Atmospheric plasmas for thin film deposition: A critical review

Atmospheric plasmas for thin film deposition: A critical review

Thin Solid Films 520 (2012) 4219–4236 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage:

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Thin Solid Films 520 (2012) 4219–4236

Contents lists available at SciVerse ScienceDirect

Thin Solid Films journal homepage:

Critical Review

Atmospheric plasmas for thin film deposition: A critical review Delphine Merche, Nicolas Vandencasteele, François Reniers ⁎ Université Libre de Bruxelles, Faculty of Sciences, Analytical and Interfacial Chemistry Department, Avenue F. D. Roosevelt 50, B-1050 Brussels, Belgium

a r t i c l e

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Available online 21 January 2012 Keywords: Atmospheric plasma Deposition APECVD Organic coatings Inorganic coatings Hybrid coatings Plasma-polymerization

a b s t r a c t An overview of the possibilities of atmospheric plasma for the deposition of inorganic and organic coatings is presented. Some particularities of the atmospheric discharges and their consequences on the synthesis of films are presented and discussed. © 2012 Published by Elsevier B.V.

Contents 1. Introduction and historical background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Atmospheric pressure plasma for the deposition of coatings. . . . . . . . . . . . . . . . . . . . . . . 3. Basic facts about atmospheric plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. The pressure–distance constraint in atmospheric plasma . . . . . . . . . . . . . . . . . . . . . . . . 5. Coatings deposited by atmospheric plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Synthesis of hybrid coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Co-deposition: organic sulfonated membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Polymerization using atmospheric pressure plasmas: general mechanisms and experimental parameters. . 9. Effect of the power on coatings properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Effect of the nature of the plasma gas and of the nature of the substrate material on the chemical structure 11. Post-discharge or “in discharge” plasma polymerization? . . . . . . . . . . . . . . . . . . . . . . . . 12. Pulsed plasmas at atmospheric pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13. Effect of the injection mode on the resulting chemistry of a coating . . . . . . . . . . . . . . . . . . . 14. Comparison: coatings under vacuum/liquid/plasma. . . . . . . . . . . . . . . . . . . . . . . . . . . 15. Deposition rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16. Nucleation in the gas phase or at the gas substrate interface . . . . . . . . . . . . . . . . . . . . . . 17. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction and historical background Plasmas, a word proposed by Langmuir [1], are often called the 4th state of matter and represent 97% of the universe. A commonly accepted definition is that a plasma is a partially or fully ionized gas. Although at the beginning, plasmas were considered as a topic of fundamental research for physicists; very quickly, the energy available in a plasma discharge was thought to be useful for applications. In parallel, the deposition of coatings has always been a technological and ⁎ Corresponding author. E-mail address: [email protected] (F. Reniers). 0040-6090/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.tsf.2012.01.026

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fundamental challenge for academia and industry. More specifically, in order to modify the surface properties of a material, scientists have developed many technologies to deposit the appropriate coatings on essentially every substrate. The challenges are numerous: control of the chemistry and structure of the layer, adhesion of the layer on a substrate, deposition rate, geometric concerns, minimizing the energy injected…. It was therefore normal that sooner or later, the energy available in a plasma discharge was used to help depositing a coating. In the first part of this introduction, we will briefly describe some other major existing deposition techniques. We will then introduce atmospheric plasmas, and basic concerns about plasmas in general, while stressing the particularities of atmospheric plasmas.


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Fig. 3. General setup of an industrial in-line atmospheric plasma deposition system.

The core part of this paper will present a list of typical coatings that can be deposited at atmospheric pressure, followed by some particular applications. Then, some specificities of atmospheric plasma deposition will be presented and discussed. Although the history of the deposition of thin films goes back to the mid-20th century, where physical (PVD) and chemical vapor (CVD) deposition techniques were developed, Michael Faraday was using PVD to deposit coatings as far back as 1838 [2]. PVD is a variety of vacuum deposition technologies involving a physical process to deposit thin coatings by the condensation of a vaporized form (transportable vapor) of a solid precursor. There are different variants of PVD, including cathodic arc deposition, electron beam, thermal evaporation deposition (where the material to be deposited, often a metal, is heated to a high vapor pressure by electrically resistive heating in “low” vacuum), pulsed laser deposition, and sputter deposition, this latter using a glow discharge plasma. Glow discharge DC sputtering technique has been used to deposit conductive (mostly metals and alloys) films on substrates, from the ion bombardment of a metal target. By heating the substrate, and carefully controlling the deposition conditions (mostly the deposition rate), one could obtain dense films, sometimes crystalline and epitaxially grown [3–6]. The radio frequency (RF) sputtering technique was used to deposit films from a sputtered non-conductive target. Variations of these techniques were realized by introducing a reactive gas in the previously inert atmosphere (reactive sputtering).

In such conditions, one could deposit carbides [7–11], nitrides [11–14] and oxides [14–19]. Typical operating conditions for the deposition of hard coatings (such as CrN) are a vacuum ranging from 10 2 to 10 − 1 Pa in a background of inert gas (such as Ar), in the presence of nitrogen. Typical cathode voltages are in the range of 1 to 4 kV. Being non-equilibrium deposition conditions, metastable phase thin films can be deposited by such techniques. These PVD deposition techniques had, however, some major drawbacks; namely, a low deposition rate, in the range of 1 nm/s, the requirement of a clean low pressure background vacuum, combined with a relatively “high” working pressure from 1 Pa up to 100 Pa necessary to sustain a stable and intense glow discharge, and the use of high quality sputtering targets. Subsequent annealing time was also a major issue. In order to circumvent the low deposition rate problem, (reactive) magnetron sputtering technique has been developed. By the addition of magnets to the cathode, the trapping of the electrons just above it was greatly increased, leading to a much higher ionization yield. As a consequence, the sputtering rate of the target is much higher, and the discharge voltage lower (~500 V) [20]. Using magnetron sputtering, typical deposition rates for metals of 10–100 nm/s and more can be achieved. Moreover, magnetron sputtering operates at lower pressure than the conventional diode discharge (typically 10 − 2 Pa), which leads to higher purity final coatings. By adding a reactive gas to the discharge, one can also deposit complex compounds such as oxides and mixed compounds starting from a simple metal target [21,22]. The use of two cathodes can lead to co-deposition [23]. A great amount of research has been done in (reactive) magnetron sputtering and this technique has now left the laboratory scale for big industrial plants. One of the major drawbacks of the magnetron sputtering technique is the possible poisoning of the cathode when operating in the reactive mode. The poisoning of the cathode occurs when the cathode surface is chemically modified by the reactive gas present in the plasma. This can lead to changes of the film stoichiometry, and decrease of the deposition rate. DC, RF, and magnetron sputtering operate in low pressure conditions, making their industrial scaling a technological challenge when large substrates have to be coated. As this paper is not dedicated to these techniques, we refer to the literature for more information [24–27]. However, coatings obtained using these techniques can be

Fig. 2. Number of hits in “ISI web of knowledge” for the words “atmospheric plasma” and “deposition or coatings”, as recorded in December 2011.

Fig. 4. Classification of plasmas in terms of density and energy for different Debye lengths (λD) [55].

Fig. 1. « Historical » Siemens ozone process using the dielectric barrier discharge (DBD) technology, in 1857, extracted from Kogelschatz (above) [43]. Schema and picture of the Siemens ozone process using DBD technology for water treatment (below) (from Kogelschatz) [43].

