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Keywords:

  • atomic force microscopy (AFM);
  • dielectric barrier discharges (DBD);
  • FT-IR;
  • hexamethyldisiloxane (HMDSO);
  • plasma-polymerization

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Part
  5. Results
  6. Discussion
  7. Conclusion

In this work, an atmospheric pressure glow-like dielectric barrier discharge in argon with small admixtures of hexamethyldisiloxane (HMDSO) is employed for the deposition of thin polydimethylsiloxane (PDMS) films. The effect of discharge power and feed composition (monomer concentration) on film properties has been investigated by means of contact angle measurements, Fourier-Transform Infrared Spectroscopy (FT-IR), and Atomic Force Microscopy (AFM). The results are described by defining a W/FM value, where W is the discharge power, F the monomer flow rate, and M is the molecular weight of the monomer. This paper shows that the deposition rate and the chemical composition of the deposited films are strongly affected by the W/FM value at which plasma-polymerization is performed.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Part
  5. Results
  6. Discussion
  7. Conclusion

Plasma-polymerization is a versatile technique for the deposition of films with functional properties suitable for a wide range of applications.1 These plasma polymers have different properties than those fabricated by conventional polymerization: the plasma-polymerized films are usually branched, highly cross-linked, insoluble, pinhole-free, and adhere well to most substrates.2, 3 Due to these excellent properties, plasma-polymerized films have been utilized in a wide range of applications, such as protective coatings, biomedical materials, electronic and optical devices, adhesion promoters, etc.3

Plasma-polymerization is mostly performed at low pressure (101–102 Pa), making its application limited to batch processes.4 Moreover, low pressure plasma technology demands the use of expensive vacuum pumping systems. Recently, plasma deposition at atmospheric pressure has become a promising technology due to its reduced equipment costs and its possibility of in-line processing.5 Among various atmospheric pressure non-thermal plasmas, dielectric barrier discharges (DBDs) have received a lot of attention due to the easy formation of a stable discharge and their scalability.6 These discharges are usually generated in the gas gap between two parallel electrodes, at least one of these being covered with a dielectric layer.7, 8 The most interesting property of DBDs is that in most gases, breakdown is initiated in a large number of short-lived independent current filaments or microdischarges.7, 8

The functionality of the plasma deposited layer is mainly determined by the nature of the used vapor precursors.6 These precursors, which lead to the coating formation, are diluted in a main gas, which is usually helium, argon, or nitrogen.9 Among the wide variety of possible precursors for plasma-polymerization, considerable attention has been given to the plasma-polymerization of organo-silicon monomers, particularly hexamethyldisiloxane (HMDSO).10–13 These monomers are of great interest because of their high deposition rates and the ability to control their structure and properties by varying deposition conditions. In addition, HMDSO is an easy and safe monomer to handle, especially compared to silane compounds.13, 14 HMDSO plasma-polymerized thin films can be assayed for a large number of applications in rather different fields such as protective anti-scratch coatings on plastic substrates, barrier films for food and pharmaceutical packaging, corrosion protection layers, coatings for biocompatible materials, and low-k dielectric layers for microelectronic applications.14, 15 Plasma deposition from HMDSO mixed with different carrier gases has been extensively studied using low pressure non-thermal plasmas, 1, 2, 16, 17 however, only few authors report on the deposition of HMDSO-based thin films using a non-thermal plasma operating at atmospheric pressure.9, 18

Taking into account the above discussion, this paper presents results on the formation of coatings in an atmospheric pressure DBD using HMDSO as gaseous precursor. Plasma-polymerized films are deposited onto polyethylene terepthalate (PET) films using argon as a carrier gas. The properties of the obtained coatings will be discussed in detail using contact angle measurements, Fourier Transform Infrared Spectroscopy (FT-IR), and Atomic Force Microscopy (AFM).

