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A new linearly extended bifocal microwave plasma device
Surface and Coatings Technology 142᎐144 Ž2001. 939᎐942
A new linearly extended bifocal microwave plasma device b M. Kaiser a,U , H. Urban a , R. Emmericha , P. Elsner a , K.-M. Baumgartner , ¨ c E. Rauchle ¨ a
Fraunhofer Institute of Chemical Technology, Joseph-¨ on-Fraunhofer-Str. 7, D-76327 Pfinztal, Germany b Muegge Electronic Company, Reichelsheim, Germany c Institut fur ¨ Plasmaforschung der Uni¨ ersitat ¨ Stuttgart, Paffenwaldring 31, 70569 Stuttgart, Germany
Abstract A new kind of linearly extended plasma source was developed. The prototype of such a device was built up as a barrel with elliptical cross-section. This bifocal shape has two focus lines. A linearly extended microwave antenna is placed along one focus line and the sector of plasma treatment is placed along the second focus line. The antenna is a linear emitter of radial divergent microwave radiation. In this work, a frequency of 2.45 GHz in pulsed and continuous wave ŽCW. mode was used. The power up to 6 kW CW and 10 kW pulsed is adjustable, pulse onroff times are variable in a wide field. The microwave is reflected by the metallic walls of the barrel and concentrated in the second focus line apart from the antenna. There a quartz tube is placed, that can be evaporated and filled with a working gas at a pressure between 10 2 and 10 5 Pa. The presented experiments have been done with Argon up to atmospheric pressure at a gas flow up to 6 slm. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: PACVD; PAPVD; Atmospheric plasma
1. Introduction Plasma technology for activation, treatment, etching and cleaning of surfaces and plasma-assisted CVD is of interest for many technical processes and applications w1᎐4x. The process of deposition and etching needs in most fields of application a good control of gas kinetics. That means a defined gas mixture, gas flow and mean free path. Working conditions at low pressure with a lot of technological and financial expenditure is usually necessary w5,6x. In the field of surface treatment and activation, the methods of plasma treatment at atmospheric pressure are well established. Beside the conventional flame treatment, the so-called ‘cold plasma’ treatment from corona or barrier discharges finds more and more success in technical applications. The high electric field
U
Corresponding author.
of such a discharge leads to a particular ionisation of the gas Žcold plasma.. With air at atmospheric pressure these ions can be formed from oxygen or nitrogen and in second order from nitrogen oxides and others. The ionised gas may destroy hazardous gases, clean surfaces or may introduce polar groups into the interacting surface of polymers to improve wetability and adhesion w7,8x. Furthermore, the deposition in barrier discharges becomes more and more popular w9x. For many practical problems the plasma density of corona and barrier discharges is insufficient. The increase of the plasma density, especially cold plasma density, in discharges is complex and usually leads to a superlinear increase of expenditure that typically ends up with a pulsed discharge at high frequency and very high voltage w10x. If this trend is continued, we come to microwave frequency where especially at 915 and 2.45 GHz a sufficient support of power with reasonable expense is available.
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 1 9 - 6
940
M. Kaiser et al. r Surface and Coatings Technology 142᎐144 (2001) 939᎐942
In this paper we would like to introduce a microwave driven ‘cold plasma’ source at atmospheric pressure with easy up-scaling and with a wide field of possible industrial applications.
