Membrane facilitated separation of NF3 and CF4
Branken, David Jacobus
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Nitrogen trifluoride (NF3) is frequently used as a source of fluorine in the electronics device manufacturing industry as a dry etchant during plasma assisted etching of silicon wafers, or during the plasma cleaning of chemical vapor deposition chambers. As a result of the electrochemical synthesis procedures in which carbon anodes are used in a fluorine-rich environment, NF3 product streams are frequently contaminated with ppm-amounts of carbon tetrafluoride (CF4). The electronics manufacturing industry, however, requires NF3 of exceptional purity, i.e. so-called VLSI-grade (very large scale integration) NF3, with CF4 concentrations of 20 ppm and below. Due to the close chemical and physical similarities of the two compounds, the removal of CF4 from NF3 has proven to be rather difficult, and current NF3 purification technologies are relatively inefficient. Although membrane gas separation has proven to be competitive in terms of operating costs and energy efficiency, its use for the purification of NF3 seems to have remained unexplored to date. In this study, the use of high free volume glassy perfluoropolymers of Teflon AF2400, Teflon AF1600, and Hyflon AD60 was therefore investigated. To be able to measure the pure and mixed gas permeabilities and selectivities of the solution-cast membranes towards NF3 and CF4, a custom built experimental setup was used, in which a newly developed gas chromatographic (GC) analysis method was implemented. Using divinylbenzene-styrene co-polymer stationary phases in the form of Super Q, a reliable quantification of mixtures of NF3 and CF4 were achieved without requiring additional fluorocarbon liquid stationary phases, as is commonly used in NF3 production environments. Furthermore, by implementing a dual-channel configuration it was possible to quantify a wide range of NF3 and CF4 concentrations. Using the newly developed technique, NF3 and CF4 concentrations of ca. 1 mol% and upwards could be quantified using a Thermal Conductivity Detector (TCD) on one channel, and NF3 and CF4 concentrations of between ca. 40 vppm and 4000 vppm could be measured using a Pulsed Discharge Helium Ionisation Detector (PDHID) on the second channel of the GC method. The glassy perfluoropolymer membranes of Teflon AF2400, Teflon AF1600, and Hyflon AD60 were prepared by a solution casting method, and it was found that annealing at sufficiently high temperatures (170 – 200 °C) ensured optimum permeability selectivity. In contrast, thermal analysis of the solution-cast Hyflon AD60 membranes that were heated to only 95 °C confirmed that the polymer matrix was significantly swollen due to a residual amount of the casting solvent. Consequently, considerably reduced selectivity and increased permeability of both NF3 and CF4 were observed for such solvent-swollen Hyflon AD60 membranes in comparison with the non-swollen membranes that were annealed at 170 °C. Nonetheless, the measured He/N2 permeability and permeability selectivity of all the membranes studied compared favourably with literature values, and selectively permeated NF3 rather than CF4 wherein the pure and mixed gas permeability selectivity displayed a clear dependence on the fractional free volume (FFV) of the polymer matrices. Thus, in accordance with the decreasing FFV of the perfluoropolymers in the order Teflon AF2400 > Teflon AF1600 > Hyflon AD60, the NF3 permeability decreased from 227 Barrer for Teflon AF2400, to 29 Barrer for Teflon AF1600, to 1.9 Barrer for Hyflon AD60. In contrast, the NF3/CF4 selectivity, α(NF3/CF4), increased inversely from 4.5 for Teflon AF2400, to 6.0 for Teflon AF1600, to the highest selectivity of 12 which was obtained using Hyflon AD60. To elucidate the mechanism of separation, the transport properties of NF3 and CF4 in Teflon AF2400 and Teflon AF1600 w.r.t. diffusion and solubility were studied using Molecular Dynamics (MD), Grand Canonical Monte Carlo (GCMC), and statistical thermodynamic techniques. The results indicated that NF3/CF4 diffusion selectivity (DNF3/DCF4) was favoured by the lower free volume of Teflon AF1600, whereas poor correlation was achieved between the GCMC calculated sorption isotherms of CF4 and the experimentally determined isotherms as reported in the literature. Consequently, the non-equilibrium lattice fluid (NELF) model, which more accurately described the sorption isotherms of CF4, was used to evaluate the solubility selectivity. It was found that by adjusting the NELF model interaction parameter, Ψ, favourable NF3/CF4 solubility selectivities (SNF3/SCF4) were predicted. Furthermore, by combining the solubility selectivity values with the diffusion selectivities calculated from the MD results, permeability selectivity predictions that correlated well with the experimentally determined values were obtained. Based on a semi-quantitative technological evaluation, it was concluded that although good NF3/CF4 mixed gas permeability selectivity was obtained with Hyflon AD60, further research into improving the NF3 solubility, and hence permeability will aid in the development of an efficient membrane gas separation process for the purification of NF3.