Abatement of Perfluorocompounds by Tandem ... - ACS Publications

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Ind. Eng. Chem. Res. 2005, 44, 5526-5534

Abatement of Perfluorocompounds by Tandem Packed-Bed Plasmas for Semiconductor Manufacturing Processes How Ming Lee,* Moo Been Chang, and Rung Feng Lu Graduate Institute of Environmental Engineering, National Central University, Chung-Li 320, Taiwan

The technological feasibility and chemical kinetics of carbon tetrafluoride (CF4) decomposition with tandem packed-bed plasmas (TPBPs) are investigated in this study. Relevant parameters including types of packing dielectric materials, applied voltage, discharge gap, and Ar additives are extensively investigated. Results demonstrate that the addition of Ar is beneficial to CF4 abatement. Operating at a higher voltage results in higher removal efficiencies. The effect of packing dielectric materials on CF4 abatement is in the order of BaTiO3 > Al2O3 > glass pellets, being correlative with their dielectric constants. An overall kinetic model is also developed. Experimental results and kinetic analysis indicate that the CF4 removal efficiency achieved with a TPBP obeys a first-order rate law. The overall removal rate constant (k, s-1) can be expressed in an Arrhenius form, k ) 0.1313Ps0.6887, where Ps represents the specific power (W/ cm3). The requirement of 90% reduction of perfluorocompounds for a semiconductor manufacturing process can be accomplished by combining five TPBP reactors in series, being operated at a residence time of 3.2 s and a specific power of 1.2 W/cm3. Introduction

Table 1. Atmospheric Lifetimes and Global-Warming Potentials (GWP100) of Greenhouse Gases1

The Semiconductor Industry Association and the U.S. Environmental Protection Agency (EPA) declared a new partnership to reduce perfluorocompound (PFC) greenhouse gas emissions from semiconductor manufacturing processes. This initiative is part of a global effort taken by semiconductor manufacturers to effectively limit greenhouse gas emissions. According to the U.S. EPA, the semiconductor industry is the first and only industry to work at the international level to reduce climate change-related emissions so far. PFCs, including CF4, C2F6, C3F8, CHF3, SF6, and NF3, are the most potent and persistent of all global-warming gases. On average, PFCs have 10 000 times the globalwarming potential of carbon dioxide (CO2) over a period of 100 years, and they can exist in the atmosphere for long times, ranging from 2000 to 50 000 years, as shown in Table 1.1 PFCs are widely used to clean semiconductor manufacturing equipment and in the etching of circuitry patterns on silicon wafers. The goal of the agreement is to reduce annual absolute PFC emissions of the participating companies, collectively, by 10% below the 1995 baseline emission before 2010. It is expected to reduce PFC emissions by an estimated 15% by 2010. From a technological perspective, a removal efficiency of 90% is the minimum requirement for PFC control devices.2 In semiconductor manufacturing processes, the exhaust gases can be treated either before or after the vacuum pump. Treatment of PFCs at the low-pressure side (i.e., upstream of the vacuum pump) possibly damages the vacuum pumps because of acid formation and particle accumulation. A reasonable alternative is to treat the waste gases downstream of the vacuum pump. Technologies available for reducing PFC emissions from semiconductor manufacturing processes in* To whom correspondence should be addressed. Tel.: (+886)3-4227151 ext. 34694. Fax: (+886)-3-4279455. Email: hmlee@ cc.ncu.edu.tw or [email protected].

greenhouse gases (GHGs) CO2 CH4 N2O CF4 C2F6 C3F8 CHF3 SF6 NF3

atmospheric lifetime (year) Traditional GHGs 50-200 12 120 PFCs 50000 10000 2600-7000 250-390 3200 50-740

GWP100 1 21 310 6500 9200 7000 11700 23900 8000

clude combustion, catalytic oxidation, and nonthermal plasmas (NTPs). NTP technologies have the advantages of high electron temperature and low energy consumption. NTPs have attracted great attention in the fields of air pollution control3-7 and clean energy reform8-10 in light of these advantages. In regards to CF4 abatement, various NTP technologies have been proposed and evaluated as a viable means, such as surface wave plasma,11,12 arc discharges,13 dielectric barrier discharges,14,15 combined plasma catalysis,14,15 radiofrequency plasmas,16,17 microwave plasmas,18 inductively coupled plasmas,19-21 and plasma torch.22 Tandem packed-bed plasma (TPBP) reactors had been successfully applied to decompose volatile organic compounds,23 NF3,24 and C2F6.25 A TPBP reactor is constructed by filling dielectric material within the parallel tandem-like electrodes. Plasmas are typically generated by a high-voltage low-frequency ac power. As external ac voltage is applied across the dielectric layers, the dielectric pellets are polarized at each half-cycle. The spherical pellets can enhance the electric field in the contact regions between adjacent pellets, leading to microbreakdowns in the gaps.23,26 Smaller pellets have more surface areas and form more uniform plasmas throughout the reactor volume,23 resulting in higher

10.1021/ie0402923 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/08/2005

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materials, applied voltage, frequency of the ac voltage, discharge gap, and Ar additives are extensively investigated. Experimental Section

Figure 1. Illustration of a TPBP reactor. Dielectric pellets are packed within two parallel circular electrodes.

