Paper PS-MoM5
Non-PFC Plasma Chemistries for Patterning Low-k Dielectric Materials

Monday, November 10, 2014, 9:40 am, Room 308

Low-k materials, such as fluorine-doped and carbon-doped silicon dioxide, exhibit reduced dielectric constants necessary to curtail parasitic capacitance and avoid crosstalk in devices, keeping pace with the trend of increasing device densities. SF₆ and perfluorocarbons (PFC) gases, which are primarily used in plasma etch of interlayer dielectric materials, generally have high global warming potentials (GWP), making their increased usage undesirable. This work focuses on evaluating etch chemistries from a thermodynamic standpoint for back end of line (BEOL) applications in patterning of proposed low-k carbon-doped silica compounds with varying carbon content. Group and bond additivity methods were used to estimate the Gibbs free energies of formation for these carbon-doped compounds. PFC and non-PFC etchants with H₂/NH₃ were assessed through the use of volatility diagrams comparing partial pressures of volatile etch products as a function of etchant partial pressure at 300K. Minimization of Gibbs free energy was employed to calculate the equilibrium distribution of species in the etch system across a range of temperatures. NF₃ and CF₃I were identified as potentially viable for etching carbon-doped silica. NF₃, a non-PFC gas with an atmospheric presence of 1ppt, GWP of 16,800, and much greater abatement efficacy, is the most effective etchant in pure form, producing volatile etch product pressures between six and eight times that produced by CF₄. CF₃I, despite being an iodofluorocarbon gas, exhibited a GWP of unity and displays reduced damage to doped carbon, making it preferable in etching carbon-doped silica. Pure CF₃I shows reduced product pressure with increasing carbon content; however, addition of H₂ and NH₃ improves its performance for the three most highly doped silica compounds. Given the higher cost associated with using NF₃ and CF₃I, the introduction of an additive (H₂ or NH₃) was assessed. Addition of H₂ and NH₃ generally showed an increase in partial pressures predicted by the volatility diagrams, with H₂ producing a much more significant increase than the pure etchant or the addition of NH₃. Preliminary experimental results comparing etch rates for moderately to highly carbon-doped silica samples with 20 sccm CF₄ and hydrogen addition generally agree with theorized predictions. Varying the feed composition between 0%, 20%, and 50% H₂, etch rates of 38.2, 44.8 and 48.86 nm/min were recorded for lightly doped silica, and 49.2, 73.0 and 128.2 nm/min were recorded for heavily carbon-doped silica.