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Parameters affecting the DC breakdown strength of parylene F thin films

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The effect of some experimental parameters (electrode area, thickness, temperature and voltage rising rate) on the breakdown strength of films of poly(α,α, α',α'-tetrafluoro-p-xylylene), a fluorinated parylene (PA-F), has been studied. The measurements were performed in air on 20 capacitor structures for each condition and the two parameters of the Weibull distribution (α and β) were fitted to the data. The influences of electrode area (0.28, 4.5 and 18 mm2), film thickness (1.4, 5.0, 9.8, 21.5 and 49.4 μm), for a temperature range from 25 °C to 350 °C and voltage rising rate between 10 V/s and 100 V/s, have been investigated and discussed for the thinnest films. The thickness increases up to 5 μm leads to an increase in the dielectric strength with increasing the thickness whereas a continuous decrease is observed for higher thicknesses. Those results are correlated to the thickness dependent crystallinity of parylene films. The effect of electrode area, measured on the thinnest films, on the PA-F dielectric strength does not impact the Weibull parameters at 25 °C for high dielectric breakdown field values, whereas it induces a decrease in the β values at low field with increasing electrode area. This highlights the presence of randomly distributed defects in the tested structures. The temperature dependence of the dielectric strength was also investigated between 25 °C and 350 °C for two different thicknesses (1.4 and 5 μm) and shows negative temperature dependence in both cases. Finally, the effect of the rate of the applied field rising between 0.07 and 0.7 MV/cm.s on the thin films was studied at 25 °C and 300 °C and does not show any remarkable effect on the Weibull parameters.
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Parameters Affecting the DC Breakdown Strength of
Parylene F Thin Films
R. Khazaka
1
, M. Bechara
1
, S. Diaham
1
, M.-L. Locatelli
1,2
IEEE Conference Publishing
1
Université de Toulouse; UPS, INPT; LAPLACE (Laboratoire Plasma et Conversion d'Energie);
118 route de Narbonne, F-31062 Toulouse cedex 9, France.
2
CNRS; LAPLACE; F-31062 Toulouse, France.
Abstract- The effect of some experimental parameters (electrode
area, thickness, temperature and voltage rising rate) on the
breakdown strength of films of poly(
α
αα
α
,
α
αα
α
,
α
αα
α
’,
α
αα
α
’-tetrafluoro-p-
xylylene), a fluorinated parylene (PA-F), has been studied. The
measurements were performed in air on 20 capacitor structures
for each condition and the two parameters of the Weibull
distribution (α and β) were fitted to the data. The influences of
electrode area (0.28, 4.5 and 18 mm
2
), film thickness (1.4, 5.0, 9.8,
21.5 and 49.4 µm), for a temperature range from 25 °C to 350 °C
and voltage rising rate between 10 V/s and 100 V/s, have been
investigated and discussed for the thinnest films. The thickness
increases up to 5 µm leads to an increase in the dielectric strength
with increasing the thickness whereas a continuous decrease is
observed for higher thicknesses. Those results are correlated to
the thickness dependent crystallinity of parylene films. The effect
of electrode area, measured on the thinnest films, on the PA-F
dielectric strength does not impact the Weibull parameters at 25
°C for high dielectric breakdown field values, whereas it induces
a decrease in the β values at low field with increasing electrode
area. This highlights the presence of randomly distributed defects
in the tested structures. The temperature dependence of the
dielectric strength was also investigated between 25 °C and 350
°C for two different thicknesses (1.4 and 5 µm) and shows
negative temperature dependence in both cases. Finally, the effect
of the rate of the applied field rising between 0.07 and 0.7
MV/cm.s on the thin films was studied at 25 °C and 300 °C and
does not show any remarkable effect on the Weibull parameters.
I.