Experimental Part

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Part
  5. Results
  6. Discussion
  7. Conclusion

DBD Set-up

A schematic diagram of the plasma configuration is depicted in Figure 1. Two circular copper electrodes (Ø = 4 cm) are placed within a cylindrical enclosure and both electrodes are covered with a ceramic (Al2O3) plate with a thickness of 0.7 mm. The gas gap between the two ceramic plates is 2.5 mm. The upper electrode is connected to an AC power source with a frequency of 50 kHz, while the lower electrode is connected to earth through a resistor of 50 Ω. The voltage applied to the electrodes is measured using a high voltage probe (Tektronix P6015A), whereas the discharge current is obtained by measuring the voltage over the 50 Ω resistor. This resistor can be replaced by a capacitor of 10 nF and the voltage across this capacitor is proportional to the charge stored on the electrodes. This latter measurement is widely used to obtain voltage-versus-charge plots, which form Lissajous figures.7, 8 From this figure, the discharge power P can be calculated as described in detail by De Geyter et al.17

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Figure 1. Schematic diagram of the experimental set-up used for plasma-polymerization (1, argon cylinder; 2, mass flow controller; 3, HMDSO monomer; 4, plasma reactor; 5, pressure gauge; 6, valve; 7, pump).

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Before starting plasma-polymerization, a PET film (Goodfellow, UK) is placed on the lower ceramic plate. After introduction of the substrate into the reactor, the discharge chamber is pumped down below 0.1 kPa and then filled with the working gas (argon-Air Liquide-Alphagaz 2) to atmospheric pressure. During plasma-polymerization, a gas mixture consisting of the carrier gas argon and the monomer gas is allowed to flow between the two electrodes. The main argon flow is controlled using a mass flow controller (MKS Instruments, The Netherlands) and is adjustable in the range of 0–10 slm (standard liters per minute). The monomer vapor is introduced in the reactor via a glass bubbler containing HMDSO (Sigma–Aldrich ≥98%), carried by a secondary flow of argon. The glass bubbler is placed in a water bath at 25 °C in order to ensure a constant temperature of the HMDSO liquid. The secondary flow is also controlled by a mass flow controller (MKS Instruments, The Netherlands) and is adjustable in the range of 0–100 sccm (standard cubic centimetres per minute). In all experiments, the effective loading of the reactive HMDSO vapor in the gas stream is calculated by measuring the quantity of liquid consumed within a defined time interval (120 min).

For the purpose of comparison, all plasma-polymerization experiments are performed at 3 slm argon flow rate for the main argon supply. In a first series of tests, the discharge power (obtained from a Lissajous figure) is kept constant, while the monomer concentration is varied by changing the flow rate through the glass bubbler to study the influence of monomer concentration on plasma-polymerization. In a second series of experiments, the monomer concentration is kept constant, while the discharge power is changed, enabling the study of the influence of discharge power on plasma deposition.

Coating Characterization

Contact angle measurements represent the easiest and quickest method for examining the properties of surfaces.17 In this paper, the contact angles of the uncoated and coated PET films are obtained using a commercial Krüss Easy Drop optical system (Krüss GmbH, Germany). This system is equipped with a software operated high-precision liquid dispenser to precisely control the drop size of the used liquid. The drop image is then stored, via a monochrome interline CCD video camera, using PC-based acquisition and data processing. Using the computer software provided with the instrument, measurement of the static contact angles is fully automated. In this work, distilled water drops of 2 µL are used as a test liquid. The values of the static contact angles, shown in this paper, are obtained using Laplace–Young curve fitting and are the average of eight values measured over an extended area of the deposited thin films.

The chemical structure of the deposited films is obtained using FT-IR. Infrared analysis of the coated PET films is performed on a Bruker Vertex 70 spectrometer purged with dry, CO2 free air. The spectrometer is equipped with a single reflection ATR accessory (MIRacle, Pike Technologies) using a germanium (Ge) crystal as an internal reflection element. The FT-IR spectra are recorded using a liquid nitrogen cooled Mercury–Cadmium–Telluride (MCT) detector with a resolution of 4 cm−1 and 32 scans are taken for each measurement. The FT-IR spectra shown in this paper are ATR-corrected for the wavelength dependence of the penetration depth and are normalized on the Si–O–Si peak at 1 020 cm−1.

Atomic Force Microscopy (AFM) can be employed to determine the thickness of the plasma-polymerized films. The PET substrates are partially covered by a mask before starting the plasma-polymerization process. After this polymerization step, the mask is removed and the samples are investigated by AFM, scanning both the covered and the uncovered zones. From height differences between these two areas the thickness of the plasma-polymerized films can be deduced. AFM images are obtained in ambient conditions with a Multimode scanning probe microscope (Digital Instruments, USA) equipped with a Nanoscope IIIa controller. 50 µm scans are recorded in tapping mode with a silicon cantilever (OTESPA-Veeco) at a scan rate of 0.2 Hz.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Part
  5. Results
  6. Discussion
  7. Conclusion

The first part of this section is dedicated to the electrical characterization of the discharge itself and to the deposition parameters used during plasma-polymerization. The second part of this section describes the chemical and physical properties of the plasma-polymerized HMDSO (PP-HMDSO) films using AFM, FT-IR, and contact angle measurements.

