Atmospheric pressure plasma enhanced chemical vapor deposition of hydrophobic coatings using fluorine-based liquid precursors

Surface & Coatings Technology 234 (2013) 21–32

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Atmospheric pressure plasma enhanced chemical vapor deposition of hydrophobic coatings using fluorine-based liquid precursors Jacqueline H. Yim a,⁎, Victor Rodriguez-Santiago a, André A. Williams a, Theodosia Gougousi b, Daphne D. Pappas a, James K. Hirvonen a a b

United States Army Research Laboratory, 4600 Deer Creek Loop, Aberdeen Proving Ground, MD, 21005, USA Department of Physics, University of Maryland Baltimore County (UMBC), Baltimore, MD 21250, USA

a r t i c l e

i n f o

Available online 28 March 2013 Keywords: Plasma enhanced chemical vapor deposition (PECVD) Hydrophobic coating Dielectric barrier discharge (DBD) Atmospheric pressure plasma jet (APPJ) Organofluorosilane Fluorocarbon

a b s t r a c t In this work, an atmospheric pressure plasma jet (APPJ) was investigated for developing hydrophobic thin film coatings on ultra-high molecular weight polyethylene (UHMWPE) films. Fluoroalkyl silanes, (CH3CH2O)3SiCH2CH2(CF2)7CF3 and (CH3O)3SiCH2CH2CF3 and fluoroaryl silane, F5ArSi(OCH2CH3)3 monomers with different fluorocarbon chain lengths were polymerized via plasma enhanced chemical vapor deposition (PECVD). These precursors in addition to other deposition processing conditions such as electrode-substrate gap distance and deposition time were investigated to understand the influence these parameters have on the overall deposition characteristics and hydrophobic behavior of the as-deposited thin film coatings. Attenuated total reflectance–Fourier transform infrared (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) techniques revealed that the chemical composition of the coatings retained a bulk of the monomer chemistry, signifying a low degree of fragmentation of the precursor in the plasma. This was particularly demonstrated by the coatings obtained with the fluoroaryl silane precursor, where the aromatic structure was kept intact. The hydrophobicity of the coatings was assessed using water contact angle (WCA) measurements and the thickness and morphology were examined using profilometry, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Variations in the composition of fluorocarbon coatings were observed as a result of deposition conditions, however the dominant parameter was found to be the monomer precursor. Optimal hydrophobic behavior was observed from coatings derived from the monomer with the longest fluorocarbon chain, as demonstrated from trends seen in WCA and CFn group concentrations. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Hydrophobic thin film coatings with Teflon-like compositions are attractive in applications where self-cleaning, wicking, anti-fog, and water-repellant properties are highly desirable. Teflon-like coatings are advantageous as they provide high thermal stability, chemical resistance, low flammability, and a low refractive index. Current efforts [1–3] aim towards developing a high-throughput deposition technology that yields uniform and pinhole-free coatings. Conventionally, hydrophobic thin film coatings have been deposited using wet chemical routes that introduce chemical functionalities with lower surface energies or initiate polymerization reactions to achieve a cross-linked polymer [4–7]. However, these methods are often complex, entail multiple processing steps, utilize solvents, and provide poor control of coating thicknesses on irregular-shaped surfaces. Other methods have also been explored that implement a single-step route and employ solvent-free processes such as evaporative methods, plasma and

⁎ Corresponding author. Tel.: +1 410 306 0966; fax: +1 410 306 0829. E-mail address: [email protected] (J.H. Yim). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.03.028

reactive sputtering techniques [8–12]. Of these processes, plasma enhanced chemical vapor deposition (PECVD) has been widely studied and utilized to produce thin films with nanometer to micrometer thicknesses [11,13–17]. PECVD has shown promise as a viable alternative to other methods while at the same time being a solvent-free process. Typically, PECVD techniques have been performed using low pressure systems with low molecular mass fluorocarbon gas-based precursors such as CF4 [18,19], C2F6 [18,20], C3F6 [20], C3F8 [20], and, C4F8 [10,21,22]. However, these precursors undergo fragmentation and random rearrangement in the plasma, which usually results in low deposition rates and irregularly structured polymers. The deposition of hydrophobic coatings using liquid precursors is not a new area of research with low-pressure PECVD processes. Hozumi and coworkers [23] were able to grow fluorine-containing films on polycarbonate using fluoroalkyl silane liquid precursor (heptadecafluoro-1, 1,2,2-tetrahydrodecyl)-1-trimethoxysilane) in an RF PECVD process. Kumar et al. [15] used an acrylate-based fluorinated monomer, 1H,1H,2H,2H-perfluorodecyl acrylate, owing to the accelerated polymerization rate of the precursor and orientation of the perfluorocarbon chain in a low pressure RF plasma system.

