition of the coatings is one of the most important criteria defining its performances. Therefore, scientists have focused their efforts toward fine control of the plasma polymer chemistry. It quickly appeared that this control can be obtained through the understanding of the PPF growth mechanism and more specifically of the phenomena taking place at the plasma–substrate interface. Accordingly, investigating the plasma chemistry rapidly turned out to be a necessity. Numerous works have therefore been focused on the investigation of the plasma phase during the PPF growth. Surprisingly, while other aspects of the field have been reviewed such as the growth mechanism of the layers [8,21], their behavior in liquid medium [82], their use for biological applications [72], their nanostructure [88] and their surface analysis [89,90], there are no documents summarizing the efforts made for a precise evaluation of the plasma phase. Therefore, the present paper aims at reviewing the principal works developed to evaluate the plasma chemistry during the low-pressure plasma polymerization process. Particularly, we pay special attention to describe how these works have contributed to enlarging the knowledge of plasma polymer growth at a molecular level. This review is organized as follows. In the first part, the plasma polymerization mechanism is described. Then, an overview of the main achievements obtained by several research groups in the field of plasma diagnostics related to the plasma polymerization process is presented. The most popular diagnostics methods employed for probing the species constituting the plasma are described, namely, mass spectrometry, gas phase Fourier transform infrared spectroscopy and optical emission spectroscopy. The main results obtained by Langmuir and ionic probes to determine the plasma parameters and ion flux, respectively, are also discussed. In addition, throughout the paper, it is shown how theoretical calculations based upon the density functional theory method have proven to be a powerful tool for assisting in the interpretation of

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the complex diagnostic data. Finally, conclusions and perspectives suggesting research strategies for increasing the fundamental knowledge in the plasma polymer field are given. 2. Plasma polymerization In this section, we start with a brief summary of the main features of plasmas employed in the context of plasma polymerization. For a deeper description of plasma physics in surface processing, readers are invited to consult the review of Bogaerts et al. [91] and the book of Lieberman and Lichtenberg [92]. Then, the main models developed in the literature for describing the growth mechanism of plasma polymers are summarized. Finally, we discuss the relationship between plasma parameters and film properties. 2.1. Plasma fundamentals A plasma is defined as a gas containing a mixture of electrons, ions, neutrals and photons. As the electron and ion densities are equal, the plasma is macroscopically neutral. The plasma phase was first described in 1920 by Irving Langmuir working on the development of vacuum tubes for large currents [11]. Although omnipresent in the universe and representing nearly 99% of the matter (solar corona, solar wind, earth's ionosphere), the natural presence of plasma on earth is rare (e.g., lightning, aurora borealis) [93]. Nevertheless, the plasmas can be easily generated artificially via the electrical excitation of a gas, for example. The particles (electrons, ions and neutrals) constituting the plasma are in motion and undergo collisions during which energy is transferred. Each family of particle is often characterized by a temperature related to its translational energy. In plasma science, the temperature of the particles is generally expressed using the eV as unit; the conversion factor is 1 eV = 11600 K. Depending upon potential differences in terms of temperature for the particles constituting the plasma, they are classified in two main families: namely, at thermodynamic and at nonthermodynamic equilibrium. For the former case, all the particles are characterized by a unique temperature, namely Te = Tn = Ti where Te, Ti and Tn represent the temperatures associated to electrons, ions and neutrals species, respectively. In contrast, for non-equilibrium plasmas, the electron temperature strongly differs from that associated with the heavy particles, ions and neutrals: Te ⋙ Ti ≥ Tn. Indeed, whereas the electron temperature typically ranges from 1 to 10 eV, the ion and neutral temperatures are close to room temperature, 0.025 eV (298 K). This is why such plasmas are often called “cold” plasmas. Indeed, although the temperature of the electrons is high, their low density and heat capacity allow surfaces surrounding the plasma to remain at relatively low temperatures [11]. This non-equilibrium feature is particularly attractive for materials processing as the electrons can induce several chemical reactions without altering the treated material by excessive temperature. Consequently, cold plasmas are predominantly used in surface material processing including plasma polymerization [91]. For conventional plasma sources (i.e., direct current, radio-frequency, microwave), the ionization degree is less than a few percent. Cold plasmas can be generated either at low (i.e., b1 Torr) or high pressure, even at atmospheric pressure. In the context of the plasma polymerization process, most studies are conducted at low pressure and are the subject of this review. However, there is recently an increasing interest for operating the plasma polymerization process at atmospheric pressure. More details about this aspect can be found in the review of Merche et al. [94]. Since the electrons are the energy vectors in the plasma, their density and energy are of crucial importance for processes occurring in the gas phase. The electrons are assumed to be in thermal equilibrium at a given temperature Te [92]. Following kinetic gas theory, the electrons energy distribution function (EEDF) can be, to a first approximation, expressed by a Maxwell-Boltzmann distribution as illustrated

Fig. 2. Typical Maxwell–Boltzmann electron energy distribution function for low pressure discharges. Adapted from [10,20].

in Fig. 2 [20,92]. Although in a conventional plasma polymerization process, the majority of electrons possesses a kinetic energy centered around a given value (1–2 eV in Fig. 2), the distribution extends to high energy (from 4 to 20 eV). This allows the occurrence of numerous chemical reactions in the plasma through collisional processes with electrons resulting in the formation of a great variety of species including radicals, ions and photons. Since the degree of ionization in the plasma is often very low, the most probable collisions involving electrons occur with neutrals particles. The probability for a given reaction to occur depends on the number of electrons able (from an energetic point of view) to induce the chemical reaction. On this basis, considering organic discharges, in comparison to ionization, dissociations reactions leading to the formation of radicals are more probable than ionization events since most of the electrons in the plasma possess an energy of 1–4 eV similar to the energy required to break simple organic chemical bonds (see Fig. 2); in contrast, the ionization energy of organic molecules is approximately 10 eV. As a consequence, the concentration of radicals in the plasma can be 103–105 higher than the ion density [22]. Another important aspect related to plasma technology is that owing to the significant difference in terms of thermal flow velocities between electrons and ions, a surface immersed in a plasma naturally acquires a negative charge which acts to accelerate the positive ions toward the surface and repel most of the impinging electrons [8,92]. For insulated surfaces (as generally encountered for plasma polymer layers), at the equilibrium, the ion flow perfectly counterbalance the electron flow leading to a stationary electrical potential named the floating potential (Vf). As a consequence of this phenomenon, near all surfaces including the substrate, regions called sheath characterized by a net positive charge develop. When ions enter the sheath, they are accelerated and strike the surface with a kinetic energy ranging typically from 10 to 30 eV in the case of a floating surface in the absence of collisions, i.e., when considering a low pressure plasma polymerization process [38]. The influence of the bombarding ions on plasma polymerization is obviously also crucial and is discussed below. It should be noted that when the substrate is directly connected to an RF power supply, the negative self-bias developed at the surface can be significantly higher than the floating potential and depends on the synthesis conditions [8]. Finally, another source of energy is also provided to the growing film due to the irradiation of ultraviolet (UV) photons emitted by excited atoms/molecules. 2.2. Growth mechanism Their unique properties make the PPF a specific class of materials arousing, over many decades, the curiosity of surface scientists who have developed strong efforts to understand their growth mechanism.

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The overall plasma polymerization mechanism involves both gas phase and surface reactions [95]. The first step consists in the vaporization of an organic precursor in a deposition chamber. The activation takes place in the plasma phase through collisional processes between energetic electrons and precursor molecules. As already mentioned, the most probable reaction consists of precursor dissociations reactions resulting in the formation of radicals. Historically, it was therefore assumed that the growth of the layer mainly occurs through either radical-radical or radical-molecule reactions. Indeed, the radicals possess an unpaired electron and therefore are highly reactive toward termination reactions with other radicals or toward addition reactions with unsaturated molecules (i.e., with double or triple bonds) [8]. Owing to the similarity between organic chemical bonds energy and the fact that the electron energy is characterized by a distribution function, numerous fragmentation pathways are possible [66]. As a consequence, a great variety of radicals is generally produced in the plasma whatever the chemical nature of the organic precursor, hence contributing to the difficulty to obtain fine control of the PPF chemistry. Some of the processes involved in the synthesis of a PPF are summarized in the “Rapid Step Growth Polymerization” (RSGP) model proposed by Yasuda in 1985. The model is built on the concept of recombination of reactive species and the subsequent reactivation of the resulting products [96,97]. The overall mechanism is schematically described in Fig. 3. Cycle 1 involves the reaction of monoradicals whereas cycle 2 concerns biradical species, both originating from the interaction of the precursor with the plasma. Steps (1), (3), (4) and (5) correspond to addition reactions between the reactive species and (i) a stable molecule (which can be the precursor molecule) containing a reactive site such as a double or triple bond (steps (1) and (4)) or (ii) other reactive species including radicals and biradicals (steps (3) and (5)). In both cases, the products formed can undergo other propagation reactions. It is obvious that the extent of addition reactions in the gas phase depends on the pressure. In the plasma polymerization mechanism, the molecules formed through the recombination between two radicals (step (2), termination reaction) can be reactivated via electron impact in contrast with the situation encountered in conventional polymerization. Consequently, the RSGP mechanism can be viewed as a succession of termination reactions followed by the reactivation of the products. When a surface is exposed to such a plasma, a solid organic thin film arising from the condensation of the reactive species produced in the plasma is deposited on the substrate. However, further “polymerization” reactions (initiation, addition, termination, reactivation) can still occur at the plasma growing-film interface. In order to provide a more complete description of the plasma polymerization mechanism, Yasuda added in his model the CAP (“Competitive ablation and polymerization”) principle, namely the simultaneous occurrence of film etching and deposition processes [21,39,96]. The synthesis of the coating thus results from a balance between both

Fig. 3. Schematic description of the rapid step-growth polymerization mechanism. Adapted from [96].

phenomena. Indeed, highly reactive radicals (e.g., .O., .F, .S., etc) can be produced and react at the interface to form stable molecules (e.g., CO2, CO, CS2, CF4). The products may desorb from the growing film leading to the removal of part of the coating (ablation). These stable molecules cannot take part in the growth of the film anymore and are either (i) pumped out of the reactor or (ii) reactivated through electron collisions. The occurrence of such surface reactions directly depends on the amount of energy provided to the growing film interface by ion bombardment and UV photon irradiation. It should be noticed that these stable molecules can also be formed via gas phase reactions reducing the amount of reactive species which can potentially take part in layer growth. The latter process is named the “scavenger effect” and can also affect the PPF chemistry [40]. At this stage of the discussion, owing to their relative low abundance in the discharge compared to radicals and neutrals, the influence of ions in the growth mechanism has been totally excluded. Nevertheless, as already mentioned, in plasma processing, a surface exposed to the discharge is continuously submitted to a flow of ions for which the typical kinetic energy ranges from 10 to 30 eV for a floating surface. This supply of energy is enough to induce chemical bond breaking leading to the formation of surface dangling bonds which can act as preferential adsorption sites for reactive species in the plasma [61,98,99]. The impact of this phenomenon is taken into account in the ion-Activated Growth Model (AGM) proposed by d'Agostino in which the formation of surface defects through ion interaction is considered [60,98]. Based on the AGM, the precursor can be incorporated in the growing film via a surface reaction with a radical site through for example the opening of a double bond. Such reaction is referred to induced plasma polymerization in contrast to plasma-state polymerization for which the activation of the molecule in the plasma is an essential step. Initially, the AGM was developed for the growth of fluorine-based coatings, but it can also be applied to other PPF families [61]. In addition, the impinging ions can also be responsible for other phenomena such as ion-assisted etching or coating densification as it will be described later. For sake of completeness, it has to be mentioned that surface reactive sites can also be created through UV photons irradiation of the growing film interface. While considering ions as active species defining the PPF properties, the AGM still implies that the density of ions is so low that any mass deposited by ions would be insignificant [100]. However, experimental evidence has clearly revealed that the ions play a greater role in the formation of a PPF [100]. Indeed, recent studies pointed out that under certain experimental conditions, the contribution of condensing ions cannot be neglected [8,57,58,101–104]. In some cases, some authors even claim that the ions are the main species responsible for PPF growth [57]. The origin of the greater role played by the ions in PPF formation is justified by several factors. For instance, the presence of a sheath at the plasma growing film interface accelerates the ions toward the film surface, significantly reducing the neutral-to-ion flux ratio at the surface in comparison with that measured in the gas phase [8,102]. Considering a low pressure plasma, e.g., 1 mTorr, the surface flux ratio of neutrals to ions is estimated to be approximately 20 times lower than the corresponding density ratio in the plasma [102]. Another important factor to take into account is the sticking probability of the ions. From studies investigating layers grown by hyperthermal ions, a sticking coefficient of 0.2–0.5 can be estimated for the ionic species depending on their energy [105,106]. On the other hand, for radical-based molecules, the sticking coefficient has been estimated to vary over a wide range (~ 10−4 to ~ 1) depending on the chemical nature of the radical (e.g., unsaturation degree) [107–109] and the activation of the surface through the formation of open bond sites at the interface [99,107,110, 111]. Therefore, as integrated in the AGM, a synergetic effect between ions and neutrals has to be considered since increasing the ion flux enables the formation of a higher number of reactive sites at the surface, thus promoting the grafting of radicals. To summarize, although nowadays it is accepted that the contribution of ions in the deposited film is significant, their exact role in the

