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Reactive sputtering of iron in Ar–N2 and Ar–O2 mixtures
Surface & Coatings Technology 200 (2005) 431 – 434 www.elsevier.com/locate/surfcoat
Reactive sputtering of iron in Ar–N2 and Ar–O2 mixtures C. Petitjeana, C. Rousselota, J.F. Piersona,T, A. Billardb a
De´partement CREST, Institut FEMTO-ST (UMR 6174), Universite´ de Franche-Comte´, Poˆle Universitaire, BP 71427, 25211 Montbe´liard Cedex, France b Laboratoire de Science et Ge´nie des Surfaces (UMR 7570), Ecoles des Mines, Parc de Saurupt, 54042 Nancy, France Available online 9 March 2005
Abstract This paper reports on the preparation of iron nitride and iron oxide films by DC reactive magnetron sputtering. The reactivity of the Fe–N2 and Fe–O2 systems is compared by the measurement of the total pressure in the deposition chamber, the iron target voltage and the films’ growth rate as a function of the reactive gas flow rate. The Fe–N2 system does not exhibit an instability phenomenon and the total pressure is quite proportional to the nitrogen flow rate. This system exhibits a low reactivity as confirmed by the effect of the nitrogen flow rate on the films’ growth rate. On the other hand, a hysteresis loop occurs for the Fe–O2 system, indicating a higher reactivity for this last system. D 2005 Elsevier B.V. All rights reserved. Keywords: Reactive sputtering; Iron nitride; Iron oxide
1. Introduction Reactive magnetron sputtering is a well-known process to synthesise various metallic oxide, nitride or carbide films using a metallic target and a reactive gas mixture (i.e., Ar– O2, Ar–N2 or Ar–CH4) [1,2]. Thus, this method has been widely used for mechanical, optical, electronic or magnetic applications. Furthermore, this process can be used to deposit coatings with complex structure such as multilayers [3], nanocomposites [4], to synthesise materials that cannot be formed using other methods [5] or to deposit oxynitride films [6]. Iron-based coatings (nitrides and oxides) have been widely studied for their magnetic properties (see for example Refs. [7–10]). However, to the best of our knowledge, there is no paper related to the elaboration and to the characterisation of iron oxynitride coatings. In order to deposit iron oxynitride films, we synthesise firstly iron nitride and iron oxide films. Thus, the aim of this paper
T Corresponding author. Tel.: +33 3 81 99 46 72; fax: +33 3 81 99 46 73. E-mail address: [email protected] (J.F. Pierson). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.028
is to compare the reactivity of sputtered iron atoms in Ar–N2 or Ar–O2 reactive mixtures.
2. Experiment Reactive sputtering experiments were performed using Alliance Concept AC450 sputter equipment with a vacuum chamber volume of about 70 l. Base pressure of 10 5 Pa was obtained with a turbomolecular pump backed with a mechanical one. The working Ar pressure was kept constant at 0.3 Pa using mass flow rate controllers and a constant pumping speed S = 10 L s 1. A pure (99.5%) metallic iron disc of 50 mm in diameter, which was located at 60 mm from the substrate, was DC sputtered with a constant current density of 100 A m 2. The substrates were cleaned with acetone and alcohol before charging in the deposition chamber. Pre-sputtering for 10 min in pure argon was carried out to clean the iron target. The reactive gas flow rate (N2 or O2) was systematically changed from 0 to 18 sccm using mass flow rate controllers. Total sputtering pressure and target voltage were measured when the reactive gas flow rate increased and/or decreased. Iron nitride and oxide films were deposited without external
C. Petitjean et al. / Surface & Coatings Technology 200 (2005) 431–434
heating. Thus, the deposition temperature was expected to be lower than 323 K. The thickness of the films deposited on glass and (100) silicon substrates were measured with Dektak 3030 profilometer. The deposition rates are calculated from the sputtering time.
