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Carbon layers cleaning from inside of narrow gaps by a RF glow discharge
Surface & Coatings Technology 205 (2011) S435–S438
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Carbon layers cleaning from inside of narrow gaps by a RF glow discharge C. Stancu a, M. Teodorescu a, A.C. Galca b, G. Dinescu a,⁎ a b
National Institute for Laser, Plasma and Radiation Physics, Atomistilor Str. 409, PO Box Mg36, Magurele-Bucharest, 077125, Romania National Institute of Materials Physics, Atomistilor 105bis, PO Box Mg7, Magurele-Bucharest, 077125, Romania
a r t i c l e
i n f o
Available online 29 March 2011 Keywords: Plasma cleaning Discharges in gaps Radiofrequency discharges
a b s t r a c t The paper focuses on the utilization of radiofrequency discharges at intermediate pressures for plasma removal of carbon residuals or layers from narrow gaps and channels. It is shown that by proper handling of the discharge power and pressure it is possible to control the development of plasma inside narrow gaps, in the proximity of surfaces to be cleaned. The discharge operation inside gaps having the width in the range 0.6–2 mm was demonstrated. The cleaning effectiveness is exemplified for castellated surfaces having rectangular gaps coated with hydrogenated amorphous carbon layers. Cleaning of large surfaces can be approached with a movable “plasma shower” type device. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Applications like removal of organic residuals from small size polymer molding dies, surface modification inside narrow spaces (e.g., for MEMS) or removal of the co-deposited layers enriched in tritium from the inside gaps of castellated tiles of the fusion machines can be approached by plasma cleaning. In particular fuel accumulation in the wall, critical for long term operation of fusion machines like Tokamaks, is more prominent for surfaces with grooves and voids, due to increase of surface area and co-deposition with the eroded wall material in the inside of such narrow spaces [1]. Various cleaning techniques already developed like laser [2], flash-lamp [3], oxidation [4] and glow discharge at low pressure [5,6] have the drawback of limited access in narrow spaces. In this paper we present an approach of removal of organic contaminants, residuals from inside of narrow gaps by using radiofrequency glow discharges. To this aim we use the peculiarity of plasmas to mold around surfaces having complex shapes, like curvatures, voids and grooves, if certain conditions are fulfilled. We have checked the ability of low pressure radiofrequency plasma discharges for cleaning surfaces with gaps or holes by experiments of removal of carbonic layers from the inside of narrow channels. We have proved that i) the radiofrequency discharge can be generated in stable conditions inside narrow spaces of various widths (0.5–2 mm); ii) it is possible to generate a localized plasma which can be moved over large surface
⁎ Corresponding author. Tel.: + 40 21 4574470; fax: + 40 214574243. E-mail address: dinescug@infim.ro (G. Dinescu). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.090
areas while maintaining the discharge inside the narrow spaces; and iii) the discharge is effective in removal of carbonic layers. 2. Principle of inside gap plasma generation By assuming that the cleaning process is enhanced by the presence of plasma in the proximity of the contaminated surface/deposited wall, the problem which has to be solved is the plasma sustaining inside the small width gaps. The transition region between the volumetric plasma and wall (plasma sheath), is the place of phenomena assuring the discharge maintenance through electron emission processes stimulated by plasma particles surface bombardment. A critical condition for plasma existence is that the given gap volume offers enough room for sheath development. The important length scales that control the plasma system behavior are the plasma sheath thickness (Lsh), and the size (width) of gap (D). In the usual case D≫Lsh, the gap width is much larger than sheath thickness and the plasma border is conformal in respect to the gap surface topography. Contrary, for D≪Lsh the border cannot follow the surface topography and plasma does not develop inside the gap. The problem of plasma development inside the gap is then translated in finding solutions to handle the sheath thickness for becoming smaller than the gap width. Roughly, the sheath thickness behaves like the Debye length: this one scales with plasma parameters as λd ~ (Te/ne)1/2 [7,8]. In most of the discharges, and for given ranges of applied power and gas pressure the increase of power leads to the increase of the electron density (via increase of the ionization rate) and the increase of pressure leads to the decreases of the electronic temperature (via thermalization of electrons by collisions with the cold gas). Thus, the injected RF power and gas pressure in the discharge can be used as handling parameters to
