WDS'7 Proceedings of Contributed Papers, Part II, 124 128, 27. ISBN 978-8-7378-24-1 MATFYZPRESS Modification of Glass Surface by Atmospheric Pressure Plasma T. Homola, A. Buček, A. Zahoranová Comenius University, Department of Experimental Physics, Mlynská dolina, 842 48 Bratislava, Slovakia. Abstract. A Diffuse Coplanar Surface Barrier Discharge (DCSBS) operating in air at atmospheric pressure has been used to induce changes in the surface properties of glass. The contact angles of water drop on the surface of glass, which were measured by drop shape analysis, decreased markably with plasma treatment for 1 s. With the increasing of the plasma treatment time, the contact angles of the samples treated by air plasma decreased to lower values, and by using the dynamic treatment mode, the values of contact angles decreased rapidly below unmeasurable limit. We also investigated the durability of the treatment. Introduction Material processing techniques such as cleaning of material surfaces, removing organic or films, and modifying polymer surface properties are one of the well established plasma application areas. In particular, the glow discharge plasma was known to be the appropriate discharge mode for material processing because of its high density and flux of active species for chemical reactions on material surfaces. As a consequence, various low-pressure plasma sources which easily produce glow-like plasma have been widely used in the industry. However, plasma sources operating at atmospheric pressure have recently received increased attention since they need no expensive high vacuum equipment and have in-line processing capability [J.R.Roth, 21]. Simply way to generate nonequilibrium plasma at atmospheric pressure is the use of dielectric barrier discharge (DBD). DBD occurs in arrangements where at least one dielectric is positioned in the gas space in between conducting electrodes. Quite unique type of the DBD generator, so called Diffuse Coplanar Surface Barrier Discharge (DCSBD), can produce very thin layer of non-equilibrium plasma with high power density (up to 1 W/cm 3 ) and, practically, in any working gas [M. Šimor et al., 22]. Comparing to other surface DBD schemes (as e.g. the one invented by Masuda et al. [S. Masuda et al., 1988]), major advantage of the DCSBD is that surface micro-discharges are in no contact with metallic electrodes, thus ensuring long lifetime of the system. The aim of this work is to increase the surface energy of glass surface by DCSBD. Freshly cleaned glass surfaces have a high surface energy and are well wettable. They, however, have a tendency to adsorb organic contamination from the ambient environment. Adsorbed organic contaminant molecules, generally less than the full monolayer coverage, of the order of nm thickness, will generate heterogeneous wettability. This may lead to non-uniform glass coatings, in particular if deposited from liquid media as in the case of widely used sol-gel coatings, silane coupling agent coatings, electroless metal plating, as well as in jetting fluid dispensing technology that is gaining popularity in the flat panel display manufacturing [Thomas Ratledge]. Experimental setup DCSBD electrode geometry consists of many parallel stripline electrodes embedded in 96% alumina as schematically shown in Figure 1. Thickness of the ceramic layer between the upper surface and electrodes is ~.4 mm. The discharge was powered by AC (~ 14 khz, up to 2 kv peak-to-peak) high voltage. Electrical measurements were performed using a digitizing oscilloscope Tektronix TDS 2 (bandwidth 2 MHz), a Tektronix P615A (1:1) high voltage probes and a Pearson Electronic 41 current probe. The configuration of electrical measurement is also showed in Figure 1. The glass samples (microscopic glass slides 76 mm x 26 mm) were treated in air plasma with the treatment time varying from 1 to 8 s. The distance between sample and ceramics was,3 mm, the discharge power was 355 W. 124
Figure 1. Sketch of the DCSBD electrode geometry and electric measurements. Results Electrical characteristics of DCSBD When the electric field on (or close to) the surface becomes high enough to cause electrical breakdown, a number of H-shaped luminous micro-discharges can be observed on the surface. Depending on both applied power and dynamics of air gas feeding, micro-discharges may be either static or travelling on the dielectric surface along the embedded strip electrodes and their density may vary from a single to many micro-discharge events produced per AC half-cycle. DCSBD voltage and current waveforms at power 43 W are shown in Figure 2. It can be seen that the real current in plasma has an impulse character with a relatively fast starting edge shorter than 1 µs and duration no longer as quarter-period of voltage. The current reaches peak value of about 2 ma. From the measurements of voltage and current we can compute the power dissipated in plasma. However, during experiments with material processing it is complicated to measure the discharge power with voltage and current probes, so we determined the functional dependency between the power dissipated in plasma and the input power of HV generator. The input power has been measured by multimeter Metex M466-M and we found that the HV generator had energy loss of about 15%. 12 3 9 6 2 Voltage [V] 3-3 -6-9 1-1 -2 Current [ma] -12-3 25 5 75 1 125 15 175 2 225 25 Time [µs] Firure 2. The waveforms of DCSBD voltage (red line) and current (blue line). 125
