- Email: [email protected]
Study of the surface mechanisms in an Ar–N2 post-discharge cleaning process
Surface and Coatings Technology 169 – 170 (2003) 181–185
Study of the surface mechanisms in an Ar–N2 post-discharge cleaning process a ´ ´ David Mezerette , Thierry Belmontea,*, Robert Hugonb, Gerard Henrionb, Thierry Czerwieca, Henry Michela a
´ Laboratoire de Science et Genie des Surfaces (U.M.R. CNRS-EDF 7570), Ecole des Mines, Parc de Saurupt, F-54042 Nancy, Cedex, France b ´ et Applications (U.M.R. CNRS 7040) Universite´ Henri Poincare´ Nancy I, B.P. 239, Laboratoire de Physique des Milieux Ionises ` Nancy, France Boulevard des Aiguillettes F-54506 Vandoeuvre-les
Abstract The purpose of the present investigation is to gain understanding of the mechanisms responsible for the cleaning properties of Ar–N2 post-discharges at room temperature. The evolution of the gas-phase composition is monitored by optical emission spectroscopy, and X-ray photoelectron spectroscopy is used to characterize the surface of iron samples initially covered by a contamination layer at various treatment times. Both techniques suggest that cleaning is at least a two-step process, beginning with the removal of aliphatic carbon from the surface. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Impurities; Glow discharge optical spectroscopy; Photoelectron spectroscopy; Nitrogen; Iron oxide; Plasma cleaning
1. Introduction In the range of applications of plasma technologies, the use of flowing afterglows in several gas mixtures (Ar–N2 w1x, Ar–O2 w2x and Ar–N2 –O2 w3x) for surface cleaning purpose has recently undergone many investigations. The mechanism on which such a cleaning process is based is still the purpose of many investigations. The reactive species—such as N or O atoms— that are present in post-discharges intervene in this process, but in a way which is not clear yet. Other phenomena have to be considered: UV emission w3x and heat w2,4x. Indeed, at room temperature, Ar–N2 postdischarges can even passivate the surface of iron substrates, probably by reticulation of the carbon-based impurities adsorbed on the surface w4x. In order to understand the mechanisms of either cleaning or passivation, reactions occurring on the surface have to be identified. It is the aim of the experimental work presented herein.
2. Experimental set-up Treatment is performed in the Lewis–Rayleigh afterglow of an Ar–5% N2 low pressure microwave plasma on 15=15 mm2 pure iron foils (99.5 wt.%). The apparatus used for treatment is represented in Fig. 1. A more complete description is given elsewhere w4x. The experimental conditions are the following: total rate flow: 1050 sccm; pressure: 500 Pa; microwave power: 130 W. Before introduction into the reactor, samples are cleaned with methanol, then with acetone and dried, in order to guarantee the reproducibility of the surface state. It is here important to notice that the surface of the samples is not made of pure iron, but of a layer of native iron oxide Fe2 –O3, as was previously confirmed by X-ray photoelectron spectroscopy (XPS) analyses w1 x . Optical emission spectroscopy (OES), carried out on the first positive system of nitrogen—known to account for the recombination of nitrogen in the gas phase by the three-body process: N(4S)qN(4S)qM™N2qMqhn
*Corresponding author. Tel.: q33-3-83-58-4091; fax: q33-3-8353-4764. E-mail address: [email protected] (T. Belmonte).
(1)
and CN bands, is used to investigate the composition of the gas phase above the substrate. At given treatment
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 0 4 1 - 0
182
´ D. Mezerette et al. / Surface and Coatings Technology 169 – 170 (2003) 181–185
Fig. 1. Experimental apparatus.
times, the plasma is turned off, and the substrate is transferred into the XPS analysis chamber for the determination of its surface composition w4x. Four elements have been studied by XPS: Fe 2p, C 1s, O 1s and N 1s. Atomic ratios are calculated by integrating the respective peak areas, corrected by the appropriate sensitivity factor, after a base-line derivation using the Shirley procedure. Gauss–Lorentz peak fitting functions are applied to the peaks, whence binding energies and contributions due to the differences in chemical environment are derived. Binding energies, and chemical bonds to which they are attributed are given in Table 1. Because the N 1s peak can be fitted almost perfectly by a single Gauss–Lorentz curve with a full width at half maximum of 2.95 eV, no accurate information about the nitrogen chemical environment could be gained.
