- Email: [email protected]
Studies on atmospheric plasma abatement of PFCs
ARTICLE IN PRESS
Radiation Physics and Chemistry 69 (2004) 113–120
Studies on atmospheric plasma abatement of PFCs Marilena T. Radoiu* BOC Edwards, Exhaust Gas Management, Kenn Business Park, Kenn Road, Clevedon, North Somerset BS21 6TH Clevedon, UK Received 17 March 2003; accepted 24 June 2003
Abstract Microwave plasma at 2.45 GHz frequency operating at atmospheric pressure in synthetic gas mixtures containing N2, CF4, C2F6, CHF3, and SF6 were investigated experimentally for various gas mixture constituents and operating conditions, with respect to their ability to destroy perfluorocompounds. It was found that the destruction and removal efficiency (DRE) of the process is highly dependent on the total gas flow. DREs of up to 99.9% have been achieved using 1.8 kW of microwave power at 20 l/min total flow rate. r 2003 Elsevier Ltd. All rights reserved. Keywords: Atmospheric plasmas; PFCs; Destruction and removal efficiencies; Microwaves
1. Introduction Gases in the atmosphere can contribute to the greenhouse effect both directly and indirectly. Direct effects occur when the gas itself is a greenhouse gas like water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). Indirect radiative forcing occurs when chemical transformations of the original gas produce other greenhouse gases, when a gas influences the atmospheric lifetimes of other gases, and/ or when a gas affects atmospheric processes that alter the radiative balance of the earth. Very powerful greenhouse gases that are not naturally occurring include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), which are generated in a variety of industrial processes. Each greenhouse gas differs in its ability to absorb heat in the atmosphere. In order to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas, the concept of a global warming potential (GWP) has been developed. Carbon dioxide (CO2) was chosen as the reference gas. Methane traps over 23 times more heat per molecule than carbon *Tel.: +44-1275-3371-73; fax: +44-1275-337-200. E-mail address: [email protected] (M.T. Radoiu).
dioxide, and CF4 absorbs 5700 times more heat per molecule than carbon dioxide. GWP values based upon a 100-year horizon and atmospheric lifetime for selected greenhouse gases are listed in Table 1. PFCs are considered the most heat-absorbent. The largest PFC emission source is primary aluminum manufacture, which generates mainly CF4 and some C2F6 as unintentional by-products. However, the semiconductor industry is the largest user and therefore emitter of intentionally produced PFCs (Houghton et al., 2001). In order to reduce the overall PFC emission to the lowest economically feasible limit, over the course of the past several years numerous studies of multiple PFC emission control technologies have been reported globally. There are three main routes for PFC emissions reduction: decomposition (combustion, plasma and thermal-chemical) of the PFCs to non-hazardous materials, recycle and recovery of the unused PFCs (Cummins et al., 1997; Yabuhara, 1997) and process optimization and/or replacement of the PFCs with other gases (Allgood, 1998; Chatterjee et al., 2001; Karecki et al., 2001; Saito et al., 2001). At the moment, the only field-proven technique for treating PFC-containing exhaust streams is thermal, either by direct thermal oxidation or catalytic oxidation. In thermal treatment the object is to transfer heat to the
0969-806X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00455-9
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
114
Table 1 Global warming potentials (100 Years Horizon) and atmospheric lifetime (years) of selected greenhouse gases Global warming gas
Global warming potential
Atmospheric lifetime
CO2 CH4 N2O CF4 C3F8 C2F6 CHF3 SF6
1 23 296 5700 8600 11,900 12,000 22,000
5–200 12 120 50,000 2600 10,000 260 3200
exhaust stream with either electrical resistance losses or burning fuel providing the original source of heat. This method is indirect and can be somewhat fuel or electricity intensive. Also, the system must be operating nearly continuously—not just when PFCs are flowing. Of the established abatement methods—combustion, plasma, reactive adsorption, and catalytic decomposition—plasma (Vitale and Sawin, 2000; Xu et al., 2000; Tonnis et al., 2000; McAdams, 2001; Chang and Yu, 2001; Chen et al., 2000) and catalytic (Ichikawa et al., 2000; Brown et al., 2001) studies have continued to dominate published reports. The use of atmospheric plasma to treat industrial exhaust is a relatively new area of work and it is intended to improve upon some of the shortcomings of thermal systems. Non-thermal atmospheric plasma (or more correctly non-equilibrium plasma) chemistry provides an extremely attractive solution compared to classical combustion, electrothermal and thermocatalytic systems for efficient chemical conversion of waste PFCs to reactive fluorinated products that can be subsequently scrubbed by proven, classical means. The original interest in highpressure plasmas is related to the following features: (a) bulk gas temperature is much higher than attainable by combustion; (b) can be operated in a controlled atmosphere, and (c) a high throughput rate of materials in a variety of forms is possible. Also, the plasma system is electrically driven and can be operated in a batch mode. The system does not need to be operating if not required. The response to the application of power is immediate with no warm-up period needed (compared to catalytic systems). The level of destruction can be readily adjusted by simply varying the power input to the system and so can adjust to varying inlet concentrations. The electrons in non-thermal plasmas can reach temperatures of 10,000–100,000 K (1–10 eV) while the gas temperature can remain as low as the ambient temperature. It is the high electron temperature that determines the chemistry of non-thermal plasmas. In non-thermal atmospheric plasmas most of the electrical energy is consumed to produce free radicals, which have
a much greater reactivity than atoms and molecules in the ground state. According to Penetrante (1993), non-thermal plasmas at atmospheric pressure can be sustained through various methods. Examples are pulsed corona discharges summarized by Chatterjee et al. (2001) and McAdams (2001), dielectric barrier or silent discharges described by Hackman and Akiyama (2000), Urashim et al. (2001) and Veldhuizen (2000), and electron beam induced plasmas (Penetrante et al., 1993; Rea, 1995; Radoiu et al., 2003). In pulsed corona discharges and barrier discharges, the discharge extinguishes before ions and neutral gas components can heat up. In electron beam techniques, the gas to be converted is irradiated with high-energy electrons from an electron beam accelerator and reactions can be initiated by electron impact dissociation, excitation and ionization at low gas temperature. These types of non-thermal plasmas are used to abate low waste gas concentrations (o1000 ppm) in large gas volume flows. Plasma techniques are proposed to clean flue gases of coal-fired electrical power plants, to remove NOX from the exhaust of Otto or Diesel engines, or to improve the emission values occurring during painting and surface cleaning. In this case, the chemically active species, which are mainly produced in the carrier gas, react with the waste gas molecules. The conversion degree rapidly decreases with increasing the waste concentration. Microwave excited plasmas belong to a fundamentally different category. Non-equilibrium results from the fact that at a sufficiently high frequency (usually 2.45 GHz), only light electrons can follow the oscillations of the electric field of the applied electromagnetic field. At atmospheric pressure, microwave plasmas exhibit homogeneous electron densities of 1012–1015 cm3 and therefore appear much more attractive for the abatement of toxic gases. In addition, a large number of inelastic electron-neutral collisions result in very efficient dissociation at relatively high concentrations of the gas to be abated. Such inelastic collisions proceed continuously preventing the reformation of the initial molecules. Microwave plasmas can be excited inside resonant cavities, within waveguide microwave circuits or by means of surface wave field applicators (Moisan et al., 1992; LePrince and Marec, 1992). This paper describes the development and application of a non-thermal plasma system sustained by 2.45 GHz frequency microwaves (MW) and operated at atmospheric pressure that can effectively remove PFCs, HFCs and SF6 from gas streams. The technology has been tested on gas flows containing C2F6, CHF3, SF6 and particularly CF4 to illustrate its effectiveness. As it will be presented in the following paper, successful abatement is dependent on the total gas flow, the total power level and the concentration of CF4.
