Stresa, Italy, 26-28 April 2006
?TIMA Editions/DTIP 2006 -page- ISBN: 2-916187-03-0
DESIGN AND FABRICATION OF A MICRO ELECTROSTATIC VIBRATION-TO-
ELECTRICITY ENERGY CONVERTER
Yi Chiu*, Chiung-Ting Kuo and Yu-Shan Chu
Department of Electrical and Control Engineering, National Chiao Tung University
1001 Ta Hsueh Road, Hsinchu 300, Taiwan, R.O.C.
Tel: +886-3-573-1838, Fax: +886-3-571-5998, Email: yichiu@mail.nctu.edu.tw
ABSTRACT
This paper presents a micro electrostatic vibration-to-
electricity energy converter. For the 3.3 V supply voltage
and 1cm
2
chip area constraints, optimal design parameters
were found from theoretical calculation and Simulink
simulation. In the current design, the output power is 200
?W/cm
2
for the optimal load of 8 M?. The device was
fabricated in a silicon-on-insulator (SOI) wafer.
Mechanical and electrical measurements were conducted.
Residual particles caused shortage of the variable
capacitor and the output power could not be measured.
Device design and fabrication processes are being
refined.
1. INTRODUCTION
Due to the advance of CMOS VLSI technology, the
power consumption of electronic devices has been
reduced considerably. The low power technology enables
the development of such applications as wireless sensor
networks [1] or personal health monitoring [2], where
remote or independent power supply is critical for
building more compact or longer-life-time systems. In
particular, energy scavenging from ambient natural
sources, such as vibration [3], radioisotope [4] and
ambient heat [5], is attracting many recent interests as the
self-sustainable power source for these applications.
Among various approaches, electrostatic vibration-to-
electricity conversion using the micro-electro- mechanical
systems (MEMS) technology is chosen in this study due
to its compatibility to IC processes and the ubiquity of the
energy source in nature.
The output power of a vibration driven converter is
related to the nature of the vibration source, which must
be known in order to estimate the generated power. The
vibration spectra of several household appliances were
measured. A typical vibration source has a peak
acceleration of about 2.25 m/s
2
at about 120 Hz, as shown
in Fig. 1. These values are used in the following static
and dynamic analysis for the design of the converter.
2. DESIGN
A variable capacitor C
v
formed by an in-plane gap-
closing comb structure is the main component in the
energy converter [3, 6], as shown in Fig. 2. Fig. 3 shows a
schematic circuit that can be used to extract the converted
energy. The variable capacitor C
v
is charged by an
external voltage source V
in
through the switch SW1 when
C
v
is at its maximum C
max.
When C
v
is charged to V
in
,
SW1 is opened and then the capacitance is changed form
C
max
to C
min
due to the electrode displacement caused by
vibration. In this process, the charge Q on the capacitor
remains constant (SW1 and SW2 both open). Therefore,
the terminal voltage on the capacitor is increased and the
vibration energy is converted to the electrostatic energy
stored in the capacitor. When the capacitance reaches C
min
(V
max
), SW2 is closed and the charge on C
v
is transferred
to a storage capacitor C
stor
. SW2 is then opened and C
v
goes back to C
max
, completing one conversion cycle.
During the period when SW2 is open, the charge on C
stor
is discharged by the load resistance R
L
with a time
constant ? = R
L
C
stor
before it is charged again by C
v
. In
the steady state, the initial and final terminal voltages V
L
of the discharge process become constant, as shown in
Fig. 4.
Figure 1 Typical vibration spectrum of a household
appliance
Yi Chiu, Chiung-Ting Kuo and Yu-Shan Chu
Design and Fabrication of a Micro Electrostatic Vibration-To-Electricity Energy Converter
?TIMA Editions/DTIP 2006 -page- ISBN: 2-916187-03-0
It can be shown that the steady-state final terminal
voltage V
sat
in the charge-discharge cycle can be
expressed as
max
in
stor
sat
min
Lstor
stor
C
V
C
V = , (1)
C
1+ exp( t/R C ) - 1
C
??
??
??