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However, due to the high costs and time intensiveness of depositing coatings using low pressure PECVD, atmospheric pressure plasmas have gained more interest as an alternative source for obtaining similar coatings. Recent efforts have focused on the use of atmospheric pressure plasma systems with liquid-based monomer precursors to carry out atmospheric pressure plasma enhanced chemical vapor deposition (AP-PECVD) processes to develop hydrophobic coatings [24,25]. Borcia et al. [26] reported the use of atmospheric pressure dielectric plasma with liquid chlorine- and fluorine-containing precursors yielding polymerized films with hydrophobic properties. In addition, large molecular mass liquid monomers commonly used in layer by layer (Lbl) and self-assembled monolayer (SAM) techniques such as organofluorosilanes [27–29] have been explored with AP-PECVD. This study explored the use of a one-step atmospheric pressure plasma enhanced chemical vapor deposition process with fluorinated monomer liquid precursors to achieve a hydrophobic coating on a polymer substrate. The deposition process consisted of a dielectric barrier discharge (DBD) in an atmospheric pressure plasma jet (APPJ) configuration in order to utilize the plasma afterglow, as previous efforts [30] have verified higher concentrations of chemically reactive species in this region. Several studies have carried out PECVD by introducing the precursor vapor in the afterglow or post-discharge of the APPJ [31,32]; however, we explored the possibility of directly administering the vapor into the plasma to obtain thin film coatings. Utilizing the vapors of large molecule-based monomer precursors with APPJ systems is a fairly new topic in the area of plasma deposition and exploiting the technology warrants the need for additional studies to gain a better understanding of the plasma chemistry and how deposition parameters affect the properties of the coatings. Therefore, several different deposition conditions other than precursor chemistry were investigated. These operating parameters include the gap distance from the APPJ to the UHMWPE film, hereafter referred to as gap distance and the deposition time. The study also looked at the prospects of using a dynamic mode in addition to that of a static mode of deposition towards the development of these hydrophobic thin film coatings. 2. Material and methods 2.1. Materials All liquid precursors: (3,3,3-trifluoropropyl) trimethoxysilane (≥ 97.0%), (pentafluorophenyl) triethoxysilane (97%), and heptadecafluoro-1,1,2,2-tetrahydrodecyl triethoxysilane (97%), hereafter referred to as FAS-3, FAS-5, and FAS-17, respectively, were acquired from Sigma Aldrich and were used as received. The chemical structure of these fluoroalkyl silane and fluoroaryl silane precursors is shown in Fig. 1. All precursors were specifically chosen to study the influence of the fluorocarbon chain length and structure (straight chain vs. aromatic) on the final properties of the coating. Ultra high purity helium gas (Praxair, 99.999%) was used as the carrier gas. The polymer substrate used in this study was an ultra-high molecular weight polyethylene (UHMWPE) film with a thickness of 75 μm (Goodfellow Co.). The UHMWPE films were rinsed with ethanol to remove surface contaminates and dried in air prior to deposition. Silicon wafers (Silicon Quest International) were used as substrates to deposit coatings for thickness measurements. 2.2. Atmospheric pressure plasma jet (APPJ) and deposition process Plasma enhanced chemical vapor deposition was carried out using a custom-built atmospheric pressure plasma jet. The plasma jet is comprised of a high-voltage (HV) electrode made up of a 12 cm long copper wire (1 mm OD) embedded within an alumina dielectric (3 mm OD). The HV electrode is encapsulated within a 15 cm long, 0.30 mm thick glass cylindrical tube (5 mm OD) with the ground electrode attached at the end of the tube, as shown in Fig. 2a. A copper coil wrapped around

Fig. 1. Chemical structures of fluoroalkyl silane liquid precursors: a) (3,3,3-trifluoropropyl) trimethoxysilane (FAS-3), b) (pentafluorophenyl) triethoxysilane (FAS-5), and c) heptadecafluoro-1,1,2,2-tetrahydrodecyl triethoxysilane (FAS-17).

the encapsulating glass cylindrical tube, positioned at the end of the HV electrode, acts as the ground. The HV electrode is positioned at the top of the glass tube, creating a 2.5 cm gap from the end of the HV electrode and nozzle/outlet of the cylindrical glass tube. The active discharge zone in the APPJ is confined to the length of the HV electrode while the afterglow region was found to extend at a length of 2 cm. The plasma jet is powered by a microsecond-pulsed power supply that generates bipolar 30 kV pulses with a width of 30–40 μs (Fig. 2b). The peak power density of the plasma with the FAS-3, FAS-5, and FAS-17 precursors was measured to be 6.0–6.5 W cm−2. An 8% duty cycle was used throughout the experiment. The power was kept constant throughout all experiments. The liquid monomer precursor was placed in a bubbler set at room temperature and helium is flowed over the precursor in the bubbler at high flow rates to transport the precursor vapor to the APPJ. A fixed helium flow-rate to transport the precursor vapor throughout the study was set at 9500 sccm. In conjunction, a base helium flow of 2000 sccm was used to maintain a stable plasma jet. Using the set carrier flow-rate, the mass flow-rate of the liquid monomers fed to the APPJ was 280.0, 2.9, and 0.7 mg min−1 for the FAS-3, FAS-5, and FAS-17, respectively, owing to the different vapor pressures associated with the precursors. No purge gas was used prior the deposition and all depositions using the APPJ were conducted in an enclosure equipped with ventilation to exhaust the spent vapors of the liquid precursor. The setup also incorporates a 38 cm × 54 cm × 0.6 cm stainless steel moveable stage covered in glass used to mount the substrate. It provides a dynamic mode in addition to a static mode of depositing coatings onto large area substrates. Depositions conducted dynamically consisted of moving the stage back and forth at a speed of 7.5 cm s−1. The influences of several deposition conditions such as gap distance and deposition time were studied. Gap distances of 2 mm and 5 mm were used to determine the optimum distance that would provide a coverage area highly concentrated with chemically reactive fluorinated species within the plasma

