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Surface modification of polyimide using dielectric barrier discharge treatment
Surface and Coatings Technology 142᎐144 Ž2001. 455᎐459
Surface modification of polyimide using dielectric barrier discharge treatment R. Seebock ¨ a,U , H. Esroma , M. Charbonnier b, M. Romandb, U. Kogelschatz c a
b
Uni¨ ersity of Applied Sciences Mannheim (HTG), Windeckstr. 110, D-68163 Mannheim, Germany Laboratoire des Sciences et Ingenierie ´ des Surfaces, Uni¨ ersite´ Claude Bernard-Lyon 1, F-69622 Villeurbanne Cedex, France c ABB Corporate Research Ltd, CH-5405 Baden, Switzerland
Abstract We report on a novel method for the surface modification of polymers by direct exposure to a dielectric barrier discharge ŽDBD. at atmospheric pressure and room temperature. The polymer under treatment is located directly on the grounded electrode and serves at the same time as the discharge barrier. So far DBD treatment of the unfilled and Al 2 O 3-filled commercially available polyimide foils have been investigated as a function of specific discharge energy. The morphological effects of the DBD treatment have been analysed by optical microscopy, SEM, AFM, and chromatic coding distance measurement while XPS was used for analysis of the chemical surface composition. The results can be summarised as follows: Firstly, the etching rate of the polyimide ŽPI. surface by DBD in air is rather high leading to a pronounced roughening within some tens of seconds. Secondly, the attack is dependent on whether the polyimide contains a filler, which is added to improve the thermal conductivity of the material. In this case the etching lays bare the grains of the filler but is spatially rather uniform. The surface roughening increases the bond strength to coating layers. Finally, in the unfilled material crater-like structures are observed which are attributed to the repetitive ignition of discharge filaments in the same location. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyimide; Dielectric barrier discharge; Chemical modification; Morphological modification; Polymers
1. Introduction Polymers play an increasing role as structural materials, as foils for protective and packaging applications and as coatings for corrosion protection or sealing applications. PI has outstanding properties like high tensile strength and dielectric strength above 22 kVrmm w1x. Today it is used in the electronic industry as a material for flexible chip carriers. One type of PI, commercially available in web form, is Kapton 䊛 , manufactured by DuPont de Nemours, to which all the investigations in this work refer. In order to act as a chip carrier the PI surface has to be metallised by copper. Here the poor adhesion of PI to metals, which is a consequence of the low specific U
Corresponding author.
surface energy, has to be overcome w2x. Adhesion is strongly influenced by the chemical nature of the surface layer, because the surface layer bonds to a metal layer mainly by physi- or chemisorption w3x. A wellestablished method to increase adhesion physio-chemically is to expose the surface to a corona discharge in air at atmospheric pressure. In this way the surface energies of many polymer surfaces are increased considerably shown by a drastic decrease of the water contact angles. The discharge energies applied per treated surface area Žcalled ‘specific discharge energies’. needed to achieve a certain surface energy vary over two orders of magnitude from one polymer to the other w4x. Chemical changes in the surface layer may also be effected by exposure to low pressure gas discharges. The specific discharge energies reported for such processes are usually three orders of magnitude higher than those in the corona case. Therefore besides
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 0 8 5 - 4
456
R. Seebock ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
chemical changes also structural changes of the surface topography like roughening are observed in low-pressure experiments w5x. Besides chemical effects also the physical surface roughening plays an important role in adhesion improvement w6x. In two preceding papers w7,8x we have given first results on DBD processing of PI. In the present work, we present results on the modification of PI surfaces that are obtained in dielectric barrier discharges ŽDBD. in air at atmospheric pressure under specific discharge energies ranging from 8 = 10 3 to 10 6 Wminrm2 . Due to these high specific energies morphological changes of the surfaces are induced and shall be discussed.