3. Results and discussion The times at which the treatment is stopped are chosen according to changes in the gas phase composition observed by OES. Typical evolutions of the intensities of CN bands and of the first positive system of nitrogen and the times for XPS analyses are shown in Fig. 2. During the first seconds of the treatment, a sharp decrease of the intensity of both emissions is observed. While the CN emission almost disappears, the intensity of the first positive system of nitrogen goes on diminishing. Four minutes after the beginning of the treatment, this decrease lowers, and an XPS analysis is performed. This sample is referred to as S1, S0 corresponding to the untreated surface. A second step occurs at approximately 8 min, with the interesting rise of the first positive system of nitrogen, followed by another fall. At the end
Table 1 Possible bonds identified by XPS Element
Binding energy (eV)
Compound
References
C 1s
285.0 286.5 288.5 289.5
C–C, C–H (aliphatic carbon) C–O (alcohol, ether) C–N C_ O (ketone), N–C_ O O–C_ O (ester)
w5–8,10x w8,10x w5,10,12x w6x
O 1s
530.0 531.5 532.4
O in Fe2 –O3 FeOH, C_ O FeOOH, H2O ads.
w5,13,14x w5x w11x
N 1s
399.7
N–C
w10x
´ D. Mezerette et al. / Surface and Coatings Technology 169 – 170 (2003) 181–185
183
Fig. 2. Evolution with treatment time of OES intensities.
of this step, that is 15 min after the beginning of treatment, another XPS analysis is performed (S2). The last XPS analysis (S3) is done at the steady-state, after 30 min. Eq. (1) describes the main loss process for N atoms in the afterglow. N atom depletion occurs through recombination on the reactor walls and on the sample surface, and through chemical reactions with species on the surface as well. If one considers the model developed by Kim and Boudart w9x of gas-phase atoms recombining with chemisorbed atoms by either the Eley–Rideal or the Langmuir–Hinshelwood mechanism, the recombination time involved is between 3 and 4 orders of magnitude shorter than necessary to explain the phenomena observed in Fig. 2. It may be concluded here that the cleaning mechanism involves effects with longer time scales than chemical etching by nitrogen. The evolution of the surface composition derived from XPS analyses for Fe 2p, C 1s, O 1s and N 1s by integrating the peak areas, assuming an homogeneous excitation zone, is drawn on Fig. 3. Sample S0 is obviously covered by a contamination layer, the importance of which decreases with treatment time, as shown by the decrease of the carbon peak and the increase in Fe- and O- surface contents, as more photoelectrons originate from the Fe2O3 layer. After 15 min, no further evolution of surface composition is observed. However, it is easily deduced from the composition of S3 that, at room temperature, cleaning is not yet complete. It was indeed demonstrated in a previous paper w4x that an additional heating at 325 K is sufficient for removing the remaining impurities. A clearer insight into cleaning mechanisms can be gained by comparing, for each binding energy, the ratio
of peak areas of samples S0, S1, S2, S3 to the maximum peak area (Fig. 4). Those results put forward the fact that cleaning is at least a two-step process, as already deduced from Fig. 2. Due to the choice of the times for the XPS analyses, the surface composition of the iron foil is not known between 4 and 8 min. Nevertheless, XPS analysis clearly shows that the etching of aliphatic carbon has almost ended after 4 min, i.e. before the beginning of the second step previously observed by OES. In Fig. 4, all the XPS peaks evolve as that of nitrogen on the surface, except the C 1s peak at 285.0 eV. During the first seconds of the treatment, the aliphatic carbon contribution strongly decreases, matching the CN emission observed by OES. N atoms are incorporated
Fig. 3. Evolution with treatment time of elemental surface composition determined by XPS.