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2.3.2. Attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) Attenuated total reflectance–Fourier transform infrared spectroscopy was utilized to study the chemical composition of each coating. Spectra were recorded using a Thermo-Nicolet 670 FT-IR spectrometer equipped with a ZnSe multi-bounce ATR crystal. Spectra were acquired using a resolution of 4 cm −1 and a total of 32 scans. 2.3.3. X-ray photoelectron spectroscopy (XPS) Near-surface compositional depth profiling of the as-deposited coatings was performed using the Kratos Axis Ultra X-ray photoelectron spectroscopy system, equipped with a hemispherical analyzer. A 100 W monochromatic Al Kα (1486.7 eV) beam irradiated a 1 mm × 0.5 mm sampling area with a take-off angle of 90°. The base pressure in the XPS chamber was held between 10−9 and 10−10 Torr. Elemental high resolution scans for C1s, F1s and O1s were taken in the constant analyzer energy mode with 20 eV pass energy. For binding energy calibration of the FAS-3, FAS-5, FAS-17 coatings, and the pristine UHMWPE C1s spectra, 292.6 eV, 289.3 eV, 291.7 eV, and 285.0 eV corresponding to the CF3, fluorobenzene ring, CF2, and C\C, C\H groups respectively, were used as references. Deconvolution of the high resolution C1s spectra was done using the Casa XPS fitting software with a Gaussian–Lorentzian fit. A minimum of 2 areas on the same sample were analyzed. 2.3.4. Film thickness and deposition rate Film thicknesses associated with each coating were measured using scanning electron microscopy (SEM) images and profilometry in order to calculate the deposition rate. Thickness measurements were acquired using a Tencor Alpha-Step IQ surface stylus profilometer and a FEI NanoSEM in both field-emission and immersion modes. Thicknesses obtained from SEM were done by taking the cross-sectional images of fractured pieces of silicon wafers with the as-deposited FAS coatings. The surface morphology of the coating was examined to assess the conformity of the coating.

Fig. 2. (a) Schematic drawing of dielectric barrier discharge (DBD) atmospheric pressure plasma microjet (APPJ) system used to deposit hydrophobic coatings. (b) Current and voltage waveform of microsecond pulse power supply.

2.3.5. Atomic force microscopy (AFM) Atomic force microscopy was used to study the morphological changes of the coatings. The AFM system used was a Dimension 3100 microscope with a Nanoscope V controller (Digital Instruments/Veeco). Imaging was done in tapping mode, using TESP (silicon) cantilevers (Veeco Probes) with an oscillation frequency of 300 kHz at a 0.5 Hz scan rate of 10 × 10 μm areas. Using Nanoscope software (v7.30), images were 1st order x-y plane fitted and then 1st order flattened. Surface roughness measurements were based on the root mean square (RMS) roughness values averaged from 3 images scanned at different locations on each sample. 3. Results and discussion

afterglow. Deposition times of 60, 180, and 360 s were used for static treatments and the number of passes used to deposit coatings when operating in dynamic mode was equivalent to the deposition times used for static treatments. In addition, we also investigated the viability of conducting deposition at room-temperature conditions, where no heat was administered to the monomer precursors to obtain thin film coatings. 2.3. Characterization of thin film coating 2.3.1. Static water contact angle (WCA) measurements To gauge the hydrophobicity of the as-deposited coatings, static water contact angles based on the sessile drop method were recorded using a goniometer equipped with a CCD camera and LabView software for image capture. WCA was measured using HPLC grade water and a total of six drops (5 μL) were taken and averaged to obtain a characteristic value of the contact angle for the control and the coated UHMWPE film surface.

