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Stabilization of chlorofluorocarbons (CFCs) by plasma copolymerization with hydrocarbon monomers
Thin Solid Films 390 Ž2001. 159᎐164
Stabilization of chlorofluorocarbons Ž CFCs. by plasma copolymerization with hydrocarbon monomers Osamu TsujiU , Takeshi Minaguchi, Hirohiko Nakano SAMCO International Inc., Research Center, 36 Waraya-cho, Takeda, Fushimi-ku, Kyoto 612-8443, Japan
Abstract We carried out the plasma copolymerization of CFCs Žtrichlorofluorocarbon. and PFC Žperfluorocarbon. with C 2 hydrocarbons. For the purposes of improving the recovery rate of the copolymerization process, we developed a cascade-type plasma reaction system. The reaction system was equipped with a maximum of 12 reaction tubes, each with an internal diameter of 32 mm and 390 mm in length. In the copolymerization experiments using the cascade type plasma reaction system, the CFC recovery rate reached a level of 95%, which represents a vast improvement of the 30᎐40% recovery rate previously achieved with parallel plate and tubular reaction systems. We found that when performing plasma copolymerization of hydrocarbons and CFCs, which contain chlorine and fluorine, the results varied, depending on the dwell time in the reactor. The level of importance of the parameters that influence the recovery rate could be ranked as follows: CFCrhydrocarbon mixing ratio ) RF power) reactor pressure. When the CFCrhydrocarbon mole ratio is CFC-113: C 2 H 4 s 1:1, the recovery rate reaches the maximum level where 95% of the CFCs are recovered in the form of copolymers and related substances. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma copolymerization; Chlorofluoro carbons; Perfluorocarbons; Cascade type plasma reaction
1. Introduction In the past, CFCs Žchlorofluorocarbons. were widely used as cooling and cleaning agents in the precision and electronics industries. However, due to their role in global warming, it was decided in the Montreal Agreement that the use of CFCs should be eliminated globally by 1995. In order to meet this goal, various techniques are being evaluated for collecting the remaining reserves of CFCs. Several methods have been proposed to transform the collected CFCs into harmless substances, which do not damage the ozone layer. In our research, we carried out the plasma copolymerization and stabilization of CFC-113 ŽCl 2 FCrCClF2 . and CFC-12 ŽCCl 2 F2 . by forming a solid film or powder U
Corresponding author. Tel.: q81-75-621-7841; fax: q81-75-6210936. E-mail address: [email protected] ŽO. Tsuji..
w1᎐3x. The copolymerization process involved mixing the gases during deposition with substances having C 2 hydrocarbon monomers, such as C 2 H 2 , C 2 H 4 , and C 2 H 6 . In order to maximize the CFC recovery rate, we developed a cascade type plasma reaction system with a 6-stage tubular plasma reactor. We performed electrical and material characterization of the plasma deposited copolymer films, and we found that the films are thermally stable and have excellent electrical properties such as low dielectric constant and high breakdown voltage w4,5x. We report and discuss the influence of process conditions such as monomer composition, RF power, and pressure on the polymer film deposition. In addition, we monitored the plasma copolymerization process using a quadrupole mass analyzer for the individual plasma reactors to optimize the plasma copolymerization process. We found that chlorine and fluorine atoms contribute significantly to the copolymerization reaction and process.
