Surface activation of glass fiber-reinforced composite by microwave plasma
in nitrogen-containing gases. Application to electroless metallization.
J. Coulm, D. Léonard, C. Bordes, P. Lanteri, Y. Clément and F. Bessueille.
Université de Lyon - Institut des Sciences Analytiques, UMR 5280 CNRS, Université Lyon1, 5 rue de la Doua,
69100 Villeurbanne, France.
Abstract: 30% glass fiber-reinforced Polyamide12 (PA12) needs to be functionalized to obtain
metal deposition using electroless metallization. This work describes a screening design of
experiments based on various operating parameters (gas mixture, relative percentage of the
nitrogen-containing gas, pressure, power density and treatment duration) of a microwave
plasma treatment. The plasma treatment is intended to graft relevant nitrogen-containing
groups at the surface of the composite to obtain both Ni metallization and significant practical
adhesion of this Ni coating. Results indicate two sets of parameters to be candidates for further
optimization as well as a specific detrimental role of oxygen incorporation, all of this being
currently further investigated to identify the overall best microwave plasma treatment
conditions of this specific composite material for its electroless metallization.
Keywords: microwave plasma functionalization, Polyamide12, screening design of
experiments, electroless metallization, contact angle measurement, ToF-SIMS.
1. Introduction
Black 30% glass fiber-reinforcement Polyamide12
(PA12) is a polymer-based composite material of
great interest because of its low cost, lightweight
and high production throughput. It is widely used in
application fields such as automotive and
electronics thanks to high mechanical properties
and chemical inertness. In particular, applications
like molded interconnect device (MID), an
injection-molded thermoplastic part with integrated
electronic circuit lines, are very promising.
However, it requires to be metallized for such
applications.
A very convenient way to obtain metallized patterns
is to combine lithography techniques with
electroless metallization. In the case of insulating
substrates, a pre-treatment to electroless
metallization is requested to graft a specific
catalyst, e.g. typically Pd(0). The pre-treatment
process which is used in industry is based on an
immersion in strong oxidizing solution and then in
a Sn/Pd colloids solution. However alternative Sn
free processes have been proposed both at industrial
and lab scales. Among those, environmental
friendly plasma processes allowing grafting of
nitrogen-containing species exhibiting strong
affinity for Pd have been proposed1.
Despite its nitrogen-containing amide function,
PA12 is unable to graft significant amount of Pd
species to activate electroless metallization without
any pre-treatment step. Plasma treatments leading
to efficient functionalization of polymers exhibit
highly variable plasma conditions (geometry,
frequency, etc.) and this also the case for nitrogencontaining functions.1-5 This even includes the
nature of the plasma gas (N21, NH31, NH3/H22
N2/H23, Ar/N24 and Ar/NH35).
In this work, operating parameters of a microwave
plasma treatment based on nitrogen-containing
gases were studied to identify the basic sets of
plasma parameters allowing the PA12 surface to be
metallized. Metallization is not the only selection
parameter as a significant practical adhesion of the
metal deposit is required to improve the reliability
of the electronic functions. The following
parameters were studied: nature of the gas mixture
(N2/H2, N2/Ar, NH3/H2, NH3/Ar), relative
percentage of the nitrogen-containing gas, pressure,
power density and treatment duration. Considering
the high numbers of parameters, a screening
design of experiments was built and used to
estimate their effect on the metallization efficiency
as well as on the practical adhesion of
the electroless Nickel films (evaluated using the
standard ASTM D3359 Scotch® tape test). Surface
functionalization was followed by ToF-SIMS and
contact angle measurements.
2. Experimental
NemrodW software was used to generate the
screening design of experiments reported in Table
1. For all the compared parameters, three levels
were selected (low, medium and high) for the four
different gas mixtures. Values were selected based
on conditions used to successfully metallize other
polymers in the plasma set-up.
Prior to plasma treatment, all the samples were
degreased with isopropanol in an ultrasonic bath
during 15min and then dried under nitrogen. The
samples were plasma treated with a capacitively
coupled microwave plasma reactor Pico-UHP-MWPC (Diener, Germany), operated at 2.45GHz and a
maximum power of 850W.
