PLASMA-SURFACE INTERACTIONS DURING REACTIVE PLASMA PROCESSING OF HYDROCARBON FILMS

Abstract

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/H₂ plasmas) we explored etch rates and the formation of modified layers. Facing Ar and H₂ 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 H₂ plasmas cause incorporation/saturation of H atoms within the film surface. For hard amorphous carbon, we find that small amounts of H₂ added to Ar plasma can completely negate ion-induced densification. Plasmas are also drastically changed by small impurities of H₂ atoms. We investigated the plasma property effects of adding H₂, D₂, CH₄, and surface derived hydrocarbon gases. We find that small amounts (as low as 1%) of H₂/D₂ 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 CH₄ 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 C₄F₈ 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.