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 0 9 2 9 - 4
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O. Tsuji et al. r Thin Solid Films 390 (2001) 159᎐164
2. Experimental
2.2. Plasma copolymerization system
2.1. Reactor design
Fig. 1 shows the schematic of the cascade type plasma reaction system used in the experiments discussed in this paper. The reaction tubes were made from Pyrex w6x, were 390 mm long, and had an internal diameter of 32 mm. Each level of the reaction system had two reaction tubes, with a total of 12 reaction tubes in the six levels. The reaction tubes were installed and removed using Ultra-Torr w7x. RF plasma was generated using a hot electrode in the center of each reactor, which released plasma towards a ground electrode. A 100-W RF generator from Adtec ŽModel AX-1000IIP. was used as the RF power source. The generator was equipped with an Adtec impedance matching circuit, the Model AM-2000S. The CFCs and the hydrocarbon monomers were supplied to the reactors using mass flow controllers from STEC ŽModel SEC-400 Mark II.. The reactors were evacuated using an Edwards mechanical rotary pump, Model E2M40 Ž840 lrmin.. A liquid nitrogen cooling trap was installed between the mechanical rotary pump and the reactor. Pressure within the reactor was monitored using a Pirani vacuum gauge from Ulvac, Model GP-1S. A quadripole mass analyzer ŽModel WIN300D from Leda Ltd.. was installed at both ends of each level of the system, for a
In order to improve the recovery of CFCs which are copolymerized into thin films Žcontaining fluorine and chlorine. and recovered, the parallel plate and tubular type reactors which were used in previous research efforts w2᎐5x were replaced with a cascade type plasma reaction system for the experiments and discussions in this paper. Three issues were given special priority in designing the cascade type of plasma reaction system. Ž1. The system should increase the length of time which the CFCsrhydrocarbon gas mixture residents in the reactor. This was achieved by using a tubular reactor construction, which was designed to ensure that the reaction has sufficient time to progress. Ž2. The system was designed to enable the hydrocarbon monomers to be supplied from the ends of each of the reaction tubes, so that the supply of monomers that are consumed in the plasma copolymerization reaction can be controlled. Ž3. The system was designed so that the individual reactors are easily installed and removed, which makes it possible to measure the weight of the resultant solids and the copolymerization efficiency within each reactor.
Fig. 1. Schematic of plasma copolymerization system.
O. Tsuji et al. r Thin Solid Films 390 (2001) 159᎐164
161
total of six measurement points. The quadrupole mass analyzers were used to perform in-situ monitoring of each reactor. 2.3. Materials and measurements The CFC-113 used in the experiments was provided by Mitsui Chemical, and CFC-12 was provided by Showa Denko Ltd. Both were of 99.9% purity or higher. The C 2 F6 used in the experiments was provided by Sumitomo Seika Ltd., and was of 99.9% purity or higher. In addition, the hydrocarbons ŽCH 4 , C 2 H 4 , C 2 H 6 and C 3 H 8 . used in the experiments were provided by Sumitomo Seika Ltd., and were of 99.9% purity or higher. The thickness of the polymers and thin films deposited inside the reaction tubes was measured using an electro balance system from Chyo Balance Ltd., Model JPN-200W. In order to measure the composition of the films deposited in the reaction tube, a 1 = 1-cm silicon chip was placed in the tube. XPS was then used to analyze the sample. The XPS used in the experiments was the Shimadzu ESCA-3200. 3. Results and discussion
Fig. 2. Distribution of polymer films at each reactor position.
3.1. Distribution of deposited polymer films In order to analyze the polymers and thin film deposition amount in each level of the reaction tubes, we selected the parameters that were previously w2x shown to yield the highest deposition rate, which is a gas mixing ratio of CFCSrC 2 H 4 s 50r50, at RF power of 100 W and pressure of 40 Pa. Deposition time was 1 h. Fig. 2 shows the increase in the weight of the reaction tube after the deposition. We observed that nearly 90% of the polymer deposition weight is located in the first
four reaction tubes. We also observed some deposition in tube 5, but the deposition amount was extremely small. We found that when the gas mixture is changed to CFCsrC 2 H 4 s 60r40, the deposition location shifts to the reaction tubes towards the back of the system, and is completed in reaction tubes number 11 and 12. Based on these results, we concluded that the length of the reaction tubes and the number of reaction tubes required to completely collect CFC-113rC 2 H 4 is highly
Fig. 3. Dependency on recovery of CFC-113rC 2 H 4 gas mixture, RF power, and reactor pressure.
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O. Tsuji et al. r Thin Solid Films 390 (2001) 159᎐164
dependent on the mole ratio of CFCs and hydrocarbons. Therefore, by selecting the optimum gas mixture, it should be possible to reduce the length of the reaction tubes. However, while at this point it is not yet clear what ratio of FrC and ClrC exists in the CFC-113 and C 2 H 4 copolymer deposited as shown in Fig. 2, the results show that the CFC recovery rates vary depending on the position of the reaction tube within the system.
