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Surface modification of polyimide films using unipolar nanosecond-pulse DBD in atmospheric air
Applied Surface Science 256 (2010) 3888–3894
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface modification of polyimide films using unipolar nanosecond-pulse DBD in atmospheric air Tao Shao a,b,*, Cheng Zhang a, Kaihua Long a, Dongdong Zhang a, Jue Wang a, Ping Yan a, Yuanxiang Zhou b a b
Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China State Key Laboratory of Control and Simulation of Power Systems and Generation Equipments, Electrical Engineering Department, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 December 2009 Received in revised form 13 January 2010 Accepted 13 January 2010 Available online 22 January 2010
DBD-induced surface modification is very versatile to increase the adhesion or hydrophilicity of polymer films. In this paper, the DBD is produced by repetitive unipolar nanosecond pulses with a rise time of 15 ns and a full width at half maximum of about 30 ns. The power densities of the homogeneous and filamentary DBDs during plasma treatment are 158 and 192 mW/m2, respectively, which are significantly less than that using ac DBD processing, and the corresponding plasma dose is also mild compared to AC DBD treatment. Surface treatment of polyimide films using the homogeneous and filamentary DBDs is studied and compared. The change of chemical and physical modification of the surface before and after plasma processing has been evaluated. It can be found that both surface morphology and chemical composition are modified, and the modification includes the rise of hydrophilicity, surface oxidation and the enhancement of surface roughness. Furthermore, the homogeneous DBD is more effective for surface processing than the filamentary DBD, which can be attributed to the fact that the homogeneous DBD can modify the surface more uniformly and introduce more polar functional groups. ß 2010 Elsevier B.V. All rights reserved.
Keywords: Dielectric barrier discharge (DBD) Polyimide Surface modification Nanosecond pulse Homogeneous DBD Filamentary DBD
1. Introduction Polymer materials have been extensively used for various industrial applications, for example, polyimide (PI) has been applied to act as substrate material in flexible electronic technology because it has excellent characteristics of high tensile strength, good thermal stability and chemical resistance [1,2]. In order to enhance the adhesion between PI and metal films, surface modification is accordingly required. Compared with other methods such as chemical technique, electrochemical process, and photo-irradiation and so on, the treatment using largevolume non-thermal plasma produced by dielectric barrier discharge (DBD) is an economic, reliable and convenient method [4–6]. Advances in the use of atmospheric pressure discharges, particularly such as DBD, have made it possible to treat polymer surfaces rapidly, continuously and uniformly without using vacuum equipment. Surface modification of polymer films using DBD has been reported by some researchers [7–16], and the adhesion or hydrophilicity properties have been dramatically
* Corresponding author at: Institute of Electrical Engineering, Chinese Academy of Sciences, PO Box 2703, Beijing 100190, China. Tel.: +86 1082547118; fax: +86 1082547116. E-mail address: [email protected] (T. Shao). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.01.045
improved. Non-thermal plasma resulted from DBD generates abundant excited species, free radicals and ions, which leads to the reactions with the first few nanometers of the film and results in chemical and physical changes [14]. In most cases, DBD is driven by an AC power source of 50 Hz or kHz of periodic sine or square waves [7–23]. DBD often works on a filament mode via many streamer micro-discharges, which would result in inhomogeneous treatment and partial thermal degradation of the treated films [18–20]. From a practical point of view, homogeneous discharge at atmospheric pressure is a very desirable condition. However, DBD treatment in the homogeneous mode requires special arrangements [7,17]. Compared with the common DBD using AC power source, DBD using unipolar pulse voltage can avoid the local overheat of micro-discharges, and improve discharge efficiency under certain conditions [24–33]. In our previous papers [34–36], it is found that electrical characteristics of the DBD excited by repetitive unipolar nanosecond pulses are different from that reported ever, such as discharge voltage and current across air gap behaves bipolar pulses and the peak of discharge current can be on an order of hundreds of amperes. In the present work, considering the potential predominance of DBD treatment using repetitive nanosecond-pulse power source, surface modification by the filamentary and homogeneous DBDs is achieved in atmospheric air. The effect of the DBD plasma treatment on surface property and adhesion characteristic of PI films is evaluated by means of water contact angle measurement, scanning electron microscopy (SEM),
T. Shao et al. / Applied Surface Science 256 (2010) 3888–3894
atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). 2. Experimental 2.1. Material The material to be treated was a commercial PI film with a thickness of 0.075 5 mm and an area of 40 mm 40 mm. All samples were rinsed first with alcohol, and then cleaned with deionized water using an ultrasonic cleaner, finally dried in vacuum drying setup before plasma treatment. PI samples were placed on the lower glass plane covering the grounded planar aluminum electrodes. For all these experiments, plasma treatments were performed in air at atmospheric pressure and room temperature. 2.2. Experimental setup for DBD treatment A schematic diagram of the experimental arrangement is shown in Fig. 1. A solid-state pulse generator was used to produce repetitive nanosecond pulses in the treatments [34–36]. Output high voltage pulse of the generator has a rise time of about 15 ns and a full width at half maximum of 20–30 ns. Pulse repetition frequency varied from single shot to 2 kHz and was controlled by a trigger modulator. The generator was an inductive energy storage type, and output voltage was changed by cycled saltwater solution via a parallel connection with the DBD treatment reactor. Plasma treatment was conducted by fixing applied voltage at about 50 kV and pulse repetition frequency at 250 Hz. The DBD was created between two circular plane-parallel aluminum electrodes with a diameter of 7 cm, and both the electrodes were covered by glass planes with different thickness between 1 mm and 4 mm, and an area of 100 mm 100 mm. Similar to the measurement described in [34,35], wide bandwidth voltage and current probes were used to monitor the electrical parameters of the DBD circuit. The voltage probe was a capacitive voltage divider connected to the high-voltage output of SPG200N. The current probe was a current diverter made of a coaxial tubular high-frequency resistor shunt. A Lecroy oscilloscope (WR204Xi, with a bandwidth of 2 GHz and a time resolution of 10 GS/s) was used to record the electrical signals. In addition, discharge images were recorded by a commercial digital camera SONY DSC-H9, which has an exposure time of 0.5 s.