3.1. Proposed reaction mechanism Several possible reaction mechanisms in which the monomer undergoes plasma polymerization on the surface of UHMWPE films are proposed in Fig. 3. The pathways, based on the free radical polymerization via chain transfer, are possible due to the presence of multiple monomer radical intermediates (1–4), which would be produced by the exposure of fluoroalkyl and fluoroaryl silanes to the plasma. Each monomer has a trialkoxysilane group and is likely to produce similar alkyl or aryl (1), siloxy (2), silyl (3), and silylalkoxy (4) radicals. Any of these radicals can then undergo chemisorption or chemical bonding with polyethylene to yield a selected variety of fluorinated polyethylene surfaces. These radicals can chemically react with the polyethylene surface as it is assumed that the energetic species in the afterglow also promotes the formation of active sites in the polyethylene backbone via chain scissioning of the C\C backbone and the abstraction of hydrogens. The chemical composition of the coatings obtained with the

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Fig. 3. Proposed PECVD plasma polymerization reaction mechanisms of fluoroalkyl silane and fluoroaryl silane monomer precursors.

pulsed APPJ setup in this study should be characteristic of the coatings achieved with pulsed plasma systems as reported in literature [15,18]. The process in which these coatings are form is referred to as pulsed plasma polymerization as described by Friedrich [3]. Short pulses (few μs) can activate the monomer molecules, generate radicals, initiate polymerization, and reduce the degree of monomer fragmentation. After the pulse, radicals partake in chain-propagation reactions during the “off” period (μs to ms) and new radicals are re-generated and re-initiation of the polymerization reactions takes places with every new pulse. Through the use of pulses, polymerized coatings possess a structure and composition that resemble their counterparts achieved through radical polymerization. In this study, a duty cycle of 8% can be presumed to provide sufficient off-time to allow plasma-generated radicals to react during the “off” period of the cycle.

The gap distance and more notably the chemical structure of the monomer precursors have a profound impact on the WCA. The influence of the gap distance between the UHMWPE film and the ground electrode is more pronounced with the FAS-17 coating. A 5% increase in the WCA is observed with FAS-17 coatings deposited from a gap distance of 2 mm when compared with coatings obtained with the 5 mm gap. However, the effect of gap distance with the FAS-3 and FAS-5 coating is not as significant when taking into account the margin of error associated with each WCA measurement. Though the gap distance does play a role on the degree of hydrophobicity of the coatings, the precursor has a more pronounced effect. Higher WCA was recorded for the FAS-17 coating whereas coatings deposited using the FAS-3 and FAS-5 precursor exhibited a 4–13% lower WCA than that of the control film. Takai et al. [13] reported a similar behavior

3.2. Water repellency Measured static water contact angles of FAS-3, FAS-5, and FAS-17 coatings deposited statically on UHMWPE films are shown in Fig. 4. UHMWPE film is inherently hydrophobic due to the long hydrocarbon chains in the polymer backbone, therefore, the baseline used to gauge the hydrophobicity of the coating is the WCA of the control film. The WCA of the control UHMWPE film is 97.5° and the WCA of the hydrophobic coating ranged from 85° to 116°. The maximum WCA observed was ca. 91°, 94°, and 116° for FAS-3, FAS-5, and FAS-17, respectively. As a comparison, Hozumi et al. [23] reported a maximum contact angle of 112° on Si substrates with FAS-17 monolayers deposited using a chemical vapor surface modification technique. Despite a 13% decrease in the contact angle observed with some of the coatings, the hydrophobicity of the surface is retained. It is well documented that exposing UHMWPE to a pure helium plasma results in a 60% decrease (~40°) in the water contact angle after 30 s [33]. The decrease in WCA with the coatings deposited using smaller molecular mass precursors signifies that competitive processes involving deposition and chemical modification occur between the monomer-based species with the atomic and molecular species formed in the plasma as a result of plasma interactions with the open atmosphere.

Fig. 4. Recorded static WCA measurements of FAS-3, FAS-5, and FAS-17 coatings at electrode-substrate gap distances of 2 mm and 5 mm.

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with the WCA with respect to the precursors. The higher angles recorded for the FAS-17 coatings signify that using a precursor containing a long fluorocarbon chain length is the dominant factor in achieving hydrophobic properties. This observation is in accordance with the work done by Tao [34], where higher contact angles were observed with longer chain lengths as a result of increased cohesive interactions and packing density. This observation is also in agreement with the SAM literature. Longer chains have been found to shield the substrate better and are attributed to cross-linking [35]. 3.3. Surface morphology Representative AFM micrographs of the control UHMWPE film and each of the FAS coating deposited under static conditions are presented in Fig. 5. AFM images verify that a coating is present on UHMWPE films. Surface roughness was used as a metric in determining the conformity of the coatings on the UHMWPE film substrate. The surface roughness of the control UHMWPE film was measured to be c.a. 34 nm and the surface roughness of the coatings at different deposition times ranged from 27 to 33 nm, 30 to 50 nm, and 18 to 27 nm for the FAS-3, FAS-5 and FAS-17 coatings, respectively. The roughness values for the FAS-3 and FAS-17 coatings lie within the roughness value of the control sample, indicating that the coatings are conformal as they take on the topography of the UHMWPE film. The FAS-5 coating, however, exhibited a slight increase in surface roughness as the AFM image shows the presence of a much thicker coating in comparison to the FAS-3 and FAS-17 coatings, where distinguishable features of the UHMWPE substrate are no longer discernible. This observation suggests that the aromatic structure of the FAS-5 precursor chemistry can have a direct impact on the coverage and film growth. 3.4. Thin film coating chemical composition ATR-FTIR spectra of the control UHMWPE film, FAS-3, FAS-5, and FAS-17 deposited coatings on the UHMWPE film in static mode are shown in Fig. 6. Each spectrum shows distinctive absorption bands in each of the coatings. These bands are highlighted in Fig. 7 and chemical