3. Results and discussion
(a) 500 480
Target voltage (V)
432
3.1. The Fe–N2 system
460 440 420 400 Increasing N2 flow rate Decreasing N2 flow rate
380 360 0
2
4
6
8
10
12
14
16
18
Nitrogen flow rate (sccm)
(b) 500 480
Target voltage (V)
As shown in Fig. 1a, the increase of nitrogen flow rate ( Q(N2)) produces a quite proportional increase of the total pressure. The reversibility of the phenomenon, while decreasing nitrogen flow rate and the very close discharge-off and discharge-on curves, are significant of a poor gettering of N2 by Fe atoms and, therefore, of a poor reactivity of the Fe–N2 system. The DC target voltage is also measured during the variation of Q(N2). Fig. 2a shows the evolution of this electrical parameter versus the nitrogen inlet when the iron target is DC sputtered. Before introducing nitrogen, the Fe target voltage is about 450 V. It corresponds to a pure elemental sputtering mode (ESM). When Q(N2) = 1 sccm, it
460 440 420 400
Increasing O2 flow rate Decreasing O2 flow rate
380 360 340 320 300 0
(a)
6
8
10
12
14
16
18
Fig. 2. Influence of the reactive gas flow rate on the iron target voltage for the Fe–N2 system (a) and for the Fe–O2 one (b).
2.8
Total pressure (Pa)
4
Oxygen flow rate (sccm) 3.2
2.4 2.0 1.6 1.2 Discharge off Discharge on, increasing Q(N2) Discharge on, decreasing Q(N2)
0.8 0.4 0
2
4
6
8
10
12
16
14
18
Nitrogen flow rate (sccm)
(b) 1.2
Total pressure (Pa)
2
1.0
0.8 0.6
Discharge off Discharge on, increasing Q(O2) Discharge on, decreasing Q(O2)
0.4 0.2 0
1
2
3
4
5
6
Oxygen flow rate (sccm) Fig. 1. Evolution of the total pressure versus the reactive gas flow rate for the Fe–N2 system (a) and for the Fe–O2 one (b).
reaches an optimum value of 485 V. At this time, the iron target may be partially nitrided by nitrogen and the FeNx compound formed on the target surface is expected to reduce the sputtering yield. It correlates with a weak change of the slope in the evolution of the total pressure versus Q(N2). For higher nitrogen flow rates, a continuous decrease of the Fe target voltage down to 360 V is observed. Contrary to the Ti–N2 system [11], where the titanium target voltage stabilises after reaching a critical value of Q(N2) (complete titanium target nitriding), the Fe target voltage does not reach a saturation value in the tested nitrogen flow rate range (from 0 to 18 sccm). The continuous decrease of Fe target voltage with increasing Q(N2) is attributed to the gradual increase of its poisoning and to the related significant increase of deposition pressure due to the rather low pumping speed. Fig. 3a presents the variation of the deposition rate as a function of the nitrogen flow rate. A significant decrease from 2.5 to 1.3 Am h 1 can be observed when Q(N2) increases from 0 to 6 sccm. In these flow rate conditions and taking into account the results in Fig. 2a, it can be concluded that the Fe–N2 system gradually changes from an elemental sputtering mode to a reactive one when nitrogen is introduced up to Q(N2) = 6 sccm. Over 6 sccm N2, the deposition rate remains nearly constant at 1.3 Am h 1, which is consistent with the full target nitriding,
C. Petitjean et al. / Surface & Coatings Technology 200 (2005) 431–434
(a)
2.6
Deposition rate (µm h-1)
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0
2
4
6
8
10
12
14
16
Nitrogen flow rate (sccm)
(b) 3.2
Deposition rate (µm h-1)
2.8 2.4
433
and then rises up to 1.6 sccm (Fig. 2b). The slight increase in deposition rate from 2.5 to 2.9 Am h 1 in ESM (Fig. 3b) is expected to proceed from the coating oxygen enrichment leading to the deposition of less dense coatings. The further introduction of oxygen in the plasma at 1.8 sccm O2 yields an abrupt decrease of the discharge voltage associated with ESM-to-RSM above-mentioned transition. This assumption is confirmed by Fig. 3b, which also shows an abrupt decrease of deposition rate. For higher oxygen flow rates, the target voltage continuously decreases. As with nitrogen, the continuous voltage decrease, while the target is expected to be wholly poisoned, is ascribed to the resulting increase in discharge pressure. Note that the ratio of deposition rates between RSM and ESM (10%) is in agreement with the higher reactivity of Fe with O2 than with N2 and is consistent with results obtained with stainless steel targets sputtered either in Ar–N2 or in Ar–O2 reactive mixtures [12].
