Minliang Shih

Decomposition of SF6 in an RF Plasma Environment

Abstract Sulfur hexafluoride (SF6)-contained gas is a common pollutant emitted during the etching process used in the semiconductor industry. This study demonstrated the application of radio-frequency (RF) plasma in the decomposition of SF6. The decomposition fraction of SF6 [ηSF6 (Cin–Cout)/Cin x 100%] and the mole fraction profile of the products were investigated as functions of input power and feed O2/SF6 ratio in an SiO2 reactor. The species detected in both SF6/Ar and SF6/O2/Ar RF plasmas were SiF4, SO2, F2, SO2F2, SOF2, SOF4, S2F10, S2OF10, S2O2F10, and SF4. The results revealed that at 40 W, ηSF6 exceeded 99%, and the reaction products were almost all converted into stable compounds such as SiF4, SO2, and F2 with or without the addition of oxygen. Sulfur oxyfluorides such as SO2F2, SOF2, SOF4, S2OF10, and S2O2F10 were produced only below 40 W. The results of this work can be used to design a plasma/chemical system for online use in a series of a manufacturing process to treat SF6-containing exhaust gases.

Difference in Conversions Between Dimethyl Sulfide and Methanethiol in a Cold Plasma Environment

This study compared the conversion of two malodorous substances, dimethyl sulfide (CH3SCH3, DMS) and methanethiol (CH3SH) in a cold plasma reactor. The DMS and CH3SH were successfully destroyed at room temperature. DMS decomposed less than CH3SH at the same conditions. In oxygen-free condition, CS2 and hydrocarbons were the major products, while SO2 and COx were main compounds in oxygen-rich environments. The DMS/Ar plasma yielded more hydrocarbons and less CS2 than that of CH3SH/Ar plasma. In the CH3SH/O2/Ar plasma, rapid formation of SO and CO resulted in the yields much more amounts of SO2 and CO2 than those in the DMS/O2/Ar plasma; and remained only a trace of total hydrocarbons, CH2O, CH3OH, CS2, and OCS. The major differences between the reaction mechanisms of DMS and CH3SH were also proposed and discussed.

An integrated approach for identification of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) pollutant sources based on human blood contents

Background, aim, and scopeThis study developed an integrated approach to identify pollutant sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) of workers based on their blood contents.Materials and methodsWe first measured blood PCDD/F contents of sinter plant workers and residents living near the plant. By comparing those blood indicatory PCDD/Fs found for residents with those for sinter plant workers, exposure-related blood indicatory PCDD/Fs were identified for each selected worker. We then measured PCDD/F concentrations of four different sinter plant workplaces and three different ambient environments of the background. By comparing those airborne indicatory PCDD/Fs found for ambient environments with those for sinter plant workplaces, exposure-related airborne indicatory PCDD/Fs for each workplace were obtained. Finally, by matching exposure-related blood indicatory PCDD/Fs with exposure-related airborne indicatory PCDD/Fs, all suspected pollutant sources were identified for each selected worker.ResultsPoor Pearson correlations were found between workers’ blood contents and their corresponding PCDD/F exposures. Significant differences were found in the top three blood indicatory PCDD/Fs among the selected workers. By matching exposure-related blood indicatory PCDD/Fs with exposure-related airborne indicatory PCDD/Fs, two to three suspected pollutant sources were identified for each selected worker.DiscussionThe poor Pearson correlation found between workers’ airborne PCDD/Fs exposures and their blood contents was because workers’ blood PCDD/Fs contents were contributed not only by workers’ occupational exposures, but also by other exposure sources and exposure routes. The difference in blood indicatory PCDD/Fs among the selected workers were obviously due to the intrinsic differences in their time/activity patterns in the involved workplaces. While workers used a dust respirator to perform their jobs, gas phase exposure-related airborne indicatory PCDD/Fs played an important role on identifying suspected pollutant sources. But if a dust respirator was not used, the gas + particle phase exposure-related airborne indicatory PCDD/Fs would become the key factor for identifying suspected pollutant sources.ConclusionsThe developed integrated approach could identify all suspected pollutant sources effectively for selected workers based on their blood contents. The identified pollutant sources were theoretically plausible since they could be verified by examining workers’ time/activity patterns, their status in using dust respirators, and the concentrations of PCDD/Fs found in the selected workplace atmospheres.Recommendations and perspectivesThe developed technique can be used to identify possible pollutant sources not only for workers but also for many other exposure groups associated with various emission sources and exposure routes in the future.

Decomposition of ethylene oxide in the RF plasma environment.

A radio frequency (RF) plasma system was used to decompose the ethylene oxide (EO) contained gas in the EO/Ar, and EO/O2/Ar system, respectively. The reactants and final products were analyzed by using FTIR (Fourier transform infrared spectroscopy). The effects of plasma operational parameters, including input power wattage (W), total gas flow rate (Q), feeding concentration (C) of EO and operational pressure for EO decomposition were evaluated. Due to the importance of the high-energy electrons in the RF plasma system, the EO decomposition fraction in plasma reaction increased with decreasing operational pressure, while that of thermal reaction, reported by previous investigations, increased with increasing operational pressure. However, owing to the electrophilic characteristic of oxygen atoms in the EO molecule causing the effect of electron attachment, in conditions of higher EO feeding concentration, the pressure dependence became the same for both plasma- and thermal- reaction. The EO oxidation reaction has also been investigated, the result shows that EO almost completely oxidized at 600–692 K gas temperature. The main products for the EO/Ar system are CO, CH4, C2H6, C2H4, and C2H2, and those for the EO/O2/Ar system are CO2 and H2O.

Reaction mechanism of 1,2-dichloroethane/O2/Ar in the cold plasma environment

Dichloroethane (EDC) is known to be hazardous to the environment and public health. In this study, application of radio frequency (rf) plasma as an alternative technology for the decomposition of EDC was demonstrated. The species detected in the effluent gas stream included CO2, CO, HCl, CCl4 ,C 2HCl3 ,C 2H3Cl, C2Cl4, CHCl3 ,C 2HCl5, COCl2 ,C 2H2 ,C 2H4 ,C 2H6, and HCOOH. The decomposition fraction of EDC (ηC2H4Cl2 ,( Cin − Cout)/Cin × 100 (%)) was dependent on input power. When input power was 80 W, stable products such as CO, CO2, and HCl were dominant and other product species were inhibited. Equivalence ratio (ϕ = (C2H4Cl2/O2)actual × (O2/C2H4Cl2)stoichiometric) was the other important operational parameter in a plasma system. When the chlorocarbon/oxygen flow was fuel rich, more soot formation was found in the plasma reactor. When it was fuel lean, CO2 and CO dominated over other product species. Within the mixture of EDC and dichloromethane (DCM), the competition of DCM with EDC could affect the decomposition fraction of EDC.