??
where ?t = conversion cycle time = 1/2f and f is the
vibration frequency. When the voltage ripple of the
charge-discharge cycle is small, as will be shown
subsequently, the output power can be estimated by
2
sat
out
L
V
P = , (2)
R
which is in general proportional to C
max
2
. In the comb
structure, C
max
is determined by the minimum finger
spacing. In a previous design [7], the minimum finger
spacing is kept at 0.5 ?m to prevent shortage of the
uninsulated fingers (Fig. 5(a)). If a dielectric coating can
be applied to the side walls of the fingers (Fig. 5(b)), they
become insulated and the minimum spacing can be
further reduced to increase C
max
and P
out
. In this design,
the total capacitance becomes C
dielectric
|| C
air
|| C
dielectric
(Fig.
6).
Silicon nitride will be used as the dielectric material
due to its process compatibility and high dielectric
constant (?
r
~ 7). With a 500-?-thick nitride coating, C
max
can be increased by a factor of four, compared to the
previous design. It should be noted that the dielectric
coating barely increases C
min
. Therefore, the expected
increase of output power will not be affected by the
change of C
min
.
2.1. Static analysis
In Eq. (1), R
L
and C
stor
can be chosen so that the
discharge time constant ? = R
L
C
stor
is much larger than the
conversion cycle time ?t. The output voltage ripple in the
Figure 2 Variable capacitor schematic
C
min
C
max
displacement due
to vibration
displacement due
to vibration
y
y
z
z
SW1 SW2
V
in
C
stor
C
v
R
L
Figure 3 Operation of the electrostatic energy converter
+
V
L
_
V
L
SW2 open
(discharging)
SW2 close
(charging)
Time
V
sat
Figure 4 Output terminal voltage V
L
in the charge-
discharge cycle
dielectric
coating
(a)
(b)
Figure 5 Variable capacitor at C
max
position: (a)
without coating, (b) with dielectric coating
C
dielectric
C
dielectric
C
air
Figure 6 Equivalent C
total
Yi Chiu, Chiung-Ting Kuo and Yu-Shan Chu
Design and Fabrication of a Micro Electrostatic Vibration-To-Electricity Energy Converter
?TIMA Editions/DTIP 2006 -page- ISBN: 2-916187-03-0
steady state can therefore be neglected. In this case, V
sat
can be approximated as
max in
sat
min
L min L stor
CV
V . (3)
tt
C1+
RC RC
?
????
+
??
??
Usually C
min
is a small value (in the order of 100 pF).
The other circuit components in Eq. (3) can then be
chosen so that C
stor
>> C
min
and R
L
C
min
<< ?t and the
expression can be simplified as
max in max in
sat L
min
Lmin
CV CV
V = R . (4)
t
t
C
RC
?
?
?
The power output becomes
2
sat max in
out L2
L
VCV
P R . (5)
tR
??
??
??
?
??
For a typical low-power sensor node or module, the
minimum output power requirement is about 200 ?W. In
addition, a power management circuit is needed to
convert the high output voltage to lower ones for various
sensor and signal processing units. To be compatible
with the power management circuit, the maximum output
voltage should be limited to about 40 V. Inserting these
constraints into Eq. (2), one can obtain the range of R
L
,
L
R8 M?.?
Even though a smaller R
L
can be used, this would require
increasing C
max
in order to satisfy the voltage and power
requirement (Eqs. (4) and (5)), which in turn will have
adverse effects in the dynamic behavior of the converter.
Therefore, R
L
= 8 M? and hence C
max
= 7 nF are used in
the following calculation.
The output power P
out
for various C
stor
and R
L
is
shown in Fig. 7 for C
stor
>> C
min
. It can be seen that the
output power does not depend strongly on the storage
capacitor C
stor
when it is relatively large. Nevertheless, a
large C
stor
will result in long initial charge time when the
converter starts to work from a static status. Hence, a
reasonable C
stor
of 20 nF is used.
From Eq. (1) and with the values of C
stor
and R
L
obtained from above, input voltage V
in
of 3.3 V, vibration
frequency of 120 Hz, and chip area size of 1 cm
2
, Fig. 8
shows the calculated output saturation voltage and power
as a function of the initial finger gap distance and the
thickness of the silicon nitride layer. The finger thickness,
length, and width are 200 ?m, 1200 ?m and 10 ?m,
respectively [7]. The dimensions of the fingers are based
on the available deep etching process capability. The
minimum gap distance is assumed to be 0.1 ?m, which is
controlled by mechanical stops. It can be seen that with a
500-?-thick nitride, the initial finger gap has an optimal
value of 35 ?m for a power output of 200 ?W and output
voltage of 40 V.