S436
C. Stancu et al. / Surface & Coatings Technology 205 (2011) S435–S438
control the sheath thickness and consequently the penetration of the discharge inside the gaps. 3. Discharge experiments 3.1. Discharge experiments with a static electrode
gauge
matching box
RF electrode RF generator
4. Cleaning experiments These experiments were performed in order to assess the effectiveness of the system in removal of carbon layers from inside of narrow gaps. Another set of experiments was performed on castellated surfaces, vacuum coated with amorphous hydrogenated carbon. The castellated substrate was assembled from separate polished aluminum cubic pieces spaced at 1.5 mm distances, so defining between them linear gaps with 1.5 mm width. Prior to mounting the cubes were vacuum coated with amorphous hydrogenated carbon by Plasma Assisted Chemical Vapor Deposition Technique (PACVD). The coating was realized on all the cube's sides in a PACVD reactor, with argon plasma injected with acetylene [9]. The coating thickness was 1.2 μm, as has been determined by Atomic Force Microscopy and ellipsometry. The coated assembled castellated specimen was submitted to cleaning in conditions of discharge generation inside the gaps in a mixture of 94% argon with 6% air, at a pressure of 27 mbar and 50 W power. The cleaning effect of the discharge is visible in the image from Fig. 4, on the whole ensemble and on a separate cube which was dismantled from the castellated substrate.
100
80 d=0.8 mm
These experiments were based on a discharge configuration similar to that presented in Fig. 1, consisting of a cooled disk-shaped RF electrode placed at 3–8 cm distance from a large area grounded castellated surface defined by machined crossed channels of 1 mm width, separated by 1 cm distance. In addition, the active RF electrode was firmly fixed to the arm of a translation stage, actuated from the outside of the discharge chamber. This way the active electrode can be moved over the castellated surface, in vacuum tight conditions, and the plasma column size and discharge penetration inside of gaps can be visually monitored during the electrode translation. These experiments have proved that the electrode, the plasma column and the contact area of plasma column with the surface move all together,
like a mobile “plasma shower”, while the plasma remains penetrating inside the gaps.
60 d=0.9 mm
40
MFC
d=0.6
3.2. Discharge experiments with a movable active electrode
Fig. 2. Image illustrating the penetration of the discharge in gaps.
Pressure [mbar]
These experiments were performed in order to check the possibility to control the development of the discharge inside gaps of various sizes. A set-up consisting of a vacuum chamber, with glass front end window, and provided with pumping, gas feeding and pressure monitoring systems was built-up (Fig. 1). The discharge chamber was provided with two parallel electrodes facing each other at a distance of 60 mm. The powered RF electrode is shaped like a disk and covered on its upper part with Teflon in order to prevent the discharge extension behind it. The other electrode is a milled aluminum piece with narrow grooves of 80 mm length and 5 mm deepness, having widths in the range 0.6–2 mm. An image of the vacuum chamber with the two electrodes during plasma generation, illustrating the discharge penetration in grooves having different widths is presented in Fig. 2. An interesting phenomenon should be mentioned at this point (not visible in the image): the plasma segmentation inside the channels of the grounded electrode is sometimes observed. The segmentation can be described as the appearing of striation-like alternating bright and dark zones distributed along the channel, which destroy the homogeneity of the discharge. In Fig. 3 we present the domains of discharge existence inside the gaps of various widths, in (power, pressure) coordinates. The operation domains are defined by the areas enclosed inside the polygonal figures connecting the points corresponding to appearance/ disappearance of plasma in the gaps. In the present experiments the upper values of the power and pressure parameters were set to less than 150 W and 100 mbar in order to prevent the plasma column contraction and the overheating of the active electrode, respectively.
d=1 mm
d=1.7 mm
20 d=2 mm
grooved electrode
0 vacuum chamber
0
20
d=1.4 mm
40
60
80
100
120
140
160
Power [W] to pump
Fig. 1. Schematic view of the experimental set-up used for discharge experiments.
