(254d) Thermodynamic-Aided Selection of Non-PFC Plasma Chemistries

Continued reduction in the size of microelectronics and nanoscale features has necessitated the use of low-k dielectric interlayer materials in an effort to curtail parasitic capacitance and RC delay. Patterning these low-k films requires consideration of both etching efficacy and environmental impact. To address these issues, a generalized methodology is developed based on a thermodynamic approach to analyze etchants and additive gases to assist in selection of plasma chemistries whose environmental effects can be more easily mitigated.

Thermodynamics is an enabling tool for assessing a reacting system, such as plasma etching of carbon doped porous silica, specifically through analysis of Gibbs free energy. A system at equilibrium has reached a minimal Gibbs free energy which can be expressed as the sum of its constituents and their corresponding chemical potentials. With known reactants, potential products, and free energies of formation as inputs, the total Gibbs energy is minimized to calculate an output quantity of each species. This calculation was then repeated across a range of temperatures at fixed pressure. Using CF₄ etching of silica as the reference and monitoring the formation of volatile etch product SiF₄(g) via volatility diagrams, a range of carbon doped porous silica, a list of viable etchants including perfluorocarbon gases, NF₃(g), CF₃I(g), as well as additive gases such as H₂(g) and NH₃(g) are examined.  Based on thermodynamic calculations, NF₃(g), a non-PFC gas with high abatement efficiency was predicted to generate the highest pressure of SiF₄(g) overall.  CF₃I(g), though calculated to be not as effective as NF₃(g), is another alternative due to its short atmospheric lifetime and low global warming potential. On the other hand, H₂(g) was found to be the most effective additive with fluorocarbon etchants.

CF₄(g) and CHF₃(g) were studied separately with varying hydrogen addition to validate the thermodynamic calculations. Optical emission spectroscopy was used in parallel to monitor atomic fluorine intensities at 685.6 and 703.7 nm as a function of H₂(g) feed percentage. Discharges of CF₄(g) mixed with 20% H₂(g) and CHF₃ with 10% H₂(g) resulted in maximal etch rates of 215 nm/min and 166 nm/min respectively. A trend similar to etch rate dependence on feed composition was seen in the spectra of atomic fluorine, with maximal intensities recorded for CF₄ and CHF₃ at 20% and 10% H₂ respectively.