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.

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).

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.

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

Vout

Ni 63 Radioisotope emitter

Silicon beam

Port 1

Lp

δ0

Lc

hp

hs

Piezoelectric unimorph section

δ

Piezoelectric plate (PZT)

Figure 1 Cross-section showing the geometrical parameters of the cantilevered piezoelectric unimorph and a model of the air-gap 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.

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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. load across the piezoelectric element for better understanding of the power generation characteristics.

The radiated kinetic energy Er for one reciprocation cycle is

recTavgErNvibTrecTavgErNrE ≅+= )( , (1)

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

rr

finalrec I

Ak

I

QT 02 δε

== , (2)

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

20

220 2

122

1δδ k

C

QkEEE

p

pqmem ≅+=+= , (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

∫=vib

dtR

tVE

T

l

outext

0

2 )( , (4)

where Vout(t) is the output voltage. The ratio of the extracted electrical energy to radiated

kinetic energy ? is

r

ext

em

ext

r

emmer E

EEE

EE

=== ηηη , (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

q

E

A

kV avg

cap ==ε

δ 30

max 278 . (6)

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

64.01627

21

max ==rη . (7)

Modeling the resonant system as a single degree of freedom system [8], it can be shown that ?me can be maximized to

fcCkk

pme

meme π

η22+

= , (8)

by using an optimum value of Rl given by

fCR

popt π2

1= . (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]

134

( )smmk

f+

=23.02

1π

, (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,

1| 0max == →cmeme ηη . Therefore, the maximum possible conversion efficiency for the device is

%64100100 maxmaxmax =×=× mer ηηη . (11) 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.

3 DESIGN AND FABRICATION

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)).

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-on-Sapphire (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

oscillator and the bias voltage dependence of the oscillation frequency is illustrated in Figure 3.

0.5 1 1.5 2

101

102

103

104

Fre

quen

cy (

Hz)

0.5 1 1.5 210

-2

10-1

100

101

102

103

Cur

rent

(nA

)

Voltage (V)

o: Frequency +: Current

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.

4 TESTING AND RESULTS 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.

PZT plate Silicon beam

Ni63 source

Microscope PZT Microgenerator

Si p-i-n diode

SOS chip

15 mm

(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. 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 %.

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Vout

Buffer

External Power Supply

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).

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ode

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ut v

olta

ge (

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ode

outp

ut (

V)

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0

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r di

ode

driv

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sign

al (

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CInput from PZT Vout

Photodiode

Cstorage

Sensor operation period

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.

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

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ode

driv

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sign

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: Ring oscillator output --- : Laser driving signal

Cstorage Input from PZT

Ring O

sc.

Vout

Photo diode

(a)

(b) 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.

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.

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.

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