Surface and Coatings Technology 142᎐144 Ž2001.

455᎐459

Surface modification of polyimide using dielectric barrier

discharge treatment

¨ a,U , H. Esroma , M. Charbonnier b, M. Romandb, U. Kogelschatz c

R. Seebock
a
Uni¨ ersity of Applied Sciences Mannheim (HTG), Windeckstr. 110, D-68163 Mannheim, Germany
b
´ des Surfaces, Uni¨ ersite´ Claude Bernard-Lyon 1, F-69622 Villeurbanne Cedex, France
Laboratoire des Sciences et Ingenierie
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 surface energy, has to be overcome w2x. Adhesion is

strongly influenced by the chemical nature of the sur-
Polymers play an increasing role as structural materi- face layer, because the surface layer bonds to a metal
als, as foils for protective and packaging applications layer mainly by physi- or chemisorption w3x. A well-
and as coatings for corrosion protection or sealing established method to increase adhesion physio-chem-
applications. PI has outstanding properties like high ically is to expose the surface to a corona discharge in
tensile strength and dielectric strength above 22 air at atmospheric pressure. In this way the surface
kVrmm w1x. Today it is used in the electronic industry energies of many polymer surfaces are increased con-
as a material for flexible chip carriers. One type of PI, siderably shown by a drastic decrease of the water
commercially available in web form, is Kapton 䊛 , manu- contact angles. The discharge energies applied per
factured by DuPont de Nemours, to which all the treated surface area Žcalled ‘specific discharge energies’.
investigations in this work refer. needed to achieve a certain surface energy vary over
In order to act as a chip carrier the PI surface has to two orders of magnitude from one polymer to the other
be metallised by copper. Here the poor adhesion of PI w4x. Chemical changes in the surface layer may also be
to metals, which is a consequence of the low specific effected by exposure to low pressure gas discharges.
The specific discharge energies reported for such
processes are usually three orders of magnitude higher
U
Corresponding author. 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 ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
R. Seebock

chemical changes also structural changes of the surface

topography like roughening are observed in low-pres-
sure 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. Fig. 1. Schematic view of the experimental set-up used for DBD-
treatment in air at atmospheric pressure.

2. Experiments
range to determine topography and roughness. Finally,
we also present first results on contact-less distance
2.1. Experimental set-up for DBD-processing
measurements by chromatic coding ŽJURCA CHR
150N.. This method possesses a depth resolution on
All experiments were performed on samples of Kap-
the order of 10 nm with a scanning range up to many
ton 䊛 200 HN and Kapton 䊛 150 MT foils with a thick-
centimetres.
ness of 50 and 38 ␮m, respectively. The HN material
represents the pure PI-polymer, while MT is alumina-
filled and possesses three times the thermal conductiv- 2.3. Chemical surface analysis
ity of HN. The foils were cut into square shaped
samples of sizes between 2.0= 2.0 and 4.0= 4.0 cm2 In order to study the chemical changes induced in
and cleaned ultrasonically in acetone before discharge the polymer surface by the DBD processing, investiga-
treatment. tions were carried out on the samples with an XPS-
For DBD processing the samples were placed on a spectrometer ŽRIBER SIA 200.. All XPS-peaks were
grounded planar Cu electrode. A schematic view of the referenced to the C 1s signal at a binding energy of
experimental arrangement is shown in Fig. 1. A load approximately 285 eV representing the C᎐C and C᎐H
Cu ring was used to prevent the polymeric samples bonds in hydrocarbons.
from bending upwardly. The PI film itself forms the
dielectric barrier of the discharge. The upper elec-
trodes were made of stainless steel or Cu and had 3. Results and discussion
diameters between 0.6 and 2.0 cm according to the
desired treatment area. The discharge gap is built 3.1. Morphology-uniform surface appearance
between the planar bottom surface of the upper elec-
trode and the PI foil. The gap width was adjusted with The DBD treatment of Kapton 䊛 results in a surface
the help of a motorised vertical precision translator roughening of the PI samples visible with the naked
ŽNEWPORT M-UTM50.. In the experiments a gap eye. In Fig. 2 we show two Kapton 䊛 samples, one
width of 100 ␮m was used. The discharge was driven at untreated, the second DBD treated. In the case of the
¨
a frequency of 125 kHz. Details are given in Seebock HN material the untreated surface is very smooth with
and Esrom w7x. The specific discharge energy was calcu- a roughness below 10 nm. After DBD treatment in the
lated as the ratio of load power read at the generator HN samples only some isolated grain-like structures
and upper electrode area. occur with widely separated grains, while the surface in
between remains rather smooth. These grains may be
2.2. Morphological surface analysis zones of different chemical composition compared to
the bulk or crystallites.
The surface morphology of the samples was investi- The untreated surfaces of Kapton 䊛 MT are not as
gated by four different methods. An optical microscope smooth as those of HN, since some of the filler grains
ŽCARL ZEISS. served as a tool for quick inspection. In must intersect the macroscopic surface. In Fig. 3 a
order to get information on the surface topography, we SEM micrograph of a Kapton 䊛 MT sample treated
used a SEM ŽLEITZ DSM 950.. We furthermore em- with 3.4= 10 3 Wminrm2 is shown, which corresponds
ployed AFM ŽDIGITAL INSTRUMENTS Nanoscope to a treatment time of only 0.5 s. The surface topogra-
III. as a high-resolution method with limited scanning phy is very similar to that of the totally untreated case,
 ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
R. Seebock 457