Fig. 3. Operation domains inside gaps in pressure–power coordinates (discharge in argon).
C. Stancu et al. / Surface & Coatings Technology 205 (2011) S435–S438
S437
Fig. 4. a) Image of the assembled castellated electrode after a partial cleaning; b) image of a partially cleaned single cube.
In order to obtain time dependent removal rates the castellated piece was disassembled in cubes, after cleaning at various times. A separate cube (Fig. 4b) was chosen for measurement of the effectiveness of the cleaning process. The quantitative measurement of the material removal was performed using spectroscopic ellipsometry. The change in light polarization (given by the quantities Ψ and Δ) of the incident linearly polarized light beam [10], was measured after the reflection on the frontal or a lateral cube face. Amorphous hydrogenated carbon is transparent at wavelengths over 600 nm. Over this value the refractive index of the deposited layer can be generated using a Cauchy dispersion [11]. Finally, the ellipsometry quantities are converted in refractive index (n) and layer thickness (d), by using a standard procedure [10,12]. The obtained dependence of the thickness of the non-removed layer upon the position (i.e. upon gap deepness) for different times is shown in Fig. 5a, while the dependence of thickness upon time for the front side is shown in Fig. 5b. From Fig. 5a, results show that the initial 1.2 μm thick carbon layer is completely removed up to 3 mm deepness in less than 5 min. In this region the removal rate is about 0.24 μm/min. As concerning the front side, the removal rates vary between 0.01 and 0.04 μm/min. The etching homogeneity along the gaps is not important for applications where removing is the main purpose, whatever the place from where the material is removed, but can be important for specific applications. The non-homogeneity of the etching is related to discharge non-homogeneity inside the gaps. The effect of discharge non-homogeneity is visible in Fig. 4b, where the border line between the upper cleaned zone and the bottom partially non-cleaned zone is not straight. Deeper cleaned zones are observed at the crossings points of the channels, where a larger space was available for dis-
a
charge penetration, and also in the middle of the lateral faces of the cubes, where a brighter plasma zone was present due to segmentation. For the given example here the non-homogeneity of the etching rate on position along the sample was estimated by the relative standard deviation of the distances from the upper cube face to the border line. The obtained value, of 45%, is a measure of the deviation of the border from a straight line, in presence of crossing channels and segmentation effect. 5. Conclusions Experiments were performed in order to check the possibility to control the development of a radiofrequency discharge inside gaps with various widths. We have shown that by gradually modifying the radiofrequency power forwarded to plasma and the discharge pressure it is possible to create and maintain the plasma inside narrow gaps for well defined domains of the pressure-power parameter space. The cleaning process is favored by plasma presence in the proximity of the surface to be cleaned. Cleaning experiments were performed in a mixture of argon/air on castellated surfaces having rectangular gaps 1.5 mm in width and 6 mm in depth, coated with hydrogenated amorphous carbon layers. Comparing to the frontal surface exposed to plasma the cleaning process is faster on the insidegap's surfaces, probably due to a hollow electrode type process enhancement. Also, the cleaning process is faster at the edges, as example at the gap entrance. Experiments with a movable active electrode have proved that the plasma column size and its contact area moves over surfaces, like a
b 1.2
1.2
t=0 min
front 1.0
1.0 t=20 min
0.8 t=10 min
0.6 t=15 min
0.4
Thickness [µm]
Thickness [m]
t=5 min
0.8 0.6 0.4
t=25 min
0.2
0.2 t=30 min
0.0
t=35 min
1
2
3
4
Position [mm]
5
6
0.0 -5
0
5
10
15
20
25
30
35
40
Time [min]
Fig. 5. a) Dependence of layer thickness upon deepness, on lateral side and for different treatment times; b) dependence of layer thickness, on the front side, upon the time.
S438
C. Stancu et al. / Surface & Coatings Technology 205 (2011) S435–S438
movable “plasma shower”, while the plasma remains penetrating inside the gaps. This result has relevance for cleaning of large nonuniform surfaces which present defects, holes, or gaps. Acknowledgments This work was partially supported in the frame of the EURATOM programme, Romanian Association MEdC and under CNCSIS contract ID_1999/2008. References [1] K. Krieger, W. Jacob, D.L. Rudakov, R. Bastasz, G. Federici, A. Litnovsky, H. Maier, V. Rohde, G. Strohmayer, W.P. West, J. Whaley, C.P.C. Wong, J. Nucl. Mater. 363–365 (2007) 870.