184
´ D. Mezerette et al. / Surface and Coatings Technology 169 – 170 (2003) 181–185
Fig. 4. Evolution with treatment time of chemical bonds determined by XPS.
into the contamination layer. C_ O contribution remains constant during the first 4 min of the treatment. Analysis of samples S2 and S3 shows that nitrogen atoms do not graft permanently on the surface. They are quickly eliminated and a simultaneous break of C–O bonds occurs. It is yet to determine how the N and O atoms removal from the surface are linked. The following mechanism can be proposed: in a first step, which lasts for approximately 4 min, aliphatic carbon is removed under CN radicals. The second step, which arises in Fig. 2 at approximately 8 min could be described as a new phase of contamination and cleaning. The intermediary phase between these two-steps very likely corresponds to changes in the chemical structure of pollutants, as O–C_ O bonds appear, by a process which is not yet identified. We believe it could be due to continuous deposition of energy by metastable species, or by bonds created by nitrogen grafting. This would explain why cleaning takes such a long time. The induced modification of carbonaceous groups create new chemical functions which are responsible for the second cleaning step. Further experiments are in progress to validate this assumption. Surface compositions of samples S2 and S3 differ only slightly, which proves that a stable layer is formed during treatment at room temperature, which has a passivating effect, as previously demonstrated w4x. The observed changes may indicate a reconstruction of the carbonaceous species on the surface. 4. Conclusion Iron samples whose surface was covered by a contamination layer have been treated at room temperature in
an Ar–N2 flowing post-discharge. Gas-phase composition was monitored by OES, and surface composition of samples at different treatment times were analysed by XPS. A correlation between results brought by those two techniques was established, and the following conclusion was drawn: the cleaning of carbonaceous impurities by an Ar–N2 post-discharge is clearly identified as a two-step process: (1) during the first minute of treatment, corresponding to strong CN-bands emission, most of carbon which is not chemically bound to oxygen is removed from the surface, the reconstruction of which enables the grafting of nitrogen atoms. (2) In a second step, the grafted nitrogen is eliminated too, alongside with oxygen, but the mechanisms responsible for these effects are not clear yet, and needs further investigation.
References w1x L. Lefevre, ` T. Belmonte, T. Czerwiec, A. Ricard, H. Michel, Appl. Surf. Sci. 153 (2000) 85. w2x M. Gaillard, P. Raynaud, A. Ricard, Plasma Polymers 4 (2–3) (1999) 241. w3x S. Moreau, M. Moisan, T. Tabrizian, et al., J. Appl. Phys. 88 (2000) 1966. w4x D. Mezerette, ´ T. Belmonte, R. Hugon, T. Czerwiec, G. Henrion, H. Michel, Surf. Coat. Technol. 142–144 (2001) 761. w5x D.R. Cousens, B.J. Wood, J.Q. Wang, A. Allen, Surf. Interface Anal. 29 (2000) 23. w6x B.W. Callen, M.L. Ridge, S. Lahooti, A.W. Neumann, R.N.S. Sodhi, J. Vac. Sci. Technol. A 13 (1995) 2023. w7x J.M. Burkstrand, J. Vac. Sci. Technol. 16 (1979) 363. w8x M. Tabbal, P. Merel, ´ S. Moisa, et al., Surf. Coat. Technol. 98 (1998) 1092. w9x Y.C. Kim, M. Boudart, Langmuir 7 (1991) 2999.
´ D. Mezerette et al. / Surface and Coatings Technology 169 – 170 (2003) 181–185 w10x S. Vallon, A. Hofrichter, L. Guyot, J. Adhea, Sci. Technol. 10– 12 (1996) 1287. w11x D.L. Cocke, M. Jurcik-Rajman, C. Veprek, J. Electrochem. Soc. 136–12 (1989) 3655. w12x T.H. Lee, J.W. Rabalais, J. Electron Spectrosc. Relat. Phenom. 11 (1977) 123.
185
w13x C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, in: G.E. Muilenberg (Ed.), Handbook of X-Ray Photoelectron Spectroscopy, First ed., Perkin-Elmer Corporation (Physical Electronics), 1979, 38 (for carbon) 42 (for oxygen) 76 (for iron). w14x D. Brion, Appl. Surf. Sci. 5 (1980) 133.