2. Experiments 2.1. Experimental set-up for DBD-processing All experiments were performed on samples of Kapton 䊛 200 HN and Kapton 䊛 150 MT foils with a thickness of 50 and 38 m, respectively. The HN material represents the pure PI-polymer, while MT is aluminafilled and possesses three times the thermal conductivity of HN. The foils were cut into square shaped samples of sizes between 2.0= 2.0 and 4.0= 4.0 cm2 and cleaned ultrasonically in acetone before discharge treatment. For DBD processing the samples were placed on a grounded planar Cu electrode. A schematic view of the experimental arrangement is shown in Fig. 1. A load Cu ring was used to prevent the polymeric samples from bending upwardly. The PI film itself forms the dielectric barrier of the discharge. The upper electrodes were made of stainless steel or Cu and had diameters between 0.6 and 2.0 cm according to the desired treatment area. The discharge gap is built between the planar bottom surface of the upper electrode and the PI foil. The gap width was adjusted with the help of a motorised vertical precision translator ŽNEWPORT M-UTM50.. In the experiments a gap width of 100 m was used. The discharge was driven at a frequency of 125 kHz. Details are given in Seebock ¨ and Esrom w7x. The specific discharge energy was calculated as the ratio of load power read at the generator and upper electrode area. 2.2. Morphological surface analysis The surface morphology of the samples was investigated by four different methods. An optical microscope ŽCARL ZEISS. served as a tool for quick inspection. In order to get information on the surface topography, we used a SEM ŽLEITZ DSM 950.. We furthermore employed AFM ŽDIGITAL INSTRUMENTS Nanoscope III. as a high-resolution method with limited scanning
Fig. 1. Schematic view of the experimental set-up used for DBDtreatment in air at atmospheric pressure.
range to determine topography and roughness. Finally, we also present first results on contact-less distance measurements by chromatic coding ŽJURCA CHR 150N.. This method possesses a depth resolution on the order of 10 nm with a scanning range up to many centimetres. 2.3. Chemical surface analysis In order to study the chemical changes induced in the polymer surface by the DBD processing, investigations were carried out on the samples with an XPSspectrometer ŽRIBER SIA 200.. All XPS-peaks were referenced to the C 1s signal at a binding energy of approximately 285 eV representing the C᎐C and C᎐H bonds in hydrocarbons.
3. Results and discussion 3.1. Morphology-uniform surface appearance The DBD treatment of Kapton 䊛 results in a surface roughening of the PI samples visible with the naked eye. In Fig. 2 we show two Kapton 䊛 samples, one untreated, the second DBD treated. In the case of the HN material the untreated surface is very smooth with a roughness below 10 nm. After DBD treatment in the HN samples only some isolated grain-like structures occur with widely separated grains, while the surface in between remains rather smooth. These grains may be zones of different chemical composition compared to the bulk or crystallites. The untreated surfaces of Kapton 䊛 MT are not as smooth as those of HN, since some of the filler grains must intersect the macroscopic surface. In Fig. 3 a SEM micrograph of a Kapton 䊛 MT sample treated with 3.4= 10 3 Wminrm2 is shown, which corresponds to a treatment time of only 0.5 s. The surface topography is very similar to that of the totally untreated case,
R. Seebock ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
Fig. 2. Untreated Žleft. and DBD treated Žright. Kapton ples Župper electrode diameter 6 mm..
䊛
MT sam-
concerning the grain size distribution. However, chemical changes occur already after such short times Žsee Section 3.3.. When the treatment is extended in time, the number of grains visible at the surface increases, as can be seen from the micrograph in Fig. 4. Here the specific energy was 2.0= 10 5 Wminrm2 corresponding to 30-s treatment time. The surface shows a fine grainlike structure with a wide spread in grain diameters. According to the manufacturer these grains consist of Al 2 O 3 , therefore their etch rate in an O containing plasma is low. The net result of the treatment is the removal of PI material between the filler grains. To confirm the SEM results some of the samples were also investigated by AFM. The average roughness R a was obtained on an area of 1 m2 . Fig. 5 shows an AFM micrograph of Kapton 䊛 MT DBD treated with 5.1= 10 5 Wminrm2 . The R a value increased from 9.6" 5.6 nm for the untreated case to 85 " 20 nm. Since the scanning area is smaller than the average grain area, however, these value may differ from the macroscopic surface roughness observed in the SEM. Therefore we also used chromatic coding contactless roughness measurement, a result of which is shown in Fig. 6 for a specific energy of 5 = 10 5 Wminrm2 and a scanning range of 300 = 300 m2 . This technique also yields R a between 50 and 100 nm confirming the AFM result. The DBD-induced surface roughening may be very versatile to increase the adhesion to the filled PI.
