- Email: [email protected]
Dynamics of materials processing by RF plasmas
ARTICLE IN PRESS
Radiation Physics and Chemistry 68 (2003) 221–226
Dynamics of materials processing by RF plasmas P.T. Murraya,*, E. Shinb a
Research Institute and Graduate Materials Engineering, University of Dayton, Dayton, OH 45469-0160, USA b Electro Optics Program, University of Dayton, Dayton, OH 45469-0245, USA
Abstract We have investigated the species present in RF plasmas of O2 and Ar/C2H2. The results for O2 suggest that symmetric charge transfer reactions within the plasma sheath cause a decrease in the ion flux and in the mean ion kinetic energy with increasing pressure. The results also indicate that plasma polymerization efficiently dissociates the C2H2 feed gas and that ionization of C2H2 is accompanied by little fragmentation. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Plasma; Mass spectrometry; Charge transfer; Polymerization; Acetylene; Oxygen; Sheath
1. Introduction Plasmas contain numerous species including positive ions, negative ions, and neutrals, all in various degrees of internal excitation and with a range of kinetic energies. Acetylene polymerization plasmas typically contain a mixture of Ar (or other rare gas) and C2H2, the former being the majority species. The species expected to be present in such mixtures include Ar, C2H2, C2H2* (electronic, vibrational, rotational excita + tion), Ar*, Ar+, C2H+ 2 , C2H2 , C2H, C2H , etc). We have investigated the plasmas generated during plasma polymerization of acetylene and, because the first step in forming films of polymerized acetylene involves cleaning the substrate with an oxygen plasma, have extended the investigations to include oxygen plasmas. Very little has been reported on acetylene plasmas that are operated under plasma polymerization conditions. In contrast, there are numerous reports on RF and microwave plasmas that give diamond or diamondlike films. For example, molecular beam mass spectrometry was used (McMaster et al., 1995) to measure the gas-phase composition near a growing diamond surface in a microwave plasma-assisted chemical vapor deposition reactor. Reported were the dependencies of gas composition on changes in the carbon mole fraction in *Corresponding author. Fax: +1-937-229-3433. E-mail address: [email protected] (P.T. Murray).
the reactor feed, inlet carbon source (CH4 versus C2H2), and surface temperature. The gas composition was independent of inlet hydrocarbon source. Films grown using either CH4 or C2H2 had similar growth rates, morphology, and Raman spectra. The decomposition of CH4 in an RF diamond deposition plasma was investigated (Okeke and Stori, 1991) by mass spectrometry. Electron impact dissociation was suggested as the rate-determining step in the plasma-chemical decomposition of methane. Mass spectrometry was used (Doyle, 1997) to determine the product gas yields for an RF glow discharge of C2H2. Doyle proposed that film growth was dominated by radicals the C4H3, C6H3, and C2H under the high-power conditions suitable for depositing diamond-like carbon. The plasma polymerization of acetylene raises a number of questions. These include: (1) the composition of the plasma and the extent to which it changes with plasma conditions, (2) the identity of those species that are necessary for producing films with useful properties, (3) determining whether the key species are formed in the gas phase or at the growth surface, (4) the most important factors for producing ‘‘good’’ films, and (5) the best way to use this information to control the plasma. We have addressed these questions by carrying out time of flight mass spectrometry (TOFMS) and residual gas analysis on the plasmas generated during plasma polymerization of acetylene. This combination of characterization techniques is expected to give us
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00288-3
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insight into the chemistry of acetylene plasmas. The eventual goal is to correlate information on the composition of the plasmas to information concerned with the molecular structure and properties of the films.
2. Experimental Shown in Fig. 1 is a schematic of the apparatus that was used for this investigation. The apparatus was attached to a large (B100 l), commercially available plasma reactor, and the plasma population was sampled through a 1 mm diameter orifice. Residual gas analysis involved ionization, by electron impact, of the neutral species present in a sample gas, followed by mass analysis of the positive ions thus generated. Mass analysis was carried out by using a quadrupole mass spectrometer with a mass range from 0 to 300 AMU and an electron kinetic energy that is programmable from 25 to 105 eV. These experiments provided information primarily on the neutral species in the plasma. The TOFMS system was mounted in an in-line configuration and included a (grounded) guard plate, filter plate, acceleration plate, second ground plate, and a field-free drift region. Following the drift region was a deflector that was biased at a large positive potential (typically +3 kV) and a microchannel plate (MCP) detector that was mounted off-axis in order to minimize the background signal due to UV photons and fast neutral particles generated in the plasma. Operation of the TOFMS entailed repetitively (30 kHz) applying a (+800 V, 200 ns) voltage pulse to the acceleration plate.