INTRODUCTION
Electronic power devices are required to operate in harsh
environments under different kinds of stresses such as high
temperature and high voltage. Therefore, materials used in the
packaged assembly, among others the polymer insulating
layers, must operate reliably under those stresses. Among the
polymers that can be used as insulating layers in power
modules, the parylene F (PA-F), commercially available as
parylene HT, seems to be a good candidate due to its
interesting intrinsic properties. Actually, this material has a
good thermal stability even in air (1000 h at 350 °C and
withstands temperature peaks of 450 °C) [1] Moreover, the
PA-F presents excellent electrical properties at 25 °C with a
volume resistivity of 2×10
17
.cm, a dielectric strength > 2
MV/cm, a relative permittivity ε
r
=2.35 (between 60 Hz and 1
MHz), and a dielectric loss factor tanδ < 2×10
-3
. Generally, the
dielectric breakdown in polymers is an important limiting
phenomenon in most electrical devices and has been
extensively investigated and reviewed. However, the degree of
understanding of the breakdown process is not sufficient.
Since the failure is a weakest link type of problem. The
weakest spots and not the average behavior of the whole
insulation set the dimensioning of the voltage breakdown [2].
Therefore, the electrode area effect is an important parameter
related to the bulk and surface defects and may affect the
breakdown strength. On the other hand, the increasing demand
for polymers for high temperature / high voltage performance
requires studying the dielectric strength of the polymers in
different conditions. Hence, measurements were done over a
wide temperature range of uses, for different dielectric
thicknesses and voltage rising rates. The aim of this paper is to
describe an investigation on the dielectric strength of PA-F
films with the variation of some experimental parameters such
as the electrode area, the thickness, the temperature and the
rising rate of the applied field, in order to validate its ability
for high temperature and/or high voltage applications.
II. EXPERIMENTAL DETAILS AND DATA ANALYSIS
A. Samples preparation
PA-F films have been deposited on cleaned stainless steel
substrates by VDP (vapor deposition polymerization) at 25 °C
using the Gorham technique [3]. This deposition technique
consists in three different steps (see Fig. 1). In the first one,
the dimer is heated at 150 °C in primary vacuum room
chamber of 1 torr. Then, the vapor is transmitted into a
pyrolysis room chamber where the dimer is separated into two
monomers at 680 °C under a pressure of 0.5 torr. Finally, the
monomer enters the room temperature deposition chamber
where it simultaneously adsorbs and polymerizes on the
substrate under 0.1 torr. The thickness of the films was
adjusted by the deposition time and different thicknesses
varying from 1.4 µm up to 49.4 µm were obtained.
CF
2
F
2
C
CF
2
F
2
C
F
2
C CF
2
CF
2
F
2
C*
*
n
PA-F dimer PA-F monomer PA-F
Fig.1 : The different steps for the PA-F film deposition.
978-1-4577-0986-9/11/$26.00 ©2011 IEEE
740
For the dielectric breakdown characterizations, metal-
insulator-metal (MIM) structures have been achieved. The top
electrodes were gold evaporated with an approximate
thickness of 150 nm and their geometrical shape and size were
given by chemical etching through a photo-resist mask owning
circular patterns of three different diameters (0.6, 2.4 and 4.8
mm). Electrode diameter of 0.6 mm and rising field rate of
0.35 MV/cm.s were used in the breakdown tests, except
contrary indication.
B. Measurements
The dielectric breakdown measurements have been
performed under a linearly increasing applied voltage at
different rising rates from 0V up to the breakdown event,
using a DC voltage supply controlled by a Labview program.
The dielectric breakdown field has been calculated in the
plane-plane electrode configuration by using the following
relation:
d
V
E
BR
BR
=
where V
BR
is the breakdown voltage (leading to the layer
short-circuit) and d is the film thickness.
20 capacitive structures have been tested for each condition
to have an acceptable statistic analysis. At room temperature,
tests were done in an insulating fluorinated liquid (FC-72) in
order to avoid the flash-over occurrence under high applied
voltages. At high temperature, measurements were performed
in air. Several possible statistical distributions were available,
among which, the Weibull distribution is to be prefered, and
widely used [4,5]. So, experimental data have been
statistically analysed using the Weibull distribution law:
β
α
γ
=
x
exF 1)(
where F(x) is the cumulative probability of failure, α is the
scale parameter (V/cm) corresponding to the breakdown field
for a probability of failure F(α)=63.2%, β is the shape
parameter. A high β value is related to a low scattering of the
data. The location (or threshold) parameter γ has been set to
zero [2]. The cumulative probability of breakdown was
obtained using the median rank function [6]:
4
.