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
2. Experimental method 2.1. Experimental system A schematic diagram of the experimental system is shown in Fig. 1. Carbon tetrafluoride (CF4,>99.7%), trifluoromethane (CHF3,>99.9%), hexafluoroethane (C2F6,>99.7%), and sulphurhexafluoride (SF6,>99.9%) were obtained from BOC Edwards. Individual mass flow controllers were used to achieve the desired concentration of CF4, C2F6, CHF3 and SF6 in nitrogen used as carrier gas. The flow was passed through a mixing reactor where water was added in stoichiometric amount or slight excess calculated as follows: CH2 O ¼ 3CiC2 F6 þ 2CiCF4 þ CiCHF3 þ 3CiSF6 ;
ð1Þ
where CiC2 F6 ; CiCF4 ; CiCHF3 and CiSF6 are the initial molar concentration of C2F6, CF4, CHF3 and SF6, respectively. The conditioned gas was then introduced into the plasma reactor, neutralized in a wet scrubber, and evacuated into the acid exhaust. 2.2. Analysis The analyses of the gas mixtures before and after reaction were monitored on-line by mass spectrometry.
115
The quadrupole mass spectrometer (QMS) used was a 300 amu Fisons Sensorlab system. Mass spectra spanning the mass range 30–200 were recorded. Gas was extracted from the sample port through heated sample lines and a chemically inert diaphragm pump. The typical fragment ions observed above m=e=20 amu and their relative abundances during all of the experiments are reproduced in Table 2. Measurements were taken using the fragment ions in bold characters in Table 2; the destruction and removal efficiency (DRE) was calculated: DRE ð%Þ ¼
Ci Cf 100; Ci
ð2Þ
where Ci is the initial concentration of the recorded species and Cf is its final concentration after the plasma abatement, estimated from the relative ion intensities. 2.3. Microwave system The microwave system consists of a 2.45 GHz frequency microwave generator GMP 20 K/SM (SAIREM, France) with variable power output from 0 to 1950 W, a 20 dB isolator (circulator+water load) for magnetron protection against the reflected power, a three-stub tuner and a sliding short circuit for impedance matching, and a microwave resonant cavity (plasma reactor)—Fig. 2.
Added water vapour N2 + test gas (CF4, CHF3, C2F6, SF6)
Microwave & Plasma Reactor
Mass spectrometer Wet scrubber
Exhaust
Fig. 1. Schematic diagram of the experimental system.
Table 2 Mass spectra of the studied gas species and ions percent abundance (%) in the mass spectra, on-line at webbook.nist.gov/chemistry C2F6
SF6 a
m/e
Ion
127 108 89 70 51
SF+ 5 SF+ 4 SF+ 3 SF+ 2 +
a
SF
% 100 9.1 26.1 5.6 7.5
Mass to charge (amu).
a
CHF3
m/e
Ion
119 69 50 31
C2F+ 5 CF+ 3 CF+ 2 + CF
% 42.2 100 10.8 18.7
a
CF4
m/e
Ion
69 51 50 31
CF+ 3 CHF+ 2 CF+ 2 CF+
% 100 90.7 14.6 49.6
m/ea
Ion
%
69 50 31
CF+ 3 CF+ 2 +
100 10 5.2
CF
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
116
N2 + test gas (CF4, CHF3, C2F6, SF6)
Crystal detector
Electrodes 2.45 GHz Microwave Generator
Isolator
Sliding short circuit
Plasma reactor To wet scrubber
Tuner
Fig. 2. Schematic of the microwave/plasma system. Table 3 Dependence of C2F6 destruction and CF4 formation on the total N2 flow rate and microwave power level N2 (l/min)
1800 W DRE C2F6 (%)
CF4 formed (ppm)
1500 W DRE C2F6 (%)
CF4 formed (ppm)
1200 W DRE C2F6 (%)
CF4 formed (ppm)
25 30 40
100 100 100
1600 3200 3500
100 100 92
2800 4100 3675
95 78 76
3900 4700 4050
Reaction conditions: C2F6=300 cm3 min1; molar ratio H2O:C2F6=3.5.
The isolator is provided with a crystal detector for reflected power measurements. Accentus’ patented microwave plasma reactor consists mainly on a resonant microwave cavity which has been described in detail elsewhere (Bayliss, 1995). It is basically a stainless steel enclosure (V ¼ 220 cm3) within which there is a pair of opposed field-enhancing electrodes. The cavity is provided with an efficient water-cooling circuit.
3. Results and discussion Microwave induced atmospheric plasma destruction of C2F6, CHF3, CF4 and SF6 in the presence of excess water (source of hydrogen and oxygen) was tested over a range of flow rates, power levels and PFC concentrations. The dilution gas was nitrogen. It is noteworthy that the perfluorocarbon compounds often produce CF4 as an ultimate by-product. In many respects the abatement of CF4 provides the highest challenge due to its stability [D(CF3-F)B130 kcal/mol] (Lide, 1992) its large infrared absorption cross-section and corresponding large GWP (5700) which makes it particularly difficult to destroy effectively. As CF4 is the most stable of all PFCs, its destruction was intensively investigated, assuming that if it could be destroyed, all the other fluorocompounds should also decompose under the same conditions. According to reactions (3)–(6) shown below, the ideal chemistry for the PFCs abatement would involve the formation of only CO2 and HF, stable final by-products that could be effectively converted to non-hazardous
salts in a caustic water scrubber. Note that the product CO2 is a GWP compound itself, but its GWP is significantly less than either of the PFCs involved. C2 F6 þ 3H2 O-CO2 þCO þ 6HF;
ð3Þ
CHF3 þ H2 O-CO þ 3HF;
ð4Þ
CF4 þ 2H2 O-CO2 þ 4HF;
ð5Þ
SF6 þ 3H2 O-SO2 þ 6HF þ 12O2 :
ð6Þ
Our experiments were carefully optimized for maximum DREs of the tested gases. To obtain this, excess water is required. The maximum molar ratio of water to PFCs (SF6) used was 3.5 to 1. 3.1. Destruction of C2F6 The destruction of C2F6 was first tested over a range of power levels using total nitrogen flow of 25, 30 and 40 l/min in the presence of water vapour as reactant. C2F6 is readily decomposed in the plasma reactor. Mixtures of water and C2F6 at 3.5 M ratio (H2O:C2F6) were fed into the reactor, and the decomposition characterized as a function of flow rate and microwave power level. The results of C2F6 decomposition versus input power and flow rate are given in Table 3. For a given amount of input power, the extent of C2F6 decomposition increases with decreasing flow rate. This implies that the extent of C2F6 decomposition is determined by the residence time of the reactant in the plasma.