This caused positive ions that were located between the acceleration and ground plates to be accelerated toward the drift region. Once entering the drift region, the ions traveled with constant velocity and were deflected offaxis and into the MCP, where they were detected. The ion TOF was determined by using the leading edge of the acceleration pulse as the start and the ion pulse at the MCP as the stop to a time-to-digital converter. TOFMS spectra were acquired by collecting data for B1 min per spectrum. The TOFMS system was also equipped with a filter plate, whose purpose was to serve as a retarding potential analyzer. This allowed us to determine the ion kinetic energy distributions by recording TOFMS with various (positive) voltages applied to the filter plate, determining the total ion intensity at each applied filter voltage, and by taking the first derivative of the intensity-voltage data. The O2 plasma data were acquired with 300 W of RF power and with O2 pressures ranging from 6 to 14 Pa. The Ar/C2H2 data were acquired from a plasma that was formed with 500 W of RF power, with a 60/40 ratio of Ar/C2H2, and with a total gas pressure of 13 Pa.
3. Results and discussion 3.1. O2 plasmas Shown in Fig. 2 are TOFMS of the positive ions detected with various O2 pressures in the reactor. The spectra exhibit an intense peak due to O+ and 2
Fig. 1. Schematic of the experimental setup.
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sheath to a final kinetic energy that is less than of the primary ion. At the pressures at which our experiments took place (several Pa), it is expected that several sequential charge transfer reactions, per incident ion, will take place within the sheath. The final ionic product from such a sequence will have a velocity that is considerably less than the initial ion velocity. In fact, the final ion may have a velocity that approaches thermal velocity, and such an ionic product may have a small probability of escaping the sheath region. The O+ signal, in contrast to O+ 2 , increases with increasing pressure. This can be understood by realizing that the most probable collision partner of O+ is O2. At low kinetic energies (less than several keV’s), the crosssection for charge transfer is a sensitive function of the energy defect in the reaction. Thus, resonant symmetric charge transfer reactions (for which the energy defect is identically zero) have the largest cross sections. In contrast, the energy defect for the asymmetric charge transfer reaction Oþ þ O2 -O þ Oþ 2 is 1.55 eV (Lias et al., 1988), suggesting that such a reaction will have a significantly smaller cross-section. In addition, since the concentration of neutral atomic O is small with our experimental conditions, neutralization of O+ through the symmetric reaction Oþ þ O-O þ Oþ
Fig. 2. TOF mass spectra of the positive ions detected with 300 W of RF power at O2 pressures of: (a) 6 Pa, (b) 8 Pa, (c) 10 Pa, (d) 12 Pa, and (e) 14 Pa.