0
3.0
),(
+
=
n
i
niF
where i and n are the rank of a failed sample and the total
number of tested samples, respectively. For plotting the
Weibull distribution law, the transformation of Eq. (2) into Eq.
(4) has been realized:
[
]
)(log)(log
)(1
1
loglog
101010
αβ
=
x
xF
e
III. RESULTS AND DISCUSSION
A. Thickness effect
The thickness dependent breakdown field was measured at
25 °C for different thicknesses varying from 1.5 up to 49.4
µm. For a long time, the thickness effect has received a
statistical justification (weak links theory). From a statistical
point of view, it can be considered that small defects are
randomly spread in the dielectric material bulk. These defects
give rise to a weakness, which may reduce the dielectric
strength of a sample locally to values lower than the
theoretical dielectric strength. The dependence of the
breakdown field upon the dielectric thickness generally fits an
empirical power-law dependence:
E
BR
(d) = K × d
n
where K is a constant and n is about 0.5. However, it has been
shown that n can have different values from 0 to more than 0.5
[7]. The n parameter can be related to the presence of
macroscopic defects, to the microscopic structure (it has been
shown that n depends on the monomer electron affinity) [8]
and it may also be related to the charge transfer at the
electrode insulator interface [9]. As shown in Fig. 2, the
thickness effect on the PA-F breakdown field does not follow
the empirical law in all the thickness range. Actually, an
increase in the breakdown field is observed for increasing
thicknesses from 1.4 to 5 µm followed by a decrease for
higher thicknesses. This behavior was related to the effect of
the thickness on the increase in the crystallinity observed in
PA-F films and discussed elsewhere [10]. In fact, it is known
that the dielectric strength is higher in the crystalline areas
than in the amorphous ones [11] and that the breakdown
occurs at the boundaries of spherulites [12] wherein the
electrical path can be formed easily. Therefore, an increase in
crystallinity, in absence of spherulites, will lead to a reduction
in the amorphous areas. Consequently, a more difficult
propagation of the breakdown channel will result, leading to
the improvement when increasing thicknesses from 1.4 to 5
µm. For higher thickness, spherulites can be formed and lead
to the reduction of the breakdown field. However, the latter
remains relatively high at 25 °C (> 4.5 MV/cm) even for the
49.4 µm-thick films.
1 10 100
2
4
6
8
10
12
α (MV/cm)
Thickness (µm)
Tested at 25 °C in FC72
Electrode diameter 0.6 mm
Fig. 2. Variation of the scale parameter α with thickness at 25°C.
B. Electrode area effect
It is generally reported that larger electrode areas induce an
increase in the number of defects in the tested area which
leads to the decrease in the breakdown field. The number of
defects N
x
can be related to the α parameter by the following
relation [13]:
(1)
(2)
(3)
(4)
741
=
Ax
A
x
N
α
α
1
10
)(log
where α
A1
and α
Ax
correspond to the dielectric strength for
particular area A1 and Ax.
In order to investigate the effect of the electrode area on the
dielectric breakdown, tests were done on three different area
varying between 0.28 and 18 mm
2
at 25°C, and results are
presented in the Fig. 3. As shown in Fig. 3, the α parameter
seems to be unaffected by the increasing of the electrode area
(8.5 to 8.7 MV/cm), neither the β for the high dielectric
strength values noted β
1
(15.6 to 16.2). Contrariwise, the
lowest values are affected with the variation of the electrode
area with more dispersed results for the largest electrodes. The
shape parameter β
2
referred to the low breakdown values
varies from 3.4 to 10.2 for electrode areas of 18 mm
2
and 0.28
mm
2
, respectively.