D. Merche et al. / Thin Solid Films 520 (2012) 4219–4236


Fig. 7. A few example schemes for dielectric barrier discharge set up [43].

2. Atmospheric pressure plasma for the deposition of coatings

Fig. 5. Evolution of the temperature of the electrons (Te) and the heavy particles (Tg) as a function of the total pressure in the plasma.

of superior quality, showing excellent crystallinity and purity. The reactive magnetron technique is widely used in industry these days. In chemical vapor deposition, the gaseous precursor (or vapor of a liquid carried by gas) reacts with another molecule in the gas phase, most often at high temperature, or with the heated substrate to create the molecule of interest that will deposit to form the coating. Typical examples are carbon nanotubes [28], ZnO [29–31], diamond [32–34], TiO2[35,36], and SnO2[37,38]. Chemical vapor deposition is an easy to operate technique that does not require complicated setup. However, thermal constraints are often a disadvantage (energy cost to heat the gas phase, difficulty to deposit onto low melting point polymers…). For example, in a study by Chakraborty [39] on the growth of diamond nanoparticles by CVD, the substrate is heated to 1100 K. For the deposition of SnO2 films, substrates are heated to several hundred K [37,38]. ZnO and SnO2 are transparent conductors, they can be used in solar cell and as transparent electrodes in displays which explains their growing popularity these last few years. For more information we refer to the literature [40,41]. In PECVD (Plasma Enhanced Chemical Vapor Deposition), like in CVD process, the precursor is introduced to the reaction chamber in the gaseous state. The assistance of the plasma reactive media for the dissociation of the precursor allows the process to proceed at much lower temperatures compared to CVD, in which the gas and surface reactions occur by thermal activation. Most of the PECVD processes (organic or inorganic) are conducted at low pressure in order to stabilize the discharge plasma by increasing the mean free path of the plasma species. But recently, there has been increasing interest to operate these technologies under atmospheric pressure. This paper will focus specifically on “atmospheric pressure plasma enhanced (or assisted) chemical vapor deposition”.

Fig. 6. Schematic illustrations of a discharge current as a function of the duration of the discharge.

The purpose of this paper is not to give an extensive list of every coating that can be deposited using atmospheric plasma. We will only present selected examples where the effect of the plasma parameters (pressure, gas, frequency…) have been studied. Similarly, our goal is also certainly not to give a full description of the atmospheric plasma technology, as it has already been reported elsewhere. For instance, Tendero et al. [42] presented a full review about atmospheric plasmas and their applications in 2006, Kogelschatz [43] focused on presenting the dielectric barrier discharges (DBDs) and their applications in a review paper. Bogaerts, in 2002 [44], published a nice review about gas discharges and their applications, describing the different low and high pressure plasma systems. Napartovitch presented an overview of atmospheric-pressure discharges producing non-thermal plasmas [45]. In 2010, Bárdos [46] presented a review of the cold atmospheric plasma sources, processes, and applications and, in 2011, Pappas published a paper entitled “Status and potential of atmospheric plasma processing of materials” [47]. Although, all of these reviews present the deposition process as one of the applications, none of them focus specifically on this item, and on the particularities of atmospheric plasma on the resulting coating, which will be the topic of this review. Although the use of atmospheric plasmas discharges has expanded enormously since the late 1990s, atmospheric plasmas have been known for a long time already. Indeed the "Siemens ozone process" (for the generation of ozone from air or oxygen) was developed in 1857 [48]. A schematic picture of the instrument is presented in Fig. 1. Although the topic of this review is to discuss the recent developments in atmospheric plasma deposition of coatings, the use of atmospheric pressure discharges to deposit materials is not new. The first experiments were reported in 1796 [49] followed by many others in the late 19th century [50]. But recent advancements in technology, characterization of the film [51,52] and the plasma [53], as well as improvement of the electrical generators [54] have opened new horizons for these techniques, thus renewing the interest of the scientific community. Therefore, in recent years, researchers have been able to transfer the knowledge developed for film deposition at low pressure to atmospheric pressure systems. There is an obvious pragmatic interest in the wish to develop atmospheric plasma as a coating technology, or as a technology to assist coating. Similar to vacuum plasma techniques, they allow avoidance of the use of

Fig. 8. Schematic of an atmospheric RF discharge used for the deposition of SiO2 (from Babayan) [60].


D. Merche et al. / Thin Solid Films 520 (2012) 4219–4236

Fig. 9. Comparison of current (red)–voltage (blue) curves for a pure He DBD discharge, and He discharge with 2% and 10% O2 with both electrodes covered with a dielectric, and for a 100% O2 discharge with one electrode not covered with a dielectric [74].

organic solvents (often not environmentally friendly), and they have a relatively good energetic yield (at least for the cold atmospheric plasma, where only the electrons are energetic). Moreover, contrary to vacuum plasmas, atmospheric plasmas avoid the use of expensive pumping systems, the building of complex transfer chambers from air to vacuum and vice versa, with a high pressure gap. The plasma parameters, such as current, power, gas flow and composition, and voltage, are relatively easy to control. They can therefore easily be implemented on an industrial line, provided the reaction mechanisms are well understood.

This domain of research is indeed strongly growing, as a review of the literature shows in Fig. 2. This graph shows the number of hits in the ISI web of knowledge for the combined words in topics “atmospheric plasma” and “deposition” or “coatings”. The trend is clear. Interestingly, one should add that more than 500 patents have been deposited on this very specific subject (based on an “espacenet” search, with the items “atmospheric plasma” and “coating”), indicating that there is a huge demand from the industrial world. We stress that for both of these analyses (papers published and patents), we

D. Merche et al. / Thin Solid Films 520 (2012) 4219–4236 Table 1 Bond energies of various species commonly encountered [75]. Bond or gas molecule

Bond or dissociation energy (eV)

C\C C_C C\H C\O C_O C\F H2 N2 O2

3.65 6.35 4.3 3.74 7.78 5.35 4.5 9.8 5.1

restricted the search to the above mentioned words, neglecting all other related expressions. This paper will discuss the current state of the art, and emphasize the particularities and the challenges present in the plasma deposition of materials using cold atmospheric plasma. Indeed, one of the advantages of cold atmospheric plasmas is that they can be applied to any kind of substrate: from steel or glass, to highly temperature sensitive materials such as plastics, or even textiles [55]. If atmospheric plasmas can be used for plasma polymerization (or plasma synthesis of coatings), they can also be used to help the adherence of the coating on the substrate through plasma activation [56–58]. The substrate can be a metal, a polymer or an inorganic substrate, such as glass or silicon, or even textiles. In an in-line system, they can even be combined, as represented in Fig. 3. 3. Basic facts about atmospheric plasmas Since a plasma is defined as a partly or totally ionized gas, an atmospheric plasma refers to such media developed at ambient pressure. In some cases, the word atmospheric is replaced by “high pressure” plasma, which, to our opinion may be confusing as some plasmas used in physics are really at high pressure (hundreds of bars). Other terms used are “atmospheric glow discharge”, or “atmospheric plasma jet”. The literature often refers to the temperature of the plasma. This term is ambiguous, as plasmas are composed of neutrals, ions, and electrons, each of them having a different temperature. A more rigorous separation would deal with the plasma at local thermal equilibrium – LTE (where all species are, on average, at the same temperature), and with the plasma at non local thermal equilibrium – non-LTE (where the temperatures of the “heavy” particles, neutrals and ions, is much lower than the temperature of the electrons). The electron density in LTE plasma is high, in the range of 1021–1026 m − 3, whereas the electron density in non-LTE plasma is lower than 1019 m − 3. In this electrified gas phase, the species that are the most influenced by the electric field are the electrons, as they are charged and have a mass considerably lower than those of neutrals and ions (often referred to as the “heavy” particles). Consequently, the vector for the energy transfer in the plasma is the electrons. Electrons can cause inelastic and elastic collisions with the heavy particles. The inelastic collisions will create the plasma reactive species and are responsible for the plasma chemistry, whereas the elastic collisions will be responsible for heating the heavy particles. Because of the difference in electron density, there are much fewer elastic collisions in non-LTE plasma, which inhibits the heating of the gas. The different plasma types are usually classified as in Fig. 4. The discussion about temperature and plasma classification presented above are, of course, valid for all plasmas, low or high pressure, cold or hot. However, a particularity that must be stressed is that, under normal conditions, at atmospheric pressure plasma should be at thermal