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
In order to identify the decomposition products of the treated gas, a 1 l/min sample from the reactor exhaust was passed through a water scrubber and to the QMS afterwards. Reaction products such as F2 and COF2 will dissolve in the water and form HF. Indeed, pH measurements of the scrubbing water indicated acidic pH after a short run time. Therefore, the only products that pass on to the QMS are non-water soluble species such as C2F6 and other perfluorocarbons. The only other product identified by QMS and confirmed by GC analyses was CF4. At 1.8 kW of microwave power, C2F6 is easily removed even at high flow rates; its destruction efficiency decreases with the power level and total gas flow (residence time in the plasma zone). CF4 production is a linear function of the microwave forward power and total flow rate; the recombination of ions leading to the formation of CF4 decreases with the increase in microwave energy and the gas residence within the plasma—Fig. 3. According to Mellor (1956) and Lide (1992), in the absence of water (or any other source of hydrogen), the equilibrium of reaction (3) shifts entirely to the most stable perfluorocarbon compound, CF4. The global reaction can be written as C2 F6 þ O2 -34CF4 þ CO2 :
ð7Þ
With a higher than stoichiometric water content in the system (H2O/C2F6>4/1), the equilibrium concentrations of CF4 and C2F6 are virtually zero (Fabian, 2002). However, in the experimental conditions described
117
above, the increase of water molar ratio above 3.5 (H2O/C2F6>3.5/1) was found to affect the plasma stability. Therefore, the addition of oxygen to the gas stream was attempted. According to Eq. (8) all the fluorine of C2F6 is transformed to HF and all the carbon to CO2: C2 F6 þ 12O2 þ 3H2 O-2CO2 þ 6HF:
ð8Þ
Indeed, it was found that the addition of O2 (C2F6/ O2=2/1) improved DRE of all species present in the gas mixture—Table 4 and Fig. 3. 3.2. Destruction of CF4 The removal of CF4 was tested over the usual range of gas flows and power levels in the presence of water. It can be seen from Fig. 4 that the destruction achieved was highly dependent on the power level and total flow rate. The formation of radical recombination products, i.e. C2F6, C2F4, was not observed in any of the tests. The decomposition of carbontetrafluoride in nitrogen and water was found to increase as a function of the microwave power level and residence time (total flow rate)—Fig. 4. The highest rate of destruction was observed as 95% at a microwave power level of 1800 W for B1.2% (vol.) CF4 in nitrogen. The linearity of the plots in Fig. 4 indicates that the decomposition rate of CF4 follows a first-order dependence on energy, although clearly the rate depends on the starting concentration of CF4. 100 80
4000
DRE (%)
CF4 (ppm)
5000
25 l/min
3000
30 l/min 2000 1000 1200
60 25 l/min 40
30 l/min + O2 40 l/min
20 1200
1400 1600 1800 Microwave power (W)
Fig. 3. CF4 formation vs. microwave forward power, nitrogen flow and oxygen addition.
30 l/min 40 l/min 1400 1600 Microwave power (W)
1800
Fig. 4. DRE of CF4 in the presence of water. Reaction conditions: CF4=300 cm3/min; molar ratio H2O:CF4=3.5.
Table 4 Influence of O2 addition and microwave power level on the DRE of C2F6 and formation of CF4 O2 (cm3 min1)
1800 W DRE C2F6 (%)
CF4 (ppm)
1500 W DRE C2F6 (%)
CF4 (ppm)
1200 W DRE C2F6 (%)
CF4 (ppm)
0 150
100 100
3200 1000
100 100
4100 2200
78 98
4700 3100
Reaction conditions: N2=30 l/min; C2F6=300 cm3 min1; molar ratio H2O:C2F6=3.5.
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
118
3.3. Destruction of CHF3 The destruction of CHF3 was tested over the same range of gas flow rates and power levels as C2F6. It was again observed that decreasing N2 flow and increasing power level favored the destruction level. As expected, the abatement of this gas was very easy and takes place with very good efficiency even at high flow rates, Table 5. Very small quantities (o10 ppm) of CF4 were identified, especially at low microwave power levels and high gas speed through the plasma reactor. 3.4. Destruction of mixtures of CF4, C2F6, and CHF3 A set of experiments was performed to evaluate the efficiency of the microwave atmospheric plasma technique when PFCs were added together in the gaseous mixture.
Table 5 Abatement of CHF3 DRE dependence on the microwave power level and N2 flow rate N2 (l/min)
1800 W (%)
1500 W (%)
1200 W (%)
25 30 40
100 100 99
100 100 97
97 95 75
3.4.1. Influence of the PFCs concentration and power level on DRE The abatement of a mixture of PFCs was tested over a range of PFCs concentrations and power levels; the gas mixture was diluted with 20 l/min N2. The molar ratio H2O:PFCs was 1.5—Table 6. It can be easily observed that for the abatement of C2F6 and CHF3, DRE was less dependent on their concentrations. The abatement of CF4 was heavily influenced by concentration especially at lower power levels. 3.4.2. Total flow rate and power level influence on DRE Table 7 summarizes the removal efficiencies of CF4, C2F6 and CHF3 at different dilution levels in nitrogen and in the presence of water as reagent. It can be observed that the most reactive gases were CHF3 and C2F6. 3.5. Destruction of SF6 The destruction of SF6 in the presence of water was tested over the same range of gas flows and power levels as PFCs. It was again observed that destruction was favored by decreasing gas flows and increasing power levels—Table 8. 3.6. By-products
Reaction conditions: CHF3=320 cm3 min1; molar ratio H2O:CHF3=2.5.
Many chemical reactions can occur within an ionized gas involving atomic, radical, and complex molecular species, as well as electrons and ions.
Table 6 Dependence of DRE on the total PFCs concentration and microwave power level PFC (cm3/min)
1800 W
a
175 250b 380c
1500 W
1200 W
CHF3
CF4
C2F6
CHF3
CF4
C2F6
CHF3
CF4
C2F6
100 100 100
97 95 91
100 100 100
100 100 100
95 90 82
100 100 100
100 100 100
92 79 72
100 100 100
a
175 cm3/min PFCs consisting of 55 cm3/min CF4, 65 cm3/min CHF3, and 55 cm3/min C2F6. 250 cm3/min PFCs consisting of 115 cm3/min CF4, 95 cm3/min CHF3, and 45 cm3/min C2F6. c 380 cm3/min PFCs consisting of 170 cm3/min CF4, 80 cm3/min CHF3, and 130 cm3/min C2F6. b
Table 7 DRE (%) of PFC/N2/H2O vs. N2 flow rate and microwave power level N2 l/min
20 25 30 40
1800 W
1500 W
1200 W
CHF3
CF4
C2F6
CHF3
CF4
C2F6
CHF3
CF4
C2F6
100 100 100 100
95 92 86 72
100 100 100 100
100 100 100 100
90 81 75 52
100 100 100 75
100 100 99 90
79 72 59 40
100 100 84 53
Reaction conditions: 250 cm3/min PFCs consisting of 115 cm3/min CF4, 95 cm3/min CHF3, and 45 cm3/min C2F6; molar ratio H2O:PFCs=1.5.
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120 Table 8 Dependence of SF6 abatement on the microwave power level and total flow rate N2 (l/min)
1800 W
1500 W
1200 W
25 30 40
100 100 82
100 90 70
83 77 53
Reaction conditions: SF6 300 cm3/min; molar ratio H2O: SF6=3.5.
In the case of a gaseous mixture of CXFY and N2, the reactions that tend to dominate the behavior of the ionized gas at lower gas temperature with higher electron temperature are: ionization, attachment and dissociation by electron-impact where all the reaction constants depend on the electron temperature. However, electron impact processes are less likely to occur than N2 processes since the concentration of CXFY is much lower than for N2. The primary electron-impact processes will then be dissociation and ionization of nitrogen gas. Therefore, when oxygen is added into the N2-CXFY gas mixture possible stable by-products are CO, CO2, COF2, OF2, NO, NO2, N2O, FO, FO2, FNO, FNO2, and FONO2. The addition of water vapour to the CXFY-N2 system significantly changes the final by-products. New, stable by-products (CO2, HF) are formed in this case. Indeed, the by-products of the plasma decomposition of CF4, C2F6 and CHF3 in the presence of water vapour and N2 were identified as HF, CO2 and small quantities of NOX (>1000–2000 ppm). Oxygenated fluorocompounds as O2F2 and OF2 were only identified when oxygen was added to the system. For SF6 destruction in the presence of water, the by-products were SO3, SO2, and HF. It is noteworthy that the examination of the plasma cavity showed no solid deposits such as solid carbon soot, etc. This observation is very important in terms of cavity maintenance and also, in preventing the formation of heavier fluorocarbons.
