Download Energy Scavengers For Wireless Intelligent Microsystems

MICROSYSTEMS & NANOSYSTEMS
ASSEMBLY
SPECIAL
Q U A L I T Y A REPORT
S S U R A N C E
Energy Scavengers For Wireless
Intelligent Microsystems
by R.J.M. Vullers, V. Leonov, T. Sterken, A. Schmitz,
IMEC
Our future is evolving towards a world
where microsystems will add intelligence to almost every object that
surrounds us. Sensing and actuating
functionalities will be ‘hidden’ in the
environment. They will be aware of
context and be able to interact wirelessly with people and with each other. These small electronic microsystems will work autonomously, based
on their low-power consumption and
energy scavenging from the environment. The choice of scavenging principle depends on the application and
the environment in which it is used.
Here we will examine thermal scavengers (e.g. for human body applications) and vibrational scavengers (e.g.
for industrial applications).
The applications of today’s wireless
transducer systems are seriously limited by their size and form factor. The
integration of transducers with radios
and batteries typically results in systems of several tens of cm3 weighing
several 100g. Until now wireless sensor systems have hardly been used for
portable applications or environments
that cannot tolerate such a form factor.
Integration of the key building blocks
of autonomous transducer nodes (radio, DSP, micropower system, sensors
and actuators) in an affordable ‘system
in a package’, as being investigated at
IMEC-NL, Dutch sister company of
Belgian research centre for nanoelectronics and nanotechnology IMEC,
will pave the way to the introduction
of wireless sensor systems in portable
devices. Today, the batteries needed
to power wireless autonomous transducer systems seriously limit the possibilities of this emerging technology.
Modern electronic components are
becoming smaller, while the scaling
of traditional batteries faces technological restrictions. Either large batteries are used that give longer autonomy but make the system bigger, or
OnBoard Technology June 2006 - page 34
Figure 1a (left) - Schematic of a thermoelectric generator; Figure 1b (right) The schematic thermal circuit representing the generator and its environment
small batteries are used that make the
system less autonomous. For this a
worldwide effort is ongoing to replace
batteries with more efficient, miniaturised power sources. The research
program at IMEC-NL, which builds
on previous and current research at
IMEC, aims at generating and storing
power at the micro scale to improve
the autonomy or reduce the size of
wireless autonomous transducer systems. Research will be carried out in
the framework of the recently created
Holst Centre. The envisaged solution
takes its energy - thermal or mechanical - from the environment and converts it into electrical energy, stored
in a micro-battery.
Thermal energy scavengers
for human body applications
Thermal energy scavengers are thermoelectric generators that exploit
Figure 2 – Schematic position of the
measuring device on the wrist. The
arrows show a heat path from the
artery to the device
the Seebeck effect to transform the
temperature difference between the
environment and the human body
into electrical energy. A thermoelectric generator (TEG) is made of thermopiles sandwiched between a hot
and a cold plate. Thermopiles are, in
turn, made of large numbers of thermocouples connected thermally in
parallel and electrically in series, as
shown schematically in Figure 1a.
The red and blue pillars represent the
two types of thermoelectric materi-
Figure 3 – A thermoelectric generator fabricated using commercial
thermopiles (left) and power conditioning electronics and a transceiver
mounted on the wrist strap (right)
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The thermal resistance of the body
depends on the position of the TEG
on the wrist. The average thermal
resistance per unit area at the “outer
side” of the wrist where the watch
face sits is about 300 cm2K/W, while
on the radial artery (the ‘inner side’)
As for the equivalent thermal resistance of the air, this can be reduced by
using appropriate radiators mounted
on the cold plate of the thermoelectric generator. For example, assuming that the device thickness is limited to 1 cm, a thermal resistance of
about 500 cm2K/W can be obtained
for a person sitting in still air, and a
thermal resistance of 200 cm2K/W for
somebody who is walking. With regard to these values, about one third
of the total temperature difference
will drop across the thermoelectric
generator. This is a satisfactory result
that provides about 10 - 15 µW/cm2.
Further decreasing air and body resistance will also result in a large heat
flow from the body and in the discomfort of the user (he/she will feel cold).
The number of thermocouples corresponding to the optimal power for the
generator described above turns out
to be very small (10-20 cm2), resulting in a very low voltage (20-30 mV/
cm2). In order to drive a simple power
management system for recharging
a battery, a minimum voltage of 0.8
volts is necessary (with its following
up-conversion). To obtain this voltage
it would be necessary to increase the
number of thermocouples and, at the
same time, to decrease their crosssection in order to fulfil the maximum power condition (equivalent
heat flow through the thermocouple
and the air). If commercial thermopiles are used, their cross-section is
limited by technology to the values
mentioned above and the required
The power generated by the device
exceeds 0.1 mW and output voltage is
more than 1 V. This is enough power
to charge a small battery and to transmit the temperature of the body to
a nearby receiving station every 1–2
seconds, for example.