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plasma polymerization mechanism is still debated [112,113]. It is, however, important to shed light on their impact for defining PPF properties as recently demonstrated by Michelmore et al. for the mechanical properties of the deposited layers [58]. The combination of the different models described in this section (schematically represented in Fig. 4) allows us to provide an overview of the main reactions taking place in the plasma and at the PPF surface. This complex mechanism is at the origin of the poor control of the chemistry of such thin films. Indeed, it has been extensively reported that functionalized PPFs contain numerous chemical functions even if the organic precursor is monofunctional [21,67]. This complex mechanism also explains the high branched and cross-linking degree of plasma polymers triggering their attractive physico-chemical properties. This irregular structure results from the random recombination of the numerous plasma-generated fragments followed by successive rearrangements, fragmentations, reinitiations reactions, etc. Another consequence of this complex mechanism is the presence of radicals in the PPF network after synthesis [72,114–118]. These radicals have been identified to be at the origin of the well-known “aging” of PPFs, namely oxidation of the material [49,82,114,119]. Nevertheless, it has been recently reported that they can be advantageously exploited for inducing a radical conventional polymerization [117,120–124]. The trapped radicals can also serve as reactive sites for the grafting of proteins [125,126]. Another aging phenomenon is related to the potential presence of embedded molecules in the plasma polymer matrix [49,82,127] which can be released in air during storage of the material [66,128–132] or after immersion of the coatings in a liquid medium [36,49,68,133–135]. The latter could be detrimental for biological applications requiring the immersion of the layers in solution [82,127,136,137]. 2.3. Control of PPF properties through deposition parameters The chemical composition of the layers (e.g., the density of a particular chemical function) as well as their cross-linking degree are strongly affected by the deposition parameters: the plasma source (e.g., radiofrequency, microwave,…), the power dissipated in the discharge, the working pressure, the precursor flow rate, the substrate temperature, etc. [4,8,17,21,22,131,138–140]. In addition, some geometric parameters related to the design of the chamber also affect the coating properties. Examples include the location of the precursor inlet, the distance between the plasma source and the substrate, etc. The dependence of the process with the reactor geometry makes therefore difficult to compare the “same” PPF from one deposition chamber to another [22,141]. The need for PPFs with a precise chemical composition and structure requires a clear understanding of the relationship between the process

Fig. 4. Overall mechanism of plasma polymerization.

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parameters and PPF properties. Very often, an empirical approach is utilized and the common strategy consists in separately studying the influence of one process parameter, while keeping the other ones constant. For a given reactor configuration, the influence of the operational parameters can be understood by considering their impact on the plasma characteristics. For example, the applied power alters the electron energy distribution function and density which govern the nature and the rate of the chemicals reactions occurring in the plasma [142]. This, in turn, influences the chemistry of the coatings and their physicochemical properties. According to the plasma polymerization mechanisms, the most important factor affecting the properties of a PPF is the energy applied per molecule which governs the degree of precursor fragmentation in the discharge [96]. In this context, Yasuda proposed a composite parameter (the so-called Yasuda parameter): W/FM where W, F and M are the power dissipated in the discharge in J/s, the monomer flow rate in mol/s and the molecular weight of the monomer in kg/mol, respectively [97]. The Yasuda parameter represents the energy input per unit mass of monomer. The term W directly scales with the electron density and hence governs the collision frequency between electrons and precursor molecules [40,95]. For a fixed working pressure, the term 1/F scales with the residence time of the particles in the discharge, thus also influencing the extent of precursor fragmentation. Therefore, the concentration of activated species in the plasma directly depends on the W/FM parameter. Regarding the evolution of the deposition rate with W/FM, two principal domains of plasma polymerization can be identified (see Fig. 5). At low W/FM, the deposition rate evolves linearly with W/FM. In this region, the so-called “energy-deficient domain”, ample monomer is available and the supply of energy is the limiting factor for increasing the deposition rate. In this domain, increasing the supply of energy in the discharge results in an increase in the concentration of film-forming species through collisional processes. Above a critical W/FM value, the deposition rate becomes constant since the precursor fragmentation has attained its maximum. This regime is called “monomer-deficient” as sufficient energy is provided, but the precursor feed rate into the chamber is the limiting factor [143]. In addition, under high energy conditions, ion-induced etching reactions can be favored and this leads to a decrease in the deposition rate. The extent of ablation phenomenon strongly depends on the chemical composition of the discharge. It is, for example, well known that fluorine-based discharges are quite sensitive to ablation reactions [4,61]. Complementary to the Yasuda concept and as a further development, Hegemann et al. developed a macroscopic approach for describing the plasma polymerization process based on the concept of chemical quasi-equilibrium [39,40,95,144–147]. In this case, the plasma is divided into (i) an active zone in which the activation takes place through collisional processes leading to the production of filmforming species and (ii) a passive zone where the deposit is formed. This approach states that the mass deposition rate Rm (expressed in

Fig. 5. Typical evolution of the deposition rate as a function of the Yasuda parameter, W/FM illustrating the different regimes of plasma polymerization [143].

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g/cm2s) depends on the parameter W/F following a quasi-Arrhenius behavior according to Eq. (1) [144]:
 Rm Ea ¼ G exp − ; F W=F

ð1Þ

where G is a reactor and process dependent factor related to the maximum monomer conversion into film growth and Ea represents an apparent activation energy related to the PPF system. It should be emphasized that the W/F parameter can be adjusted depending on the reactor configuration (e.g., symmetric vs. asymmetric), thus making the macroscopic approach suitable for facilitating the transfer of a plasma polymerization system from one chamber to another [146,148,149]. A typical example of an Arrhenius-type plot (ln(Rm/F) versus the inverse specific energy, (W/F)−1), is shown in Fig. 6 for the plasma polymerization of methane. Ln (Rm/F) evolves linearly with (W/F)− 1 pointing to an Arrhenius-like behavior over the range of W/F investigated. In this regime, the film grows mainly through radical reactions and the kinetic limiting factor is the production rate of the film-forming species in the plasma. It is assumed here that the fragmentation pattern of the precursor in the plasma is identical with W/F and that the formation rate of radicals increases exponentially with energy input [148]. It has been reported that Eq. (1) holds for many monomers and gas mixtures for a certain range of W/F [146,147,150]. In Fig. 6, from the negative slope of the linear fit, Ea, which is related to an ensemble of fragmentation reactions in the active zone, can be deduced [151]. Its value is monomer specific and correlates with the bond dissociation energy of the precursor [146]. Compared to the Yasuda approach, the activation energy separates the plasma polymerization into the energy and monomer deficient regime [144,148]. When applied to several PPF families for a broad range of energy conditions, the macroscopic approach has revealed that more than two deposition regimes (i.e., the energy and monomer deficient regime at high and low energy levels, respectively) have to be considered depending on the plasma polymerization system. This could be related to different growth mechanisms. For instance, deviations from Eq. (1) at low energy levels could indicate a predominant ionic oligomerization mechanism and/or the grafting of intact monomers on reactive sites at the interface [148]. At high energy levels, the drop in the deposition rate observed for instance in the case of nitrogen- and oxygen-based gas mixtures could provide information about variations in plasma chemical pathway mechanism or ion-induced etching reactions [40]. Although the macroscopic approach has revealed its potential for describing the plasma polymerization process, defining the plasma polymer formation mechanism based on the evolution of the Rm/F as a function of (W/F)−1 in an Arrhenius-type plot is not straightforward and could lead to erroneous conclusions without additional data from, for example, plasma diagnostic measurements. This has led to a very interesting debate in the plasma polymer community regarding the suitability of the macroscopic approach [39,148,151–154].

Fig. 6. Arrhenius-like plot of the deposition rate versus the inverse energy input for a methane discharge. Adapted from [146].

The energy applied to the plasma polymerization process affects not only the deposition rate, but also the composition and the cross-linking degree of the layers. At high power, more cross-linked PPFs are formed because of the extensive fragmentation of the precursor yielding a higher quantity of small-molecular weight film-forming radicals. At the same time, ion bombardment becomes more intense and may also contribute to the densification and cross-linking of the growing film (see below). These high energy conditions are suitable for obtaining good barrier properties finding applications in food packaging and corrosion protection [155]. On the other hand, plasma polymerization conducted at low energy conditions provides a low degree of precursor fragmentation and a high retention of the functional group hosted by the precursor. This can be explained by the small amount of electroninduced collisions with precursor molecules leading therefore to an activation process rather than a complete disintegration of the initial chemical structure. This mode of operation is therefore more suitable for biological applications [8,21,41,47,48,55,72,82,156]. As already mentioned, the energetic conditions at the growing film surface due to ionic bombardment are a key factor controlling film properties. Therefore, in complement to the macroscopic approach, with the aim to rationalize the influence of the ion bombardment in plasma polymerization based on the synthesis conditions, Hegemann et al. have introduced a new concept, namely the momentum density (πsurf) defined as the momentum flux per deposition rate following Eq. (2) [157]: πsurf ¼

pffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi Γi Emean ; 2mi R

ð2Þ

where mi, Γi, Emean are the average mass, the flow and the mean energy of bombarding ions, respectively. R represents the deposition rate. For example, the authors reported a linear correlation between πsurf and the density of the coatings (directly related to the crosslinking degree) for plasma polymerization from discharges of pure C2H4 as well as NH3/C2H4 and CO2/C2H4 mixtures (Fig. 7). This trend is explained by the increase in chemical bond breaking at the surface and subsequent random recombination of radicals. At the same time, although the N/C and O/C ratios are constant for a certain range of energetic conditions, the NH2/N and COOH/COOR ratios scale inversely with densification as induced by πsurf. This example illustrates the important role played by the bombarding ions in the plasma polymerization process as reported recently for the control of coating stability in aqueous medium [50,158]. In view of the modern applications of this class of coatings, especially in the field of biotechnology, the control of the chemical composition of the PPF has become an aspect of increasing importance. In this context, the optimization of “conventional” plasma parameters (mainly the W/FM parameter) presents some limitations. Indeed, even when using low energy conditions, an irregular structure predominates and the density of the targeted function remains low. With this problem in mind, a breakthrough appeared when the pulsed plasma technique, reported in the field of plasma polymerization in 1972 by Tiller et al., was introduced [21]. The idea was to further reduce the extent of fragmentation of the precursor and hence the side reactions accounting for the formation of other functionalities than the one hosted by the precursor. Today, pulsed plasma polymerization has become a well-established method for the synthesis of functionalized PPF [37,41,42,44,46,48,54,59,68,70,159–161]. The pulsed approach consists in producing the discharge intermittently according to the pulse frequency. The mean power dissipated into the discharge can be easily modulated by adjusting the time during which the plasma is switched ON (ton) and OFF (toff) [47]. During the plasma “ON” time, electrons, ions, radicals and photons are produced. The ton period can be approximated to a discharge operating in Continuous Wave (CW) mode sustained at a power equivalent to the power applied during the plasma “ON” time [37]. During this period, the layer grows through the complex mechanism previously