4. Conclusions The application of microwave discharges is potentially a perspective technology for perfluorocarbons destruction. Original laboratory equipment consisting on a 2.45 GHz frequency microwave generator with variable power up to 1950 W and a plasma reactor has been developed for this research. The resonant structure of the plasma reactor provides a high electrical field necessary to achieve breakdown at atmospheric pressure. The results obtained show that PFCs can be significantly reduced in PFC/N2/H2O mixtures. It was established that the destruction and removal efficiencies (DRE) of PFCs and SF6 in plasma processing are
119
greater than 90% at 1.8 kW (2.45 GHz) of microwave power and 20 l/min total flow rate. Further studies on the removal of PFCs and SF6 are continuing. Although it has been demonstrated that MW atmospheric plasma can be applied as the energy source for this process, it must be pointed out that the whole system has not been totally optimized. The potential of MW sustained plasma technologies for the destruction of PFCs has been addressed in this paper. The scope of this paper is to demonstrate the ability of microwave plasma treatment to induce a variety of physical and chemical phenomena, which can play a significant role in the development of new and existing technologies in the greenhouse gases abatement. Acknowledgements Assistance by SAIREM, France for the development of the microwave power supply is gratefully acknowledged. Also, support from Accentus, Environmental Solutions, UK for the Microwave induced plasma reactor is very much appreciated.
References Allgood, C., 1998. Fluorocarbon process gases in microelectronics. Adv. Mater. 10 (15), 1239–1242. Bayliss, K.H., 1995. Plasma generator with field-enhanced electrodes. US5418430, 23 May 1995. Brown, R.S., Rossin, J.A., Aitchison, K., 2001. Catalytic technology for PFC emission control. Solid State Technol. 46 (7), 189–194. Chang, M.B., Yu, S.J., 2001. An atmospheric-pressure plasma process for C2F6 removal. Environ. Sci. Technol. 35 (8), 1587–1592. Chatterjee, R., Karecki, S., Reif, R., Sparks, T., Vartanian, V., Goolsby, B., 2001. The evaluation of hexafluorobenzene as an environmentally benign dielectric etch chemistry. J. Electrochem. Soc. 148, G721–G724. Chen, X., Holber, W., Peter, M., 2000. An efficient, highly reliable plasma tool for PFC abatement. In: Proceedings of the Electrochemical Society, Environmental Issues with Materials and Processes for the Electronics and Semiconductor Industries (3rd), Mendicino, L., Simpson, L. (Eds.), Pv 2000-7, Toronto, Canada, 1–9. Cummins, W.R., Dupuis, G., Kesari, S., Miner, D., Trilli, K., Fleming, G., 1997. PFCs in the semiconductor industry: a primer. Semicond. Int. 20 (8), 265–272. Fabian, L., 2002. Plasmachemical conversion of long-living perfluorocompounds under atmospheric pressure. Future Fab Int. 12, 61–64. Hackman, R., Akiyama, H., 2000. Air pollution control by dielectric discharges. IEEE Trans. Dielectr. Electr. Insul. 7, 657–686. Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., Van der Linden, P.J., Dai, X., Maskell, K., Johnson, C.A. (Eds.),
ARTICLE IN PRESS 120
M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
2001. Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Vol. 1. Cambridge University Press, New York. Ichikawa, S., Kanno, S., Kurokawa, H., Tamata, S., Eisenmann, M., 2000. Catalytic decomposition of perfluorocarbons (PFCs). In: Proceedings of the Seventh International Semiconductor ESH Conference, (ISEHS2000), Dresden, Germany, paper 27.5.3. Karecki, S., Chatterjee, R., Pruette, L., Reif, R., Sparks, T., Beu, L., Vartanian, V., Novoselov, K., 2001. Evaluation of oxalyl fluoride for a dielectric etch application in an inductively coupled plasma etch tool. J. Electrochem. Soc. 148, G141–G146. LePrince, P., Marec, J., 1992. Microwave Excitation Technology, Plasma Technology. Plenum Press, New York. Lide, D.R. (Ed.), 1992–1993. CRC Handbook of Chemistry and Physics, 73rd Edition, CRC Press, Boca Raton, FL. McAdams, R., 2001. Prospects for non-thermal atmospheric plasmas for pollution abatement. J. Phys. D 34, 2810–2821. Mellor, J.W., 1956. Inorganic and Theoretical Chemistry, Supplement II, Part I. F, Cl, Br, I, At, Longmans. Green and Co., London, New York, Toronto. Moisan, M., Pelletier, J. (Ed.), 1992. Microwave Excited Plasmas in Plasma Technology, Vol. 4. Elsevier Science B.V., Amsterdam. Penetrante, B.M., Schultheis, S.E. (Eds.), 1993. Non-Thermal Plasmas Techniques for Pollution Control. Part A: Overview, Fundamentals and Supporting Technologies and Part B: Electron Beam and Electrical Discharge Processing. Springer, Berlin. Radoiu, M., Martin, D., Calinescu, I., 2003. Emission control of SO2 and NOX by irradiation methods. J. Haz. Mater. 97 (1–3), 145–158.
Rea, M., 1995. Combustion flue gas treatment. In: Chang, J.S., Kelly, A.J., Crowley, J.M. (Eds.), Handbook of Electrostatic Processes. Marcel Dekker Inc., New York, pp. 607–627. Saito, Y., Yamazaki, H., Mouri, I., 2001. Remote-plasmaenhanced reaction between a silicon surface and trifluoro-acetyl-fluoride gas. J. Vac. Sci. Technol. A 19 (1), 38–40. Tonnis, E.J., Graves, D.B., Vartanian, V.H., Beu, L., Lii, T., Jewett, R., 2000. Inductively coupled, point-of-use plasma abatement of perfluorinated compounds and hydrofluorinated compounds from etch processes utilizing O2 and H2O as additive gases. J. Vac. Sci. Technol. A 18 (2), 393–400. Urashim, K., Kostov, K.G., Chang, J.C., Okayassa, Y., Ywaizumi, T., Yoshimura, K., Kato, T., 2001. Removal of C2F6 from a semiconductor process flue gas by a ferroelectric packed-bed barrier discharge reactor with an absorber. IEEE Trans. Ind. Appl. 37 (5), 1456–1463. Veldhuizen van, E.M. (Ed.), 2000. Electrical Discharges for Environmental Purposes: Fundamentals and Applications. Nova Sci. Publishers, New York, USA. Vitale, S.A., Sawin, H.H., 2000. Abatement of C2F6 in rf and microwave plasma reactors. J. Vac. Sci. Technol. A 18 (5), 2217–2223. Xu, X., Rauf, S., Kushner, A., 2000. Plasma abatement of perfluorocompounds in inductively coupled plasma reactors. J. Vac. Sci. Technol. A 18 (1), 213–231. Yabuhara, Y., 1997. A study of abatement technology on PFC gases: recent advances in the PFC emission reduction technologies. In: Proceedings of the Fourth International Environment Safety & Health Conference of the Semiconductor Industry, session 7 (Perfluorinated Compounds), Milan, Italy, paper no. 7.