equilibrium. Indeed, a decrease in the electron mean free path with a corresponding increase in pressure will result in an increase of the number of collisions (including elastic collisions). This will, in turn, lead to a heating of the gas, as represented in Fig. 5. An arc plasma is a typical example of an LTE plasma. In order to avoid reaching this state, a common approach is to reduce the number of collisions by limiting the time of the discharge (i.e. limiting the time during which the electrons are accelerated by the electric field in a single direction), as shown in Fig. 6. The technical tricks to avoid arcs are well known in the atmospheric plasma community today. They consist in applying pulses to the electrodes (pulsed voltage, AC or DC), by alternating the polarities of the electrodes with a high frequency generator, and/or by placing at least one dielectric material between the electrodes. The effect of the dielectric is to provoke an accumulation of electrons at one electrode, decreasing the apparent voltage difference between the electrodes, which, in turn, decreases the electric field responsible for the acceleration of the electrons and the subsequent collisions. Such plasmas are called dielectric barrier discharges (DBD) and they are used nowadays in most applications for plasma deposition. A schematic of a DBD is presented in Fig. 7. Other atmospheric plasma technologies developed for the deposition of coatings are capacitive radiofrequency discharges [60] or microwave plasmas [61]. A schematic of a capacitive RF discharge is presented in Fig. 8. More general information about plasma can be found in these text books [59,62–64]. Kurosawa et al. [65–67] have developed micro-discharges for micro-plasma polymerization, which are obtained by a very high frequency (438 MHz) micro-plasma jet machine at atmospheric pressure (in a thin quartz capillary, 1.5 mm in diameter). They used the micro-discharges for the local synthesis of very thin plasma polymerized-PS coatings for bio-sensors and chemical-sensors (without masks) applications. The automation of the micro-plasma jet machine allows patterns to be drawn (line art painting). By using plasma gases (i.e., noble gases like He and Ar, or N2, or air), it is possible to obtain glow and filamentary (streamers) discharges at atmospheric pressure. Despite its name, the glow discharge will never be perfectly homogeneous, such as a low pressure glow discharge, except for the case of pure helium. For this reason, helium is usually preferred for fundamental laboratory studies. Moreover, it obviously does not react to make secondary products, contrary to less noble gases such as nitrogen. The homogeneity of the He discharge is due to the existence of its high energy metastables (19.82 eV for 2 3S and 20.62 eV for 2 1S) [68,69]. The helium 2 3S state has a lifetime of 7900 s [70].

Fig. 10. Typical breakdown voltage curves for different gases between two parallel plate electrodes. p = pressure and d = electrode separation. Data taken from [76].


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Table 2 List of thin film deposited by atmospheric plasma. Coating


Precursor(s) used




SiO2 SiO2 SiO2 SiO2 SiO2





CeOx SiNx Fluoroalkyl silanes

DLC (Diamond-like carbon)

DLC Carbon nanotubes CxHy(Oz) CxHy

PS (Polystyrene)

PEG (Polyethylene glycol)

PEG (Polyethylene glycol)

PMMA (polymethyl methacrylate) PAA (polyacrylic acid)

PTFE-like PTFE-like



From pure SiO2 to SiOxCyHz depending on HMDSO/N2O ratio. DBD HMDSO/Ar or Ar + air Faster in Ar, but carbon contaminated. Slower with Ar + air, but SiO2. DBD HMDSO/N2 + Air (+O2) or controlled N2 Evidence of the oxygen role in the open atmosphere air reactor. DBD HMDSO or BTSE/Ar or Ar + O2 Evidence of the covalent bond between the surface and the SiOx coating. RF capacitive torch HMDSO, TMDSO, HMDSN, TMCTS, TEOS/ Glass coatings on plastics without carbon He + 2% O2 contamination. Atmospheric pressure TEOS/He + O2 Substrate temperature from 388 K to plasma jet 623 K. Comparable properties (FTIRa, capacitance) to those of thermally grown SiO2 films at 1173 K. Atmospheric microplasma HMDSO/Ar At low flow rate (b 0.1 sccm), no carbon jet contamination. At higher flow rate, 21% C contamination. O2 not necessary to obtain inorganic films. DBD TiCl4/N2O Deposition under a wide range of working pressure (200 Pa to atmospheric pressure). Higher photocatalytic activity for films deposited under atmospheric pressure. Single and multi plasma jet Titanium (IV) diisopropoxide bis(2,2,6,6- Influence of the substrate (quartz, Si, Ni) tetramethyl-3,5-heptanedionate [Ti(O-i- on the physical properties of the layers. Pr)2(thd)2]/He + N2 + OESb. SnOx deposited from MBTC are soluble in DBD Monobutyltin trichloride C4H9SnCl3 (MBCT) or Tetrabutyltin C16H36Sn (TBT)/ water, contrary to those deposited from TBT precursors. Amorphous coatings air with small concentrations of SnO2, SnO, and Sn crystalline phases. RF plasma jet Cerium acetate (CH3COOH)3Ce in H2O CeOx films contaminated by carbon. spray RF capacitive torch SiH4/N2/He Substrate temperature ranging from 373 K to 773 K. Precursor dissolved in isooctane and RF capacitive torch C8F17C2H2Si(OC2H5)3 (l)/He + N2 spun onto the substrate before the atmospheric plasma exposition. RF capacitive torch Acetylene/H2/He Deposition rates ~ 0.10 μm/min at substrate temperatures ~ 473 K. Solid 13 C NMRc study. DBD Methane or acetylene/H2 Coatings are close to those ones obtained by PECVD at low pressure (b 10 Torr). Atmospheric pressure Acetylene/He Vertical growth of CNT on metal nuclei plasma jet (Fe, Ni). DBD Ethylene, Ethylene oxide, Propylene/Ar Appearance and chemical structure or He different if deposited in Ar or He. DBD Acetylene Morphology and deposition rates versus discharge mode (diffuse-filamentarycorona). DBD and RF capacitive Styrene/Ar or He Films of better quality with the DBD. torch Influence of the carrier gas on the chemical structure. RF capacitive torch Tetraethylene glycol dimethyl ether Low power necessary to maintain the (tetraglyme) vapor or aerosol/Ar ether (C\O\C) functionalities for biocompatibility (if vapor precursor). Tetraethylene glycol dimethyl ether Control of the monomer fragmentation (tetraglyme) aerosol/He by controlling the plasma power and the gas feed. DBD MMA/He Study of the deposition rate. DBD AA (acrylic acid)/He IR/UV–vis spectrometry. AA (acrylic acid) New spray method. DBD (+EHDA — electrohydro dynamic atomization) DBD and HF torch CxFyAz (with A : heteroatom) + He or Ar CF2 fingerprint in XPSd + 110° contact angle. For C3F6/He mixtures, the ratio F/C ~ 1.5, DBD C3F6/He or C3F8/He + H2 whereas for C3F8/He + H2, the ratio ranged from 0.6 to 1.5 depending of the H2 gas feed. Variation of the cross-linking and the polymerization rates with the hydrogen feed. DBD CF4, C2F6, C2H2F4, C3F8, C3HF7 and c-C4F8/ Correlation between the plasma Ar + N2 or air diagnostic (OESb, OASe) and the films properties.