feed gas, we initially measured the flow rates of the CFCs, the PFC and the C 2 H 4 gas using mass flow controllers. We then individually calculated the weight of the supplied gas w Fcf xg, w Fpf xg and w Fhc xg with the following formula:
3.2. Deposition parameters
The recovery was then calculated using the weight in Eq. Ž1. above and the weight of the copolymer recovered from inside the reaction tubes. The calculation for the recovery rate w Rg x is thus:
With the objective of determining the optimum parameters for depositing CFC-113 and C 2 H 4 copolymers, we selected three sets of deposition parameters and tested to see which would be closest to optimum. When testing for RF power dependency and pressure dependency, we selected the CFC-113rC 2 H 4 mixing ratio that was thought to be optimum, which was 50r50. Based on the results shown in Fig. 3, the level of importance of the parameters that influence the recovery rate are in the following order: CFCrhydrocarbon mixing ratio ) RF power) reactor pressure. This indicates that it is essential to optimize the gas mixing ratio in order to maximize the recovery rate. In addition, we found that when the reactor pressure is low, even at a high RF power level, changes in RF power level do not have a significant influence on the recovery rate. We concluded that the cause of this phenomenon is as follows. In a plasma copolymerization reaction which contains halogen elements such as chlorine and fluorine, when the electron energy, such as the plasma parameter ‘electric field Ž E .rpressure Ž p .’, reaches a certain minimum threshold level, the presence of chlorine and fluorine radicals greatly contributes to accelerating the copolymerization reaction.
Gas flow Ž sccm . = deposition time Ž min . =molecular weight w F xg s 22.4= 10 3 Ž cm3 r mol.
w Rg x % s
Deposited copolymer Ž g. = 100 w Fcf x g q w Fhc x g
Ž1.
Ž2.
The results are shown in Fig. 4. CFC-113 and CFC-12 exhibited peaks at mixing ratios of 50r50, with a decrease in the recovery rate as the CFCs concentration exceeds the optimum level. In addition, CCl 4 , which contains a higher ratio of chlorine, produced a relatively high recovery rate. In contrast, C 2 F6 , which does not contain chlorine, exhibited a maximum recovery rate at a gas mixture of C 2 F6rC 2 H 4 s 25r75. Beyond this level, and excessive supply of hydrocarbons resulted in a decrease in the recovery rate. Based on these results, when plasma processing CFC-113 ŽCl 2 FCrCClF2 . that contains both chlorine and fluorine within the molecular structure, during the plasma polymerization process the chlorine is easily incorporated in the copolymer. In contrast, fluorine has a tendency not to be incorporated in the copolymer. The phenomenon whereby chlorine is easily incorporated into the copolymer has previously been confirmed by FTIR analysis in our previous experimental w8x.
3.3. Influence of the mixing ratio 3.4. In-situ monitoring by QMS As described above, the CFCsrhydrocarbon mixing ratio has a significant influence on the recovery rate of copolymers and related reaction by-products. Based on these findings, we went on to examine the unique properties of the plasma copolymerization of halogen species such as chlorine and fluorine with hydrocarbons such as C 2 H 4 . For these experiments, for CFCs we chose to use CFC-113 ŽCl 2 FCrCClF2 ., CFC-12 ŽCCl 2 F2 . and CCl 4 , which contains a methane structure. For PFCs that do not contain chlorine, we chose C 2 F6 . We defined the recovery rate of the 50r50 CFC-113rC 2 H 4 mixing ratio, which yields the highest recovery rate, as 100%, and based on this we then calculated the recovery rates of the different gas mixtures. The recovery rates for each gas mixture were calculated based on weight measurement. In order to calculate the weight of the
In addition to the copolymer film and powder that is found within the plasma reaction tubes, we also found traces of acids such as HCl and HF, which were formed from the chlorine and fluorine contained in the CFCs. In order to analyze the chemical composition of these substances, we installed a quadrupole mass analyzer on each side of the reaction tubes. The measurement results of the mass spectrum are shown in Fig. 5. Prior to plasma generation within the reactor, the mass spectrum shows peaks for molecules that include chlorine and fluorine, such as CF, CClF, and CCl 2 F. After beginning plasma generation with the gas mixture, the aforementioned peaks disappeared, and instead peaks for acidic substances such as HF and HCl were observed. The volume of HF and HCl, as calculated based on the mass spectrum, is equivalent to approxi-
O. Tsuji et al. r Thin Solid Films 390 (2001) 159᎐164
Fig. 4. Recovery rate variation vs. the mole ratio of CFCs to C 2 H 4 in the feed gas.