destruction efficiencies. Yamamoto et al.23 applied a TPBP reactor to decompose toluene (237 ppm), methylene chloride (500 ppm), and trichlorotrifluoroethane (500 ppm) in dry air. The maximum efficiencies achieved for those three compounds were 100%, 40%, and 30%, respectively, for a single TPBP reactor operating with a gas flow rate of 3.3 cm3/s. Chang et al.24 applied four TPBP reactors in series for decomposing NF3. When 5000 ppm of NF3 in N2 with a flow rate of 3.3 cm3/s was processed, the NF3 removal efficiency increased with an increase in the number of reactors in series. For example, the removal efficiency was ∼20% for a single reactor, ∼90% for three reactors, and >95% for four reactors. On the basis of previous studies, one finds that the destruction efficiency achieved with a single reactor is not high enough, especially for treating the compounds with strong chemical bondings. This level of removal efficiency seems to have difficulty fulfilling the minimum requirement of the semiconductor manufacturing process (i.e., >90% as mentioned previously). The removal efficiency achieved with a TPBP reactor can be enhanced by assembling reactors in series. Therefore, the main thrust of this study is to enhance the PFC abatement efficiency to a reasonable level. In this study, CF4 is selected as the target compound because it is the most stable compound among PFCs. If CF4 could be decomposed by TPBP, all other PFCs would be effectively destroyed. A kinetic approach is taken to understand how many reactors are needed in series to achieve the targeted efficiency of >90%. TPBP is designed to operate at room temperature and atmospheric pressure, which in turn is friendly to the normal operation of semiconductor manufacturing processes. Various parameters including types of packing dielectric

Figure 2. Schematic of the experimental setup.

The concept of a TPBP reactor is illustrated in Figure 1. A TPBP reactor is of a cylindrical shape. The reactor was made of a Pyrex glass tube of 100-mm diameter and 200-mm length. Two parallel electrodes were placed inside the reactor and separated with a distance varying from 2.2 to 24.2 mm depending on the experimental setup. The electrodes were made of stainless steel mesh, so that gas streams could pass through. In the center of each mesh electrode, a stainless steel rod was welded. The rod did not go through the discharge region but was extended outside of the reactor for connection with the high-voltage power supply. BaTiO3 pellets were packed into a TPBP reactor as dielectric materials in all experimental tests unless specified. BaTiO3 pellets were nonporous and of ca. 2-mm diameter. In addition to BaTiO3, either Al2O3 or glass pellets were also examined to understand the packing effect on the CF4 removal efficiency. All packings were of ca. 2-mm diameter. Dielectric constants of BaTiO3, Al2O3, and glass are ∼104, 10, and 4.5, respectively. BaTiO3 pellets were prepared by Fuji Titanium Inc., Osaka, Japan, whereas Al2O3 and glass pellets were supplied by Echo Chemical Co., Ltd., Chung-Li, Taiwan. Figure 2 shows the laboratory-scale experimental system designed and constructed to evaluate the effectiveness of TPBP for CF4 abatement. The simulated gas stream was provided by compressed gas cylinders including N2, O2, CF4, and Ar. A set of mass flowmeters (HFC 202; Teledyne Hastings, Hampton, VA) were used to regulate the flow rates of the feeding gases. The composition and flow rates of the gas streams could then be accurately controlled. The gas streams of all tests were controlled at a gas flow rate of 5.5 cm3/s. The objective of this study is to develop a technique that can operate at atmospheric pressure; therefore, the gas pressure was not controlled (1.0 ( 0.006 atm). Likewise, the gas temperatures of the feeding gases and the reactors were not controlled and were close to ambient temperature (25 ( 3 °C). An ac power supply (model 2700P; Chen-Hwa Co.) with variable voltage and frequency was used as the power source. The voltage from the power source was increased 150 times by a step-up transformer (Jui-Hsiang PTY Co. Ltd.). The power

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Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005

Figure 3. Effect of the applied voltage and specific power on the CF4 removal efficiency and energy yield. Gas compositions: CF4 ) 500 ppm, O2 ) 20%, Ar ) 4000 ppm, and N2 ) 79.55%. Gas flow rate ) 5.5 cm3/s, discharge gap ) 0.82 cm, packings ) BaTiO3.

