Decomposition of SF6 and H2S Mixture in Radio Frequency Plasma

May 28, 2003 - SF6/H2S/Ar plasma system, HF and elemental sulfur were the main produced species. ... NF3 from the semiconductor industry.10-12 The 13...
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Decomposition of SF6 and H2S Mixture in Radio Frequency Plasma Environment Minliang Shih,*,† Wen-Jhy Lee,† and Chuh-Yung Chen‡ Department of Environmental Engineering, and Department of Chemical Engineering, National Cheng Kung University, No. 1, University Road, Tainan 70101, Taiwan

Sulfur hexafluoride (SF6) is a gaseous pollutant generated in manufacturing processes in the semiconductor industry. Hydrogen sulfide (H2S), as a reductant, was used to treat SF6 in a radio frequency (RF) plasma system. In the SF6/Ar plasma system, SiF4 and SO2 were the two dominant species detected in the glass reactor; other detected species were SO2F2, SOF2, and SOF4. In the SF6/H2S/Ar plasma system, HF and elemental sulfur were the main produced species. Although the species SiF4, SO2, SO2F2, SOF2, and SOF4 were detected in the SF6/H2S/Ar plasma system, adding H2S clearly inhibited the generation of these byproducts of SiO2 etching. At an input power of 5 W and an H2S/SF6 ratio of 2.9, the mass fraction of fluorine from feed SF6 converted into HF was 95.3%, while the mass fraction of sulfur from feed SF6 and H2S converted into elemental sulfur was 96.1%. Given the advantages of recovering HF and reclaiming elemental sulfur, hydrogen sulfide can be used as an auxiliary gas in treating SF6 in an RF plasma system. 1. Introduction Sulfur hexafluoride (SF6) is widely used as an etching and etching-aid gas in radio frequency (RF) discharge in the manufacture of very large scale integration (VLSI) circuits.1-3 The consumption of SF6 has recently increased greatly with the output value of the semiconductor industry. SF6 is a pollutant gas emitted from semiconductor manufacturing plants that contributes to the greenhouse effect.4 SF6 is thermally stable up to 500 °C and does not burn in the gas phase.5 However, when SF6 is in the environment of an electrical discharge, including RF discharge, arcs, sparks, and coronas, it is dissociated into lower fluorides of sulfur as follows:6

SFx + e- f SFx-1 + F + e-

(6 g x g 1)

(1)

These products of dissociation normally tend to recombine rapidly to reform SF6. Unfortunately, they can also recombine into the most toxic product of S2F10.5,7 SF6 undergoes some decomposition and oxidation in an electrical discharge, particularly in the presence of oxygen and water vapor. Hazardous byproducts, including SO2F2, SOF2, SOF4, SF4, SO2, and HF, can be produced simultaneously.6,8-9 Plasma discharge methods have been presented for treating effluent gases such as CF4, C3F8, CHF3, and NF3 from the semiconductor industry.10-12 The 13.56 MHz RF plasma is known as a cold plasma that can increase the temperature and kinetic energy of electrons; however, the gas molecules of a reactor are near room temperature.13 Consequently, conventional combustion reactions that require high activation energy can be replaced by RF plasma technology with relatively low gas temperature and low power consumption. Breit* To whom correspondence should be addressed. Tel: 8866-27-7575, ext. 54531. Fax: 886-6-275-2790. E-mail: wjlee@mail. ncku.edu.tw. † Department of Environmental Engineering. ‡ Department of Chemical Engineering.

barth et al. presented an oxygen-based RF discharge process to decompose fluorocarbon waste gases, including C4F8 and CHF3.10 In the system, a glass reactor was connected in series to the outlet of a commercial RF parallel-plate reactor. In the presence of RF plasma, C4F8 can be decomposed into CF4 and C2F6; moreover, the glass wall provides SiO2 that reacts with these fluorocarbon products to generate stable SiF4 as follows:10

4F + SiO2 ) SiF4 + O2

(2)