2. Experimental details Usually the source of microwave for plasma excitation is an antenna with divergent characteristics. The electric field decreases with distance of the order ry2 . With respect to typical transmitter ᎐receiver systems a concentration of the microwave apart from the antenna is useful. In this paper a solution with a bifocal arrangement is introduced, where the antenna and the wave concentrator are in the focus lines of a metallic barrel with elliptical cross-section. The scheme of the plasma source is shown in Fig. 1. The dimension of the ellipse is 30 and 50 cm for the respective semi-axes. The elliptical barrel was made of aluminium sheets with a thickness of 2 mm. The shape is spanned between two planar walls with a steel superstructure. The barrel is linearly extended over the length of 1000 mm. Along the upper focus line the microwave antenna is placed. The antenna consists of a 10-mm copper rod with a length of 1000 mm. The copper rod is mounted in co-axial couplers at both sides of the rod to transform the microwave power from the waveguide to the coaxial-like antenna. To optimise the matching, variable short cuts and three stub tuners are mounted at each side. The upper part of the barrel is then like a typical cylindrical radar transmitter w11x. After reflection at the metallic walls, the microwave is concentrated in the lower focus line. This sector is where the ignition of the plasma takes place. For easy pressure and gas flow control of the plasma gases, a quartz tube is mounted along the lower focus line. The quartz tube has a diameter of 30 mm. The front end of the quartz tube is connected to a vacuum pump and a vacuum gauge for pressure control in the range of 100 Pa up to atmospheric pressure Ž10 5 Pa.. On the back end of the tube an interface for gas
Fig. 1. Schematic view of the elliptical barrel with linear extension.
Fig. 2. Photography of the experimental set up.
inlet is installed. Gas inlet and gas flow is controlled by a mass flow controller system with a maximum flow of 10 slm for each controller. In the middle of the planar front wall, a microwave sealed aperture is placed for visual observation of the plasma conditions. Fig. 2 shows a photograph of the experimental set up. The top region shows the microwave feed in and the bottom region shows the adaptation of the vacuum supply to the quartz tube. The following photographs of the plasma were taken through the window. The microwave source is a 2.45-GHz water cooled magnetron with a power supply of Muegge Electronic. The magnetron can be driven in pulsed and continuous wave ŽCW. mode. The power is variable in the range of 1᎐6 kW for the CW mode and 1᎐10 kW for the pulsed mode. The power pulse mode control allows variable ratios of on᎐off time from 20 s to 160 ms and pulse duration between 20 s and 160 ms, independently. The power can be split to both sides of the antenna with a beam splitter mounted on a high power three stub tuner. To protect the magnetron from reflected power, a circulator with water dummy load is placed between the tuner and the magnetron.
3. Results and discussion A first experiment with an argon plasma at low pressure should demonstrate the validation of the bifocal principle. Fig. 3 shows a photograph of an argon plasma at 110 Pa and a total flow of 1.5 slm, excited with pulsed microwave power of 1 kW with a symmetric pulse on᎐off time of 0.5 ms. The plasma is homogeneously extended over the whole length of the quartz tube. The experimental set up allows a continuous increase of the argon pressure up to atmospheric pressure and the plasma turned out to be stable over the whole pressure regime. The higher argon flow of 6 slm
M. Kaiser et al. r Surface and Coatings Technology 142᎐144 (2001) 939᎐942
Fig. 3. Argon plasma at 110 Pa, pulsed power, 1 kW, symmetric pulse on᎐off time 0.5 ms.