Radiation Physics and Chemistry 69 (2004) 113–120
Studies on atmospheric plasma abatement of PFCs Marilena T. Radoiu* BOC Edwards, Exhaust Gas Management, Kenn Business Park, Kenn Road, Clevedon, North Somerset BS21 6TH Clevedon, UK Received 17 March 2003; accepted 24 June 2003
Abstract Microwave plasma at 2.45 GHz frequency operating at atmospheric pressure in synthetic gas mixtures containing N2, CF4, C2F6, CHF3, and SF6 were investigated experimentally for various gas mixture constituents and operating conditions, with respect to their ability to destroy perfluorocompounds. It was found that the destruction and removal efficiency (DRE) of the process is highly dependent on the total gas flow. DREs of up to 99.9% have been achieved using 1.8 kW of microwave power at 20 l/min total flow rate. r 2003 Elsevier Ltd. All rights reserved. Keywords: Atmospheric plasmas; PFCs; Destruction and removal efficiencies; Microwaves
1. Introduction Gases in the atmosphere can contribute to the greenhouse effect both directly and indirectly. Direct effects occur when the gas itself is a greenhouse gas like water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). Indirect radiative forcing occurs when chemical transformations of the original gas produce other greenhouse gases, when a gas influences the atmospheric lifetimes of other gases, and/ or when a gas affects atmospheric processes that alter the radiative balance of the earth. Very powerful greenhouse gases that are not naturally occurring include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), which are generated in a variety of industrial processes. Each greenhouse gas differs in its ability to absorb heat in the atmosphere. In order to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas, the concept of a global warming potential (GWP) has been developed. Carbon dioxide (CO2) was chosen as the reference gas. Methane traps over 23 times more heat per molecule than carbon *Tel.: +44-1275-3371-73; fax: +44-1275-337-200. E-mail address: [email protected] (M.T. Radoiu).
dioxide, and CF4 absorbs 5700 times more heat per molecule than carbon dioxide. GWP values based upon a 100-year horizon and atmospheric lifetime for selected greenhouse gases are listed in Table 1. PFCs are considered the most heat-absorbent. The largest PFC emission source is primary aluminum manufacture, which generates mainly CF4 and some C2F6 as unintentional by-products. However, the semiconductor industry is the largest user and therefore emitter of intentionally produced PFCs (Houghton et al., 2001). In order to reduce the overall PFC emission to the lowest economically feasible limit, over the course of the past several years numerous studies of multiple PFC emission control technologies have been reported globally. There are three main routes for PFC emissions reduction: decomposition (combustion, plasma and thermal-chemical) of the PFCs to non-hazardous materials, recycle and recovery of the unused PFCs (Cummins et al., 1997; Yabuhara, 1997) and process optimization and/or replacement of the PFCs with other gases (Allgood, 1998; Chatterjee et al., 2001; Karecki et al., 2001; Saito et al., 2001). At the moment, the only field-proven technique for treating PFC-containing exhaust streams is thermal, either by direct thermal oxidation or catalytic oxidation. In thermal treatment the object is to transfer heat to the
0969-806X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00455-9
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
114
Table 1 Global warming potentials (100 Years Horizon) and atmospheric lifetime (years) of selected greenhouse gases Global warming gas
Global warming potential
Atmospheric lifetime
CO2 CH4 N2O CF4 C3F8 C2F6 CHF3 SF6
1 23 296 5700 8600 11,900 12,000 22,000
5–200 12 120 50,000 2600 10,000 260 3200
exhaust stream with either electrical resistance losses or burning fuel providing the original source of heat. This method is indirect and can be somewhat fuel or electricity intensive. Also, the system must be operating nearly continuously—not just when PFCs are flowing. Of the established abatement methods—combustion, plasma, reactive adsorption, and catalytic decomposition—plasma (Vitale and Sawin, 2000; Xu et al., 2000; Tonnis et al., 2000; McAdams, 2001; Chang and Yu, 2001; Chen et al., 2000) and catalytic (Ichikawa et al., 2000; Brown et al., 2001) studies have continued to dominate published reports. The use of atmospheric plasma to treat industrial exhaust is a relatively new area of work and it is intended to improve upon some of the shortcomings of thermal systems. Non-thermal atmospheric plasma (or more correctly non-equilibrium plasma) chemistry provides an extremely attractive solution compared to classical combustion, electrothermal and thermocatalytic systems for efficient chemical conversion of waste PFCs to reactive fluorinated products that can be subsequently scrubbed by proven, classical means. The original interest in highpressure plasmas is related to the following features: (a) bulk gas temperature is much higher than attainable by combustion; (b) can be operated in a controlled atmosphere, and (c) a high throughput rate of materials in a variety of forms is possible. Also, the plasma system is electrically driven and can be operated in a batch mode. The system does not need to be operating if not required. The response to the application of power is immediate with no warm-up period needed (compared to catalytic systems). The level of destruction can be readily adjusted by simply varying the power input to the system and so can adjust to varying inlet concentrations. The electrons in non-thermal plasmas can reach temperatures of 10,000–100,000 K (1–10 eV) while the gas temperature can remain as low as the ambient temperature. It is the high electron temperature that determines the chemistry of non-thermal plasmas. In non-thermal atmospheric plasmas most of the electrical energy is consumed to produce free radicals, which have
a much greater reactivity than atoms and molecules in the ground state. According to Penetrante (1993), non-thermal plasmas at atmospheric pressure can be sustained through various methods. Examples are pulsed corona discharges summarized by Chatterjee et al. (2001) and McAdams (2001), dielectric barrier or silent discharges described by Hackman and Akiyama (2000), Urashim et al. (2001) and Veldhuizen (2000), and electron beam induced plasmas (Penetrante et al., 1993; Rea, 1995; Radoiu et al., 2003). In pulsed corona discharges and barrier discharges, the discharge extinguishes before ions and neutral gas components can heat up. In electron beam techniques, the gas to be converted is irradiated with high-energy electrons from an electron beam accelerator and reactions can be initiated by electron impact dissociation, excitation and ionization at low gas temperature. These types of non-thermal plasmas are used to abate low waste gas concentrations (o1000 ppm) in large gas volume flows. Plasma techniques are proposed to clean flue gases of coal-fired electrical power plants, to remove NOX from the exhaust of Otto or Diesel engines, or to improve the emission values occurring during painting and surface cleaning. In this case, the chemically active species, which are mainly produced in the carrier gas, react with the waste gas molecules. The conversion degree rapidly decreases with increasing the waste concentration. Microwave excited plasmas belong to a fundamentally different category. Non-equilibrium results from the fact that at a sufficiently high frequency (usually 2.45 GHz), only light electrons can follow the oscillations of the electric field of the applied electromagnetic field. At atmospheric pressure, microwave plasmas exhibit homogeneous electron densities of 1012–1015 cm3 and therefore appear much more attractive for the abatement of toxic gases. In addition, a large number of inelastic electron-neutral collisions result in very efficient dissociation at relatively high concentrations of the gas to be abated. Such inelastic collisions proceed continuously preventing the reformation of the initial molecules. Microwave plasmas can be excited inside resonant cavities, within waveguide microwave circuits or by means of surface wave field applicators (Moisan et al., 1992; LePrince and Marec, 1992). This paper describes the development and application of a non-thermal plasma system sustained by 2.45 GHz frequency microwaves (MW) and operated at atmospheric pressure that can effectively remove PFCs, HFCs and SF6 from gas streams. The technology has been tested on gas flows containing C2F6, CHF3, SF6 and particularly CF4 to illustrate its effectiveness. As it will be presented in the following paper, successful abatement is dependent on the total gas flow, the total power level and the concentration of CF4.