457
Fig. 3. SEM micrograph of a Kapton 䊛 MT sample treated with 3.4= 10 3 Wminrm2 .
between 3.4= 10 3 and 5.1= 10 5 Wminrm2 showed a slight increase in the O and N percentages to 70.0 and 11.7 at.% while the C fraction decreased to 70.0 at.%. For more details refer to Charbonnier et al. w9x. The main feature of the XPS-results for the MT samples is the presence of Al from the filler and of some P. During DBD processing the percentages of Al and O increase while that of C decreases. The increase in P suggests that P is also a constituent of the filler. As the matrix material is the same as in the MT type samples, we expect only a slight oxidation of this fraction. We assume therefore that the O increase is essentially accounted for by the laying bare of the filler grains by the DBD processing. 3.3. Morphology-crater-like surface appearance In some of the experiments on Kapton 䊛 HN samples at high discharge power we observed structures that resemble craters on the surface after DBD processing. We never observed these structures in the MT samples.
3.2. Chemical surface composition Kapton 䊛 200 HN and 150 MT samples were investigated by XPS after different treatment times by using the DBD. In the HN material only its constituents C, O and N were found. In the untreated sample the relative percentages of these elements were 78.6, 15.0 and 6.4 at.%. The DBD treatment with specific energies
Fig. 4. SEM micrograph of a Kapton 䊛 MT sample treated with 2.0= 10 5 Wminrm2 .
458
R. Seebock ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
Fig. 7. Craters on Kapton 䊛 HN surface generated by DBD treatment.
Fig. 5. AFM micrograph of a Kapton 䊛 MT surface Žarea 1 m2 . DBD treated with 5.1= 10 5 Wminrm2 ; Scales: X,Y: 0.2 mrdiv; Z: 0.1 mrdiv.
An example is shown in Fig. 7. The craters are growing in diameters with increasing treatment time while their centres stay fixed at a distance of approximately 300 m. We assume that the origin of this appearance is the microstructure of the discharges. The DBD in air at 10 5 Pa is filamentary. Under a wide range of discharge conditions the filaments are ignited at random spatial positions in different cycles of the ac voltage waveform. In the case of crater formation we assume
Fig. 6. Chromatic coding distance measurement scan of a Kapton 䊛 MT surface Žarea 300 = 300 m2 . DBD treated with 5 = 10 5 Wminrm2 ; z-Scale: 1 mrdiv.
that filaments ignite repetitively at the same position in subsequent cycles of the driving voltage, so that a stationary filament pattern is formed that reflects itself in the crater structure on the dielectric. On the crater bottom a radial topographic structure is observed. This can be clearly seen in Fig. 7. In the crater bottom between the radially aligned structures the surface is rather smooth resembling the etching results in the uniform case. The strongest etching action in the direction perpendicular to the surface is obviously found in the craters, i.e. in a circular area centred at the discharge filaments. This would mean that the etching is essentially caused by the surface gliding discharge connected with each filament. It is well known from DBD modelling that the radical density in the gliding discharge is very high supporting the assumption. We further note that the electric field in the gliding discharge is radially directed away from the filament. Under its action particle motion occurs which leads to a radial component of the etching rate. The texture would then be the result of the radial etching rate together with a shadowing effect caused by zones of lower etching rate. The origin of the craters is further elucidated by the observation of a second type of surface structures after very short DBD treatment of HN samples, one example of which is shown in Fig. 8. These structures are found only in the Žunfilled. HN samples and for very low specific energy of the order of 10 3 Wminrm2 . They show a radial alignment in the direction of one ore several centres close to each other. The fine traces going out from these centres have led to a permanent damage of the PI surface. Whether this was accomplished by partial melting or by very intensive etching reaction by a high radical and UV photon density or by a combined action of both effects cannot be decided at present. The features of these structures resemble posi-
R. Seebock ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
459
unfilled material a fast etching is achieved in the DBD, too. If a smooth surface is to be maintained, care has to be taken that the non-stationary filament mode is established. On the other hand, if the planarity is not of crucial importance, centres for very strong anchoring of surface layers can be generated in the stationary filament mode.