made an increase of power necessary. Fig. 4 shows a photograph of an argon plasma at atmospheric pressure Ž10 5 Pa., excited with pulsed microwave power of 7 kW with a symmetric pulse on᎐off time of 0.5 ms. The plasma is all in all homogeneous, but structured as streamers. The extension of the plasma is reduced and has a length of approximately 900 mm. It is remarkable that there was no need for cooling the tube. The upper result at low pressure is not very surprising. In previous work w12,13x a linearly extended lowpressure plasma source, called Duo-Plasmaline 䊛 , was introduced. The main feature of that Plasmaline is a linearly extended axially homogeneous plasma. The
941
line can be treated like an inverted Ž1rr . luminescent tube where the plasma is outside the quartz tube in a vacuum chamber. Inside the tube are, air at atmospheric pressure and the same microwave antenna as described in Fig. 1. The running Plasmaline is comparable to a co-axial waveguide where the outer conductor is particularly replaced by the low-pressure plasma. The linear extension of this kind of plasma depends mainly on the microwave power and might reach several meters. Although the plasma of these sources is very effective because of the high electron density Žof the order of the cut off density 7.46= 10 10 cmy3 ., the highest electron density is close to the quartz tube with a radial decay w13x. This fact is accompanied by some disadvantages. The recombination processes of the plasma are accelerated by plasma wall interactions with highest plasma activity, deposition rates and thermal or radiation strain of the antenna or the corresponding device. This may change the properties of the line and for conducting layer deposition, the microwave transmission. In high vacuum applications, this kind of problem can be overcome by separation of the antenna and the plasma by ECR techniques. Besides the ECR technique, another method for separation of antenna and plasma is published w14x. Wild et al. have introduced a plasma device of ellipsoidal geometry. It is used for diamond plasma synthesis featuring a separation of the antenna from the plasma area in the two focuses of the ellipsoid. The working pressure is up to several 10 000 Pa. The presented bifocal plasma source is a combination of the above mentioned linear source and the separation of antenna and plasma. The Duo-Plasmaline is now the antenna for microwave power and the elliptical shaped barrel is a separator of the plasma zone from the antenna. The shaped barrel works as a very efficient linearly extended concentrator of the microwave power by inverting the 1rr decay of the antenna field strength in the upper focus line towards the plasma area in the lower focus line. The bifocal plasma source may combine the advantages of the plasmaline and the ellipsoid for a new generation of plasma applications in the medium pressure regime up to atmospheric pressure.
4. Outlook
Fig. 4. Argon plasma at atmospheric pressure, pulsed power, 7 kW, symmetric pulse on᎐off time 0.5 ms.
Many established methods for atmospheric plasma generation are described, e.g. w15᎐17x. Barrier discharge, flame treatment or plasma jets are well known examples for atmospheric plasma states. For many applications the thermal non-equilibrium between electrons and neutral gas, radicals and ions is of interest. The electron energy of the order of some electron volts
942
M. Kaiser et al. r Surface and Coatings Technology 142᎐144 (2001) 939᎐942
is responsible for the plasma reactivity, while the neutrals, radicals and ions are at room temperature Ž0.025 eV.. This so-called ‘cold plasma’ can be reached easily at low pressure. At atmospheric pressure the mean free path of the electrons is much shorter, and the interaction between electrons and neutral gas leads to a decreasing electron temperature and an increasing neutral gas temperature. With a high frequency electric field activation of the electrons some kind of ‘cold plasma’ can be observed mainly in dielectric barrier discharges. The linearly extended bifocal plasma source allows plasma excitation up to atmospheric pressure, but with higher excitation densities the validity of the term ‘cold plasma’ gets more and more lost. The pulsed power mode and a variable on᎐off ratio may allow to adjust a reasonable average power, to keep the high electron temperature and a moderate gas temperature. The plasma chemistry, however, is different to the processes in low-pressure plasmas and may offer future surprises. With the assumption of a stable homogeneous free standing plasma along the focus line at atmospheric pressure opens a wide field of applications in the field of activation, cleaning and deposition. The linear extension, the well established microwave technology, the easy up scaling in power and size and the avoidance of vacuum expense is very promising for large scale applications and further investigations on characterising studies. The first prototype already opens the field of optical plasma observations where the easy set up, the long optical path through the plasma, the high plasma density in the focus, the avoidable plasma wall interactions and the adjustable plasma conditions give a promising tool for basic optical plasma investigations. The geometry of the described barrel depends on the wavelength of the radiation. It is possible to build up smaller barrels when using dielectric filling compounds or higher frequencies than 2.45 GHz. On the other