Micromachined thermopiles
The use of commercial thermopiles
has proven that human heat can be
used to power a sensor node. Nevertheless, the solution is non-optimal
for two reasons: firstly because it does
not offer the possibility of optimising
the power and the voltage at the same
time, and secondly, since thermopile
fabrication techniques cannot be easily automated, it is very expensive. One
possible solution could be the use of
micromachined thermopiles.
Micromachined thermopiles have already been presented in scientific literature, and are used in miniaturised
commercial thermoelectric coolers.
Micromachining has the potential advantage of reducing the lateral size of
the thermocouple. This means that a
much larger number of thermocouples can be fabricated per unit area
thus maintaining the condition of
equal thermal conductance of thermocouple and air required for power
optimisation. Such an approach enaFigure 5 – Principle of a vibration
scavenger
Q U A L I T Y
Figure 4a (left) - Schematic of the TEG capable of combining large power
and large voltage (rim and thermopiles are not scaled to overall device
dimensions)
Figure 4b (right) - Simulated performance of the TEG shown in (a)
voltage is obtained by increasing the
number of thermopiles at the expense
of reduced power. A system based on
this ‘compromise’ has been developed
and is shown Figure 3. The power
conditioning electronics, together
with a low power radio transmitter,
are mounted on a flexible substrate
glued to the wrist strap.
ASSEMBLY
MICROSYSTEMS & NANOSYSTEMS
In commercially available thermopiles, typically based on Bi2Te3, the
pillars have a lateral size of 0.3 – 1mm
and a height of 1 – 3 mm. Considering
the lowest values of the interval, a TEG
optimised to obtain the maximum
power will have a thermal resistance
of about 200 cm2K/W per cm2 of surface. In operating conditions, the generator is inserted in a thermal circuit
that includes the thermal resistance
of the body and the equivalent thermal resistance of the air (Figure 1b).
In order to have a sizeable temperature drop on the device, these series
resistances must not be too large with
respect to that of the generator.
it is over twice as small (Figure 2).
A S S U R A N C E
al, and the metal interconnects are
drawn in gold. The pink and blue
plates represent the cold and hot
sides of the device. The maximum
electrical power is generated when
the load matches the electrical resistance of the generator and when
the thermal conductance of the
thermocouples equals that of the air
between the plates (this is perfectly
true if we consider that the heat
flow from the body is not influenced
by the thermoelectric generator - a
condition well approximated in our
case). In these conditions power increases when the height of the pillars is increased.
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OnBoard Technology June 2006 - page 35
MICROSYSTEMS & NANOSYSTEMS
ASSEMBLY
Electrostatic conversion
Figure 6a (left) - Principle of electrostatic conversion; Figure 6b (right)
- Schematic drawing of the IMEC MEMS-based electrostatic vibration
scavenger
SPECIAL
Q U A L I T Y A REPORT
S S U R A N C E
Generators based on vibrations
The implementation of the vibration
scavenger is based on a specially designed variable capacitor, fabricated
using two wafers and MEMS technology, and on the use of an electret
layer as polarisation source, implemented in a third wafer, also with
the function of providing 0-level
packaging. A schematic of the device is shown in Figure 6b. The core
of the device is a variable capacitor
with sliding fin-type electrodes. The
fins of the moveable electrode are
sculpted in the seismic mass, while
the fixed electrode is patterned on a
glass wafer. The two electrodes are
maintained at a quasi-fixed, small
distance (about 1µm) by a thin adhesive layer that also keeps the two
wafers together. This variable sliding capacitor exhibits a dC/dz more
than 10 times larger than that of a
comb capacitor of equivalent dimensions.
The most common approach to extracting mechanical energy from
vibrations and converting it to electrical power, is to use an inertial
system, schematised as a spring connected to the vibrating frame and a
mass (Figure 5). The motion of the
mass with respect to the frame causes
the movement of the different parts
of an electromechanical generator,
which delivers power to an external
load. Three types of generators can
be used: electromagnetic, electrostatic and piezoelectric. Below the
focus will be on the electrostatic and
piezoelectric generators under development at IMEC.
The polarisation voltage, meanwhile, is provided by an electret
layer implemented in a third wafer.
The electret used at IMEC is formed
by a stack of two layers - silicon oxide and silicon nitride - deposited
onto the Silicon wafer. Charges injected by corona discharge into the
electret are trapped at the interface
between the two dielectrics and
polarise the two capacitors formed
by the moveable electrode with the
electret wafer and the fixed electrode, respectively. The use of an
electret instead of batteries allows
voltages as high as 400 volts per 1
µm thickness of silicon oxide to be
Figure 7 - Capacitance and capacitance change as a function of position.
The points are experimental values, while the lines are modelled
bles a large voltage and a large power
to be combined. Unfortunately, micromachined thermocouples have
a height of just a few microns, and
this drastically reduces the thermal
resistance of the generator. The temperature drop on the device is consequently small and the power generated negligible.