Acknowledgements
Fig. 8. TraceŽs. of streamers andror DBD filamentŽs. on Kapton 䊛 HN sample resembling Lichtenberg figure Žarea 2.0= 2.7 mm2 ..
tive Lichtenberg figures w10x. Lichtenberg figures of positive point coronas are taken by placing a photographic film between the corona tip and the opposite plane electrode. The streamer propagating from the tip hits the film and induces a surface discharge. The dielectric in a barrier discharge plays the role of the film in the corona. The difference is that the DBD between parallel electrodes is not of the classical corona type, since no low field drift region is present w11x. Nevertheless, streamers propagate from one electrode and hit the dielectric inducing a surface discharge. We assume that the structure observed in Fig. 8 was created by the action of streamers or discharge filaments on the dielectric. If this process is further continued it might lead to crater formation. The detailed conditions of crater formation will be the subject of further research.
4. Conclusions We have shown that barrier discharges are able to effectively etch filled PI material of the Kapton 䊛 MT type. In this case a uniform etching is possible laying bare the top of the filler grains. We have already shown that Kapton 䊛 MT treated in this way may well be metallised with good adhesion w9,12,13x. In the case of
The authors gratefully acknowledge the funding by the Karl Volker Stiftung. We are indebted to Dr B. ¨ Michelt of Jurca GmbH for taking the chromatic coding scans and to I. Deppner for assistance during the experiments. References w1x Polyimide, data sheet, Goodfellow Co., Berwin, USA, 1998. w2x M. Strobel, C.S. Lyons, K.L. Mittal ŽEds.., Plasma Surface Modification of Polymers: Relevance to Adhesion, VSP, Utrecht, The Netherlands, 1994. w3x E.M. Liston, L. Martinu, M.R. Wertheimer, in: M. Strobel, C.S. Lyons, K.L. Mittal ŽEds.., Plasma Surface Modification of Polymers: Relevance to Adhesion, VSP, Utrecht, The Netherlands, 1994, pp. 3᎐39. w4x Softal Report. No.110, Softal Electronic GmbH, 21107 Hamburg, Germany. w5x N. Inagaki, S. Tasaka, K. Hibi, in: M. Strobel, C.S. Lyons, K.L. Mittal ŽEds.., Plasma Surface Modification of Polymers: Relevance to Adhesion, VSP, Utrecht, The Netherlands, 1994, pp. 275᎐290. w6x K.L. Mittal, J. Vac. Sci. Technol. 13 Ž1976. 19. w7x R. Seebock, ¨ H. Esrom, Proc. Of the 6th Int. Symp. on High Pressure, Low Temperature Plasma Chem, Plasma Ireland, Ltd, Cork, Ireland, 31 August᎐2 September 1998. w8x H. Esrom, R. Seebock, ¨ M. Charbonnier, M. Romand, Polymer Surface Modification: Relevance to Adhesion, Vol. 2, K.L. Mittal Žed.., VSP, Utrecht, The Netherlands, 2000, p. 335. w9x M. Charbonnier, M. Romand, U. Kogelschatz, H. Esrom, R. Seebock, ¨ J. Adhes. Sci. Technol. Ž2000. in press. w10x L.B. Loeb, Electrical Coronas, University of California Press, Berkeley, 1965. w11x M. Goldman, A. Goldman, R.S. Sigmond, Pure Appl. Chem. 57 Ž1985. 1353᎐1362. w12x H. Esrom, R. Seebock, ¨ M. Charbonnier, M. Romand, Surf. Coat. Technol. 125 Ž2000. 19᎐24. w13x M. Romand, M. Charbonnier, H. Esrom, R. Seebock, ¨ in: Proc. of the 23 rd Annual Meeting of the Adhesion Society, Myrtle Beach, USA, 20᎐23 February 2000 ŽISSN 1086-9506..