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Static contact angle measurement was made immediately after DBD treatment by dropping 2 ml distilled water on the PI surface. The values of static contact angle were obtained using Laplace– Young curve fitting based on the imaged water drop profile and were the average of eight measured data on different locations. Surface morphology was investigated by two methods. Firstly, a SEM (JEOL JSM-6701F) observation was used, and the film samples were sputtered and coated with a thin layer of gold on the surface before analysis. Furthermore, an AFM (Digital Instrument D3100) image in tapping mode was performed as a high-resolution method to determine topography and roughness, and the corresponding analysis was done using Nanoscope software. In order to investigate surface chemical characterization, an XPS (VG Scientific ESCALab220i-XL) analysis was carried out on PI surface. The analysis used nonmonochromatic Mg Ka radiation operating at 300 W and the pressure in the analyzing chamber was maintained at 3 10 9 mbar. All XPS-peaks were referred to the C 1s signal at a binding energy of approximately 285 eV [10], and curve fitting of the C 1s peak was done using XPSpeak 4.1 software. 3. Results 3.1. Optical and electrical characterization of the DBD For the treatment, two discharge modes are used, which are referred to as the homogeneous DBD (HDBD) and filamentary DBD (FDBD). Fig. 2 gives images of the typical discharge modes. In the case of the HDBD, two glass planes of 3 mm in thickness covered both electrodes respectively and the air gap spacing was 2 mm, no filament was observed and the discharge was homogeneous in the whole discharge regime. In contrast, in the case of the FDBD, both electrodes were covered by two glass layers of 2 mm in thickness and the air gap spacing was 6 mm. It can be found that discharge phenomenon in Fig. 2a is different from that in Fig. 2b, where the filaments perpendicular to the electrodes are randomly distributed in the air gap. It should be pointed out that the photographs are taken by a commercial digital camera and not recorded by a highspeed CCD camera. In this paper, the photographs taken by a commercial digital camera are used to distinguish the two discharge modes [34,36].
2.3. Measurement for surface analysis In view of the measurement requirement of surface analysis, contact angle of water, SEM, AFM, and XPS were used in surface analysis.
Fig. 1. Schematic view of the DBD set-up used for surface modification (1, repetitive nanosecond-pulse power supply; 2, capacitive voltage divider; 3, upper aluminum electrode; 4, glass planes; 5, PI films; 6, digital camera; 7, lower aluminum electrode; 8, Lecroy WR204Xi oscilloscope; 9, current viewing resistor; 10, trigger modulator).
Fig. 2. Two typical DBD images during the treatment experiments: (a) the HDBD mode and (b) the FDBD mode.
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Fig. 3. Voltage and current waveforms for the HDBD (a) and FDBD (b) (1, applied voltage; 2, calculated voltage on air gap; 3, measured DBD current; 4, calculated discharge current across air gap).
The most common electrical parameters determining the DBD characterization consist of applied voltage and measured DBD current, which are shown in Fig. 3. The experimental conditions are the same as that described in Fig. 2. According to the equivalent electrical model [34,37], discharge voltage and current across the air gap are calculated and shown in Fig. 3, respectively. It can be seen that though applied voltage is a positive pulse, measured DBD current appears bipolar and has positive and negative pulses. The measured DBD current is on an order of 100 A even if both of the electrodes are covered with a glass of 3 mm in thickness, which is significantly different from discharge current in AC DBD with an amplitude below one ampere, typically on the order of tens to hundreds of mA. The bipolar DBD current indicates that two consecutive discharges per applied pulse voltage occur. The primary discharge starts at the rising edge of applied voltage and the secondary discharge begins at the falling edge of applied voltage. Moreover, the time interval from the quenching of the primary discharge to the start of the secondary discharge is almost zero, which is also different from that ever reported by other researchers within about tens to hundreds of nanoseconds [28,30– 33]. According to the equations for discharge power, energy deposition in the air gap can be calculated. In the case of the surface modification using HDBD and FDBD, the power densities during plasma processing are 158 and 192 mW/cm2, respectively. In view of the equation of power density for the two cases, the energy density is controlled by treatment time [9]. If the treatment time is determined, the energy density is proportional to the treatment time for the two cases. So we describe the changes of surface in the plasma processes as a function of treatment time in this paper.
using the HDBD remains low and indicates the uniformity of the HDBD treatment. As to DBD plasma processing of polymer films, it has been known that water contact angle would recover and lead to a reduction in hydrophilicity after plasma treatment with exposure to ambient air for an ageing time. Fig. 5 shows this variation of water contact angles measured on the treated films for various treatment times as a function of ageing time, and the experimental results using the FDBD and HDBD treatments are presented in Fig. 5a and b, respectively. For both treatments, it can be found that contact angle rapidly increases during the first 3 days, and then reaches a stable value of about 608. This behavior can be attributed to the chemical reactions of the remaining active radicals on the PI surface with oxygen or moisture in ambient air, or movement of the oxygen-containing hydrophilic polar groups into the bulk of PI films [8,18–21]. It should be pointed out that the power densities in these treatments are rather low to form intermolecular bonds between the newly produced oxygen complex and the film surfaces compared to other studies [18–21]. Similar result of contact angle increasing with ageing time was also obtained, and it was concluded that the ageing problem depended on the kind of polymer material, treatment gas, and ambient conditions rather than plasma treatment process [38,39]. During ageing, the dispersion of the contact angle data remains low among various treatment times, perhaps suggesting that the surface is not significantly degraded after short duration treatment and has a
3.2. Water contact angle measurements Measurement of static water contact angle can be used to evaluate the adhesion or hydrophilicity of polymer surface. Fig. 4 depicts the evolution of contact angle of water on PI surface as a function of treatment time. It is seen that water contact angle decreases with increasing treatment time, and it indicates an increase in hydrophilicity. The water contact angle of the untreated film is 71.08, and decreases to minimum 29.98 and 27.38 for 10 s FDBD and HDBD treatments respectively. However, when the PI film is treated over 10 s, the contact angle does not change anymore with treatment time. This suggests that there is a saturation effect of plasma treatment, and some similar results can be found on other polymer films [9–14,17]. In addition, compared to the FDBD treatment, the dispersion of the contact angle data
Fig. 4. Water contact angle on the surface of PI films with the FDBD and HDBD treatments.