2.2. Dynamic analysis
After the dimensions of the variable capacitor are
determined from the static analysis, the dynamics of the
micro structure is analyzed so that the desired maximum
displacement, and hence C
max
, can be achieved by the
target vibration source. The electro-mechanical dynamics
of the converter can be modeled as a spring-damper-mass
system. The dynamic equation is
em
mz+b (z)+b (z,z)+kz= my, (6)-&& & &&
where z is the displacement of the shuttle mass m with
respect to the device frame, y is the displacement of the
device frame caused by vibration,
m
b (z,z)& is the equiva-
lent mechanical damping representing energy loss caused
mainly by the squeezed film effect, and b
e
(z) is the
electrostaitc force acting on the MEMS structure. Notice
that the mechanical damping
m
b is a function of both the
displacement z of the shuttle mass and its velocity z& [3].
Figure 8 Output saturation voltage and power vs.
initial finger gap (R
L
= 8 M?, C
stor
= 20 nF)
Figure 7 Output power for various R
L
and C
stor
Yi Chiu, Chiung-Ting Kuo and Yu-Shan Chu
Design and Fabrication of a Micro Electrostatic Vibration-To-Electricity Energy Converter
?TIMA Editions/DTIP 2006 -page- ISBN: 2-916187-03-0
A Simulink model was built to simulate the system
behavior based on Fig. 3 and Eq. (6), as shown in Fig. 9.
The charge redistribution box calculates the charging and
discharging events when C
v
reaches C
max
or C
min
. This
process represents the power output. Due to the limited
shuttle mass that can be achieved in a MEMS process
using only silicon, an external attached mass m is
considered in order to increase the displacement of the
variable capacitor and the energy conversion efficiency.
For various attached mass, Fig. 10 shows the
maximum achievable displacement and corresponding
spring constant. It can be seen that a mass of 7.2 gram is
required to achieve the maximum of 34.8 ?m according
to the static design. The corresponding spring constant,
4.3 kN/m, will used to design the spring structures. With
these values, the output voltage simulated by the
Simulink model as a function of time is plotted in Fig. 11.
The charge-discharge cycles are evident and the
saturation voltage V
sat
is close to the expected value of 40
V. Table 1 summarizes the important device design
parameters according to both the static and dynamic
analysis.
3. FABRICATION
A schematic device layout is shown in Fig. 12. The
center hole is used to fix the position of the attached steel
ball. A SOI wafer with a 200-?m-thick device layer was
used for large capacitance. The oxide layer and the handle
wafer are 2 ?m and 500 ?m thick, respectively. Fig 13
shows an earlier fabrication process without the dielectric
coating. The variable capacitor structure is first defined
by deep reactive ion etching (Deep RIE) (Fig. 13(a)).
After the sacrificial oxide layer is removed using HF
solution (Fig. 13(b)), aluminum is evaporated for contact
(Fig. 13(c)). A steel ball is then attached to the central
plate to adjust the resonant frequency to match the
Table 1 Design parameters of the energy converter
Parameter Description
W Width of shuttle mass 10 mm
L Length of shuttle mass 8 mm
L
f
Length of finger 1200 ?m
W
f
Width of finger 10 ?m
m Shuttle mass 7.2 gram
d Initial finger gap 35 ?m
d
min
Minimum finger gap 0.1 ?m
C
stor
Storage capacitance 20 nF
k Spring constant 4.3 kN/m
t Dielectric layer thickness 500 ?
?
r
Dielectric constant 7 (SiN)
R
L
Load resistance 8 M?
V
sat
Output voltage ~ 40 V
P
out
Output power ~ 200 ?W
Figure 9 Dynamic simulation model
Figure 10 Maximum displacement and spring constant
for various attached mass
Figure 11 Output voltage vs. time
Yi Chiu, Chiung-Ting Kuo and Yu-Shan Chu
Design and Fabrication of a Micro Electrostatic Vibration-To-Electricity Energy Converter
?TIMA Editions/DTIP 2006 -page- ISBN: 2-916187-03-0
vibration source and improve the conversion efficiency
(Fig. 13(d)).
