PARYLENE-C AS AN ELECTRET MATERIAL FOR
MICRO ENERGY HARVESTING
S. Genter and O. Paul
Department of Microsystems Engineering (IMTEK), University of Freiburg, Germany
Abstract: This paper reports the results of a systematic study about the suitability of Parylene-C as an electret
material in micro energy harvesting devices to transduce vibrational energy into the electrical domain. The aim is
the optimization of the long-term stability of electrical charges in the electret, implanted by a corona discharge
setup. With electret layer thicknesses between 4 µm and 23 µm, charging potentials up to í1200 V and charging
temperatures up to 120°C the time-dependent discharging behavior was recorded over a period of 285 days. The
experiments showed a charge stability increased by 18% at an optimal charging temperature of 100°C in comparison to room temperature charging. By using lower charging potentials and thicker electret layers, the relative
charge loss is reduced by 30% and 65% respectively.
Keywords: Parylene-C, long-term charge stability, energy harvesting
nar electrode. Primarily, CO3í ions emerge from the
discharge [9,10]. Due to their low kinetic energy, the
penetration depth of the ions is small and they are
stored close to the electret surface. A mesh-shaped
grid electrode between the needle and the planar electrode makes it possible to control the implanted
charge carrier density. The surface potential resulting
from the implanted charge carriers is limited to the
potential of the grid electrode with respect to the planar electrode.
INTRODUCTION
The development of micro energy harvesting devices able to transduce ambient energy into the electrical domain has recently received much attention
[1-3]. The harvested electrical energy is used to extend battery lifetime or even to avoid the need of batteries of e.g. wireless sensor nodes or implanted medical devices such as cardiac pacemakers.
One approach among others to harvest vibrational
energy exploits electrostatic transduction using
charged electret materials [4]. For this type of micro
generators, the stability of the ionic charges implanted
into the electret is mandatory for guaranteeing the
long-term functionality of the device.
Parylene-C is a material with several attractive features. Due to its high dielectric constant of 3.15,
charge carrier densities larger than 2.0 mC/m² can be
achieved. This value is more than 50% higher than
that of Cytopȹ[5,6] and thus offers a better device performance. The ability of Parylene-C to store electrical
charges over extended periods has thus to be investigated. Other groups have already used Parylene-C
[7,8]. However, a detailed study of its storage properties has been lacking and is provided by this work.
Discharging Mechanisms
Electret materials lose their charge carriers for several reasons [9]. First, they are non-ideal insulators.
Thus, resistive losses occur depending on the positive
or negative intrinsic carriers available in the valence
or conduction bands, respectively. The implanted
charge carriers build up an electrical field causing the
drift of ions within the material towards the planar
electrode where they are neutralized. A third phenomenon is diffusion. By implanting charge carriers at the
electret surface one creates a concentration gradient.
THEORY
Electret Charging
Electrets can be charged using several techniquesȹ[9], one of which is the corona discharge
method. Hereby, the inhomogeneous electrical field
between a sharp-tipped needle and a planar electrode
produces a discharge in air at atmospheric pressure.
When a negative potential is applied between needle
and planar electrode, negative charge carriers are accelerated towards the electret layer covering the pla978-0-9743611-9-2/PMEMS2012/$20©2012TRF
Figure 1: Photograph of two test chips with ParyleneC coating for the characterization of the charge stability of this electret material.
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PowerMEMS 2012, Atlanta, GA, USA, December 2-5, 2012
achievable surface potential is limited to the potential
UGrid applied to the grid electrode placed between
needle and electret. When USurf = UGrid, the accelerating force vanishes and no further ions are implanted.
Both potentials are provided by the high-voltage
sources LNC 10000-2 neg and LNC 1200-20 neg
from Heinzinger, Germany. Further, the charging station comprises the temperature controller CT15 from
Minco, USA, enabling the temperature to be varied
between room temperature (RT) and 120°C.
Figure 2: Photograph of the corona discharge setup
(left) and a schematic view of the charging chamber
(right).
Surface Potential Measurement
Figure 3 shows the setup for measuring the surface
potential after the test chips have been charged. The
main component is the electrostatic voltmeter Isoprobe 279 from Monroe Electronics, USA. This instrument is able to measure potentials up to ±3000 V
with an accuracy of 0.1% and a temperature stability
of ±0.003%/°C. Up to 16 test chips can be placed on a
vacuum chuck mounted on a xyș-stage. A LabView
routine scans the voltmeter probe over the sample surface in steps of 200 µm, mapping the surface potential
and determining the maximum and average surface
potential values.
Figure 3: Photograph of the measurement setup
of the surface potential of charged electret layers.
RESULTS
The initial distribution tends towards a homogeneous
distribution, whereby charge carriers again reach the
planar electrode. Further charge decay occurs also due
to the attraction of compensation charges from the
environment.
Electret Thickness
In a first series of experiments, the electret thickness is varied between 4 µm and 23 µm to determine
its influence on the long-term stability of the charges.
All chips were charged at RT at UNeedle = í8 kV and
UGrid values between í100 V and í1200 V for a duration of 3 min. The surface potentials were measured
directly after charging. The results for five different
thicknesses including a common linear fit of all the
data are shown in Figure 4. Apparently, the thickness
of the electret layer has no significant influence on the
resulting surface potential at any value of UGrid. The
TEST CHIP FABRICATION
Test chips for the purpose of electret charging experiments and surface potential measurements were
fabricated. A metal stack of 20 nm Cr, 300 nm Au and
40 nm Cr is deposited on a four-inch Pyrex substrate
and structured by wet etching. Then, Parylene-C layers with different thicknesses are deposited at a pressure of 22 mTorr. The Parylene-C layers are structured into 8×8 mm² squares by reactive ion etching in
oxygen plasma. Finally, the wafer is diced into
11.5×11.5-mm²-large chips. Two of them are shown
in Figure 1.