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Fig. 5. Contact angle of water on PI film versus treatment time and ageing: (a) the FDBD treatment and (b) the HDBD treatment.
rapid recovery. From a practical point of view, it suggests that too long treatment time is not necessary, and a continuous operation should be used to avoid the aging effect immediately after the treatment. 3.3. Morphology analysis of surface appearance SEM analysis before and after treatment gives information on the etching function of FDBD and HDBD. Fig. 6a shows the SEM picture before plasma treatment. Little spots can be seen on the SEM picture and the untreated surface is very smooth. Fig. 6b shows the SEM micrograph after 10 s FDBD treatment. It can be seen that the surface has been etched, and some isolated grain-like structures occur with widely separated grains. The SEM micrograph of PI surface treated by HDBD is shown in Fig. 6c, which also corresponds to a treatment time of 10 s. The surface shows a fine grain-like structure with a wide spread in grain diameters, and the distribution of grain-like structures is widely uniform compared to that observed in Fig. 6b. It indicates that the HDBD treatment seems to be more uniform and indirectly suggests that HDBD is more homogeneous than FDBD. To confirm the better treatment using HDBD than FDBD indicated by the SEM observation, some samples were also investigated by AFM. Fig. 7a–c presents AFM images before and after the FDBD and HDBD treatments, respectively, which corresponds to a treatment time of 10 s and a scanning area of 1 mm 1 mm. The values of the mean surface roughness (Ra), root mean square surface roughness (Rrms), and maximum surface roughness (Rmax) obtained from the AFM scans are tabulated in Table 1. A clear contrast between the surface topographies
Fig. 6. SEM observation of the untreated and treated PI films. (a) Untreated, (b) after 10 s FDBD treatment and (c) after 10 s HDBD treatment.
resulting from different treatments can be seen. The untreated PI shown in Fig. 7a presents a very smooth surface with no distinguishable features and the roughness is lower (Rmax = 9.158 nm) than the treated films. Fig. 7b shows a new granular structure of the PI surface treated by FDBD. The surface is very rough (Rmax = 44.315 nm) and almost five times rougher than the untreated sample. Compared with the FDBD treatment in Fig. 7b, the distribution of the granular structure presented in Fig. 7c is much proportional spacing and homogeneous, and the roughness is relatively smaller (Rmax = 23.196 nm). The topography and roughness of PI films are modified after plasma
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Fig. 7. AFM images of the untreated and treated PI films (1 mm 1 mm). (a) Untreated, (b) after 10 s FDBD treatment and (c) after 10 s HDBD treatment.
modification, which is due to the very energetic active species that are capable of etching the surface and breaking the polymer chains. Furthermore, both SEM and AFM prove that the HDBD treatment is more uniform than that using the FDBD plasma.
groups of the untreated and treated films. Fig. 8a shows the XPS spectra before and after 10 s FDBD and HDBD plasma treatments. The XPS spectra of the untreated surface show peaks corresponding to binding energies of C 1s (285 eV), N 1s (400 eV) and O 1s (532 eV). It can be seen that, after the FDBD or HDBD plasma treatment, the intensity of the C 1s peak is decreased, whereas that of N 1s and O 1s is increased. Furthermore, compared to the intensities of the C 1s, O 1s, and N 1s peaks after the FDBD treatment, the peak of C 1s using the HDBD treatment is lower and that of the N 1s and O 1s peaks is larger. Table 2 shows C, N, O atomic compositions for the untreated samples and treated samples. It can be found that the surface modification leads to an increase in the O/C atomic ratio from 0.238 for the untreated sample to 0.351 for 10 s FDBD treated sample, and to 0.427 for 10 s FDBD treated sample. The oxygen increase indicates that oxygencontaining polar groups are formed in the surface. Table 2 also shows a slight increase in N/C atomic ratio, and it suggests that. One source could be some nitrogen-containing groups are also formed due to the nitrogen in atmospheric air. Another is probable that the surface cleaning allows the fingerprint of the material bulk to appear on the spectra and the nitrogen percentage thus appears to increase [22]. It is well known that the chemical functional groups responsible for the increase in the adhesion or hydrophilicity of the polymer are mainly related to the C 1s peak. According to the XPS survey scans shown in Fig. 8a, high-resolution XPS-spectra of the C 1s peaks are performed. The C 1s spectra before and after the plasma treatment are represented in Fig. 8b. The C 1s spectrum from the untreated and treated samples is decomposed into 4 components: one at 284.7 eV due to C–C groups, one at 285.6 eV due to C–N groups, one at 286.2 eV due to C–O groups, and 288.6 eV due to C5 5O groups [40]. The calculated concentration of each chemical component by deconvolution using Gaussian functions is also summarized in Table 2. It can be found from Table 2 that after the DBD plasma treatment the C–C group decreases, while the C–O, C5 5O, and C–N groups increase. This means that air plasma mainly attacks the C–C groups, and forms more oxygen-containing groups. The introduction of the polar oxygen-containing groups is responsible for the higher adhesion or hydrophilicity of PI surface after plasma treatment. The above results indicate that the DBD plasma treatment introduces oxygencontaining functional groups into the molecular chain and enhances surface energy of PI surface. In this case, the HDBD treatment results in higher adhesion than the FDBD treatment, which is attributed to more polar functional groups introduced into PI surface. 4. Discussions
3.4. Surface composition analysis DBD in atmospheric air can produce a wide range of active species, air plasma increases surface energy by introducing oxygen-containing polar group into the polymer surface, and atomic oxygen is thought to be the main reactive species responsible for this oxygen inclusion [9–14]. XPS can be used to elaborate the change of the atomic compositions and possible Table 1 Ra, Rrms, and Rmax obtained by AFM for the untreated and treated (at the FDBD and HDBD modes) PI films. Sample
Untreated FDBD HDBD
Surface roughness (nm) Ra
Rrms
Rmax
0.762 1.937 2.035
0.988 3.894 2.910
9.158 44.315 23.196
Surface analysis before and after plasma treatment indicates that HDBD is more effective than FDBD. This can be attributed to the discharge characteristic of this treatment. On the one hand, the treatment using HDBD can provide more active species than that of FDBD. Surface modification is a processing of interaction between the plasma and surface of the polymer to be treated. Compared with the air gap spacing of 6 mm in the FDBD treatment, the air gap spacing is only 2 mm in the HDBD treatment. In the case of the FDBD treatment, the filamentary discharge bridges the air gap and the active species are produced along the filamentary channel, and therefore lots of them cannot reach the PI surface. However, in the case of HDBD, the discharge was homogeneous in the whole discharge regime and the majority of the active species can reach PI surface and interact with it. On the other hand, the discharge uniformity can account for better treatment effect using the HDBD treatment, and Figs. 2 and 6–7 can prove HDBD is more homogeneous that FDBD.