consumption of the whole system (i.e., plug-in power) was determined by integration of the voltage and current (both in a root-mean-square value), which were measured by a power meter installed in the power supply (model 2700P; Chen-Hwa Co.). Identification and measurement of CF4 and associated end products were carried out by an online Fourier transform infrared (FTIR) spectrometer (FTS-165; BioRad, Hercules, CA). The IR spectra spanning wavenumbers ranging from 400 to 4000 cm-1 were recorded with a resolution of 2 cm-1. Each data point represents the average of 32 times’ scans. A gas chromatograph combined with a thermal conductivity detector and a flame ionization detector (GC-TCD/FID, China Chromatography 9800) was used to check the accuracy of the IR analysis. The simulated gas compositions were designed based on the exhausts from a typical semiconductor manufacturing process. Typical compositions of processing gases contain 20-50 vol % CF4 in Ar. The total gas flow rate was several hundreds of cubic centimeters per second. The utilization of CF4 in the process is ca. 50%. In other words, around half of CF4 is unused and needs to be treated. The exhausts are induced by a dry pump with dry N2 (ca. 800 cm3/s) as the carrier gas. Roughly speaking, the exhaust is diluted around 200-1000 times. Hence, the compositions of the gas flows downstream of the dry pump are about CF4 ) 500 ppm, Ar ) 4000 ppm, and 99.55% N2. If one uses dry air to substitute N2 as the carrier gas for a dry pump, the gas composition varies to about CF4 ) 500 ppm, Ar ) 4000 ppm, O2 ) 20.91%, and 78.64% N2. Our experiment is conducted with this simulated gas composition. The removal efficiency (η) is defined as η (%) ) 100(Cin - Cout)/Cin, where Cin and Cout are the concentrations of CF4 in the influent and effluent (ppm), respectively. The energy yield (ηE, g/kW‚h) denotes the mass of CF4 removed by unit electrical energy input (kW‚h). The specific power (Ps, W/cm3) represents the electrical energy dissipated into a unit volume of a TPBP reactor. Results and Discussion Experiments were first carried out with a TPBP reactor having a discharge gap of 0.82 cm (Figure 3). The gas streams, with a gas flow rate of 5.5 cm3/s, contained 500 ppm of CF4, 4000 ppm of Ar, and balance air. The ac 60-Hz voltages ranging from 0 to 6 kV were applied to the reactor to generate plasmas. Plasmas take

place when the applied voltage is greater than 2.3 kV. As the applied voltage is increased, the removal efficiency increases gradually. The maximum removal efficiency achieved is 16%. Figure 3 also indicates the specific power (Ps), which is defined as the plug-in power divided by the volume of the TPBP reactor. The specific power means the electrical power supplied for a 1-cm3 reactor. Specific powers are 0.2, 0.34, 0.59, and 0.91 W/cm3, respectively, when applied voltages are controlled at 3.0, 3.8, 4.5, and 5.3 kV. The specific power increases with an increase in the applied voltage, indicating that more energy is deposited into the reactor as a higher voltage is applied, resulting in a higher removal efficiency. However, the energy yield does not follow exactly with the trend of increasing applied voltage (or specific power). The maximum energy yield (ηE ) 0.78 g/kW‚h) is achieved when a voltage of 3.8 kV is applied. Operating at either a higher or lower voltage results in a lower energy yield. The energy yield has an optimum value depending on the applied voltage, although the removal efficiency increases with an increase in the applied voltage. Effect of Ar on CF4 Abatement. CF4 is a very stable greenhouse gas. The inertness retards chemical reactions and causes difficult abatement. The reaction between CF4 and a highly active species such as an OH radical is relatively slow; e.g., the reaction rate constant (k) of OH + CF4 f HOF + CF3 at 298 K and 1.0 atm is less than 1.2 × 106 cm3/mol/s.27 To accelerate CF4 decomposition, more active and/or energetic species should be provided. In this study, Ar is selected as a potential additive to enhance the CF4 removal efficiency because its ions and metastables can quickly react with CF4, as shown in reactions (2) and (3).

e + Ar f Ar+ or Ar* k depends on the electron energy (1) Ar+ + CF4 f CF3+ + F + Ar k ) 4.21 × 1013 (cm3/mol/s)28 (2) Ar* + CF4 f CF2 + F2 + Ar k ) 2.41 × 1013 (cm3/mol/s)29 (3) The reaction rate constants of CF4 with Ar+ and Ar* are several orders higher than that with OH. On the other hand, experimental results by Krasnoperov and Krishtopa30 indicated that the removal efficiency could be enhanced by the charge-transfer mechanism if the ionization potential of the contaminant is lower than that of the background gases. The ionization potentials of Ar+ and CF4+ are 15.76 and 14.36 eV, respectively.31 Hence, the existence of Ar in the gas streams is beneficial to CF4 decomposition. Figure 4 illustrates how the CF4 removal efficiency changes with the Ar content, varying from 0.4 to 40 vol % for applied voltages ranging from 3 to 5.25 kV. Experimental results demonstrate that adding Ar favors CF4 decomposition. Its removal efficiency increases as more Ar is added into the gas streams. Figure 5 shows the enhancement degree of ηCF4 with respect to the Ar content. The enhancement factor is defined as the ratio of ηCF4 at a selected Ar content to ηCF4 at Ar ) 4000 ppm. It is observed that the enhancement factor decreases as the applied voltage is increased. In other words, enhancement of ηCF4 resulting from Ar addition is more effective at a lower applied voltage. For example, ηCF4


Figure 6. Effect of the dielectric materials (BaTiO3, Al2O3, and glass) on the CF4 removal efficiency. The packing pellets are 2 mm in diameter, and the discharge gap is 8.2 mm. Gas compositions: CF4 ) 500 ppm, O2 ) 20%, Ar ) 40%, and N2 ) 39.95%. Gas flow rate ) 5.5 cm3/s.

Figure 4. Effects of Ar addition on the CF4 removal efficiency. Gas compositions: CF4 ) 500 ppm, selected Ar concentration, O2 ) 20%, and N2 balance. Gas flow rate ) 5.5 cm3/s, discharge gap ) 0.82 cm, packings ) BaTiO3.

