Measurements of the Plasma Parameters in RF Torch Discharge Used for Deposition of Oxide Thin Films

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1 WDS'06 Proceedings of Contributed Papers, Part II, , 006. ISBN MATFYZPRESS Measurements of the Plasma Parameters in RF Torch Discharge Used for Deposition of Oxide Thin Films M. Chichina, P. Kudrna, M. Tichý, A. Grinevich Charles University in Prague, Faculty of Mathematics and Physics, V Holešovičkách, Prague,Czech Republic. O. Churpita, Z. Hubička, S. Kment, J. Olejníček Institute of Physics, Division of Optics, Academy of Sciences of the Czech Republic, Na Slovance, 18 1 Prague 8, Czech Republic. Abstract. We report on the application of barrier-torch plasma jet system - the novel plasma deposition system capable of working at atmospheric pressure in the open air - for deposition of ZnO and TiO layers. Properties of deposited thin films were analyzed by XRD and AFM. ZnO thin films had a pronounced hexagonal crystal structure and chemical composition of the films was very close to that of stoichiometric ZnO. We did not detect any other crystalline but the Anatase phase of TiO deposited using the barrier torch discharge. The typical average surface roughness of TiO films was nm and roughness of the ZnO was nm. In this paper we present the first results of spectroscopic measurements of Balmer H β line width aimed at estimating the electron density in plasma jet at atmospheric pressure and rough estimation of electron density in the same system by means of plasma jet column impedance measurements. Introduction A large number of systems for atmospheric PECVD (Plasma Enhanced Chemical Vapour Deposition) have been developed recently. Low temperature dielectric-barrier discharges [Eliasson et al., 1987, Kanazawa et al., 1988, Kogelschatz et al., 1997, 000, Massines et al., 1998, Samoilovich et al., 1997] are very often applied for PECVD of polymer and other kinds of thin films [Barankova et al., 000, Donohoe et al., 1979, Klages et al., 000, Massines et al., 1995]. We applied single RF plasma jet system for deposition of ZnO and TiO oxide thin films at atmospheric pressure on quartz glass and silicon substrates. ZnO films recently gained wide interest due to their stability in hydrogen plasma as transparent conductive oxide films. TiO has been a subject of intensive research because of its outstanding physical and chemical characteristics and biocompatibility. The electron concentration is one of the basic plasma parameters. The knowledge of this parameter is important in order to achieve good reproducibility of the deposition process, because the rate of precursor decomposition depends on electron concentration. Usually, the electron concentration is estimated from the Langmuir probe measurements. At the pressures close or equal to the atmospheric one it is, however, difficult to use single Langmuir probe for plasma monitoring. We will present here that electron concentration in the atmospheric plasma jet system can be very roughly estimated from the plasma column impedance measurement. In this contribution we present results of spectroscopic measurements aimed at estimating the plasma jet parameters at atmospheric pressure. The emission spectra were used for determination of the electron concentration in plasma jet. Results of plasma parameters obtained from emission spectroscopy were in a fair agreement with parameters measured by impedance monitoring of the plasma jet column at atmospheric pressure. Experiment The atmospheric RF barrier-torch plasma jet apparatus has been described in [Hubička et al., 00]. The system in Figure 1 works at atmospheric pressure; it is just encased in acrylic 108

2 glass box, which is provided by a pipe leading the used working gases out of the building. We applied single-jet system for deposition of ZnO and TiO thin films at atmospheric pressure on quartz glass substrates. The quartz glass tube with internal diameter 1.5 mm is surrounded by the stainless steel RF powered electrode. The RF electrode was connected with RF power generator via the matching unit. As an RF source the generator with maximum output RF power 600 W and frequency f=13.56 MHz is used. Helium and nitrogen were fed into the quartz nozzle. The distance between nozzles outlet and the substrate varied within 4-13 mm. Due to the high RF field at the edge of the powered electrode the RF barrier-torch discharge was generated in the mixture of