mately 20᎐25% of the total volume of CFCs introduced during the experiment. When this is combined with the fact that the amount of solid filmrpowder copolymer accounts for 70᎐75% Žweight. of the total volume of CFCs, then the total recovery rate has reached approximately 95%. However, due to the presence of these acidic substances, it is necessary to neutralize the ex-
163
Fig. 5. Mass spectra of ions found in the reaction gases of non-discharge ŽI. and discharge ŽII. of CFC-113rC 2 H 4 .
haust with an alkaline substance, or to dilute the exhaust with a large volume of water. 3.5. Reactor position ¨ s. polymer composition We determined the distribution of chlorine and fluorine contained in the copolymer film deposited during copolymerization with the gas mixing ratio that is optimized and yields the highest recovery rate. After
Fig. 6. Elemental composition of deposited copolymer films at each reactor positions.
164
O. Tsuji et al. r Thin Solid Films 390 (2001) 159᎐164
depositing the copolymer film on 10 = 10-mm silicon wafers that were placed at the center of the reaction tubes in each level of the system, XPS was used to measure the ratio of chlorine, fluorine, and carbon in the film deposited at each level of the reactor. The results of the XPS analysis are summarized in Fig. 6. An interesting phenomenon is that while chlorine is found deposited in the films up to the reaction tubes in the center area of the reactor, chlorine is almost nonexistent in the films in the tubes towards the end of the reactor. Furthermore, based on the XPS data, it appears that when compared with carbon a large amount of fluorine is present in the copolymer. However, in reality, the absolute amount of fluorine present is limited, and it is not a significant factor that influences the recovery rate. In any case, it is important to note that when evaluating the presence of multiple halogen elements such as chlorine and fluorine in polymers deposited via plasma copolymerization, the composition ratio may vary significantly depending on the location within the deposition system.
fluorine. This is due not only to the difference in the energy of the C᎐Cl and C᎐F bonds of the chlorine and fluorine contained in CFCs, but also because of the differences in the way that the chlorine radicals and fluorine radicals disassociate with the hydrocarbons. Based on these findings, we can conclude that when designing a system for the copolymerization of halogen compounds containing elements such as chlorine and fluorine with hydrocarbons, it is essential to give consideration to the reactor geometry, positioning and flow system. In our experiments, we found that 75% of the CFCs were recovered in the form of a film or powder produced by plasma copolymerization. In addition, approximately 20% of the CFCs were accounted for by the acids HCl and HF that were generated from the chlorine and fluorine as by-products of the reaction and were evacuated from the reaction tubes. As a result, we found that a combined total of 95% of the CFCs were recovered by the plasma system used for this experiment.
4. Conclusions
Acknowledgements
In this study, with the objective of improving the recovery rate of the copolymerization process, we developed a prototype of a cascade type plasma reaction system. In order to perform in-situ monitoring of the plasma process, we installed quadrupole mass analyzers on both sides of each reaction tube. In order to analyze the structure and composition of the copolymer deposited during the plasma process, a silicon sample was placed in each reaction tube. This enabled us to perform a comparative evaluation of the influence that location has on copolymer composition. If one is to obtain a high CFC recovery rate, it is not sufficient to simply make a long reaction tube. It is also essential to optimize the gas mixing ratio for the CFCs or PFCs and the hydrocarbon gases. We have reached the conclusion that the optimum mole ratio for CFC113rC 2 H 4 is 50r50. Furthermore, our research has also shown that it is easier for hydrocarbons to combine with chlorine than
The authors would like to thank the RITE ŽResearch Institute of Innovative Technology for the Earth. for their support of this research project and the experimentals. References w1x O. Tsuji, U.S. Pat. 5,569,810 w2x O. Tsuji, M. Sawai, H. Nakano, K. Miyashita, 7th Symp. Plasma Mater. Conf. Abstr. Ž1994. 48. w3x O. Tsuji, M. Sawai, H. Nakano, T Minaguchi, T. Wydeven, 12th Int. Symp. Plasma Chem., Minneapolis, Minn., Abstr. Ž1995. 983. w4x O. Tsuji, T Minaguchi, H. Nakano, T Tatsuta, Jpn. J. Appl. Phys. 36 Ž1997. 4964. w5x O. Tsuji, T Minaguchi, H. Nakano, T Tatsuta, J. Photopolym. Sci. Technol. 10 Ž1997. 143. w6x Pyrex is a trademark of Corning Inc. w7x Ultra-Torr is a trademark of Cajon Corp. w8x O. Tsuji, T Minaguchi, K. Yoshimura, H. Nakano, T. Tatsuta, J. Photopolym. Sci. Technol. 11 Ž1998. 307.