Figure 5. Enhancement of the CF4 removal efficiency (ηCF4) by the Ar content. Experimental conditions are kept the same as those in Figure 4. The enhancement factor is defined as the ratio of ηCF4 at selected Ar concentration to ηCF4 at Ar ) 4000 ppm.

increases 1.5 times as Ar addition changes from 4000 ppm to 40% at voltage ) 5.3 kV, while ηCF4 increases 2.5 times at voltage ) 3.0 kV. This phenomenon is possibly caused by the fact that higher Ar concentrations tend to initiate spark discharges in a BaTiO3 packed-bed reactor.32 Hence, the effect of the addition of Ar on CF4 abatement is more significant when TPBP is operated at low applied voltages compared with high applied voltages. Although the effect of Ar addition varies with the applied voltage, our experimental results indicate that adding Ar into the gas streams is beneficial to CF4 abatement for all CF4 concentrations tested. However, the trend is somewhat different from the results presented by Chang et al.24 Their works showed that Ar addition had an optimum value depending on the applied voltages. The biggest difference between their work and ours is the Ar content tested, for instance, 0-10% Ar for Chang’s work and 0.4-40% in this study. The details regarding Ar addition need further investigation. Effect of Packing Materials on CF4 Abatement. Experimental tests were carried out with three kinds

of dielectric materials including BaTiO3, Al2O3, and glass pellets with diameters of 2 mm. Experiments were conducted with gas streams containing 500 ppm of CF4, 40% Ar, and 20% O2, with N2 as the carrier gas. Experimental results indicate that the removal efficiency increases with an increase in the applied voltage for all three dielectric materials tested (Figure 6). Packing glass pellets results in the lowest removal efficiency among those three dielectric materials applied. The removal efficiency achieved by packing Al2O3 pellets is slightly higher than that achieved by packing glass pellets. Significant enhancement of the removal efficiency is observed when BaTiO3 is packed. In short, the removal efficiency achieved is in the order of BaTiO3 . Al2O3 > glass. This trend parallels their dielectric constants, e.g., ∼104, 10, and 4.5 for BaTiO3, Al2O3, and glass, respectively. A high dielectric constant induces a high electric field24 and also derives intensive discharges.33 Both phenomena are beneficial to CF4 decomposition, resulting in a higher removal efficiency. Effect of the Discharge Gap on CF4 Abatement. This study investigated six sets of discharge gaps ranging from 0.22 to 2.42 cm. Experimental tests were conducted with gas streams containing 500 ppm of CF4, 40% Ar, and 20% O2, with N2 as the carrier gas. The gas temperature and pressure of the inlet gas streams were maintained at 25 ( 3 °C and 1.0 ( 0.006 atm, respectively. Figure 7a illustrates the dependence of the CF4 removal efficiency on the discharge gap and applied voltage. Experimental results demonstrate that the CF4 removal efficiency increases as the applied voltage is increased regardless of variation of the discharge gap. A linear relationship between the applied voltage and removal efficiency is observed regardless of variation of the discharge gap, except for the data of discharge gaps larger than 1.92 cm. Nevertheless, operating at a higher applied voltage results in not only a higher removal efficiency but also a higher power consumption. Figure 7b shows the relationship between the applied voltage and power consumption. Power consumption increases linearly as the applied voltage is stepped up. On the other hand, power consumption decreases when the gap is increased for a given applied voltage. The relationships between the applied voltage, discharge gap, and power seem complex. For a better understand-

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Figure 8. CF4 and product concentrations (a) and product selectivity (b) as a function of the applied voltage. The gas streams contained 500 ppm of CF4, 40% Ar, 20% O2, and 39.95% N2. The TPBP reactor was packed with BaTiO3 pellets. Figure 7. Influence of the applied voltage on the CF4 removal efficiency (a) and on power consumption (b), respectively, for selected discharge gaps. The gas streams contained 500 ppm of CF4, 40% Ar, 20% O2, and 39.95% N2. The TPBP reactor was packed with BaTiO3 pellets.

ing of these relationships, an overall kinetic study is carried out in this work and will be discussed in the Overall Kinetic Model of CF4 Abatement section. Byproducts of CF4 Abatement with a TPBP Reactor. In an oxygen-rich environment, the carbon of CF4 is mainly oxidized into CO2, carbon monoxide (CO), and carbonyl difluoride (COF2); on the other hand, the fluorine could be converted into F2, COF2, and fluorine monoxide (OF2). In this study, three compounds including CO2, COF2, and CO were identified by FTIR spectrum analysis. This result agrees with the works of Hartz et al.,11 Yu and Chang,14 and Lee et al.15 Figure 8a shows the end-product concentrations as a function of the applied voltages. Experimental tests were conducted for gas streams containing 500 ppm of CF4, 40% Ar, 20% O2, and 39.95% N2. The TPBP reactor was packed with BaTiO3 pellets. The most abundant intermediate was CO2, with a yield of 60-70%. The concentrations of CO2, COF2, and CO all increase with an increase in the applied voltage. The carbon balance was in the range of 0.95-1.05. Coke was not observed on the surface of the packings. In the viewpoint of global warming, CO2 is also a greenhouse gas, but its globalwarming potential is greatly lower compared with that of CF4 (see Table 1). COF2, a toxic chemical, can be easily removed by wet scrubbing. Figure 8b shows the product selectivity of carboncontaining species as a function of the applied voltage. CO2 selectivity increases with an increase in the applied voltage, while COF2 selectivity decreases with an increase in the applied voltage. Consequently, CF4 decomposition in a TPBP reactor speculatively corresponds