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Fig. 8. XPS taken on the PI surface before and after 10 s FDBD or HDBD treatment: (a) the O 1s, N 1s, C 1s peaks and (b) the C 1s peak.
Table 2 Chemical composition of the PI surface before and after 10 s FDBD and HDBD treatment. Sample
Untreated FDBD HDBD a
Atomic composition (%)
C 1s possible groups (%)
C
O
N
C–C
C–N
C–O
C5 5O
285a
532a
400a
284.7a
285.6a
286.3a
288.6a
75.37 69.03 64.39
17.93 24.22 27.48
6.7 6.76 8.13
74.53 67.85 63.33
3.84 3.87 4.42
13.30 18.27 21.88
8.33 10.10 10.37
Binding energy (eV).
It can be found that electrical parameters in repetitive unipolar nanosecond-pulse DBD are much better than that in the DBD using AC excitation and are also superior compared with those in submicrosecond pulse DBD [28–32]. Nanosecond-pulse discharge is a typical over-voltage breakdown, and breakdown voltage can exceed that using DC, AC, or microsecond-pulse source. The over-voltage breakdown will result in a higher initial electric field or reduced electric field E/N and lead to plasma-chemical processes such as atomic or molecular electronic excitation and dissociation enhancing rapidly with E/N [27]. Accordingly, the DBD produced by unipolar high-voltage pulses can produce much powerful plasma and active particles, and the application in surface modification of polymers will be more efficient. This efficiency has also been experimentally confirmed by many investigations [27–32]. Compared with the common DBD treatment of polymer surface using ac power source, the treatment using FDBD or HDBD is much mild. In this case, the power density during the FDBD processing (192 mW/m2) is slightly larger than that using the HDBD treatment (158 mW/m2). Moreover, plasma dose is an important parameter, which controls the treatment efficiency, and it can be calculated from the production of the power density and treatment time. For a treatment time of 10 s, the corresponding plasma doses are 1.58 103 mJ/cm2 and 1.92 103 mJ/cm2 in the HDBD and FDBD treatment respectively. In addition, for polymer or
fabrics, many studies have reported that a localized strong degradation may take place. Some substantial heavy particle heating takes place in the gas phase and lead to the heating of the surface, and then, direct surface damage by partial melting would occur [18,19]. It is probable due to the non-homogeneity of the DBD and localized melt resulting from micro-discharges. However, in this case, even if the instantaneous peak power is very high and can reach a megawatt order [34], the mean energy dissipated by the DBD plasma is relatively mild and no degradation of the surface of PI film takes place. The treated samples show no visual difference compared to the untreated case, even for an extended treatment time of 30 s or longer. 5. Conclusion Surface properties and adhesion characteristics of PI surface by the DBD plasma treatment using repetitive nanosecond pulses are investigated. Adhesion properties of PI surfaces can be improved not only by enhancing surface energy by introducing some functional groups, but also by increasing surface roughness by plasma etching. From the results of water contact angle measurement and XPS analysis, surface energy is enhanced and some oxygen-containing functional groups are incorporated. As shown in the observations of SEM and AFM, PI surface has been etched and
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the surface roughness is evidently increased. It can be seen that surface morphology and chemical composition of the PI films are modified, and HDBD is more effective for surface processing than FDBD. Acknowledgements This work was supported by the National Natural Science Foundation of China under contract NSFC 50707032, the Knowledge Innovation Program of the Chinese Academy of Sciences under Contract KGCX2-YW-339, and State Key Laboratory of Control and Simulation of Power System and Generation Equipment in Tsinghua University under contract SKLD09KZ05. References [1] A. Bruno Frazier, C.H. Ahn, M.G. Allen, Sens. Actuators A-Phys. 45 (1994) 47–55. [2] J.P. Moliton, T. Trigaud, B. Mazie`re, C. Aubry, Mater. Sci. Semicond. Process 3 (2000) 419–425. [4] U. Kogelschatz, Plasma Chem. Plasma Process 23 (2003) 1–46. [5] A. Fridman, A. Chirokov, A. Gutsol, J. Phys. D: Appl. Phys. 38 (2005) R1–24. [6] R. Morent, N. De Geyter, J. Verschuren, K. De Clerck, P. Kiekens, C. Leys, Surf. Coat. Technol. 202 (2008) 3427–3449. [7] F. Massines, R. Messaoudi, C. Mayoux, Plasmas Polym. 3 (1998) 43–59. [8] N. Cui, M.D. Brown, Appl. Surf. Sci. 189 (2002) 31–38. [9] Z. Fang, L. Hao, H. Yang, X. Xie, Y. Qiu, K. Edmund, Appl. Surf. Sci. 255 (2009) 7279– 7285. [10] G. Borcia, C.A. Anderson, N. Brown, Plasma Sources Sci. Technol. 12 (2003) 335– 344. [11] N. De Geyter, R. Morent, C. Leys, L. Gengembre, E. Payen, S. Van Vlierberghe, E. Schacht, Surf. Coat. Technol. 202 (2008) 3000–3010. [12] R. Morent, N. De Geyter, C. Leys, Nucl. Instrum. Methods Phys. Res. Sect. B-Beam Interact. Mater. Atoms 266 (2008) 3081–3085. [13] N. De Geyter, R. Morent, C. Leys, L. Gengembre, E. Payen, Surf. Coat. Technol. 201 (2007) 7066–7075. [14] D.J. Upadhyay, N. Cui, B.J. Meenan, M.D. Brown, J. Phys. D: Appl. Phys. 38 (2005) 922–929. [15] C. Liu, M.D. Brown, B.J. Meenan, Appl. Surf. Sci. 252 (2006) 2297–2310. [16] C. Sarra-Bournet, S. Turgeon, D. Mantovani, G. Laroche, Plasma Process. Polym. 3 (2006) 506–515.