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group assignments of these bands are given in Table 1. The IR spectra of FAS-3 coatings show the existence of 10 new absorbance bands when compared to the IR spectrum of the control UHMWPE film. The more intense bands lie in the range of 1268–1073 cm −1 as shown in Fig. 7a [36]. These peaks correspond to the C\F stretching vibration in CF3 and the overlapping bands of the Si\O stretching of the Si\O\Si and the asymmetric Si\O\C stretching. Weaker adsorption bands due to C\F stretching were measured at 1370 and 1316 cm − 1, along with Si\O stretching at 904 and 841 cm − 1 and Si\C stretching at 802 cm − 1. Spectra of FAS-5 coatings exhibit several strong absorption peaks in the 975–1800 cm−1 region (Fig. 7b). In this region, a sharp, intense peak at 1100 cm−1 in addition to the shoulder peak at 1140 and 1150 cm−1 was identified as the C\F stretching vibration. Another distinct peak was measured at 1470 cm−1, corresponding to the aromatic C_C stretching vibration. The intensity of this band in addition to the bands at 1645 cm−1 and 1522 cm−1 suggests that the aromatic ring in the monomer remained intact during the deposition process. The broad Si\OH stretching vibration absorption band at 3230 cm−1 seen in Fig. 6c alludes that the ethoxy groups, which are more prone to undergo abstraction and chain scissioning in the afterglow, are reactive site for the formation of a siloxane cross-linked network. The presence of this band also suggests that the silane-bearing end group affixes to the surface of the film through chemical bonding while the bulky fluorobenzene ring orientates outwards, away from the surface. The FTIR spectra of FAS-17 coatings (Fig. 7c) are different from those of the FAS-3 and FAS-5 as they exhibit fewer bands. Strong absorption peaks at 1050–1250 cm − 1 were identified as the C\F stretching vibration of the \CF2 group [36]. Within this region, there is also a peak at 1110 cm−1 that indicates an Si\O\C stretching band is present. Weak absorption due to the C\F stretching in the CF3\CF2 was measured at 1368–1351 cm − 1. Less intense bands were also found at 1110, 1080, and 958 cm − 1 and were assigned as the asymmetric Si\O\C stretching vibration, Si\O\Si stretching, and Si\O\C stretching bands, respectively. To further verify the chemical groups identified through ATR-FTIR, XPS analysis was performed on the coatings. Atomic concentrations

Fig. 5. AFM images of UHMWPE film surfaces: (a) before deposition, and with coatings deposited for 300 s and at a gap distance of 2 mm using (b) FAS-3, (c) FAS-5, and (d) FAS-17 precursors.

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Fig. 6. Representative ATR-FTIR spectra of (a) control UHMWPE film, (b) FAS-3, (c) FAS-5, and (d) FAS-17 coatings deposited on UHMWPE for 300 s.

recorded from survey scans of the coatings show clear differences in the concentration of chemical groups found in the coatings as a function of monomer precursor and deposition time (Fig. 8). Looking at

the atomic concentration of fluorine functional groups (F1s) with respect to deposition time in Fig. 9, a visible trend is observed for the FAS-17 coating deposited at gap distances of 2 mm and 5 mm. The

Fig. 7. Zoomed-in spectral regions of (a) FAS-3, (b) FAS-5, and (c) FAS-17 coatings obtained by ATR-FTIR.

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Table 1 List of chemical groups assigned to ATR-FTIR absorbance bands present in plasma polymerized FAS-3, FAS-5, and FAS-17 coatings [15,23,36,41]. Wavenumbers (cm−1)

Functional group assignment

3700–3200 1645 1525–1470 1485–1445 1420–1205 1350–1120 1400–1100 1365–1325 1225–1090 1110–1000 1135 1095 1090–1010 1014 990–945 955–830 810–600

Si\OH stretching C_C aromatic stretching, ring bending C_C aromatic stretching \(CH2)n-deformation C\F stretching in CF3 C\F stretching in CF3 C\F stretching in CF2 C\F stretching in CF3-CF2 C\F stretching in (sat)-CF3 Asymmetric Si\O\C stretching Ring and C\F stretching in aromatic-F Si-aromatic (Si\Ar) stretching Si\O stretching in Si\O\Si C\F stretching (monofluorinated aliphatic group) Si\O\C stretching Si\O stretching in Si\OH Si\C stretching