considerably smaller peaks due to O+ and O+ 3 . Within the plasma, positive ions are generated by electron impact. The small O+/O+ 2 peak ratio (o1%) indicates that the mean electron kinetic energy in the plasma is marginally above the 18.9 eV threshold for O+ formation from O2 (Samson et al., 1982). The data of Fig. 2 show a monotonic decrease in the O+ signal with 2 increasing O2 pressure. The decrease in O+ intensity is attributed to 2 symmetric charge transfer reactions within the plasma sheath. The plasma sheath is a region of space at the plasma/chamber wall interface across which the plasma potential is applied. Charge transfer reactions are represented in Reaction (1). þ Oþ 2f þ O2 -O2f þ O2 :
ð1Þ
ion (denoted Oþ In these reactions, a fast 2f ) interacts with a thermal energy O2 within the sheath. An electron is transferred from the (slow) neutral to the (fast) ion, resulting in the formation of a fast neutral species and a nascent ion which is accelerated within the O+ 2
is expected to be minimal. Thus, because of the significantly lower probability for charge transfer to occur with O+, its intensity can be used as a reference to monitor the initial O+ 2 concentration within the plasma. Such an argument suggests that we can model the plasma sheath as a reaction cell, and that a plot of + ln(I[O+ 2 ]/I[O ]) vs pressure will be linear with a slope equal to sx: In this case, s is the cross-section for the charge transfer reaction, and x is the length of the sheath. Baer and Murray (1981) used a similar procedure to explore the dynamics of charge and proton + transfer reactions of NH+ 3 . The quantities I[O2 ] and + I[O ] are the intensities of the molecular and atomic oxygen ions, respectively. Shown in Fig. 3 is such a plot, which is clearly linear. From the slope of the line (0.439 Pa1), we can estimate the length of the plasma sheath if we make the simplifying assumption that the symmetric charge transfer cross-section for O+ is 2 constant (i.e. independent of kinetic energy). The kinetic energy (as well as vibrational state) dependence of the symmetric charge transfer cross-section have been reported previously (Baer et al., 1978). Using a value (Baer et al., 1978) for s of 2.5 1019 m2 yields a value of 7.3 mm as the length of the plasma sheath. This number is approximately a factor of two smaller than the value reported (Mizutani and Hayashi, 2000) previously for an inductively coupled Ar/O2 plasma.
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Nevertheless, it illustrates that our approach is reasonable. A more thorough analysis, in which the kinetic energy dependence of s is explicitly taken into account, is currently underway. An additional factor that characterizes a plasma is the kinetic energy distribution of the ions. If our charge
transfer hypothesis is correct, there should be a decrease in the mean O+ 2 kinetic energy as the pressure in the reactor increases. Shown in Fig. 4 are the kinetic energy distributions of O+ 2 that were recorded at the indicated pressures. The data indicate that the most probable O+ 2 kinetic energy decreases from a value of B300 eV at 5 Pa to less than 50 eV at 14 Pa. It is also interesting to note that the distributions become more Maxwellian with increasing pressure. These results represent the ionized species that leave the plasma reactor. These species are also those that strike the surface of a substrate placed within the plasma reactor. This suggests that substrate surfaces are exposed to a flux of molecular ions (and neutrals) during cleaning in an O2 plasma and that the mean kinetic energy and flux of the ionized species decrease with increasing pressure. 3.2. Ar/C2H2
+ Fig. 3. Plot of ln(I[O+ 2 ]/I[O ]) versus pressure of O2 with 300 W of RF power.
Thin films of polymerized acetylene are formed by plasma polymerization of a mixture of Ar and C2H2. Shown in Fig. 5 is a TOFMS of the positive ions detected from a plasma formed by applying 500 W of RF power to a Ar/C2H2 mixture. The spectrum indicates + + the presence of H+, H+ 2 , CHn (0ono4), H2O , + + + C2Hn (0ono6), Ar , C3Hn (0ono8), and possibly C4H+ n (0ono10). The mass resolution is not sufficient
Fig. 4. Kinetic energy distributions of O+ 2 at: (a) 5 Pa, (b) 6 Pa, (c) 10 Pa, and (d) 14 Pa with 300 W of RF power.
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Fig. 5. TOF mass spectrum of positive ions detected from 500 W of RF power applied to an Ar/C2H2 (60/40) mixture. The total gas pressure was 15 Pa.
to distinguish the individual species in the various hydrocarbon series. This is due to the high kinetic energy of the ions. The spectrum suggests that C2H+ n (0ono6) are the predominant ionized species in the plasma. These results are also consistent with the FTIR results on polymerized films (not shown), which exhibit signal from CH2 and CH3 species on the surface. The results suggest that the conditions within the plasma, even at 500 W, are sufficiently gentle to produce C2H+ n (0ono6) ions, with fragmentation to produce CH+ n (0ono4) to be less probable. Shown in Fig. 6 are residual gas mass spectra data taken from the same plasma. The spectrum presented in Fig. 6(a) was acquired after the Ar/C2H2 mixture was fed into the plasma reactor, but with no RF power applied to the reactor. Peaks due to Ar+ and C2H+ n (n ¼ 0; 1; 2) are observed in the spectrum, as well as peaks due to OH+, H2O+, CO+, and CO+ 2 , all of which are typical of those detected in unbaked vacuum chambers. The spectrum presented in Fig. 6(b) was acquired with the same conditions used to acquire the data in Fig. 6(a), but 500 W of RF power was applied to the reactor. The most intriguing aspect of this spectrum is the virtual elimination of peaks associated with C2H2. The position of the C2H2 peak is indicated in Fig. 6(b) by the arrow. These results suggest that the plasma reactor is very efficient in dissociating the C2H2 feed gas. The significant decrease of C2Hn species in this spectrum is also attributed to the placement of the RGA system.
