Time-resolved mass and energy spectral investigation of a pulsed polymerising plasma struck in acrylic acid

Abstract

This paper details an electrical diagnostic study of the discharge used in the pulsed plasma polymerisation of acrylic acid. The capacitively coupled RF discharge was operated at a frequency of 1 kHz and a pressure of 1.3 Pa. Using a retractable deposition-protected Langmuir probe the time-resolved electron temperature, density and plasma potential in the bulk were obtained. The time-resolved ion energy distribution functions (IEDFs) relative to ground potential for a selection of masses ( m/ z = 19, 55, 73) and the mass spectra (2–100 amu) have been measured by the energy-resolved mass spectrometer using an improved double gating technique. In both the “on” and “off” times of the pulse, the IEDFs for the heavier mass ions are characterised by a single peak ( E p – hereafter – the energy which corresponds with the peak intensity), the position and width of which are determined by the time-dependent plasma potential V p and its distribution across the plasma to the spectrometer, while a weakly bimodal ion energy distribution is present in the IEDF for m/ z = 19 in the “on” time. In the steady “on” time, the relative ion flux (detector count rate) of each species and the energy corresponding to the peak intensity in the IEDFs are constant, the latter ( E p ∼ 47 eV) correlating very well to the probe measured plasma potential. At pulse initiation, V p rises to values > 90 V, however, these energies are not detected in IEDF with our time resolution of 10 μs. In the “off” time, this energy falls with a two-fold decay time, initially sharply to ∼ 23 eV (at the first measured point which was made 1 μs after beginning of the “off” period) and then slowly in the next 150 μs to ∼ 17–18 eV closer to the end of the cycle. The elevation of the E p well above ground is in contrast to that usually observed in pulsed Ar discharges (∼ 10 V higher), due to in this case, charging of the acrylic acid deposited materials on the walls. The time-history of the integrated area under the IEDFs reveals that after the initiation of the “on” time the concentration N of lighter species rises more quickly than the heavier ones. In the “off” time, however, although N 19 becomes negligible after 50 μs, N 73 decreases more quickly than N 55 which is still non-zero at 150 μs. The time resolved mass-spectral reveal the general mass dependency in the concentration rise and fall times of the detected species.