2.0 1.6
4. Conclusion
1.2 0.8 0.4 0.0 0
1
2
3
4
5
Oxygen flow rate (sccm) Fig. 3. Deposition rate of iron nitride (a) and iron oxide (b) compounds during DC deposition versus reactive gas flow rate.
therefore operating in the so-called reactive sputtering mode (RSM). The rather low decrease of deposition rate (about 50%) between ESM and RSM is in agreement with the poor reactivity of the Fe–N2 system.
Iron nitride and iron oxide coatings were deposited on silicon and glass substrates by DC reactive magnetron sputtering of an iron target in Ar–N2 and Ar–O2 mixtures, respectively. This paper was focused on the reactivity of sputtered iron atoms in Ar–N2 and Ar–O2 reactive mixtures. Instability phenomena of DC reactive sputtering process involving iron target and N2 or O2 were studied from gas total pressure and target voltage. The Fe–N2 system did not show an instability phenomenon in the tested experimental conditions, indicating a low reactivity. On the other hand, with changing of the oxygen flow rate a hysteresis loop occurred for the Fe–O2 system. The structural characterisation and the study of the films’ properties will be detailed further in another paper.
3.2. The Fe–O2 system Fig. 1b illustrates the dependence of the total pressure in the deposition chamber as a function of the oxygen flow rate ( Q(O2)) during DC sputtering of the Fe target in reactive Ar–O2 mixtures. When Q(O2) is lower than about 1.5 sccm, most of oxygen is gettered on the condensation areas by sputtered iron atoms and the total pressure remains constant at 0.33 Pa. The target thus operates in ESM. Over this critical flow rate, a slight increase of oxygen flow rate drives the pressure up to 0.5–0.6 Pa and the process turns abruptly into RSM. For higher oxygen flow rates, the total pressure exhibits a linear dependence with Q(O2), the slope of which is close to the one of the discharge-off curve. While decreasing oxygen flow rate, the RSM-to-ESM transition occurs around 0.8 sccm. The hysteresis reveals an unstable behaviour of the Fe–O2 system, which reveals more reactive than the Fe–N2 one. Contrary to the Fe–N2 system, when Q(O2) increases, the iron target voltage remains constant up to about 1.2 sccm O2
Acknowledgement The work described in this paper is supported by the European Union through the NMP3-CT-2003-505948 project bHARDECOATQ and by ITSFC (Surface Treatments Institute of Franche-Comte´), Region of Franche-Comte´, and DRIRE and FEDER funds.
References [1] M. Ohring, The Materials Science of Thin Films, Academic Press, San Diego, CA, 1992. [2] R.F. Bunshah, Handbook of Deposition Technologies for Films and Coatings, Noyes Publications, Park Ridge, NJ, 1994. [3] A. Thobor, C. Rousselot, C. Clement, J. Takadoum, N. Martin, R. Sanjines, F. Levy, Surf. Coat. Technol. 124 (2000) 210. [4] D. Pilloud, J.F. Pierson, A.P. Marques, A. Cavaleiro, Surf. Coat. Technol. 180/181 (2004) 352.
434
C. Petitjean et al. / Surface & Coatings Technology 200 (2005) 431–434
[5] J.F. Pierson, A. Thobor-Keck, A. Billard, Appl. Surf. Sci. 210 (2003) 359. [6] J.-M. Chappe´, N. Martin, J.F. Pierson, G. Terwagne, J. Lintymer, J. Gavoille, J. Takadoum, Appl. Surf. Sci. 225 (2004) 29. [7] J.F. Bobo, M.J. Casanove, L. Hennet, E. Snoeck, M. Piecuch, J. Magn. Magn. Mater. 164 (1996) 61. [8] X. Wang, W.T. Zheng, H.W. Tian, S.S. Yu, W. Xu, S.H. Meng, X.D. He, J.C. Han, C.Q. Sun, B.K. Tay, Appl. Surf. Sci. 220 (2003) 30.
[9] Y.K. Kim, M. Oliveria, J. Appl. Phys. 75 (1994) 431. [10] A. Mun˜oz-Martin, C. Prieto, C. Ocal, J.L. Martinez, J. Colino, Surf. Sci. 482/485 (2001) 1095. [11] N. Martin, C. Rousselot, J. Vac. Sci. Technol., A, Vac. Surf. Films 17 (1999) 2869. [12] P. Gutier, PhD Thesis, INPL, France, 1999.