[2] C. Grisolia, A. Semerok, J.M. Weulersse, F. Le Guern, S. Fomichev, F. Brygo, P. Fichet, P.Y. Thro, P. Coad, N. Bekris, M. Stamp, S. Rosanvallon, G. Piazza, J. Nucl. Mater. 363–365 (2007) 1138. [3] C. Grisolia, G. Counsell, G. Dinescu, A. Semerok, N. Bekris, P. Coad, C. Hopf, J. Roth, M. Rubel, A. Widdowson, E. Tsitrone, Fusion Eng. Des. 82 (2007) 2390. [4] I. Tanarro, J.A. Ferreira, V.J. Herrero, F.L. Tabarés, C. Gómez-Aleixandre, J. Nucl. Mater. 390–391 (2009) 696. [5] J.A. Ferreira, F.L. Tabarés, D. Tafalla, J. Nucl. Mater. 363–365 (2007) 888. [6] C. Hopf, W. Jacob, V. Rohde, J. Nucl. Mater. 374 (2008) 413. [7] A. Grill, Cold Plasma in Materials Fabrication, IEEE Press, New York, 1994. [8] C.-K. Kim, D.J. Economou, J. Appl. Phys. 91 (2002) 2594. [9] B. Mitu, S.I. Vizireanu, C. Petcu, G. Dinescu, M. Dinescu, R. Birjega, V.S. Teodorescu, Surf. Coat. Technol. 180 (2004) 238. [10] R.M.A. Azzam, N.H. Bashara, Ellipsometry and Polarized Light, North Holland, Amsterdam, 1987. [11] L. Cauchy, Bull. Sci. Math. 14 (1830) 6. [12] V. Ion, A.C. Galca, N.D. Scarisoreanu, M. Filipescu, M. Dinescu, Phys. Status Solidi C 5 (2008) 1180.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Carbon layers cleaning from inside of narrow gaps by a RF glow discharge C. Stancu a, M. Teodorescu a, A.C. Galca b, G. Dinescu a,⁎ a b
National Institute for Laser, Plasma and Radiation Physics, Atomistilor Str. 409, PO Box Mg36, Magurele-Bucharest, 077125, Romania National Institute of Materials Physics, Atomistilor 105bis, PO Box Mg7, Magurele-Bucharest, 077125, Romania
a r t i c l e
i n f o
Available online 29 March 2011 Keywords: Plasma cleaning Discharges in gaps Radiofrequency discharges
a b s t r a c t The paper focuses on the utilization of radiofrequency discharges at intermediate pressures for plasma removal of carbon residuals or layers from narrow gaps and channels. It is shown that by proper handling of the discharge power and pressure it is possible to control the development of plasma inside narrow gaps, in the proximity of surfaces to be cleaned. The discharge operation inside gaps having the width in the range 0.6–2 mm was demonstrated. The cleaning effectiveness is exemplified for castellated surfaces having rectangular gaps coated with hydrogenated amorphous carbon layers. Cleaning of large surfaces can be approached with a movable “plasma shower” type device. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Applications like removal of organic residuals from small size polymer molding dies, surface modification inside narrow spaces (e.g., for MEMS) or removal of the co-deposited layers enriched in tritium from the inside gaps of castellated tiles of the fusion machines can be approached by plasma cleaning. In particular fuel accumulation in the wall, critical for long term operation of fusion machines like Tokamaks, is more prominent for surfaces with grooves and voids, due to increase of surface area and co-deposition with the eroded wall material in the inside of such narrow spaces [1]. Various cleaning techniques already developed like laser [2], flash-lamp [3], oxidation [4] and glow discharge at low pressure [5,6] have the drawback of limited access in narrow spaces. In this paper we present an approach of removal of organic contaminants, residuals from inside of narrow gaps by using radiofrequency glow discharges. To this aim we use the peculiarity of plasmas to mold around surfaces having complex shapes, like curvatures, voids and grooves, if certain conditions are fulfilled. We have checked the ability of low pressure radiofrequency plasma discharges for cleaning surfaces with gaps or holes by experiments of removal of carbonic layers from the inside of narrow channels. We have proved that i) the radiofrequency discharge can be generated in stable conditions inside narrow spaces of various widths (0.5–2 mm); ii) it is possible to generate a localized plasma which can be moved over large surface
⁎ Corresponding author. Tel.: + 40 21 4574470; fax: + 40 214574243. E-mail address: dinescug@infim.ro (G. Dinescu). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.090