Electroless catalyst grafting step was realized by
immersion in a 1g/L PdCl2 solution with 10mL/L
HCl for 30 seconds. Samples were then rinsed with
DI water and dried under nitrogen. The electroless
metallization step was performed by immersion in a
laboratory made electroless bath consisting of 36
g/L nickel sulphate hexahydrate, 29 mL/L lactic
acid (90%), 10 g/L sodium hypophosphite, pH
being adjusted at 5 with sodium hydroxide and
deposition temperature being 85°C. All samples
were immersed for 5 minutes in the electroless
bath, leading to a film thickness of 1µm,
determined by AFM.
Contact angle measurements were performed on a
Digidrop Contact Angle Meter (GBX Instrument,
France). The dispersive and polar components as
well as surface energy were calculated using the
Owens/Wendt equation and three different liquids:
water, diiodomethane and formamide. Six drops
were measured on different areas to qualify the
homogeneity of the treatment over the entire
surface.
ToF-SIMS measurements were carried out using a
Physical Electronics TRIFT III ToF-SIMS
instrument (Physical Electronics, Chanhassen,
USA) operated with a pulsed 22keV Au+ ion gun
(ion current of about 2nA) rastered over a
300µm*300µm area. An electron gun was operated
in pulsed mode at low electron energy for charge
compensation. Ion dose was kept below the static
conditions limits. Data were analyzed using the
WinCadence software. Mass calibration was
performed on hydrocarbon secondary ions. The
mean and standard deviations were calculated from
three measurements on different areas of each
sample.
Practical adhesion of the nickel deposit film was
evaluated using the standard ASTM D3359
Scotch® tape test. On each sample, a grid pattern
with six stripes in each direction was obtained with
a scalpel. The tape (Scotch® Making Tape
2503710, 3M, USA) was placed on the center of the
grid and sharply peeled off to an angle of 90°. Then
a picture was taken and the percentage of removed
Ni coating was calculated using image analysis
(GNU Image Manipulation Program, freeware).
3. Results
Figure 1 shows pictures for all the plasma treated
PA12 after the metallization step. Evidently, all the
samples plasma treated in a NH3 based gas mixture
led to complete surface metallization. However, it
must be noted that it is not the only conditions to
obtain metallization: one plasma treatment in a N2
based gas mixture also led to PA12 surface
metallization, it is the treatment corresponding to
the highest power and H2 proportion. This result is
consistent with previous results from Wang et al
who studied the influence of different operating
parameters in a N2-H2 microwave (433MHz)
reactor, and showed that high power and H2 relative
percentage lead to a decrease in nitrogen grafting,
but an increase in amine function incorporation.3 In
the NH3-Ar gas mixture, practical adhesion seems
to be a function of the treatment duration. For the
30sec treatment, 2.9% and 2.6% of metal was
removed against 6.5% for the 2min. treatment and
11.3% for the 5 min. treatment. In the NH3-H2 gas
mixture, plasma treatments at low pressure lead to
better practical adhesion with 1.9% and 1.7%
removed metal against 3.5% and 4.1% removed
metal for higher pressure.
Table 1 also exhibits the practical adherence results.
It is interesting to note that the best practical
adhesion was obtained on the sample treated in the
N2-H2 gas mixture (see also figure 2).
Table 1 : Screening design of experiments and percentage of removed metal after the Scotch® tape test.
Sample
Time (sec)
Power (%)
Pressure (Pa)
Nature of the gas mixture
Untreated
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0
30
30
30
30
120
120
120
120
300
300
300
300
30
30
30
30
0
25
60
100
25
25
60
100
25
25
60
100
25
25
60
100
25
0
80
120
150
80
120
80
80
150
150
80
80
120
80
150
120
80
0
N2/H2
N2/Ar
NH3/H2
NH3/Ar
NH3/Ar
NH3/H2
N2/Ar
N2/H2
N2/Ar
N2/H2
NH3/Ar
NH3/H2
NH3/H2
NH3/Ar
N2/H2
N2/Ar
Percentage of the
nitrogen gas
0
50
75
90
50
90
50
50
75
50
90
75
50
75
50
50
90
Removed metal after the
Scotch® tape test (%)
x
x
3.5
2.9
6.5
1.9
x
x
x
x
11.3
4.1
1.7
2.6
0.9
x
Figure 4: PCA on contact angle results.
Figure 1 : Pictures of all the plasma treated samples
after the electroless metallization step.
Figure 2 : Picture of the sample # 15 (50%N2-50%H2,
50% power, 120Pa, 30s) after the electroless
metallization and the Scotch® tape test.
