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Optical emission spectroscopy characterization of inductively coupled RF hydrogen plasma during etching of composite materials

Low pressure inductively coupled RF hydrogen plasma was utilized for etching of dental posts (FRC Postec). Plasma was created in a linear borosilicate glass tube by an 8 turn coil with an EM field oscillat-ing at 13.56 MHz. Dental posts were treated at various plasma conditions: hydrogen gas pressure (10-50 Pa), and discharge powers (up 300 W). Real-time spectral analysis was performed by a LIBS2000+ spec-trometer system from Ocean Optics, which consists of seven miniature spectrometers (resolution 0.1 nm in the range 200-980 nm). The effect of plasma treatment on the sample was monitored by JSM 7000F Field Emission Scanning Electron Microscope (FE SEM) equipped with energy-dispersive spectrometer (EDS), INCA 350, Oxford Instruments for elemental analysis. The experimental apparatus is shown in figure 1. Spectral signature of hydrogen radicals and H2 Fulcher band were observed during the plasma etching of posts. The most prominent hydrogen radical lines are the Balmer series (H at 656 nm, H at 486 nm and H at 434 nm). Molecular hydrogen band was observed at around 602 nm. Additionally, a well known sodium line was observed (actually sodium D-lines at 589 and 589.6 nm) as a signature of the etching process. All these spectral features were monitored thought the whole experiment, and both the spectrum and the temporally resolved acquisition are shown in figure 2. Three different plasma species have several distinct trends during the etching time (120 s). Molecular hydrogen band remains practically constant during etching, atomic lines (such as H) decrease while sodium line increases. This points to the fact that hydrogen atoms are consumed in the dental posts, and sodium is released from the posts. We note that other possible products such as ytterbium fluoride or CO or CH could not be observed in spectra. The FRC posts are example of complex materials that consist of glass fibers and a composite matrix [2]. Plasma treated posts have different surface and structural properties (figure 3). Plasma treatment in-creases surface wettability and could lead to a replacement for current methods of acid etching and post-silanization [3]. OES monitoring of plasma treatment provides important information during optimi-zation procedure.

optical emission spectroscopy; rf plasma; hydrogen; composite materials

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