Fig. 6. RGA spectra from (a) Ar/C2H2 (60/40) mixture with no RF applied, (b) Ar/C2H2 mixture (60/40) with 500 W of applied RF, (c) O2 with no RF applied, and (d) O2 with 500 W of applied RF. In all cases, the total pressure was 15 Pa.
The RGA system is located in a non-line-of-sight position with respect to the plasma. Thus, species must undergo several collisions with vacuum chamber walls before being scattered into the RGA. At each collision there is a high probability that reactive species such as C2 or H will react (stick) and will thus not scatter into the RGA. The RGA system, therefore, is most sensitive to permanent gases such as CO and C2H2. By way of comparison, shown in Fig. 6(c) and (d) are RGA spectra that were acquired with O2 in the reactor (with no Ar/C2H2). Fig. 6(c) was acquired after O2 was fed into the plasma reactor, but no RF power was applied. The dominant peak is due to O2. The spectrum presented in Fig. 6(d) was acquired with the same conditions used to acquire the data in Fig. 6(c), but 500 W of RF was applied to the reactor. There is no detectable change in the O2 peak height after RF power was applied. These results suggest either that (1) electron impact dissociation of O2 is less efficient than dissociation of C2H2 under the reactor conditions, or that (2) recombination of O atoms is sufficiently efficient to produce a large concentration of molecule oxygen.
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4. Summary We have investigated the species present RF plasmas of O2 and Ar/C2H2. The results for O2 suggest that symmetric charge transfer reactions within the plasma sheath cause a decrease in the ion flux and in the mean ion kinetic energy with increasing pressure. The results also indicate that plasma polymerization efficiently dissociates the C2H2 feed gas and that ionization of C2H2 is accompanied by little extensive fragmentation. Finally, the results illustrate the limited usefulness of RGA analysis in such measurements.
Acknowledgements This work was funded by the National Science Foundation.
References Baer, T., Murray, P.T., 1981. Cross sections for symmetric charge and proton transfer reactions of internal energy selected NH3(v)+. J. Chem. Phys. 75, 4477.
Baer, T., Murray, P.T., Squires, L., 1978. Total cross sections for symmetric charge transfer reactions of O+ 2 in selected translational and internal energy states. J. Chem. Phys. 68, 4901–4906. Doyle, J.R., 1997. Chemical kinetics in low pressure acetylene radio frequency glow discharges. J. Appl. Phys. 82, 4763–4771. Lias, S.G., Bartmess, J.E., Liebman, J.F., Holmes, J.L., Levin, R.D., Mallard, W.G., 1988. J. Phys. Chem. Ref. Data 17, Suppl. 1. p. 634 McMaster, M.C., Hsu, W.L., Coltrin, M.E., Dandy, D.S., Fox, C., 1995. Dependence of the gas composition in a microwave plasma-assisted diamond chemical vapor deposition reactor on the inlet carbon source: CH4 versus C2H2. Diam. Relat. Mater. 4, 1000. Mizutani, N., Hayashi, T., 2000. Ion energy distribution at an r.f.-biased electrode in an inductively coupled plasma affected by collisions in a sheath. Thin Solid Films 374, 167–174. Okeke, L., Stori, H., 1991. Plasma-chemical decomposition of methane during diamond synthesis. Plasma Chem. Plasma Process. 11, 489–499. Samson, S., Rayborn, G.H., Pareek, P.N., 1982. Dissociative photoionization cross sections of O2 from threshold to ( J. Chem. Phys. 76, 393–397. 120 A.