4 5 6 7 8 9 10 11
1.0
0.5
0.0
-0.5
-1.0
18mm
2
β
2
=3.4
4.5mm
2
β
2
=7.1
0.28mm
2
β
2
=10.2
log
10
(log
e
(1/1-F))
0.28 mm
2
4.5 mm
2
18 mm
2
E
B
r
(MV/cm)
Tested at 25 °C in FC 72
18mm
2
α=8.5 MV/cm, β
1
=16.2
4.5mm
2
α=8.64 MV/cm, β
1
=16.5
0.28mm
2
α=8.7MV/cm, β
1
=15.6
-1.5
Fig. 3. Weibull plot for 1.4 µm PA-F films for different electrode areas at 25
°C. The extracted Weibull parameters are represented showing two different
shape parameters for high and low values noticed β
1
and β
2
respectively.
This can lead to suppose the existence of two different
Weibull distributions [14], one for the high breakdown values
which is not affected by the additional defects in the material
induced by larger electrodes, and the other one for the low
values related to additional defects dispersed in the film under
the tested area. A similar behavior of the Weibull distribution
for different electrode areas was observed by Laihonen et al.
[2] for thin (4 µm) polypropylene films. However, the effect
of electrode area on the low breakdown values is not very
clear and requires further studies. Based on those results, one
can also conclude that the defects in the material are dispersed
randomly with a low density, which does not affect the scale
parameter α.
C. Temperature effect
The thermal energy induces changes in polymers leading to
the acceleration of the free electrons. Also, when the
temperature increases the trapped electrons are freed more
easily with less electric field needed. Furthermore, when the
temperature increases the overall volume of the polymer
expands and the material density decreases due to the free
volume increase, which leads to a decrease in the breakdown
strength. Therefore, it is expected that the dielectric strength
decreases with increasing temperature [15,16] as illustrated in
Fig. 4. The independence of the breakdown strength with
thickness at high temperature can be related to the breakdown
mechanism that occurs and which is not thickness dependent
such as impulse thermal breakdown where the thermal
conduction of the film does not has any role [17], or some
other electronic breakdown processes [12]. The breakdown
field at 350 °C remains higher than 3 MV/cm indicating that
the material is suitable for high temperature applications.
0 50 100 150 200 250 300 350 400
2
3
4
5
6
7
8
9
1
0
Electrode diameter 0.6 mm
α (MV/cm)
Temperature (°C)
1.4 µm
5 µm
Fig. 4. Variation of the scale parameter α with temperature for two different
thicknesses.
D. Rising field rate effect
Several assumptions can explain the variation of the Weibull
parameters with the rising rate. The first one is related to the
charge accumulated with time in the dielectric. Dielectric
breakdown occurs when a critical amount of charges is
accumulated in the dielectric. When the total charge exceeds
this value, the irreversible dielectric breakdown occurs. It can
be understood that as more time is available at each field at
low rising rates, charge accumulation is higher and hence
breakdown field is lower. The second assumption is related
to the induced heat by Joule effect. It was shown in [17] and
[18] that the rising rate of the applied field, in the range of 0.2
MV/cm.s and 3 MV/cm.s in the former and 0.08 MV/cm.s and
0.8 MV/cm.s, does not affect the breakdown strength at low
temperature (ambient) but it has an effect at higher
temperature. This behavior supports their discussions of the
two different breakdown mechanisms at low and high
temperature corresponding to an electronic breakdown, which
is unaffected by the rising applied field rate, and an impulse
thermal breakdown, related to the heat by Joule effect in the
bulk of the material. In the latter case, the breakdown strength
increases when the rising rate increases. In the third
assumption, the bulk and surface effects on the dielectric
strength was investigated and it was reported that at low rising
rate the final breakdown process is screened by an excessive
amount of space charges from suffering surface defects as
compared with the high rising rate. This effect was already
shown for thin films of polyethylene for rising rate from 10
2
V/s to 10
6
V/s (for different thicknesses of about 1 µm) in
(6)
742
[19], where, the variation of the results dispersion for the
different rising rates (an improvement in β values must be
shown with decreasing the rising field) is due to a smaller
concentration of weak spots in the bulk of the material than
that at the surface. In order to investigate the effect of the
rising field on our material, the Weibull statistical
representation for three rising field rates at ambient and high
temperature (300 °C) on the 1.4 µm-thick films are
represented in Fig. 5. The α values seem to be unaffected by
the rising rate at the two tested temperatures which leads to
ignore the presence of the first two assumptions related to the
charge accumulation and Joule effect. Also the β value seems
to be unaffected by the rising rate and more clearly at 25 °C
which can lead to say, by referring to the third assumption that
the defect density at the material surface is very low and does
not affect the shape parameter.