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
2. Experimental method 2.1. Experimental system A schematic diagram of the experimental system is shown in Fig. 1. Carbon tetrafluoride (CF4,>99.7%), trifluoromethane (CHF3,>99.9%), hexafluoroethane (C2F6,>99.7%), and sulphurhexafluoride (SF6,>99.9%) were obtained from BOC Edwards. Individual mass flow controllers were used to achieve the desired concentration of CF4, C2F6, CHF3 and SF6 in nitrogen used as carrier gas. The flow was passed through a mixing reactor where water was added in stoichiometric amount or slight excess calculated as follows: CH2 O ¼ 3CiC2 F6 þ 2CiCF4 þ CiCHF3 þ 3CiSF6 ;
ð1Þ
where CiC2 F6 ; CiCF4 ; CiCHF3 and CiSF6 are the initial molar concentration of C2F6, CF4, CHF3 and SF6, respectively. The conditioned gas was then introduced into the plasma reactor, neutralized in a wet scrubber, and evacuated into the acid exhaust. 2.2. Analysis The analyses of the gas mixtures before and after reaction were monitored on-line by mass spectrometry.
115
The quadrupole mass spectrometer (QMS) used was a 300 amu Fisons Sensorlab system. Mass spectra spanning the mass range 30–200 were recorded. Gas was extracted from the sample port through heated sample lines and a chemically inert diaphragm pump. The typical fragment ions observed above m=e=20 amu and their relative abundances during all of the experiments are reproduced in Table 2. Measurements were taken using the fragment ions in bold characters in Table 2; the destruction and removal efficiency (DRE) was calculated: DRE ð%Þ ¼
Ci Cf 100; Ci
ð2Þ
where Ci is the initial concentration of the recorded species and Cf is its final concentration after the plasma abatement, estimated from the relative ion intensities. 2.3. Microwave system The microwave system consists of a 2.45 GHz frequency microwave generator GMP 20 K/SM (SAIREM, France) with variable power output from 0 to 1950 W, a 20 dB isolator (circulator+water load) for magnetron protection against the reflected power, a three-stub tuner and a sliding short circuit for impedance matching, and a microwave resonant cavity (plasma reactor)—Fig. 2.
Added water vapour N2 + test gas (CF4, CHF3, C2F6, SF6)
Microwave & Plasma Reactor
Mass spectrometer Wet scrubber
Exhaust
Fig. 1. Schematic diagram of the experimental system.
Table 2 Mass spectra of the studied gas species and ions percent abundance (%) in the mass spectra, on-line at webbook.nist.gov/chemistry C2F6
SF6 a
m/e
Ion
127 108 89 70 51
SF+ 5 SF+ 4 SF+ 3 SF+ 2 +
a
SF
% 100 9.1 26.1 5.6 7.5
Mass to charge (amu).
a
CHF3
m/e
Ion
119 69 50 31
C2F+ 5 CF+ 3 CF+ 2 + CF
% 42.2 100 10.8 18.7
a
CF4
m/e
Ion
69 51 50 31
CF+ 3 CHF+ 2 CF+ 2 CF+
% 100 90.7 14.6 49.6
m/ea
Ion
%
69 50 31
CF+ 3 CF+ 2 +
100 10 5.2
CF
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
116
N2 + test gas (CF4, CHF3, C2F6, SF6)
Crystal detector
Electrodes 2.45 GHz Microwave Generator
Isolator
Sliding short circuit
Plasma reactor To wet scrubber
Tuner
Fig. 2. Schematic of the microwave/plasma system. Table 3 Dependence of C2F6 destruction and CF4 formation on the total N2 flow rate and microwave power level N2 (l/min)
1800 W DRE C2F6 (%)
CF4 formed (ppm)
1500 W DRE C2F6 (%)
CF4 formed (ppm)
1200 W DRE C2F6 (%)
CF4 formed (ppm)
25 30 40
100 100 100
1600 3200 3500
100 100 92
2800 4100 3675
95 78 76
3900 4700 4050
Reaction conditions: C2F6=300 cm3 min1; molar ratio H2O:C2F6=3.5.
The isolator is provided with a crystal detector for reflected power measurements. Accentus’ patented microwave plasma reactor consists mainly on a resonant microwave cavity which has been described in detail elsewhere (Bayliss, 1995). It is basically a stainless steel enclosure (V ¼ 220 cm3) within which there is a pair of opposed field-enhancing electrodes. The cavity is provided with an efficient water-cooling circuit.
3. Results and discussion Microwave induced atmospheric plasma destruction of C2F6, CHF3, CF4 and SF6 in the presence of excess water (source of hydrogen and oxygen) was tested over a range of flow rates, power levels and PFC concentrations. The dilution gas was nitrogen. It is noteworthy that the perfluorocarbon compounds often produce CF4 as an ultimate by-product. In many respects the abatement of CF4 provides the highest challenge due to its stability [D(CF3-F)B130 kcal/mol] (Lide, 1992) its large infrared absorption cross-section and corresponding large GWP (5700) which makes it particularly difficult to destroy effectively. As CF4 is the most stable of all PFCs, its destruction was intensively investigated, assuming that if it could be destroyed, all the other fluorocompounds should also decompose under the same conditions. According to reactions (3)–(6) shown below, the ideal chemistry for the PFCs abatement would involve the formation of only CO2 and HF, stable final by-products that could be effectively converted to non-hazardous
salts in a caustic water scrubber. Note that the product CO2 is a GWP compound itself, but its GWP is significantly less than either of the PFCs involved. C2 F6 þ 3H2 O-CO2 þCO þ 6HF;
ð3Þ
CHF3 þ H2 O-CO þ 3HF;
ð4Þ
CF4 þ 2H2 O-CO2 þ 4HF;
ð5Þ
SF6 þ 3H2 O-SO2 þ 6HF þ 12O2 :
ð6Þ
Our experiments were carefully optimized for maximum DREs of the tested gases. To obtain this, excess water is required. The maximum molar ratio of water to PFCs (SF6) used was 3.5 to 1. 3.1. Destruction of C2F6 The destruction of C2F6 was first tested over a range of power levels using total nitrogen flow of 25, 30 and 40 l/min in the presence of water vapour as reactant. C2F6 is readily decomposed in the plasma reactor. Mixtures of water and C2F6 at 3.5 M ratio (H2O:C2F6) were fed into the reactor, and the decomposition characterized as a function of flow rate and microwave power level. The results of C2F6 decomposition versus input power and flow rate are given in Table 3. For a given amount of input power, the extent of C2F6 decomposition increases with decreasing flow rate. This implies that the extent of C2F6 decomposition is determined by the residence time of the reactant in the plasma.
