Experimental study of the plasma gas phase in low-pressure radiofrequency discharges of oxygen and hexamethyldisiloxane is presented. The plasma phase has been studied by means of optical emission spectroscopy. Mass spectroscopy of the neutral and of the charged species has been performed too, directly sampling the plasma gas phase, by a dedicated spectrometer. We also measured the ion energy distribution. We have studied the influence of the operating conditions on the plasma gas-phase composition which plays a primary role in the formation process of SiO2 films, which are known for their important applicative uses. 1. Introduction Plasma-enhanced chemical vapour deposition (PECVD) of silicon oxides is one of the most promising techniques allowing industrial-scale deposition of high-quality, transparent, and thin coatings [1, 2]. SiO2-like films deposited by plasma polymerisation was first developed for microelectronics. Then, they have been applied also as protective coatings in optoelectronics and packaging [3]. The most favourable option is to use organosilicon compounds as precursor, like hexamethyldisiloxane (HMDSO) mixed with O2 and/or noble gases. It was found that composition and structure of films change with the deposition conditions [1]. A systematic control of plasma is then needed to obtain films with the desired properties. Better understanding of the chemical kinetics processes happening in the discharge is then mandatory and requires suitable diagnostics to probe directly the plasma gas phase. Previous investigations employed mass spectroscopy (MS), optical emission spectroscopy (OES), and infrared absorption spectroscopy (FT-IR) [1, 4–7]. However, many points are still open, for instance, concerning the role of ions in the gas phase, the deposition and the film growth process [1]. We have investigated O2/HMDSO plasmas produced by a resonant inductive radiofrequency (RF) plasma source and used to deposit SiO2-like films on polyethylene terephthalate (PET). Diagnostics include OES and on-line MS of neutrals and ions from the discharged gas phase. Our mass spectrometer allows to change the electron energy in the ionization source, in order to better identify the parent radical sampled from the plasma. We were able to measure also ion energy spectra. The properties of the film have been studied by attenuated total reflectance infrared (ATR) spectroscopy, electron microscopy, and by contact angle measurements. 2. Experimental Setup and Diagnostics A scheme of the setup is shown in Figure 1. The plasma source was developed by CCR
References
[1]
H. Biederman, Plasma Polymer Films, Imperial College Press, London UK, 2004.
[2]
P. Esena, S. Zanini, and C. Riccardi, “Plasma processing for surface optical modifications of PET films,” Vacuum, vol. 82, no. 2, pp. 232–235, 2007.
[3]
R. Lamendola and R. D'Agostino, “Process control of organosilicon plasmas for barrier film preparations,” Pure and Applied Chemistry, vol. 70, no. 6, pp. 1203–1208, 1998.
[4]
D. S. Wavhal, J. Zhang, M. L. Steen, and E. R. Fisher, “Investigation of gas phase species and deposition of SiO2 films from HMDSO/O2 plasmas,” Plasma Processes and Polymers, vol. 3, no. 3, pp. 276–287, 2006.
[5]
M. Creatore, Y. Barrell, J. Benedikt, and M. C. M. Van De Sanden, “On the hexamethyldisiloxane dissociation paths in a remote Ar-fed expanding thermal plasma,” Plasma Sources Science and Technology, vol. 15, no. 3, pp. 421–431, 2006.
[6]
D. Magni, C. Deschenaux, C. Hollenstein, A. Creatore, and P. Fayet, “Oxygen diluted hexamethyldisiloxane plasmas investigated by means of in situ infrared absorption spectroscopy and mass spectrometry,” Journal of Physics D, vol. 34, no. 1, pp. 87–94, 2001.
[7]
M. R. Alexander, F. R. Jones, and R. D. Short, “Mass spectral investigation of the radio-frequency plasma deposition of hexamethyldisiloxane,” Journal of Physical Chemistry B, vol. 101, no. 18, pp. 3614–3619, 1997.
[8]
M. Weiler, K. Lang, E. Li, and J. Robertson, “Deposition of tetrahedral hydrogenated amorphous carbon using a novel electron cyclotron wave resonance reactor,” Applied Physics Letters, vol. 72, no. 11, pp. 1314–1316, 1998.
[9]
R. Barni, S. Zanini, and C. Riccardi, “Diagnostics of reactive RF plasmas,” Vacuum, vol. 82, no. 2, pp. 217–219, 2007.
[10]
R. W. B. Pearse and A. G. Gaydon, The Identification of Molecular Spectra, John Wiley & Sons, New York, NY, USA, 1976.
[11]
A. Schwabedissen, E. C. Benck, and J. R. Roberts, “Langmuir probe measurements in an inductively coupled plasma source,” Physical Review E, vol. 55, no. 3, pp. 3450–3459, 1997.
[12]
C. Riccardi, R. Barni, and M. Fontanesi, “Experimental study and simulations of electronegative discharges at low pressure,” Journal of Applied Physics, vol. 90, no. 8, pp. 3735–3742, 2001.
[13]
F. F. Chen, Introduction to Plasma Physics, Plenum, New York, NY, USA, 1984.
[14]
J. M. Smith, Introduction to Chemical Engineering Thermodynamics, McGraw-Hill, New York, NY, USA, 1987.
[15]
C. Riccardi, R. Barni, F. De Colle, and M. Fontanesi, “Modeling and diagnostic of an SF6 RF plasma at low pressure,” IEEE Transactions on Plasma Science, vol. 28, no. 1, pp. 278–287, 2000.
[16]
S. Zanini, C. Riccardi, M. Orlandi et al., “Surface properties of HMDSO plasma treated PET,” Surface and Coatings Technology, vol. 200, no. 1–4, pp. 953–957, 2005.
[17]
S. Zanini, C. Riccardi, M. Orlandi, and E. Grimoldi, “Characterisation of SiOxCyHz thin films deposited by low-temperature PECVD,” Vacuum, vol. 82, no. 2, pp. 290–293, 2007.
[18]
R. A. Siliprandi, S. Zanini, E. Grimoldi, F. S. Fumagalli, R. Barni, and C. Riccardi, “Atmospheric pressure plasma discharge for polysiloxane thin films deposition and comparison with low pressure process,” Plasma Chemistry and Plasma Processing, vol. 31, no. 2, pp. 353–372, 2011.
[19]
A. Sonnenfeld, A. Bieder, and P. R. von Rohr, “Influence of the gas phase on the water vapor barrier properties of films deposited from RF and dual-mode plasmas,” Plasma Processes and Polymers, vol. 3, no. 8, pp. 606–617, 2006.
