DEPOSITION OF THIN FILMS ON POLYCARBONATES BY PULSE DIELECTRIC BARRIER DISCHARGE

Transcription

1 a DEPOSITION OF THIN FILMS ON POLYCARBONATES BY PULSE DIELECTRIC BARRIER DISCHARGE T. Opalińska a, B. Ulejczyk a, L. Karpiński b, K. Schmidt-Szałowski c a) Industrial Chemistry Research Institute, Rydygiera 8, Warszawa, POLAND b) Institute of Plasma Physics and Laser Microfusion, Hery 23, Warszawa, POLAND c) Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 664 Warszawa, POLAND Abstract: Thin films with content of Si, C and O were deposited on polycarbonates from tetraethoxysilane by pulse dielectric barrier discharge at atmospheric pressure. Influences of plasma parameters, such as the energy of single pulse of discharge and the position PC plates on electrodes on the deposition rate, were investigated. Results shown that the deposition rate of thin film increased with increasing energy of single pulse, for example: deposition rate was 4.6 or 37 nm/min when the single pulse energy was 6.2 or 9.1 mj. The structure of thin films was investigated using scanning electron microscopy, atomic force microscopy and infrared spectroscopy. This plasma technology can be used for successfully depositing transparent, smooth and homogeneous films. 1. INTRODUCTION Polycarbonates (PC) have very useful properties of the bulk, such as low-density and thus reduced weight, high elasticity and transparency. They have been widely used in the industry to replace glasses in many applications, such as headlights, windscreens, lenses and compact discs. However, their use is sometimes limited by unwanted properties of the surface such as low hardness, low scratch resistance and degradation by ultraviolet. Various deposition technologies of producing thin, protective film containing silicon for remedying these limits are investigated. Till now, the best effects were achieved by plasma methods. These methods are based on microwave plasma techniques under low pressures from.1 to 1 hpa. Low pressure is a practical limit of this way of depositing thin film in large scale [1-3]. For that reason, investigations about depositing of thin film containing silicon in dielectric barrier discharge (DBD) under atmospheric pressure were begun [4]. DBD is inhomogeneous and consists of many micro-discharges. The properties of the microdischarge depend on the dielectric barrier permittivity, dielectric barrier thickness, plasma-generating gas and the others [5]. In our early studies, we observed that using a pulse power supply instead of a sinusoidal power supply made of DBD more homogenous [6]. In this work, we present the results of thin film deposition in the pulse dielectric barrier discharge (PDBD) at atmospheric pressure. 2. EXPERIMENTAL Plasma deposition was performed in the PDBD reactor, which is showed in FIGURE 1. The reactor consists of two round stainless steel electrodes enclosed in Plexiglas casing. Dielectric barrier was made of a PC plate characterized by the relative electric permittivity of 2.3 (measured by a Precision LCR Meter HP 4284A). We investigated two kinds of arrangements. One arrangement: high-voltage electrode and grounded electrode were covered with PC plates. Thin films were deposited on both PC plates. Second arrangement: only the grounded electrode was covered with PC plate and the highvoltage electrode was not shielded. Thin films were deposited on PC plate and on the surface of highvoltage electrode. In all experiments PC plates of thickness.75 mm were used. The discharge gap distance always was.75 mm. All depositions were performed at the room temperature under the atmospheric pressure. The reactor was powered by a pulsed electric system with consisted of autotransformer, high-voltage transformer, high-voltage resistor, Blumlein line and spark-gap. a Electronic address: Teresa.Opalinska@ichp.pl

2 The gas supply system is showed on FIGURE 2. The reactor was feded by plasma-generating gas which consisted helium (85% by vol.), oxygen (15% by vol.) and the vapor of tetraethoxysilane (TEOS - 32 ppm). Helium and oxygen were taken from gas cylinder through mass flow controllers and they were mixed. Total flow rate of the gas mixture was kept at 1 L(S.T.P.)/h in all the experiments. Next step was to pass gases through a scrubber with liquid TEOS and then let them to enter into the reactor. The concentration of TEOS in the plasma-generating gas was calculated basing on flow rates of helium and oxygen and the mass decrease of liquid TEOS in the scrubber. gas inlet mass flow controlers pulse power supply Plexiglas casing grounded electrode high-voltage electrode PC plate He gas outlet O 2 scrubber with TEOS reactor FIGURE 1. Schema of PDBD reactor. FIGURE 2. Block schema of gas supply system. Mass deposition of the thin film was determined gravimetrically. PC plates were weighed before and after the deposition. Changes of the mass of PC plates were determined with Sartorius BP 221S balance. The thickness of the thin film was calculated from the following formula: D=m (s* g) -1 (1) where: D thickness [mm], m change of the mass of PC plate [mg], s* arbitrary defined of PC plate surface [cm 2 ], g density of deposited thin film [average 2 g/cm 3 ] The morphology of films was investigated with using LEO 153 scanning electron microscope (SEM) and Nanoscop 13 Digital Instrument atomic force microscope (AFM/TM). The chemical composition of the deposited films was investigated using Spectrum 2 infrared spectrometer with Fourier transform (FTIR) and the energy disperse X-ray detector (EDX) attached to the LEO 153. Electric parameters, such as voltage and current, were recorded using Tektronix TDS 354 oscilloscope. The real energy released in a single pulse was calculated from the following formula: E= τ 2 U(t) I(t) dt (2) τ 1 where: E real energy released in a single pulse of the discharge [J], U voltage [V], I current [A], t time [s], τ 2 -τ 1 duration of the single pulse (FIGURE 6) 3. RESULT AND DISCUSSION PC plates remained transparent after deposition of the thin film. The surfaces of the deposited films were very smooth and homogenous. All the surfaces of the PC plates were uniformly coated. PDBD did not change the surface roughness and left the original structure of the surface. The amplitude of deflection does not exceed 3 nm. The deposited films were not the pure SiO 2. EDX spectra of the thin films was exhibited Si, C and O. FTIR spectra of thin films deposited in PDBD from TEOS (upper) and original PC (down) are showed in FIGURE 3. The broad band in the range 3-37 cm -1 is assigned to SiOH group and COOH group and water. The peak at 17 cm -1 is attributed to C=O stretching vibration, the peak at 16 cm -1 is due to H 2 O and the peak at 97 cm -1 is corresponded to Si-OH stretching vibration. Moreover, the thin films are characterized by strong absorption in the range cm -1. There are few peaks which are due to silylmethyl, ethoxy and aliphatic groups like: 15 cm -1 C-O-C, Si-O-C and Si-O-Si symmetric deformation; 111 cm -1 SiO 2 network vibration; 117 cm -1 C-O-C, Si-O-C and Si-O-Si asymmetric deformation; 12 cm -1 CH 3 rocking in Si-O-CH 3 and 124 cm -1 CH 3 symmetric deformation in Si-(CH 3 ) x. The peak at 8 cm -1 is