In order to overcome this difficulty,
IMEC has designed a special micromachined thermoelectric generator for application on humans, which
combines the large thermal resistance
of the device with a large number of
thermopiles. The schematic is shown
in Figure 4a. Several thousands of
thermocouples are mounted on a Silicon rim. The function of this rim is
to increase the parasitic plate-to-plate
thermal resistance of the generator. If
Bi2Te3 is used as thermoelectric material, an optimised device fabricated according to this scheme and positioned
on the human wrist can generate up
to 30 µW/cm2 at a voltage exceeding
4 volts in indoor applications. Figure
OnBoard Technology June 2006 - page 36
The principle of electrostatic conversion is shown in Figure 6a. The
polarisation voltage determines the
charge on the two capacitors. The
capacitance changes due to the external vibrations cause a redistribution of the charge and hence a
current flows through the load. Key
factors controlling the performance
characteristics of the scavengers
are the polarisation voltage, Vp,
and the change in capacitance per
unit displacement of the moveable
electrode, dC/dz. IMEC has concentrated on maximising these two parameters.
4b show a realisation of such a device
based on SiGe thermocouples. Because of the inferior thermoelectric
properties of this material with respect to Bi2Te3, an optimised device is
expected to generate 4.5 µW/cm2 at a
voltage of 1.5 Volts.
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The design of the piezoelectric devices
is similar to the classical design of accelerometers: a bending structure (a
beam, see Figure 8a) is connected to
a vibrating frame. The beam supports
a piezoelectric capacitor and a mass.
The vibration of the frame induces a
vibration of the mass and a bending of
the cantilever. The strained piezoelectric layer generates charges that flow
in the external circuit (see Figure 8).
The piezoelectric generator consists
of a piezoelectric layer sandwiched
between a bottom and top electrode.
The thickness of the bulk Silicon is
about 630 µm whereas the beam has
a targeted thickness of 25 µm. Several
designs of piezoelectric devices differing in geometry and electrical connection are fabricated on one wafer.
The devices are equipped with masses
of different dimensions (3x3 mm2,
5x5 mm2, and 7x7 mm2). A variation
of resonance frequencies (300 Hz, 700
Hz, and 1000 Hz) is realised by varying the length of the beam. The devices are designed as single piezoelectric
Figure 8a (left) - Schematic drawing of the piezoelectric scavenger
Figure 8b (right) - Top view of a series-connection of four piezoelectric
generators (dimension of the mass: 3x3 mm2)
generators or as a series connection
of four piezoelectric generators as
shown in Figure 8b.
Two different process flows have been
developed for using PZT or AlN as piezoelectric material. The advantages
of PZT include its higher piezoelectric
and dielectric constants as compared
to AlN. However, the deposition of the
PZT layer is rather time-consuming
as it is deposited in multiple steps.
Moreover the sputtering process of
AlN is more compatible to a batch
MEMS-process. The fabrication of the
devices is currently ongoing.
Extensive simulations of the electromechanical behaviour of this scavenger type were carried out using a
two-dimensional transient finite element model. The model includes the
description of the mechanical straindisplacement relations between mechanical and electrical quantities as
well as the interaction of the piezoelectric device with an external electrical load for the dissipation of the
generated electrical power. By using
this model, several operation characteristics can be calculated such as
output power versus external load
and output power versus frequency.
Typical shapes of these operation
characteristics are shown in Figures
9a and b. The developed model can be
used as a tool to optimise the design
of piezoelectric scavengers in terms
of power and energy density as well
as frequency response. According
to simulation results, the fabricated
piezoelectric devices are expected to
generate electrical power in the range
of 0.01 to 0.1 mW.
Q U A L I T Y
Figure 9a (left) - Typical characteristics of the output power as a function of the electrical load resistance for a
single piezoelectric device having a mass of 5x5mm2; Figure 9b (right) - as a function of the excitation frequency
for a single piezoelectric device with a resonance frequency of about 600 Hz
ASSEMBLY
MICROSYSTEMS & NANOSYSTEMS
The measured capacitance, as well as
the simulated capacitance, is reported in Figure 7. The measurements
show a slightly higher peak value of
capacitance as well as a higher stray
field capacitance. This might indicate the presence of a parasitic capacitance. The experimental points
demonstrate a capacitance change
of 0.09 pF/µm, while a maximal
value of 0.23 pF/µm was expected.
These values can be improved by optimising the design towards larger
surface, because currently the capacitances occupy only 0.6 mm2.
Enlargement of the surface will proportionally scale both capacitance
and capacitance changes.
Piezoelectric conversion
A S S U R A N C E
reached. Furthermore, the electret’s
polarisation voltage is stable during its aging (for a potential below
150 volts, for example, the lifetime
exceeds 200 years) and short process steps at high temperatures are
possible without significant change
in the potential. After a 10-minute
annealing step at 450°C, the potential is still around 93% of its initial
value.
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OnBoard Technology June 2006 - page 37