He and N. Vapours of precursors were fed directly into the nozzle powered by RF electrode. As the thin film growth precursors, vapours of Zn-acetylacetonate (Zn[C 5 H 7 O ] ) and Ti-izopropoxide (Ti [OCH (CH 3 ) ] 4 )were used. Figure 1. RF plasma jet system at atmospheric pressure used for deposition of ZnO thin films. These chemicals were placed into the containers, kept at electronically stabilized temperature up to 3 C for deposition TiO thin films and up to 105 C for deposition of ZnO thin films. The stable precursor temperature resulted in stable precursor flow rate (stable He - N - precursor mixture ratio) even if we could not directly measure the precursor flow rate. Quartz glass with thickness. mm was used as substrate for both types of thin films. Coating of the larger area was provided by motor-driven x-y movement of the grounded Al substrate holder, which was also provided by the water-cooling system. The RF source worked in pulse modulated mode. This modulation allowed excitation of the high density plasma in the active part of the duty cycle and simultaneously keeping the neutral gas in the plasma jet near the substrate reasonably cold thus protecting polymer substrate from thermal damages. Properties of thin films ZnO films in the role of transparent conductive oxide films gained recently wide interest due to their stability. They have applications in solar cell technology, gas detection, and many others TCO (Transparent Conducting Oxide) applications. For understanding of the thin film structures we investigated the ZnO films with help of X-Ray Diffraction (XRD) method. These analyses have shown that the ZnO thin films on quartz glass substrates have a pronounced hexagonal crystal structure with preferable crystallites orientation with c axis perpendicular to the substrate surface, see Figure. The chemical composition of the films was very close to that of stoichiometric ZnO. XRD analysis of TiO thin films showed that apart from the amorphous material only the Anatase crystalline phase is present in films. Post-deposition annealing at high temperature is usually required to produce the amorphous - Anatase transformation. Hence, with increasing 109

3 CHICHINA ET AL.: PLASMA JET SYSTEM 1500 TiO 00 Intensity [a.u.] Intensity [a.u.] 600 temperature 5 C ZnO 00 temperature 450 C temperature 500 C 500 temperature 550 C ZnO standard Q TiO 0 o [ ] Figure. XRD pattern of ZnO film. 40 Q o 80 Anatase standard 100 [ ] Figure 3. XRD pattern of TiO film. Figure 5. AFM scan of TiO thin film. Figure 4. AFM scan of ZnO thin film. the annealing temperature the amount of Anatase phase also in our samples increased, see Figure 3. The sloped part of the XRD spectra at lower Bragg angles is the effect of the amorphous substrate. We conclude that we did not detect any other crystalline but the Anatase phase of TiO deposited using the barrier torch discharge. Part of the layer material deposited in the additional amorphous phase was transformed into Anatase crystals by annealing at temperatures between 300 C to 550 C after deposition for hours. In other deposition methods, which we found in literature, mixtures of different TiO phases or only the Rutile phase were found. In addition, while in the other titanium oxide deposition procedures the TiO layers without annealing usually contain solely amorphous titanium dioxide, the deposition using barrier torch discharge yields substantial amount of Anatase phase already at room temperature. Figures 4 and 5 show AFM morphology for ZnO and TiO thin films. The typical average surface roughness of ZnO films is between nm and size of patterns between nm. The typical average surface roughness of TiO films is between nm and size of patterns between nm. Determination of the electron concentration by means of plasma jet column impedance measurements In order to measure the RF instant power PRF absorbed in the plasma jet during active part of the duty cycle, we attempted to directly measure the instantaneous values of the RF voltage and current by means of digital phosphor oscilloscope (DPO). For that we attached the RF capacitive voltage probe to the metallic RF electrode placed around nozzles and inserted Rogowski coil as an RF current sensor in current path between the substrate and the grounded 110