Study of the surface mechanisms in an Ar–N2 post-discharge cleaning process a ´ ´ David Mezerette , Thierry Belmontea,*, Robert Hugonb, Gerard Henrionb, Thierry Czerwieca, Henry Michela a
´ Laboratoire de Science et Genie des Surfaces (U.M.R. CNRS-EDF 7570), Ecole des Mines, Parc de Saurupt, F-54042 Nancy, Cedex, France b ´ et Applications (U.M.R. CNRS 7040) Universite´ Henri Poincare´ Nancy I, B.P. 239, Laboratoire de Physique des Milieux Ionises ` Nancy, France Boulevard des Aiguillettes F-54506 Vandoeuvre-les
Abstract The purpose of the present investigation is to gain understanding of the mechanisms responsible for the cleaning properties of Ar–N2 post-discharges at room temperature. The evolution of the gas-phase composition is monitored by optical emission spectroscopy, and X-ray photoelectron spectroscopy is used to characterize the surface of iron samples initially covered by a contamination layer at various treatment times. Both techniques suggest that cleaning is at least a two-step process, beginning with the removal of aliphatic carbon from the surface. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Impurities; Glow discharge optical spectroscopy; Photoelectron spectroscopy; Nitrogen; Iron oxide; Plasma cleaning
1. Introduction In the range of applications of plasma technologies, the use of flowing afterglows in several gas mixtures (Ar–N2 w1x, Ar–O2 w2x and Ar–N2 –O2 w3x) for surface cleaning purpose has recently undergone many investigations. The mechanism on which such a cleaning process is based is still the purpose of many investigations. The reactive species—such as N or O atoms— that are present in post-discharges intervene in this process, but in a way which is not clear yet. Other phenomena have to be considered: UV emission w3x and heat w2,4x. Indeed, at room temperature, Ar–N2 postdischarges can even passivate the surface of iron substrates, probably by reticulation of the carbon-based impurities adsorbed on the surface w4x. In order to understand the mechanisms of either cleaning or passivation, reactions occurring on the surface have to be identified. It is the aim of the experimental work presented herein.
2. Experimental set-up Treatment is performed in the Lewis–Rayleigh afterglow of an Ar–5% N2 low pressure microwave plasma on 15=15 mm2 pure iron foils (99.5 wt.%). The apparatus used for treatment is represented in Fig. 1. A more complete description is given elsewhere w4x. The experimental conditions are the following: total rate flow: 1050 sccm; pressure: 500 Pa; microwave power: 130 W. Before introduction into the reactor, samples are cleaned with methanol, then with acetone and dried, in order to guarantee the reproducibility of the surface state. It is here important to notice that the surface of the samples is not made of pure iron, but of a layer of native iron oxide Fe2 –O3, as was previously confirmed by X-ray photoelectron spectroscopy (XPS) analyses w1 x . Optical emission spectroscopy (OES), carried out on the first positive system of nitrogen—known to account for the recombination of nitrogen in the gas phase by the three-body process: N(4S)qN(4S)qM™N2qMqhn
*Corresponding author. Tel.: q33-3-83-58-4091; fax: q33-3-8353-4764. E-mail address: [email protected] (T. Belmonte).
(1)
and CN bands, is used to investigate the composition of the gas phase above the substrate. At given treatment
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 0 4 1 - 0
182
´ D. Mezerette et al. / Surface and Coatings Technology 169 – 170 (2003) 181–185
Fig. 1. Experimental apparatus.
times, the plasma is turned off, and the substrate is transferred into the XPS analysis chamber for the determination of its surface composition w4x. Four elements have been studied by XPS: Fe 2p, C 1s, O 1s and N 1s. Atomic ratios are calculated by integrating the respective peak areas, corrected by the appropriate sensitivity factor, after a base-line derivation using the Shirley procedure. Gauss–Lorentz peak fitting functions are applied to the peaks, whence binding energies and contributions due to the differences in chemical environment are derived. Binding energies, and chemical bonds to which they are attributed are given in Table 1. Because the N 1s peak can be fitted almost perfectly by a single Gauss–Lorentz curve with a full width at half maximum of 2.95 eV, no accurate information about the nitrogen chemical environment could be gained.
