Effect of Local Gas Chemistry and Type of Cold Atmospheric Plasma Source on VUV Exposure

Different cold atmospheric plasma (CAP) sources are used for a variety of applications and produce a variety of different reactive species based on operating conditions such as gas chemistry and operating voltage. Direct comparisons of these different sources and understanding of how local environment near the target can affect the flux of species and high energy photons are lacking. Experiments with a kHz driven ring-APPJ show direct interactions between the plasma and the sample surface can cause local damage to polymer films whereas more remote treatments showed uniform thickness reduction [1]. For an Ar-based kHz driven ring-APPJ the source of thickness reduction is largely due to vacuum ultraviolet (VUV). This observation is supported by increased modification of a 193 nm as opposed to a 248 nm photoresist polymer. Modification was also seen with line-of-sight by placing a mesh directly over the sample. The VUV is produced from Ar 2 * at 125 nm as shown by using MgF 2 and alumina optical filters with 114 nm and 140 nm cutoff wavelengths. Fourier transform infrared spectroscopy (FTIR) of treated polymer films showed similar reductions in CO -C and C=O bonding as for low pressure VUV treatments [2]. (See Fig. 1) Modification rates decrease with increasing O 2 in the local gas environment. This modification rate increases over the duration of experiment as Ar becomes the dominant gas in the chamber. The VUV emission from the APPJ is absorbed by O 2 in the local environment and significantly less by N 2 and Ar. This was confirmed by using finite element modeling (COMSOL) to investigate the gas mixture over time between the APPJ and the sample. Using this O 2 concentration, the Beer-Lambert law was used to estimate the decrease of the modification with increasing O 2 , which agreed with experimental data. VUV modification was different depending on the type of source. The kHz driven ring APPJ showed the largest modification through a MgF 2 filter compared to without. All jet-type sources which use Ar showed effects due to VUV reaching the surface to various degrees. In contrast, the surface microdischarge (SMD) source operates without noble gas and showed no thickness loss for either condition. Adding 1% O 2 to the Ar flow in a MHz jet source also effectively eliminated VUV effects as the O 2 strongly reduces the formation of excimers. Figure 1 – Comparison of CAP sources shown …