Radiation Physics and Chemistry 68 (2003) 221–226
Dynamics of materials processing by RF plasmas P.T. Murraya,*, E. Shinb a
Research Institute and Graduate Materials Engineering, University of Dayton, Dayton, OH 45469-0160, USA b Electro Optics Program, University of Dayton, Dayton, OH 45469-0245, USA
Abstract We have investigated the species present in RF plasmas of O2 and Ar/C2H2. The results for O2 suggest that symmetric charge transfer reactions within the plasma sheath cause a decrease in the ion flux and in the mean ion kinetic energy with increasing pressure. The results also indicate that plasma polymerization efficiently dissociates the C2H2 feed gas and that ionization of C2H2 is accompanied by little fragmentation. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Plasma; Mass spectrometry; Charge transfer; Polymerization; Acetylene; Oxygen; Sheath
1. Introduction Plasmas contain numerous species including positive ions, negative ions, and neutrals, all in various degrees of internal excitation and with a range of kinetic energies. Acetylene polymerization plasmas typically contain a mixture of Ar (or other rare gas) and C2H2, the former being the majority species. The species expected to be present in such mixtures include Ar, C2H2, C2H2* (electronic, vibrational, rotational excita + tion), Ar*, Ar+, C2H+ 2 , C2H2 , C2H, C2H , etc). We have investigated the plasmas generated during plasma polymerization of acetylene and, because the first step in forming films of polymerized acetylene involves cleaning the substrate with an oxygen plasma, have extended the investigations to include oxygen plasmas. Very little has been reported on acetylene plasmas that are operated under plasma polymerization conditions. In contrast, there are numerous reports on RF and microwave plasmas that give diamond or diamondlike films. For example, molecular beam mass spectrometry was used (McMaster et al., 1995) to measure the gas-phase composition near a growing diamond surface in a microwave plasma-assisted chemical vapor deposition reactor. Reported were the dependencies of gas composition on changes in the carbon mole fraction in *Corresponding author. Fax: +1-937-229-3433. E-mail address: [email protected] (P.T. Murray).
the reactor feed, inlet carbon source (CH4 versus C2H2), and surface temperature. The gas composition was independent of inlet hydrocarbon source. Films grown using either CH4 or C2H2 had similar growth rates, morphology, and Raman spectra. The decomposition of CH4 in an RF diamond deposition plasma was investigated (Okeke and Stori, 1991) by mass spectrometry. Electron impact dissociation was suggested as the rate-determining step in the plasma-chemical decomposition of methane. Mass spectrometry was used (Doyle, 1997) to determine the product gas yields for an RF glow discharge of C2H2. Doyle proposed that film growth was dominated by radicals the C4H3, C6H3, and C2H under the high-power conditions suitable for depositing diamond-like carbon. The plasma polymerization of acetylene raises a number of questions. These include: (1) the composition of the plasma and the extent to which it changes with plasma conditions, (2) the identity of those species that are necessary for producing films with useful properties, (3) determining whether the key species are formed in the gas phase or at the growth surface, (4) the most important factors for producing ‘‘good’’ films, and (5) the best way to use this information to control the plasma. We have addressed these questions by carrying out time of flight mass spectrometry (TOFMS) and residual gas analysis on the plasmas generated during plasma polymerization of acetylene. This combination of characterization techniques is expected to give us
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00288-3
ARTICLE IN PRESS 222
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insight into the chemistry of acetylene plasmas. The eventual goal is to correlate information on the composition of the plasmas to information concerned with the molecular structure and properties of the films.
2. Experimental Shown in Fig. 1 is a schematic of the apparatus that was used for this investigation. The apparatus was attached to a large (B100 l), commercially available plasma reactor, and the plasma population was sampled through a 1 mm diameter orifice. Residual gas analysis involved ionization, by electron impact, of the neutral species present in a sample gas, followed by mass analysis of the positive ions thus generated. Mass analysis was carried out by using a quadrupole mass spectrometer with a mass range from 0 to 300 AMU and an electron kinetic energy that is programmable from 25 to 105 eV. These experiments provided information primarily on the neutral species in the plasma. The TOFMS system was mounted in an in-line configuration and included a (grounded) guard plate, filter plate, acceleration plate, second ground plate, and a field-free drift region. Following the drift region was a deflector that was biased at a large positive potential (typically +3 kV) and a microchannel plate (MCP) detector that was mounted off-axis in order to minimize the background signal due to UV photons and fast neutral particles generated in the plasma. Operation of the TOFMS entailed repetitively (30 kHz) applying a (+800 V, 200 ns) voltage pulse to the acceleration plate.