3 characterized to the symmetrical stretching of the Si-C bond. The band in the range cm -1 is associated to the aliphatic groups vibration. FIGURE 3. Typical FTIR spectra of the deposited thin film (upper) and original PC (under). The deposition rate and the thickness of deposited films depended strongly on the energy of the single pulse of the discharge (FIGURE 4, TABLE 1) and the position PC plates on the electrodes (FIGURE 5, TABLE 1). Thickness, nm Deposition time, min A B FIGURE 4. Variation of film thickness with deposition time under process conditions of: A energy of single pulse 9.1 mj, PC plate placed at the grounded electrode; B energy of single pulse 6.2 mj, PC plate placed at the grounded electrode.

4 A Thickness, nm B Deposition time, min FIGURE 5. Variation of film thickness with deposition time under process conditions of: A PC plate placed at the high-voltage electrode, energy of single pulse 6.2 mj; B PC plate placed at the grounded electrode, energy of single pulse 6.2 mj. In all the experiments the thickness of deposited films increased with increasing deposition time. It was simple and linear dependence (FIGURES 4, 5). TABLE 1. Comparison of deposited rate of thin film by different parameters Energy of single pulse, mj Deposition rate, nm/min. PC plates covered electrodes grounded grounded high-voltage The real energy released in the single pulse of the discharge, the voltage and current depended on the arrangement of the reactor, and its electric capacity. The single pulse s current depends on the electric capacity strongly (FIGURE 6). We investigated two kinds of the reactor arrangement of different capacities: with single dielectric barrier and with double dielectric barriers. Runs of the voltage and current of these two kinds of the reactor arrangements are showed on FIGURE current voltage Time, ns Current, A voltage current τ 1 τ 2 τ 1 τ Time, ns FIGURE 6. Runs of the voltage and the current of PBDB witch one PC plate (left) and two PC plates (right). Electric capacity of the reactor with single PC plate was 23.2 pf and the value of the real energy was 9.1 mj, calculated according the formula 2. Electric capacity of the reactor with two PC plates was 17.8 pf and the real energy was 6.2 mj. The quite small change of the real energy released in the single pulse of the discharge (~5%) caused great increase of the thickness of the deposited film (FIGURE 4) and the film deposition rate (TABLE 1) Current, A

5 In the experiments, the films were deposited faster on the PC plate placed at the high-voltage electrode than at the grounded electrode (FIGURE 5, TABLE 1). This effect is probably depended of nonsymmetric work of the electric power supply, which is showed on FIGURE 7. Each tooth form on the recorded voltage line is correlated to the single pulse of the discharge. This non-symmetric work of two electrodes depends on construction of the electric power supply. It is not still possible to remove it Time, ms FIGURE 7. Run of the voltage, which supply the reactor with two PC plates. 4. CONCLUSION 1. Pulse dielectric barrier discharge (PDBD) can by used for thin filmsdeposition on polycarbonates (PC) under the atmospheric pressure and at the room temperature. 2. Thin films deposited by PDBD of tetraethoxysilane (TEOS) were transparent and smooth. 3. The deposition rate and the thickness of deposited film depended on the real energy released in the single pulse of the discharge and on the position PC plates on electrodes. The films were deposited faster on the PC plates, which covered high-voltage electrode than PC plates, which covered grounded electrode. The deposition rate and thickness of thin film increased with the increasing energy of single pulse. REFERENCES [1] Wróbel A. M., Wertheimer M. R., Plasma Deposition, Treatment, and Ething of Polymers, R. d Agostino, ACADEMIC PRESS INC., San Diego, 199, Chapter 3 [2] Hatanaka Y., Sano K., Aoki T., Wróbel A. M., Thin Solid Films, 368, (2) [3] Behnish J., Tyczkowski J., Gazicki M., Pela I., Holländer A., Ledzion R., Surface and Coatings Technology, 98, (1998) [4] Schmidt-Szałowski K., Rżanek-Boroch Z., Sentek J., Rymuza Z., Kusznierewicz Z., Misiak M., Plasmas and Polymers, 5, (2) [5] Opalińska T., Effect of the Permittivity of Ferroelectric Electrodes on Macroscopic Characteristic of the Barrier Discharge. In: Proc. 7 th Int. Symp. on High Pressure Low Temperature Plasma Chemistry, ed. by H.-E. Wagner, J. F. Behnke and G. Babucke, Greiswald, 2, pp [6] Ulejczyk B., Opalińska T., Schmidt-Szałowski K., Polaczek J., Karpiński L., Pawłowski S., Polish Patent Appln. P