Stabilization of chlorofluorocarbons Ž CFCs. by plasma copolymerization with hydrocarbon monomers Osamu TsujiU , Takeshi Minaguchi, Hirohiko Nakano SAMCO International Inc., Research Center, 36 Waraya-cho, Takeda, Fushimi-ku, Kyoto 612-8443, Japan
Abstract We carried out the plasma copolymerization of CFCs Žtrichlorofluorocarbon. and PFC Žperfluorocarbon. with C 2 hydrocarbons. For the purposes of improving the recovery rate of the copolymerization process, we developed a cascade-type plasma reaction system. The reaction system was equipped with a maximum of 12 reaction tubes, each with an internal diameter of 32 mm and 390 mm in length. In the copolymerization experiments using the cascade type plasma reaction system, the CFC recovery rate reached a level of 95%, which represents a vast improvement of the 30᎐40% recovery rate previously achieved with parallel plate and tubular reaction systems. We found that when performing plasma copolymerization of hydrocarbons and CFCs, which contain chlorine and fluorine, the results varied, depending on the dwell time in the reactor. The level of importance of the parameters that influence the recovery rate could be ranked as follows: CFCrhydrocarbon mixing ratio ) RF power) reactor pressure. When the CFCrhydrocarbon mole ratio is CFC-113: C 2 H 4 s 1:1, the recovery rate reaches the maximum level where 95% of the CFCs are recovered in the form of copolymers and related substances. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma copolymerization; Chlorofluoro carbons; Perfluorocarbons; Cascade type plasma reaction
1. Introduction In the past, CFCs Žchlorofluorocarbons. were widely used as cooling and cleaning agents in the precision and electronics industries. However, due to their role in global warming, it was decided in the Montreal Agreement that the use of CFCs should be eliminated globally by 1995. In order to meet this goal, various techniques are being evaluated for collecting the remaining reserves of CFCs. Several methods have been proposed to transform the collected CFCs into harmless substances, which do not damage the ozone layer. In our research, we carried out the plasma copolymerization and stabilization of CFC-113 ŽCl 2 FCrCClF2 . and CFC-12 ŽCCl 2 F2 . by forming a solid film or powder U
Corresponding author. Tel.: q81-75-621-7841; fax: q81-75-6210936. E-mail address: [email protected] ŽO. Tsuji..
w1᎐3x. The copolymerization process involved mixing the gases during deposition with substances having C 2 hydrocarbon monomers, such as C 2 H 2 , C 2 H 4 , and C 2 H 6 . In order to maximize the CFC recovery rate, we developed a cascade type plasma reaction system with a 6-stage tubular plasma reactor. We performed electrical and material characterization of the plasma deposited copolymer films, and we found that the films are thermally stable and have excellent electrical properties such as low dielectric constant and high breakdown voltage w4,5x. We report and discuss the influence of process conditions such as monomer composition, RF power, and pressure on the polymer film deposition. In addition, we monitored the plasma copolymerization process using a quadrupole mass analyzer for the individual plasma reactors to optimize the plasma copolymerization process. We found that chlorine and fluorine atoms contribute significantly to the copolymerization reaction and process.
