Optical emission spectroscopy characterization of oxygen plasma during treatment of a PET foil

Optical emission spectroscopy (OES) analysis of inductively coupled RF oxygen plasma during plasma treatment of a 23 mu m thick polyethylene terephthalate (PET) foil is presented. Plasma was generated in pure oxygen at a pressure of 75Pa with a RF generator at a frequency of 27.12MHz and an output power of 300W. The electron temperature was about 6 eV, the density of charged particles about 10(16) m(-3) and the density of neutral O atoms about 10(22) m(-3). Spectra were measured in the range from 250 to 950 nm by means of an optical spectrometer. For the first 10s of plasma treatment the OES showed the presence of oxygen radicals only. Later, the OES spectra became richer with significant emission from CO and OH, which was attributed to PET oxidation. Simultaneously, the O peaks decreased significantly. After prolonged plasma treatment, the O peaks recovered, the CO band vanished while the OH and H peaks still persisted. In the final period of the treatment only atomic oxygen lines remained. The results showed that OES analysis was a powerful method for studying the evolution of PET oxidation by plasma treatment.