Radioisotope Micropower Generator for
CMOS Self-powered Sensor Microsystems
R. Duggirala, H. Li, A. M. Pappu*, Z. Fu*, A. Apsel* and A. Lal
SonicMEMS Laboratory
* OEVLSI Laboratory
School of Electrical and Computer Engineering, Cornell University
122 Philips Hall, Ithaca, NY-14850
Tel 001-607-255-1815, Fax 001-607-254-3508, e-mail rd92@cornell.edu†
Abstract
We present a long lasting (10’s of years) radioisotope micropower generator for self-powered sensor microsystems that enables
truly autonomous operation. In this paper, we demonstrate powering of a microsystem comprising an optical sensor that
modulates a silicon-on-sapphire+ (SOS) ring-oscillator for data logging. The generator employs direct charging to convert
radiated ß-particle kinetic energy into stored electromechanical energy in a piezoelectric unimorph, and piezoelectricity to
convert the stored electromechanical energy to extractable electrical energy, with an overall conversion efficiency of 2.78 %.
The generator goes through a charge-discharge-vibrate cycle, integrating the energy collected during the charging phase (115
min at 48 nW input energy from a weak 0.5 milliCurie source). This enables high power output (16 µW peak across a 1 MO
load impedance) for a short time (7 s) during the vibration cycle. The AC signal from the piezoelectric element is rectified
using SOS diodes, and stored across an external capacitor. The voltage bias (~ 2 V) thus realized is used to drive low-power
sensors and electronics.
Keywords: radioisotope, self-powered ,microsystem, micropower, sensor.
The low penetration depth of the emitted particles
combined with the solid state nature of the source
eliminates the need for elaborate measures for shielding,
further enhancing the applicability of the micropower
generator for microsystems such as remote sensor nodes.
Here we integrate this micropower generator with an
optical sensor microsystem comprising of an optical sensor
modulating a ring oscillator for data logging.
1 INTRODUCTION
Major
advances
in
microelectronics
and
microfabrication have enabled the realization of wireless
sensor and actuator microsystems, which can potentially
be used in applications including environment monitoring
[1], surveillance [2], etc. A major constraint in the
realization of such nodes is the amount of energy that can
be packed into the power unit for autonomous operation,
as these nodes are often not rechargeable. Currently used
battery technologies [3], and the fuel cell technologies
under investigation [4] are limited by the low energy
storage density (1-2 kJ/m3 for batteries and 20 kJ/m3 for
hydrocarbon fuels) of the fuels used. In comparison,
radioisotopes can have very high energy densities (~105
kJ/m3) [5]. Additionally, since the half-life of radioisotope
thin films can range from 100’s of years to a few seconds,
a power source with optimal lifetime can be designed with
a suitable choice of radioisotope.
We have previously reported a self-reciprocating direct
charging cantilever [6] to show the feasibility of capturing
the kinetic energy of emitted particles to actuate a
cantilever, and convert the radiated kinetic energy to stored
mechanical energy in the cantilever. We recently realized a
milliscale micropower generator that utilizes the direct
charging process to actuate a piezoelectric unimorph, which
in turn generates electrical power [7]. The piezoelectric
unimorph supplies the load circuit with a directly usable
voltage signal while shielding it from the high voltage
generated due to direct charging. The generator uses a 0.5
millicurie thin-film 63Ni solid source emitting low-energy ß
radiation (average electron energy 17 keV, half-life = 100.2
years) with very low penetration depth (14 µm in copper).