Contact angle measurement results are displayed in
Figure 3. For the samples plasma treated in a NH 3
based gas mixture, the polar component and surface
energy values are somewhat variable. Some
treatments like the treatment # 3 (90%NH3-10%H2,
100% power, 1.5mbar, 30s) even induce lower
surface energy values than that observed for the
untreated PA12 sample.
Principal Component Analysis (PCA) was
performed on the contact angle measurement results
(Figure 4). It clearly shows that the gas mixture
rules the surface energy. Most of the samples
treated in a N2 based gas mixture and in a NH3
based gas mixture are detected in separate areas of
the PCA plot. Furthermore, in our experimental
conditions, polar component and surface energy
appear to be correlated.
PCA was also performed on ToF-SIMS negative
spectra (Figure 5). As in the case of contact angle
measurement results, the PCA plot separates
samples mostly as a function of the plasma gas
mixture. The first component (F1) explains 61% of
the peak variations, and is mainly associated to
oxygen based secondary ion peaks (on the positive
part) and nitrogen based secondary ion peaks (on
the negative part). In our conditions, the more the
H2 relative percentage in N2-H2, the more the
treatment is localized on the right side of F1 (high
relative intensity for oxygen based secondary ion
peaks).
Figure 3 : Contact angle measurements, dispersive
component, polar component and surface energy
determined from contact angle measurements using
the Owens/Wendt equation.
Figure 5 : PCA on ToF-SIMS negative mode data.
4. Discussion
All the plasma treatments selected in the screening
design of experiments did not lead to surface
metallization. However two sets of parameters
appear to be candidates for further optimization. On
the one side, most of the treatments performed in
N2 based gas mixtures were not efficient to graft
nitrogen functions allowing for the chemisorption
of Pd2+ species, except for the sample #15 (50%N250%H2, 50% power, 1.2mbar, 30s) which turns out
to be the sample exhibiting the best practical
adhesion result among all tested. This shows that an
optimization can be found around that set of data
and as indicated by Wang et al3 for a very different
plasma set-up, it could be interesting for future
work to test lower relative percentages of N2. On
the other side, most of the treatments performed in
NH3 based gas mixtures led to surface metallization
but the practical adhesion extent is depending on
the experimental parameters. NH3-H2 lower
pressures seem to enhance the practical adhesion,
while in NH3-Ar plasmas, the main contributing
parameter is the treatment duration, short time lead
to best practical adhesion.
Contact angle measurements clearly show that
treatments performed in N2 based gas mixtures led
to a significant increase in polar component and
surface energy values, while it is not the case for
treatments performed in NH3 based gas mixtures.
ToF-SIMS analyses allow assigning this trend to a
significant oxygen uptake. This may be attributed to
impurities in the plasma chamber and/or postfunctionalization after contact with air. This is
evidenced by the ToF-SIMS CNO-/CN- ratio
(Figure 6) which seems to be a relevant signature of
best conditions for metallization. All the samples
that further metallized exhibit values lower than
0.50. It is interesting to note that in the case of
treatments performed in N2 based gas mixtures,
only the sample #15 exhibits a value for this ratio
under 0.50, confirming the relevance in our
experimental plasma treatment conditions of this
characterization parameter to identify best
conditions for metallization. However it is not an
absolute signature as evidenced by the ToF-SIMS
CNO-/CN- ratio value for the untreated sample
which is 0.32 and still does not lead to
metallization. This can be explained by the fact that
the plasma treated surface does exhibit several
chemical functions while the original PA12 surface
is characterized by only amide functions.
Figure 6: Relative intensity of CNO-, CN-, and CNO/CN- ratio.
5. Conclusion
Thanks to a screening design of experiments, two
sets of plasma parameters (N2/H2 and NH3/H2) for
plasma pre-treatment of 30% glass fiber-reinforced
Polyamide12 (PA12) were identified to allow
subsequent metallization with significant practical
adhesion results. For the N2/H2 mixture treatment
conditions, a H2 relative percentage higher than
50% is of interest, while in the NH3-H2 mixtures,
pressure is a key parameter to ensure the practical
adhesion of the deposit. In order to further optimize
these plasma pre-treatments, a Doehlert design of
experiments was set up and experimentation is
currently in progress. The same approach is also
investigated on other polymers in our laboratory.
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