The effect of plasma on the glass surface Wettability (hydrophilic property) defined as the ability to absorb liquid on a solid surface is one of the most important properties of materials and can be measured by the shape of a water droplet placed on the surface, which is characterised by the contact angle. The higher the surface energy, the smaller the contact angle, which means an improvement in wettability. Plasma treatment of a surface can lead to improved wettability of this surface. Since the wettability of surface is strongly related with adhesion, surface modification by plasma is useful in many industrial areas. In this work the surface of glass was treated by plasma generated by DCSBD in air at atmospheric pressure. Figure 3 shows the change of distilled water contact angle according to plasma treatment. It is apparent that the plasma treatment leads to lower values of contact angle. The measurement was done in static treatment mode, that means the sample of glass is placed over the surface of discharge in optimal distance.3 mm without moving it during the treatment. The contact angle decreases from 37 to 1 with approximately 5 s of plasma exposure time at the discharge power of 355 W. Discharge during treatment was not operated in flowing regime. From observations of treated glass samples by electron microscope, it can be seen that this method of plasma modification of glass surface is not homogeneous. It is possible to eliminate this problem by using the dynamic treatment mode. This means, that sample is moving through or above the plasma layer with some speed, which determines the treatment time. We constructed a device which enables such dynamic treatment mode. The results are much better than that of static treatment, because at treatment time of about 1s, the contact angle decreased under measurable limit (2 ) the water droplet perfectly wetted the surface and the surface energy became higher than surface energy of water (72.8 mj/m 2 ). 4 35 Contact angle [deg] 3 25 2 15 1 5 1 2 3 4 5 6 7 8 9 Treatment time [s] Figure 3. Effect of plasma treatment time on contact angle in the static treatment mode. We also investigated durability of the treatment in static treatment mode. Three different treated glass samples (treatment time: 1s, 3s, 7s) at the power of 355 W were exposed to the ambient atmosphere for four days. At the end of each day, some samples were taken and the contact angle measured. The results are shown in Figure 4. It is clear that contact angle increases for all three samples even after 1 day. Discussion From the results it is clear that an increase in the treatment time leads to a decrease of contact angle. This is due to the cleaning effect of the plasma [R. A. DiFelice, 21], which degrades the organic contaminants, the end products of such action being CO 2 and H 2 O. In addition, the concentration of surface OH groups could be increased, which again would improve the wettability [T. Yamamoto et al., 24]. 126
4 35 value before treatment Contact angle [deg] 3 25 2 15 1 5 1s 3s 7s 2 4 6 8 1 Time [h] Figure 4. Durability test of glass samples (treatment time: 1 s, 3 s, 7 s). The results in Figure 3 show that even very short plasma exposure time by DCSBD is sufficient to reach significant increase in wettability of glass surface. We achieved better results by using device allowing treatment in dynamic mode. This could be due to non-homogeneous treatment in the case of static mode. The reached plasma modification, however, is not stable and with the time tends to degrade backwards to initial values, which could be the result of adsorbtion of organic contaminants from ambient atmosphere [S. Takeda et al., 1999]. Conclusion In this work the possibility of using plasma generated by DCSBD to modify glass surfaces was investigated. We conducted experiments to determine change of contact angle after plasma treatment with respect to plasma exposure time and durability of the treatment. The results show that DCSBD has potential to be used in applications of glass surface modification and also modification of other silica materials. In comparison to other plasma sources (e.g. volume, surface barrier discharges) [T. Yamamoto et al., 24] DCSBD treatment time of glass surface at atmospheric pressure is significantly shorter with much less energy consumption. It follows than that the application of DCSBD plasma treatment in the glass industry can be inexpensive, high-speed, and accomplished in-line. Acknowledgments. Contract 1/41/7. The present work was supported by the Scientific Grant Agency VEGA under References J.R.Roth, Industrial Plasma Engineering vol.2: Applications to Nonthermal Plasma Processing, IOP Publishing, Bristol, 21 M. Šimor et al., Atmospheric-pressure diffuse coplanar surface discharge for surface treatments, Appl. Phys. Lett., 81, 22 S. Masuda et al., IEEE Trans. Ind. Appl., 24 (1988), 223 Thomas Ratledge, Liang Fang, Floriana Suriawidjaja, Asymtek, 2762 Loker Avenue West, Carlsbad, CA: Advances in jet Dispensing for Flat Panel Applications http://www.asymtek.com/news/articles/25_12_advances_in_jet%2dispensing_fpd.pdf R. A. DiFelice, An investigation of plasma pretreatments and plasma polymerized thin films for titanium/polyimide adhesion, Doctoral thesis, Faculty of the Virginia Polytechnic Institute and State University, 21. 127
T. Yamamoto, M. Okubo, N. Imai, Y. Mori: Improvement on Hydrophilic and Hydrophobic Properties of Glass Surface Treated by Nonthermal Plasma Induced by Silent Corona Discharge, Plasma Chemistry and Plasma Processing, Vol. 24, No. 1, 24, pp. 1 12. S. Takeda, M. Fukawa, Y. Hayashi, K. Matsumoto: Surface OH group governing adsorption properties of metal oxide films, Thin Solid Films, 339, 1999, pp. 22 224 128