A multi-technique investigation of the pulsed plasma and plasma polymers of acrylic acid: Millisecond pulse regime

Acrylic acid plasma polymers have been fabricated from pulsed 13.56 MHz RF plasmas in a "capacitively" coupled plasma deposition chamber. Plasma "on" and "off" (t(on/off)) times in the millisecond (ms) pulse time regime have been investigated using a peak plasma power of 50 W. Employing a fixed t(on) (5 ms), the effect of t(off) (2-1000 ms) on the solid-phase plasma polymer has been investigated using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF SIMS). For the first time, mass spectrometry has been employed to monitor the neutral and charged species in pulsed plasma (gas-phase) as a function of tiff. These measurements are supported by quartz crystal mass balance (QCMB) deposition rate measurements. Throughout the study, comparison with continuous (CW) wave plasmas of equivalent "average" power has been made XPS analyses revealed that with increased t(off) (i.e., lower "average" power) the extent of carboxyl retention increased from <33% at a t(off) of 5 ms to ca. 66% at a t(off) of 1000 ms. The SIMS data indicate at low-average power (0.2 W) a linear polymer, closely resembling conventional acrylic acid, is deposited. The mass spectral data indicate that the principal affect of t(off) is on the amount of intact (uncharged) acrylic acid in the system. This is shown by monitoring the signal from the molecular ion of acrylic acid (m/z 72, M.+) in the electron impact mass spectra of the neutral plasma species. Positively charged species were readily detected with ions of m/z >73 [M + H](+) present at all t(off). These ions correspond to oligomers of the form [nM + H](+), where n = 2 or 3, or fragments resulting from the loss of, for example, H2O. The extent of oligomerization is inversely proportional to the average plasma power. In pulsed plasma, the deposition rate (ng/s) increased with average plasma power. A plot of deposition mass per cycle (ng/cycle) shows the increasing importance of radical chemistry in plasma polymer growth at longer t(off). Deposition continues for up to 500 ms after the plasma is switched off. The gas-phase mass spectrometric and QCMB data substantiate the importance of radicals in plasma polymer growth at longer t(off) and we estimate that under the plasma operating conditions employed the radicals' (average) lifetime t(1/2) is about 250 ms.