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discharge in the CW mode can be employed, thus allowing a significant reduction in the degree of fragmentation of the precursor and in turn enhancing the retention of the chemical group of interest [37,46]. 3. Which tools can we use to probe the plasma polymerization process? As detailed in the previous section, in plasma polymerization, the numerous species present in the discharge (i.e., electrons, ions, radicals, stable molecules and photons) react with each other and with the growing film via a multitude of interaction pathways [117]. This complex mechanism makes the assessment of each individual reaction as well as the prediction of the coating properties very challenging. Furthermore, depending on the chemical nature of the precursor and the synthesis conditions, especially at low energy conditions, some specific reactions can predominate. This would also influence the final features of the formed layers. Therefore, it has rapidly become obvious that for obtaining a good control of the PPF chemistry, a knowledge of how a PPF grows at a molecular level is essential [8]. 3.1. Diagnostic techniques

Fig. 7. (a) Plasma polymer film (from C2H4, NH3/C2H4 and CO2/C2H4) density vs. the momentum density during film growth. The linear evolution indicates densification by momentum transfer. (b) Chemical composition of a plasma polymer from NH3/C2H4 and CO2/C2H4 depending on the momentum density during film growth. The filled symbols indicate the relative amount of oxygen and nitrogen. The open symbols show the functional group density which is reduced with the increasing densification. Adapted from [157].

described. When the plasma is switched OFF, due to the recombination of the electrons and ions at the reactor walls, the electron density as well as the floating potential naturally developed at the substrate rapidly decrease as experimentally measured [41]. Consequently, in this case, ion bombardment and photon irradiation rapidly disappear, limiting the side reactions at the surface and thus favoring the reaction of radicals present in the gas volume with the nucleation sites generated during ton. Depending on the chemistry of the starting molecule, the precursor itself can participate in addition reactions with the radicals sites present at the surface of the growing film even during toff. This has been experimentally demonstrated for the pulsed plasma polymerization of acrylic acid [37]. This precursor, containing a double bond, can therefore easily undergo a propagation reaction through a radical mechanism involving the opening of the double bond. In this case, the pulsed mode can ideally be viewed as a succession of activation reactions (ton) followed by propagations steps during toff, thus favoring the incorporation of the targeted chemical group in the coatings [45]. If non-classical polymerizable molecules are considered, growth events during toff can be excluded. Indeed, in this case, during toff, the radicals rapidly recombine at the interface and the precursor itself does not participate in the formation of the film, as revealed for the pulsed plasma polymerization of 3-fluoroaniline [160]. Nevertheless, even in this case, the use of the pulsed plasma polymerization strategy is beneficial as mean powers lower than those required to sustain the

When concluding one of his papers dealing with the development of the macroscopic approach, Hegemann wrote: “ It is expected that further clarification of the complex processes taking place during plasma polymerization can be achieved by the use of plasma diagnostics in combination with the macroscopic approach” [146]. Therefore, in addition to the development of empirical models (e.g., Yasuda, Hegemann), the plasma polymer community has more and more attempted to characterize the process by employing state-of-the-art plasma analysis (mass spectrometry, gas phase Fourier Transformed Infrared Spectroscopy, optical emission spectroscopy, electrostatic and ion probes) and surface analysis (X-ray Photoelectron Spectroscopy-XPS, Time of Flight Secondary Ion Mass Spectrometry-ToF-SIMS) tools. Several diagnostic methods have been developed and employed during the last 30 years for detailed comprehensive study of the plasma chemistry. For evaluating the chemical composition of the plasma, nonintrusive optical diagnostics methods including optical emission spectroscopy (OES) and gas-phase Fourier Transform Infrared Spectroscopy (GFTIR) as well as mass spectrometry (MS) have been used extensively. In complement to these techniques, ion probes, especially designed for operating in organic discharges, have also been developed with the aim to measure absolute ion fluxes toward surfaces facing the plasma. Finally, governing the production rate of species in the plasma, the electron density and temperature can be evaluated using electrostatic probes. Table 1 summarizes the main features of the diagnostic methods employed in the context of low-pressure plasma polymerization. The techniques are described in more detail in the next section of this review. 3.2. Theoretical support for a better description of the data If the evaluation of the plasma chemistry is a challenging task, the interpretation of the accumulated data is often, at least, as difficult. This is why the use of theoretical tools is relevant in this context when used in close synergy with experimental measurements [179]. Modeling the plasma chemistry is a formidable task in view of the large number of species involved and the large diversity of intervening processes (interaction between light and plasma species, heat propagation, chemical reactions, thermal diffusion of the plasma species, etc.). Many theoretical studies rely on mechanistic models involving generally only a fraction of the processes and/or species, without any atomistic detail; in practice, kinetic equations are solved to describe, as a function of time, for instance the generation of the different species in the plasma (together with their degree of charging) and their spatial distribution. The main limitation of such approaches is that the results heavily depend on the

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Table 1 Main features of the plasma diagnostics methods described in this review. The last column refers to the relevance of the diagnostic tool as a plasma polymerization diagnostic: + = low relevance tool, ++ = medium relevance tool, +++ = high relevance tool. Method

Species probed or measurable parameters

Time-resolved

Electrostatic or Langmuir probe

Electron density/temperature, plasma/floating Potential [142,162]

Yes [41,162,163]

OES

Mass spectrometry Ion probe GFTIR

Excited neutrals/ionic species [69,70,132,156,164,165] –Neutrals and ions depending on the analysis mode [37,38,54,57,58,66,68,101,128,169,170]

Yes for ions [41,163]

actual values chosen for the kinetic rates, in particular for the underlying activation energies. Those rates can be inferred from experimental measurements or estimated from sophisticated quantum-chemical calculations. A critical comparison between reference data and the results of the simulations is required to validate the chosen parameters. A refinement is to account for the chemical structures by using molecular dynamics simulations based on force fields (i.e., expressions yielding the relative energies of a given system in different geometries based on a series of bonded and non-bonded energetic terms). These methods cannot account for electronic excitations of the species or for the presence of free electrons; moreover, although standard force fields cannot describe the formation of chemical bonds, the implementation of reactive force fields (such as ReaxFF force fields [180]) allows one to circumvent this limitation. Such simulations are deterministic in the sense that they can describe the trajectory of all species. There are, however, two main limitations associated with such simulations: (i) the quality of the results critically depends on the nature of the interatomic potentials used to describe the van der Waals interactions and the charge assignment on the atoms to evaluate the Coulomb energies; (ii) those simulations cannot be solved analytically and require a time discretization, with a very small time step (of the order of a femtosecond). Accordingly, the simulations are typically run over a limited time (typically a few hundred nanoseconds) and cannot account for slow kinetic processes. However, exploration of the conformational space can be accelerated by increasing the temperature or by coupling the force field to Monte Carlo simulations based on a random (compared to thermodynamic) sampling of the system. A theoretical approach much less adopted in the field of plasma chemistry is to perform quantum-chemical calculations to shed light on important properties of the plasma chemistry. The most popular method used is Density Functional Theory (DFT) which limits the size of the system to be considered (typically up to one hundred heavy atoms), but provides key information such as the free enthalpy of chemical reactions or activation energies. DFT is not a universal theory and actually comes with many different flavors based on the choice of the functional and basis set. It is thus always recommended to validate the selected DFT approach by comparison to experimental data or highly sophisticated Hartree–Fock based calculations. DFT can also be coupled to molecular dynamics simulations, for instance using the Car–Parrinello method though at a much more extensive computational cost compared to force-field calculations [181]. DFT typically sheds light on a very specific aspect in the plasma, generally to assess the change in the electronic energy (or in the enthalpy or free enthalpy) associated with a reaction and activation energies related to computed reaction pathways. For illustration, DFT has been used to model the growth of CVD diamond, showing the different steps allowing for the insertion

Relevance in view of the elucidation of the growth mechanism of PPF +

Semi-quantitative in some cases if using an internal standard [156,166–168] –Semi-quantitative [48,54,66,68,131,172]

Yes [166]

–Ion energy distribution function of ions [38,104,171] Absolute ion flux toward a surface [10,101,102,104,176] No Vibration frequency of chemical bonds [52,99,177,178] No

Comments

–Quantitative for neutrals in some particular cases [173–175]

++

+++

++ Quantitative using a calibration procedure [52,99] +++

of an additional CH2 group on the surface starting from a CH3 radical [182]; in this study, DFT was actually coupled with a force field (within a Quantum Mechanics/Molecular Mechanics—QM/MM—approach) in order to treat the bulk diamond material at a lower level of theory to account for the electrostatic environment in the simulations and the part of the surface where the grafting occurs with DFT to access electronic properties. Another study reported the use of DFT calculations to select the best precursors for the plasma polymerization of organosilicates by computing bond dissociation energies and free enthalpies of reactions [183]. Interestingly, algorithms to find transition states (and hence to estimate activation energies) are now implemented in many quantum-chemical packages, using, for example the intrinsic coordinate reaction (IRC) theory [184]. The transition state can be validated by: (i) a frequency analysis showing a negative frequency for one mode, and (ii) by a steepest descent algorithm from the transition state ensuring that the correct reactants and products are reached. This approach has been exploited in a study modeling the production of ethane from methane in a plasma [185]. Interestingly, all these studies involved the B3LYP functional of DFT owing to its good performance in reproducing experimental enthalpies of formation [186]. Based on the previous considerations, the DFT method appears to be promising for a detailed description of the chemical reactions taking place in the plasma. For a decade, our group has developed several strategies using DFT calculations in combination with standard diagnostic techniques to enhance the understanding of plasma chemistry, as it will be exemplified in the next section. 4. Overview of the diagnostics methods In this section, each diagnostic technique is individually presented discussing its advantages and drawbacks. After a brief description of the basic principle of the method, particular attention is devoted to the link between diagnostic data and growth mechanisms. Furthermore, in several examples, we also show how the use of DFT calculations can be employed for assisting and completing the complex diagnostic data. 4.1. Electrostatic probe The electrons are the energy vehicles in the plasma and thus govern the production rate of reactive species through the dissociation/ionization of the precursor. Therefore, the knowledge of the electron temperature, density and electron energy distribution function (EEDF) are mandatory for clarifying chemical reactions pathways. These plasma parameters, together with the floating potential, can be obtained by