4 holder. This configuration was experimentally beneficial since we measured RF current on the conductor one terminal of which was grounded and consequently the error due to capacitive coupling of RF voltage to sensed current signal was relatively low. Voltage and current sensors were calibrated prior to their installation. RF signals from these calibrated sensors were fed to digital oscilloscope which was able to directly process the sampled data. We were able from the voltage and current waveforms to calculate instant RF power P RF absorbed in the plasma over one RF cycle in active modulation period and real Z R and imaginary Z im parts of the plasma impedance. Typical example of RF voltage and current waveforms obtained in our system is given in Figure 6. We have used the described plasma impedance monitoring for determination of discharge current density, absorbed RF power, real and imaginary part of the plasma impedance and, consequently, for the rough estimation of electron concentration. Figure 6. Example of RF current and voltage waveform in barrier-torch discharge 37 watts deposited in the torch-discharge ( Q He =900 sccm=1.5 Pa m 3 s 1 ). Figure 7. Electron concentration and RF power density in dependence on the RF current amplitude. It can be considered, see [Shi et al., 003], that the real part of plasma jet column impedance Z R represents its ohmic resistance. Since the excitation frequency of RF generator ω is much less than the collision frequency ν of electrons with heavy particles, ω = πf << ν, we can write in accord with [Shi et al., 003] for the electron concentration n e : n e = d eaz R µ e. (1) In this expression d is the length of the resistive plasma jet column, e is the electron charge, A is the cross section of plasma jet, Z R is the real part of the single plasma jet column impedance and µ e = m /(Vs) is the electron mobility in He at atmospheric pressure 10 5 Pa. Electron concentration n e calculated according formula (1) is depicted in Figure 7 in dependence on the RF discharge current density. It shows that n e was relatively stable in magnitude n e = cm 3. We suppose that n e calculated here is only rough estimate (approximately factor two accuracy) of some average value in the plasma jet. Our presumptions used for n e calculation were based on approximation and simplification of the real situation. We can see in Figure 7 the dependences of n e and volume RF power density p on the RF discharge current density i. While the power density p rises with the current density i as expected, the n e is almost independent of i. The reason for such behaviour of n e consists most probably just in the crude method of n e estimation, which is unable to resolve the variations of n e with i. Estimation of the electron concentration using spectroscopic measurements In this paper we present the first results of the electron density estimation by means of the spectroscopic measurements at atmospheric pressure in Ar. The electron concentration was 111

5 Figure 8. Scheme of the spectroscopic measurements of the electron concentration in the plasma jet at atmospheric pressure.(1) metal nozzle, () grounded electrode, (3) power generator, (4) humidifier, (5) lens system, (6) spectrograph, (7) photomultiplier, (8) amplifier. 0 H B lineprofile(486.16nm) Voigtprofile cm -3 ] Intensity[a. u.] Wavelength [nm] Electronconcentration[ Power[W] Figure 9. Experimental H β line profile ( nm) and its approximation by the Voigt profile. Figure 10. Dependence of the electron concentration in Ar plasma jet at atmospheric pressure on the power delivered into the plasma. estimated from the H β line broadening. A small amount of water vapour was added to the Ar gas flow to obtain sufficient intensity of H β emission spectral line (Balmer series, nm) by means of humidifier. We used the method based on the Stark broadening of H β spectral line spontaneously emitted from the plasma. For the n e calculation we adopted the simple relation between λ Stark (H β ) and n e derived and exploited already by [Jasiǹski et al., 005]: n e = [ λ Stark (H β )] 1.55 () where the Stark broadening λ Stark (H β ) is in nm and n e in cm 3. The profile of an emission line can be affected by different broadening mechanisms: natural, thermal Doppler, Stark (collisional), instrumental etc. We assumed together with [Jasiǹski et al., 005] that the profile of H β spectral line was affected in our case by only two kinds of the broadening mechanisms: instrumental and Stark. To eliminate the instrumental broadening we used the profile of Hg spectral line (435.8 nm) fitted by Gaussian profile as an instrumental function. The H β line profile was fitted by Voigt profile, see Figure 9. The Voigt profile is the convolution of Gaussian and Lorentzian profiles. Using the deconvolution we obtained the Lorentzian profile of H β spectral line and thus we were able to measure the Stark broadening. In Figure 8 there are depicted the main parts of the used experimental setup: power generator (3), Ar gas supply, humidifier (4), lens system (5), spectrograph (6) and PC computer. In the preliminary experiments described here we powered the discharge by a DC source using the power range W. Ar gas mass flow rate was Q Ar = 700 sccm = 1. Pa m3 s 1 11