to a two-step decomposition given by the following equation: CF4 f COF2 f CO2. CO is probably formed via electron-impact dissociations of either CO2 or COF2. In addition to carbon-containing species, fluorinecontaining species were also observed in the experimental tests. COF2 was identified in FTIR analysis, but OF2 was not; F2 is a diatomic molecule and cannot be detected by FTIR. Fluorine-containing products need further investigation; a mass spectrometer should be a good instrument to identify the byproducts.9,18 Nitrogen-containing species such as NO2, NO, and N2O were also found in FTIR analysis during processing of CF4 with NTPs. NO2 and N2O were the dominant species of nitrogen-containing species, whereas the N2O concentration was around 20-33% of the NO2 concentration. Nitrogenous species were mainly formed through interactions between air molecules such as N2 and O2 and chemically active species such as O and N from plasma processes. However, the nitrogenous species were not quantified. It is estimated that as high as 150 ppm of NO2 and 50 ppm of N2O were generated by comparing the signals of their characteristic wavenumbers with a reference CF4 signal, when high voltages were applied to the gas stream containing 500 ppm of CF4, 20% O2, and 40% Ar, with N2 as the carrier gas. It should be noted that NOx formation might cause problems if not well handled. The dominant nitrogenous end products for plasma treatment in this study (NO2) and for a traditional combustion system (NO) are remarkably different. These two species own distinct characteristics; e.g., NO2 is water-soluble, but NO is not. Hence, NO2 can be removed with a wet-scrubbing device. It can simultaneously remove COF2. In addition, there are at least two possible approaches to minimizing NOx formation by NTPs: (1) abating PFCs in either a N2 or O2 environment instead of in an air environment; (2) using H2O(g) as the additive to replace O2. Minimi-


zation of undesired products is an important issue for developing an air pollution control technology. Further works are deemed necessary for this goal. Overall Kinetic Model of CF4 Abatement. Reactions for abating gas pollutants in NTPs are far from elementary but consist of a complex set of opposing, consecutive, and parallel reactions. For instance, CF4 molecules are destroyed and created by consecutive opposing reactions of the form

e + CFx T CFx-1 + F + e, x ) 1-4

(4)

The processing time is long compared to the reaction or transport times for the gas-phase species of interest; therefore, the chemical reaction mechanism can be assumed to complete in pseudo steady state. In pseudo steady state, the flow rates of the feeding gas stream, dissipated power, and all gas-phase species are constant, being independent of time. However, the concentrations of these gas species cannot be determined from equilibrium thermodynamics because the system of NTP is not in thermal equilibrium. If the reaction rate constants (k) are known, then the concentrations can be found by solving the rate equations for particle conservation of each species. Because the kinetics of plasma chemistry are complex and usually involve tens of species and hundreds of reactions, the set of rate equations is generally solved numerically. Even worse, some of these reaction rate constants are incomplete and not available to date. However, insights can be gained by considering simplified reaction sets under both timevarying and steady-state conditions. CF4 abatement in NTPs can be simplified as follows:

e + CF4 f intermediates f end products

(5)

We assume that the kinetic energy of electrons can be referred to the energy deposited into the system, just like the elevated temperature. Therefore, reaction (5) can be rewritten as the consecutive time-varying firstorder reactions kA

kB

A 98 B 98 C

(6)

A first-order destruction is reasonably assumed in this study because only one reactant is bombarded with excess energetic electrons in the plasma reactor. Then, one can obtain34

[

nC ) nA0 1 +

1 (k e-kAt - kAe-kBt) kA - k B B

]

(7)

There are two special cases: (a) kA , kB and (b) kA . kB. In NTPs, the reaction rate constant of electronimpact reactions is essentially much greater than that of gas-phase radical reactions, in general being several orders larger. Therefore, case b, kA . kB, is logical and acceptable for NTPs. In other words, A creates B before B creates C. Hence, there are, approximately, two uncoupled first-order reactions having solutions

{

nA ) nA0e-kAt nB ≈ nA0(1 - e-kAt) nC ≈ 0

(8)

Figure 9. Plot of -ln(nA/nA0) vs residence time. Table 2. Overall Reaction Rate Constants of CF4 Abatement in NTPs (Results of Figure 7) eqs of linear fitting

specific power Ps (W/cm3)

kA (s-1)

determination coefficient R2

Ps ) 0.0660kA Ps ) 0.0871kA Ps ) 0.1095kA Ps ) 0.1333kA Ps ) 0.1588kA

0.4 0.6 0.8 1.0 1.2

0.0660 0.0871 0.1095 0.1333 0.1588

0.9782 0.9925 0.9965 0.9966 0.9951

for 0 < t < ht and

{

nA ≈ 0 nB ≈ nA0e-kBt nC ≈ nA0(1 - e-kBt)