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 0 9 2 9 - 4
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O. Tsuji et al. r Thin Solid Films 390 (2001) 159᎐164
2. Experimental
2.2. Plasma copolymerization system
2.1. Reactor design
Fig. 1 shows the schematic of the cascade type plasma reaction system used in the experiments discussed in this paper. The reaction tubes were made from Pyrex w6x, were 390 mm long, and had an internal diameter of 32 mm. Each level of the reaction system had two reaction tubes, with a total of 12 reaction tubes in the six levels. The reaction tubes were installed and removed using Ultra-Torr w7x. RF plasma was generated using a hot electrode in the center of each reactor, which released plasma towards a ground electrode. A 100-W RF generator from Adtec ŽModel AX-1000IIP. was used as the RF power source. The generator was equipped with an Adtec impedance matching circuit, the Model AM-2000S. The CFCs and the hydrocarbon monomers were supplied to the reactors using mass flow controllers from STEC ŽModel SEC-400 Mark II.. The reactors were evacuated using an Edwards mechanical rotary pump, Model E2M40 Ž840 lrmin.. A liquid nitrogen cooling trap was installed between the mechanical rotary pump and the reactor. Pressure within the reactor was monitored using a Pirani vacuum gauge from Ulvac, Model GP-1S. A quadripole mass analyzer ŽModel WIN300D from Leda Ltd.. was installed at both ends of each level of the system, for a
In order to improve the recovery of CFCs which are copolymerized into thin films Žcontaining fluorine and chlorine. and recovered, the parallel plate and tubular type reactors which were used in previous research efforts w2᎐5x were replaced with a cascade type plasma reaction system for the experiments and discussions in this paper. Three issues were given special priority in designing the cascade type of plasma reaction system. Ž1. The system should increase the length of time which the CFCsrhydrocarbon gas mixture residents in the reactor. This was achieved by using a tubular reactor construction, which was designed to ensure that the reaction has sufficient time to progress. Ž2. The system was designed to enable the hydrocarbon monomers to be supplied from the ends of each of the reaction tubes, so that the supply of monomers that are consumed in the plasma copolymerization reaction can be controlled. Ž3. The system was designed so that the individual reactors are easily installed and removed, which makes it possible to measure the weight of the resultant solids and the copolymerization efficiency within each reactor.
Fig. 1. Schematic of plasma copolymerization system.
O. Tsuji et al. r Thin Solid Films 390 (2001) 159᎐164
161
total of six measurement points. The quadrupole mass analyzers were used to perform in-situ monitoring of each reactor. 2.3. Materials and measurements The CFC-113 used in the experiments was provided by Mitsui Chemical, and CFC-12 was provided by Showa Denko Ltd. Both were of 99.9% purity or higher. The C 2 F6 used in the experiments was provided by Sumitomo Seika Ltd., and was of 99.9% purity or higher. In addition, the hydrocarbons ŽCH 4 , C 2 H 4 , C 2 H 6 and C 3 H 8 . used in the experiments were provided by Sumitomo Seika Ltd., and were of 99.9% purity or higher. The thickness of the polymers and thin films deposited inside the reaction tubes was measured using an electro balance system from Chyo Balance Ltd., Model JPN-200W. In order to measure the composition of the films deposited in the reaction tube, a 1 = 1-cm silicon chip was placed in the tube. XPS was then used to analyze the sample. The XPS used in the experiments was the Shimadzu ESCA-3200. 3. Results and discussion
Fig. 2. Distribution of polymer films at each reactor position.
3.1. Distribution of deposited polymer films In order to analyze the polymers and thin film deposition amount in each level of the reaction tubes, we selected the parameters that were previously w2x shown to yield the highest deposition rate, which is a gas mixing ratio of CFCSrC 2 H 4 s 50r50, at RF power of 100 W and pressure of 40 Pa. Deposition time was 1 h. Fig. 2 shows the increase in the weight of the reaction tube after the deposition. We observed that nearly 90% of the polymer deposition weight is located in the first
four reaction tubes. We also observed some deposition in tube 5, but the deposition amount was extremely small. We found that when the gas mixture is changed to CFCsrC 2 H 4 s 60r40, the deposition location shifts to the reaction tubes towards the back of the system, and is completed in reaction tubes number 11 and 12. Based on these results, we concluded that the length of the reaction tubes and the number of reaction tubes required to completely collect CFC-113rC 2 H 4 is highly
Fig. 3. Dependency on recovery of CFC-113rC 2 H 4 gas mixture, RF power, and reactor pressure.
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dependent on the mole ratio of CFCs and hydrocarbons. Therefore, by selecting the optimum gas mixture, it should be possible to reduce the length of the reaction tubes. However, while at this point it is not yet clear what ratio of FrC and ClrC exists in the CFC-113 and C 2 H 4 copolymer deposited as shown in Fig. 2, the results show that the CFC recovery rates vary depending on the position of the reaction tube within the system.