3. Results and discussion The times at which the treatment is stopped are chosen according to changes in the gas phase composition observed by OES. Typical evolutions of the intensities of CN bands and of the first positive system of nitrogen and the times for XPS analyses are shown in Fig. 2. During the first seconds of the treatment, a sharp decrease of the intensity of both emissions is observed. While the CN emission almost disappears, the intensity of the first positive system of nitrogen goes on diminishing. Four minutes after the beginning of the treatment, this decrease lowers, and an XPS analysis is performed. This sample is referred to as S1, S0 corresponding to the untreated surface. A second step occurs at approximately 8 min, with the interesting rise of the first positive system of nitrogen, followed by another fall. At the end
Table 1 Possible bonds identified by XPS Element
Binding energy (eV)
Compound
References
C 1s
285.0 286.5 288.5 289.5
C–C, C–H (aliphatic carbon) C–O (alcohol, ether) C–N C_ O (ketone), N–C_ O O–C_ O (ester)
w5–8,10x w8,10x w5,10,12x w6x
O 1s
530.0 531.5 532.4
O in Fe2 –O3 FeOH, C_ O FeOOH, H2O ads.
w5,13,14x w5x w11x
N 1s
399.7
N–C
w10x
´ D. Mezerette et al. / Surface and Coatings Technology 169 – 170 (2003) 181–185
183
Fig. 2. Evolution with treatment time of OES intensities.
of this step, that is 15 min after the beginning of treatment, another XPS analysis is performed (S2). The last XPS analysis (S3) is done at the steady-state, after 30 min. Eq. (1) describes the main loss process for N atoms in the afterglow. N atom depletion occurs through recombination on the reactor walls and on the sample surface, and through chemical reactions with species on the surface as well. If one considers the model developed by Kim and Boudart w9x of gas-phase atoms recombining with chemisorbed atoms by either the Eley–Rideal or the Langmuir–Hinshelwood mechanism, the recombination time involved is between 3 and 4 orders of magnitude shorter than necessary to explain the phenomena observed in Fig. 2. It may be concluded here that the cleaning mechanism involves effects with longer time scales than chemical etching by nitrogen. The evolution of the surface composition derived from XPS analyses for Fe 2p, C 1s, O 1s and N 1s by integrating the peak areas, assuming an homogeneous excitation zone, is drawn on Fig. 3. Sample S0 is obviously covered by a contamination layer, the importance of which decreases with treatment time, as shown by the decrease of the carbon peak and the increase in Fe- and O- surface contents, as more photoelectrons originate from the Fe2O3 layer. After 15 min, no further evolution of surface composition is observed. However, it is easily deduced from the composition of S3 that, at room temperature, cleaning is not yet complete. It was indeed demonstrated in a previous paper w4x that an additional heating at 325 K is sufficient for removing the remaining impurities. A clearer insight into cleaning mechanisms can be gained by comparing, for each binding energy, the ratio
of peak areas of samples S0, S1, S2, S3 to the maximum peak area (Fig. 4). Those results put forward the fact that cleaning is at least a two-step process, as already deduced from Fig. 2. Due to the choice of the times for the XPS analyses, the surface composition of the iron foil is not known between 4 and 8 min. Nevertheless, XPS analysis clearly shows that the etching of aliphatic carbon has almost ended after 4 min, i.e. before the beginning of the second step previously observed by OES. In Fig. 4, all the XPS peaks evolve as that of nitrogen on the surface, except the C 1s peak at 285.0 eV. During the first seconds of the treatment, the aliphatic carbon contribution strongly decreases, matching the CN emission observed by OES. N atoms are incorporated
Fig. 3. Evolution with treatment time of elemental surface composition determined by XPS.