2 THE RADIOISOTOPE THIN-FILM MICROPOWER
GENERATOR
Figure 1 shows the schematic of the piezoelectric
radioisotope micropower generator. The collector at the tip
of the cantilever beam traps the charged particles emitted
from the thin-film radioactive source. By charge
conservation, the radioisotope film is left with equal and
opposite charges. This leads to electrostatic force between
the cantilever and the radioactive source, bending the
cantilever and converting the radiated kinetic energy to
stored mechanical energy. For suitable initial gap
separations, the tip of the cantilever eventually makes
contact with the radioactive thin-film and the accumulated
charges get neutralized via charge transfer. As the
electrostatic force is nulled, the cantilever is released. The
sudden release excites oscillations, which lead to charges
induced in the piezoelectric element at the base of the
cantilever. The AC signal from the piezoelectric power
source can be used directly across a load impedance
(Figure 2) or rectified using diodes, and filtered across an
external capacitor (Figure 2 Inset). The voltage bias thus
realized is used to drive low-power sensors and electronics
(Figures 5, 6, 7). The analysis below assumes a resistive
133
by modeling the air-gap capacitor as a current controlled
electrostatic actuator and assuming a linear spring. The
electromechanical energy Eem stored in the cantilever just
before discharge is
Piezoelectric unimorph section
Lc
Lp
Vout
hp
Port 1
hs
Piezoelectric
plate (PZT)
δ
E em = E m + E q =
δ0
Figure 1 Cross-section showing the geometrical parameters of
the cantilevered piezoelectric unimorph and a model of the airgap capacitor. The current source is due to emitted charges, R is
due to ionization current/ secondary electron emission. The
source plate and the collector plate form a capacitor.
Tvib V 2 (t )
Eext = ∫ out dt ,
Rl
0
6
Voltage (V)
1
0
η = η rη me =
-2
0.95
1
Time (s)
0
2
-2
1.5
1
0.5
-4
0
0
1
2
Time (s)
0
1
3
2
3
Time (s)
4
E em E ext
E
= ext ,
E r E em
Er
(5)
where ?r is the ratio of the stored electromechanical energy
to radiated kinetic energy and ?me is the ratio of the
extracted electrical energy to stored electromechanical
energy. The ratio ?r can be maximized by designing the
peak charging voltage of the air-gap capacitor, Vcapmax, to
satisfy the condition
1.05
2.5
Voltage (V)
Voltage (V)
-1
2
(4)
where Vout(t) is the output voltage.
The ratio of the extracted electrical energy to radiated
kinetic energy ? is
2
4
(3)
where Em is the stored mechanical energy, Eq is the stored
dielectric energy in the piezoelectric element, k is the
stiffness of the cantilever beam, d0 is the initial gap height,
Qp is the charge induced in the piezoelectric element just
before contact due to the bending deformation and Cp is
the capacitance of the piezoelectric element. For the
devices discussed here, Em~1000Eq. The extracted
electrical energy per cycle Eext, across a load resistor Rl, is
given by
Ni 63
Radioisotope
emitter
Silicon beam
2
1 2 Qp
1
kδ 0 +
≅ kδ 02 ,
2
2C p 2
5
V cap max =
4
3
E avg
8 kδ 0
.
=
q
27 εA
(6)
Figure 2 Measured voltage characteristics of the microgenerator
designed for a capacitive load, immediately after discharge,
across a 1 MO load. The oscillation frequency is 35 Hz and the
peak output power is 30 µW at an overall conversion efficiency of
1 %. Upper Inset: Close-up of the waveform at t= 1 s from the
discharge showing the sinusoidal nature of the oscillations.
Lower Inset: Measured rectified voltage across a 470 nF external
capacitance. The capacitor is discharged in 3 sec in the graph
due to the loading (µA) of the oscilloscope.
The above equation is based on the simplifying assumption
that the peak charging voltage is not limited by voltage
breakdown in the gap and all the emitted particles have a
kinetic energy of Eavg. Writing d0 in terms of Vcapmax and
substituting the expression for Trec in the expression for ?r
we get
load across the piezoelectric element for better
understanding of the power generation characteristics.
The radiated kinetic energy Er for one reciprocation
cycle is
(1)
E r = N r E avg (Trec + Tvib ) ≅ N r E avg Trec ,
Modeling the resonant system as a single degree of
freedom system [8], it can be shown that ?me can be
maximized to
η r max =
η me =
where Nr is the rate of collection of charged particles, Eavg
is the average kinetic energy of the charged particles, Trec
is the reciprocation period, and Tvib is the duration for
which the vibrations are sustained. The vibration period
Tvib is negligible compared to Trec for the devices with high
efficiency, as high charge voltages requiring long
reciprocation times lead to high efficiency. The
reciprocation period Trec can be calculated using
Trec =
Q final
Ir
=
2εAkδ 0
,
1 27
= 0.64 .
2 16
k me
,
k me + 2C p c 2πf
(7)
(8)
by using an optimum value of Rl given by
R opt =
1 .