The SiF4 can be converted into nontoxic solid products using a CaO/Ca(OH)2 absorber. However, waste sludge raises another concern of this process. Several researchers have studied SF6 decomposition mechanisms in O2, noble gases, and O2/noble gas plasmas.6,14-16 However, the final products are toxic and require further treatment. Wang et al. used hydrogen-based RF plasma to examine the decomposition mechanisms of chlorofluorocarbons, which are also greenhouse gases that are extensively used in the semiconductor industry.11,17 Comparing H2/Ar with a O2/Ar plasma system, Wang et al. determined that H2/Ar plasma is preferred over O2/Ar plasma for treating chlorofluorocarbons. Hydrogen sulfide is a well-known toxic gas that is present in the natural environment (for example, in volcanic gases, marshes, swamps, sulfur springs, and decaying matter). It is a harmful impurity in industrial gases (natural gas and coke oven gas).18,19 The acceptable concentration ceiling for hydrogen sulfide in the workplace is 20 000 ppb, as determined by the Occupational Safety and Health Administration. The minimum risk level (MRL) for hydrogen sulfide is 70 ppb, assuming daily 24-hour exposure over a period of 14 days or less, as determined by the Agency for Toxic Substances and Disease Registry (ATSDR).20 Sulfur can be traditionally recovered from H2S-containing gases by the Claus process. However, the tail gas of the Claus process still contains 1 vol % H2S and must be treated with oxygen or H2S absorption/recycling technologies to satisfy strict requirements.21 H2S is also used as a

10.1021/ie0208063 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/28/2003

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Figure 1. Schematic of the RF plasma system.

processing gas in the electrical industry. Recently, the shrinking of particle sizes in semiconductors has enhanced the strength of the oscillator of various optical transitions.22,23 To this end, an RF sputtering technique has been selected for synthesizing the nanocrystalline CdS thin film generally used as a window material because of its large band gap.24,25 A low-pressure (10-4 Torr) H2S atmosphere has been selected for depositing a CdS thin film to prevent pinhole production in the device. This process yields superior uniformity and crystallinity.26 However, H2S effluent is a concern in relation to human health and the environment.18 In this work, H2S was used as a reducing agent to identify a treatment that could effectively decompose SF6 and simultaneously solve the problem of H2S. RF plasma was selected as the technology for treating such toxic gases. The decomposition fractions of both SF6 and H2S [ηSF6, ηH2S; (Cin - Cout)/Cin × 100%] and the distribution of stable products (mass of sulfur or fluorine of the detected product/mass of sulfur or fluorine of feed SF6 and feed H2S × 100%) were examined as functions of input power and feed H2S/SF6 ratio in a glass reactor. The recovery of decomposition products, including hydrogen fluoride and elemental sulfur, were evaluated. Additionally, the reaction mechanisms in both SF6/Ar and SF6/H2S/Ar plasma systems were discussed. 2. Experimental Description Figure 1 presents the experimental apparatus. The flow rates of SF6, H2S, and Ar were each monitored using a calibrated mass flow controller (Brooks 5850 E). These gases were mixed in a gas mixer and then introduced into a 15-cm-long, cylindrical VYCOR glass reactor with an inner diameter of 4.14 cm, at a fixed flow rate of 100 sccm (standard cm3/min). The plasma discharge zone was surrounded by two external copper electrodes. This configuration prevented the metal electrodes from participating in the reaction. One electrode was coupled to a 13.56-MHz RF generator (Dressler, Cesar) connecting a matching network (Dressler, Vario Match); the other was grounded. These apparatuses constituted an inductively coupled plasma system.27 The pressure in this system was maintained below 10-3 Torr until the experiment began using a mechanical vacuum pump and a diffusion oil pump to prevent contamination. Each design requirement was met by

measuring the concentration of reactants and products at least three times to ensure that the plasma reactions were in a steady state. The reactants and end products were first identified by gas chromatography/mass spectrometry (GC/MS, Varian Star/Saturn 2000) and Fourier transform infrared (FTIR) spectrometry (Nicolet, Avatar 360) using a 9.6-m path-length gas cell (CIC, Ranger) and then quantified using on-line FTIR. The analysis conditions for FTIR were a resolution of 0.5 cm-1 and a scan number of 8. Only neutral products were detected by IR spectroscopy and are presented herein. Reactants and products were both calibrated by withdrawing unreacted gases directly through a sampling line connected to the FTIR. The molar fraction of product species was determined by comparing the peak heights with that of the standard gas at the same wavenumber.28 Depositions in and behind the glow discharge zone were gathered by several tiny glass plates attached to the inner wall of the reactor. After cooling to room temperature, the plate samples were stored in an N2-filled bag. The deposition structures were obtained by X-ray diffraction (XRD, RIGAKU model D/MAX III-V) spectroscopy with CuKR radiation, scanned from 5 to 75° (2θ). Meanwhile, the elements within the deposits were determined by scanning electron microscopy energydispersive X-ray spectrometry (SEM-EDS, Noran, Voyager 1000) analysis. An SF6 feed concentration of 0.2% was used to imitate the effluent concentration in industrial processes, since an SF6 concentration of as low as 1% in argon can yield a high etching rate.2 The other parameters were as follows: The applied RF power was between 5 and 50 W; the total gas flow rate was 100 sccm; the feed H2S/SF6 ratio (R) was between 0.0 and 9.9, and the operating pressure was 10 Torr. 3. Results and Discussion 3.1. SF6 and H2S Decomposition. The initial reactions of the plasma system involved excitation of the dominant Ar as follows:

Ar + e- f Ar* + e-

+

Ar + e f Ar + 2e

-

(3) (4)

Argon, as the carrier gas and as the energy transfer

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Figure 3. XRD patterns of the S8 and the sample (H2S/SF6 feed ratio ) 9.9, input power ) 50 W). (JCPDS Card No. 83-2284).

Figure 2. Decomposition fraction of SF6 (A) and H2S (B) with power input of between 5 and 50 W under various H2S/SF6 feed ratios of between R ) 0.0 and R ) 9.9.

medium, was excited and ionized under electrical discharge. Within a swarm of electrons, Ar*, and a third molecules (M), SF6 was excited and dissociated in the plasma. Ryan et al. proposed that SF6 dissociated extremely rapidly into fragments smaller than SF4.9 Ryan has also suggested that SF2 is a precursor in the sequence of dissociation reactions.14

SF6 + e- f SF2 + 4F + ek ) 28 (s-1, P ) 1 Torr) (5) Similarly, H2S can also be dissociated as follows:29

H2S + e- f SH + H + e-

(6)

H2S + H ) SH + H2 k ) 2 × 10-11 exp(-1710/RT) (cm3 molecule-1 s-1) (7) Previous model sensitivity analyses indicate that applied power is the most important parameter that positively affects the decomposition of reactants in an RF plasma reactor.11,30 Figure 2 presents the correlation between the ηSF6 and the ηH2S with an input power from 5 to 50 W at various H2S/SF6 feed ratios of 0.0, 0.6, 1.7, 2.0, 2.9, 5.1, and 9.9. The ηSF6 increased sharply from 26.4 to 98.7% as the power increased from 5 to 30 W, reaching a steady state at a power of 50 W in the SF6/ Ar system (R ) 0.0), as shown in Figure 2A. A lower E/P ratio (electric field/reduced pressure) is associated with a lower mean electron energy and reduces the reaction rate constants.31,32 Consequently, the dissociation of SF6 by electron impact is inhibited at low power

input. Therefore, a power supply that enables effective generation of electrons is essential in such a system. Another factor that affects the ηSF6 in the SF6/Ar system is the SiO2 etching process. Since F atoms formed by the dissociation of SF6 react with an SiO2 reactor wall and release oxygen, the production of stable SiF4 and oxidation species such as sulfur oxyfluorides can supress the recombination of F atoms into SF6 and increase ηSF6. When H2S (as a reductant) was added to the system (R ) 0.6-2.9), hydrogen or hydrogen radicals were generated by electron impact dissociation reactions (reactions 6 and 7). These reducing agents compete with SiO2 for fluorine atoms and, thus, inhibit etching. Consequently, ηSF6 clearly increased from 62.6 to 99.0%, even at a power of 5 W. As the H2S/SF6 ratio was increased to 9.9 at a power of 5 W, reducing electron density, ηSF6 declined slightly to 90.9%. These findings imply that input power does not remain the most important parameter when the H2S/SF6 ratio exceeds 5.1; that is, when H2S is added, ηSF6 is increased at a particular power input. However, some of the kinetic energy of the electrons is lost because it is transferred to more reaction species. Hence, the power must suffice to cause more SF6 decomposition. When the H2S/SF6 feed ratio was under 2.9 (the mole fraction of H2S was 0.58%), H2S disappeared at all input powers (Figure 2B). ηH2S slightly decreased to 97.0% at a power of 5 W and an H2S/SF6 feed ratio of 2.9; it then decreased to 91.5% when the H2S/SF6 ratio was 9.9 at the same power input. However, H2S disappeared from the plasma at all feed ratios when the power exceeded 30 W. 3.2. Deposition. A thin film formed on the wall of the reactor inside the plasma zone in both SF6/Ar and SF6/H2S/Ar systems. However, a clear light yellow substance was deposited downstream of the plasma zone in the SF6/H2S/Ar system; this substance was not observed in the SF6/Ar systems. SEM-EDS analyses showed that sulfur is its dominant element and represented between 92 and 100% (w/w) of the deposit under various experimental parameters of the SF6/H2S/Ar system. In the SF6/Ar system, the sulfur element was not detected by SEM-EDS analyses. However, balancing the masses of the input and output sulfur showed that 9-1% of sulfur was lost at an input power of between 5 and 50 W. XRD determined that the deposited sulfur was elemental sulfur (Sn). Thermodynamically stable S8 (R) was the dominant sulfur allotrope in the SF6/H2S/ Ar system (Figure 3).

