Space- and time-resolved diagnostics of microwave plasma process used for hydrogenated amorphous carbon film coating of PET bottles

The addition of a coating on the inside walls of a standard polyethylene teraphthalate (PET) bottle provides a gas barrier improvement of the packaging for beer, juice and carbonated soft drinks. Sidel Inc. has developed a plasma-assisted deposition process of hydrogenated amorphous carbon (a-C:H) films, which uses pulsed microwave discharges with acetylene gas precursor. We report here the space- and time-resolved optical diagnostics of this process, aiming to assess the microwave discharge characteristics and behavior during the deposition process. Time-resolved fast imaging of the discharge enables the investigation of plasma ignition, expansion and extinction inside the bottle along the microwave power pulses of 100 Hz frequency. Space- and time-resolved optical emission spectroscopy is used in order to examine the feature of the main radiative species within the plasma and to estimate the gas temperature. Measurements showed that the plasma emission intensity is always higher in the base and around the neck of the bottle, which comes from the discharge spatial inhomogeneity and from the successive phases of plasma development always involving the lower part of the bottle. The spatio-temporal intensity distribution of radiative species, namely H-atom and C₂ molecule, is in good agreement with fast imaging, whereas the profile of CH radical is found to be more homogeneous. Rotational temperatures of excited states of C₂ molecule and CH radical measured during a microwave pulse were found to be relatively high, between 1700 and 2700 K, and slightly dependent of time and position. A short heating time of only a few hundreds of microseconds is noticed and gives evidence of non equilibrium between the rotational temperatures and the gas temperature. This preliminary plasma diagnostic is a first step towards drawing possible connections between plasma characteristics and a-C:H coating properties and PET bottle post-treatment temperature, aiming at optimizing the deposition process.