hand, one can use lower frequencies, which leads to longer wavelength. The 915-MHz technology is well established in industrial application when very high power up to several 100 kW is necessary. The easy geometry and the easy construction of the system leads to a low cost upscaling when larger wavelengths and therefore higher power applications are necessary. References w1x H.V. Boening, Fundamentals of Plasma Chemistry and Technology, Technomic Publishing Company, Lancaster, UK, 1988. w2x H. Yasuda, Plasma Polymerisation, Academic Press, Orlando, FL, 1985. w3x R. d’Agostino, Treatment and Etching of Polymers, Academic Press, San Diego, CA, 1990. w4x F. Garbassi, M. Morra, E. Occichello, Polymer Surfaces, From Physics to Technology, Wiley, Chichester, UK, 1994. w5x T.D. Bestwick, G.S. Oehrlein, J. Vac. Sci. Technol. A 8 Ž3. Ž1990. 1696. w6x E. Ghanbari, I. Trigor, T. Nguyen, J. Vac. Sci. Technol. A 7 Ž3. Ž1989. 918. w7x J.-S. Chang, Jpn. Soc. Appl. Phys 69 Ž3. Ž2000. 268. w8x D. Zhang, Q. Sun, L.C. Wadsworth, Polymer engineering and science, USA, Soc. Plastics Eng. 38 Ž1998. 965. w9x Thyen R., JOT q Oberflache. J. Oberflachentech. 38Ž5. 1998, ¨ ¨ Sonderteil Dunne Schichten, ISSN 0940-8789 pp. 12᎐14 ¨ w10x S. Masuda, S. Hosokawa, X. Tu, Z. Wang, J. Electrostat. 34 Ž1995. 415. w11x E. Pehl, Huthig Verlag, Heidelberg, Mikrowellentechnik, 2, ¨ 1989. ISBN 3-7785-1667-1, p. 36 ff w12x German patent, DE 19503205 C1, 2 February 1995. w13x W. Petasch, E. Rauchle, H. Muegge, K. Muegge, Surf. Coat. ¨ Technol. 93 Ž1997. 112. w14x M. Funer, C. Wild, P. Koidl, Surf. Coat. Technol. 116r119 ¨ Ž1999. 853. w15x G. Kruger, U. Meyer, Adhesion Kleben u. Dichten, Jg. 42, ¨ 5r98. w16x J.B. Class, Tackifying rubber compositions, Rubber World 219 Ž1998. 1. w17x D. Grasme, Beschichten von aluminiumteilen durch hochleistungs-plasmaspritzverfahren, WT-Z Ind. Fertigung 68 Ž1978. 265᎐267.
A new linearly extended bifocal microwave plasma device b M. Kaiser a,U , H. Urban a , R. Emmericha , P. Elsner a , K.-M. Baumgartner , ¨ c E. Rauchle ¨ a
Fraunhofer Institute of Chemical Technology, Joseph-¨ on-Fraunhofer-Str. 7, D-76327 Pfinztal, Germany b Muegge Electronic Company, Reichelsheim, Germany c Institut fur ¨ Plasmaforschung der Uni¨ ersitat ¨ Stuttgart, Paffenwaldring 31, 70569 Stuttgart, Germany
Abstract A new kind of linearly extended plasma source was developed. The prototype of such a device was built up as a barrel with elliptical cross-section. This bifocal shape has two focus lines. A linearly extended microwave antenna is placed along one focus line and the sector of plasma treatment is placed along the second focus line. The antenna is a linear emitter of radial divergent microwave radiation. In this work, a frequency of 2.45 GHz in pulsed and continuous wave ŽCW. mode was used. The power up to 6 kW CW and 10 kW pulsed is adjustable, pulse onroff times are variable in a wide field. The microwave is reflected by the metallic walls of the barrel and concentrated in the second focus line apart from the antenna. There a quartz tube is placed, that can be evaporated and filled with a working gas at a pressure between 10 2 and 10 5 Pa. The presented experiments have been done with Argon up to atmospheric pressure at a gas flow up to 6 slm. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: PACVD; PAPVD; Atmospheric plasma
1. Introduction Plasma technology for activation, treatment, etching and cleaning of surfaces and plasma-assisted CVD is of interest for many technical processes and applications w1᎐4x. The process of deposition and etching needs in most fields of application a good control of gas kinetics. That means a defined gas mixture, gas flow and mean free path. Working conditions at low pressure with a lot of technological and financial expenditure is usually necessary w5,6x. In the field of surface treatment and activation, the methods of plasma treatment at atmospheric pressure are well established. Beside the conventional flame treatment, the so-called ‘cold plasma’ treatment from corona or barrier discharges finds more and more success in technical applications. The high electric field
U
Corresponding author.