The fabricated first-generation device is shown in
Fig. 14 [7]. The width of the finger is reduced to 6.8 ?m
due to the tolerance in photolithography and RIE
processes. The deviation will affect the characteristics of
the converter such as the resonant frequency, output
power, and output voltage.
4. MEASUREMENT
4.1. Mechanical measurement
The displacement of the device without the attached mass
was measured using a PROWAVE JZK-1 shaker. The
measured response is shown in Fig. 15. Since the mass
was not attached, the vibration acceleration was increased
to 40 m/s
2
for easy observation. The maximum displace-
ment is about 10 ?m at 800 Hz, and the quality factor
0
Q = ??? is about 9.6, where
0
? is the resonant
frequency and ?? is the resonant bandwidth shown in
Fig. 15. The mass of the center plate is approximately
0.038 gram, thus the spring constant can be calculated as
2
0
k = ? m = 960 N/m . The measured spring constant is
different from the design mainly due to the feature size
shrink in the fabrication process, as shown in Fig 14.
4.2 Electrical measurement
The electrical measurement was conducted using an
INSTEK-LCR-816 LCR meter and a HP-4192A
Figure 12 Layout schematic
a) c)
b) d)
Si Al
Figure 13 Fabrication process: (a) define structure by
Deep RIE, (b) etch oxide by HF solution, and (c) apply
Al by thermal evaporation, (d) attach external mass
Figure 15 Frequency response of the device
Figure 14 Fabricated device: (a) top view, (b) cross
section of comb fingers, (c) overview of the converter
with attached mass
width ~
6.8 ?m
depth
~ 200 ?m
(b)
(c)
center
plate
spring
mecha-
nical stop
variable
capacitor
(a)
Yi Chiu, Chiung-Ting Kuo and Yu-Shan Chu
Design and Fabrication of a Micro Electrostatic Vibration-To-Electricity Energy Converter
?TIMA Editions/DTIP 2006 -page- ISBN: 2-916187-03-0
impedance analyzer. The measured capacitance without vibration was about 500 ~ 600 pF, while the calculated
capacitance C
min
is about 50 pf. The major contribution of
the large measured capacitance is the parasitic
capacitance C
par
between the center plate and the
substrate beneath it.
Besides the parasitic capacitance, there is also a
parallel parasitic conductance. The measured conductance
varies from die to die with an average resistance of 2.5
k?. It is suspected to be caused by the residual particles
left in the device after the release step. The presence of
the parasitic capacitance and conductance had hindered
the measurement of output power. New devices are being
fabricated with the substrate underneath the combs
removed to prevent residual particles.
5. CONCLUSION
The design and analysis of a micro vibration-to-electricity
converter are presented. The device was fabricated in a
SOI wafer. The reduced feature size of the fabricated
device resulted in the decrease of spring constants.
Mechanical and electrical measurements of the fabricated
device were conducted. Impedance measurements
showed an unwanted parasitic conductance which
resulted in the failure of output power measurement.
Improvement of the design and fabrication processes is
being conducted.
This project is supported in part by the National
Science Council, Taiwan, ROC, under the grant No. NSC
93-2215-E-009-066.
6. REFERENCES
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low-power wireless networking?, IEEE Computer, Vol.
33, pp. 42-48, 2000.
[2] R. Tashiro, et al., ?Development of an electrostatic
generator that harnesses the motion of a living body: (use
of a resonant phenomenon)?, JSME International Journal
Series C, Vol. 43, No. 4, pp. 916-922, 2000.
[3] S. Roundy, et al., ?Micro-electrostatic vibration-to-
electricity converters,? Proc. IMECE 39309, 2002.
[4] R. Duggirala, et al., ?Radioisotope micropower
generator for CMOS self-powered sensor microsystems?,
Proc. PowerMEMS, pp. 133-136, 2004.
[5] T. Douseki, et al., ?A batteryless wireless system uses
ambient heat with a reversible-power-source compatible
CMOS/SOI dc-dc converter?, Proc. IEEE International
Solid-State Circuits Conference, pp. 2529-33, 2003.
[6] C.B. William, et al., ?Analysis of a micro-electric
generator for microsystems?, Sensors and Actuators, A52,
pp. 8-11, 1996.
[7] Y.S. Chu, et al., ?A MEMS electrostatic vibration-to-
electricity energy converter?, Proc. PowerMEMS, pp. 49-
52, 2005.