Surface modification of polyimide using dielectric barrier discharge treatment R. Seebock ¨ a,U , H. Esroma , M. Charbonnier b, M. Romandb, U. Kogelschatz c a
b
Uni¨ ersity of Applied Sciences Mannheim (HTG), Windeckstr. 110, D-68163 Mannheim, Germany Laboratoire des Sciences et Ingenierie ´ des Surfaces, Uni¨ ersite´ Claude Bernard-Lyon 1, F-69622 Villeurbanne Cedex, France c ABB Corporate Research Ltd, CH-5405 Baden, Switzerland
Abstract We report on a novel method for the surface modification of polymers by direct exposure to a dielectric barrier discharge ŽDBD. at atmospheric pressure and room temperature. The polymer under treatment is located directly on the grounded electrode and serves at the same time as the discharge barrier. So far DBD treatment of the unfilled and Al 2 O 3-filled commercially available polyimide foils have been investigated as a function of specific discharge energy. The morphological effects of the DBD treatment have been analysed by optical microscopy, SEM, AFM, and chromatic coding distance measurement while XPS was used for analysis of the chemical surface composition. The results can be summarised as follows: Firstly, the etching rate of the polyimide ŽPI. surface by DBD in air is rather high leading to a pronounced roughening within some tens of seconds. Secondly, the attack is dependent on whether the polyimide contains a filler, which is added to improve the thermal conductivity of the material. In this case the etching lays bare the grains of the filler but is spatially rather uniform. The surface roughening increases the bond strength to coating layers. Finally, in the unfilled material crater-like structures are observed which are attributed to the repetitive ignition of discharge filaments in the same location. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyimide; Dielectric barrier discharge; Chemical modification; Morphological modification; Polymers
1. Introduction Polymers play an increasing role as structural materials, as foils for protective and packaging applications and as coatings for corrosion protection or sealing applications. PI has outstanding properties like high tensile strength and dielectric strength above 22 kVrmm w1x. Today it is used in the electronic industry as a material for flexible chip carriers. One type of PI, commercially available in web form, is Kapton 䊛 , manufactured by DuPont de Nemours, to which all the investigations in this work refer. In order to act as a chip carrier the PI surface has to be metallised by copper. Here the poor adhesion of PI to metals, which is a consequence of the low specific U
Corresponding author.
surface energy, has to be overcome w2x. Adhesion is strongly influenced by the chemical nature of the surface layer, because the surface layer bonds to a metal layer mainly by physi- or chemisorption w3x. A wellestablished method to increase adhesion physio-chemically is to expose the surface to a corona discharge in air at atmospheric pressure. In this way the surface energies of many polymer surfaces are increased considerably shown by a drastic decrease of the water contact angles. The discharge energies applied per treated surface area Žcalled ‘specific discharge energies’. needed to achieve a certain surface energy vary over two orders of magnitude from one polymer to the other w4x. Chemical changes in the surface layer may also be effected by exposure to low pressure gas discharges. The specific discharge energies reported for such processes are usually three orders of magnitude higher than those in the corona case. Therefore besides
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 0 8 5 - 4
456
R. Seebock ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
chemical changes also structural changes of the surface topography like roughening are observed in low-pressure experiments w5x. Besides chemical effects also the physical surface roughening plays an important role in adhesion improvement w6x. In two preceding papers w7,8x we have given first results on DBD processing of PI. In the present work, we present results on the modification of PI surfaces that are obtained in dielectric barrier discharges ŽDBD. in air at atmospheric pressure under specific discharge energies ranging from 8 = 10 3 to 10 6 Wminrm2 . Due to these high specific energies morphological changes of the surfaces are induced and shall be discussed.