of such a discharge leads to a particular ionisation of the gas Žcold plasma.. With air at atmospheric pressure these ions can be formed from oxygen or nitrogen and in second order from nitrogen oxides and others. The ionised gas may destroy hazardous gases, clean surfaces or may introduce polar groups into the interacting surface of polymers to improve wetability and adhesion w7,8x. Furthermore, the deposition in barrier discharges becomes more and more popular w9x. For many practical problems the plasma density of corona and barrier discharges is insufficient. The increase of the plasma density, especially cold plasma density, in discharges is complex and usually leads to a superlinear increase of expenditure that typically ends up with a pulsed discharge at high frequency and very high voltage w10x. If this trend is continued, we come to microwave frequency where especially at 915 and 2.45 GHz a sufficient support of power with reasonable expense is available.
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 1 9 - 6
940
M. Kaiser et al. r Surface and Coatings Technology 142᎐144 (2001) 939᎐942
In this paper we would like to introduce a microwave driven ‘cold plasma’ source at atmospheric pressure with easy up-scaling and with a wide field of possible industrial applications.
2. Experimental details Usually the source of microwave for plasma excitation is an antenna with divergent characteristics. The electric field decreases with distance of the order ry2 . With respect to typical transmitter ᎐receiver systems a concentration of the microwave apart from the antenna is useful. In this paper a solution with a bifocal arrangement is introduced, where the antenna and the wave concentrator are in the focus lines of a metallic barrel with elliptical cross-section. The scheme of the plasma source is shown in Fig. 1. The dimension of the ellipse is 30 and 50 cm for the respective semi-axes. The elliptical barrel was made of aluminium sheets with a thickness of 2 mm. The shape is spanned between two planar walls with a steel superstructure. The barrel is linearly extended over the length of 1000 mm. Along the upper focus line the microwave antenna is placed. The antenna consists of a 10-mm copper rod with a length of 1000 mm. The copper rod is mounted in co-axial couplers at both sides of the rod to transform the microwave power from the waveguide to the coaxial-like antenna. To optimise the matching, variable short cuts and three stub tuners are mounted at each side. The upper part of the barrel is then like a typical cylindrical radar transmitter w11x. After reflection at the metallic walls, the microwave is concentrated in the lower focus line. This sector is where the ignition of the plasma takes place. For easy pressure and gas flow control of the plasma gases, a quartz tube is mounted along the lower focus line. The quartz tube has a diameter of 30 mm. The front end of the quartz tube is connected to a vacuum pump and a vacuum gauge for pressure control in the range of 100 Pa up to atmospheric pressure Ž10 5 Pa.. On the back end of the tube an interface for gas
Fig. 1. Schematic view of the elliptical barrel with linear extension.
Fig. 2. Photography of the experimental set up.
inlet is installed. Gas inlet and gas flow is controlled by a mass flow controller system with a maximum flow of 10 slm for each controller. In the middle of the planar front wall, a microwave sealed aperture is placed for visual observation of the plasma conditions. Fig. 2 shows a photograph of the experimental set up. The top region shows the microwave feed in and the bottom region shows the adaptation of the vacuum supply to the quartz tube. The following photographs of the plasma were taken through the window. The microwave source is a 2.45-GHz water cooled magnetron with a power supply of Muegge Electronic. The magnetron can be driven in pulsed and continuous wave ŽCW. mode. The power is variable in the range of 1᎐6 kW for the CW mode and 1᎐10 kW for the pulsed mode. The power pulse mode control allows variable ratios of on᎐off time from 20 s to 160 ms and pulse duration between 20 s and 160 ms, independently. The power can be split to both sides of the antenna with a beam splitter mounted on a high power three stub tuner. To protect the magnetron from reflected power, a circulator with water dummy load is placed between the tuner and the magnetron.