(9)

for t > ht, where ht ) (kAkB)-1/2 is the characteristic time that divides the fast and slow time scales. The characteristic time (th) for plasma processing of CF4 abatement is estimated to be larger than the magnitude of 10-6 s (µs). In atmospheric-pressure NTPs, time duration (t) of a microdischarge is generally in the magnitude of 10-8 s (tens of nanoseconds), which is significantly smaller than the characteristic time ht. Hence, eq 8 is an appropriate expression of rate equations for CF4 abatement by NTPs. Our following efforts are then to develop an overall kinetic model that can be used to predict the abatement efficiency of CF4 with TPBPs. First, removal rate constant k is determined by analyzing our experimental data shown in Figure 7. The data set is based on the specific power, ranging from 0.4 to 1.2 W/cm3, to avoid the effects of power and discharge gap. k can be obtained by the slope of a plot of -ln(nA/nA0) vs residence time (t). Figure 9 shows a linear relationship between -ln(nA/nA0) and the residence time (t) for the experimental data of t < 4 s. As a result, the fact that CF4 abatement in NTPs obeys a first-order rate law is confirmed. The specific-power-dependent k is summarized in Table 2. The regressions of the specific power with k are statistically acceptable because the determination coefficients (R2) are all greater than 0.97. Because specific power serves as the energy supplied to the system, specific power parallels the temperature to activate the reactions in traditional chemistry. For this reason, there exists a relationship between the specific power and the CF4 removal efficiency. As illustrated in Figure 10, the CF4 destruction rate constants [-ln(k)] vs specific power [-ln(Ps)] can be represented by a linear regression as follows: -ln(k) ) -0.6887 ln(Ps) + 2.0306. Substituting this into the

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Figure 10. Plot of CF4 abatement rate constants, -ln(k), vs specific power, -ln(Ps).

Figure 11. Plots of CF4 specific power vs removal efficiency. Dots: experimental data. Lines: model results.

equation, one obtains

k(Ps) ) 0.1313Ps0.6887

(10)

where k is the overall removal rate constant of CF4 (s-1) and Ps is the specific power (W/cm3). The expression of eq 10 is like a modified Arrhenius form also found in the works of Zhao et al.,7 Rosocha,35 and Agnihotri et al.36 In this way, the overall removal efficiency of CF4 (ηCF4) can be rewritten from eq 8 and expressed as

ηCF4 ) 1 - exp(-0.1313Ps0.6887t)

(11)

Equation 11 is the overall kinetic model that can be used to predict the removal efficiency of CF4 for TPBPs. Figure 11 shows the experimental data and the model results of ηCF4, with a specific power ranging from 0.2 to 1.2 W/cm3 and a residence time varying from 0.5 to 3.2 s, respectively. Effects of the ac frequency on the CF4 removal efficiency are illustrated in Figure 12. Dots are experimental data; solid lines are model results based on eq 11. The discharge gap of the TPBP reactor is 1.42 cm, corresponding to a gas residence time of 3.2 s. Both experimental results and model prediction indicate that the CF4 removal efficiency increases with an increase in the frequency of the ac voltage. It is noted that experimental data shown in Figure 12 were not taken

Figure 12. Effects of the ac frequency on the CF4 removal efficiency for selected applied voltages. The discharge gap of the TPBP reactor is 1.42 cm, corresponding to a gas residence time of 3.2 s. Dots: experimental data. Lines: model results.

to determine the parameters in the model. A Pearson correlation of 0.973 demonstrates a good correlation between the experimental and predicted data. The prediction agrees well with the experimental results. Hence, the model developed is capable of and reliable for predicting the CF4 removal efficiency for a TPBP reactor. Our experimental results demonstrate that the maximum removal efficiency of CF4 by a single packed-bed reactor is around 35%, being achieved with packing at an applied voltage of 6.75 kV and a residence time of 3.2 s (see Figure 7a). This level of removal efficiency cannot fulfill the requirement of a semiconductor manufacturing process. For current etching and chamber cleaning processes, 90% or even higher reduction of PFCs is needed.2 Although the removal efficiency achieved with a TPBP reactor can be enhanced by increasing the length of the reactor (i.e., extending the residence time), this approach works well only when the discharge gap is smaller than 1.92 cm (see Figure 7). From this perspective, more TPBP reactors in series are needed to achieve the criterion of >90% PFC reduction. On the basis of the overall kinetic model developed, if a TPBP reactor has a residence time of 3.2 s and is supplied with a specific power of 1.2 W/cm3, the overall removal rate constant of CF4 is 0.149 s-1. Thus, the requirement of more than 90% CF4 reduction can be achieved with five TPBP reactors in series. Conclusions This study employs TPBPs to decompose CF4. TPBP is designed to operate at room temperature and atmospheric pressure, which is suitable for application in semiconductor manufacturing processes. Results demonstrate that Ar addition is beneficial to CF4 abatement. However, the effect of Ar addition on CF4 abatement varies with the applied voltage. Ar addition is more effective when TPBP operates at a low applied voltage compared to when it operates at high applied voltage. The effect of packing dielectric materials on CF4 abatement is in the order of BaTiO3 . Al2O3 > glass pellets, attributed to their dielectric constants. Increasing the length of the reactor (i.e., extending the residence time) can enhance the CF4 removal efficiency. However, this approach works well only when the discharge gap is smaller than 1.92 cm.


