Pulsed and continuous wave acrylic acid radio frequency plasma deposits: plasma and surface chemistry.

Plasma polymers have been formed from acrylic acid using a pulsed power source. An on-pulse duration of 100 micros was used with a range of discharge off-times between 0 (continuous wave) and 20,000 micros. X-ray photoelectron spectroscopy (XPS) has been used in combination with trifluoroethanol (TFE) derivatization to quantify the surface concentration of the carboxylic acid functionality in the deposit. Retention of this functionality from the monomer varied from 2% to 65%. When input power was expressed as the time-averaged energy per monomer molecule, E(mean), the deposit chemistry achieved could be described using a single relationship for all deposition conditions. Deposition rates were monitored using a quartz crystal microbalance, which revealed a range from 20 to 200 microg m(-2) s(-1), and these fell as COOH functional retention increased. The flow rate was found to be the major determinant of the deposition rate, rather than being uniquely defined by E(mean), connected to the rate at which fresh monomer enters the system in the monomer deficient regime. The neutral species were collected in a time-averaged manner. As the energy delivered per molecule in the system (E(mean)) decreased, the amount of intact monomer increased, with the average neutral mass approaching 72 amu as E(mean) tends to zero. No neutral oligomeric species were detected. Langmuir probes have been used to determine the temporal evolution of the density and temperature of the electrons in the plasma and the plasma potential adjacent to the depositing film. It has been found that even 500 micros into the afterglow period that ionic densities are still significant, 5-10% of the on-time density, and that ion accelerating sheath potentials fall from 40 V in the on-time to a few volts in the off-time. We have made the first detailed, time- and energy-resolved mass spectrometry measurements in depositing acrylic acid plasma. These have allowed us to identify and quantify the positive ion species in the acrylic acid plasma during both the on- and the off- periods. The relative intensities of oligomeric species of the type [nM + H]+ as large as n = 3 were observed to increase in the off-time suggesting vapor phase polymerization after power input to the plasma was ceased. The energy distribution functions of these ions demonstrated that they were produced in the plasma in both the on- and the off-times. This remarkable observation contradicts the assumptions usually made when speculating on pulsed plasma that ions have very short lifetimes, although it is anticipated that radicals still have significantly longer lifetimes, estimated from calculation to be in excess of 1 ms. The increase in average positive ion mass during the off-period can be related to the lower mobility of the heavier components, reducing their relative loss to surfaces, and the polymer chain growth in the gas phase due to the ion-neutral collisions. The implications of these observations are discussed in light of polymerization mechanisms proposed from continuous acrylic acid and millisecond pulsing plasmas.