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Figure 4. Mass fraction of input fluorine converted into SiF4 (A) and mass fraction of input sulfur converted into SO2 (B) with power input of between 5 and 50 W under various H2S/SF6 feed ratios of between R ) 0.0 and R ) 9.9.

3.3. Products Distribution. The main species in the SF6/H2S/Ar plasma system were SiF4, SO2, HF, and elemental sulfur; the minor detected species were SO2F2, SOF2, and SOF4. SF6 can be dissociated to release fluorine atoms in plasma. Without reacting with other elements, SFx can recombine with fluorine to form SF6. However, when the walls of the system have surfaces of SiO2, chemical reaction or ion bombardment (probably by Ar+ in plasma) induces desorption of oxygen from the SiO2 surface, producing a Si-like overlayer on the SiO2, onto which fluorine atoms can be adsorbed.33 Then the formation and desorption of stable SiF4 can be spontaneously induced (reaction 2). Figure 4A showed that the mass fraction of input fluorine converted into SiF4 (MSiF4) increased with power input when no H2S was added in the system. When input power exceeded 30 W and ηSF6 reached its the maximum value, the SiO2 etching rate and MSiF4 were stabilized. When H2S was added to the system at R ) 0.6, the MSiF4 was smaller than that at R ) 0.0. However, MSiF4 values at these two H2S/SF6 ratios exhibited the same upward trend as the power input. The reducing agent inhibited the etching of the SiO2 wall, resulting in a clear reduction in the SiF4 formation throughout the power range, when more H2S was added to the system. However, MSiF4 increased slightly as the power input exceeded 30 W at H2S/SF6 feed ratios between 1.7 and 2.9. This mechanism also influenced the formation of SO2 and sulfur oxyfluorides, including SOF2, SOF4, and SO2F2, which were the byproducts of oxidation reactions. The mass fraction of the input sulfur converted into SO2 (MSO2) was far exceeded by that of sulfur oxyfluorides in the SF6/Ar system (Figures 4B and 5). Ryan et al. proposed the following oxidation pathways in plasma (pressure ) 1 Torr):6

Figure 5. Mass fraction of input sulfur converted into SO2F2 (A), SOF2 (B), and SOF4 (C) with power input of between 5 and 50 W under various H2S/SF6 feed ratios of between R ) 0.0 and R ) 9.9.

O2 + e- f O + O + e-

k ) 23 (s-1)

(8)

SF2 + O ) SOF + F k ) 1.1 × 10-10 (cm3 molecule-1 s-1) (9) SF + O ) SO + F k ) 1.7 × 10-10 (cm3 molecule-1 s-1) (10) S + O2 ) SO + O k ) 2.3 × 10-12 (cm3 molecule-1 s-1) (11) As suggested by Ryan, SF2 is a precursor to reactions following the dissociation of SF6. When oxygen was released from the etching process, it reacted rapidly with fragments of dissociated SF6. After the etching process reached its maximum at a power of over 30 W, the release of more oxygen species favored oxidation, leading to the formation of SO2 as the dominant product of the sulfur sink in the SF6/Ar system as follows:6

SOF + O ) SO2 + F k ) 7.9 × 10-11 (cm3 molecule-1 s-1) (12) SO + O + M ) SO2 + M k ) 1.4 × 10-13 (cm3 molecule-1 s-1) (13) SO + SO ) SO2 + S k ) 8.3 × 10-15 (cm3 molecule-1 s-1) (14) Stable products such as SOF4, SOF2, and SO2F2 could be produced by these reactions as follows:6