Fig. 3. SEM micrograph of a Kapton 䊛 MT sample treated with

Fig. 2. Untreated Žleft. and DBD treated Žright. Kapton 䊛
MT sam- 3.4= 10 3 Wminrm2 .
ples Župper electrode diameter 6 mm..
between 3.4= 10 3 and 5.1= 10 5 Wminrm2 showed a
concerning the grain size distribution. However, chemi- slight increase in the O and N percentages to 70.0 and
cal changes occur already after such short times Žsee 11.7 at.% while the C fraction decreased to 70.0 at.%.
Section 3.3.. When the treatment is extended in time, For more details refer to Charbonnier et al. w9x.
the number of grains visible at the surface increases, as The main feature of the XPS-results for the MT
can be seen from the micrograph in Fig. 4. Here the samples is the presence of Al from the filler and of
specific energy was 2.0= 10 5 Wminrm2 corresponding some P. During DBD processing the percentages of Al
to 30-s treatment time. The surface shows a fine grain- and O increase while that of C decreases. The increase
like structure with a wide spread in grain diameters. in P suggests that P is also a constituent of the filler. As
According to the manufacturer these grains consist of the matrix material is the same as in the MT type
Al 2 O 3 , therefore their etch rate in an O containing samples, we expect only a slight oxidation of this frac-
plasma is low. The net result of the treatment is the tion. We assume therefore that the O increase is essen-
removal of PI material between the filler grains. To tially accounted for by the laying bare of the filler
confirm the SEM results some of the samples were also grains by the DBD processing.
investigated by AFM. The average roughness R a was
obtained on an area of 1 ␮m2 . Fig. 5 shows an AFM 3.3. Morphology-crater-like surface appearance
micrograph of Kapton 䊛 MT DBD treated with 5.1=
10 5 Wminrm2 . The R a value increased from 9.6" 5.6 In some of the experiments on Kapton 䊛 HN samples
nm for the untreated case to 85 " 20 nm. Since the at high discharge power we observed structures that
scanning area is smaller than the average grain area, resemble craters on the surface after DBD processing.
however, these value may differ from the macroscopic We never observed these structures in the MT samples.
surface roughness observed in the SEM. Therefore we
also used chromatic coding contactless roughness mea-
surement, 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.

3.2. Chemical surface composition

Kapton 䊛 200 HN and 150 MT samples were investi-

gated 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 Fig. 4. SEM micrograph of a Kapton 䊛 MT sample treated with
at.%. The DBD treatment with specific energies 2.0= 10 5 Wminrm2 .
458 ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
R. Seebock

Fig. 7. Craters on Kapton 䊛 HN surface generated by DBD treat-

ment.

that filaments ignite repetitively at the same position in

Fig. 5. AFM micrograph of a Kapton 䊛 MT surface Žarea 1 ␮m2 . subsequent cycles of the driving voltage, so that a
DBD treated with 5.1= 10 5 Wminrm2 ; Scales: X,Y: 0.2 ␮mrdiv; Z: stationary filament pattern is formed that reflects itself
0.1 ␮mrdiv. 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
An example is shown in Fig. 7. The craters are growing between the radially aligned structures the surface is
in diameters with increasing treatment time while their rather smooth resembling the etching results in the
centres stay fixed at a distance of approximately 300 uniform case. The strongest etching action in the direc-
␮m. We assume that the origin of this appearance is tion perpendicular to the surface is obviously found in
the microstructure of the discharges. The DBD in air the craters, i.e. in a circular area centred at the dis-
at 10 5 Pa is filamentary. Under a wide range of dis- charge filaments. This would mean that the etching is
charge conditions the filaments are ignited at random essentially caused by the surface gliding discharge con-
spatial positions in different cycles of the ac voltage nected with each filament. It is well known from DBD
waveform. In the case of crater formation we assume modelling that the radical density in the gliding dis-
charge is very high supporting the assumption. We
further note that the electric field in the gliding dis-
charge 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 accom-
plished by partial melting or by very intensive etching
Fig. 6. Chromatic coding distance measurement scan of a Kapton 䊛
reaction by a high radical and UV photon density or by
MT surface Žarea 300 = 300 ␮m2 . DBD treated with 5 = 10 5 a combined action of both effects cannot be decided at
Wminrm2 ; z-Scale: 1 ␮mrdiv. present. The features of these structures resemble posi-
 ¨ et al. r Surface and Coatings Technology 142᎐144 (2001) 455᎐459
R. Seebock 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

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 cod-
ing scans and to I. Deppner for assistance during the
experiments.
Fig. 8. TraceŽs. of streamers andror DBD filamentŽs. on Kapton 䊛
HN sample resembling Lichtenberg figure Žarea 2.0= 2.7 mm2 ..
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