This caused positive ions that were located between the acceleration and ground plates to be accelerated toward the drift region. Once entering the drift region, the ions traveled with constant velocity and were deflected offaxis and into the MCP, where they were detected. The ion TOF was determined by using the leading edge of the acceleration pulse as the start and the ion pulse at the MCP as the stop to a time-to-digital converter. TOFMS spectra were acquired by collecting data for B1 min per spectrum. The TOFMS system was also equipped with a filter plate, whose purpose was to serve as a retarding potential analyzer. This allowed us to determine the ion kinetic energy distributions by recording TOFMS with various (positive) voltages applied to the filter plate, determining the total ion intensity at each applied filter voltage, and by taking the first derivative of the intensity-voltage data. The O2 plasma data were acquired with 300 W of RF power and with O2 pressures ranging from 6 to 14 Pa. The Ar/C2H2 data were acquired from a plasma that was formed with 500 W of RF power, with a 60/40 ratio of Ar/C2H2, and with a total gas pressure of 13 Pa.
3. Results and discussion 3.1. O2 plasmas Shown in Fig. 2 are TOFMS of the positive ions detected with various O2 pressures in the reactor. The spectra exhibit an intense peak due to O+ and 2
Fig. 1. Schematic of the experimental setup.
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sheath to a final kinetic energy that is less than of the primary ion. At the pressures at which our experiments took place (several Pa), it is expected that several sequential charge transfer reactions, per incident ion, will take place within the sheath. The final ionic product from such a sequence will have a velocity that is considerably less than the initial ion velocity. In fact, the final ion may have a velocity that approaches thermal velocity, and such an ionic product may have a small probability of escaping the sheath region. The O+ signal, in contrast to O+ 2 , increases with increasing pressure. This can be understood by realizing that the most probable collision partner of O+ is O2. At low kinetic energies (less than several keV’s), the crosssection for charge transfer is a sensitive function of the energy defect in the reaction. Thus, resonant symmetric charge transfer reactions (for which the energy defect is identically zero) have the largest cross sections. In contrast, the energy defect for the asymmetric charge transfer reaction Oþ þ O2 -O þ Oþ 2 is 1.55 eV (Lias et al., 1988), suggesting that such a reaction will have a significantly smaller cross-section. In addition, since the concentration of neutral atomic O is small with our experimental conditions, neutralization of O+ through the symmetric reaction Oþ þ O-O þ Oþ
Fig. 2. TOF mass spectra of the positive ions detected with 300 W of RF power at O2 pressures of: (a) 6 Pa, (b) 8 Pa, (c) 10 Pa, (d) 12 Pa, and (e) 14 Pa.