2. Experiments 2.1. Experimental set-up for DBD-processing All experiments were performed on samples of Kapton 䊛 200 HN and Kapton 䊛 150 MT foils with a thickness of 50 and 38 m, respectively. The HN material represents the pure PI-polymer, while MT is aluminafilled and possesses three times the thermal conductivity of HN. The foils were cut into square shaped samples of sizes between 2.0= 2.0 and 4.0= 4.0 cm2 and cleaned ultrasonically in acetone before discharge treatment. For DBD processing the samples were placed on a grounded planar Cu electrode. A schematic view of the experimental arrangement is shown in Fig. 1. A load Cu ring was used to prevent the polymeric samples from bending upwardly. The PI film itself forms the dielectric barrier of the discharge. The upper electrodes were made of stainless steel or Cu and had diameters between 0.6 and 2.0 cm according to the desired treatment area. The discharge gap is built between the planar bottom surface of the upper electrode and the PI foil. The gap width was adjusted with the help of a motorised vertical precision translator ŽNEWPORT M-UTM50.. In the experiments a gap width of 100 m was used. The discharge was driven at a frequency of 125 kHz. Details are given in Seebock ¨ and Esrom w7x. The specific discharge energy was calculated as the ratio of load power read at the generator and upper electrode area. 2.2. Morphological surface analysis The surface morphology of the samples was investigated by four different methods. An optical microscope ŽCARL ZEISS. served as a tool for quick inspection. In order to get information on the surface topography, we used a SEM ŽLEITZ DSM 950.. We furthermore employed AFM ŽDIGITAL INSTRUMENTS Nanoscope III. as a high-resolution method with limited scanning
Fig. 1. Schematic view of the experimental set-up used for DBDtreatment in air at atmospheric pressure.
range to determine topography and roughness. Finally, we also present first results on contact-less distance measurements by chromatic coding ŽJURCA CHR 150N.. This method possesses a depth resolution on the order of 10 nm with a scanning range up to many centimetres. 2.3. Chemical surface analysis In order to study the chemical changes induced in the polymer surface by the DBD processing, investigations were carried out on the samples with an XPSspectrometer ŽRIBER SIA 200.. All XPS-peaks were referenced to the C 1s signal at a binding energy of approximately 285 eV representing the C᎐C and C᎐H bonds in hydrocarbons.
3. Results and discussion 3.1. Morphology-uniform surface appearance The DBD treatment of Kapton 䊛 results in a surface roughening of the PI samples visible with the naked eye. In Fig. 2 we show two Kapton 䊛 samples, one untreated, the second DBD treated. In the case of the HN material the untreated surface is very smooth with a roughness below 10 nm. After DBD treatment in the HN samples only some isolated grain-like structures occur with widely separated grains, while the surface in between remains rather smooth. These grains may be zones of different chemical composition compared to the bulk or crystallites. The untreated surfaces of Kapton 䊛 MT are not as smooth as those of HN, since some of the filler grains must intersect the macroscopic surface. In Fig. 3 a SEM micrograph of a Kapton 䊛 MT sample treated with 3.4= 10 3 Wminrm2 is shown, which corresponds to a treatment time of only 0.5 s. The surface topography is very similar to that of the totally untreated case,
R. Seebock ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
Fig. 2. Untreated Žleft. and DBD treated Žright. Kapton ples Župper electrode diameter 6 mm..
䊛
MT sam-
concerning the grain size distribution. However, chemical changes occur already after such short times Žsee Section 3.3.. When the treatment is extended in time, the number of grains visible at the surface increases, as can be seen from the micrograph in Fig. 4. Here the specific energy was 2.0= 10 5 Wminrm2 corresponding to 30-s treatment time. The surface shows a fine grainlike structure with a wide spread in grain diameters. According to the manufacturer these grains consist of Al 2 O 3 , therefore their etch rate in an O containing plasma is low. The net result of the treatment is the removal of PI material between the filler grains. To confirm the SEM results some of the samples were also investigated by AFM. The average roughness R a was obtained on an area of 1 m2 . Fig. 5 shows an AFM micrograph of Kapton 䊛 MT DBD treated with 5.1= 10 5 Wminrm2 . The R a value increased from 9.6" 5.6 nm for the untreated case to 85 " 20 nm. Since the scanning area is smaller than the average grain area, however, these value may differ from the macroscopic surface roughness observed in the SEM. Therefore we also used chromatic coding contactless roughness measurement, a result of which is shown in Fig. 6 for a specific energy of 5 = 10 5 Wminrm2 and a scanning range of 300 = 300 m2 . This technique also yields R a between 50 and 100 nm confirming the AFM result. The DBD-induced surface roughening may be very versatile to increase the adhesion to the filled PI.