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Surface modification of polyimide films using unipolar nanosecond-pulse DBD in atmospheric air Tao Shao a,b,*, Cheng Zhang a, Kaihua Long a, Dongdong Zhang a, Jue Wang a, Ping Yan a, Yuanxiang Zhou b a b
Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China State Key Laboratory of Control and Simulation of Power Systems and Generation Equipments, Electrical Engineering Department, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 December 2009 Received in revised form 13 January 2010 Accepted 13 January 2010 Available online 22 January 2010
DBD-induced surface modification is very versatile to increase the adhesion or hydrophilicity of polymer films. In this paper, the DBD is produced by repetitive unipolar nanosecond pulses with a rise time of 15 ns and a full width at half maximum of about 30 ns. The power densities of the homogeneous and filamentary DBDs during plasma treatment are 158 and 192 mW/m2, respectively, which are significantly less than that using ac DBD processing, and the corresponding plasma dose is also mild compared to AC DBD treatment. Surface treatment of polyimide films using the homogeneous and filamentary DBDs is studied and compared. The change of chemical and physical modification of the surface before and after plasma processing has been evaluated. It can be found that both surface morphology and chemical composition are modified, and the modification includes the rise of hydrophilicity, surface oxidation and the enhancement of surface roughness. Furthermore, the homogeneous DBD is more effective for surface processing than the filamentary DBD, which can be attributed to the fact that the homogeneous DBD can modify the surface more uniformly and introduce more polar functional groups. ß 2010 Elsevier B.V. All rights reserved.
Keywords: Dielectric barrier discharge (DBD) Polyimide Surface modification Nanosecond pulse Homogeneous DBD Filamentary DBD
1. Introduction Polymer materials have been extensively used for various industrial applications, for example, polyimide (PI) has been applied to act as substrate material in flexible electronic technology because it has excellent characteristics of high tensile strength, good thermal stability and chemical resistance [1,2]. In order to enhance the adhesion between PI and metal films, surface modification is accordingly required. Compared with other methods such as chemical technique, electrochemical process, and photo-irradiation and so on, the treatment using largevolume non-thermal plasma produced by dielectric barrier discharge (DBD) is an economic, reliable and convenient method [4–6]. Advances in the use of atmospheric pressure discharges, particularly such as DBD, have made it possible to treat polymer surfaces rapidly, continuously and uniformly without using vacuum equipment. Surface modification of polymer films using DBD has been reported by some researchers [7–16], and the adhesion or hydrophilicity properties have been dramatically
* Corresponding author at: Institute of Electrical Engineering, Chinese Academy of Sciences, PO Box 2703, Beijing 100190, China. Tel.: +86 1082547118; fax: +86 1082547116. E-mail address: [email protected] (T. Shao). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.01.045
improved. Non-thermal plasma resulted from DBD generates abundant excited species, free radicals and ions, which leads to the reactions with the first few nanometers of the film and results in chemical and physical changes [14]. In most cases, DBD is driven by an AC power source of 50 Hz or kHz of periodic sine or square waves [7–23]. DBD often works on a filament mode via many streamer micro-discharges, which would result in inhomogeneous treatment and partial thermal degradation of the treated films [18–20]. From a practical point of view, homogeneous discharge at atmospheric pressure is a very desirable condition. However, DBD treatment in the homogeneous mode requires special arrangements [7,17]. Compared with the common DBD using AC power source, DBD using unipolar pulse voltage can avoid the local overheat of micro-discharges, and improve discharge efficiency under certain conditions [24–33]. In our previous papers [34–36], it is found that electrical characteristics of the DBD excited by repetitive unipolar nanosecond pulses are different from that reported ever, such as discharge voltage and current across air gap behaves bipolar pulses and the peak of discharge current can be on an order of hundreds of amperes. In the present work, considering the potential predominance of DBD treatment using repetitive nanosecond-pulse power source, surface modification by the filamentary and homogeneous DBDs is achieved in atmospheric air. The effect of the DBD plasma treatment on surface property and adhesion characteristic of PI films is evaluated by means of water contact angle measurement, scanning electron microscopy (SEM),
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atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). 2. Experimental 2.1. Material The material to be treated was a commercial PI film with a thickness of 0.075 5 mm and an area of 40 mm 40 mm. All samples were rinsed first with alcohol, and then cleaned with deionized water using an ultrasonic cleaner, finally dried in vacuum drying setup before plasma treatment. PI samples were placed on the lower glass plane covering the grounded planar aluminum electrodes. For all these experiments, plasma treatments were performed in air at atmospheric pressure and room temperature. 2.2. Experimental setup for DBD treatment A schematic diagram of the experimental arrangement is shown in Fig. 1. A solid-state pulse generator was used to produce repetitive nanosecond pulses in the treatments [34–36]. Output high voltage pulse of the generator has a rise time of about 15 ns and a full width at half maximum of 20–30 ns. Pulse repetition frequency varied from single shot to 2 kHz and was controlled by a trigger modulator. The generator was an inductive energy storage type, and output voltage was changed by cycled saltwater solution via a parallel connection with the DBD treatment reactor. Plasma treatment was conducted by fixing applied voltage at about 50 kV and pulse repetition frequency at 250 Hz. The DBD was created between two circular plane-parallel aluminum electrodes with a diameter of 7 cm, and both the electrodes were covered by glass planes with different thickness between 1 mm and 4 mm, and an area of 100 mm 100 mm. Similar to the measurement described in [34,35], wide bandwidth voltage and current probes were used to monitor the electrical parameters of the DBD circuit. The voltage probe was a capacitive voltage divider connected to the high-voltage output of SPG200N. The current probe was a current diverter made of a coaxial tubular high-frequency resistor shunt. A Lecroy oscilloscope (WR204Xi, with a bandwidth of 2 GHz and a time resolution of 10 GS/s) was used to record the electrical signals. In addition, discharge images were recorded by a commercial digital camera SONY DSC-H9, which has an exposure time of 0.5 s.