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
In order to identify the decomposition products of the treated gas, a 1 l/min sample from the reactor exhaust was passed through a water scrubber and to the QMS afterwards. Reaction products such as F2 and COF2 will dissolve in the water and form HF. Indeed, pH measurements of the scrubbing water indicated acidic pH after a short run time. Therefore, the only products that pass on to the QMS are non-water soluble species such as C2F6 and other perfluorocarbons. The only other product identified by QMS and confirmed by GC analyses was CF4. At 1.8 kW of microwave power, C2F6 is easily removed even at high flow rates; its destruction efficiency decreases with the power level and total gas flow (residence time in the plasma zone). CF4 production is a linear function of the microwave forward power and total flow rate; the recombination of ions leading to the formation of CF4 decreases with the increase in microwave energy and the gas residence within the plasma—Fig. 3. According to Mellor (1956) and Lide (1992), in the absence of water (or any other source of hydrogen), the equilibrium of reaction (3) shifts entirely to the most stable perfluorocarbon compound, CF4. The global reaction can be written as C2 F6 þ O2 -34CF4 þ CO2 :
ð7Þ
With a higher than stoichiometric water content in the system (H2O/C2F6>4/1), the equilibrium concentrations of CF4 and C2F6 are virtually zero (Fabian, 2002). However, in the experimental conditions described
117
above, the increase of water molar ratio above 3.5 (H2O/C2F6>3.5/1) was found to affect the plasma stability. Therefore, the addition of oxygen to the gas stream was attempted. According to Eq. (8) all the fluorine of C2F6 is transformed to HF and all the carbon to CO2: C2 F6 þ 12O2 þ 3H2 O-2CO2 þ 6HF:
ð8Þ
Indeed, it was found that the addition of O2 (C2F6/ O2=2/1) improved DRE of all species present in the gas mixture—Table 4 and Fig. 3. 3.2. Destruction of CF4 The removal of CF4 was tested over the usual range of gas flows and power levels in the presence of water. It can be seen from Fig. 4 that the destruction achieved was highly dependent on the power level and total flow rate. The formation of radical recombination products, i.e. C2F6, C2F4, was not observed in any of the tests. The decomposition of carbontetrafluoride in nitrogen and water was found to increase as a function of the microwave power level and residence time (total flow rate)—Fig. 4. The highest rate of destruction was observed as 95% at a microwave power level of 1800 W for B1.2% (vol.) CF4 in nitrogen. The linearity of the plots in Fig. 4 indicates that the decomposition rate of CF4 follows a first-order dependence on energy, although clearly the rate depends on the starting concentration of CF4. 100 80
4000
DRE (%)
CF4 (ppm)
5000
25 l/min
3000
30 l/min 2000 1000 1200
60 25 l/min 40
30 l/min + O2 40 l/min
20 1200
1400 1600 1800 Microwave power (W)
Fig. 3. CF4 formation vs. microwave forward power, nitrogen flow and oxygen addition.
30 l/min 40 l/min 1400 1600 Microwave power (W)
1800
Fig. 4. DRE of CF4 in the presence of water. Reaction conditions: CF4=300 cm3/min; molar ratio H2O:CF4=3.5.
Table 4 Influence of O2 addition and microwave power level on the DRE of C2F6 and formation of CF4 O2 (cm3 min1)
1800 W DRE C2F6 (%)
CF4 (ppm)
1500 W DRE C2F6 (%)
CF4 (ppm)
1200 W DRE C2F6 (%)
CF4 (ppm)
0 150
100 100
3200 1000
100 100
4100 2200
78 98
4700 3100
Reaction conditions: N2=30 l/min; C2F6=300 cm3 min1; molar ratio H2O:C2F6=3.5.
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
118
3.3. Destruction of CHF3 The destruction of CHF3 was tested over the same range of gas flow rates and power levels as C2F6. It was again observed that decreasing N2 flow and increasing power level favored the destruction level. As expected, the abatement of this gas was very easy and takes place with very good efficiency even at high flow rates, Table 5. Very small quantities (o10 ppm) of CF4 were identified, especially at low microwave power levels and high gas speed through the plasma reactor. 3.4. Destruction of mixtures of CF4, C2F6, and CHF3 A set of experiments was performed to evaluate the efficiency of the microwave atmospheric plasma technique when PFCs were added together in the gaseous mixture.
Table 5 Abatement of CHF3 DRE dependence on the microwave power level and N2 flow rate N2 (l/min)
1800 W (%)
1500 W (%)
1200 W (%)
25 30 40
100 100 99
100 100 97
97 95 75
3.4.1. Influence of the PFCs concentration and power level on DRE The abatement of a mixture of PFCs was tested over a range of PFCs concentrations and power levels; the gas mixture was diluted with 20 l/min N2. The molar ratio H2O:PFCs was 1.5—Table 6. It can be easily observed that for the abatement of C2F6 and CHF3, DRE was less dependent on their concentrations. The abatement of CF4 was heavily influenced by concentration especially at lower power levels. 3.4.2. Total flow rate and power level influence on DRE Table 7 summarizes the removal efficiencies of CF4, C2F6 and CHF3 at different dilution levels in nitrogen and in the presence of water as reagent. It can be observed that the most reactive gases were CHF3 and C2F6. 3.5. Destruction of SF6 The destruction of SF6 in the presence of water was tested over the same range of gas flows and power levels as PFCs. It was again observed that destruction was favored by decreasing gas flows and increasing power levels—Table 8. 3.6. By-products
Reaction conditions: CHF3=320 cm3 min1; molar ratio H2O:CHF3=2.5.
Many chemical reactions can occur within an ionized gas involving atomic, radical, and complex molecular species, as well as electrons and ions.
Table 6 Dependence of DRE on the total PFCs concentration and microwave power level PFC (cm3/min)
1800 W
a
175 250b 380c
1500 W
1200 W
CHF3
CF4
C2F6
CHF3
CF4
C2F6
CHF3
CF4
C2F6
100 100 100
97 95 91
100 100 100
100 100 100
95 90 82
100 100 100
100 100 100
92 79 72
100 100 100
a
175 cm3/min PFCs consisting of 55 cm3/min CF4, 65 cm3/min CHF3, and 55 cm3/min C2F6. 250 cm3/min PFCs consisting of 115 cm3/min CF4, 95 cm3/min CHF3, and 45 cm3/min C2F6. c 380 cm3/min PFCs consisting of 170 cm3/min CF4, 80 cm3/min CHF3, and 130 cm3/min C2F6. b
Table 7 DRE (%) of PFC/N2/H2O vs. N2 flow rate and microwave power level N2 l/min
20 25 30 40
1800 W
1500 W
1200 W
CHF3
CF4
C2F6
CHF3
CF4
C2F6
CHF3
CF4
C2F6
100 100 100 100
95 92 86 72
100 100 100 100
100 100 100 100
90 81 75 52
100 100 100 75
100 100 99 90
79 72 59 40
100 100 84 53
Reaction conditions: 250 cm3/min PFCs consisting of 115 cm3/min CF4, 95 cm3/min CHF3, and 45 cm3/min C2F6; molar ratio H2O:PFCs=1.5.
ARTICLE IN PRESS M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120 Table 8 Dependence of SF6 abatement on the microwave power level and total flow rate N2 (l/min)
1800 W
1500 W
1200 W
25 30 40
100 100 82
100 90 70
83 77 53
Reaction conditions: SF6 300 cm3/min; molar ratio H2O: SF6=3.5.
In the case of a gaseous mixture of CXFY and N2, the reactions that tend to dominate the behavior of the ionized gas at lower gas temperature with higher electron temperature are: ionization, attachment and dissociation by electron-impact where all the reaction constants depend on the electron temperature. However, electron impact processes are less likely to occur than N2 processes since the concentration of CXFY is much lower than for N2. The primary electron-impact processes will then be dissociation and ionization of nitrogen gas. Therefore, when oxygen is added into the N2-CXFY gas mixture possible stable by-products are CO, CO2, COF2, OF2, NO, NO2, N2O, FO, FO2, FNO, FNO2, and FONO2. The addition of water vapour to the CXFY-N2 system significantly changes the final by-products. New, stable by-products (CO2, HF) are formed in this case. Indeed, the by-products of the plasma decomposition of CF4, C2F6 and CHF3 in the presence of water vapour and N2 were identified as HF, CO2 and small quantities of NOX (>1000–2000 ppm). Oxygenated fluorocompounds as O2F2 and OF2 were only identified when oxygen was added to the system. For SF6 destruction in the presence of water, the by-products were SO3, SO2, and HF. It is noteworthy that the examination of the plasma cavity showed no solid deposits such as solid carbon soot, etc. This observation is very important in terms of cavity maintenance and also, in preventing the formation of heavier fluorocarbons.