457
Fig. 3. SEM micrograph of a Kapton 䊛 MT sample treated with 3.4= 10 3 Wminrm2 .
between 3.4= 10 3 and 5.1= 10 5 Wminrm2 showed a slight increase in the O and N percentages to 70.0 and 11.7 at.% while the C fraction decreased to 70.0 at.%. For more details refer to Charbonnier et al. w9x. The main feature of the XPS-results for the MT samples is the presence of Al from the filler and of some P. During DBD processing the percentages of Al and O increase while that of C decreases. The increase in P suggests that P is also a constituent of the filler. As the matrix material is the same as in the MT type samples, we expect only a slight oxidation of this fraction. We assume therefore that the O increase is essentially accounted for by the laying bare of the filler grains by the DBD processing. 3.3. Morphology-crater-like surface appearance In some of the experiments on Kapton 䊛 HN samples at high discharge power we observed structures that resemble craters on the surface after DBD processing. We never observed these structures in the MT samples.
3.2. Chemical surface composition Kapton 䊛 200 HN and 150 MT samples were investigated by XPS after different treatment times by using the DBD. In the HN material only its constituents C, O and N were found. In the untreated sample the relative percentages of these elements were 78.6, 15.0 and 6.4 at.%. The DBD treatment with specific energies
Fig. 4. SEM micrograph of a Kapton 䊛 MT sample treated with 2.0= 10 5 Wminrm2 .
458
R. Seebock ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
Fig. 7. Craters on Kapton 䊛 HN surface generated by DBD treatment.
Fig. 5. AFM micrograph of a Kapton 䊛 MT surface Žarea 1 m2 . DBD treated with 5.1= 10 5 Wminrm2 ; Scales: X,Y: 0.2 mrdiv; Z: 0.1 mrdiv.
An example is shown in Fig. 7. The craters are growing in diameters with increasing treatment time while their centres stay fixed at a distance of approximately 300 m. We assume that the origin of this appearance is the microstructure of the discharges. The DBD in air at 10 5 Pa is filamentary. Under a wide range of discharge conditions the filaments are ignited at random spatial positions in different cycles of the ac voltage waveform. In the case of crater formation we assume
Fig. 6. Chromatic coding distance measurement scan of a Kapton 䊛 MT surface Žarea 300 = 300 m2 . DBD treated with 5 = 10 5 Wminrm2 ; z-Scale: 1 mrdiv.
that filaments ignite repetitively at the same position in subsequent cycles of the driving voltage, so that a stationary filament pattern is formed that reflects itself in the crater structure on the dielectric. On the crater bottom a radial topographic structure is observed. This can be clearly seen in Fig. 7. In the crater bottom between the radially aligned structures the surface is rather smooth resembling the etching results in the uniform case. The strongest etching action in the direction perpendicular to the surface is obviously found in the craters, i.e. in a circular area centred at the discharge filaments. This would mean that the etching is essentially caused by the surface gliding discharge connected with each filament. It is well known from DBD modelling that the radical density in the gliding discharge is very high supporting the assumption. We further note that the electric field in the gliding discharge is radially directed away from the filament. Under its action particle motion occurs which leads to a radial component of the etching rate. The texture would then be the result of the radial etching rate together with a shadowing effect caused by zones of lower etching rate. The origin of the craters is further elucidated by the observation of a second type of surface structures after very short DBD treatment of HN samples, one example of which is shown in Fig. 8. These structures are found only in the Žunfilled. HN samples and for very low specific energy of the order of 10 3 Wminrm2 . They show a radial alignment in the direction of one ore several centres close to each other. The fine traces going out from these centres have led to a permanent damage of the PI surface. Whether this was accomplished by partial melting or by very intensive etching reaction by a high radical and UV photon density or by a combined action of both effects cannot be decided at present. The features of these structures resemble posi-
R. Seebock ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
459
unfilled material a fast etching is achieved in the DBD, too. If a smooth surface is to be maintained, care has to be taken that the non-stationary filament mode is established. On the other hand, if the planarity is not of crucial importance, centres for very strong anchoring of surface layers can be generated in the stationary filament mode.
