Elsevier

Thin Solid Films

Volume 520, Issue 13, 30 April 2012, Pages 4219-4236
Thin Solid Films

Critical Review
Atmospheric plasmas for thin film deposition: A critical review

https://doi.org/10.1016/j.tsf.2012.01.026Get rights and content

Abstract

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.

Section snippets

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 fundamental

Atmospheric pressure plasma for the deposition of coatings

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

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

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:Vbreakdown=B×p×dC+lnp×dWith:

    d

    inter-electrode spacing

    p

    pressure

    B and C

    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

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

Synthesis of hybrid coatings

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

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

Polymerization using atmospheric pressure plasmas: general mechanisms and experimental parameters

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

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.E=WFMwith:

    W

    discharge power (J/s)

    F

    monomer flow rate (μl/min)

    M

    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

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

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

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,

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 post-discharge. 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

Comparison: coatings under vacuum/liquid/plasma

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

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

Nucleation in the gas phase or at the gas substrate interface

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

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

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.

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