Reference Massines [70] Morent [86] Zhu [98] Batan [84] Moravej [85] Babayan [60]

Raballand [99]

Zhang [91]

Kment [90]

Korotkov [100]

Soukup [101] Moravej [85] Barankin [102]

Ladwig [97]

Bugaev [96] Lee et Overzet et al. [95] Goossens [103] Jiang [104]

Merche [105]

Nisol [106]

Da Ponte [107]

De Geyter [93] Topala [94] Tatoulian [108]

Reniers [109] Fanelli [110]

Vinogradov [111]

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Table 2 (continued) Coating


Precursor(s) used




C4Cl6 or C2H2Cl4






C8H8 + CF3SO3H/Ar or He

Ratio Cl/C ~ 1. C\Cl and C\Cl2 bonds Hubert [112] observed by XPSd. Coatings were significantly different Heyse [113] from those deposited in vacuum plasmas. Merche [114] Good preservation of the sulfonic acid groups grafted inside the entire membrane.

a b c d e


Fourier Transform Infrared Spectroscopy. Optical Emission spectroscopy. Nuclear Magnetic Resonance. X-ray photoelectron spectroscopy. Optical absorption spectroscopy.

The metastable states of helium can be reached by collision with energetic electrons: −


He þ e →He þ 2e −

He þ e →He þ e 


2He →He2 þ e 

He þ


ð1Þ ð2Þ ð3Þ

þ He:


Where the He* is the 2 3S1 state, with a potential energy of 19.82 eV and the He2* molecular metastable has an energy of 18– 19 eV. These high energy states cannot return to the ground state by direct photoemission, they usually do so by transfer of energy by collision. For other applications, argon is an alternative of choice, as it also possesses high-energy metastable states (11.5 eV for ( 3P2) and 11.7 eV for ( 3P0) [71,72]). Argon metastables have a shorter lifetime than helium, but the availability of argon makes it much more interesting for industrial applications, although it is more difficult to obtain a homogeneous discharge with Ar. The existence of neutral metastable states is of huge importance for the homogeneity of the discharge. Indeed, these atoms, being formed in a microchannel in the discharge, are not sensitive to the electric field and they can travel freely in the plasma. They can later decay by a collision generating a new electron that will in turn induce ionization. This homogenization effect is of enormous importance for the deposition of coatings, as it insures a good distribution of the energy within the entire discharge. The effect is not observed in air and with corona discharges for instance. A good homogeneity of the discharge will help to obtain, in turn, a homogeneous coating. Moreover the existence of metastables, and the presence of other excited species of the plasma gas will induce a

major difference between the deposition mechanisms of coatings at atmospheric pressure and those at low pressure: the metastables and the other excited species can collide with precursor molecules, to initiate the nucleation and growth mechanisms, such as electrons are doing at low pressure. The following equations show the typical Penning effect of a metastable (M*) on a diatomic molecule (A2): 



M þ A2 →M þ A2 þ e

ð5Þ −

M þ A2 →M þ A þ A þ e :


To illustrate the change in homogeneity of a discharge with the gas composition and the electrode nature, Fig. 9 shows the evolution of the current–voltage curves for a DBD with 2 dielectrics and filled with pure He and contaminated with up to 10% oxygen. The last curve shows results of a 100% O2 discharge with only one electrode covered with a dielectric. One can clearly see the changes in the homogeneity of the discharge both in the images and in the current vs. time curves. A filamentary discharge yields multiple and very short current pulses (one for each streamer), while a homogeneous glow discharge has only one large current peak per half period of voltage, like for pure He. The other discharges are filamentary (even if they look homogeneous by eye). The oxygen discharge is a fully filamentary discharge. The proportion of impurities plays an important role because it controls the density of metastables species at the time of discharge ignition. Oxygen (like N2) disturbs the discharge by quenching the He metastables which stabilize the discharge [73]. The presence of two dielectrics (instead of one in the case of pure oxygen) also contributes to render the discharge more homogeneous. Most chemical bonds can be broken by these metastable neutral atoms. One should also mention that O2 and N2 also present metastable states, but they are of lower energy and/or lower lifetime. Table 1

Fig. 11. Commonly used monomers for the deposition of SiO2 coatings. With TMCTS = tetramethylcyclotetrasiloxane, HMDSO = hexamethyldisiloxane, HMDSN = hexamethyldisilazane, TEOS = tetraethoxysilane, and TMDSO = tetramethyldisiloxane.


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Fig. 12. (a) Secondary Electron Microscopy (SEM) image of a well-aligned Multiwall Carbon nanotubes (MWCNT) forest on an Fe film with the Atmospheric Pressure Plasma Jet (APPJ) off (inset, high resolution SEM image), and (b) SEM image of a well-aligned MWCNT forest on an Fe film with the APPJ on (inset, high resolution SEM image) [95].

gives the bond energy (or dissociation energy) for a few bonds or gases. 4. The pressure–distance constraint in atmospheric plasma One of the major drawbacks of atmospheric plasma is the very strong dependence of the voltage that one should apply to the electrodes with the pressure and the distance, as shown in Fig. 10. This is expressed by the Paschen law: V breakdown ¼

Bpd C þ lnðp  dÞ


With: d p B and C

inter-electrode spacing pressure constants depending of the nature of the gas and the electrode material.

Eq. (7) and Fig. 10 show that the breakdown voltage strongly depends on the nature of the plasma gas, and on the inter-electrode spacing, the pressure being fixed when operating at atmospheric pressure. This limits the geometry that can be used in plasma reactors, as the inter-electrode spacing must remain small, typically in the range of a few mm, in order to avoid elevated breakdown voltages. It also reveals that the use of air is generally not recommended if one wants to reduce energy consumption, as its breakdown voltage

Fig. 13. Schematic of the DBD system used for the deposition of plasma polymerized sulfonated polystyrene [114].

is considerably higher than that of noble gases, for example. This is another reason why many studies dealing with atmospheric plasmas are realized with a noble gas, whether for surface activation or PECVD of inorganic or organic coatings (plasma-polymerization). These limitations have also contributed to the development of post-discharge deposition processes, where the substrate can be placed further away, and does not have to be inserted in the tiny inter-electrode gap. 5. Coatings deposited by atmospheric plasmas Table 2 presents a non-exhaustive list of coatings that were deposited using atmospheric plasma technology. The choice of these coatings is mostly dictated by the industrial applications. The first column identifies the coating obtained, and the second describes the technology used (mostly DBD). The third column lists the most important results of the paper. For inorganic coatings, silicon oxide coatings are certainly the most studied. Indeed, silica — like coatings are of great industrial interest, as silicon oxide layers have many valuable chemical, thermal, and optical properties such as being a good permeation barrier to gas diffusion (when dense enough) and good wettability (when covered by silanol groups). They can also be used in the fabrication of photovoltaic solar cells, as layers resisting corrosion, and in precision engineering (aeronautical and automotive). These coatings can be deposited by magnetron sputtering [77–79], chemical vapor deposition [80–82], and now by atmospheric plasma [70,83–86]. In all these techniques, a precursor monomer is used and reacts with oxygen in the gas phase. The most frequently used monomers are presented in Fig. 11. All these precursors are characterized by their symmetrical chemical structure which results in a low enthalpy of vaporization (because of the weak intermolecular interactions) and by the fact that they all contain silicon already bonded to oxygen, except for

Fig. 14. Yasuda's mechanism for plasma polymerization at low pressure [120].