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Static contact angle measurement was made immediately after DBD treatment by dropping 2 ml distilled water on the PI surface. The values of static contact angle were obtained using Laplace– Young curve fitting based on the imaged water drop profile and were the average of eight measured data on different locations. Surface morphology was investigated by two methods. Firstly, a SEM (JEOL JSM-6701F) observation was used, and the film samples were sputtered and coated with a thin layer of gold on the surface before analysis. Furthermore, an AFM (Digital Instrument D3100) image in tapping mode was performed as a high-resolution method to determine topography and roughness, and the corresponding analysis was done using Nanoscope software. In order to investigate surface chemical characterization, an XPS (VG Scientific ESCALab220i-XL) analysis was carried out on PI surface. The analysis used nonmonochromatic Mg Ka radiation operating at 300 W and the pressure in the analyzing chamber was maintained at 3 10 9 mbar. All XPS-peaks were referred to the C 1s signal at a binding energy of approximately 285 eV [10], and curve fitting of the C 1s peak was done using XPSpeak 4.1 software. 3. Results 3.1. Optical and electrical characterization of the DBD For the treatment, two discharge modes are used, which are referred to as the homogeneous DBD (HDBD) and filamentary DBD (FDBD). Fig. 2 gives images of the typical discharge modes. In the case of the HDBD, two glass planes of 3 mm in thickness covered both electrodes respectively and the air gap spacing was 2 mm, no filament was observed and the discharge was homogeneous in the whole discharge regime. In contrast, in the case of the FDBD, both electrodes were covered by two glass layers of 2 mm in thickness and the air gap spacing was 6 mm. It can be found that discharge phenomenon in Fig. 2a is different from that in Fig. 2b, where the filaments perpendicular to the electrodes are randomly distributed in the air gap. It should be pointed out that the photographs are taken by a commercial digital camera and not recorded by a highspeed CCD camera. In this paper, the photographs taken by a commercial digital camera are used to distinguish the two discharge modes [34,36].
2.3. Measurement for surface analysis In view of the measurement requirement of surface analysis, contact angle of water, SEM, AFM, and XPS were used in surface analysis.
Fig. 1. Schematic view of the DBD set-up used for surface modification (1, repetitive nanosecond-pulse power supply; 2, capacitive voltage divider; 3, upper aluminum electrode; 4, glass planes; 5, PI films; 6, digital camera; 7, lower aluminum electrode; 8, Lecroy WR204Xi oscilloscope; 9, current viewing resistor; 10, trigger modulator).
Fig. 2. Two typical DBD images during the treatment experiments: (a) the HDBD mode and (b) the FDBD mode.
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Fig. 3. Voltage and current waveforms for the HDBD (a) and FDBD (b) (1, applied voltage; 2, calculated voltage on air gap; 3, measured DBD current; 4, calculated discharge current across air gap).
The most common electrical parameters determining the DBD characterization consist of applied voltage and measured DBD current, which are shown in Fig. 3. The experimental conditions are the same as that described in Fig. 2. According to the equivalent electrical model [34,37], discharge voltage and current across the air gap are calculated and shown in Fig. 3, respectively. It can be seen that though applied voltage is a positive pulse, measured DBD current appears bipolar and has positive and negative pulses. The measured DBD current is on an order of 100 A even if both of the electrodes are covered with a glass of 3 mm in thickness, which is significantly different from discharge current in AC DBD with an amplitude below one ampere, typically on the order of tens to hundreds of mA. The bipolar DBD current indicates that two consecutive discharges per applied pulse voltage occur. The primary discharge starts at the rising edge of applied voltage and the secondary discharge begins at the falling edge of applied voltage. Moreover, the time interval from the quenching of the primary discharge to the start of the secondary discharge is almost zero, which is also different from that ever reported by other researchers within about tens to hundreds of nanoseconds [28,30– 33]. According to the equations for discharge power, energy deposition in the air gap can be calculated. In the case of the surface modification using HDBD and FDBD, the power densities during plasma processing are 158 and 192 mW/cm2, respectively. In view of the equation of power density for the two cases, the energy density is controlled by treatment time [9]. If the treatment time is determined, the energy density is proportional to the treatment time for the two cases. So we describe the changes of surface in the plasma processes as a function of treatment time in this paper.