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using the well-known electrostatic probe (also referred to as a Langmuir probes). This method is relatively easy to implement during PECVD experiments. The probe itself typically consists of a tungsten wire, which is biased with respect to the ground potential by a DC generator. By sweeping the voltage from negative to positive values, e.g., from −50 V to +50 V, the probe surface first collects positive ions (regardless of their charge state or their chemical nature) and, as the voltage becomes positive, plasma electrons. By processing the current–voltage characteristics, one can obtain plasma parameters as a function of process parameters. More information on the theory as well as technical aspects of probe diagnostics can be found in Refs. [92,187]. Langmuir probes have been used for a long time to analyze deposition plasmas whatever the process, i.e., Physical Vapor Deposition or PECVD. However, in each case, scientists have to face the same problems, namely: the deposition of the coating onto the probe surface and the probe body and the fact that the measurement is spatially localized. The coating deposition issue might be somehow exacerbated during the deposition of plasma polymers owing to the relatively large deposition rate (as compared to, e.g., magnetron sputter deposition). Obviously, researchers have devised and upgraded their tools in order to minimize coating deposition related issues, e.g., by programming cleaning routines between data acquisition steps. The later can be achieved by positively biasing the probe for several seconds. Biederman et al. used a Langmuir probe during the DC deposition of hydrophilic films using hexane/Ar/H2O mixtures [188]. Fig. 8 shows how the (absolute) probe current varies as a function of the probe voltage for various gas mixtures. From their measurements, one can learn that the electron population is divided into two groups, slow and fast electrons, when the plasma is ignited in pure argon gas (24 Pa). The low energy group is characterized by a Maxwellian-like EEDF. By the addition of water in the Ar plasma, the electron temperature decreases from 0.70 to 0.15 eV, and the EEDF is no longer Maxwellian. The electron density, the floating and plasma potentials also change. The electron density (ne) varied from ~3 × 1015 m−3 to less than ~1 × 1015 m−3 as the percentage of H2O was increased from 0 to 50%. The floating potential has a relatively constant value around 2.5 V as the H2O was varied in the same proportion. However, the plasma parameters changed dramatically as hexane was added to the mixture; the electron temperature decreased to less than 0.1 eV and the plasma and floating potentials varied by more than 20 V. To summarize these observations, one can say that a small addition of hexane dramatically modifies the properties of the plasma. Obviously, these modifications will change how the plasma species interact with the chamber walls and the growing plasma polymer film. Time-resolved Langmuir probe studies were also reported for acrylic acid pulsed discharges in order to determine the temporal evolution of

27

the density and temperature of the electrons as well as the plasma potential adjacent to the deposited film [41,162]. An example of such a study is depicted in Fig. 9. In this work, the probe was either compensated or uncompensated. Most of the time, when used to analyze RF plasmas, Langmuir probes must be compensated by adding an RC circuit to the probe circuit. This modification allows for filtering the RF harmonics, which may lead to a distortion of the I-V characteristics. With such a filter, I–V curves can be processed as if they were obtained in DC plasma. From Fig. 9, it can be learned that in such a pulsed discharge, the plasma parameters are time-dependent. Electron densities and temperatures peak during the ON-time, when the electrical energy is transferred to the plasma species. The electron temperature and density then quickly decay as the power supply is switched off. As a consequence, the gas phase chemistry, which is mainly promoted through electron impact, can be modulated by varying the duty cycle (ton/ toff + ton). From Fig. 9, it can also be seen that the electron density and temperature increase when the power applied to the plasma is increased. The pulse amplitude and the duty cycle are thus key parameters when devising the synthesis of plasma polymer films through PECVD processes. In order to minimize the probe coating deposition issue, a modification of the classic probe setup in which a loop of fine wire is heated has been implemented [142]. Such probes are also referred to as emissive probes because biasing the wire with DC or AC voltages induces strong electron emission from the probe itself. More details can be found in Ref. [142]. For sake of completeness, it has to be mentioned that diagnostic techniques other than Langmuir probes are available to obtain relevant information on the electron population. As an example, Guimond et al. used microwave interferometry to estimate the electron density in organic plasmas [95]. More information on this interferometry technique can be found in Ref. [189]. This method offers some advantages as compared to the more conventional electrostatic probes, namely (i) the measurement is not perturbed by film deposition and (ii) it is not necessary to take into account the ion composition in the plasma sheath in order to deduce electron density of the plasma. The main disadvantage lies in the lack of information about the electron temperature. Using microwave interferometry, Guimond et al. measured more than a ten-fold increase in electron density, which increased from ~ 2.5 × 1015 m−3 to 3.5 × 1015 m−3 as the power was increased from 5 to 100 W in C2H4 and NH3/C2H4 discharges [95]. It should be noted that the evolution of the electron density saturated as the power delivered to the plasma reached ~ 60 W. Gas heating is invoked to explain this behavior. Similar trends in the electron density vs applied power were obtained by the same authors in C2H4 and C2H4/CO2 discharges [39]. These experimental results were used to further describe the growth mechanism and to distinguish the relative influence of both chemical and physical plasma processes. In conclusion, Te, ne and EEDF are key data that should be known in order to understand the fragmentation and excitation/ionization reactions taking place in the plasma, and in fine, to better understand the mechanistic formation of the film. Electrostatic probes represent a simple and rather cheap way to obtain this information. However, research scientists must keep in mind the limitations of the technique such as the covering of the probe issue. In order to overcome these limitations, more sophisticated, non-intrusive diagnostic methods can be implemented, such as the microwave interferometry. 4.2. Optical emission spectroscopy

Fig. 8. Evolution of the probe characteristic for Ar/H2O and Ar/H2O/hexane gas mixtures [188].

Investigating plasma chemistry is essential for identifying reactive species taking part in PPF formation, and hence controlling film properties. Among other techniques (i.e., mass spectrometry and the gas phase Fourier transform infrared spectroscopy detailed in other sections), this can be achieved by optical emission spectroscopy (OES) which is based

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Fig. 9. Evolution of (a) the electron density and (b) electron temperature during the pulse period for plasma polymerization of acrylic acid. In each case, the pulse frequency equals 500 Hz and the duty cycle is set to 50%. Black circles, lozenges and triangles correspond to a peak power of 5, 20 and 40 W, respectively. Adapted from [162].

on the collection, by a spectrometer, of the radiation coming from the plasma. Optical emission from a plasma occurs primarily through electron impact excitation of atoms or molecules according to Eq. (3): N þ e− →N þ e− ;

ð3Þ

where N* is the excited state of the species N. This is followed by a relaxation to a lower energy state releasing a photon of energy equal to the difference between the two energy states (Eq. (4)): N →N þ hν;

ð4Þ

where h is Planck constant and ν is the frequency of the emitted photon. The emission of specific frequencies can be used to identify the species present in the studied plasma. More details about the technique can be found in the following Refs. [92,187,190,191]. OES studies are well documented in the literature related to low pressure sputtering plasmas such as magnetron discharges because, in this situation, the spectra are usually sparse and relatively easy to analyze. Indeed, most of the excited species inside such plasmas are sputtered metal atoms and argon atoms. Diatomic molecules such as oxygen or nitrogen are added to the argon background gas if oxide or metal nitride compounds have to be synthesized. In the case of plasma polymerization, which makes use of more complex organic molecules,

Fig. 10. Emission spectrum recorded in the ultraviolet–visible range during the plasma polymerization of acetone/CO2 [164].

the optical spectra becomes crowded by emission bands originating from the multiple electronic, vibrational, and rotational excited states of the precursors. Fig. 10 shows a typical emission spectrum of acetone/CO2 plasma. Although, numerous emission bands and lines overlap and are poorly, or not, resolved, some lines can be unambiguously identified such as Hα, OH, CH, CO, CO2 and N. The presence of nitrogen emission might originate from impurities incorporated during the deposition process. The presence of the other species result from the dissociation/rearrangement reactions taking place in the plasma, hence highlighting the complex gas-phase chemistry in this kind of plasma. Therefore, OES allows for a qualitative description of the plasma chemistry enabling the investigation of the chemical reaction pathways occurring in the discharge as reported for nitrogen [23,192–195], fluorine [196,197], sulfur [69,70] and oxygen-based [156,165–167,198–201] discharges. Another example that deserves to be pointed out is the detailed study of Granier et al. dealing with the investigation of plasma chemistry by OES of hexamethyldisiloxane (HMDSO) and tetraethoxysilane (TEOS) discharges with and without oxygen [199]. It has been shown that HMDSO plasmas are dominated by Si, SiO and SiH species whereas OH, CO, CO+ and CO+ 2 emission lines are mainly identified in TEOS plasma. From these data, the authors concluded that the presence of CO and OH molecules in TEOS plasmas originates from surface reactions at the plasma/growing film interface followed by their desorption. These findings have allowed providing relevant information on the growth mechanism of the coatings, an essential step for understanding film properties. Nevertheless, it has to be stressed that given the complexity of organic plasmas in terms of the diversity of the molecules present in the discharge, a complete identification of all species constituting the gas is extremely difficult by using solely OES. On the other hand, OES can also be used to monitor other important features of the plasma, for instance the capacitive-to-inductive transition when working with inductively-coupled plasma discharges. This has been demonstrated in our group for the plasma polymerization of propanethiol using a copper coil connected to an RF power supply as a plasma source [66,132]. It is known that, in such a working environment, the capacitive discharge is characterized by a weak global emission intensity since the energy transfer from the generator to the plasma electrons is not efficient. On the other hand, as the plasma enters the inductive regime, i.e., by increasing the RF power to the coil, the gas becomes much brighter. Fig. 11 shows how the capacitive-to-inductive transition is accompanied by: i) an increase in the global emission intensity, ii) an increase in the PPF deposition rate and iii) a decrease in the sulfur content in the PPF. Such behavior is related to the intense fragmentation of the precursor when more energy is dissipated in the plasma.

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the relative concentration of species such as H, CH, CO, OH, and CN. Examples of such analyses can also be found in the literature for nitrogen [192,205–208] fluorine [197], hydrocarbon [168] and oxygen [156,166,167] containing discharges. The actinometry method is explained in the book of Lieberman and Lichtenberg [92]. Briefly, the technique consists in calculating the line intensity ratios Ix/It of two plasma species. The first one, Ix, is related to the particle of unknown concentration, the other one, It, is emitted by the tracer (of known concentration). The excitation threshold energy and the excitation cross section of these two emitters must be nearly the same. In this way, the ratio of intensities Ix/It is directly proportional to the concentration of the emitting species nx. This line intensity ratio is equal to: IX nX ne KX ¼ ; It nt ne K t

ð6Þ

In Eq. (6), ne is the electron density and Kx, Kt are related to:

Fig. 11. Influence of the RF power on the capacitive-to-inductive transition for plasma polymerization of propanethiol accompanied by: (a) an increase in the global emission intensity, (b) an increase in the deposition rate and (c) a decrease in the sulfur content in the PPF. Inset: Typical emission spectrum of the ICP discharge taken in the inductive mode for a power of 100 W [132].

To the contrary, it is difficult to obtain quantitative data through OES measurements, e.g., determining the absolute density of species identified from the data displayed in Fig.10. The detected intensity of the line Ix, appearing at a wavelength λ, and emitted by an excited specie x can be expressed according to Eq. (5): Ix ¼ nx Aij kðλÞ;

ð5Þ

where nx⁎ is the number density of the excited species x and k(λ) characterizes the optical setup response at the wavelength λ. Aij is the frequency of spontaneous emission of a photon at the wavelength λ following the radiative decay from the excited state j toward the lower energy state i. In cold low-pressure plasmas, the production of excited levels typically involves electron collisions with the species. The rate constant of this collision depends on the collision cross section σ(E), which is a function of the kinetic energy of the electrons, and of the EEDF. The later is therefore a key property that should be determined in order to extract quantitative information from the OES measurements. Although one can find collision cross section data in the literature, e.g., for elastic collisions with polyatomic molecules relevant to plasma processing [202] and inelastic cross section for electron with hydrogen [203] or oxygen molecules [204], measuring the EEDF is often problematic. Nevertheless, careful Langmuir probe measurements can be carried out to provide this information (see previous section). Furthermore, if the atom or molecule is excited to a metastable energy level, which is characterized by a much longer radiative lifetime as compared to radiative excited states, another loss mechanism, involving diffusion outside the detection area (e.g., toward the chamber walls), must also be taken into account. This situation would make a quantitative description of the plasma even more complicated. This is the reason why OES should not be considered as the tool of choice to provide a quantitative insight on the plasma chemistry during plasma polymerization. Nevertheless, in some special experimental conditions, actinometry can be adapted to plasma polymerization in order to estimate

i) The production rate of the considered excited states (considering electron impact excitation, the later depend on the EEDF and the electron-specie collision excitation cross section), ii) The radiative lifetime of the considered excited species, and iii) The global response of the optical setup (e.g., the transmission of the lenses, optical fibers, the sensitivity of the detector, etc. depend on the wavelength). If the tracer t is properly chosen, the line ratio is proportional to nx. Since Kx, Kt, ne cancel out and nt is known, nx can be deduced. In Ref. [168], actinometry was implemented to quantify the production of H2, H and CH species in an Ar/styrene plasma. The fragmentation of the aromatic precursor was found to increase with the RF power delivered in the plasma (Fig. 12). In their publication, Choudhury et al. varied the RF power from 20 W to 130 W. They found that the properties of the styrene – based PPF are improved at a RF power of 100 W. OES and film characterization data pointed out that improvement at this specific value of the power is due to the predominance of CH radicals in the plasma and an enhanced cross-linking density due to the presence of a highest percentage of carbon content in the film (Fig. 12). These researchers concluded their study by suggesting the possibility of using styrene-based PPF, deposited at RF power of 100 W, as high performance protective coatings for metal surfaces. Using a similar methodology, Palumbo et al. has highlighted an inverse correlation between the relative proportion of CO in the discharge and the retention degree of the precursor in acrylic acid PPF [167].

