Etching of Dielectrics: Fundamental Plasma-Surface Interactions Through Mass-Filtered, Energy-Tuned Ion Beams

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SummaryThis research seeks to understand the fundamental plasma-surface interactions responsible for the etching of silicon, silicon dioxide (silica), and other novel dielectric materials (zirconia, porous silicon oxide, black diamond) in fluorocarbon plasmas or other appropriate chemistries. To this end, plasma-extracted, mass-filtered ion beams of adjustable energy are directed at surfaces of the aforementioned dielectric materials using a unique ion beamline combined with scattering and surface diagnostics. Neutral radicals of the same or different chemical composition are also supplied simultaneously at varying neutral to ion ratios. The emitted reaction products are monitored by time-of-flight quadrupole mass spectrometry in situ and in real time. The stable surface products of the interaction are analyzed using surface analysis techniques. Comparisons are made with scattering at silicon surfaces to understand selective etching of dielectric materials over silicon. Specific goals include the understanding of the scattering dynamics of energetic, inert, and reactive ions on surfaces of relevance to the semiconductor industry; the reasons for etch selectivity between silicon and silica in fluorocarbon plasmas; distinguishing non-thermal reaction pathways and measuring their contribution to etching relative to that of thermal reactions; monitoring the extent and chemical nature of the surface modification facilitated by the ion bombardment as a function of the translational energy and chemical identity of the ions; measurement of etch yields as functions of translational energy and incident angle of the projectile ions and developing phenomenological models of the observed dependencies; using the etch-yield models for beam scattering to simulate profile evolution; and performing etch experiments in the beam path for comparison and validation purposes.Controlled ion-beam surface experiments are performed that target the fundamental interactions occurring when etching silicon, silicon dioxide, and novel dielectric materials in high-density plasmas. A detailed picture of the scattering interaction is produced, including etch yields and reaction products as functions of incident energy and angle. Complex ions [CFx+ and SiFx+ (x=1-3)] typical of the complex fluorocarbon chemistries employed in the etching of oxides and low-k dielectrics are employed. A rapid screening of the role of a number of ions in etching various materials is performed. This establishes a new paradigm to select the plasma chemistry to achieve a desired etch rate and etch profile in contrast to the time-consuming and costly recipe development by trial-and-error now practiced by industry. The experiments are used to validate results from molecular dynamics simulations for improved understanding and to produce phenomenological models of scattering needed by the industry for truly predictive profile evolution.Broader impactThe results produced are fundamental enough to be used by theorists in the validation of simulations of beam-surface interactions and practical enough to be useful to process engineers in the selection of chemistries and operating conditions that permit rapid optimization of etch tools. A new paradigm in etch process development is established through a combination of fundamental beam-scattering experiments and etch-profile-evolution simulations. The knowledge and understanding obtained is incorporated in courses and tutorials on plasma-surface interactions to educate students and engineers on the underlying principles of chemical reaction dynamics. A new, low-cost experiment involving atmospheric microplasmas in direct patterning of silicon is developed to introduce plasma-surface interactions to undergraduates.