Plasma Deposition of Fluorocarbon Thin Films from c-C4F8 using Pulsed and Continuous rf Excitation

Fluorocarbon films of varying composition have been deposited from pulsed and continuous plasmas of octafluorocyclobutane (c-C4F8) and c-C4F8/Ar. Deposition rates are a nearly linear function of applied rf power, although the pulsed plasma deposition rates are significantly lower than continuous plasma rates at the same average power. The pulsed plasma deposition rates can be attributed almost entirely to the plasma on time during the pulse, but there is a dependence on the pulse off time, indicating the plasma chemistry for the pulsed systems are different than the continuous systems. Ar addition affects the deposition rates through a residence time effect, but also plays a significant role in the deposition chemistry by reducing the degree of C4F8 dissociation, resulting in more fluorinated films. Refractive indices for all films increase linearly with applied rf power, with the pulsed plasma-deposited films falling on the same curve. Carbon 1s x-ray photoelectron spectroscopy shows that the continuous plasma-deposited films become increasingly fluorinated as the rf power is decreased, exhibiting a much larger range of compositions over applied power than usually seen for continuous fluorocarbon plasma systems. A step change in the %CF2 (37 - 49%) occurs between 50 W continuous and 10/100 (400 W during on time) pulsed plasma films which is attributed to the different plasma chemistry of the pulsed vs. continuous system. Evidence suggests that the C4F8 is not fully dissociated in either plasma system and that larger order species in the gas phase play a significant role in the deposition mechanisms.