using the HDBD remains low and indicates the uniformity of the HDBD treatment. As to DBD plasma processing of polymer films, it has been known that water contact angle would recover and lead to a reduction in hydrophilicity after plasma treatment with exposure to ambient air for an ageing time. Fig. 5 shows this variation of water contact angles measured on the treated films for various treatment times as a function of ageing time, and the experimental results using the FDBD and HDBD treatments are presented in Fig. 5a and b, respectively. For both treatments, it can be found that contact angle rapidly increases during the first 3 days, and then reaches a stable value of about 608. This behavior can be attributed to the chemical reactions of the remaining active radicals on the PI surface with oxygen or moisture in ambient air, or movement of the oxygen-containing hydrophilic polar groups into the bulk of PI films [8,18–21]. It should be pointed out that the power densities in these treatments are rather low to form intermolecular bonds between the newly produced oxygen complex and the film surfaces compared to other studies [18–21]. Similar result of contact angle increasing with ageing time was also obtained, and it was concluded that the ageing problem depended on the kind of polymer material, treatment gas, and ambient conditions rather than plasma treatment process [38,39]. During ageing, the dispersion of the contact angle data remains low among various treatment times, perhaps suggesting that the surface is not significantly degraded after short duration treatment and has a
3.2. Water contact angle measurements Measurement of static water contact angle can be used to evaluate the adhesion or hydrophilicity of polymer surface. Fig. 4 depicts the evolution of contact angle of water on PI surface as a function of treatment time. It is seen that water contact angle decreases with increasing treatment time, and it indicates an increase in hydrophilicity. The water contact angle of the untreated film is 71.08, and decreases to minimum 29.98 and 27.38 for 10 s FDBD and HDBD treatments respectively. However, when the PI film is treated over 10 s, the contact angle does not change anymore with treatment time. This suggests that there is a saturation effect of plasma treatment, and some similar results can be found on other polymer films [9–14,17]. In addition, compared to the FDBD treatment, the dispersion of the contact angle data
Fig. 4. Water contact angle on the surface of PI films with the FDBD and HDBD treatments.
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Fig. 5. Contact angle of water on PI film versus treatment time and ageing: (a) the FDBD treatment and (b) the HDBD treatment.
rapid recovery. From a practical point of view, it suggests that too long treatment time is not necessary, and a continuous operation should be used to avoid the aging effect immediately after the treatment. 3.3. Morphology analysis of surface appearance SEM analysis before and after treatment gives information on the etching function of FDBD and HDBD. Fig. 6a shows the SEM picture before plasma treatment. Little spots can be seen on the SEM picture and the untreated surface is very smooth. Fig. 6b shows the SEM micrograph after 10 s FDBD treatment. It can be seen that the surface has been etched, and some isolated grain-like structures occur with widely separated grains. The SEM micrograph of PI surface treated by HDBD is shown in Fig. 6c, which also corresponds to a treatment time of 10 s. The surface shows a fine grain-like structure with a wide spread in grain diameters, and the distribution of grain-like structures is widely uniform compared to that observed in Fig. 6b. It indicates that the HDBD treatment seems to be more uniform and indirectly suggests that HDBD is more homogeneous than FDBD. To confirm the better treatment using HDBD than FDBD indicated by the SEM observation, some samples were also investigated by AFM. Fig. 7a–c presents AFM images before and after the FDBD and HDBD treatments, respectively, which corresponds to a treatment time of 10 s and a scanning area of 1 mm 1 mm. The values of the mean surface roughness (Ra), root mean square surface roughness (Rrms), and maximum surface roughness (Rmax) obtained from the AFM scans are tabulated in Table 1. A clear contrast between the surface topographies
Fig. 6. SEM observation of the untreated and treated PI films. (a) Untreated, (b) after 10 s FDBD treatment and (c) after 10 s HDBD treatment.
resulting from different treatments can be seen. The untreated PI shown in Fig. 7a presents a very smooth surface with no distinguishable features and the roughness is lower (Rmax = 9.158 nm) than the treated films. Fig. 7b shows a new granular structure of the PI surface treated by FDBD. The surface is very rough (Rmax = 44.315 nm) and almost five times rougher than the untreated sample. Compared with the FDBD treatment in Fig. 7b, the distribution of the granular structure presented in Fig. 7c is much proportional spacing and homogeneous, and the roughness is relatively smaller (Rmax = 23.196 nm). The topography and roughness of PI films are modified after plasma
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Fig. 7. AFM images of the untreated and treated PI films (1 mm 1 mm). (a) Untreated, (b) after 10 s FDBD treatment and (c) after 10 s HDBD treatment.
modification, which is due to the very energetic active species that are capable of etching the surface and breaking the polymer chains. Furthermore, both SEM and AFM prove that the HDBD treatment is more uniform than that using the FDBD plasma.