2 3 4 5 6 7 8 9 10
Electrode diameter 0.6 mm
E
BR
(MV/cm)
log
10
(log
e
(1/1-F))
0.07 MV/cm.s-2C
0.35 MV/cm.s-2C
0.7 MV/cm.s-2C
0.07 MV/cm.s-300°C
0.35 MV/cm.s-300°C
0.7 MV/cm.s-300°C
1.0
0.5
0.0
-0.5
-1.0
-1.5
Fig. 5. Effect of the rising applied field rate on the Weibull distribution at 25
°C and 300 °C.
IV. CONCLUSION
The dielectric strength of fluorinated parylene (PA-F) films
was studied at high temperature up to 350 °C under DC
conditions and shows negative temperature dependence. The
effect of electrode area, thickness and rising field rate were
also investigated and the breakdown strength seems to be
unaffected by all these parameters in the studied range, which
allows us to conclude that the defect density at the material
surface and in the bulk is very low and does not affect the
scale parameter values. Results presented in this paper indicate
also that PA-F can be a suitable material for high temperature
applications, at least for short times, with a breakdown
strength higher than 3.5 MV/cm at 350°C measured for thin
films (1.4 and 5 µm), which is in good agreement with silicon
carbide power device applications, where the electric field
stress can reach locally values between 2 and 3 MV/cm.
AC
KNOWLEDGEMENTS
The authors gratefully acknowledge the support of the FRAE
(Foundation de Recherche pour l’Aéronautique et l’Espace).
Special thanks are expressed to Mr. Benoît Schlegel for his
help in experimental samples preparation.
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... The observed relatively weak ramp-rate-dependence of the un-filled BOPP films is consistent with previous studies on e.g. BOPP [16] and parylene F [17] films. At the slowest ramp rates (0.25 and 1 Vs -1 µm -1 ) the 63.2 % the Cap-BOPP film shows a slight decrease in the high-probability breakdown region and emergence of weak point population. ...
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The effect of voltage ramp rate on the short-term dielectric breakdown strength of polymer nanocomposites is not well-documented. In this paper, the effect of DC field ramp rate on the large-area breakdown performance of melt-extruded bi- axially oriented polypropylene (BOPP) films incorporating 4.5 wt-% of nano-silica is studied. By utilizing a self-healing multi- breakdown measurement method with a variable DC voltage ramp rate, a statistically large amount of breakdown data was obtained from a large total sample film area as a function of DC field ramp rate (0.1-50 Vs-1µm-1)). With a decreasing ramp rate, Weibull statistical analysis of the breakdown data suggests a systematically decreasing trend in the breakdown strength (Weibull Į) and an increase in the Weibull shape parameter of time (a>1) for the nanocomposite. The observed behavior is speculated to be attributable to highly altered internal charge dynamics of the silica-BOPP nanocomposite. The results exemplify the importance of careful breakdown strength assessment when dielectric films of more complex internal structure are studied.
... Consequently, the following section only gives the main experimental observable tendencies on the breakdown field of thermo-stable dielectrics. Recently, the influence of several parameters on the dielectric strength has been reported for BPDA/PDA PI and PA-F films (Diaham, 2010b;Khazaka, 2011a). ...
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