PLASMA-SURFACE INTERACTIONS DURING REACTIVE PLASMA PROCESSING OF HYDROCARBON FILMS
Oehrlein, Gottlieb S
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Reactive plasma interactions with hydrocarbon-based surfaces play a critical role in future biological-plasma applications and for microelectronic device manufacture. As device dimensions get smaller and we require fine control of surfaces during plasma processing we will need to develop more understanding of fundamental plasma surface interactions. Through the use of plasma-deposited amorphous carbon films interacting inert/reactive plasmas (Ar/H<sub>2</sub> plasmas) we explored etch rates and the formation of modified layers. Facing Ar and H<sub>2</sub> plasmas mixtures, hydrocarbon surfaces can exhibit widely different properties, depending on plasma composition (ions, reactive species, fast neutrals) and initial film composition (graphitic, polymeric). Ar plasmas cause densification of hydrocarbon surface by selectively sputtering H atoms, while H<sub>2</sub> plasmas cause incorporation/saturation of H atoms within the film surface. For hard amorphous carbon, we find that small amounts of H<sub>2</sub> added to Ar plasma can completely negate ion-induced densification. Plasmas are also drastically changed by small impurities of H<sub>2</sub> atoms. We investigated the plasma property effects of adding H<sub>2</sub>, D<sub>2</sub>, CH<sub>4</sub>, and surface derived hydrocarbon gases. We find that small amounts (as low as 1%) of H<sub>2</sub>/D<sub>2</sub> in Ar cause a large decrease in electron density, increase in electron temperature, Ar metastable atoms, and radically different ion mass distributions. These effects are intensified at higher pressures, as neutral molecule-ion interactions in the plasma increase. These changes can be related to the surface modification caused by the plasma. Surface derived impurities into inert plasmas were also investigated. Hydrocarbon flow from the surface causes changes to plasma properties similar to the addition of CH<sub>4</sub> gas. We applied the learning from these fundamental plasma-surface interaction studies to an applied problem of plasma-assisted shrink of asymmetric photoresist features. Using fluorocarbon-based plasmas, we successfully shrink asymmetric pattern features and find that lower concentrations of C<sub>4</sub>F<sub>8</sub> in plasmas and shorter deposition thicknesses lead to more uniform shrink in L and W dimensions. To improve future plasma-assisted shrink processes, careful tuning of plasma composition and feature dimensions is critical.