groups of the untreated and treated films. Fig. 8a shows the XPS spectra before and after 10 s FDBD and HDBD plasma treatments. The XPS spectra of the untreated surface show peaks corresponding to binding energies of C 1s (285 eV), N 1s (400 eV) and O 1s (532 eV). It can be seen that, after the FDBD or HDBD plasma treatment, the intensity of the C 1s peak is decreased, whereas that of N 1s and O 1s is increased. Furthermore, compared to the intensities of the C 1s, O 1s, and N 1s peaks after the FDBD treatment, the peak of C 1s using the HDBD treatment is lower and that of the N 1s and O 1s peaks is larger. Table 2 shows C, N, O atomic compositions for the untreated samples and treated samples. It can be found that the surface modification leads to an increase in the O/C atomic ratio from 0.238 for the untreated sample to 0.351 for 10 s FDBD treated sample, and to 0.427 for 10 s FDBD treated sample. The oxygen increase indicates that oxygencontaining polar groups are formed in the surface. Table 2 also shows a slight increase in N/C atomic ratio, and it suggests that. One source could be some nitrogen-containing groups are also formed due to the nitrogen in atmospheric air. Another is probable that the surface cleaning allows the fingerprint of the material bulk to appear on the spectra and the nitrogen percentage thus appears to increase [22]. It is well known that the chemical functional groups responsible for the increase in the adhesion or hydrophilicity of the polymer are mainly related to the C 1s peak. According to the XPS survey scans shown in Fig. 8a, high-resolution XPS-spectra of the C 1s peaks are performed. The C 1s spectra before and after the plasma treatment are represented in Fig. 8b. The C 1s spectrum from the untreated and treated samples is decomposed into 4 components: one at 284.7 eV due to C–C groups, one at 285.6 eV due to C–N groups, one at 286.2 eV due to C–O groups, and 288.6 eV due to C5 5O groups [40]. The calculated concentration of each chemical component by deconvolution using Gaussian functions is also summarized in Table 2. It can be found from Table 2 that after the DBD plasma treatment the C–C group decreases, while the C–O, C5 5O, and C–N groups increase. This means that air plasma mainly attacks the C–C groups, and forms more oxygen-containing groups. The introduction of the polar oxygen-containing groups is responsible for the higher adhesion or hydrophilicity of PI surface after plasma treatment. The above results indicate that the DBD plasma treatment introduces oxygencontaining functional groups into the molecular chain and enhances surface energy of PI surface. In this case, the HDBD treatment results in higher adhesion than the FDBD treatment, which is attributed to more polar functional groups introduced into PI surface. 4. Discussions
3.4. Surface composition analysis DBD in atmospheric air can produce a wide range of active species, air plasma increases surface energy by introducing oxygen-containing polar group into the polymer surface, and atomic oxygen is thought to be the main reactive species responsible for this oxygen inclusion [9–14]. XPS can be used to elaborate the change of the atomic compositions and possible Table 1 Ra, Rrms, and Rmax obtained by AFM for the untreated and treated (at the FDBD and HDBD modes) PI films. Sample
Untreated FDBD HDBD
Surface roughness (nm) Ra
Rrms
Rmax
0.762 1.937 2.035
0.988 3.894 2.910
9.158 44.315 23.196
Surface analysis before and after plasma treatment indicates that HDBD is more effective than FDBD. This can be attributed to the discharge characteristic of this treatment. On the one hand, the treatment using HDBD can provide more active species than that of FDBD. Surface modification is a processing of interaction between the plasma and surface of the polymer to be treated. Compared with the air gap spacing of 6 mm in the FDBD treatment, the air gap spacing is only 2 mm in the HDBD treatment. In the case of the FDBD treatment, the filamentary discharge bridges the air gap and the active species are produced along the filamentary channel, and therefore lots of them cannot reach the PI surface. However, in the case of HDBD, the discharge was homogeneous in the whole discharge regime and the majority of the active species can reach PI surface and interact with it. On the other hand, the discharge uniformity can account for better treatment effect using the HDBD treatment, and Figs. 2 and 6–7 can prove HDBD is more homogeneous that FDBD.
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Fig. 8. XPS taken on the PI surface before and after 10 s FDBD or HDBD treatment: (a) the O 1s, N 1s, C 1s peaks and (b) the C 1s peak.
Table 2 Chemical composition of the PI surface before and after 10 s FDBD and HDBD treatment. Sample
Untreated FDBD HDBD a
Atomic composition (%)
C 1s possible groups (%)
C
O
N
C–C
C–N
C–O
C5 5O
285a
532a
400a
284.7a
285.6a
286.3a
288.6a
75.37 69.03 64.39
17.93 24.22 27.48
6.7 6.76 8.13
74.53 67.85 63.33
3.84 3.87 4.42
13.30 18.27 21.88
8.33 10.10 10.37
Binding energy (eV).
It can be found that electrical parameters in repetitive unipolar nanosecond-pulse DBD are much better than that in the DBD using AC excitation and are also superior compared with those in submicrosecond pulse DBD [28–32]. Nanosecond-pulse discharge is a typical over-voltage breakdown, and breakdown voltage can exceed that using DC, AC, or microsecond-pulse source. The over-voltage breakdown will result in a higher initial electric field or reduced electric field E/N and lead to plasma-chemical processes such as atomic or molecular electronic excitation and dissociation enhancing rapidly with E/N [27]. Accordingly, the DBD produced by unipolar high-voltage pulses can produce much powerful plasma and active particles, and the application in surface modification of polymers will be more efficient. This efficiency has also been experimentally confirmed by many investigations [27–32]. Compared with the common DBD treatment of polymer surface using ac power source, the treatment using FDBD or HDBD is much mild. In this case, the power density during the FDBD processing (192 mW/m2) is slightly larger than that using the HDBD treatment (158 mW/m2). Moreover, plasma dose is an important parameter, which controls the treatment efficiency, and it can be calculated from the production of the power density and treatment time. For a treatment time of 10 s, the corresponding plasma doses are 1.58 103 mJ/cm2 and 1.92 103 mJ/cm2 in the HDBD and FDBD treatment respectively. In addition, for polymer or
fabrics, many studies have reported that a localized strong degradation may take place. Some substantial heavy particle heating takes place in the gas phase and lead to the heating of the surface, and then, direct surface damage by partial melting would occur [18,19]. It is probable due to the non-homogeneity of the DBD and localized melt resulting from micro-discharges. However, in this case, even if the instantaneous peak power is very high and can reach a megawatt order [34], the mean energy dissipated by the DBD plasma is relatively mild and no degradation of the surface of PI film takes place. The treated samples show no visual difference compared to the untreated case, even for an extended treatment time of 30 s or longer. 5. Conclusion Surface properties and adhesion characteristics of PI surface by the DBD plasma treatment using repetitive nanosecond pulses are investigated. Adhesion properties of PI surfaces can be improved not only by enhancing surface energy by introducing some functional groups, but also by increasing surface roughness by plasma etching. From the results of water contact angle measurement and XPS analysis, surface energy is enhanced and some oxygen-containing functional groups are incorporated. As shown in the observations of SEM and AFM, PI surface has been etched and
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