C p 2πf
(9)
Here, kme is the mechanical to electrical coupling
coefficient dependant on the geometry of the cantilever
beam system and the characteristics of the piezoelectric
element, Cp is the dielectric capacitance of the
piezoelectric element, c is the mechanical damping
coefficient and f is the resonance frequency of the
cantilever beam system given by [14]
(2)
Ir
134
1
2π
k
,
(0.23m + m s )
oscillator and the bias voltage dependence of the
oscillation frequency is illustrated in Figure 3.
(10)
where ?i = 1.875 for fundamental bending mode, m is the
mass of the cantilever beam, ms is the mass of the collector
plate. The collector plate mass provides an additional
degree of freedom in adjusting the resonant frequency
without changing the stiffness of the beam (for keeping
Trec constant). From Equation 8, ?me approaches unity in
the absence of mechanical damping, or,
η me max = η me | c→0 = 1 .
Therefore, the maximum possible conversion efficiency
for the device is
(11)
η max × 100 = η r maxη me max × 100 = 64% .
For improving efficiency, the stiffness of the beam
and the gap height should be designed to satisfy Equation
6. Unfortunately, that is not always possible due to the
electrical breakdown of air between the air-gap capacitor
plates before the voltage reaches 17 kV, the voltage
corresponding to 17 keV average energy of 63Ni. Higher
beam stiffness leads to higher ?r, but degrades ?me
(Equation 8), thus requiring careful optimization of the
design parameters.
10
4
10
Frequency (Hz)
10
10
2
3
10
10
3
10
2
o: Frequency
+: Current
1
10
0.5
1
Voltage (V)
10
10
2
1.5
1
0
Current (nA)
f =
-1
-2
Figure 3 Measured dependency of oscillation frequency and
input current of the ring oscillator on voltage bias.
3.3 Optical Sensor
A commercial off-the-shelf silicon p-i-n photodiode is
used in the work presented here. The photodiode has a
responsivity of 0.5 A/W, an active area of 13 mm2, dark
current of 10 nA, spectral response of 350-1100 nm and a
diode capacitance of 20 pF. A 780 nm VCSEL is used as a
light source to test the sensor microsystem.
3 DESIGN AND FABRICATION
4 TESTING AND RESULTS
3.1 Radioisotope Power Generator
The micropower generator is designed for power transfer
to a capacitive load via a rectifier bridge, keeping in view
the voltage bias requirements for the electronics and the
photodiode. A 0.7 V drop across the diode bridge entails
large current output from the micropower generator to
realize 1 - 2 V across the storage capacitor. Voltage
characteristics of the resulting device are shown in Figure
2. The fabrication of the piezoelectric generator is
described elsewhere [7]. A ß-particle emitting 63Ni
radioisotope is used for all the experiments. The 63Ni is
electroplated as a 1 cm × 1 cm thin-film on a 1 mm thick
Ni plate. A large device (5 cm × 5 mm) and a small device
(15 mm × 2 mm) are fabricated and tested in a vacuum
chamber Figure 4(a)). The smaller device is fabricated to
demonstrate the feasibility of packaging the device in a
ceramic DIP package (Figure 4(b)).
Figure 4(a) shows the experimental test setup for the
micropower generator. The piezoelectric unimorph beam is
clamped between two ceramic plates for electrical
insulation.
3.2 Silicon on Sapphire (SOS) Ring Oscillator
The ring oscillator was custom designed for low power (1
nW), low voltage (0.7 V) operation by using long channel
transistors in the ultra-low parasitics 0.5 µm Silicon-onSapphire (SOS) CMOS process. The ring oscillator
consists of five stages with a tail current source to control
the power consumption of the circuit. The current source is
biased using a diode stack between the oscillator Vdd and
GND. The first and the last inverter stages of the ring
oscillator were modified to accommodate the load of the
output buffer and optimize the speed-energy performance
of the oscillator. The I-V characteristics of the ring
The source is clamped by two Teflon plates, which are
mounted on a linear motion stage used to control the initial
distance between the source and the cantilever collector.
The setup is placed inside a vacuum chamber (p ~ 1
mTorr) sealed with a glass top. A microscope connected to
a CCD camera outside the chamber is used to monitor the
motion of the cantilever tip.