areas while maintaining the discharge inside the narrow spaces; and iii) the discharge is effective in removal of carbonic layers. 2. Principle of inside gap plasma generation By assuming that the cleaning process is enhanced by the presence of plasma in the proximity of the contaminated surface/deposited wall, the problem which has to be solved is the plasma sustaining inside the small width gaps. The transition region between the volumetric plasma and wall (plasma sheath), is the place of phenomena assuring the discharge maintenance through electron emission processes stimulated by plasma particles surface bombardment. A critical condition for plasma existence is that the given gap volume offers enough room for sheath development. The important length scales that control the plasma system behavior are the plasma sheath thickness (Lsh), and the size (width) of gap (D). In the usual case D≫Lsh, the gap width is much larger than sheath thickness and the plasma border is conformal in respect to the gap surface topography. Contrary, for D≪Lsh the border cannot follow the surface topography and plasma does not develop inside the gap. The problem of plasma development inside the gap is then translated in finding solutions to handle the sheath thickness for becoming smaller than the gap width. Roughly, the sheath thickness behaves like the Debye length: this one scales with plasma parameters as λd ~ (Te/ne)1/2 [7,8]. In most of the discharges, and for given ranges of applied power and gas pressure the increase of power leads to the increase of the electron density (via increase of the ionization rate) and the increase of pressure leads to the decreases of the electronic temperature (via thermalization of electrons by collisions with the cold gas). Thus, the injected RF power and gas pressure in the discharge can be used as handling parameters to
S436
C. Stancu et al. / Surface & Coatings Technology 205 (2011) S435–S438
control the sheath thickness and consequently the penetration of the discharge inside the gaps. 3. Discharge experiments 3.1. Discharge experiments with a static electrode
gauge
matching box
RF electrode RF generator
4. Cleaning experiments These experiments were performed in order to assess the effectiveness of the system in removal of carbon layers from inside of narrow gaps. Another set of experiments was performed on castellated surfaces, vacuum coated with amorphous hydrogenated carbon. The castellated substrate was assembled from separate polished aluminum cubic pieces spaced at 1.5 mm distances, so defining between them linear gaps with 1.5 mm width. Prior to mounting the cubes were vacuum coated with amorphous hydrogenated carbon by Plasma Assisted Chemical Vapor Deposition Technique (PACVD). The coating was realized on all the cube's sides in a PACVD reactor, with argon plasma injected with acetylene [9]. The coating thickness was 1.2 μm, as has been determined by Atomic Force Microscopy and ellipsometry. The coated assembled castellated specimen was submitted to cleaning in conditions of discharge generation inside the gaps in a mixture of 94% argon with 6% air, at a pressure of 27 mbar and 50 W power. The cleaning effect of the discharge is visible in the image from Fig. 4, on the whole ensemble and on a separate cube which was dismantled from the castellated substrate.
100
80 d=0.8 mm
These experiments were based on a discharge configuration similar to that presented in Fig. 1, consisting of a cooled disk-shaped RF electrode placed at 3–8 cm distance from a large area grounded castellated surface defined by machined crossed channels of 1 mm width, separated by 1 cm distance. In addition, the active RF electrode was firmly fixed to the arm of a translation stage, actuated from the outside of the discharge chamber. This way the active electrode can be moved over the castellated surface, in vacuum tight conditions, and the plasma column size and discharge penetration inside of gaps can be visually monitored during the electrode translation. These experiments have proved that the electrode, the plasma column and the contact area of plasma column with the surface move all together,
like a mobile “plasma shower”, while the plasma remains penetrating inside the gaps.