Fig. 12. Relative concentration of H, H2 and CH species in an Ar/styrene plasma, as a function of the RF power injected to the plasma [168].

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In another study, Bousquet et al. used the actinometry method for investigating the temporal evolution of the relative density of atomic oxygen by time-resolved OES measurements in HMDSO/O2 plasmas [166]. In order to monitor the reactive species during the postdischarge (i.e., when light is no longer emitted), the authors employed a short pulse excitation technique. Briefly, this method consists in applying a second shorter probing pulse at a RF power similar to the one applied during the main pulse. The aim is to create electrons for exciting the remaining long-lived species. More details can be found in Refs. [166,209]. This approach has allowed measuring the O-atom loss coefficient on surfaces as a function of the chemical composition of the gas. Furthermore, it has been shown that deposition events during the plasma “OFF” time occur due to the dissociation of HMDSO molecules by long-lived oxygen atoms. Although extensively used in the field of organic plasma diagnostic, most of the time, the OES technique does not allow drawing a complete picture of the plasma chemistry. Therefore, its impact in the understanding of the growth mechanism is limited to specific cases. However, the relatively low cost of the equipment and its ease of implementation justify the use of the OES technique for a rapid screening of plasma chemistry. 4.3. Mass spectrometry As explained in the previous section, only a partial picture of the chemical composition of the plasma can be accessed from OES. In this context, the mass spectrometry (MS) technique has emerged as the most widely employed diagnostic method for investigating the plasma polymerization process. Indeed, in comparison to the OES method, a deeper knowledge of the chemical composition of the discharge including ions, stable molecules and radicals can be obtained. The first reports of the analysis of organic plasmas by MS date from the 1980s for polymerizing/etching fluorine based-discharges [197,210]. Nevertheless, the use of the mass spectrometry technique as a tool for a mechanistic understanding of the growth of functionalized PPF has expanded in the 1990s with numerous contributions coming from the group of R.D. Short in Sheffield [38,57,102,169,170]. In order to avoid: (i) failure of some electronic part of the apparatus and (ii) collisions of analyzed ions during their transport inside the equipment, a pressure less than 5.10− 6 Torr is necessary in the MS [175]. Therefore, for plasma polymerization processes operating at a pressure less than 100 mTorr, the MS is connected to the deposition chamber through a small orifice (generally 100 μm in diameter) and is independently pumped while for plasma polymerization conducted at higher pressure, a multistep differential pumping is required [211]. Depending on the analysis mode, this technique enables the detection of neutral (residual gas analysis, RGA mode) and ionic (glow discharge mass spectrometry, GDMS mode) species present in the gas phase. Since the mass analyzer only discriminates ions according to their mass-to-charge ratio (m/z), the sampling of neutral species requires an ionization source. The most common approach consists in heating a filament to generate an electron beam normal to the neutral particle flow; this results in ionization of the neutral species by electron impact and leads to the formation of cations. Obviously, the kinetic energy of the incident electrons has to be higher than the threshold ionization energy of the molecules (typically 10 eV for organic species). It should be noted that electron attachment processes can also be employed leading to the formation of negative ions [212]. In this case, the kinetic energy of the electrons is lower than the threshold ionization energy. Nevertheless, this ionization method is only restricted to highly electronegative species (e.g., fluorine-based) and has therefore been significantly less employed. We will therefore focus only on the electron impact ionization mode for the detection of neutrals. For both analysis modes of the spectrometer (i.e., RGA and GDMS), the ions are separated according to their m/z ratio by means of a quadrupole analyzer, the most commonly used mass analyzer for plasma

analysis [175]. Its working principle relies on the combination of DC and RF potentials applied to four conducting rods to define stable trajectories in oscillating electrical fields which allow separating ions according to their m/z ratio. It should be noted that the quadrupole analyzer is characterized by a transmission function which strongly decreases with mass. For most of mass spectrometers, the manufacturer found empirically that this function ranges between m−1 and m−2. In the literature, an m−1 correction factor is generally applied to the mass spectra [41,47, 101,104,128,170]. Once the ions have been separated according to their m/z ratio, they are converted to a measurable signal using a secondary electron multiplier (SEM) detector. More details about the instrumentation can be found in the book of de Hoffmann and Stroobant [213] and the review of Benedikt et al. [175]. For the GDMS mode, the technique also enables time and energy-resolved measurements. In contrast, in RGA mode, time-resolved measurements cannot be obtained since the transit time of neutral species across the plasma-instrument boundary layer and the ionization chamber is typically longer than the pulse duration when the process is operated in pulsed mode [41]. 4.3.1. Neutral species analysis With regard to the RGA detection mode, a non-negligible drawback is related to the ionization process taking place in the ionization source of the equipment. Indeed, the ions are formed in an excited state and the release of excess of energy may lead to the fragmentation of the molecular ion, hence resulting in the appearance of additional peaks in the mass spectrum as schematically described in Fig. 13. In other words, the detection of a signal in the mass spectrum does not necessarily imply that the corresponding species are formed in the plasma. In conventional MS, a kinetic energy of 70 eV is usually employed in order to maximize the signal intensity since for organic molecules, the maximum of the electron beam ionization cross section is located around 70 eV [175,213]. Nevertheless, for plasma analysis, an electron kinetic energy of 20 eV is preferable in order to limit excessive precursor fragmentation in the ionization source of the mass spectrometer [41,47,48,54,55,66,68,131]. This value corresponds to the best compromise between reduction of fragmentation in the ionization source and the threshold ionization energy of various organic-based molecules and fragments [41]. Based on previous considerations, when operating in the RGA mode, the main challenge is then to differentiate the molecules/fragments produced through dissociation/rearrangement reactions in the plasma from those formed in the ionization source of the spectrometer. Although this aspect is often neglected, a very simple relation (Eq. (7)) can be applied to subtract the fragmentation undergone by the precursor in the apparatus itself following [41,54,68,131]: Ic ðmÞ ¼ Im ðPlasma ONÞ−Im ðPlasma OFFÞ:

IMonomer ðPlasma ONÞ ; IMonomer ðPlasma OFFÞ

ð7Þ

Fig. 13. Schematic description of the principle of mass spectrometry in RGA mode. The ionization process taking place in the ionization source induces, for some species, the fragmentation of the parent ion, hence resulting in the appearance of additional peaks in the mass spectrum (in red) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

D. Thiry et al. / Thin Solid Films 606 (2016) 19–44

Fig. 14. Neutral mass spectra of acrylic acid: (a) plasma OFF, (b) plasma ON (100 μs ontime, 1000 μs off-time, 50 W), (c) corrected for both fragmentation of the acrylic acid precursor in the ionization source according to Eq. (7) and mass transmission [41].

where Ic (m) is the calculated peak intensity for m/z = m, Im (Plasma ON) and Im (Plasma OFF) represent the experimental peak intensity for mass m when the plasma is switched ON and OFF, respectively. IMonomer is the intensity corresponding to the precursor signal. An illustrative example of the application of Eq. (7) is shown in Fig. 14 for the plasma polymerization of acrylic acid. Together with allylamine, this PPF is probably the most studied in the literature. Fig. 14 a-b present the mass spectra of acrylic acid (in the absence of plasma) and when the plasma is switched ON in pulsed mode (50 W, ton = 100 μs, toff = 1000 μs), respectively. The peaks can be assigned as follows [38,163]: m/z = 72 the acrylic acid precursor; m/z = 55 to .+ CH2CHCO+; m/z = 44 to C2H4O.+; C3H.+ 8 and CO2 ; m/z = 28 to .+ + .+ C2H4 and CO ; m/z = 27 to C2H3 ; m/z = 26 to C2H.+ 2 ; m/z = 18 to + H2O.+; m/z = 2 to H.+ 2 ; and m/z = 1 to H . The mass spectrum of the plasma corrected according to Eq. (7) is depicted in Fig. 14c. After the correction, peaks previously identified at m/z = 55 and 27 almost disappear, indicating that these signals originate from dissociations reactions in the ionization source. This example illustrates the importance of the correction method for obtaining the right picture of the neutral plasma chemistry.

31

In Fig. 14, it is worth noting the presence of numerous peaks in the mass spectrum indicating a complex neutral-gas phase chemistry even at low power as also reported for other plasma polymer families [47,55,66,104,170,214]. Although traces of molecules with m/z higher than the precursor have been observed in some studies, no “dimer” or “trimer” based on the monomeric repeat unit is identified in contrast to positive ions (see hereafter) [37,66,104]. One exception is the plasma polymerization of methyl isobutyrate where a peak associated with a neutral dimer has been observed [215]. A closer look to the mass spectrum in Fig. 14 also reveals the production of stable hydrocarbon based molecules (e.g. C2H2, C2H4), as frequently encountered whatever the precursor employed [38,47,54,55, 104,128]. As proposed by Thiry et al. and supported by theoretical calculations, these hydrocarbon molecules result from rearrangement reactions of the radicals produced from dissociation reactions in the gas phase [66]. In addition to the formation of stable hydrocarbon-based molecules, stable molecules containing a heteroelement (CO, CO2 and H2O in the present example) are also formed [37,38,128]. Similarly, NH3 and N2 species are identified in nitrogen-based discharges [47,48, 104,170] as well as H2S and CS2 in sulfur containing organic plasmas [66,68–70,216]. The concentration of such molecules in the gas phase is generally inversely correlated to the heteroelement concentration in the films [217]. Indeed, considering that these unreactive molecules cannot be trapped within the PPF, these species do not take part in film growth, hence reducing the amount of reactive molecules containing the heteroelement available for the deposit. Another complication for the treatment of mass spectrometry data is related to the poor mass resolution encountered for the quadrupole analyzer, i.e., 1 amu. This leads to numerous isobaric interferences (i.e., several species contributing to the same ion signal). For example, the peak at m/z = 28 in Fig. 14 can be attributed to C2H4 and/or CO. To overcome this problem, several strategies can be employed. In their work, Haddow et al. used a plasma of both 13C-labeled and unlabeled acrylic acid in order to facilitate the attribution of the peaks in the mass spectra [38]. In some particular cases, we can take advantage of the isotopic abundance of the heteroelement to discriminate molecules presenting similar m/z ratios, as shown in the case of propanethiol discharges [66]. Finally, another strategy consists in measuring the intensity of the signal at a given m/z as a function of the kinetic energy of the colliding electrons [198]. This method enables one to deduce the ionization energy of the specie(s) contributing to a given signal and then to determine their chemical composition. Ultimately, the combination of other diagnosis methods such as gas phase Fourier transform infrared spectroscopy and OES can also assist the interpretation of the mass spectra data [53,200]. Although the intensity of a signal in an RGA mass spectrum is related to the density of the corresponding species, calculation of their absolute concentration is not straightforward. Among other things, this requires the knowledge of the ionization cross-section which, for many stable molecules, are available in the literature [175]. In this case, the absolute density of the probed molecules can be obtained by means of a calibration gas with a known density and electron impact ionization crosssection [175]. With respect to radical species, determination of their absolute density is much more demanding as they can be lost through collisions with the walls of the reactor [173,175]. Most of the time, the corresponding cross-sections are not available and several assumptions have therefore to be made [175]. Furthermore, in some cases, specific experimental configurations such as multistep differential pumping are needed for an accurate measurement [173,174]. Based on these considerations, in the context of the plasma polymerization process, the great diversity of reactions (including fragmentations, rearrangements, etc.) highly complicates measurements of the absolute densities of the numerous species constituting the plasma. Nevertheless, some simple methods can be employed for obtaining quantitative or semi-quantitative information. For instance, the absolute precursor concentration can be easily measured using a calibration

