Time-resolved electric probe measurements in the pulsed-plasma polymerisation of acrylic acid

Using a combination of radio frequency (RF)-compensated and -uncompensated probes, the temporal evolution of the electron density Ne, electron temperature Te and ion impact energy Ei (at an electrically isolated surface) have been determined in the pulsed-plasma polymerisation of acrylic acid. The discharge (excited by 13.56 MHz RF, coupled through an external coil) was operated at a fixed pulse frequency of 500 Hz and pressure of 2.6 Pa, but a variation of powers (5, 20 and 40 W) and duty cycles, (35%, 50% and 75%). The probes were heated prior to measurement to eliminate the formation of insulating deposits on the surface. Experiments have also been performed in pure argon at the same conditions, to act as a comparison. In the acrylic acid afterglow plasma, the electron densities were observed to fall from a maximum of 6×1016 m−3 (in the ‘on’ time) to below 2×1015 m−3 with characteristic decay times of τn∼50 μs, much faster than in argon where τn∼200 μs was found. The density in the ‘on’ time of the pulse was strongly dependent on the discharge power Prf, with a 250% increase in Ne as Prf was increased from 5 to 40 W. As the duty cycle was reduced, the plasma density decreased in the ‘on’ time, giving over a full cycle, lower plasma densities for longer ‘off’ times. Typically, at the start of the ‘on’ time, the electron temperature peaked at values up to 8 eV; however, this decreased is the remainder of the ‘on’ time to about 4 eV. This value was only weakly dependent on the discharge power and duty cycle. During the ‘off’ time, Te fell much more quickly than the density, with characteristic decay times of several 10ʹs of μs or less. The ion bombarding energy to the floating probe, given by the time-averaged potential drop across the sheath, fell sharply from values up to 65 eV in the ‘on’ time to less than 2 eV in the ‘off’ time, the precise values varying slowly with power and duty. This fall happened with a characteristic time of ∼10 μs (for both acrylic acid and argon), controlled by the drop in Te. The results show that at the higher powers, a significant fraction of the sheath potential drop (up to 25 V) and therefore the ion energy was due to the presence of large RF potential fluctuations in the discharge. These preliminary results show the interesting and highly complex nature of the pulsed-polymerising plasma, particularly the highly modulated ion energies at a boundary or substrate.