3. Results and discussion A first experiment with an argon plasma at low pressure should demonstrate the validation of the bifocal principle. Fig. 3 shows a photograph of an argon plasma at 110 Pa and a total flow of 1.5 slm, excited with pulsed microwave power of 1 kW with a symmetric pulse on᎐off time of 0.5 ms. The plasma is homogeneously extended over the whole length of the quartz tube. The experimental set up allows a continuous increase of the argon pressure up to atmospheric pressure and the plasma turned out to be stable over the whole pressure regime. The higher argon flow of 6 slm
M. Kaiser et al. r Surface and Coatings Technology 142᎐144 (2001) 939᎐942
Fig. 3. Argon plasma at 110 Pa, pulsed power, 1 kW, symmetric pulse on᎐off time 0.5 ms.
made an increase of power necessary. Fig. 4 shows a photograph of an argon plasma at atmospheric pressure Ž10 5 Pa., excited with pulsed microwave power of 7 kW with a symmetric pulse on᎐off time of 0.5 ms. The plasma is all in all homogeneous, but structured as streamers. The extension of the plasma is reduced and has a length of approximately 900 mm. It is remarkable that there was no need for cooling the tube. The upper result at low pressure is not very surprising. In previous work w12,13x a linearly extended lowpressure plasma source, called Duo-Plasmaline 䊛 , was introduced. The main feature of that Plasmaline is a linearly extended axially homogeneous plasma. The
941
line can be treated like an inverted Ž1rr . luminescent tube where the plasma is outside the quartz tube in a vacuum chamber. Inside the tube are, air at atmospheric pressure and the same microwave antenna as described in Fig. 1. The running Plasmaline is comparable to a co-axial waveguide where the outer conductor is particularly replaced by the low-pressure plasma. The linear extension of this kind of plasma depends mainly on the microwave power and might reach several meters. Although the plasma of these sources is very effective because of the high electron density Žof the order of the cut off density 7.46= 10 10 cmy3 ., the highest electron density is close to the quartz tube with a radial decay w13x. This fact is accompanied by some disadvantages. The recombination processes of the plasma are accelerated by plasma wall interactions with highest plasma activity, deposition rates and thermal or radiation strain of the antenna or the corresponding device. This may change the properties of the line and for conducting layer deposition, the microwave transmission. In high vacuum applications, this kind of problem can be overcome by separation of the antenna and the plasma by ECR techniques. Besides the ECR technique, another method for separation of antenna and plasma is published w14x. Wild et al. have introduced a plasma device of ellipsoidal geometry. It is used for diamond plasma synthesis featuring a separation of the antenna from the plasma area in the two focuses of the ellipsoid. The working pressure is up to several 10 000 Pa. The presented bifocal plasma source is a combination of the above mentioned linear source and the separation of antenna and plasma. The Duo-Plasmaline is now the antenna for microwave power and the elliptical shaped barrel is a separator of the plasma zone from the antenna. The shaped barrel works as a very efficient linearly extended concentrator of the microwave power by inverting the 1rr decay of the antenna field strength in the upper focus line towards the plasma area in the lower focus line. The bifocal plasma source may combine the advantages of the plasmaline and the ellipsoid for a new generation of plasma applications in the medium pressure regime up to atmospheric pressure.
4. Outlook
Fig. 4. Argon plasma at atmospheric pressure, pulsed power, 7 kW, symmetric pulse on᎐off time 0.5 ms.
