435764 Thermodynamic-Aided Selection of Non-PFC Plasma Chemistries

Monday, November 9, 2015
Exhibit Hall 1 (Salt Palace Convention Center)
Nicholas Altieri1, Jack Kun-Chieh Chen1, Luke Minardi2 and Jane P. Chang2, (1)Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, CA, (2)Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, CA

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.

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