feed gas, we initially measured the flow rates of the CFCs, the PFC and the C 2 H 4 gas using mass flow controllers. We then individually calculated the weight of the supplied gas w Fcf xg, w Fpf xg and w Fhc xg with the following formula:
3.2. Deposition parameters
The recovery was then calculated using the weight in Eq. Ž1. above and the weight of the copolymer recovered from inside the reaction tubes. The calculation for the recovery rate w Rg x is thus:
With the objective of determining the optimum parameters for depositing CFC-113 and C 2 H 4 copolymers, we selected three sets of deposition parameters and tested to see which would be closest to optimum. When testing for RF power dependency and pressure dependency, we selected the CFC-113rC 2 H 4 mixing ratio that was thought to be optimum, which was 50r50. Based on the results shown in Fig. 3, the level of importance of the parameters that influence the recovery rate are in the following order: CFCrhydrocarbon mixing ratio ) RF power) reactor pressure. This indicates that it is essential to optimize the gas mixing ratio in order to maximize the recovery rate. In addition, we found that when the reactor pressure is low, even at a high RF power level, changes in RF power level do not have a significant influence on the recovery rate. We concluded that the cause of this phenomenon is as follows. In a plasma copolymerization reaction which contains halogen elements such as chlorine and fluorine, when the electron energy, such as the plasma parameter ‘electric field Ž E .rpressure Ž p .’, reaches a certain minimum threshold level, the presence of chlorine and fluorine radicals greatly contributes to accelerating the copolymerization reaction.
Gas flow Ž sccm . = deposition time Ž min . =molecular weight w F xg s 22.4= 10 3 Ž cm3 r mol.
w Rg x % s
Deposited copolymer Ž g. = 100 w Fcf x g q w Fhc x g
Ž1.
Ž2.
The results are shown in Fig. 4. CFC-113 and CFC-12 exhibited peaks at mixing ratios of 50r50, with a decrease in the recovery rate as the CFCs concentration exceeds the optimum level. In addition, CCl 4 , which contains a higher ratio of chlorine, produced a relatively high recovery rate. In contrast, C 2 F6 , which does not contain chlorine, exhibited a maximum recovery rate at a gas mixture of C 2 F6rC 2 H 4 s 25r75. Beyond this level, and excessive supply of hydrocarbons resulted in a decrease in the recovery rate. Based on these results, when plasma processing CFC-113 ŽCl 2 FCrCClF2 . that contains both chlorine and fluorine within the molecular structure, during the plasma polymerization process the chlorine is easily incorporated in the copolymer. In contrast, fluorine has a tendency not to be incorporated in the copolymer. The phenomenon whereby chlorine is easily incorporated into the copolymer has previously been confirmed by FTIR analysis in our previous experimental w8x.
3.3. Influence of the mixing ratio 3.4. In-situ monitoring by QMS As described above, the CFCsrhydrocarbon mixing ratio has a significant influence on the recovery rate of copolymers and related reaction by-products. Based on these findings, we went on to examine the unique properties of the plasma copolymerization of halogen species such as chlorine and fluorine with hydrocarbons such as C 2 H 4 . For these experiments, for CFCs we chose to use CFC-113 ŽCl 2 FCrCClF2 ., CFC-12 ŽCCl 2 F2 . and CCl 4 , which contains a methane structure. For PFCs that do not contain chlorine, we chose C 2 F6 . We defined the recovery rate of the 50r50 CFC-113rC 2 H 4 mixing ratio, which yields the highest recovery rate, as 100%, and based on this we then calculated the recovery rates of the different gas mixtures. The recovery rates for each gas mixture were calculated based on weight measurement. In order to calculate the weight of the
In addition to the copolymer film and powder that is found within the plasma reaction tubes, we also found traces of acids such as HCl and HF, which were formed from the chlorine and fluorine contained in the CFCs. In order to analyze the chemical composition of these substances, we installed a quadrupole mass analyzer on each side of the reaction tubes. The measurement results of the mass spectrum are shown in Fig. 5. Prior to plasma generation within the reactor, the mass spectrum shows peaks for molecules that include chlorine and fluorine, such as CF, CClF, and CCl 2 F. After beginning plasma generation with the gas mixture, the aforementioned peaks disappeared, and instead peaks for acidic substances such as HF and HCl were observed. The volume of HF and HCl, as calculated based on the mass spectrum, is equivalent to approxi-
O. Tsuji et al. r Thin Solid Films 390 (2001) 159᎐164
Fig. 4. Recovery rate variation vs. the mole ratio of CFCs to C 2 H 4 in the feed gas.
mately 20᎐25% of the total volume of CFCs introduced during the experiment. When this is combined with the fact that the amount of solid filmrpowder copolymer accounts for 70᎐75% Žweight. of the total volume of CFCs, then the total recovery rate has reached approximately 95%. However, due to the presence of these acidic substances, it is necessary to neutralize the ex-
163
Fig. 5. Mass spectra of ions found in the reaction gases of non-discharge ŽI. and discharge ŽII. of CFC-113rC 2 H 4 .
haust with an alkaline substance, or to dilute the exhaust with a large volume of water. 3.5. Reactor position ¨ s. polymer composition We determined the distribution of chlorine and fluorine contained in the copolymer film deposited during copolymerization with the gas mixing ratio that is optimized and yields the highest recovery rate. After
Fig. 6. Elemental composition of deposited copolymer films at each reactor positions.