60 d=0.9 mm
40
MFC
d=0.6
3.2. Discharge experiments with a movable active electrode
Fig. 2. Image illustrating the penetration of the discharge in gaps.
Pressure [mbar]
These experiments were performed in order to check the possibility to control the development of the discharge inside gaps of various sizes. A set-up consisting of a vacuum chamber, with glass front end window, and provided with pumping, gas feeding and pressure monitoring systems was built-up (Fig. 1). The discharge chamber was provided with two parallel electrodes facing each other at a distance of 60 mm. The powered RF electrode is shaped like a disk and covered on its upper part with Teflon in order to prevent the discharge extension behind it. The other electrode is a milled aluminum piece with narrow grooves of 80 mm length and 5 mm deepness, having widths in the range 0.6–2 mm. An image of the vacuum chamber with the two electrodes during plasma generation, illustrating the discharge penetration in grooves having different widths is presented in Fig. 2. An interesting phenomenon should be mentioned at this point (not visible in the image): the plasma segmentation inside the channels of the grounded electrode is sometimes observed. The segmentation can be described as the appearing of striation-like alternating bright and dark zones distributed along the channel, which destroy the homogeneity of the discharge. In Fig. 3 we present the domains of discharge existence inside the gaps of various widths, in (power, pressure) coordinates. The operation domains are defined by the areas enclosed inside the polygonal figures connecting the points corresponding to appearance/ disappearance of plasma in the gaps. In the present experiments the upper values of the power and pressure parameters were set to less than 150 W and 100 mbar in order to prevent the plasma column contraction and the overheating of the active electrode, respectively.
d=1 mm
d=1.7 mm
20 d=2 mm
grooved electrode
0 vacuum chamber
0
20
d=1.4 mm
40
60
80
100
120
140
160
Power [W] to pump
Fig. 1. Schematic view of the experimental set-up used for discharge experiments.
Fig. 3. Operation domains inside gaps in pressure–power coordinates (discharge in argon).
C. Stancu et al. / Surface & Coatings Technology 205 (2011) S435–S438
S437
Fig. 4. a) Image of the assembled castellated electrode after a partial cleaning; b) image of a partially cleaned single cube.
In order to obtain time dependent removal rates the castellated piece was disassembled in cubes, after cleaning at various times. A separate cube (Fig. 4b) was chosen for measurement of the effectiveness of the cleaning process. The quantitative measurement of the material removal was performed using spectroscopic ellipsometry. The change in light polarization (given by the quantities Ψ and Δ) of the incident linearly polarized light beam [10], was measured after the reflection on the frontal or a lateral cube face. Amorphous hydrogenated carbon is transparent at wavelengths over 600 nm. Over this value the refractive index of the deposited layer can be generated using a Cauchy dispersion [11]. Finally, the ellipsometry quantities are converted in refractive index (n) and layer thickness (d), by using a standard procedure [10,12]. The obtained dependence of the thickness of the non-removed layer upon the position (i.e. upon gap deepness) for different times is shown in Fig. 5a, while the dependence of thickness upon time for the front side is shown in Fig. 5b. From Fig. 5a, results show that the initial 1.2 μm thick carbon layer is completely removed up to 3 mm deepness in less than 5 min. In this region the removal rate is about 0.24 μm/min. As concerning the front side, the removal rates vary between 0.01 and 0.04 μm/min. The etching homogeneity along the gaps is not important for applications where removing is the main purpose, whatever the place from where the material is removed, but can be important for specific applications. The non-homogeneity of the etching is related to discharge non-homogeneity inside the gaps. The effect of discharge non-homogeneity is visible in Fig. 4b, where the border line between the upper cleaned zone and the bottom partially non-cleaned zone is not straight. Deeper cleaned zones are observed at the crossings points of the channels, where a larger space was available for dis-