Electrical Characterization of the Discharge and Deposition Parameters

The most common electrical diagnostic of a DBD consists of the measurement of the current–voltage waveforms. For all operating conditions used in this paper, the discharge current consists of two parts: the sinusoidal current waveform corresponds to the capacitive current, while two wide peaks superimposed on this capacitive current represent the current of a glow-like discharge.18, 19 It is known that in the case of a filamentary DBD, a large number of spike-like current pulses with nanosecond durations are randomly appearing at every half cycle of the applied voltage.7, 8, 19 On the other hand, an atmospheric pressure glow discharge is characterized by a single current pulse with much longer duration.9, 16, 19 However, Sublet et al. have shown that at atmospheric pressure, increasing the applied voltage in helium causes a discharge transition from an atmospheric pressure glow discharge to a multi-peak mode, which they call the “multi-glow” mode, since the current–voltage characteristic for each current pulse is similar to the one obtained in the glow mode.18 Therefore, due to the presence of only two wide peaks superimposed on the capacitive current, we conclude that the DBD used in this paper is a glow-like discharge. It is also important to mention that the different operating conditions do not modify the characteristic of the discharge: the process always operates in a glow-like discharge regime.

In a first series of experiments, the monomer concentration is varied by using three different argon flow rates (8, 14, and 20 sccm) through the glass bubbler containing the HMDSO monomer. In this way, plasma-polymerization can be performed using three different monomer concentrations (1.3, 3.0, and 4.0 ppm, respectively). The main argon supply and the discharge power is kept constant at 3.0 slm and 9.5 W, respectively. In a second series of experiments, plasma-polymerization is performed with three different discharge powers (5.8, 9.5, and 15.2 W), while the HMDSO monomer concentration is kept constant at 3.0 ppm.

Contact Angle Measurements

Contact angle measurements are performed on the uncoated PET film and on PET films covered with PP-HMDSO films with a thickness of 700 nm. The average contact angle of the uncoated PET film is 72.6° and the average contact angle for the PET films covered with different PP-HMDSO films is shown in Table 1. Table 1 clearly shows that when the monomer concentration is decreased from 4.0 to 3.0 ppm at 9.5 W, the contact angle remains constant at ∼108–109°. However, when the monomer concentration is further decreased to 1.3 ppm, the contact angle decreases to ∼99°. Table 1 also shows that when the discharge power is increased from 5.8 to 9.5 W at a constant monomer concentration of 3.0 ppm, the contact angle remains stable at ∼108°. However, when the discharge power is increased to 15.2 W, the contact angle decreases to ∼100°.

Table 1. Average contact angle of the PP-HMDSO films (thickness = 700 nm) using various deposition parameters.
Monomer concentrationDischarge powerContact angle
ppmW°
1.39.599.4
3.09.5107.7
4.09.5109.1
3.05.8108.3
3.015.299.9

AFM Analysis: Coating Thickness

The influence of the monomer concentration and the discharge power on the thickness of the PP-HMDSO films is shown in Figure 2. It can be seen that for all deposition parameters the thickness of the PP-HMDSO films linearly increases with the deposition time. From the slope of the straight lines, the PP-HMDSO deposition rate can be calculated and the results are also depicted in Figure 2. Figure 2 clearly shows that the deposition rate significantly increases from 3.05 to 4.82 nm·s−1 when the monomer concentration is increased from 1.3 to 3.0 ppm at 9.5 W. However, when the monomer concentration is further increased to 4.0 ppm, the deposition rate decreases to 4.04 nm·s−1. In Figure 2, it can also be seen that when the discharge power is increased from 5.8 to 9.5 W at 3.0 ppm, the deposition rate increases from 2.21 to 4.82 nm·s−1. However, when the discharge power is further increased to 15.2 W, the deposition rate decreases to 4.37 nm·s−1.

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Figure 2. Film thickness as a function of deposition time for different deposition parameters.