D. Merche et al. / Thin Solid Films 520 (2012) 4219–4236


TiO2 was also deposited using an atmospheric plasma torch [90], and by a DBD discharge [91]. Matsumoto [92] deposited silicon nitride coatings by dielectric barrier discharge starting from silane, nitrogen, and hydrogen. They present optical applications (as for the fabrication photovoltaic fuel cells), and excellent barrier properties; they are also used for the fabrication of electronic components. For organic coatings, acrylates are similarly very popular. Coatings issued from acrylates can be used in a large domain of applications, as resistant layers to electron beams, membranes for gas separation, humidity sensors, optical components, etc. [93,94], and they are often the starting point for the post-deposition of other materials, due to their good adhesion properties. More exotic coatings like nanotube forests [95] as shown in Fig. 12, or diamond-like carbon (DLC) are nowadays also deposited by atmospheric plasma [96,97]. The DLC coatings (doped or not) deposited on a variety of industrial components (on metals, glasses, ceramics, plastics) provide protection due to their high hardness, and can be used as electrical passivation layers. 6. Synthesis of hybrid coatings Fig. 15. Schematic comparison between conventional polymers and plasma polymers [125].

HMDSN. For all these precursors, like for low pressure deposition and for CVD, oxygen must be added in the reacting plasma phase to reach the stoichiometric SiO2 composition and to efficiently remove the remaining carbonaceous contaminants. Amongst them, HMDSO and TEOS are the most popular nowadays. In atmospheric plasma, the main parameters to be controlled are the relative ratios among the monomer flux, the oxygen flux, and the electron density. Titanium oxide is also a coating bringing more and more interest, as the anatase and rutile phases are widely used for photocatalysts, chemical sensors, solar cells, dielectric materials, etc. More generally, TiO2 coatings can exhibit self-cleaning properties, present an antibacterial effect, and are superhydrophilic. They can already be easily prepared by CVD or by magnetron sputtering and a few studies report their preparation by cold plasmas [87,88]. Nie et al. [89] managed to synthesize nanocrystalline anatase TiO2 using TiCl4 and O2 as precursors by atmospheric cold plasmas in a DBD without extra heating or thermal treatment. Only one phase, anatase (with a nanocrystal size about 10–15 nm), was observed by the means of X-Ray Diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), and SAED (Selected Area Electron Diffraction) measurements.

The possibility to work at atmospheric pressure makes it easier also to combine two precursors in different phases (liquid or gas + solid). Atmospheric plasmas can also be used easily to synthesize hybrid multifunctional inorganic coatings. For instance, Bardon et al. [115] dispersed solid AlCeO3 nanoparticles in a hexamethyldisiloxane solution (HMDSO), and then sprayed this mixture in an aerosol form into the gas flow of a DBD system. The cerium-based nanoparticles inserted in an organosilicon plasma coating were deposited on steel for anticorrosion purposes. 7. Co-deposition: organic sulfonated membranes As already mentioned, atmospheric plasma systems allow simultaneously deposition of more than one molecule, allowing the formation of hybrid, multifunctional compounds. Other techniques such as reactive magnetron sputtering [116] or magnetron sputtering with multiple targets also allow to do so [23], but they are usually restricted to inorganic materials and metals, and organic polymers are prohibited. As an example, we recently synthesized sulfonated polystyrene at atmospheric pressure in one step starting from styrene and trifluoromethane sulfonic acid monomers using a DBD plasma chamber. This could be a major advance in the synthesis of complex, multifunctional membranes that are currently made by conventional organic chemistry, requiring time consuming multiple steps, organic non-eco-

Fig. 16. CAP-model for plasma-polymerization [124].


D. Merche et al. / Thin Solid Films 520 (2012) 4219–4236

Fig. 17. (Left) pp-sulfonated polystyrene synthesized by DBD with He as plasma gas; (right) with Argon as plasma gas.

friendly solvents, and delicate operations. The plasma polymerizedsulfonated PS can be used as proton exchange membranes for miniaturized PEMFC (Polymer Electrolyte Membrane Fuel Cell) using hydrogen or methanol. XPS results have shown that the sulfonated groups (ionizable groups) content is tunable by varying the temperature of the acid monomer (so the acid-to-styrene ratio) or by increasing the voltage (leading to more radical sites available for the grafting of sulfonated groups). The simultaneous injection of the both precursors allows an homogeneous distribution of the sulfonated groups inside the whole membrane thickness, as observed by dynamic Secondary Ion Mass Spectrometry (dynamic SIMS) which is necessary for its good conductivity. The schematic of the system used in this case is presented in Fig. 13. Atmospheric plasma systems can also be successfully used for the deposition of hybrid coatings by introducing vapor of a hybrid organic–inorganic monomer (as vinyltriethoxysilane) and an organic monomer (such as hexene). These synthesized coatings present excellent barrier properties due the synergistic effect of the organic and inorganic network [117]. Hody et al. [118] used a similar DBD system for the synthesis of functionalized hybrid coatings carrying carboxylic groups (able to create covalent bonds with various chemical groups) from the simultaneous injection of vinyltrimethoxysilane and maleic anhydride.

8. Polymerization using atmospheric pressure plasmas: general mechanisms and experimental parameters

pressure processes. However, at low pressure, the monomer is often injected alone in the plasma (especially for highly volatile monomers), whereas at atmospheric pressure, the use of a plasma gas is most often required. Note that the presence of a plasma gas is necessary for plasma polymerization at low pressure, when the monomer is introduced in the afterglow region. The polymerization process is induced by the formation of an activated species, usually in the radical form, in the gas phase. This radical is typically generated by a collision with a high-energy particle (electrons, ions, or metastables) or can be generated by irradiation using UV light. A bond in the precursor is broken, generating active radicals that can polymerize. The plasma polymerization process that takes place at atmospheric pressure is usually considered as being almost similar to the one proposed by Yasuda [119] for low pressure plasmas (Fig. 14). Other mechanisms for plasma polymerization have been proposed, for instance by Tibbitt [121], Kobayashi [122] and Stille [123]. A critical review of the mechanisms for plasma polymerization was recently presented by Friedrich [50]. A major difference with the processes operating at low pressure is the “plasma” excitation step. At low pressure, the excitation mechanism leading to the activated monomer (Mi• or Mk• ) is mostly due to collisions between the monomer (Mi) with an electron inducing the breaking of a chemical bond leading to two radicals: −

Mi þ e →Mj þ Mk þ e :


Generally, the precursors that can be used for atmospheric pressure plasma polymerization are identical to those used for low

Fig. 18. AFM 3D image obtained in tapping mode on a modified SiO2 sample deposited in He DBD. From [118].

