IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 6, JUNE 2009 961 Comparing Deposition Properties in an Atmospheric Pressure Plasma System Operating in Uniform and Nonuniform Modes Barry Twomey, Denis P. Dowling, Gerry Byrne, William G. Graham, Lucas F. Schaper, Damian Della Croce, Alan Hynes, and Liam O’Neill Abstract—A large-scale atmospheric pressure plasma has been generated in helium, and the time-resolved optical and electrical properties have been shown to produce a homogeneous dielectric barrier discharge. Introducing tetraethyl orthosilicate as a liquid aerosol into this plasma produced clear, uniform, and smooth plasma polymerized coatings. Optical imaging studies have shown that adding 1% oxygen to the gas mixture induced a switch from a homogeneous plasma to a filamentary or microdischarge mode of operation, and this has been shown to dramatically alter the morphology of the deposited coatings. Surface analysis reveals significant particulate inclusions in coatings deposited from the filamentary mode of operation. Index Terms—CVD, optical imaging, plasma properties, thin films. I. I NTRODUCTION P LASMA polymerization has been widely exploited under vacuum conditions for many years to produce a range of industrial coatings [1], [2]. By altering the plasma parameters and the choice of precursor, the chemistry of the deposited coatings can be tailored to deliver coatings that meet the requirements of industries as diverse as microelectronics [3], biotechnology [4], food packaging [5], and textiles [6]. While many of these applications use simple coating chemistry de- posited by continuous wave plasmas, research has shown that even higher levels of functional chemistry could be retained in the deposited coatings by moving from continuous wave to a pulsed plasma operation [7], [8]. In recent years, coating technology has expanded to en- compass coatings deposited using atmospheric pressure plasma techniques [9]–[11]. Further enhancements arose from the combination of atmospheric pressure glow discharge (APGD) plasmas [12], [13] with the introduction of the precursor as an aerosol. The combination of a nonthermal APGD with a Manuscript received December 4, 2008. First published May 12, 2009; current version published June 10, 2009. B. Twomey, D. P. Dowling, and G. Byrne are with the University College Dublin, Dublin 4, Ireland (e-mail: barry.twomey@ucd.ie; gerald. byrne@ucd.ie; denis.dowling@ucd.ie). W. G. Graham, L. F. Schaper, and D. Della Croce are with Queen’s University Belfast, BT7 1NN Belfast, U.K. (e-mail: b.graham@qub.ac.uk; lschaper01@qub.ac.uk; ddellacroce01@qub.ac.uk). A. Hynes and L. O’Neill were with Dow Corning Plasma Solutions, Cork, Ireland (e-mail: alanhynespro@gmail.com; liam.m.oneill@gmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2009.2015226 Fig. 1. (Left) ICCD image of interelectrode (5 mm) He discharge at 1000-W applied power obtained over (right) a half-period (≈ 25 μs) and a light intensity profile. low-energy liquid precursor allows coatings to be deposited which retain the chemical structure of the precursor mole- cule in a manner similar to that reported for pulsed vacuum plasma systems [14], [15]. The APGD is generally the preferred plasma option as it provides a nonthermal, homogeneous and pulsed plasma that can be readily produced under ambient pres- sure [16]. Uniform dielectric barrier discharges, both glow (APGD) and Townsend (APTD) discharges are normally characterized as having a current pulse each half-cycle of the applied voltage. Each pulse should have a duration of a few microseconds, and a current density of approximately equal to milliamperes per cubic centimeter. A band of uniform emission parallel to the electrode with a negative polarity is generally seen as an indicator of an APGD, while if the uniform emission is concentrated parallel to the positive electrode, the plasma is described as an atmospheric pressure Townsend discharge [17]. The polarity of the electrode is determined by both the applied voltage and the charge stored on the dielectric. Unfortunately, it is not possible in these large area discharges to measure the gap voltage since the capacitance of the capacitor required for the measurement [18] would significantly distort the electrical performance of the system. It is therefore difficult to determine conclusively if the present discharges are APGDs or ATGD. However, at intermediate power (1000 W), there is evidence of a luminous area followed by additional dark space and the positive column glow associated with APGDs [17], as shown in Fig. 1. Recently, the authors have published a detailed characteri- zation of a helium plasma produced within a industrial scale commercial atmospheric pressure plasma coating system and concluded that it behaves in a manner similar to other plasmas 0093-3813/$25.00 © 2009 IEEE Authorized licensed use limited to: University College Dublin. Downloaded on July 20, 2009 at 05:38 from IEEE Xplore. Restrictions apply.