highest concentration of fluorine groups observed was 52 at.% corresponding to the FAS-17 coating deposited for 3 min and positioned at a gap distance of 2 mm. Coatings formed from FAS-3 and FAS-5 monomers possessed slightly lower fluorine concentrations, ranging from 24 to 28 at.%, about half the concentration of fluorine associated with FAS-17 coatings. The same trend was observed with the WCA of the coatings in regards to monomer precursor and deposition gap distance. This reaffirms the dependence of hydrophobicity to fluorine concentration. The influence of the gap distance of the electrode to the film can be noted by the increase in the concentration of oxygencontaining functional groups as well as a decrease in the F1s concentration. Increasing the gap distance from 2 mm to 5 mm resulted in twice the amount of atomic oxygen incorporated into the FAS-17 coating, whereas with the other coatings, the oxygen content remains the same. The influence of the gap distance with the FAS-3 and FAS-5 precursors had little to no effect on the atomic concentration. The influence of gap distance can be further described by comparing the theoretical ratios of the atomic species in the monomer precursor with those of the plasma polymerized coatings. Table 2 summarizes the calculated oxygen, fluorine, and silicon atomic concentrations relative to carbon for all coatings measured at different deposition times and gap distances. Based on these ratios (O/C, F/C, and Si/C), FAS-3 and FAS-17 coatings deposited at the 2 mm gap distance gave values close to the theoretically expected values of the FAS-3 and FAS-17 precursor molecules when compared to the same coatings deposited at the 5 mm gap distance. At the 5 mm gap distance, these coatings tend to have higher concentration of oxygen, fluorine, and silicon. Coatings produced from the FAS-5

Fig. 9. The dependence of atomic concentration of fluorine groups (F1s) on precursor monomer and gap distance as a function of deposition time.

monomer at the 2 and 5 mm gap distances showed no difference in the composition with one another as both coatings yield atomic ratios close to that of the monomer. The effect that these processing conditions have on the coatings needs to be further investigated through the quantification of specific functional groups. Representative deconvoluted high resolution C1s spectra of the FAS-3, FAS-5, and FAS-17 coatings are shown in Fig. 10. The C1s curves were peak-fitted with different peaks that correspond to specific functional groups. The assignments of these functional groups are summarized in Table 3. Depending on the monomer precursor used to deposit the coating, the number of peaks fitted varied. The C1s spectra of the FAS-3 coatings show that the composition is mainly comprised of \CF3 (C5) and C\O (C2) groups, where \CF3 and C\O groups make-up 13–18% and 40–50% of the composition, respectively. Other peaks used to fit the FAS-3 C1s spectra were CF2 (C4), CF (C3), and C\Si (C6). Looking at the effect of processing conditions (Fig. 11a), the deposition time plays a key role in the composition of the coatings. Longer deposition times resulted in a significant increase in the concentration of C\O and a gradual increase of CF3 groups after 60 s of exposure to the afterglow. High amounts of oxidize groups can be attributed to the oxygen in the precursor as well as from the afterglow interacting with the open atmosphere. With longer exposure to the afterglow and other competing reactions that occur within this

Fig. 8. Comparison of C, O, F, and Si atomic concentration in (a) FAS-3, (b) FAS-5, and (c) FAS-17 coatings deposited using electrode-substrate gap distances of 2 mm and 5 mm at different deposition time under.

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Table 2 Comparison of O/C, F/C, and Si/C ratios of the monomer precursor and the plasma polymerized coatings at different deposition times and electrode-substrate gap distances. Precursor

FAS-3

FAS-5

FAS-17

Table 3 Assigned chemical components in deconvoluted high resolution C1s spectrum of PECVD FAS-3, FAS-5, and FAS-17 coatings [15,23,39,42].

Deposition time (seconds)

2 mm gap distance

5 mm gap distance

Component

Peak position (eV)

Functional group

O/C

F/C

Si/C

O/C

F/C

Si/C

Monomer 60 180 300 Monomer 60 180 300 Monomer 60 180 300

0.50 0.64 0.62 0.55 0.25 0.44 0.58 0.43 0.19 0.25 0.53 0.25

0.50 0.77 0.62 0.58 0.42 0.60 0.58 0.51 1.06 1.20 1.87 1.26

0.17 0.18 0.32 0.24 0.08 0.09 0.20 0.11 0.06 0.06 0.07 0.25

0.50 0.86 0.72 0.64 0.25 0.46 0.51 0.42 0.19 1.33 0.56 0.90

0.50 0.98 0.71 0.68 0.42 0.57 0.51 0.56 1.06 1.42 1.47 1.41

0.17 0.28 0.39 0.25 0.08 0.11 0.18 0.11 0.06 0.46 0.30 0.36

C1 C2 C3 C4 C5 C6 C7 C8

285 286.5–286.7 289.5–289.7 291.4–291.7 292.6, 294.1 283.6, 284.3 288.1 289.3

C\C, C\H C\O CF CF2 CF3 C\Si C\CF Fluorobenzene ring

region governs the composition of the polymerized films. Depending on the gap distance used, the rate at which the C\O and CF3 groups are incorporated in the coating is affected. At a distance of 5 mm, the concentration of CF3 and C\O functional groups increases by a factor of 2 after 60 s of deposition time as compared to coatings deposited at a distance of 2 mm. However, after 180 and 300 s, the concentration of the two groups does not vary between the coatings formed at gap distances of 2 mm and 5 mm as the concentrations peak-off at around 50–57 at.% for C\O groups and 17 at.% for CF3 groups. The main peak in the C1s curve of FAS-5 coatings was the fluorobenzene ring (C8) at 289.3 eV [37–39], with a peak concentration of 55–58 at.% after a deposition time of 180 s. This confirms what was previously observed from ATR-FTIR analysis. This suggests that a low degree of fragmentation of the \CF3 group in the precursor occurs in the plasma. In addition to the fluorobenzene ring, other fitted peaks

