389736 Non-PFC Plasma Chemistries for Patterning Low-k Materials
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, 45 and 49 nm/min were recorded for lightly doped silica, and 49, 73 and 128 nm/min were measured for heavily carbon-doped silica.
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