Nanostructure and Composition Control of Fluorocarbon Films from Modulated Tetrafluoroethylene Plasmas.

To study plasma enhanced chem. vapor deposition (PECVD) of tetrafluoroethylene under modulated discharge (MD) conditions, the effect of the duty cycle (DC) on the time-resolved CF2* emitting species in MD C2F4 plasmas and on the compn. and morphol. of the deposited CFx films was examd. PECVD conditions (substrate: cryst. silicon) were kept const. with respect to power (30 W), pressure (300 mTorr), substrate temp. (room temp.), C2F4 flow rate (6 sccm), and deposition time (90 min) but were varied with respect to modulation parameters period (20-200 ms, only 100 ms period results are reported in the article) and DC (2-100%). The chem. compn., the crystallinity, and the morphol. of the deposited fluorocarbon films were detd. by FT-IR and XPS as well as x-ray diffractometry and SEM. Data of plasma phase and results of film characterization are presented and discussed. The ratios of the CFx fractions (i.e., CF3, CF2, CF, C-CF, and C-C) and the F/C ratio were detd. for various DCs. A decrease in DC from 100% (= continuous wave (CW) condition) to 5% increased CF2 fraction from 29.84 to 62.33% and simultaneously decreased all other fractions. F/C ratio also increased (1.40/100% to 1.90/5%). Under MD plasma conditions, the film structure became more ordered and from the compositional point of view led to structure retention, and linear PTFE chains were obtained. A transition from amorphous (CW condition) to cryst. morphol. depending strongly from DC value was obsd. The results are kinetically and mechanistically discussed. The modulation of C2F4 plasmas was a useful tool to control the kinetics of the heterogeneous reaction of CF2 species and to shift it by tuning the DC values.