include the C\CF (C7), C\O (C2) and CF (C3) groups at 288.1, 286.7, and 290 eV, respectively. No distinct changes were observed in the composition with respect to gap distance as the trend with individual concentrations was found to be very similar to one another (Fig. 11b). With the FAS-17 coating, a total of 7 curves corresponding to the CF3, CF2, CF, C\CF, C\O, C\C, and C\Si were used to resolve the C1s peak. The main chemical group found throughout the coating was CF2 (C4) with concentrations ranging as high as 54 at.% in some of the coatings. In addition to the CF2 groups, other fluoro-containing constituents found in the coatings included CF3 (C5) and CF (C3) bearing groups. These groups make up the other 30–40 at.% of the coating composition. Corroborating observations from WCA and spectroscopic analyses, the composition of FAS-17 coatings can vary based on the distance of the APPJ from the substrate and the deposition time as seen in Fig. 11c. When gap distances of 2 mm and 5 mm were used, the concentration of CF2 groups increased significantly to 30 at.% and 20 at.% after 60 s, respectively. Another group that was influenced by the gap distance was C\O. A sharp increase in the concentration of the group was observed after 60 s when a gap distance of 2 mm was used. In the case with

Fig. 10. Deconvolution of high resolution C1s spectra of the (a) control UHMWPE film, (b) FAS-5, (c) FAS-3, and (d) FAS-17 coatings deposited for 300 s at a gap distance of 2 mm.

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Fig. 11. Comparison of atomic percentages of chemical groups obtained from C1s peak-fitting of plasma polymerized (a) FAS-3, (b) FAS-5, and (c) FAS-17 coatings deposited at gap distances of 2 mm and 5 mm.

coatings deposited at a gap distance of 5 mm, the concentration of C\O is gradual over time. The increase in C\O bearing groups in the 2 mm gap distance coating could be a result of the chain scissioning of the ethyl groups in the silane end group of the monomer with long exposure to the afterglow or the incorporation of atomic oxygen at the active sites of the polymer chain through interactions with the atmosphere.

3.5. Film thickness and deposition rate SEM and profilometry were utilized to obtain estimates of the film thickness grown at different deposition times and electrode-substrate gap distances at static conditions. For consistency, all profilometry measurements were taken from the deposited area positioned directly under

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Fig. 12. Representative SEM images of (a) FAS-3, (b) FAS-5, and (c) FAS-17 coatings used to determine film thickness. All coatings shown were grown using a deposition time of 300 s with an electrode-substrate distance of 2 mm.

Fig. 13. (a) Film thickness as a function of deposition time of FAS-3, FAS-5, and FAS-17 coatings obtained from profilometry and SEM. The inset highlights the 0–150 nm thickness range measured for the FAS-17 coatings deposited at a gap distance of 2 mm and 5 mm gap. (b) Representative spatial profile of fluorocarbon-like coatings, FAS-5 precursor deposited at a gap distance of 2 mm for 60 s.

the APPJ. SEM was used to verify thickness measurements by looking at the cross-section of FAS-coated Si wafers. Representative SEM images are shown in Fig. 12. The film thickness of FAS coatings deposited at different gap distances is shown in Fig. 13a as a function of different deposition times. Comparing all 3 coatings, the FAS-3 monomer yielded the

thickest films, with a thickness c.a. 1.3 μm after 180 s of deposition time, whereas the FAS-17 monomer gave the thinnest coating of the 3 precursors. This was expected as the concentration of the FAS-3 monomer fed to the APPJ was 400 times more than that of the FAS-17 monomer. The thicknesses of FAS-5 coatings were found to lie between that of

Fig. 14. Comparison of C, O, F, and Si atomic concentrations in (a) FAS-3, (b) FAS-5, and (c) FAS-17 coatings deposited using a substrate-electrode gap distance of 2 mm and 5 mm as a function of deposition time. A dynamic deposition mode was used to apply the coatings.

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Fig. 15. Comparison of atomic percentages of chemical groups obtained from C1s peak-fitting of plasma polymerized (a) FAS-3, (b) FAS-5, and (c) FAS-17 coatings deposited at gap distances of 2 mm and 5 mm in dynamic mode.

the FAS-3 and FAS-17 coatings, with thicknesses ranging from 0.2 to 1.0 μm. The coating thickness was found to vary spatially due to the spreading of the afterglow region upon contact with the substrate as seen in the profile of the FAS-5 coating seen in Fig. 13b. The resultant shape of the profile due to this spreading effect alludes that a more localized deposition of the precursor radicals occurs. This is seen from the area of the UHMWPE film located directly under the jet as it is thicker than the surrounding area due to the direct exposure of the afterglow.