6 and that of mixture of Ar+H O, Q Ar+H O = 300 sccm = 0.5 Pa m 3 s 1. Metal nozzle (1) with inner diameter 1.4 mm and outlet diameter 6 mm was used as electrode. Distance between the metal nozzle and the grounded electrode () was variable in the range 3 6 mm. To measure the H β spectral profile (see Figure 9), the light emitted by plasma jet was focused onto entrance slit of the spectrograph (SPM, 100 grooves/mm) by means of a lens system (5). The width of the entrance slit of the spectrograph was 10 µm. The measurements were reproducible within approximately 6 % error limit. The measured electron densities ranged around cm 3. It was observed that the plasma density increased with increasing of the power supplied into the plasma; see Figure 10. The magnitude of the electron density was approximately by one order of magnitude greater compared to the results of plasma column impedance measurements in He, presented in this article and in [Chichina et al., 005]. Measurements of the H β spectral profile in atmospheric plasma jet with He as working gas did not give so far satisfactory results. The most probable reason consists in smaller electron density in comparison to Ar at similar deposited power [Kousal et al., 00] and consequently in too low H β line width; further experiments are necessary. Conclusion RF plasma jet system at atmospheric pressure was used for ZnO and TiO thin films deposition. Such ZnO thin films had stochiometric ZnO chemical composition and hexagonal crystal structure what could be seen from XRD spectra. The deposited TiO thin films had only one crystalline phase - Anatase. The ratio of Anatase to amorphous TiO has increased after post-deposition annealing. We determined the electron concentration (n e ) in atmospheric plasma jet by two independent methods. The value of n e in DC powered argon plasma jet obtained from Stark broadening of H β spectral line was around cm 3. The plasma column impedance measurements yielded on the other hand the magnitude of n e around cm 3 in helium RF powered plasma jet. The experimental conditions (deposited power, mass flow rate of the working gas, nozzle diameter) were similar in both experiments. However, a strong effect of the working gas on electron concentration is not surprising, since analogous values of n e in atmospheric DC plasma jet in Ar (around cm 3 ) and in He (around cm 3 ) at otherwise similar experimental conditions have also been found in [Kousal et al., 00]. Acknowledgments. This work is a part of the project MSM that is financed by the Ministry of Education, Youth and Sports of the Czech Republic. Thanks are also due to Czech Science Foundation, grants No. 0/03/H16, 0/05/4 and 0/06/0776. References Barankova, H. et al., Appl. Phys. Lett. 76 (000) 85. Chichina, M.et al., Plasma Processes and Polymers (005) 501. Donohoe, K. G. et al., J. Appl. Polymer Sci. 3 (1979) 591. Eliasson, B. et al J. Phys. D: Applied Phys. 0 (1987) 141. Hubička, Z. et al., Plasma Sources Science and Technology 11 (00) 195. Jasiǹski, M. et al., XXVIIth ICPIG. Eindhoven, the Netherlands 005. Kanazawa, S. et al., J. Phys. D: Appl. Phys. 1 (1988) 838. Klages, C. P. et al., Int. Symp. High Pressure Low Temperature Plasma Chemistry. Hakone VII, 000. Kogelschatz, U., J. Phys. IV. France, Kogelschatz, U., Int. Symp. High Pressure Low Temperature Plasma Chemistry. Hakone VII, 000. Kousal, J.et al., Czechoslovak Journal of Physics 5 (00) D571. Massines, F. et al., XXII. Int. Conf. on Phenomena in Ionised Gases. Hoboken, New Jersey, Massines, F. et al., J. Appl. Phys. 83 (1998) 950. Samoilovich, V. G. et al., Physical Chemistry of the Barrier Discharge (in Russian), Moscow State Univ., 1989, English transl., J. P. F. Conrads and F. Leipold, DVS-Verlag GmbH, Düsseldorf Shi, J. J. et al., J. Appl. Phys. 94 (003)