considerably smaller peaks due to O+ and O+ 3 . Within the plasma, positive ions are generated by electron impact. The small O+/O+ 2 peak ratio (o1%) indicates that the mean electron kinetic energy in the plasma is marginally above the 18.9 eV threshold for O+ formation from O2 (Samson et al., 1982). The data of Fig. 2 show a monotonic decrease in the O+ signal with 2 increasing O2 pressure. The decrease in O+ intensity is attributed to 2 symmetric charge transfer reactions within the plasma sheath. The plasma sheath is a region of space at the plasma/chamber wall interface across which the plasma potential is applied. Charge transfer reactions are represented in Reaction (1). þ Oþ 2f þ O2 -O2f þ O2 :
ð1Þ
ion (denoted Oþ In these reactions, a fast 2f ) interacts with a thermal energy O2 within the sheath. An electron is transferred from the (slow) neutral to the (fast) ion, resulting in the formation of a fast neutral species and a nascent ion which is accelerated within the O+ 2
is expected to be minimal. Thus, because of the significantly lower probability for charge transfer to occur with O+, its intensity can be used as a reference to monitor the initial O+ 2 concentration within the plasma. Such an argument suggests that we can model the plasma sheath as a reaction cell, and that a plot of + ln(I[O+ 2 ]/I[O ]) vs pressure will be linear with a slope equal to sx: In this case, s is the cross-section for the charge transfer reaction, and x is the length of the sheath. Baer and Murray (1981) used a similar procedure to explore the dynamics of charge and proton + transfer reactions of NH+ 3 . The quantities I[O2 ] and + I[O ] are the intensities of the molecular and atomic oxygen ions, respectively. Shown in Fig. 3 is such a plot, which is clearly linear. From the slope of the line (0.439 Pa1), we can estimate the length of the plasma sheath if we make the simplifying assumption that the symmetric charge transfer cross-section for O+ is 2 constant (i.e. independent of kinetic energy). The kinetic energy (as well as vibrational state) dependence of the symmetric charge transfer cross-section have been reported previously (Baer et al., 1978). Using a value (Baer et al., 1978) for s of 2.5 1019 m2 yields a value of 7.3 mm as the length of the plasma sheath. This number is approximately a factor of two smaller than the value reported (Mizutani and Hayashi, 2000) previously for an inductively coupled Ar/O2 plasma.
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Nevertheless, it illustrates that our approach is reasonable. A more thorough analysis, in which the kinetic energy dependence of s is explicitly taken into account, is currently underway. An additional factor that characterizes a plasma is the kinetic energy distribution of the ions. If our charge
transfer hypothesis is correct, there should be a decrease in the mean O+ 2 kinetic energy as the pressure in the reactor increases. Shown in Fig. 4 are the kinetic energy distributions of O+ 2 that were recorded at the indicated pressures. The data indicate that the most probable O+ 2 kinetic energy decreases from a value of B300 eV at 5 Pa to less than 50 eV at 14 Pa. It is also interesting to note that the distributions become more Maxwellian with increasing pressure. These results represent the ionized species that leave the plasma reactor. These species are also those that strike the surface of a substrate placed within the plasma reactor. This suggests that substrate surfaces are exposed to a flux of molecular ions (and neutrals) during cleaning in an O2 plasma and that the mean kinetic energy and flux of the ionized species decrease with increasing pressure. 3.2. Ar/C2H2
+ Fig. 3. Plot of ln(I[O+ 2 ]/I[O ]) versus pressure of O2 with 300 W of RF power.
Thin films of polymerized acetylene are formed by plasma polymerization of a mixture of Ar and C2H2. Shown in Fig. 5 is a TOFMS of the positive ions detected from a plasma formed by applying 500 W of RF power to a Ar/C2H2 mixture. The spectrum indicates + + the presence of H+, H+ 2 , CHn (0ono4), H2O , + + + C2Hn (0ono6), Ar , C3Hn (0ono8), and possibly C4H+ n (0ono10). The mass resolution is not sufficient
Fig. 4. Kinetic energy distributions of O+ 2 at: (a) 5 Pa, (b) 6 Pa, (c) 10 Pa, and (d) 14 Pa with 300 W of RF power.
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Fig. 5. TOF mass spectrum of positive ions detected from 500 W of RF power applied to an Ar/C2H2 (60/40) mixture. The total gas pressure was 15 Pa.
to distinguish the individual species in the various hydrocarbon series. This is due to the high kinetic energy of the ions. The spectrum suggests that C2H+ n (0ono6) are the predominant ionized species in the plasma. These results are also consistent with the FTIR results on polymerized films (not shown), which exhibit signal from CH2 and CH3 species on the surface. The results suggest that the conditions within the plasma, even at 500 W, are sufficiently gentle to produce C2H+ n (0ono6) ions, with fragmentation to produce CH+ n (0ono4) to be less probable. Shown in Fig. 6 are residual gas mass spectra data taken from the same plasma. The spectrum presented in Fig. 6(a) was acquired after the Ar/C2H2 mixture was fed into the plasma reactor, but with no RF power applied to the reactor. Peaks due to Ar+ and C2H+ n (n ¼ 0; 1; 2) are observed in the spectrum, as well as peaks due to OH+, H2O+, CO+, and CO+ 2 , all of which are typical of those detected in unbaked vacuum chambers. The spectrum presented in Fig. 6(b) was acquired with the same conditions used to acquire the data in Fig. 6(a), but 500 W of RF power was applied to the reactor. The most intriguing aspect of this spectrum is the virtual elimination of peaks associated with C2H2. The position of the C2H2 peak is indicated in Fig. 6(b) by the arrow. These results suggest that the plasma reactor is very efficient in dissociating the C2H2 feed gas. The significant decrease of C2Hn species in this spectrum is also attributed to the placement of the RGA system.
