Paper PS+SS-ThA6
Control of Hydrocarbon Surface Density during H₂/D₂/Ar Low Temperature Plasma Interaction

Thursday, November 3, 2011, 3:40 pm, Room 202

Control of surface properties of hydrocarbon materials during interaction with H₂/D₂/Ar low temperature plasma has applications in diverse areas including: thin film synthesis, electronic device manufacturing, nuclear fusion reactor design, and plasma sterilization. Plasma processing/exposure of hydrocarbon materials can cause large changes to the surface chemistry and morphology due to interaction with ions, reactive neutrals, and UV/VUV photons. Hard hydrocarbon materials such as amorphous hydrocarbon (a-C:H) and diamond-like carbon (DLC), suffer a loss of density and a thick modified layer with reactive plasma etching through reactant saturation of the surface and an increase in density and formation of a thin modified layer by inert plasma etching through selective sputtering of H, whereas soft hydrocarbon materials (polymers, biomaterials) exposed to reactive and inert plasmas have been shown to increase in density and become chemically modified. In this work we have explored the plasma/surface interaction with hard/soft hydrocarbon films using different H₂/D₂/Ar gas feedstock compositions along with different ion energy/fluence to the surface. The time dependent changes in optical properties and the etch yields were found using real time in-situ ellipsometry. Using multilayer films (e.g., soft a-C:H over hard a-C:H) we monitored dynamic changes in the penetration/modification depths for different plasma chemistries.

Shallow modifications by inert plasmas (such as Ar) are well understood. Ar plasma depletes H from the surface of a-C:H films through selective sputtering, scaling with ion energy. H₂ plasmas have been shown to cause deep hydrogenation of the surface not predicted with TRIM models. Mixing the processing gas (Ar/H₂) can be used to control the surface density from H depleted to H saturated. To better understand the roles of ion mass, etch rate and diffusion on the depth/degree of modification seen for H₂ plasmas, we performed comparable studies using D₂. We find that increasing the mass of the ions (by using H isotopes such as D) causes a large change in the etching and surface modification behavior. The etch rate of hard a-C:H in D₂ plasma is ~2 times the rate in H₂ plasma, and surfaces show a lower degree of modification (hydrogenation) than for H₂ plasma. In soft hydrocarbon materials exposed to D₂, H₂, and Ar plasmas (listed in order of modification, least to greatest), we find that the density increase is dependent on the ion chemistry, energy, penetration depth, and mass. The data will be compared with molecular dynamics simulation results.