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In a previous study, Pinto Mota et al. have deposited HMDSO films using a radiofrequency (RF) discharge operated in HMDSO vapor with small percentages of argon at very low pressures (15 Pa).20 These authors have found a deposition rate of 1.0–2.17 nm·s−1 when the discharge power is varied between 6 and 35 W. Zanini et al. also deposited PP-HMDSO films using a capacitively coupled RF plasma operating in pure HMDSO vapor at 100 Pa.14 They have found a film thickness of ∼120 nm after a deposition time of 10 min at 60 W. Comparing the thickness results obtained in this paper with the ones obtained by Zanini et al. and Pinto Mota et al., it can be concluded that plasma-polymerization of HMDSO using an atmospheric pressure DBD is very efficient.14, 20

FT-IR Analysis

The chemical structure and the chemical bonds present in the deposited films can be studied by means of FT-IR spectroscopy. Figure 3 depicts the FT-IR spectra of several PP-HMDSO films deposited using different monomer concentrations at a constant discharge power of 9.5 W. The thickness of the different PP-HMDSO films is kept constant at 700 nm; as a result, the thickness of the deposited films exceeds the penetration depth (660 nm) of the infrared beam and detection of background signals from PET substrates is prevented. Figure 3 clearly shows that when deposition occurs at 4.0 ppm, the FT-IR spectrum exhibits strong absorption bands at 2 965, 1 260, 1 020, 850, and 800 cm−1. The absorption bands at 2 965 and 1 260 cm−1 can be attributed to CH3 stretching and CH3 deformation vibrations in Si–CH3 groups, respectively.21 The absorption band at 1 020 cm−1 can be assigned to Si–O–Si stretching vibrations, while the peak at 850 cm−1 can be attributed to Si–C and CH3 rocking vibrations.21 The absorption band at 800 cm−1 is due to CH3 rocking vibrations and Si–O–Si stretching vibrations. The FT-IR spectrum also exhibits a small absorption peak at 1 410 cm−1, which can be attributed to the asymmetric deformation vibration of CH3 groups.21

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Figure 3. FT-IR spectra of PP-HMDSO films deposited with different monomer concentrations (discharge power = 9.5 W, film thickness = 700 nm).

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Figure 3 also clearly shows that the FT-IR spectrum of the PP-HMDSO films is not significantly changed when the monomer concentration decreases from 4.0 to 3.0 ppm. However, when the monomer concentration is further decreased to 1.3 ppm, two small peaks at 1 717 and 897 cm−1 appear, which are due to C=O stretching vibrations and Si–OH deformation vibrations, respectively. Moreover, a new peak appears at 727 cm−1, which can be attributed to OH deformation vibrations.21 In addition, a small broad peak between 3 700 and 3 200 cm−1, which is due to OH stretching vibrations, can be observed. Figure 3 also shows that the peaks due to Si–C and CH3 vibrations (2 965, 1 260, 850, and 800 cm−1) are considerably smaller at 1.3 ppm.

Taking into account the above results, it can be concluded that the HMDSO films plasma-polymerized at high monomer concentration retain almost all characteristics of the monomer and that they have a chemical structure similar to polydimethylsiloxane (PDMS). However, at low monomer concentration, the deposits contain some oxidized groups (Si–OH, C=O, etc.) and a smaller amount of CH3 groups. Therefore, at low monomer concentration the chemical structure is close to that of slightly oxidized PDMS. This behavior could also explain the lower contact angle value obtained at low monomer concentration.

Figure 4 represents the FT-IR spectra of several PP-HMDSO films (thickness = 700 nm) deposited at various discharge powers using a constant monomer concentration of 3.0 ppm. It can be seen in Figure 4 that increasing the discharge power from 5.8 to 9.5 W does not give significant variations in the chemical structure of the deposited films. However, if the PP-HMDSO films are deposited at a discharge power of 15.2 W, a different FT-IR spectrum is observed quite similar to the FT-IR spectrum of the films deposited at low monomer concentration (only the small peak at 1 717 cm−1 is not observed in the FT-IR spectrum at high discharge power). Therefore, at low discharge power, the chemical structure of the PP-HMDSO films is similar to the one of PDMS, while at higher discharge power, the chemical structure is slightly oxidized PDMS, which could again explain the lower contact angle obtained at high discharge power.

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Figure 4. FT-IR spectra of PP-HMDSO films for different discharge powers (monomer concentration = 3.0 ppm, film thickness = 700 nm).