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SOF3 + F + M ) SOF4 + M k ) 5.0 × 10-11 (cm3 molecule-1 s-1) (15) SOF2 + F + M ) SOF3 + M k ) 5.2 × 10-14 (cm3 molecule-1 s-1) (16) SOF + F + M ) SOF2 + M k ) 3.3 × 10-15 (cm3 molecule-1 s-1) (17) SO2 + F + M ) SO2F + M k ) 1.0 × 10-14 (cm3 molecule-1 s-1) (18) SO2F + F + M ) SO2F2 + M k ) 1.0 × 10-11 (cm3 molecule-1 s-1) (19) However, at a high input power, the dominant etching process consumes much fluorine and produces stable SiF4 that can retard these reactions. Normally, the mass fractions of the input sulfur converted into SO2F2 (MSO2F2), SOF4 (MSOF4), and SOF2 (MSOF2) increased with the power below 20 W in the SF6/Ar system. MSO2F2 and MSOF2 remained constant as the power increased (Figure 5A,B). However, MSOF4 decreased to the steady state when the power exceeded 20 W at R ) 0 (Figure 5C). When H2S was added to the system, other formation pathways of oxidation products could be as follows:5,34

O + H2 ) HO + H k ) 1.1 × 10-10 (cm3 molecule-1 s-1) (20) H + O2 ) HO2 + H k ) 7.5 × 10-11 (cm3 molecule-1 s-1) (21) H + HO2 ) 2HO k ) 7.2 × 10-11 (cm3 molecule-1 s-1) (22) SF5 + OH ) SOF4 + HF k ) 1.6 × 10-12 (cm3 molecule-1 s-1) (23) SOF3 + OH ) SO2F2 + HF k ) 1.0 × 10-13 (cm3 molecule-1 s-1) (24) However, MSOF4 and MSO2F2 of the SF6/H2S/Ar plasma system were not consistent with these formation pathways (reactions 23-24). When H2S was added to the system, HF was clearly the main sink of fluorine. The mass fraction of the input fluorine converted into HF (MHF) was 56.4% at an H2S/SF6 feed ratio of 0.6 and a power of 5 W; it then declined as input power increased (Figure 6A). At an H2S/SF6 ratio of up to 1.7, MHF clearly increased with the amount of H2S added, reaching a maximum of 2.9, decreasing slightly when the ratio exceeded 5.1 (Figure 6B). These results follow from the direct reaction of fluorine atoms, obtained from SF6 dissociation, with the H2S or hydrogen species; then the thermodynamically stable product, HF, is produced as follows:5,35,36

H2S + F ) HF + HS k ) 1.28 × 10-10 (cm3 molecule-1 s-1) (25) HS + F ) HF + S k ) 2.0 × 10-10 (cm3 molecule-1 s-1) (26)

Figure 6. Mass fraction of input fluorine converted into HF (A, B) and mass fraction of input sulfur converted into sulfur deposition (C, D) with power input of between 5 and 50 W under various H2S/SF6 feed ratios of between R ) 0.0 and R ) 9.9.

F + H2 ) HF + H k ) 1.4 × 10-10 exp[-(500 ( 200)/T] (cm3 molecule-1 s-1) (27) F + H ) HF

k ) 1.0 × 10-13 (cm3 molecule-1 s-1) (28)

When the feed ratio exceeded 5.1, increasing the H2S concentration reduced the mean electron energy, yielding a low ηSF6 at a low power input (Figure 2A). MHF declines slightly with the amount of H2S added, because fewer fluorine atoms are produced (Figure 6B). At a high H2S/SF6 ratio, the dominant hydrogen species can compete with oxygen or hydroxyl species, leading to the steady production of HF at a power above 30 W. Meanwhile, the formation of SO2, SOF2, SOF4, and SO2F2 were inhibited. The deposition mechanism of sulfur may involve the precursors of diatomic sulfur S2 (probably S and SF) that survive from the recombination of free fluorine and SFx radicals.37 As more precursors are maintained in the system, the probability of formation of S2 in the gas phase is increased. Then the S2 diffuses away from the powered electrode zone and is converted into stable S8 after reacting with downstream surfaces. The mass

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Figure 7. The predominant reaction pathway for the SF6/Ar RF plasma.