Fig. 6. RGA spectra from (a) Ar/C2H2 (60/40) mixture with no RF applied, (b) Ar/C2H2 mixture (60/40) with 500 W of applied RF, (c) O2 with no RF applied, and (d) O2 with 500 W of applied RF. In all cases, the total pressure was 15 Pa.
The RGA system is located in a non-line-of-sight position with respect to the plasma. Thus, species must undergo several collisions with vacuum chamber walls before being scattered into the RGA. At each collision there is a high probability that reactive species such as C2 or H will react (stick) and will thus not scatter into the RGA. The RGA system, therefore, is most sensitive to permanent gases such as CO and C2H2. By way of comparison, shown in Fig. 6(c) and (d) are RGA spectra that were acquired with O2 in the reactor (with no Ar/C2H2). Fig. 6(c) was acquired after O2 was fed into the plasma reactor, but no RF power was applied. The dominant peak is due to O2. The spectrum presented in Fig. 6(d) was acquired with the same conditions used to acquire the data in Fig. 6(c), but 500 W of RF was applied to the reactor. There is no detectable change in the O2 peak height after RF power was applied. These results suggest either that (1) electron impact dissociation of O2 is less efficient than dissociation of C2H2 under the reactor conditions, or that (2) recombination of O atoms is sufficiently efficient to produce a large concentration of molecule oxygen.
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P.T. Murray, E. Shin / Radiation Physics and Chemistry 68 (2003) 221–226
4. Summary We have investigated the species present RF plasmas of O2 and Ar/C2H2. The results for O2 suggest that symmetric charge transfer reactions within the plasma sheath cause a decrease in the ion flux and in the mean ion kinetic energy with increasing pressure. The results also indicate that plasma polymerization efficiently dissociates the C2H2 feed gas and that ionization of C2H2 is accompanied by little extensive fragmentation. Finally, the results illustrate the limited usefulness of RGA analysis in such measurements.
Acknowledgements This work was funded by the National Science Foundation.
References Baer, T., Murray, P.T., 1981. Cross sections for symmetric charge and proton transfer reactions of internal energy selected NH3(v)+. J. Chem. Phys. 75, 4477.
Baer, T., Murray, P.T., Squires, L., 1978. Total cross sections for symmetric charge transfer reactions of O+ 2 in selected translational and internal energy states. J. Chem. Phys. 68, 4901–4906. Doyle, J.R., 1997. Chemical kinetics in low pressure acetylene radio frequency glow discharges. J. Appl. Phys. 82, 4763–4771. Lias, S.G., Bartmess, J.E., Liebman, J.F., Holmes, J.L., Levin, R.D., Mallard, W.G., 1988. J. Phys. Chem. Ref. Data 17, Suppl. 1. p. 634 McMaster, M.C., Hsu, W.L., Coltrin, M.E., Dandy, D.S., Fox, C., 1995. Dependence of the gas composition in a microwave plasma-assisted diamond chemical vapor deposition reactor on the inlet carbon source: CH4 versus C2H2. Diam. Relat. Mater. 4, 1000. Mizutani, N., Hayashi, T., 2000. Ion energy distribution at an r.f.-biased electrode in an inductively coupled plasma affected by collisions in a sheath. Thin Solid Films 374, 167–174. Okeke, L., Stori, H., 1991. Plasma-chemical decomposition of methane during diamond synthesis. Plasma Chem. Plasma Process. 11, 489–499. Samson, S., Rayborn, G.H., Pareek, P.N., 1982. Dissociative photoionization cross sections of O2 from threshold to ( J. Chem. Phys. 76, 393–397. 120 A.