184
´ D. Mezerette et al. / Surface and Coatings Technology 169 – 170 (2003) 181–185
Fig. 4. Evolution with treatment time of chemical bonds determined by XPS.
into the contamination layer. C_ O contribution remains constant during the first 4 min of the treatment. Analysis of samples S2 and S3 shows that nitrogen atoms do not graft permanently on the surface. They are quickly eliminated and a simultaneous break of C–O bonds occurs. It is yet to determine how the N and O atoms removal from the surface are linked. The following mechanism can be proposed: in a first step, which lasts for approximately 4 min, aliphatic carbon is removed under CN radicals. The second step, which arises in Fig. 2 at approximately 8 min could be described as a new phase of contamination and cleaning. The intermediary phase between these two-steps very likely corresponds to changes in the chemical structure of pollutants, as O–C_ O bonds appear, by a process which is not yet identified. We believe it could be due to continuous deposition of energy by metastable species, or by bonds created by nitrogen grafting. This would explain why cleaning takes such a long time. The induced modification of carbonaceous groups create new chemical functions which are responsible for the second cleaning step. Further experiments are in progress to validate this assumption. Surface compositions of samples S2 and S3 differ only slightly, which proves that a stable layer is formed during treatment at room temperature, which has a passivating effect, as previously demonstrated w4x. The observed changes may indicate a reconstruction of the carbonaceous species on the surface. 4. Conclusion Iron samples whose surface was covered by a contamination layer have been treated at room temperature in
an Ar–N2 flowing post-discharge. Gas-phase composition was monitored by OES, and surface composition of samples at different treatment times were analysed by XPS. A correlation between results brought by those two techniques was established, and the following conclusion was drawn: the cleaning of carbonaceous impurities by an Ar–N2 post-discharge is clearly identified as a two-step process: (1) during the first minute of treatment, corresponding to strong CN-bands emission, most of carbon which is not chemically bound to oxygen is removed from the surface, the reconstruction of which enables the grafting of nitrogen atoms. (2) In a second step, the grafted nitrogen is eliminated too, alongside with oxygen, but the mechanisms responsible for these effects are not clear yet, and needs further investigation.
References w1x L. Lefevre, ` T. Belmonte, T. Czerwiec, A. Ricard, H. Michel, Appl. Surf. Sci. 153 (2000) 85. w2x M. Gaillard, P. Raynaud, A. Ricard, Plasma Polymers 4 (2–3) (1999) 241. w3x S. Moreau, M. Moisan, T. Tabrizian, et al., J. Appl. Phys. 88 (2000) 1966. w4x D. Mezerette, ´ T. Belmonte, R. Hugon, T. Czerwiec, G. Henrion, H. Michel, Surf. Coat. Technol. 142–144 (2001) 761. w5x D.R. Cousens, B.J. Wood, J.Q. Wang, A. Allen, Surf. Interface Anal. 29 (2000) 23. w6x B.W. Callen, M.L. Ridge, S. Lahooti, A.W. Neumann, R.N.S. Sodhi, J. Vac. Sci. Technol. A 13 (1995) 2023. w7x J.M. Burkstrand, J. Vac. Sci. Technol. 16 (1979) 363. w8x M. Tabbal, P. Merel, ´ S. Moisa, et al., Surf. Coat. Technol. 98 (1998) 1092. w9x Y.C. Kim, M. Boudart, Langmuir 7 (1991) 2999.
´ D. Mezerette et al. / Surface and Coatings Technology 169 – 170 (2003) 181–185 w10x S. Vallon, A. Hofrichter, L. Guyot, J. Adhea, Sci. Technol. 10– 12 (1996) 1287. w11x D.L. Cocke, M. Jurcik-Rajman, C. Veprek, J. Electrochem. Soc. 136–12 (1989) 3655. w12x T.H. Lee, J.W. Rabalais, J. Electron Spectrosc. Relat. Phenom. 11 (1977) 123.
185
w13x C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, in: G.E. Muilenberg (Ed.), Handbook of X-Ray Photoelectron Spectroscopy, First ed., Perkin-Elmer Corporation (Physical Electronics), 1979, 38 (for carbon) 42 (for oxygen) 76 (for iron). w14x D. Brion, Appl. Surf. Sci. 5 (1980) 133.