This profile is characteristic of the other coatings, as the shape of the curves was similar with the exception in the difference in height. The presences of spikes in the profile are artifacts generated by debris on the coatings and have no physical relations to the as-deposited coatings. Another distinctive observation seen from thickness measurements is the effect of gap distance. This occurrence is more pronounced with the FAS-3 and FAS-5 coatings formed with a 2 mm gap distance as the thickness values are about 2–3 times higher than the coatings deposited with

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the 5 mm gap distance. This was expected as shorter gap distances limit the residence time and depletion of the chemical species in the afterglow. Based on thickness measurements, the deposition rates of the FAS-3, FAS-5, and FAS-17 were approximately 185, 158, and 15 nm min−1, respectively. Again, these growth rates can reflect upon the amount of precursor fed in the plasma. However, etching processes can be taken into consideration. It is known that atomic fluorine is an etchant [40], so the relation between thickness and etching of alkylfluorosilane coatings can be tied to the concentration of fluorine atoms in the monomer, however this claim needs to be further substantiated from measurements of the plasma source. The higher concentration of fluorine atoms present in the FAS-17 precursor could impose a competition between etching and polymerization processes, where etching can be prevalent thus resulting in thinner films. To verify whether etching is a significant factor in the deposition rate, further studies and characterization of the plasma source are needed. 3.6. Dynamic versus static deposition This study also looked at properties of the grown films, specifically the chemistry, when applying a transient mode of deposition. A comparison between static and dynamic depositions was done through XPS measurements of the atomic species (Fig. 14) and the chemical groups (Fig. 15) present in the coatings. One of the challenges associated with operating in open atmosphere is the control over the chemistry of the thin film as the interactions of the plasma with ambient air are inevitable. When comparing coatings that were administered on UHMWPE films statically and dynamically, minor differences were observed. For FAS-5 coatings, no discernible differences were seen. However, for FAS-3 and FAS-17 coatings, dissimilarities were found in the compositional make up of the films. FAS-3 coatings deposited at a gap distance of 2 mm showed a sharp increase in C\O and CF3 groups after 60 s of treatment time and then plateaus thereon when dynamically exposed to the APPJ compared to coatings deposited on fixed PE films. Based on these findings, the potential to deposit fluorocarbon-based coatings using a moveable platform is promising, especially for large-scale deposition applications.

exemplary in the case of the FAS-5 coatings derived from the aromaticbased precursor as the ring structure was found to be intact during the deposition process based on spectroscopic measurements. From experimental findings, processing conditions such as the APPJ-substrate gap distance played a factor in the physical properties of the coatings. The gap distance affected the deposition rate of the FAS-3 and FAS-5 coatings as well the film composition of FAS-3 and FAS-17 coatings. The shortest gap distance was found to be the optimum in the case of FAS-17 coatings. In summary, the deposition of fluorocarbon coatings with hydrophobic properties can be achieved at atmospheric conditions by using fluorinated monomers containing (CF2)n, where n ≥ 7, and regulating plasma parameters.

Acknowledgments This research was supported in part by an appointment to the Postgraduate Research Program at the U.S. Army Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USARL.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

4. Conclusions

[15] [16]

This study has shown the feasibility of depositing hydrophobic coatings at atmospheric conditions using liquid monomer precursors through a PECVD process. Based on overall observations from this study, using liquid-based precursors with an APPJ system are viable alternatives to the conventional gaseous-based small molecular weight per-fluorinated precursors reported in earlier works [26,43,44] with different atmospheric pressure plasma systems. The chemical compositions of the reported coatings are comparable with coatings in this work, specifically coatings deposited using the FAS-17 precursor as various types of fluorocarbon groups (CFn) were identified as the dominate species. Water contact angles of studied coatings (90–116°) were found to lie within the range of reported values (100–135°). The distinguishable difference observed was the achieved growth rates of the coatings, however this can be acknowledged due to different plasma systems and conditions used. Optimal hydrophobic behavior exhibited by PECVD coated UHMWPE films were achieved from large molecular mass precursor, heptadecafluoro-1,1,2,2-tetrahydrodecyl triethoxysilane (FAS-17), owing to its long alkyl chain consisting of eight fluorocarbon (CF2 and CF3) groups. This was evident from WCA measurements, as angles of 110–116° were achieved with FAS-17 coatings. XPS and ATR-FTIR analyses further support the strong dependency of the hydrophobicity to monomer chemistry. Compositional analyses of FAS-17 coatings suggest that the hydrophobic property of the polymer film is largely contingent on the concentration of CF2 groups when compared to the other coatings explored in this study. Overall, the chemical composition of plasma polymerized PECVD fluorocarbon films was found to closely resemble that of the monomer. This was true with all coatings and

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