st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Contribution of roughness form rate on PTFE adhesion strength
by nitrogen ion irradiation
A.Yusuke Nagasaka 1, B. Toru Iwao 2, C. Motoshige Yumoto 2
1
2
Tokyo City University, Tokyo, Japan
Tokyo City University, Tokyo, Japan
Abstract: In order to improve the adhesive strength of PTFE (poly-tetra-fluoro-ethylene), nitrogen ion irradiation was used in this study. Surface property was measured by AFM (Atomic
Force Microscope) and surface energy. As a result, there was no correlation between the adhesive strength and surface energy. Then, fine roughness was obtained, which was confirmed
to have good relation between the adhesive strength.
Keywords: PTFE, Ion irradiation, Adhesive force, Polar group, Roughness
1. Introduction
Printed circuit board material for high frequency is required the chemical stability thermal resistance
[1].PTFE(Poly-tetra-fluoro-ethylene) has been attracting
attention as a material satisfying the above properties[2].
However, the adhesive strength of PTFE is very poor, and
need to be improved. There are two types of method for
improving adhesion. One is forming use the roughness for
utilizing an anchor effect. The other one is introducing
polar groups to enhance the chemical bonding force.
However, the height of roughness should be less than 100
nm to suppress the loss due to the skin effect.
Plasma treatment is well known as an effective surface
modification process. Active species including radicals,
electrons, and ions generated by plasma discharge attack
the surface of the PTFE. As a result, polar groups can be
introduced to activate the surface [3].
Although the direct plasma treatment can activate the
surface of PTFE well, damage is also caused at the same
time. To avoid the damage, remote plasma treatment releasing sample from plasma source was used [4]. However, a little far from plasma source reduce the density of
active species on the sample. Therefore, remote plasma
treatment can suppress damage, but have a disadvantage
on efficiency. To both protect surface of sample and improve efficiency, it is necessary to find out what kind of
active specie influence surface property.
According to the result of high E/n (E:electric field
strength, n: particle number density) treatment using the
left part of paschen’ minimum, where is a small pd(p:
pressure, d:distance of electrodes)region [5]. By using the
discharge under high E/n in nitrogen, many polar groups
were introduced and the adhesive strength was increased
without roughness increasing [6]. And it is deduced that
nitrogen ion energy was an important factor to improve
the adhesive strength.
In high E/n discharge space, the energy of ions is from
a few eV to several hundred eV, thus, the ion irradiation
Fig. 1 Ion irradiation device.
method was used to clarify the energy dependence. The
results showed that many polar groups were introduced by
irradiation of nitrogen ions around 30 eV. The roughness
reached the maximum value around 300 eV and decreased
above 300 eV[7]. On the other hand, adhesive strength
increased with ion energy increasing up to 3000eV. From
the fact that adhesive strength increased despite the decrement of roughness, it is assumed that there is another
factor improving the adhesive strength.
This study is focus on fine roughness around several
10nm, which is supposed not to increase entirety of
roughness，but can help to increase adhesive strength.
Then, the relation between fine structure and the adhesive
strength is examined and the results are summarized in
the paper.
2. Experimental
2.1 Ion irradiation apparatus
Fig.1 shows the ion irradiation apparatus. The induction
coil wrapped around a quartz chamber, and a capacitively
coupled plasma at 13.56 MHz is generated. Accelerating
electrode and the ground electrode are placed between the
plasma space and the sample stage. The charged particles
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
generated in the plasma space are accelerated between the
electrodes applying DC voltage to the accelerating electrode and are irradiated on the sample. Now, irradiated ion
plasma potential is about 60V, flux is about uA/cm2 ,
Using a turbo molecular pump and a rotary pump , the
chamber is exhausted down to 10-4 Pa and N2(purity
99.999%) gas is introduced to 1.3×10 -2 Pa. Injected power
into plasma space is 25 W. The sample was PTFE film, its
diameter is 30 mm and its thickness is 0.1 μm.
The sample was cleaned in ethanol bath by an ultrasonic washing machine for 10 minutes to remove surface
impurities and dried it in desiccator more than 24 hours.