Fig. 19. AFM 3D image recorded in tapping mode on a modified SiO2 sample deposited in Ar DBD. From [118].

D. Merche et al. / Thin Solid Films 520 (2012) 4219–4236 Table 3 Comparison between the “in-discharge” and “post-discharge” plasma deposition. In discharge

Post discharge

(Very) High deposition rates, depends on current and precursor flow.

(Relatively) Lower deposition rates, depends also on distance between substrate and plasma, and the precursor flow. No effect of inter-electrode distance.

Strong effect of inter-electrode distance. Higher reactivity: higher fragmentation of precursor. Coating and substrate exposed to high energy species from plasma (electron, ions, molecules, UV). Easy to rapidly treat large twodimension samples in-line. Good control of the atmosphere for deposition. Poisoning of the electrodes.

Lower reactivity, lower fragmentation of precursor. Coating and substrate less exposed to high energy species from plasma (mostly UV and neutrals). Ideal for small samples or three dimensional coatings. Poorer control of the atmosphere for deposition, especially in open air environment. No effect on the electrodes.

Although this mechanism is also present at high pressure; one has to also take into account reactions involving high-energy metastable atoms (or molecules) from the plasma gas: 


M þ A2 →M þ A þ A þ e :


Because plasma-polymerized coatings are often formed by random radical recombination, it is generally accepted that their structure is different from that of conventional polymers [124]. Plasma-polymers are more disordered. They are not characterized by repeating units and they can exhibit a higher degree of crosslinking and branching. A comparative schematic often seen in the literature is shown in Fig. 15. This assertion is somewhat simplistic, as it depends strongly on the operating conditions. The chemical structure and morphology of plasma-polymerized coatings depend on the internal (as (non)homogeneity of the discharge, distribution of the various species in the plasma, energy of the species…), and the external (reactor geometry, discharge power or applied voltage, frequency, total pressure, partial pressure of the monomer, flow rates…) plasma parameters. Since the external parameters influence the internal parameters, the properties of the coatings can be tailored by controlling the external parameters. Some of these parameters are presented below. 9. Effect of the power on coatings properties The chemical regularity of the plasma-polymerized coatings decreases with an increase in the effective power as described by Yasuda (for identical flow rates) [124]. Yasuda's factor is given in the following equation. E¼



increase of the radical recombination rate) in the region of “soft conditions”, and decreases after reaching a maximum, as described by the CAP model (competition ablation-polymerization) proposed by Yasuda (see Fig. 16) [120]. 10. Effect of the nature of the plasma gas and of the nature of the substrate material on the chemical structure of an organic coating In an atmospheric plasma, the choice of the nature of the main plasma gas can have large consequences on the chemistry and the structure of the resulting coating. To illustrate this, three examples are presented. The influence of the nature of the plasma gas on the chemical structure of polystyrene coatings synthesized in a DBD under atmospheric pressure was studied in [105]. Plasma-synthesized polystyrene exhibits a higher degree of unsaturation, branching, and cross-linking, and a lower density of aromatics when using argon as plasma gas, than when using helium. This effect was reinforced when the substrate was metallic (because this asymmetry generates a much more filamentary discharge). The relative change in the degree of unsaturation, branching, and cross-linking were studied by Fourier-transform infrared spectrometry (FTIR) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). These two techniques were proposed by Luo [126], Chen [127], and Oran [128]. More generally, the texture of the coating and its roughness will also depend on the nature of the plasma gas at high pressures [105]. Fig. 17 shows surface images of pp-sulfonated polystyrene realized in helium or in argon. While the left image reveals a homogeneous and smooth coating, the right one reveals a structured layer. This is induced by the change in the nature of the discharge. The heliumbased plasma is indeed more homogeneous than the argon one, which is mostly filamentary (glow discharge with many streamers). The streamers are volumes in the plasma where the density of electrons is higher, therefore where the reactivity is increased, with more radicals and ions created. These streamers are direct consequences of the presence and consumption of metastables in a high pressure plasma, as shown in Fig. 9, and will have dramatic consequences on the bulk structure of the deposited coating and its surface. The key effects are summarized below: – because of the higher reactivity, the fragmentation of the precursor will be higher, leading to a higher degree of branching and cross-linking, more unsaturation, and to a decrease of the density of aromatics (in the case of the presence of aromatic groups). – because of local higher reactivity, the deposition rate will be higher under the streamers leading to coatings of non-uniform thickness. Moreover, the deposition rate of pp-sulfonated PS is slightly higher in the presence of Ar than with He (260–280 nm/ min in the presence of He as carrier gas, vs. 360–580 nm/min


with: W F M

discharge power (J/s) monomer flow rate (μl/min) molecular weight of monomer (kg/mol).

The factor introduced by Yasuda corresponds to the energy supplied per unit of monomer. For low ratios (low discharge power and high flow rates), the structure of the coatings is closer to the conventional polymer than for high ratios, where the films are more disordered (more fragmentation). In the same way, the deposition rate increases with this ratio (more activated radicals leading to an

Fig. 20. Cross-sectional schematic of an APPLD system [Dow Corning Plasma Solutions, Ireland] [108].


D. Merche et al. / Thin Solid Films 520 (2012) 4219–4236

Fig. 21. Synthesis of plasma polymerized polyethylene glycol by injection of (left) liquid tetraglyme droplets into the post-discharge of an atmospheric RF argon plasma and (right) tetraglyme vapor in the post-discharge.

with Ar at a voltage of 1 kV). Jiang [104] showed similarly that the polymerization rate was higher along the streamers in a DBD than outside the filaments. He compared by SEM three types of microscopic structures of films corresponding to three different DBD discharges modes (a diffuse glow discharge, a “corona-like”, and a filamentary discharge). The self-organized pattern obtained under the discharge filaments can be of interest in many applications such as electronic devices fabrication, bionic materiel growth, etc. – because of the higher electron density and the presence of streamers at the growing film surface, local electron and ion bombardment may occur inducing surface damage such as pitting. Hody et al. [118] also highlighted the importance of the carrier gas nature (He or Ar) for coatings synthesized from maleic anhydride and vinyltrimethoxysilane. Coatings deposited in continuous Ar plasmas show a highly rough surface, contrary to those deposited in He. The roughness can be attributed to the inhomogeneous discharge observed in Ar. They observed particles on top of the film for deposits in the presence of He. Moreover the degree of carboxylic functionalization is higher in the presence of Ar instead of He. Fig. 18 is an AFM image of a sample deposited in He, and Fig. 19 an image of a sample deposited under the same conditions in Ar. Similarly, Goossens et al. [103] observed that polyethylene films deposited in a He DBD are different in polymer structure, from those deposited in an Ar DBD. The ones deposited with He yield a low density, sticky, and opaque polymer that can be dissolved in chloroform, whereas Ar gives a clear and more solid polymer, with good adhesion to all substrates, and not soluble in chloroform.

