Characterization of thin-film deposition in a pulsed acrylic acid polymerizing discharge

In this study, thin-film deposition in a pulsed rf polymerizing discharge $(13.56MHz)$ struck in acrylic acid has been investigated by mass spectrometry, x-ray photoelectron spectroscopy, and quartz crystal microbalance techniques. The experiment was conducted at a fixed acrylic acid pressure of $1.3Pa$ and “on” pulse duration of $0.1ms$⁠, whereas the “off” time was varied between 0 and $20ms$⁠. The rf input power in the “on” time and gas flow rate were varied between 10 and $50W$ and 1.5 and $4.8sccm$ (sccm denotes cubic centimeter per minute at STP), respectively. These changes of the discharge conditions resulted in large-scale progressive variations in film and gas-phase plasma composition. In particular, the –COOH functionality of the monomer was increasingly retained in the plasma-generated thin films as the duty cycle was lowered (i.e., with lowered time-averaged powers). The monomer retention reached its maximum value of 66% for “off” times exceeding $5ms$⁠, when the discharge was operating in the power-deficient regime. The results show that the film deposition rate is a strong function of the monomer flow rate, whereas –COOH retention is correlated to the amount of unfragmented monomer in the plasma, controlled by the applied power.