Reactive sputtering of iron in Ar–N2 and Ar–O2 mixtures C. Petitjeana, C. Rousselota, J.F. Piersona,T, A. Billardb a
De´partement CREST, Institut FEMTO-ST (UMR 6174), Universite´ de Franche-Comte´, Poˆle Universitaire, BP 71427, 25211 Montbe´liard Cedex, France b Laboratoire de Science et Ge´nie des Surfaces (UMR 7570), Ecoles des Mines, Parc de Saurupt, 54042 Nancy, France Available online 9 March 2005
Abstract This paper reports on the preparation of iron nitride and iron oxide films by DC reactive magnetron sputtering. The reactivity of the Fe–N2 and Fe–O2 systems is compared by the measurement of the total pressure in the deposition chamber, the iron target voltage and the films’ growth rate as a function of the reactive gas flow rate. The Fe–N2 system does not exhibit an instability phenomenon and the total pressure is quite proportional to the nitrogen flow rate. This system exhibits a low reactivity as confirmed by the effect of the nitrogen flow rate on the films’ growth rate. On the other hand, a hysteresis loop occurs for the Fe–O2 system, indicating a higher reactivity for this last system. D 2005 Elsevier B.V. All rights reserved. Keywords: Reactive sputtering; Iron nitride; Iron oxide
1. Introduction Reactive magnetron sputtering is a well-known process to synthesise various metallic oxide, nitride or carbide films using a metallic target and a reactive gas mixture (i.e., Ar– O2, Ar–N2 or Ar–CH4) [1,2]. Thus, this method has been widely used for mechanical, optical, electronic or magnetic applications. Furthermore, this process can be used to deposit coatings with complex structure such as multilayers [3], nanocomposites [4], to synthesise materials that cannot be formed using other methods [5] or to deposit oxynitride films [6]. Iron-based coatings (nitrides and oxides) have been widely studied for their magnetic properties (see for example Refs. [7–10]). However, to the best of our knowledge, there is no paper related to the elaboration and to the characterisation of iron oxynitride coatings. In order to deposit iron oxynitride films, we synthesise firstly iron nitride and iron oxide films. Thus, the aim of this paper
T Corresponding author. Tel.: +33 3 81 99 46 72; fax: +33 3 81 99 46 73. E-mail address: [email protected] (J.F. Pierson). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.028
is to compare the reactivity of sputtered iron atoms in Ar–N2 or Ar–O2 reactive mixtures.
2. Experiment Reactive sputtering experiments were performed using Alliance Concept AC450 sputter equipment with a vacuum chamber volume of about 70 l. Base pressure of 10 5 Pa was obtained with a turbomolecular pump backed with a mechanical one. The working Ar pressure was kept constant at 0.3 Pa using mass flow rate controllers and a constant pumping speed S = 10 L s 1. A pure (99.5%) metallic iron disc of 50 mm in diameter, which was located at 60 mm from the substrate, was DC sputtered with a constant current density of 100 A m 2. The substrates were cleaned with acetone and alcohol before charging in the deposition chamber. Pre-sputtering for 10 min in pure argon was carried out to clean the iron target. The reactive gas flow rate (N2 or O2) was systematically changed from 0 to 18 sccm using mass flow rate controllers. Total sputtering pressure and target voltage were measured when the reactive gas flow rate increased and/or decreased. Iron nitride and oxide films were deposited without external
C. Petitjean et al. / Surface & Coatings Technology 200 (2005) 431–434
heating. Thus, the deposition temperature was expected to be lower than 323 K. The thickness of the films deposited on glass and (100) silicon substrates were measured with Dektak 3030 profilometer. The deposition rates are calculated from the sputtering time.