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curve relying on the intensity of the monomer signal as a function of the pressure when the plasma is switched OFF [41]. Using this calibration curve, the intensity of the precursor measured in the mass spectrometer, whatever the plasma parameters, can be directly related to the partial pressure of the monomer. For example, a decrease in the partial pressure of the acrylic acid precursor as a function of the energy delivered in the plasma has been reported [37,38,41]. This expected behavior is attributed to an increase in ne with power resulting in an increase in precursor fragmentation [142]. It should be noted that the amount of unfragmented chemical precursors remaining in the discharge is generally related to the concentration of the chemical group hosted by the monomer [47,48]. To monitor the relative proportion of radicals and stable molecules produced through fragmentation/rearrangement reactions in the plasma, a depletion function can be employed according to Eq. (8) [172, 218]: D ð%Þ ¼



 IOFF − ION ION  100 ¼ 1−  100; IOFF IOFF

ð8Þ

where ION is the intensity of a peak at a given plasma condition and IOFF is the intensity of the same peak in the absence of a plasma. Based on Eq. (8), when a peak intensity increases with power, the depletion of that peak is negative. Negative values indicate that the corresponding species are formed by the action of the plasma. In other words, the higher the negative value of the depletion function, the higher is the production of the corresponding species in the plasma. Using this methodology, Hazrati et al. have studied the formation of key fragments for the plasma polymerization of ethanol and demonstrated the production of radicals such as .CH3 and .OH to the detriment of the precursor molecule upon increasing the power [172]. It is important to stress here that Eq. (8) has to be employed with care depending on the plasma polymer investigated. Indeed, species formed from the dissociation of the precursor ion in the source of the spectrometer affect the measured intensities and therefore the conclusions drawn from the depletion function. Another strategy has been recently implemented for directly correlating plasma and film chemistries in view of the elucidation of the growth mechanism of propanethiol PPF [68]. Briefly, under certain experimental conditions, it has been shown that propanethiol PPF contains a sulfur content much higher than in the precursor. This particular feature is explained by the trapping of H2S molecules within the plasma polymer matrix [66,131,132,219]. To validate this hypothesis, the relative proportion of H2S (Irel. H2S) with respect to the sum of signals associated with carbon-based species (m/z = 15,26–29,39–43), which could potentially take part to the growth of the PPF, has been calculated. The chosen species refer to radicals or molecules containing an unsaturation which could potentially be grafted at dangling bonds

Fig. 15. Evolution of the sulfur to carbon ratio of propanethiol PPF as a function of Irel. (H2S) for different powers [68]. See the text for details.

present at the growing film surface. Irel. (H2S) has then been calculated following Eq. (9) and compared to the sulfur to the carbon ratio (S/C) measured by XPS in the corresponding PPF (Fig. 15): Irel: ðH 2 SÞ ¼ IC ðm=z ¼ 15Þ þ

IC ðH2 SÞ m=z¼29 X

m=z¼43 X

m=z¼26

m=z¼39

IC ðm=zÞ þ

;

ð9Þ

IC ðm=zÞ

where Ic (m/z) corresponds to the corrected intensity calculated following Eq. (7) for the signal at m/z. The obtained linear correlation between Irel. (H2S) and the S/C ratio as a function of power validates the trapping hypothesis, revealing the attractiveness of the mass spectrometry technique for a deeper understanding of the PPF growth at a molecular level. We understand from these examples that the interpretation of mass spectrometry data is often challenging. As already mentioned, one strategy to make it easier consists in exploiting DFT calculations. As an example, the study of the plasma polymerization of ethyl lactate, finding applications in the design of biodegradable coatings, is presented [54]. Briefly, the objective of this work was to correlate the ester content in the films, a key parameter for controlling the biodegradability of the material, with the plasma chemistry. A particular feature of the mass spectra of ethyl lactate plasma in RGA mode is the absence of a signal corresponding to the precursor (m/z = 118), thus making the interpretation of the data highly complicated. This is explained by the “brittleness” of the ethyl lactate molecule which suffers of extended fragmentation reactions in the ionization source of the spectrometer. Based on mass spectrometry data, a thorough mechanistic study using DFT was undertaken. A detailed description of the complex calculated fragmentation pathway occurring in the ionization source of the spectrometer as well as in the plasma (Fig. 16a–b) is beyond the scope of this review and interested readers can consult Ref. [54]. We will concentrate on the species at m/z = 75 (i.e., C3H7O+ 2 ) which deserve peculiar attention (see highlighted rectangle in Fig. 16). Using the DFT calculations, it can be learned that this species can only be formed through an ionic mechanism involving the undamaged ethyl lactate ion and thus takes place in the ionization source of the spectrometer (Fig. 16 a). Indeed, considering a radical mechanism occurring preferentially in the plasma (Fig. 16 b), the intermediate of the corresponding reaction is unstable and rearranges spontaneously to a more stable form at m/ z = 74. Comparing both fragmentation patterns (i.e., ionic and radical), this implies that the intensity of the peak at m/z = 75 is proportional to the density of undamaged precursor in the plasma. Therefore, this peak can be correlated with the evolution of the ester content in the PPF depending on the synthesis conditions. This example highlights the significance of theoretical tools for extracting relevant information from complex diagnostic data. A similar approach has been successfully employed for deriving the chemical reactions pathways encountered by the precursor in propanethiol [66,68] and benzene/cyclohexane [220] plasmas. Another example revealing the strong interest in combining DFT calculations with MS data is related to the plasma polymerization of allylamine and cyclopropylamine. The aim of this work was to study the role of the nature of the precursor on the density of primary amines group in the PPF [48]. It has been shown that above a critical value of the mean power (around 30 W) delivered in the plasma, both RGA mass spectrometry and XPS measurements indicate that the two precursors behave in a very similar way, showing significant fragmentations and poor incorporation of primary amines in the films. However, below this value, the content of amine groups increases in the films in a larger proportion for the cyclopropylamine precursor. These data have been understood based on DFT calculations. Fig. 17 displays the enthalpies of reactions associated with various initial fragmentation schemes of allylamine and cyclopropylamine, as calculated by DFT. Note that: (i) all calculations have been performed in the unrestricted scheme

D. Thiry et al. / Thin Solid Films 606 (2016) 19–44

33

Fig. 16. Proposed fragmentation pathways based on DFT calculations of ethyl lactate dissociation in (a) the ionization source of the spectrometer and (b) in the plasma. Adapted from [54].

and, (ii) activation and bond dissociation energies are almost equivalent for bond dissociation processes [221]. In the case of allylamine, the more thermodynamically favorable reaction involves the rupture of the C\\N bond to form allyl and NH2 radicals. The lower bond dissociation energy is rationalized by the stabilization of the allyl radical by resonance effects. In contrast, the easiest reaction in the case of cyclopropylamine is the opening of the ring, thus retaining the amine chemical functionality on the precursor. This different behavior is fully consistent with the increased amount of amino group detected in the film upon plasma polymerization of cyclopropylamine. Accounting for the entropic effects would systematically decrease by about 0.5 eV, the calculated bond dissociation energies while keeping unchanged the previous conclusions. The full consistency between the experimental and theoretical results thus paves the way toward a theoretical screening and even the design of precursors aimed at optimization of the degree of PPF functionalization.

4.3.2. Ion analysis As already mentioned, the ions also play a major role in the PPF growth mechanism. The ionic gas phase chemistry has therefore also been extensively investigated by MS. Fig. 18 represents a typical example of the chemical composition of positive ions in an acrylic acid discharge. Similar to the RGA analysis mode, numerous peaks are identified. It is important to stress that in this case, the ions during their transport in the mass spectrometer obviously do not suffer of fragmentation before their detection. Therefore, all peaks appearing in the spectrum correspond to ions formed in the plasma. The base peak at m/z = 73 is ascribed to the protonated monomer ([M + H]+) while the most prominent fragments are observed at m/z = 39 (C3H+ 3 ), 55 (CH2CHCO+) and 57 (CH3CH2CO+). A relevant observation is the detection of oligomeric ions of the form (2 M + H)+ at m/z = 145 and (3 M + H)+ at m/z = 217, where M corresponds to the molecular weight of the starting material as also observed in other works [101,

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Fig. 17. DFT-calculated bond dissociation energies of the initial fragmentations of allylamine (top) and cyclopropylamine (bottom). Adapted from [48].

128,215,222]. Oligomer ions corresponding to [4 M + H]+ have also been identified in Ref. [38]. The dimer and trimer ions can lose H2O giving rise to additional signals separated by a value of 18. Such oligomer ions formed through gas phase neutral/ion reactions have also been identified in propanoic acid [215,222], allylalcohol [214], propanol [214], allylamine [104,170], HMDSO [102,223,224], methyl isobutyrate, methyl methacrylate, n-butyl methacrylate [169], ethanol [172,225], γ-terpinene [171] and thiophene [218] plasmas. Most of the time, the formation of these oligomeric ions involves the addition of a hydrogen ion on the neutral precursor followed by the successive additions of uncharged monomers. However, the exact mechanism remains unclear in some aspects. For example, gas phase oligomerization reactions do not require the presence of a double bond in the organic precursor marking a clear difference with classical ionic polymerization [169,215]. The

Fig. 18. Positive-ion mass spectrum of acrylic acid plasma sustained at a power of 3 W. The region under the horizontal bar has been expanded by a factor of 5 in intensity. Adapted from [215].

reason why neutral/radical addition reactions in the gas phase is ruled out lies in the kinetics of the reactions as proposed by Benedikt [226]. Indeed, ion-neutral reactions are typically 10 times faster than reactions between neutral particles due to the attractive potential created by the induced dipole moment in the neutral particle. The intensities of the peaks associated with the oligomeric cations were found to evolve with the power dissipated in the discharge. Most of the time, a decrease in the relative proportion of these ions in the plasma upon increasing the power has been reported [37,169, 171]. This behavior can be ascribed to a decrease in the concentration of the precursor, hence reducing the probability of addition reactions with ions. Since a correlation between the relative proportion of ions and the degree of retention of the precursor in the films has been highlighted for several PPF systems, it has been proposed that these ions significantly contribute to the film formation especially at low power [128]. It should be noted that for allylamine plasmas, a nearly constant proportion of oligomeric ions has been reported whatever the power. This points out the influence of the chemical nature of the precursor on the oligomerization reactions [104,170]. For sake of completeness, negative ions (fragments and oligomers) have also been identified in silane [227] and oxygen containing discharges [228,229] using MS. Their detection is only possible in the afterglow at sufficient long OFF time for enabling their extraction from the bulk plasma. In continuous wave (CW) plasma polymerization, their contribution in the condensing material is excluded since they are confined in the bulk of the gas due to the positive electrical potential drop between the plasma and the surrounding surfaces. When operating the discharge in the pulsed mode, they are able to reach the film surface only for sufficient long plasma “OFF” times. Their role in the overall deposited mass has been estimated to be quite low (b 1%) in comparison to positive ions and neutrals [228].

D. Thiry et al. / Thin Solid Films 606 (2016) 19–44

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Fig. 20. Energy distribution function at the grounded orifice of the mass spectrometer of the precursor in γ-terpinene plasma sustained at a power of 50 W [171].

Fig. 19. Time-resolved ion mass spectra of acrylic acid discharges obtained at 50 W RF power (ton = 100 μs and toff = 10,000 μs). Adapted from [41].