2.2 Method
Surface energy can be divided into two components
which are the surface energy of γsp and γsd , by using
formula proposed by Owens et al,[8]. Magnitude of γsp
depends on the amount of the polar groups. Magnitude of
γsd depends on the surface roughness. To evaluate γsp and
γsd , and are used which surface energy is summarized in
tabel 1.
Fig. 2 180 ° peel test
3. Results and discussion
3.1 Relationship between surface energy and adhesion
force
Fig 3 shows that γsp was increased by irradiation of nitrogen ions around 30 eV. On the other hand, γsd reached
the maximum value around 300 eV and decreased above
300 eV. However, adhesive force increased depending on
ion energy .
(1)
(1+cosθ)
Liquid
＝
(2)
Surface energy [mJ/m2]
γLd
γLp
γL
Deionized
water
22.0
50.2
72.2
Methylene
iodide
48.5
2.3
50.8
Table 1 . Surface energy of liquids[8] .
Adhesive strength measures by the 180 degrees peel test
method. The sample after processing (10 mm × 50 mm) is
pasted up with a tape (Sumitomo 3M Ltd) using epoxy
adhesive (Nichiban Co., Ltd.) . Adhesive strength
measured at a speed of 100mm / min as shown in Fig 2.
From the result obtained up to now, adhesive strength
increased
by irradiation of nitrogen ion energy up to
2000eV. However, surface energy decreased above 300eV.
Accordingly, it is assumed that surface energy evaluated
by the contact angle cannot evaluate the small scale
roughness, because of the super water repellent.
Thus, surface roughness was observed by using the AFM
(Atomic Force Microscope, Seiko Instruments, SPA-400)
The tapping method is used to suppress the influence of
charging.
Fig. 3 Relationship between surface energy and adhesion force
Fig. 4 Relationship between surface energy and adhesion force
(total of dispersion force component and the polar force component)
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig 6 shows the relation between the adhesive strength
and the roughness form rate. From Fig 6, correlation coefficient was 0.99.
Fig. 5. PTFE cross-sectional view
(a)untreated (b)30 eV (c)1000 eV (d)3000 eV
Fig 4 shows the correlation between adhesion force and
surface energy that is sum of γsd and γsp. Correlation coefficient is -0.83. From the fact that adhesive strength increased despite the decrement of roughness, it is assumed
that there is another factor improving the adhesive
strength.
Fig 5 shows a cross-sectional view of the surface roughness measured by AFM (a) untreated (b) 30eV (c) 1000
eV (d) 3000 eV . From Fig 5，it is clear that fine roughness is formed with increasing ion energy. Each average
roughness is Ra=110 nm(untreated), Ra= 178 nm (30eV),
Ra= 63 nm (1000 eV), Ra= 82 nm (3000 eV).
From the result, the authors conducted that the contact
angle obtained is affected by the superhydrophobic effect
and the net surface energy is not evaluated. In other words,
γsd is not suitable for evaluating fine roughness.
3.2 Roughness form rate
To evaluate fine roughness, roughness form rate is calculated from the sectional view. Roughness form rate is
defined as the value obtained by dividing the average
roughness with intervals.
Fig. 6. Adhesion to form roughness rate
4. Conclusion
PTFE (poly-tetra-fluoro-ethylene) has excellent characteristics such as low dielectric constant .However, adhesive property is poor. Nitrogen ions were irradiated to
improve the adhesive strength of PTFE, and its energy
was changed from 30eV to 3000eV. The results showed
that positive relation between the surface energy and the
adhesive strength was not obtained. The contact angle
obtained is affected by the superhydrophobic effect and
the net surface energy is not evaluated. When measuring
the surface roughness with AFM, fine roughness was observed. To evaluate fine roughness, roughness form rate is
calculated from the sectional view, which is defined as the
value obtained by dividing the average roughness with
intervals. As a result correlation coefficient was 0.99.
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Department of Electrical and Electronic Engineering,
Tokyo City University, Setagaya Japan
Yusuke Nagasaka
[email protected]