Figures 2 shows the voltage generation characteristics
of the micropower generator designed for a capacitive
load. A second design for designed for resistive load
yielded a peak output power of 16 µW for a reciprocation
period of 115 minutes at an overall conversion efficiency
of 2.78 %.
Microscope
PZT Microgenerator
Si p-i-n diode
Ni63 source
PZT plate
15 mm
Silicon beam
SOS chip
(a)
(b)
Figure 4. (a) The experimental setup used for testing the
prototype device. (b) Photograph of a prototype sensor
microsystem packaged in an IC package.
135
1
Cstorage
Vout
Vout
(a)
0.5
0.4
1.5
Ring oscillator output (V)
0.3
0.2
0.1
0
-20
0
20
40
60 80 100 120 140 160 180
Time (s)
Figure 5 Measured output of the ring oscillator at the end of a
reciprocation cycle. The oscillations were observed to last for 6
minutes (amplitude >50 mV). The frequency was found to be
stable to 20 % from 180 sec to 240 sec. The ring oscillator output
decay follows the bias output decay (Figure 2).
Cstorage
C
Photodiode
1
Photodiode output (V)
1
Input
from
PZT
2
0.8
Vout
0.6
0
0.4
0.2
Sensor operation
period
0
-5
0.5
0
Time (ms)
5
2
1
0
0.5
0
: Ring oscillator output
--- : Laser driving signal
-10
(b)
-5
0
Time (ms)
5
10
Figure 7 (a) Schematic of the optical sensor microsystem. (b)
Measured output from the ring oscillator driven by the
photodiode sensing an optical signal from a pulse modulated
laser (Incident optical power~ 500 nW @ 780 nm) (Figure 2(a)).
The frequency of the ring oscillator is 1.58 kHz. The waveform
was captured 200 ms after the discharge. The sensor microsystem
can be readily used in light sensing applications as the ring
oscillator shuts off in the light off state, and the frequency of
oscillation in the light-on state depends on the intensity of the
incident light.
Laser diode driving signal (V)
1.5
Photodiode output voltage (V)
Cstorage
Laser diode driving signal (V)
Voltage (V)
0.6
Input
from
PZT
Ring Osc.
0.7
Buffer
Input
from
PZT
Ring
Oscillator
0.8
Photo diode
External
Power Supply
0.9
0
0
5
10
bias a silicon p-i-n diode modulating a low power ring
oscillator for a complete optical sensor microsystem with
data logging. Future work will involve designing
microfabricated radioisotope micropower sources and
exploring low-power electronics for use with the power
source.
15
Time (s)
Figure 6 Measured output of the photodiode driven by the
micropower generator and sensing an input from a pulse
modulated laser (Incident optical power~ 250 nW @ 780 nm).
Close-up measured at t=3 s from discharge
The AC output signal from the micropower generator
can be rectified using a full wave rectifier and used to
charge a storage capacitor, thus generating a bias for
driving the low power ring oscillator (Figure 5) or a
photodiode (Figure 6). The photodiode output current for
an incident optical power of 500 nW at 780 nm is 250 nA,
and the ring oscillator oscillates at 1.58 kHz for a input
bias current of 250 nA at 1.4 V. Recognizing this, the ring
oscillator was connected at the photodiode output and a
square wave modulated VCSEL laser beam was shone on
the photodiode. Figure 7 shows the resulting modulation of
the ring oscillator, demonstrating both sensing and data
logging capabilities of the microsystem.
ACKNOWLEDGMENT S
This work was supported by DARPA-MTO under the
MPG program, and contracted under the U. S. Army
Aviation and Missile Research, Development, and
Engineering Center.
[1]
[2]
[3]
[4]
5 CONCLUSIONS
A radioisotope micropower generator capable of
driving a optical sensor microsystem comprising of a
photodiode and low power electronics has been
demonstrated. Using a weak 0.5 milliCurie ß-particle 63Ni
source, energy generation in the pulse mode (7 s every 2
hours) at an overall efficiency of 2.78 % has been
demonstrated. The charges generated from the micropower
generator have been stored across a capacitor and used to
[5]
[6]
[7]
[8]
136
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