164
O. Tsuji et al. r Thin Solid Films 390 (2001) 159᎐164
depositing the copolymer film on 10 = 10-mm silicon wafers that were placed at the center of the reaction tubes in each level of the system, XPS was used to measure the ratio of chlorine, fluorine, and carbon in the film deposited at each level of the reactor. The results of the XPS analysis are summarized in Fig. 6. An interesting phenomenon is that while chlorine is found deposited in the films up to the reaction tubes in the center area of the reactor, chlorine is almost nonexistent in the films in the tubes towards the end of the reactor. Furthermore, based on the XPS data, it appears that when compared with carbon a large amount of fluorine is present in the copolymer. However, in reality, the absolute amount of fluorine present is limited, and it is not a significant factor that influences the recovery rate. In any case, it is important to note that when evaluating the presence of multiple halogen elements such as chlorine and fluorine in polymers deposited via plasma copolymerization, the composition ratio may vary significantly depending on the location within the deposition system.
fluorine. This is due not only to the difference in the energy of the C᎐Cl and C᎐F bonds of the chlorine and fluorine contained in CFCs, but also because of the differences in the way that the chlorine radicals and fluorine radicals disassociate with the hydrocarbons. Based on these findings, we can conclude that when designing a system for the copolymerization of halogen compounds containing elements such as chlorine and fluorine with hydrocarbons, it is essential to give consideration to the reactor geometry, positioning and flow system. In our experiments, we found that 75% of the CFCs were recovered in the form of a film or powder produced by plasma copolymerization. In addition, approximately 20% of the CFCs were accounted for by the acids HCl and HF that were generated from the chlorine and fluorine as by-products of the reaction and were evacuated from the reaction tubes. As a result, we found that a combined total of 95% of the CFCs were recovered by the plasma system used for this experiment.
4. Conclusions
Acknowledgements
In this study, with the objective of improving the recovery rate of the copolymerization process, we developed a prototype of a cascade type plasma reaction system. In order to perform in-situ monitoring of the plasma process, we installed quadrupole mass analyzers on both sides of each reaction tube. In order to analyze the structure and composition of the copolymer deposited during the plasma process, a silicon sample was placed in each reaction tube. This enabled us to perform a comparative evaluation of the influence that location has on copolymer composition. If one is to obtain a high CFC recovery rate, it is not sufficient to simply make a long reaction tube. It is also essential to optimize the gas mixing ratio for the CFCs or PFCs and the hydrocarbon gases. We have reached the conclusion that the optimum mole ratio for CFC113rC 2 H 4 is 50r50. Furthermore, our research has also shown that it is easier for hydrocarbons to combine with chlorine than
The authors would like to thank the RITE ŽResearch Institute of Innovative Technology for the Earth. for their support of this research project and the experimentals. References w1x O. Tsuji, U.S. Pat. 5,569,810 w2x O. Tsuji, M. Sawai, H. Nakano, K. Miyashita, 7th Symp. Plasma Mater. Conf. Abstr. Ž1994. 48. w3x O. Tsuji, M. Sawai, H. Nakano, T Minaguchi, T. Wydeven, 12th Int. Symp. Plasma Chem., Minneapolis, Minn., Abstr. Ž1995. 983. w4x O. Tsuji, T Minaguchi, H. Nakano, T Tatsuta, Jpn. J. Appl. Phys. 36 Ž1997. 4964. w5x O. Tsuji, T Minaguchi, H. Nakano, T Tatsuta, J. Photopolym. Sci. Technol. 10 Ž1997. 143. w6x Pyrex is a trademark of Corning Inc. w7x Ultra-Torr is a trademark of Cajon Corp. w8x O. Tsuji, T Minaguchi, K. Yoshimura, H. Nakano, T. Tatsuta, J. Photopolym. Sci. Technol. 11 Ž1998. 307.