Acknowledgements
Fig. 8. TraceŽs. of streamers andror DBD filamentŽs. on Kapton 䊛 HN sample resembling Lichtenberg figure Žarea 2.0= 2.7 mm2 ..
tive Lichtenberg figures w10x. Lichtenberg figures of positive point coronas are taken by placing a photographic film between the corona tip and the opposite plane electrode. The streamer propagating from the tip hits the film and induces a surface discharge. The dielectric in a barrier discharge plays the role of the film in the corona. The difference is that the DBD between parallel electrodes is not of the classical corona type, since no low field drift region is present w11x. Nevertheless, streamers propagate from one electrode and hit the dielectric inducing a surface discharge. We assume that the structure observed in Fig. 8 was created by the action of streamers or discharge filaments on the dielectric. If this process is further continued it might lead to crater formation. The detailed conditions of crater formation will be the subject of further research.
4. Conclusions We have shown that barrier discharges are able to effectively etch filled PI material of the Kapton 䊛 MT type. In this case a uniform etching is possible laying bare the top of the filler grains. We have already shown that Kapton 䊛 MT treated in this way may well be metallised with good adhesion w9,12,13x. In the case of
The authors gratefully acknowledge the funding by the Karl Volker Stiftung. We are indebted to Dr B. ¨ Michelt of Jurca GmbH for taking the chromatic coding scans and to I. Deppner for assistance during the experiments. References w1x Polyimide, data sheet, Goodfellow Co., Berwin, USA, 1998. w2x M. Strobel, C.S. Lyons, K.L. Mittal ŽEds.., Plasma Surface Modification of Polymers: Relevance to Adhesion, VSP, Utrecht, The Netherlands, 1994. w3x E.M. Liston, L. Martinu, M.R. Wertheimer, in: M. Strobel, C.S. Lyons, K.L. Mittal ŽEds.., Plasma Surface Modification of Polymers: Relevance to Adhesion, VSP, Utrecht, The Netherlands, 1994, pp. 3᎐39. w4x Softal Report. No.110, Softal Electronic GmbH, 21107 Hamburg, Germany. w5x N. Inagaki, S. Tasaka, K. Hibi, in: M. Strobel, C.S. Lyons, K.L. Mittal ŽEds.., Plasma Surface Modification of Polymers: Relevance to Adhesion, VSP, Utrecht, The Netherlands, 1994, pp. 275᎐290. w6x K.L. Mittal, J. Vac. Sci. Technol. 13 Ž1976. 19. w7x R. Seebock, ¨ H. Esrom, Proc. Of the 6th Int. Symp. on High Pressure, Low Temperature Plasma Chem, Plasma Ireland, Ltd, Cork, Ireland, 31 August᎐2 September 1998. w8x H. Esrom, R. Seebock, ¨ M. Charbonnier, M. Romand, Polymer Surface Modification: Relevance to Adhesion, Vol. 2, K.L. Mittal Žed.., VSP, Utrecht, The Netherlands, 2000, p. 335. w9x M. Charbonnier, M. Romand, U. Kogelschatz, H. Esrom, R. Seebock, ¨ J. Adhes. Sci. Technol. Ž2000. in press. w10x L.B. Loeb, Electrical Coronas, University of California Press, Berkeley, 1965. w11x M. Goldman, A. Goldman, R.S. Sigmond, Pure Appl. Chem. 57 Ž1985. 1353᎐1362. w12x H. Esrom, R. Seebock, ¨ M. Charbonnier, M. Romand, Surf. Coat. Technol. 125 Ž2000. 19᎐24. w13x M. Romand, M. Charbonnier, H. Esrom, R. Seebock, ¨ in: Proc. of the 23 rd Annual Meeting of the Adhesion Society, Myrtle Beach, USA, 20᎐23 February 2000 ŽISSN 1086-9506..