EXPERIMENTAL
Sample Charging
To charge the test chips, the custom-made threeelectrode corona discharge setup [4] shown in Figure
2 is used. The needle potential UNeedle = í8 kV is applied with respect to the grounded planar electrode
beneath the Parylene-C. The generated ions are accelerated and implanted into the electret where they establish a surface potential USurf. The maximum
Figure 4: Measured initial surface potential USurf as a
function of the grid voltage UGrid for Parylene-C layers
of different thicknesses charged at RT.
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UGrid = í V, while UGrid = í V leads to a reduction by only 26%. Clearly, higher potentials are
preferred for a better generator performance, but
slower decay is favorable for long-term operation.
With identical layer thicknesses and increasing initial USurf, the electrical field within the layer is increased. Therefore, higher electrostatic forces act on
the charge carriers and result in a faster decay.
Influence of Charging Temperature
As mentioned, the corona discharge setup includes
a temperature controller. Use was made of this additional degree of freedom to charge 11-µm-thick
Parylene-C layers at temperatures between RT and
120°C using UGrid = í1000 V. In doing so, the glass
transition temperature TG of about 90°C [11] is exceeded by several samples. Both UNeedle and UGrid
stayed activated for 10 min. After 3 min, the heater
was turned off to let the samples cool down below TG.
Even for charging at 120°C, the temperature dropped
below 45°C after the charging cycle. The normalized
values of USurf are shown in Figure 7.
Figure 5: Normalized surface potential USurf as a function of the time for Parylene-C layers of different
thicknesses charged at RT with UGrid = í V.
linear fit has a mean slope of 616 V/kV. The standard
deviation of the experimental data from the fit curve is
±15.9 V.
By repeating the measurement of USurf at later
times, different discharging behaviors for the various
layer thicknesses are observed. In Figure 5 USurf normalized to its value directly after charging with
UGrid = í V is plotted against time. The 4-µmthin electret layer loses 84% of its initial surface potential whereas the 23-µm-thin layer only loses 26%
after a duration of 100 days. Assuming identical initial
surface potentials, the faster decay in thinner layers
can be quantitatively explained by the higher electrostatic forces acting on the charge carriers. They are
efficiently driven towards the planar electrode under
the electret layer where they are neutralized.
Comparing the differences between the thicknesses
we conclude that the gain in charge stability levels off
at large thicknesses. In other terms, the difference between 4 µm and 6 µm is much larger than e.g. between 11 µm and 23 µm. Theoretically, the surface
charge density resulting from the given USurf value is
smaller for thicker electret layers. Considering the
similar behavior of 11-µm- and 23-µm-thick layers,
11 µm seems an optimal thickness for a high surface
charge density combined with a decent charge carrier
stability.
Figure 6: Discharging behavior of an 8-µm-thick
Parylene-C layer charged at RT with different grid
voltages UGrid.
Variation of UGrid
In a further experiment, 8-µm-thick layers were
charged for 3 min at RT with grid potentials UGrid between í100 V and í1200 V. Again the timedependent discharging behavior was monitored over a
period of 285 days. Figure 6 shows the significant
influence on the discharging rate of the applied UGrid
value and therefore the initial USurf. Samples charged
at higher potentials suffer from a higher discharging
rate than samples charged at lower potentials. For example, a reduction of 58% is observed for
Figure 7: Influence of charging temperature on the
discharging behavior of 11-µm-thick electret layers
charged at UGrid = í V.
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Higher charging temperatures showed no influence
on the initial surface potentials. The values for all
charging temperatures are in good agreement with
Figure 4. Elevating the charging temperature up to
100°C steadily reduces the charge decay. The loss
after a duration of 146 days can be reduced from 38%
when charging at RT to 21% when charging at 100°C.
We conjecture that due to the elevated temperature,
the ion mobility in the polymer is increased and thus
the implanted ions are able to settle into more stable
states. The result for a charging temperature of 120°C
differs: the charge stability is lower for longer times.
Possibly, polymer degradation has occurred during
charging and the ion mobility is enhanced.
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CONCLUSION
The experiments have shown that Parylene-C is a
suitable electret material for micro generators to
transduce vibrational energy into the electrical domain. However, it has its limitations in view of the
decay which the stored charges undergo. This behavior is similar to materials such as PTFE and PET but
is inferior to Cytop. These materials are also commonly used as electret materials [7,12].
By varying the electret thickness and two parameters for charging the layers, namely the grid potential
and the temperature, the long-term stability of the
charges was quantified and improved. It turns out, that
a charging temperature of 100°C close to TG results in
the highest charge stability.
As far as the electret thickness is concerned, one
faces a tradeoff. Thicker electret layers show significantly increased charge stability. From 11 µm on, the
further enhancement is moderate. However, the surface charge density for identical USurf values is lower
for thicker layers. All aspects considered, 11 µm can
be recommended as an optimal Parylene-C layer
thickness.
Considering the grid potential, increasing its value
results in a linear increase of the surface potential independent of the electret thickness. Such higher potentials are desired. However, they result in a faster
decay due to the higher electric field within the electret and hence the higher electrostatic forces acting on
the electret charges.
ACKNOWLEDGMENTS
We gratefully acknowledge the financial support by
the German Research Foundation (DFG) through
grant no. GRK 1322.
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