Experimental results and the overall kinetic model developed in this study indicate that the CF4 removal efficiency in TPBP obeys the first-order reaction for the gas streams containing 500 ppm of CF4, 40% Ar, and 20% O2, with N2 as the carrier gas. The overall removal rate constant (k, s-1) can be expressed in a modified Arrhenius form, k ) 0.1313Ps0.6887, where Ps represents the specific power ranging from 0.2 to 1.2 W/cm3. A Pearson correlation of 0.973 demonstrates that the model’s prediction agrees well with the experimental results. Therefore, the model developed in this study is capable of and reliable for predicting the CF4 removal efficiency achieved with TPBP reactors. It is also observed that the CF4 removal efficiency in TPBP is mainly correlated with the power dissipated into TPBP reactors and the processing time correspondingly. On the basis of the overall kinetic model developed, the requirement of semiconductor manufacturing (>90% reduction of PFCs) can be fulfilled by five serial TPBP reactors, being operated at a residence time of 3.2 s and a specific power of 1.2 W/cm3. Because CF4 could be reasonably abated by TPBP, all other PFCs should be effectively decomposed as well. Hence, this study successfully demonstrates that TPBP is technologically feasible to abate PFCs from semiconductor manufacturing processes. Acknowledgment The authors acknowledge Prof. Jen-Shih Chang of McMaster University, Canada, for providing BaTiO3 pellets. Additional thanks go to Prof. Ta-Chin Wei of Chung-Yu Christian University, Taiwan, for valuable comments and discussions. This work was supported by the Center for Environmental and Energy Research, University System of Taiwan (UST-CEER-938239-12), and the National Science Council, Republic of China (NSC-93-2211-E-008-012). Literature Cited (1) Houghton, J. T.; Meira Filho, L. G.; Callander, B. A.; Harris, N.; Kattenberg, A.; Maskell, K. Climate Change 1995sthe Science of Climate Change; Cambridge University Press: New York, 1996; p 121. (2) Van Gompel, J.; Walling, T. A New Way to Treat Process Exhaust to Remove CF4. Semicond. Int. 1997, 20 (10), 95. (3) Mok, Y. S.; Koh, D. J.; Kim, K. T.; Nam, I. S. Nonthermal Plasma-Enhanced Catalytic Removal of Nitrogen-Oxides over V2O5/TiO2 and Cr2O3/TiO2. Ind. Eng. Chem. Res. 2003, 42 (13), 2960. (4) Chen, Z.; Mathur, V. K. Nonthermal Plasma for Gaseous Pollution Control. Ind. Eng. Chem. Res. 2002, 41 (9), 2082. (5) Lee, H. M.; Chang, M. B.; Wu, K. Y. Abatement of Sulfur Hexafluoride Emission from Semiconductor Manufacturing Process by Atmospheric-Pressure Plasmas. J. Air Waste Manage. Assoc. 2004, 54, 960. (6) Tsai, C. H.; Lee, W. J.; Chen, C. Y.; Liao, W. T.; Shih, M. Formation of Solid Sulfur by Decomposition of Carbon Disulfide in the Oxygen-Lean Cold-Plasma Environment. Ind. Eng. Chem. Res. 2002, 41 (6), 1412. (7) Zhao, G. B.; Hu, X.; Yeung, M. C.; Plumb, O. A.; Radosz, M. Nonthermal Plasma Reactions of Dilute Nitrogen Oxide Mixtures: NOx in Nitrogen. Ind. Eng. Chem. Res. 2004, 43 (10), 2315. (8) Tang, L.; Huang, H.; Zhao, Z. L.; Wu, C. Z.; Chen, Y. Pyrolysis of Polypropylene in a Nitrogen Plasma Reactor. Ind. Eng. Chem. Res. 2003, 42 (6), 1145-1150. (9) Yang, Y. Methane Conversion and Reforming by Nonthermal Plasma on Pins. Ind. Eng. Chem. Res. 2002, 41 (24), 5918. (10) Fincke, J. R.; Anderson, R. P.; Hyde, T. A.; Detering, B. A. Plasma Pyrolysis of Methane to Hydrogen and Carbon Black. Ind. Eng. Chem. Res. 2002, 41 (6), 1425.