Although the pulsed mode has been extensively employed in the plasma polymerization domain, only a few studies were dedicated to time-resolved mass spectrometry measurements. As aforementioned, such kinds of measurements are limited to ionic species [41]. The results obtained have allowed a better understanding of the chemistry of pulsed organic discharges. The main conclusions drawn are that some ions can “survive” during the plasma “OFF” time during at least 1000 μs after the extinction of the plasma [41,163]. This aspect is illustrated in Fig. 19 where GDMS measurements were performed in acrylic acid discharges for selected times during the plasma “OFF” time. The dominant positive ions are identified as follows: m/z = 55 to CH2CHCO+, m/z = 73 to [M + H]+ (M being the acrylic acid precursor), m/z = 127 to [M + H-H2O]+, m/z = 145 to [2 M + H]+, m/z = 199 to [3 M + H-H2O]+ and m/z = 217 to [3 M + H]+. After 500 μs, only the trimeric species (at m/z = 217) are still detected in the plasma and remain observable after 1000 μs. In addition, one can also notice that the intensity of the ions at m/z = 199 and 217 are higher after 500 μs than 200 μs. This clearly indicates the possibility of OFF-time reactions of monomers with dimers to form trimers within the time scale of the OFF-period. These results have challenged the usual view of a plasma polymerizing pulsed discharge, namely a concomitant ionic and neutral deposition mechanism during the plasma ON time and an exclusively neutral chemistry during the OFF time. In addition to the significance of determining the neutral/ionic chemical composition of the plasma, investigating the energy of the

ions bombarding the substrate is also of considerable interest for a mechanistic description of plasma polymer formation. Indeed, by means of a specific RF biasing technique enabling to control the ion energy independently of other parameters, Barton et al. have reported that the ion energy can significantly alter some important film properties such as the deposition rate as well as the layers chemistry [222,230]. The mass spectrometry technique allows us to measure the energy distribution function of the ions (IEDF) arriving at the orifice of the instrument which can be grounded or at floating potential. However, it has to be mentioned that energy measurement at a floating orifice is much more representative of the energy of ions bombarding the growing film when the substrate is not connected to an RF power supply since most of the time, plasma polymers are insulating [38]. For an accurate estimation of the bombarding ion energy, the orifice of the mass spectrometer has to be localized at the position of the substrate. Fig. 20 represents a typical example of an IEDF recorded in a γ-terpinene plasma using a spectrometer with a grounded orifice. The IEDF has a shape which is typical of those recorded in plasmas characterized by a collisionless sheath. The “peak” in the IEDF (at about 16 eV in Fig. 20) corresponds to ions accelerated across the plasma pre-sheath and sheath and entering the MS without any collision (and hence without loss of energy) [171,225]. Haddow et al. have performed a systematic study of the evolution of the ion energy as a function of the dissipated power in CW plasma polymerization of acrylic acid using an external RF inductive coil as plasma source [38]. The results are summarized in Fig. 21 depicting an increase in the ion energy from 5 eV to 30 eV when the power evolves from 1 to 15 W. At 3 W, the number of atoms in the molecular ion has been estimated to be, on average, 10 whereas the ion energy is 13 eV, thus corresponding to an energy per atom b1.3 eV [38,101]. It is reasonable to assume that at this value, fragmentation of the striking ion (and surface) will be minimal enabling a better preservation of the

Fig. 21. Evolution of the ion energy of acrylic acid cations relative to a self-biased, or floating, surface as function of the power. Adapted from [38].

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function of interest. At higher power, the peak ion energy is 30 eV while the ions contain b 10 atoms on average, leading to an energy per atom N3 eV. Since bond energies in organic molecules typically range from 3 to 5 eV, the fragmentation of the incoming ion and/or the growing film surface might be anticipated at higher powers, therefore contributing to the decrease in the degree of retention of the precursor in the coating [38,104]. To summarize, the popularity gained in the field of plasma polymerization by the MS technique over several decades lies in its ability to determine relevant properties including the chemical composition of the plasma and the energy of bombarding ions. This is probably the reason why this method has emerged as one of the most powerful diagnostic methods for deepening the fundamental understanding of the mechanistic formation of a given PPF. The main disadvantage of the MS technique is related to the cost of the equipment and the often difficult interpretation of the data. In the latter case, it is clear that theoretical support such as DFT calculations can help. 4.4. Ion probes Using the MS technique, although all ions constituting the plasma can be identified, the absolute ion flow toward the growing film cannot be deduced since MS measurements do not give access to the absolute density of ions. This aspect is, however, of crucial importance to determine the mass delivery by ions to the film surface and also in combination with other techniques to measure the contribution of ions in the total energy flux reaching the growing film. In plasma processing, a classical means to measure the ion flux to a surface is based on ordinary electrostatic probes inserted into the plasma volume. From measurements of the ion density and electron temperature in the plasma bulk, the ion flux toward the interface is deduced using the Bohm criterion. Nevertheless, in the context of plasma polymerization, this method is not well suited for reliable and routine analysis. Indeed, as explained in section 4.1, the coverage of the probe with an insulating PPF highly complicates the measurements [8,101,104,176]. In 1996, Braithwaite et al. have developed a novel probe to measure the absolute ion flux toward a surface in organic discharges which presents significant advantages: tolerant to insulating deposits, nonperturbing, easy to implement and applicable whatever the plasma source [176]. Briefly, the basic principle consists in applying a pulsed RF voltage to the electrode. Owing to the different mobility of electrons and ions, the probe naturally acquires a negative self-bias potential when an RF signal is applied. The RF signal is then chopped for approximately 5–10 ms and the change in the voltage is measured with time as the ions impact the negatively biased electrode [8,102]. The ion flux is deduced from the variation of the bias voltage as a function of time. For more details about the physical phenomena involved in the

measurement of the ion flux using this probe, the readers are invited to consult the paper of Braithwate et al. [176]. Today, ion flux probes are relatively inexpensive and easy to use [58]. An example of the evolution of the ion flux toward the substrate as a function of the power is displayed in Fig. 22 for the plasma polymerization of allylamine. The ion flux was found to increase from 6.6 × 1016 m−2 s−1 ions at 1 W to 1.4 × 1018 ions m−2 s−1 at 14 W. Similar trends and ion flux values were reported for acrylic acid [58,222], propionic acid [58,222], hydrocarbons (n-hexane and 1,7-octadiene), diethylene glycol, diethylene glycol divinyl ether [58], γ-terpinene [171] and ethanol [225] plasmas. The observed trend in Fig. 22 is ascribed to the increase in the electron density and, in turn, in the ion density in the plasma volume upon increasing the power [142]. The calculation of the average ion mass based on the ionic mass spectra combined with ion flux measurements allow us to deduce the ion mass flux toward the growing film. To evaluate the contribution of ions in the deposited mass, the latter is compared with the mass deposition rate (expressed as the total mass deposited per unit surface and time) obtained from quartz crystal microbalance. The data are summarized in Table 2 for the present example. At low power (i.e., 1 W), approximately 63% of ions could, in principle, contribute to film formation. Interestingly, at a power of 5 W, the ion flux is sufficient to account for all deposited mass. This provides a strong argument that ions significantly contribute to film formation. It is important here to stress that it does not mean that the film grows through a pure ionic mechanism and that neutral/radical surface reactions are ruled out. Estimation of the mass delivery to the film by ions by a comparison between the mass deposition rate and ion flux inherently takes into account that all ionic species exhibit a sticking coefficient equal to unity, or at least does not evolve with the process parameters. However, the real situation is much more complicated as the sticking coefficient of ions is less than unity and is affected by their chemical nature, energy and the nature of the surface. For example, it can be learned from Table 2 that the ion flux exceeds the mass deposition rate for a power of 5 W, indicating a loss of ionic mass incorporated in the film. A possible loss mechanism could be recombination events occurring at the growing film surface followed by the formation of a stable molecule unable to be chemisorbed (e.g., CO.+ + e−surface → CO (g) or CH3CH2CO+ + e−surface → CH3CH2 (surface) + CO (g)) [101]. It should be noted that a more pronounced ablation phenomenon at higher power might also be anticipated. The previous considerations also prevail for the sticking coefficient of neutral species for which it has been reported that their surface reactivity is directly related to the density of dangling bonds at the interface [99,107]. Actually, both neutrals and ions are intertwined in the overall mechanism since an increase in the ion flux causes the formation of more radical sites at the surface, thus resulting in a more efficient grafting of reactive neutral species. The emergence of ion probes has also highlighted the influence of the chemical nature of the precursor on the process. Michelmore et al. investigated several saturated/unsaturated monomers, comparing the mass deposition rate with the ion flux as a function of power [57,58]. For each pair of precursors, despite similar ion fluxes for a given set of plasma parameters, the mass deposition rate is significantly higher for the unsaturated monomer. This behavior is attributed to a more pronounced ionic deposition mechanism for the saturated precursor,

Table 2 Comparison of total mass deposition rates and positive-ion mass flux in allylamine plasmas for different powers [104]. See text for details.

Fig. 22. Evolution of the ion flux in allylamine plasmas as a function of RF power [104].

Power (W)

Total material deposition rate (μg m−2 s−1)

Positive–ion mass flux (μg m−2 s−1)

1 3 5 15

18.7 61.7 86 127.1

11.8 36.2 99.4 226.6

D. Thiry et al. / Thin Solid Films 606 (2016) 19–44

whereas the grafting of intact precursor at open bond sites present at the interface is predominant for the unsaturated one. The development of the ion probe has therefore allowed shedding light on the contribution of the ions to the PPF formation. Even though the exact role played by radicals and ions in the plasma polymer formation is still debated, it is however well-established that the actual picture of the PPF growth mechanism should consider both species.

4.5. Gas-phase Fourier transform spectroscopy As already mentioned, the mass spectrometry technique, is one of the most exploited plasma diagnostic methods for probing PECVD discharges and, specifically, the plasma polymerization process [41,48,68, 172]. Nevertheless, it is accepted that it presents two major drawbacks, namely the fact that: (i) the quantification of species is complex such that most of time only semi-quantitative data can be obtained and (ii) the additional fragmentation of the plasma generated species which likely occurs in the ionization chamber when using the RGA mode makes interpretation of the data even more complicated [99]. These limitations are in fact related to the “ex situ” feature of this plasma diagnostic technique. Therefore, in situ approaches have been developed in order to overcome these drawbacks while taking care to avoid perturbing the plasma during measurements. Optical techniques fulfill these requirements, especially the absorption spectroscopy (AS) method because they allow determining population densities in both ground and metastable states as well as information related to the gas temperature when considering the line profile of atomic bands or the ro-vibrational structure of molecular gases [231]. The fact that absolute densities can be deduced from AS measurements without instrument calibration is one of the main advantage of this technique in comparison to OES, which was discussed in Section 4.2. Among the different wavelength regions that have been scanned, the infrared (λ = 14 μm–25 μm corresponding to 700–4000 cm−1) is particularly well adapted to plasma polymerization process because this region of the electromagnetic spectrum contains the vibrational signatures of plasma-generated molecular species. Historically, dispersive instruments were first used for IR absorption measurements, but they are not well suited for detecting weakly absorbing molecular species in low pressure plasmas. Therefore, today, plasma diagnostics using infrared absorption spectroscopy is often performed through Fourier transform infrared spectroscopy (FTIR) or using tuneable diode laser absorption spectroscopy (TDLAS). Both methods have their advantages and drawbacks. Laser techniques typically offer higher sensitivity and resolution than FTIR, but cannot (even with tunable lasers) probe the broad bandwidth range required to track many IR absorption peaks simultaneously. An excellent review on the TDLAS spectroscopy can be found in Ref. [232]. Initially, before FTIR and TDLAS have emerged as plasma diagnostic tools, IR absorption was used to probe silicon [233] and a silicon dioxide dry etching processes [234], using fluorocarbon gases to take advantage of the strong absorbing nature of the fluorocarbon species and thus to overcome the limited sensitivity of dispersive instruments [231]. With the development of the above mentioned spectroscopic methods to probe low pressure plasma with sufficient sensitivity, infrared analysis of plasmas containing IR-sensitive species has become of importance in view of the quantitative character of the analysis. Indeed, in molecular and low- temperature plasmas characteristic of the plasma polymerization process, the plasma–surface interaction that governs thin film growth is controlled by the fragmentation of precursor molecules in the plasma. Therefore, a better understanding of thin film growth strongly depends on better evaluation of the plasma chemistry and reaction kinetics. This is only possible if we are able to quantitatively probe the densities of species in the plasma. From an instrument point of view, the sensitivity problem that is encountered in low-pressure discharges is often tackled by implementing White-cell multiple pass