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Part
  5. Results
  6. Discussion
  7. Conclusion

In this paper, plasma-polymerization of HMDSO in argon is studied using different deposition parameters. In a first series of experiments, the influence of monomer concentration on HMDSO plasma-polymerization is studied at a constant discharge power of 9.5 W. Results clearly show that at high monomer concentrations (3.0 and 4.0 ppm), the PP-HMDSO coatings chemically resemble conventionally polymerized PDMS. However, if the monomer concentration becomes too low (1.3 ppm), the chemical structure of the PP-HMDSO films can be compared to that of slightly oxidized PDMS. In a second series of experiments, the influence of discharge power on HMDSO plasma-polymerization is explored in detail at a constant monomer concentration of 3.0 ppm. Results have shown that at low powers, the PP-HMDSO films chemically resemble conventionally polymerized PDMS. However, if the discharge power is too high (15.2 W), the films have the chemical structure of slightly oxidized PDMS.

Polymer formation in plasma-polymerization encompasses successive reactions of the activation of monomers by plasma into radicals, of the recombination between the formed radicals and of the reactivation of the recombined molecules. The activation of monomers and reactivation of the recombined molecules by plasma, are essentially due to the fragmentation (hydrogen abstraction and bond scission) by plasma. The fragmentation process depends on how much electric energy (discharge power) is supplied to maintain the plasma, how much monomers are introduced in the plasma region and where the monomer molecules interact with activated species of the discharge.22, 23 Yasuda proposed a controlling parameter W/FM, where W is the discharge power [J·s−1], F the monomer flow rate [mol·s−1], and M is the molecular weight of the monomer [kg·mol−1], respectively. 22, 23 The W/FM parameter is an apparent input energy per unit of monomer molecule in J·kg−1, therefore, the magnitude of the W/FM parameter is considered to be proportional to the concentration of activated species in the discharge. The polymer deposition rate increases by increasing the W/FM parameter in the operational condition where the activated species have a much lower concentration than monomer molecules in the plasma (monomer sufficient region); afterwards, the polymer formation rate levels off (competition region). At higher W/FM values, the polymer deposition rate decreases with increasing W/FM values because of lack of monomer molecules (monomer deficient region).22, 23 Table 2 shows the different W/FM values used in this paper and the corresponding deposition rates. Table 2 clearly shows that at W/FM values below 82.24 MJ·kg−1, the deposition rate increases with increasing W/FM value, suggesting that deposition occurs in the monomer sufficient region. In this region, monomer molecules are almost not fragmented and plasma polymers with a high retention of the HMDSO monomer groups can be obtained. When the W/FM value is increased above 82.24 MJ·kg−1, the deposition rate decreases to 4.37 nm·s−1 at a W/FM value of 132.03 MJ·kg−1 and to 3.05 nm·s−1 at a W/FM value of 195.02 MJ·kg−1. Therefore, at high discharge power and at low monomer concentration, deposition occurs in the monomer deficient region. In this region, monomer molecules are subjected to fragmentation and plasma polymers with a loss of monomer functional groups are obtained. This behavior could explain the presence of oxidized groups in the films plasma-polymerized at high discharge power and low monomer concentration.

Table 2. Deposition parameters with corresponding W/FM values and deposition rates.
Monomer concentrationDischarge powerW/FMDeposition rate
ppmWMJ·kg−1nm·s−1
3.05.850.492.21
4.09.561.494.04
3.09.582.244.82
3.015.2132.034.37
1.39.5195.023.05

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Part
  5. Results
  6. Discussion
  7. Conclusion

In this paper, HMDSO has been plasma-polymerized using a glow-like DBD in argon at atmospheric pressure using various deposition parameters. The aim of this work was to contribute to the understanding of PP-HMDSO growth mechanisms by investigating the influence of different parameters, such as discharge power and monomer concentration on the film properties. Results clearly show that the deposition rate and the chemical structure of the thin films are strongly affected by the apparent input energy per unit of monomer molecule (W/FM value). At low W/FM values, high deposition rates of up to 4.8 nm·s−1 can be found. Moreover, using these deposition parameters, the PP-HMDSO films chemically resemble conventionally polymerized PDMS. However, at high W/FM values, the deposition rates decrease to 3.0 nm·s−1 and the PP-HMDSO films have a chemical structure of slightly oxidized PDMS. Therefore, one should carefully choose the deposition parameters in order to obtain the highest deposition rate and the highest retention of monomer groups in the plasma-polymerized thin films.