the vacuum system of RF etching. RF plasma is a good choice for these gases because it has the advantages of requiring no additional pumping energy, low power consumption, and the use of mature technology in the electrical industry. At an input power of 5 W and a H2S/ SF6 ratio of 2.9, the mass fraction of fluorine from feed SF6 converted into HF was 95.3%; meanwhile, the mass fraction of sulfur from feed SF6 and H2S converted into sulfur deposition was 96.1%. HF, an important reagent in the preparatory cleaning of the surface of silicon for fabricating integrated circuits, is recovered in a wet scrubber process.38 Elemental sulfur, an industrial material, can be collected to reduce the very large treatment volume of sulfur-containing gas. Consequently, an RF plasma system has been developed to connect on-line to a commercial RF dry-etching plasma process, followed by a recovery system. Given its advantages of recovering HF in reducing toxicity and recovering the elemental sulfur during the reduction of SO2 and H2S effluent, an RF plasma system can be used to treat a mixture of SF6 and H2S. 4. Conclusions RF plasma is a highly developed technology with a low energy consumption, commonly used in the semiconductor industry. SF6 and H2S can quickly dissociate under RF plasma discharge. Both ηSF6 and ηH2S can exceed 99% when the H2S/SF6 feed ratio is between 1.7 and 2.9, even at a power input of below 10 W. The input power promoted the etching of SiO2 by F atoms to generate the stable product SiF4. Other oxidation products of SiO2 etching were SO2 and minor sulfur oxyfluoride species, including SO2F2, SOF2, and SOF4, as observed in the SF6/Ar plasma system. When H2S was added, HF and elemental sulfur (with S8 dominant), were the main products. The generation of SiF4, SO2, and sulfur oxyfluorides and byproducts of SiO2 etching, was inhibited in the SF6/H2S/Ar plasma system. Given its advantages in recovering HF and elemental sulfur, the RF plasma system has potential for treating SF6 and H2S.

Figure 8. The predominant reaction pathway for the SF6/H2S/ Ar RF plasma.

fraction of input sulfur converted into deposited sulfur (MS) increased from 76.1 to 80.7% as the power increased from 5 to 10 W at an H2S/SF6 ratio of 0.6. Then it declined to 69.2% at a power of 50 W (Figure 6C). A higher power input results in higher ηSF6 and provides more S2 precursors. However, the etching process is also promoted by an increased power input, which resulted in more sinking of the sulfur in oxidation products, such as SO2 and SO2F2, when the power exceeded 10 W. However, when the H2S/SF6 ratio exceeded 1.7, MS was 90% and was not clearly influenced by the power input. When the feed ratio exceeded 5.1, the fluorine atoms were saturated by hydrogen species, and the MS reached 99% when the power exceeded 30 W. Finally, Figures 7 and 8 show the possible reaction pathways of SF6 decomposition in both the SF6/Ar and the SF6/H2S/Ar plasma systems, respectively. 3.4. Evaluation of SF6 Treatment Process. Since SF6, SF6 oxidation products, and H2S can cause environmental and health problems, the semiconductor industry requires a treatment system connected in series to the manufacturing process without affecting

Acknowledgment The authors thank the National Science Council, Taiwan, for financially supporting this research under Contract no. NSC 91-2211-E-006-025. We are also grateful for the insightful discussions given by both Professor Perng-Jy Tsai and Professor Cheng-Hsien Tsai. Literature Cited (1) Eisele, K. M. SF6, a Preferable Etchant for Plasma Etching Silicon. J. Electrochem. Soc. 1981, 128, 123. (2) Sheng H. Y.; Fujita, D.; Ohgi, T.; Okamoto, H.; Nejoh, H. Submicrometer Transmission Mask Fabricated by Low-Temperature SF6/O2 Reactive Ion Etching and Focused Ion Beam. J. Vac. Sci. Technol., B 1998, 16, 2982. (3) Nordheden, K. J.; Upadhyaya, K.; Lee, Y.-S.; Gogineni, S. P.; Kao, M.-Y. GaAs Etch Rate Enhancement with SF6 Addition to BCl3 Plasmas. J. Electrochem. Soc. 2000, 147, 3850. (4) Maiss, M.; Brenninkmeijer, C. A. M. Atmospheric SF6: Trends, Source, and Prospects. Environ. Sci. Technol. 1998, 32, 3077. (5) Van Brunt, R. J.; Herron, J. T. Fundamental Processes of SF6 Decomposition and Oxidation in Glow and Corona Discharges. IEEE Trans. Electron. Insul. 1990, 25, 75. (6) Ryan, K. R.; Plumb, I. C. A Model for the Etching of Silicon in SF6/O2 Plasmas. Plasma Chem. Plasma Process. 1990, 10, 207.

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Received for review October 14, 2002 Revised manuscript received February 27, 2003 Accepted April 16, 2003 IE0208063