37

optical arrangements. Fig. 23 illustrates, for example, typical experimental setups used for FTIR (Fig. 23a) and TDLAS (Fig. 23b) measurements. Numerous PECVD processes involving hydrocarbons precursors to deposit C-containing films such as Diamond Like Carbon layers or plasma polymers have been studied by IR absorption spectroscopy in order to monitor transient and stable species that are strongly involved in the film growth mechanism. For example, Takahashi et al. have reported on the use of infrared laser absorption spectroscopy to study the impact of the precursor (i.e., C4F8, C3F6, C5F8) on the growth of fluorocarbon thin films by evaluating radical kinetics [235]. The coatings were synthesized in a capacitively-coupled plasma using pressures ranging from 15 to 45 mTorr. In this work, they showed that the density of small mass radicals such as CF or CF2 (1010 and 1012 cm−3, respectively) are not impacted by the precursor nature in their experimental conditions. However, stable molecules such as CF4 and C2F6 were generated from C4F8 and C3F6 precursors suggesting a lower gas and surface polymerization for these precursors. This is consistent with the higher growth rate and fluorine content obtained when using the C5F8 precursors [236]. Fig. 24 shows the power dependence of (a) CF, (b) CF2 and (c) CF3 radical densities measured for the different fluorocarbon-based plasmas. These data, and particularly the saturation of the CF2 signal with increasing power and the low densities of CF and CF3 radicals, have allowed the authors to conclude that the contribution of these radicals to the film formation is not significant. Later, the same group extended this work by studying the copolymerization of C6F6 and C5F8 by OES and FTIR plasma diagnostics. In this work, they demonstrated that the polymerization of C6F6 and its incorporation with a ring structure in the thin film improves the thermal stability of the films in comparison with pure C5F8 films [236]. The PECVD deposition of C-containing coatings with other heteroelements than fluorine have also been studied. For example, Goujon et al., have studied plasma chemistry during synthesis of SiOx coatings in a capacitively-coupled plasma using HDMSO/O2 mixtures at relatively high pressure (i.e., 1 Torr) by using OES and FTIR (without the use of a White cell) [200]. Their results have revealed, in agreement with previous work, the existence of two regimes as a function of the HDMSO content in the gas mixture. They were also able to evaluate the degree of fragmentation of the HDMSO precursor as a function of the injected power, which was a valuable input to proposing a tentative fragmentation pattern of the precursor. Similarly, Raynaud et al. have studied the plasma polymerization of HMDSO in a microwave discharge by FTIR absorption spectroscopy [178]. In this case, the plasma was generated at low pressure (3 mTorr) using a Withe cell with up to 44 m of optical length (see Fig. 23a). These data were correlated to the chemical structure and composition of the deposited films in order to understand the growth mechanism. It has been demonstrated that, surprisingly, the coating structure evolves with an opposite trend in comparison to the plasma chemistry. At low power (Fig. 25a), the growth mechanism is affected by the surface formation of stable volatile molecules (CH4, (CH3)3SiH and pentamethyldisiloxane) from a significant part of the plasma generated radicals. This implies that only a few basic radicals are responsible for the growth of films, especially (CH3)xSiO. At high power (Fig. 25b), the plasma is dense enough to dissociate the byproducts, and hence to increase the flux of condensing species, leading to an increase in the Si\\O grafting rate in the films. Although a significant insight into the growth mechanism has been obtained thanks to plasma diagnostics using infrared spectroscopy, the authors mentioned that even if the technique is powerful, the complexity of the spectra presenting numerous bands related to the different chemical species/radicals in the plasma makes interpretation difficult. This later claim becomes even more relevant when considering the more complex plasma polymerization systems that are today studied to synthesize functionalized organic surfaces. As an example, Wells et al. have studied the plasma polymerization of ethylene glycol in a pulsed inductively-coupled plasma reactor by FTIR and OES in order to

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D. Thiry et al. / Thin Solid Films 606 (2016) 19–44

Fig. 23. Experimental setup used (a) by Raynaud et al. to probe a microwave HMDSO plasma by FTIR diagnostics [178] and (b) by Takahashi et al. to probe the discharge by TDLAS during the plasma polymerization of fluorocarbon-based precursors [235].

evaluate the dynamics of monomer fragmentation and the effective chemical feedback from the boundary walls [59]. A pressure of 60 mTorr was used and the optical length was 64 cm. The presence in the plasma of CH, CO, OH and H (by OES) and of acetylene, ethylene, methane, water, formaldehyde, CO and CO2 (by FTIR) was demonstrated. These stable volatile molecules are claimed to be the results of radical recombination and polymerization processes occurring at plasma/ walls interfaces. It is also shown that evolution of the film chemistry as a function of the applied power is correlated with the fragmentation pathway of ethylene glycol in pulsed plasmas with a higher retention of the monomer functionality at low mean power. The complexity of plasma polymerization processes clearly makes infrared data interpretation more and more difficult. In order to contribute to a better interpretation of these data, our group has recently used a DFT strategy similar to the one mentioned earlier in the mass

spectrometry section [53]. This approach has been developed in the context of the synthesis of ethyl lactate plasma polymers. In order to validate our approach, we have first compared the infrared spectrum simulated for ethyl lactate with a corresponding experimental measurement obtained from ethyl lactate vapor (plasma OFF), see Fig. 26. A comparison between the two spectra shows a very nice agreement, with a root-mean-square deviation of ~ 5 cm−1, hence validating our theoretical approach. When initiating the plasma, the fragmentation of precursor molecules generates a mixture of different species leading to a much more complex spectrum. Mass spectrometry measurements in RGA mode have allowed identifying nine dominant fragments: CH3, CH4, H2O, CO, CO2, C2H4, C2H2, CH3CHOH, and C3H7O2. Fig. 27b exhibits the sum of the DFT-calculated infrared spectra of these nine fragments, with their relative intensities weighted on the basis of the peak intensities from

D. Thiry et al. / Thin Solid Films 606 (2016) 19–44

39

Fig. 24. Power dependence of (a) CF, (b) CF2 and (c) CF3 radical densities measured by TDLAS for different fluorocarbon-based precursors [235].

the mass spectrum. The weighting factor (WFi) for a given species i is expressed as: IMS WFi ¼ Xi MS : Ii

ð10Þ

synthetic IR spectrum. This can be corrected by estimating the percentage of undamaged ethyl lactate molecules in the plasma (XEL) by comparing the intensity of the peaks related to the ester function in the ethyl lactate vapor when the plasma is switched OFF and when a power of 60 W is applied. This leads to a XEL value equal to 0.712. As a

i

the intensity in the mass spectrum of the species i. with IMS i This yields a simulated spectrum in marked discrepancy with the experimental one (Fig. 27a) due to fragmentation in the ionization source of the spectrometer. The ester-bearing molecules/fragments are not identified in the mass spectra and thus not considered to build the

Fig 25. Simplified reaction pathway for (a) low and (b) high power conditions established from FTIR diagnostic measurements for the plasma polymerization of HMDSO. Adapted from [178].

Fig. 26. (a) Experimental IR spectrum of ethyl lactate in gas phase without discharge at 10 mTorr and 5 sccm and (b) IR spectrum of ethyl lactate calculated by DFT [53].

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D. Thiry et al. / Thin Solid Films 606 (2016) 19–44

specifically, to evaluate the reaction efficiency of the ester-containing fragments (σester) in the growing film [99]. The latter was obtained by comparing the plasma chemistry to the film composition. Fig. 28 shows the evolution of σester as a function of the applied power. The measured trend highlights the higher incorporation efficiency of esterbased fragments in the growing film at high powers. This is likely related to a higher density of surface active site generated by ion and photon irradiation upon increasing the power. Gas-phase FTIR spectroscopy is a powerful tool allowing for quantitative determination of important species (radical, stable molecules) in the plasma phase. In combination with other diagnostic tools, as well as with theoretical calculations and surface analysis, it is even possible to evaluate surface reaction coefficients. One of the important drawbacks is the complexity of the instrumentation which requires the design of dedicated reactors.

5. Conclusions and perspectives

Fig. 27. (a) Experimental IR spectrum of an ethyl lactate plasma sustained by a power of 60 W. (b) Synthetic IR spectrum calculated on the basis of Eq. (10) and (c) corrected according to the degree of fragmentation of the precursor [53].

result, the peaks associated to the fragments are further corrected by a factor (1 − XEL) = 0.288. The resulting corrected spectrum, shown in Fig. 27c, appears to be in much better agreement with the recorded experimental spectrum, allowing for the assignment of most of the peaks even though the presence of fragments containing ester groups in the plasma has been neglected. This original protocol clearly provides a new diagnostic tool to probe the plasma chemistry, especially the presence of specific chemical functionalities. In the framework of this research, we were also interested in the evolution of the ester-containing species density as a function of the power which, in fine, partly determine the degradation behavior of the deposited films. These data were used to better understand the growth mechanism of the ethyl lactate plasma polymer and, more

Fig. 28. Evolution of the surface reaction coefficient of ester-bearing fragments (σester) as a function of power during the synthesis of ethyl lactate plasma polymers, as estimated from FTIR plasma diagnostic data. Adapted from [99].

The interest in organic surfaces exhibiting a well-defined and tuneable chemistry with stability is becoming increasingly important in many modern fields of application, especially in the bio-technology sector. In this context, plasma polymer thin films historically used as protection or barrier coatings have found a potential “New Age”. Nevertheless, these new opportunities will only materialize if the necessary control of the physico-chemical properties of the materials is attained. In view of the complexity of the plasma polymerization process, it is todays accepted that such a control will only be possible through a deeper understanding of the plasma chemistry and ultimately of the plasma-surface interaction in order to determine consistent growth mechanisms. In this review, it is suggested that the required detailed analysis and understanding of plasma chemistry employed in low-pressure plasma polymerization processes can only be obtained by a complementary approach combining several state-of-the-art plasma diagnostic tools. The most employed tools are mass spectrometry (in both ion and neutral modes), optical techniques such as OES and gas phase FTIR, and to a lesser extent electrostatic and ionic probes. Each of these techniques provides a piece of a puzzle that has to be handled with care. Indeed, in addition to the already challenging experimental measurements, the interpretation of the gathered data is, at least, as difficult. Therefore, in order to access the physical values that will ultimately be used to determine a convincing growth mechanism (e.g., flux of radicals, ions, sticking coefficients, etc.), the use of theoretical calculations such as those based on DFT can provide valuable support. It is clear that the research community has understood the strong need for detailed knowledge of plasma chemistry as revealed by the numerous publications on this topic. Nevertheless, effort is still required to go further in the direction of a quantitative description of this complex medium. In our opinion, effort is especially needed to better evaluate the surface reactivity of neutral and ionic film-forming species with the growing film. The knowledge of this essential physical parameter will ultimately makes possible the development of a new generation of models allowing for the design of experiments in view of a given coating application. The wise choice and even the design of precursors that can be driven by diagnostic tools and theoretical calculation as demonstrated in some work reported in this review are a good examples of the future knowledge-driven development of the plasma polymerization process. The future will also certainly see the confirmation of the rise of atmospheric plasma polymerization processes. We intentionally did not consider these processes in the present review, but we are clearly aware that they potentially represent the next step of development of this technology. Therefore, it is clear that diagnostic approaches, specific to the atmospheric pressure plasma polymerization processes, should also be on the “to-do” list of the scientists active in the field.

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