The hydrophobic of SiO x film deposited by the cold plasma arc at atmospheric pressure Zhen Zhou 1, Qiang Chen 1 *, Fuping Liu 1, Zhaoxing Ren 2 1-Laboratory of Plasma Physics and Materials, Beijing Institute of Graphic Communication, Beijing 102600, China;2- Hefei Yanfei Electric Apparatus Science and Technology Co., Ltd,hefei 230088, China) especial in surface modification[1], coating Abstract At atmospheric pressure with (CH 3 ) 3 SiOSi (CH 3 ) 3 (HMDSO) as the monomer, the SiO x films were deposited in the cold plasma arc without extra heating and oxygen source. With the variation of processing parameters such as the air flow rate, the ratio of monomer and air, and the distance of nozzle to substrate as well as the exposure time, the influence on film structure of coatings was investigated by Fourier transform infrared (FTIR). Through atomic force microscope (AFM) and digital optical microscope the surface morphology was also observed. The hydrophobic behavior of SiOx film was characterized by water contact angle measurement (DSA100, Germany) in detail. Larger than 160 of water contact angle was obtained in the conditions of 10cm distance of nozzle to substrate, 90ml/min air flow rate and 2min explosion time. The possible reason was analyzed in this paper. Keywords: the cold plasma arc, SiO x film, hydrophobic Introduction Low-temperature non-equilibrium plasma has broad prospects at the practical applications, deposition[2], surface grafting[3], plasma chemical polymerization[4], disinfection and sterilization[5], preparation of nano-materials[6] and so on. However, most plasma polymerization is at low pressure, vacuum containers with slow polymerization rate [7]. Therefore, in high pressure or at atmospheric pressure plasma polymerization for plasma chemical polymerization has become the main research directions in recent. The atmospheric discharge then becomes the highlight research topic including gas-electron collision, gas-discharge, single-molecule chemical polymerization, etc. SiOx film can be applied including microelectronics, corrosion-resistant, wear-resistant and so on. As a plastic barrier layer of flexible packaging materials SiOx, as soft glass, is the third generation of high-barrier coating. It demonstrates good barrier properties to oxygen and humid vapor permeability in a wide temperature range. It also can be used in microwave heating in food packaging with excellent chemical resistance for acid, oils and fats. The coatings are transparency. In this work, the SiOx film was deposited by cold plasma arc at atmospheric
pressure only flowing air and (CH 3 ) 3 SiOSi(CH 3 ) 3 (HMDSO) as the reactive and the monomer[8]. In the plasma discharge region, the monomer shall be collided with the high-energy electrons, the energy then will be transferred and the dissociating or chemical bond rupture reaction will be happened. Then, the formation of active free radicals of Si-O-Si precursor will be occurred to synthesize SiOx film. At the same time, the performance of SiOx film characterization. (b) Figure 1. a-schematic diagram of the cold plasma arc (1-conpress air, 2-electrical source, 3- the cold plasma arc, 4-monomer, 5- floater flow meter, 6- mass flow controller, 7- CF 4 gas); b-the structure of jet; Results and Discussion 1. Hydrophobicity and surface morphology Experiment Figure 1 (a) is home-made cold plasma arc deposition system. Compressed air controlled by the rotate meter carried HMDSO into discharge zones. Substrates for experiment are KBr pallet, aluminum and p-si wafer. The Al foil and silicon wafer were cleaned ultrasonically in acetone, alcohol, deionized water for 10 minutes, contact angle( ) 160 140 120 100 80 60 ait flow rate 90ml/min h=10cm 10s 30s 1min 1.5min 2min time respectively, then dry in the air. The KBr pallet was for FT-IR measurement, whereas aluminum Figure 2 the influence of the exposure time on the water contact angle(drawn with origin and silicon for the water contact angle measurement and the adhesion test. Figure 2 shows the influence of the exposure time on the water contact angle, air flow rate is 90ml/min, the distance of nozzle to substrate is 10cm. As can be seen from the figure, with the deposition time prolonging from 10s to 2min, the water contact angle increased in consequence. At 2min, the water contact angle reached 161.1 0, demonstrates a a super-hydrophobic behavior.
surfaces by digital microscope (a, c) and water contact angle (b, d). a and b are the photos of coatings in air flow rate 90ml/min, exposure time for 1min, distance of nozzle to substrate 10cm; c and d are the photos in air flow rate 90ml/min, exposure time for 2min, distance of nozzle to substrate 10cm. Figure 3 shows the influence from deposition time. when the air flow rate was 90ml/min, the distance of nozzle to substrate is 10cm. From the figure 3 a and c, regardless of the different exposure times, a dense structure was formed in the substrate, but the much tighter one was appeared at 2min deposition film rather than that at 1min one. From the figure 3 b and d one can see that the hydrophobic property was a b seriously depended on the deposition time, the water contact angle reached110 0 at 1min coating, and 161 0 was observed at 2min deposition one, which is certain a super-hydrophobic surface. So the deposition time affected significantly the status of film surface. 2. FTIR a 30ml/min ABS(a.u.) 60ml/min 90ml/min 500 1000 1500 2000 2500 3000 3500 4000 Wavenumber(cm-1) Figure 3.the photos of SiOx coating on glass and Si c d
film increased with the deposition time when ABS(a.u.) 20s 30s 40s the same distance of nozzle and substrate (10cm) and air flow rate(90ml/min) were fixed. 3. Surface morphology 500 1000 1500 2000 2500 3000 3500 4000 Wavenumber(cm-1) (b) Figure 4 FTIR spectra of SiO x films deposited at different air flow rates and exposure times. ( (a) is the air flow rate 30ml/min, 60ml/min, 90ml/min, exposuretime 1min, distance of nozzle to substrate 10cm. (b) is the spectrum of exposure for 20s, 30s, 40s, and air flow rate 90ml/min, distance of nozzle to substrate 10cm) As can be seen from Figure 4, the wavenumber at 1050 cm -1-1060 cm -1, a strong Si-O-Si absorption peak is appeared in the spectra, the wavenumber at 810 cm -1-820 cm -1, the fingerprint absorption peak of Si-O-Si was testified the SiOx film formation. In spectrum there are no other peaks, means the SiOx coating was very pure. At the same time, Figure 4 (a) illustrates that with the air flow rate increased from 30ml/min, 60ml/min to 90ml/min, the peak intensity also increased at wave number 1050 cm -1-1060 cm -1, indicating the thicker growth of film along with the air flow rate when the same distance of nozzle to substrate was 10cm and exposure time was 2min. With the increase of the deposition time, from 20s, 30s to 40s the peak intensity at wavenumber 1050 cm -1-1060 cm -1 was also increased, which means the thickness of the Figure 5 the surface morphology of SiO x film( the distance of nozzle to substrate was 10cm, exposure time was 2min, and air flow rate was 60ml/min) Figure 5 shows the surface morphology of SiO x film. As can be seen from the image, SiO x films is compress and uniform. The surface roughness was 23.3nm. Conclusion At atmospheric pressure, with HMDSO as the monomer, the influence of process parameters on SiO x coating properties were performed in the cold plasma arc. The water contact angle at 161 0 was obtained. The structure analysis by FTIR demonstrates that with the increased of exposure time, the distance of nozzle to substrate, or air flow rate, the growth rate of SiO x films was always increased rapidly. Through the optical and AFM image the surface morphology of films was rather tightness, the
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