11. Post-discharge or “in discharge” plasma polymerization? The number of bonds broken by the high energy particles, and the number of radicals originating from the precursor, will depend on the ratio between the precursor flow and the injected power (cf. Yasuda's factor, defined above in Eq. (10)). Nevertheless, some geometrical aspects, and some non-obvious effects must be taken into account also for the polymerization process. For example, the precursor can be injected either in the discharge or in the post-discharge of the plasma. That will result in drastically different reactivities: the plasma region contains electrons and ions (which are influenced by the electrical field), metastables and UV, whereas the post-discharge region is mostly neutral, containing as reactive species mostly neutral metastable and UV photons. The geometry will also induce a difference in the polymerization process and mechanisms. Indeed, as the “in discharge” process is more reactive,

it could create more damages to the precursor than the postdischarge deposition process. This is especially true in organic polymerization, where keeping the appropriate functionality brought by the precursor is of importance. This effect is already known from low pressure processes. The low pressure polymerization of polyethylene glycol, a biocompatible material, showed that coatings deposited in the post or pre-discharge are of better quality than a coating deposited in the discharge, because the polyether character responsible for the biocompatibility property is not destroyed in the postdischarge [129]. As already shown, deposition inside the discharge may alter the quality of the final coating, as well as the substrate, because of the presence of streamers in the plasma. The progressive poisoning of the electrodes must also be taken into account when injecting the precursor into the discharge. For online industrial coaters, that could be an issue. Table 3 presents a comparison between “in-discharge” and “postdischarge” atmospheric plasma deposition. 12. Pulsed plasmas at atmospheric pressure Although most of the studies reported used continuous-wave (example: sinusoidal wave) plasma generators (high frequency from 1 to 400 kHz for the DBDs and 13.56 or 27 MHz for RF capacitive discharges), the voltage can also be applied in the form of discrete pulses (of a few milli- to microseconds). Asymmetrical pulsed RF plasma constitutes a very promising technique to enhance the controllability of plasma-polymerized film chemistry due to better control of the energy dissipation in the plasma, which leads to chemically better-defined plasma-polymer films. Because the pulsed discharge can operate at much higher peak voltages and current for the same average power, the instantaneous precursor activation (during the light phase of the cycle — ton) can be enhanced. The reaction continues during the “off” period, as described by Yasuda [119]. The radical (co)-polymerization is favored when the plasma is off during the dark phase (toff). By playing on the duty cycle (relationship between the plasma ON and the OFF times), one can control the retention of the precursor functional groups. A few examples of the use of pulsed-plasma for the plasmadeposition at atmospheric pressure are described below. Denis et al. [130] used pulsed plasmas at atmospheric pressure for the polymerization of allylamine and cyclopropylpamine. The primarily amine-based films show potential applications in the modification of filtration membranes, treatment of carbon nanotubes, biomedical applications, etc. According to Boscher et al. [131], the use of appropriate pulsedplasma conditions in a DBD enables morphological control and

D. Merche et al. / Thin Solid Films 520 (2012) 4219–4236


Fig. 22. XPS C1s peak of (left): APPLD plasma polymerized PEG coatings as a function of the plasma power; (right) PEG coatings synthesized from tetraglyme injected as a vapor in the post-discharge of the plasma [106].

improves electrochemical properties (such as polarization resistance, passivation, etc.) of HMDSO synthesized on aluminum foils (in a nitrogen atmosphere) for corrosion protection. They showed that a higher degree of cross-linking with equivalent deposition rates can be obtained, and the pulsing of the electric signal prevents the formation of pinholes and heterogeneities in the coatings. Topala [132] obtained hydrophobic polystyrene in a He discharge (DBD) by optimizing discharge current parameters (pulses, pulse shape, duty-cycle, gap length, He gas flow…). He noticed that the electric parameters of the plasma such as the voltage pulse shape (sinusoidal or ramp wave) and the rise time have a strong influence on the discharge current maximum, and this more than the other parameters, such as the distance between the electrodes or the He flow rate carrying the styrene monomer. The coatings of Hody et al. [118] deposited from maleic anhydride and vinyltrimethoxisilane by pulsed Ar plasma are less rough than

films deposited in a continuous mode. The sample roughness is probably smoothened during the “off period”. 13. Effect of the injection mode on the resulting chemistry of a coating One of the big advantages of the use of atmospheric plasma is the ability to inject liquids directly into the discharge, or into the postdischarge. This process is called atmospheric pressure plasma liquid deposition (APPLD). The “in discharge” process was developed by Badyal and Dow Corning and is presented in Fig. 20 [133]. APPLD has the advantage that large surfaces can be treated quickly, using a simple method of injection (spraying droplets of the liquid directly in the plasma), in the absence of oxygen (in a He atmosphere), which allows complex organic chemistry. The fact that the precursor is injected in the liquid form results in less degradation,


D. Merche et al. / Thin Solid Films 520 (2012) 4219–4236

Fig. 23. SEM micrograph of the cross-section of BTSE films deposited on Al by (A) wet deposition, (B) vacuum plasma, and (C) atmospheric plasma. From [83].

as the precursor is protected inside the liquid droplet from plasma degradation. In the Dow Corning process, the surface is also preactivated by the plasma before exposure to the precursor, which insures better adhesion of the deposited layer. A comparison of the precursor injected as a liquid or as a vapor has been carried out by Nisol [106], who synthesized biocompatible PEG coatings from tetra(ethylene glycol) dimethylether also known as tetraglyme. The precursor was injected as shown in Fig. 21. FTIR, XPS and ToF-SIMS results show that the APPLD process (left) maintains the polyether character (the C\O component in the C1s peak of the coating is the signature for the polyether character), whereas the vapor phase process is strongly dependent on the plasma power, as represented in Fig. 22. APPLD systems have also been used to deposit polyacrylic acid films [134]; silicon-based films [135], and pure SiO2 could be deposited from organosilicon in a He\O2 plasma using a nebulizer system [136]. Nebulizers were also used to “spray” colloid solutions of metal nanoparticles in order to decorate various surfaces [137]. 14. Comparison: coatings under vacuum/liquid/plasma

Fig. 23 shows typical FEG-SEM pictures of these 3 coatings. From Fig. 23, it can be seen that the three techniques provide dense layers, with similar thicknesses (depending on the plasma parameters). The composition of the coatings, however, are slightly different, the one deposited by atmospheric plasma being more oxidized than the two others, as shown in Table 4. This was interpreted by the fact that the atmospheric plasma was operating using a torch running with argon, in an open air environment, which results in a higher oxidation of the precursor during the condensation reaction. The adhesion of such layers deposited (on aluminum) by atmospheric plasma proved to be excellent, due to the formation of a strong interfacial bond evidenced by ToF-SIMS (see Fig. 24). When comparing sulfonated polystyrene synthesized at low or high pressure, better retention of sulfonated groups is obtained under atmospheric pressure than at low pressure (where the XPS S 2p peak is often split [139,140]). This suggests milder plasma conditions at high pressure. This can be explained by a lower mean free path of the species. If their collision frequency is higher, their energy can become too low to fragment the sulfonated groups [114]. In this specific case, in order to preserve the chemical structure in lowpressure plasma conditions, it is preferable to operate in the post-

As the use of an atmospheric technology is more recent than the well-established use of vacuum techniques, an immediate question deals with the comparison between the quality of the coatings obtained by different techniques. This is obviously difficult to do, as many parameters, in each technique can influence the final quality of the deposited layer. Sawada et al. [138] compared SiO2 films deposited from TEOS and HMDSO at atmospheric pressure to those prepared using low pressure plasma. They came to the conclusion that the films deposited by both methods (as characterized by FTIR and XPS) were very similar. Similarly, in a joint research proposal, three groups investigated the comparison between films deposited from BTSE (bis-1,2-(triethoxysilyl) ethane) by three different techniques (wet deposition, vacuum plasma deposition, and atmospheric deposition). Table 4 Comparison of the atomic composition of BTSE films using various deposition method [83]. Binding energy (eV)

Wet deposition Vacuum plasma Atmospheric plasma

Atomic concentration (%)







102.3 102.7 103.2

284.4 284.4 284.4

532.3 532.3 532.8

27.5 28 34.7

26.7 24.3 6.7

45.8 47.6 58.6

Fig. 24. Evidence of the formation of an Al\O\Si bond at the interface between an atmospheric-plasma-deposited SiOx layer and an aluminum substrate. From [84].