3. Results and discussion
(a) 500 480
Target voltage (V)
432
3.1. The Fe–N2 system
460 440 420 400 Increasing N2 flow rate Decreasing N2 flow rate
380 360 0
2
4
6
8
10
12
14
16
18
Nitrogen flow rate (sccm)
(b) 500 480
Target voltage (V)
As shown in Fig. 1a, the increase of nitrogen flow rate ( Q(N2)) produces a quite proportional increase of the total pressure. The reversibility of the phenomenon, while decreasing nitrogen flow rate and the very close discharge-off and discharge-on curves, are significant of a poor gettering of N2 by Fe atoms and, therefore, of a poor reactivity of the Fe–N2 system. The DC target voltage is also measured during the variation of Q(N2). Fig. 2a shows the evolution of this electrical parameter versus the nitrogen inlet when the iron target is DC sputtered. Before introducing nitrogen, the Fe target voltage is about 450 V. It corresponds to a pure elemental sputtering mode (ESM). When Q(N2) = 1 sccm, it
460 440 420 400
Increasing O2 flow rate Decreasing O2 flow rate
380 360 340 320 300 0
(a)
6
8
10
12
14
16
18
Fig. 2. Influence of the reactive gas flow rate on the iron target voltage for the Fe–N2 system (a) and for the Fe–O2 one (b).
2.8
Total pressure (Pa)
4
Oxygen flow rate (sccm) 3.2
2.4 2.0 1.6 1.2 Discharge off Discharge on, increasing Q(N2) Discharge on, decreasing Q(N2)
0.8 0.4 0
2
4
6
8
10
12
16
14
18
Nitrogen flow rate (sccm)
(b) 1.2
Total pressure (Pa)
2
1.0
0.8 0.6
Discharge off Discharge on, increasing Q(O2) Discharge on, decreasing Q(O2)
0.4 0.2 0
1
2
3
4
5
6
Oxygen flow rate (sccm) Fig. 1. Evolution of the total pressure versus the reactive gas flow rate for the Fe–N2 system (a) and for the Fe–O2 one (b).
reaches an optimum value of 485 V. At this time, the iron target may be partially nitrided by nitrogen and the FeNx compound formed on the target surface is expected to reduce the sputtering yield. It correlates with a weak change of the slope in the evolution of the total pressure versus Q(N2). For higher nitrogen flow rates, a continuous decrease of the Fe target voltage down to 360 V is observed. Contrary to the Ti–N2 system [11], where the titanium target voltage stabilises after reaching a critical value of Q(N2) (complete titanium target nitriding), the Fe target voltage does not reach a saturation value in the tested nitrogen flow rate range (from 0 to 18 sccm). The continuous decrease of Fe target voltage with increasing Q(N2) is attributed to the gradual increase of its poisoning and to the related significant increase of deposition pressure due to the rather low pumping speed. Fig. 3a presents the variation of the deposition rate as a function of the nitrogen flow rate. A significant decrease from 2.5 to 1.3 Am h 1 can be observed when Q(N2) increases from 0 to 6 sccm. In these flow rate conditions and taking into account the results in Fig. 2a, it can be concluded that the Fe–N2 system gradually changes from an elemental sputtering mode to a reactive one when nitrogen is introduced up to Q(N2) = 6 sccm. Over 6 sccm N2, the deposition rate remains nearly constant at 1.3 Am h 1, which is consistent with the full target nitriding,
C. Petitjean et al. / Surface & Coatings Technology 200 (2005) 431–434
(a)
2.6
Deposition rate (µm h-1)
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0
2
4
6
8
10
12
14
16
Nitrogen flow rate (sccm)
(b) 3.2
Deposition rate (µm h-1)
2.8 2.4
433
and then rises up to 1.6 sccm (Fig. 2b). The slight increase in deposition rate from 2.5 to 2.9 Am h 1 in ESM (Fig. 3b) is expected to proceed from the coating oxygen enrichment leading to the deposition of less dense coatings. The further introduction of oxygen in the plasma at 1.8 sccm O2 yields an abrupt decrease of the discharge voltage associated with ESM-to-RSM above-mentioned transition. This assumption is confirmed by Fig. 3b, which also shows an abrupt decrease of deposition rate. For higher oxygen flow rates, the target voltage continuously decreases. As with nitrogen, the continuous voltage decrease, while the target is expected to be wholly poisoned, is ascribed to the resulting increase in discharge pressure. Note that the ratio of deposition rates between RSM and ESM (10%) is in agreement with the higher reactivity of Fe with O2 than with N2 and is consistent with results obtained with stainless steel targets sputtered either in Ar–N2 or in Ar–O2 reactive mixtures [12].