Many established methods for atmospheric plasma generation are described, e.g. w15᎐17x. Barrier discharge, flame treatment or plasma jets are well known examples for atmospheric plasma states. For many applications the thermal non-equilibrium between electrons and neutral gas, radicals and ions is of interest. The electron energy of the order of some electron volts
942
M. Kaiser et al. r Surface and Coatings Technology 142᎐144 (2001) 939᎐942
is responsible for the plasma reactivity, while the neutrals, radicals and ions are at room temperature Ž0.025 eV.. This so-called ‘cold plasma’ can be reached easily at low pressure. At atmospheric pressure the mean free path of the electrons is much shorter, and the interaction between electrons and neutral gas leads to a decreasing electron temperature and an increasing neutral gas temperature. With a high frequency electric field activation of the electrons some kind of ‘cold plasma’ can be observed mainly in dielectric barrier discharges. The linearly extended bifocal plasma source allows plasma excitation up to atmospheric pressure, but with higher excitation densities the validity of the term ‘cold plasma’ gets more and more lost. The pulsed power mode and a variable on᎐off ratio may allow to adjust a reasonable average power, to keep the high electron temperature and a moderate gas temperature. The plasma chemistry, however, is different to the processes in low-pressure plasmas and may offer future surprises. With the assumption of a stable homogeneous free standing plasma along the focus line at atmospheric pressure opens a wide field of applications in the field of activation, cleaning and deposition. The linear extension, the well established microwave technology, the easy up scaling in power and size and the avoidance of vacuum expense is very promising for large scale applications and further investigations on characterising studies. The first prototype already opens the field of optical plasma observations where the easy set up, the long optical path through the plasma, the high plasma density in the focus, the avoidable plasma wall interactions and the adjustable plasma conditions give a promising tool for basic optical plasma investigations. The geometry of the described barrel depends on the wavelength of the radiation. It is possible to build up smaller barrels when using dielectric filling compounds or higher frequencies than 2.45 GHz. On the other
hand, one can use lower frequencies, which leads to longer wavelength. The 915-MHz technology is well established in industrial application when very high power up to several 100 kW is necessary. The easy geometry and the easy construction of the system leads to a low cost upscaling when larger wavelengths and therefore higher power applications are necessary. References w1x H.V. Boening, Fundamentals of Plasma Chemistry and Technology, Technomic Publishing Company, Lancaster, UK, 1988. w2x H. Yasuda, Plasma Polymerisation, Academic Press, Orlando, FL, 1985. w3x R. d’Agostino, Treatment and Etching of Polymers, Academic Press, San Diego, CA, 1990. w4x F. Garbassi, M. Morra, E. Occichello, Polymer Surfaces, From Physics to Technology, Wiley, Chichester, UK, 1994. w5x T.D. Bestwick, G.S. Oehrlein, J. Vac. Sci. Technol. A 8 Ž3. Ž1990. 1696. w6x E. Ghanbari, I. Trigor, T. Nguyen, J. Vac. Sci. Technol. A 7 Ž3. Ž1989. 918. w7x J.-S. Chang, Jpn. Soc. Appl. Phys 69 Ž3. Ž2000. 268. w8x D. Zhang, Q. Sun, L.C. Wadsworth, Polymer engineering and science, USA, Soc. Plastics Eng. 38 Ž1998. 965. w9x Thyen R., JOT q Oberflache. J. Oberflachentech. 38Ž5. 1998, ¨ ¨ Sonderteil Dunne Schichten, ISSN 0940-8789 pp. 12᎐14 ¨ w10x S. Masuda, S. Hosokawa, X. Tu, Z. Wang, J. Electrostat. 34 Ž1995. 415. w11x E. Pehl, Huthig Verlag, Heidelberg, Mikrowellentechnik, 2, ¨ 1989. ISBN 3-7785-1667-1, p. 36 ff w12x German patent, DE 19503205 C1, 2 February 1995. w13x W. Petasch, E. Rauchle, H. Muegge, K. Muegge, Surf. Coat. ¨ Technol. 93 Ž1997. 112. w14x M. Funer, C. Wild, P. Koidl, Surf. Coat. Technol. 116r119 ¨ Ž1999. 853. w15x G. Kruger, U. Meyer, Adhesion Kleben u. Dichten, Jg. 42, ¨ 5r98. w16x J.B. Class, Tackifying rubber compositions, Rubber World 219 Ž1998. 1. w17x D. Grasme, Beschichten von aluminiumteilen durch hochleistungs-plasmaspritzverfahren, WT-Z Ind. Fertigung 68 Ž1978. 265᎐267.