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Fig. 27. SiO2 deposition rate as a function of the precursor type, and the precursor partial pressure (reprinted from Moravej et al. [85] (the plasma gas is Helium, with 2% oxygen added, injected in a RF capacitive torch operating a 13.56 MHz, with a volumetric plasma power of 20 W cm− 3).

Fig. 25. High deposition speed DBD system [70].

discharge (to avoid fragmentation due to electrons) [140,141], while, due to the milder conditions in atmospheric plasma, the deposition can take place inside the discharge, leading to much higher deposition rates [140–143]. Fei et al. [144], while comparing MoSi2 coatings deposited by atmospheric and vacuum plasma spray processes, observed that the density and the homogeneity, as well as the contamination, the Vickers Hardness, and the resistance to thermal oxidation were much better for coatings deposited using vacuum plasma than for those deposited under atmospheric pressure. 15. Deposition rates One of the interests in the development of atmospheric plasma technologies is the possibility to install them on industrial coaters. In that respect, the deposition rate is a crucial parameter. Although one should always take these numbers with great care (it depends on the parameters, on the desired purity, and structure of the layers, etc.), we give here some indications extracted from recent studies. In 2005, Massines [70] obtained deposition rates of 12 nm/min, using a DBD system reproduced in Fig. 25. However, these values were obtained inside the DBD; when going to the post-discharge,

Fig. 26. Profile of the deposition rate (r) and percent of SiO2 (g) in case of N2–N2O– HMDSO coatings deposition under standard conditions as a function of position along the direction of the gas flow (the gas entrance is at position 0), and the residence time. Reprinted from: F. Massines et al. [70].

the deposition rate dropped drastically due to a decrease of the concentration of activated precursors as shown in Fig. 26. De Geyter et al. [145] reported in 2009 deposition rates of 180 to 288 nm/min for HMDSO deposition by DBD. They used very low plasma power (5 to 12 W), but the films still contain carbon-based groups. Morent [86] studying the deposition of SiOx films from HMDSO, came to the same conclusion as many groups: deposition of a coating is fast (6 nm/s) using a pure noble gas (He, Ar), but this layer is SiOxCyHz. When an oxygen-containing source is added, the deposition rate decreases (2 nm/s), but pure SiO2 coatings are obtained. In the post-discharge, a linear log–log relationship between the precursor pressure and the deposition rate is observed, as reported by Moravej (Fig. 27) [85]. Indeed, in the post-discharge, less energetic species are able to break bonds and to activate the nucleation and growth process. Therefore, the growth process will mostly take place at the gas substrate interface. A parameter which is rarely used in atmospheric plasma deposition of coatings is the plasma gas temperature. Sawada [138] already pointed out a positive effect (increase in the deposition rate when increasing the gas temperature) and we could evidence it for the deposition of SiO2 starting from HMDSO by DBD. In our experimental conditions, going from room temperature to 200 °C led to doubling the deposition rate, reaching 100 nm/s rates for ultra-pure and dense SiO2. According to Sawada, this is because the gas-phase reaction of monomer decomposition is enhanced by atomic oxygen, and the reaction rates in the gas phase increase as the gas-phase temperature increases. A side effect of the temperature is to favor the

Fig. 28. Domains of plasma polymerization reactions of styrene obtained at atmospheric pressure. Adapted from [146].


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double bonds will have very high deposition rates, while, for example, fully saturated precursors will have very small deposition rates. 16. Nucleation in the gas phase or at the gas substrate interface

Fig. 29. Thickness profiles of plasma-polymerized PS films deposited by DBD as a function of different discharge power. Adapted from [146].

elimination of volatile carbonaceous residues from the precursor, leading to purer SiO2 coatings. De Geyter [93] studied the effect of plasma DBD parameters on the deposition rate and the structure of PMMA coatings. They show that the deposition rate is, to a large extent, increasing with the monomer concentration injected in the chamber. The deposition rate is constant with time, which is a characteristic of many well-controlled plasma techniques. They also show that the deposition rate starts to increase with the plasma power, reaches a maximum, and decreases at high plasma power. Simultaneously, the plasma power changes the distribution of chemical functions in the film. Average deposition rates range from 1 to 2 nm/s. Asandulesa et al. [146] studied the influence of the flow rate and the discharge power on the polymerization rate of plasma polymerized PS coatings synthesized in a DBD (in the presence of He). They identified different domains of plasma-polymerization and showed that the polymerization rate increases with the ratio W/FM in the energy deficient region (soft conditions) and decreases (after a critical point) in the monomer deficient region, as shown in Fig. 28. The increase of the thickness (constant deposition time) for pp-PS coatings with the power is presented in Fig. 29. More generally, for the synthesis of organic coatings, the deposition rate will also strongly depend on the chemical structure of the precursor. Precursors easy to polymerize and containing one or two

As described above, one of the challenges in the deposition of coatings by plasma technology is to obtain dense, homogeneous, and uniform films. The same rationale as the one operating at low pressure applies here: to obtain such coatings, the nucleation and growth processes must take place mostly at the gas/substrate interface, and not in the gas phase. A case leading to the formation of powders and/or to a non-dense coating is presented in Fig. 30. At high pressure conditions, characterized by a very small mean free path of the particles, many collisions can take place, leading sometimes to nucleation and growth in the gas phase, rather than at the substrate surface. The condensation in the gas phase leads, in turn, to the formation of powder instead of a dense coating. In order to avoid that, one should avoid having too high of a concentration of precursor (or monomer) molecules activated simultaneously. Therefore, careful control of the monomer injection flow is required. This will limit, to some extent, the linear dependence of the deposition rate on the monomer flow. 17. Conclusions Atmospheric plasma deposition of coatings, a relatively “new” technology, is gaining more and more interest. Nowadays, one can deposit organic, inorganic and hybrid coatings on various substrates, with different geometries, quite easily. In some cases, very high deposition rates can be reached, which are of the order of magnitude suitable for industrial applications. However, the quality of the coating is strongly dependent on experimental parameters such as the geometry used (in discharge or post-discharge), the nature of the plasma gas (He or Ar), the state of the precursor (liquid or gas) and much work still has to be done in order to understand the complex mechanisms that take place at atmospheric pressure. Understanding these phenomena will certainly help, in the future, to develop technologies as robust as the standard PVD techniques, but with different goals. For example, we are convinced that very high purity coatings can, at this moment, only be obtained with vacuum-based technologies. In this review, we have tried to illustrate a few examples of the current state of the art, trying to underline the major routes that are currently investigated in this complex research area. Acknowledgments Although this paper is a review, some of the results presented here were recently generated in our research group thanks to various funding agencies. The authors would like to thank the Belgian Federal Government "IAP -PSI (physical chemistry of plasma surface interactions) – P6-08 network", the FOMOS program (Belgian technology pole), the FRIA, the MIRAGE project (Marshall Plan, Walloon Region), and the ULB post-doctoral program. References

Fig. 30. Uncontrolled atmospheric plasma deposition of TiO2 starting from titanium isopropoxide and leading to a porous layer [147].

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