2.0 1.6
4. Conclusion
1.2 0.8 0.4 0.0 0
1
2
3
4
5
Oxygen flow rate (sccm) Fig. 3. Deposition rate of iron nitride (a) and iron oxide (b) compounds during DC deposition versus reactive gas flow rate.
therefore operating in the so-called reactive sputtering mode (RSM). The rather low decrease of deposition rate (about 50%) between ESM and RSM is in agreement with the poor reactivity of the Fe–N2 system.
Iron nitride and iron oxide coatings were deposited on silicon and glass substrates by DC reactive magnetron sputtering of an iron target in Ar–N2 and Ar–O2 mixtures, respectively. This paper was focused on the reactivity of sputtered iron atoms in Ar–N2 and Ar–O2 reactive mixtures. Instability phenomena of DC reactive sputtering process involving iron target and N2 or O2 were studied from gas total pressure and target voltage. The Fe–N2 system did not show an instability phenomenon in the tested experimental conditions, indicating a low reactivity. On the other hand, with changing of the oxygen flow rate a hysteresis loop occurred for the Fe–O2 system. The structural characterisation and the study of the films’ properties will be detailed further in another paper.
3.2. The Fe–O2 system Fig. 1b illustrates the dependence of the total pressure in the deposition chamber as a function of the oxygen flow rate ( Q(O2)) during DC sputtering of the Fe target in reactive Ar–O2 mixtures. When Q(O2) is lower than about 1.5 sccm, most of oxygen is gettered on the condensation areas by sputtered iron atoms and the total pressure remains constant at 0.33 Pa. The target thus operates in ESM. Over this critical flow rate, a slight increase of oxygen flow rate drives the pressure up to 0.5–0.6 Pa and the process turns abruptly into RSM. For higher oxygen flow rates, the total pressure exhibits a linear dependence with Q(O2), the slope of which is close to the one of the discharge-off curve. While decreasing oxygen flow rate, the RSM-to-ESM transition occurs around 0.8 sccm. The hysteresis reveals an unstable behaviour of the Fe–O2 system, which reveals more reactive than the Fe–N2 one. Contrary to the Fe–N2 system, when Q(O2) increases, the iron target voltage remains constant up to about 1.2 sccm O2
Acknowledgement The work described in this paper is supported by the European Union through the NMP3-CT-2003-505948 project bHARDECOATQ and by ITSFC (Surface Treatments Institute of Franche-Comte´), Region of Franche-Comte´, and DRIRE and FEDER funds.
References [1] M. Ohring, The Materials Science of Thin Films, Academic Press, San Diego, CA, 1992. [2] R.F. Bunshah, Handbook of Deposition Technologies for Films and Coatings, Noyes Publications, Park Ridge, NJ, 1994. [3] A. Thobor, C. Rousselot, C. Clement, J. Takadoum, N. Martin, R. Sanjines, F. Levy, Surf. Coat. Technol. 124 (2000) 210. [4] D. Pilloud, J.F. Pierson, A.P. Marques, A. Cavaleiro, Surf. Coat. Technol. 180/181 (2004) 352.
434
C. Petitjean et al. / Surface & Coatings Technology 200 (2005) 431–434
[5] J.F. Pierson, A. Thobor-Keck, A. Billard, Appl. Surf. Sci. 210 (2003) 359. [6] J.-M. Chappe´, N. Martin, J.F. Pierson, G. Terwagne, J. Lintymer, J. Gavoille, J. Takadoum, Appl. Surf. Sci. 225 (2004) 29. [7] J.F. Bobo, M.J. Casanove, L. Hennet, E. Snoeck, M. Piecuch, J. Magn. Magn. Mater. 164 (1996) 61. [8] X. Wang, W.T. Zheng, H.W. Tian, S.S. Yu, W. Xu, S.H. Meng, X.D. He, J.C. Han, C.Q. Sun, B.K. Tay, Appl. Surf. Sci. 220 (2003) 30.
[9] Y.K. Kim, M. Oliveria, J. Appl. Phys. 75 (1994) 431. [10] A. Mun˜oz-Martin, C. Prieto, C. Ocal, J.L. Martinez, J. Colino, Surf. Sci. 482/485 (2001) 1095. [11] N. Martin, C. Rousselot, J. Vac. Sci. Technol., A, Vac. Surf. Films 17 (1999) 2869. [12] P. Gutier, PhD Thesis, INPL, France, 1999.