4. Conclusions The application of microwave discharges is potentially a perspective technology for perfluorocarbons destruction. Original laboratory equipment consisting on a 2.45 GHz frequency microwave generator with variable power up to 1950 W and a plasma reactor has been developed for this research. The resonant structure of the plasma reactor provides a high electrical field necessary to achieve breakdown at atmospheric pressure. The results obtained show that PFCs can be significantly reduced in PFC/N2/H2O mixtures. It was established that the destruction and removal efficiencies (DRE) of PFCs and SF6 in plasma processing are
119
greater than 90% at 1.8 kW (2.45 GHz) of microwave power and 20 l/min total flow rate. Further studies on the removal of PFCs and SF6 are continuing. Although it has been demonstrated that MW atmospheric plasma can be applied as the energy source for this process, it must be pointed out that the whole system has not been totally optimized. The potential of MW sustained plasma technologies for the destruction of PFCs has been addressed in this paper. The scope of this paper is to demonstrate the ability of microwave plasma treatment to induce a variety of physical and chemical phenomena, which can play a significant role in the development of new and existing technologies in the greenhouse gases abatement. Acknowledgements Assistance by SAIREM, France for the development of the microwave power supply is gratefully acknowledged. Also, support from Accentus, Environmental Solutions, UK for the Microwave induced plasma reactor is very much appreciated.
References Allgood, C., 1998. Fluorocarbon process gases in microelectronics. Adv. Mater. 10 (15), 1239–1242. Bayliss, K.H., 1995. Plasma generator with field-enhanced electrodes. US5418430, 23 May 1995. Brown, R.S., Rossin, J.A., Aitchison, K., 2001. Catalytic technology for PFC emission control. Solid State Technol. 46 (7), 189–194. Chang, M.B., Yu, S.J., 2001. An atmospheric-pressure plasma process for C2F6 removal. Environ. Sci. Technol. 35 (8), 1587–1592. Chatterjee, R., Karecki, S., Reif, R., Sparks, T., Vartanian, V., Goolsby, B., 2001. The evaluation of hexafluorobenzene as an environmentally benign dielectric etch chemistry. J. Electrochem. Soc. 148, G721–G724. Chen, X., Holber, W., Peter, M., 2000. An efficient, highly reliable plasma tool for PFC abatement. In: Proceedings of the Electrochemical Society, Environmental Issues with Materials and Processes for the Electronics and Semiconductor Industries (3rd), Mendicino, L., Simpson, L. (Eds.), Pv 2000-7, Toronto, Canada, 1–9. Cummins, W.R., Dupuis, G., Kesari, S., Miner, D., Trilli, K., Fleming, G., 1997. PFCs in the semiconductor industry: a primer. Semicond. Int. 20 (8), 265–272. Fabian, L., 2002. Plasmachemical conversion of long-living perfluorocompounds under atmospheric pressure. Future Fab Int. 12, 61–64. Hackman, R., Akiyama, H., 2000. Air pollution control by dielectric discharges. IEEE Trans. Dielectr. Electr. Insul. 7, 657–686. Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., Van der Linden, P.J., Dai, X., Maskell, K., Johnson, C.A. (Eds.),
ARTICLE IN PRESS 120
M.T. Radoiu / Radiation Physics and Chemistry 69 (2004) 113–120
2001. Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Vol. 1. Cambridge University Press, New York. Ichikawa, S., Kanno, S., Kurokawa, H., Tamata, S., Eisenmann, M., 2000. Catalytic decomposition of perfluorocarbons (PFCs). In: Proceedings of the Seventh International Semiconductor ESH Conference, (ISEHS2000), Dresden, Germany, paper 27.5.3. Karecki, S., Chatterjee, R., Pruette, L., Reif, R., Sparks, T., Beu, L., Vartanian, V., Novoselov, K., 2001. Evaluation of oxalyl fluoride for a dielectric etch application in an inductively coupled plasma etch tool. J. Electrochem. Soc. 148, G141–G146. LePrince, P., Marec, J., 1992. Microwave Excitation Technology, Plasma Technology. Plenum Press, New York. Lide, D.R. (Ed.), 1992–1993. CRC Handbook of Chemistry and Physics, 73rd Edition, CRC Press, Boca Raton, FL. McAdams, R., 2001. Prospects for non-thermal atmospheric plasmas for pollution abatement. J. Phys. D 34, 2810–2821. Mellor, J.W., 1956. Inorganic and Theoretical Chemistry, Supplement II, Part I. F, Cl, Br, I, At, Longmans. Green and Co., London, New York, Toronto. Moisan, M., Pelletier, J. (Ed.), 1992. Microwave Excited Plasmas in Plasma Technology, Vol. 4. Elsevier Science B.V., Amsterdam. Penetrante, B.M., Schultheis, S.E. (Eds.), 1993. Non-Thermal Plasmas Techniques for Pollution Control. Part A: Overview, Fundamentals and Supporting Technologies and Part B: Electron Beam and Electrical Discharge Processing. Springer, Berlin. Radoiu, M., Martin, D., Calinescu, I., 2003. Emission control of SO2 and NOX by irradiation methods. J. Haz. Mater. 97 (1–3), 145–158.
Rea, M., 1995. Combustion flue gas treatment. In: Chang, J.S., Kelly, A.J., Crowley, J.M. (Eds.), Handbook of Electrostatic Processes. Marcel Dekker Inc., New York, pp. 607–627. Saito, Y., Yamazaki, H., Mouri, I., 2001. Remote-plasmaenhanced reaction between a silicon surface and trifluoro-acetyl-fluoride gas. J. Vac. Sci. Technol. A 19 (1), 38–40. Tonnis, E.J., Graves, D.B., Vartanian, V.H., Beu, L., Lii, T., Jewett, R., 2000. Inductively coupled, point-of-use plasma abatement of perfluorinated compounds and hydrofluorinated compounds from etch processes utilizing O2 and H2O as additive gases. J. Vac. Sci. Technol. A 18 (2), 393–400. Urashim, K., Kostov, K.G., Chang, J.C., Okayassa, Y., Ywaizumi, T., Yoshimura, K., Kato, T., 2001. Removal of C2F6 from a semiconductor process flue gas by a ferroelectric packed-bed barrier discharge reactor with an absorber. IEEE Trans. Ind. Appl. 37 (5), 1456–1463. Veldhuizen van, E.M. (Ed.), 2000. Electrical Discharges for Environmental Purposes: Fundamentals and Applications. Nova Sci. Publishers, New York, USA. Vitale, S.A., Sawin, H.H., 2000. Abatement of C2F6 in rf and microwave plasma reactors. J. Vac. Sci. Technol. A 18 (5), 2217–2223. Xu, X., Rauf, S., Kushner, A., 2000. Plasma abatement of perfluorocompounds in inductively coupled plasma reactors. J. Vac. Sci. Technol. A 18 (1), 213–231. Yabuhara, Y., 1997. A study of abatement technology on PFC gases: recent advances in the PFC emission reduction technologies. In: Proceedings of the Fourth International Environment Safety & Health Conference of the Semiconductor Industry, session 7 (Perfluorinated Compounds), Milan, Italy, paper no. 7.