(11) Hartz, C. L.; Bevan, J. W.; Jackson, M. W.; Wofforo, B. A. Innovative Surface Wave Plasma Reactor Technique for PFC Abatement. Environ. Sci. Technol. 1998, 32 (5), 682-687. (12) Wofford, B. A.; Jackson, M. W.; Hartz, C.; Bevan, J. W. Surface Wave Plasma Abatement of CHF3 and CF4 Containing Semiconductor Process Emissions. Environ. Sci. Technol. 1999, 33 (11), 1892. (13) Chen, D. T.; David, M. M.; Tiers, G. V. D.; Scboepfer, J. N. A Carbon Arc Process for Treatment of CF4 Emissions. Environ. Sci. Technol. 1998, 32 (20), 3237. (14) Yu, S. J.; Chang, M. B. Oxidative Conversion of PFC via Plasma Processing with Dielectric Barrier Discharges. Plasma Chem. Plasma Process. 2001, 21 (3), 311. (15) Lee, H. M.; Chang, M. B.; Chen, C. W. Evaluation of Four Kinds of Nonthermal Plasma Reactor for Abatement of Perfluorocompounds. J. Adv. Oxid. Technol. 2005, 8 (1), 33. (16) Tonnis, E. J.; Daniels, J.; Graves, D. Point-of-use Plasma Abatement of PFCs: Modeling and Experiments. Bull. Am. Phys. Soc. 1998, 43, 1477. (17) Sawin, H. H. Abatement of PFCs in a Plasma Reactor Using O2 as an Additive Gas. Bull. Am. Phys. Soc. 1998, 43, 1475. (18) Radoiu, M. T. Studies of 2.45 GHz Microwave Induced Plasma Abatement of CF4. Environ. Sci. Technol. 2003, 37 (17), 3985. (19) Xu, X. D.; Rauf, S.; Kushner, M. J. Plasma Abatement of Perfluorocompounds in Inductively-Coupled Plasma Reactors. J. Vacuum Sci. Technol. A 2000, 18 (1), 213. (20) Delattre, J. L.; Friedman, T. L.; Stacy, A. M. Destructive Abatement of CF4 and C2F6 via a Plasma-Induced Reaction with CaO. J. Vacuum Sci. Technol. B 1999, 17 (6), 2664. (21) Liao, M. Y.; Wong, K.; Mcvittie, J. P.; Saraswat, K. C. Abatement of Perfluorocarbons with an Inductively-Coupled Plasma Reactor. J. Vacuum Sci. Technol. B 1999, 17 (6), 2638. (22) Hong, Y. C.; Uhm, H. S. Abatement of CF4 by AtmosphericPressure Microwave Plasma Torch. Phys. Plasmas 2003, 10 (8), 3410. (23) Yamamoto, T.; Lawless, P. A.; Owen, M. K.; Ensor, D. S.; Boss, C. Decomposition of Volatile Organic Compounds by a Packed-bed Reactor and a Pulsed-Corona Plasma Reactor. In Nonthermal Plasma Techniques for Pollution Control; Penetrante, B. M., Schultheis, S. E., Eds.; Springer-Verlag: New York, 1993; pp 223-237. (24) Chang, J. S.; Kostov, K. G.; Urashima, K.; Yamamoto, T.; Okayasu, Y.; Kato, T.; Iwaizumi, T.; Yoshimura, K. Removal of NF3 from Semiconductor Process Flue Gases by Tandem PackedBed Plasma and Adsorbent Hybrid Systems. IEEE Trans. Ind. Appl. 2000, 36 (5), 1251. (25) Urashima, K.; Kostov, K. G.; Chang, J. S.; Okayasu, Y.; Iwaizumi, T.; Yoshimura, K.; Kato, T. Removal of C2F6 from a Semiconductor Process Flue Gas by a Ferroelectric Packed-Bed Barrier Discharge Reactor with an Adsorber. IEEE Trans. Ind. Appl. 2001, 37 (5), 1456. (26) Vercammen, K. L. L.; Berezin, A. A.; Lox, F.; Chang, J. S. Nonthermal Plasma Techniques for the Reduction of Volatile Organic Compounds in Air Streams: A Critical Review. J. Adv. Oxid. Technol. 1997, 2 (2), 312. (27) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Rossi, M. J.; Troe, J. Evaluated Kinetic, Photochemical and Heterogeneous Data for Atmospheric Chemistry: Supplement V, IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. J. Phys. Chem. Ref. Data 1997, 26, 521. (28) Brashears, H. C., Jr.; Setser, D. W. Transfer and Quenching Rate Constants for XeF (B) and XeF (C) State in Low Vibrational Levels. J. Chem. Phys. 1982, 76 (10), 4932. (29) Velazco, J. E.; Kolts, J. H.; Setser, D. W. Quenching Rate Constants for Metastable Argon, Krypton, and Xenon Atoms by Fluorine Containing Molecules and Branching Ratios for XeF* and KrF* Formation. J. Chem. Phys. 1976, 65 (9), 3468. (30) Krasnoperov, L. N.; Krishtopa, L. G. Kinetic Study in the Destruction of Hydrocarbons, NOx, and SOx in Dielectric Barrier Discharge. Proceedings of the 3rd International Symposium on Non-Thermal Plasma, Cheju Island, Republic of Korea, April 2327, 2001; pp 18-23. (31) Field, F. H.; Franklin, J. L. Electron Impact Phenomena and the Properties of Gaseous Ions; Academic Press: New York, 1970; pp 253 and 387. (32) Jogan, K.; Mizuno, T.; Yamamoto, T.; Chang, J. S. The Effect of Ar Mixture on the Reduction of CO2 from Combustion

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Flue Gases by an AC Ferroelectric Packed Bed Plasma Reactor. Polym. J. Appl. Chem. 1995, 39, 583. (33) Halliday, D.; Resnick, R. Fundamentals of Physics, 2nd ed.; Wiley: New York, 1998. (34) Lieberman, M. A.; Lichtenberg, A. J. Principles of Plasma Discharges and Materials Processing; Wiley: New York, 1994; p 269. (35) Rosocha, L. A. Nonthermal Plasma Applications to the Environment: Gaseous Electronics and Power Conditioning. IEEE Trans. Plasma Sci. 2005, 33 (1), 129.

(36) Agnihotri, S.; Cal, M. P.; Prien, J. Destruction of 1,1,1Trichloroethane Using Dielectric Barrier Discharge Nonthermal Plasma. J. Environ. Eng. 2004, 130 (3), 349.

Received for review December 13, 2004 Revised manuscript received March 15, 2005 Accepted March 21, 2005 IE0402923

Abatement of Perfluorocompounds by Tandem ... - ACS Publications

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