a
charge penetration, and also in the middle of the lateral faces of the cubes, where a brighter plasma zone was present due to segmentation. For the given example here the non-homogeneity of the etching rate on position along the sample was estimated by the relative standard deviation of the distances from the upper cube face to the border line. The obtained value, of 45%, is a measure of the deviation of the border from a straight line, in presence of crossing channels and segmentation effect. 5. Conclusions Experiments were performed in order to check the possibility to control the development of a radiofrequency discharge inside gaps with various widths. We have shown that by gradually modifying the radiofrequency power forwarded to plasma and the discharge pressure it is possible to create and maintain the plasma inside narrow gaps for well defined domains of the pressure-power parameter space. The cleaning process is favored by plasma presence in the proximity of the surface to be cleaned. Cleaning experiments were performed in a mixture of argon/air on castellated surfaces having rectangular gaps 1.5 mm in width and 6 mm in depth, coated with hydrogenated amorphous carbon layers. Comparing to the frontal surface exposed to plasma the cleaning process is faster on the insidegap's surfaces, probably due to a hollow electrode type process enhancement. Also, the cleaning process is faster at the edges, as example at the gap entrance. Experiments with a movable active electrode have proved that the plasma column size and its contact area moves over surfaces, like a
b 1.2
1.2
t=0 min
front 1.0
1.0 t=20 min
0.8 t=10 min
0.6 t=15 min
0.4
Thickness [µm]
Thickness [m]
t=5 min
0.8 0.6 0.4
t=25 min
0.2
0.2 t=30 min
0.0
t=35 min
1
2
3
4
Position [mm]
5
6
0.0 -5
0
5
10
15
20
25
30
35
40
Time [min]
Fig. 5. a) Dependence of layer thickness upon deepness, on lateral side and for different treatment times; b) dependence of layer thickness, on the front side, upon the time.
S438
C. Stancu et al. / Surface & Coatings Technology 205 (2011) S435–S438
movable “plasma shower”, while the plasma remains penetrating inside the gaps. This result has relevance for cleaning of large nonuniform surfaces which present defects, holes, or gaps. Acknowledgments This work was partially supported in the frame of the EURATOM programme, Romanian Association MEdC and under CNCSIS contract ID_1999/2008. References [1] K. Krieger, W. Jacob, D.L. Rudakov, R. Bastasz, G. Federici, A. Litnovsky, H. Maier, V. Rohde, G. Strohmayer, W.P. West, J. Whaley, C.P.C. Wong, J. Nucl. Mater. 363–365 (2007) 870.
[2] C. Grisolia, A. Semerok, J.M. Weulersse, F. Le Guern, S. Fomichev, F. Brygo, P. Fichet, P.Y. Thro, P. Coad, N. Bekris, M. Stamp, S. Rosanvallon, G. Piazza, J. Nucl. Mater. 363–365 (2007) 1138. [3] C. Grisolia, G. Counsell, G. Dinescu, A. Semerok, N. Bekris, P. Coad, C. Hopf, J. Roth, M. Rubel, A. Widdowson, E. Tsitrone, Fusion Eng. Des. 82 (2007) 2390. [4] I. Tanarro, J.A. Ferreira, V.J. Herrero, F.L. Tabarés, C. Gómez-Aleixandre, J. Nucl. Mater. 390–391 (2009) 696. [5] J.A. Ferreira, F.L. Tabarés, D. Tafalla, J. Nucl. Mater. 363–365 (2007) 888. [6] C. Hopf, W. Jacob, V. Rohde, J. Nucl. Mater. 374 (2008) 413. [7] A. Grill, Cold Plasma in Materials Fabrication, IEEE Press, New York, 1994. [8] C.-K. Kim, D.J. Economou, J. Appl. Phys. 91 (2002) 2594. [9] B. Mitu, S.I. Vizireanu, C. Petcu, G. Dinescu, M. Dinescu, R. Birjega, V.S. Teodorescu, Surf. Coat. Technol. 180 (2004) 238. [10] R.M.A. Azzam, N.H. Bashara, Ellipsometry and Polarized Light, North Holland, Amsterdam, 1987. [11] L. Cauchy, Bull. Sci. Math. 14 (1830) 6. [12] V. Ion, A.C. Galca, N.D. Scarisoreanu, M. Filipescu, M. Dinescu, Phys. Status Solidi C 5 (2008) 1180.