This paper presents a polycrystalline silicon germanium (poly-SiGe)
thermopile specially designed for thermoelectric generators used on human body.
Both the design of the single thermocouple and the arrangement of the
thermocouple array have been described. A rim structure has been introduced in
order to increase the temperature difference across the thermocouple junctions.
The modeling of the thermocouple and the thermopile has been performed
analytically and numerically. An output power of about 1 $\mu$W at an output
voltage of more than 1 V is expected from the current design of thermopiles in
a watch-size generator. The key material properties of the poly-SiGe have been
measured. The thermopile has been fabricated and tested. Experimental results
clearly demonstrate the advantage of the rim structure in increasing output
voltage. In presence of forced convection, the output voltage of a non-released
thermopile can increase from about 53 mV/K/cm2 to about 130 mV/K/cm2 after the
rim structure is formed. A larger output voltage from the thermopile is
expected upon process completion.Comment: Submitted on behalf of EDA Publishing Association
  (http://irevues.inist.fr/EDA-Publishing

Fiorini, P.

Leonov, V.

Van Hoof, C.

Wang, Z.

English

arXiv

Micro power generators and micro energy sourcesThis paper presents a polycrystalline silicon germanium (poly-SiGe) thermopile specially designed for thermoelectric generators used on human body. Both the design of the single thermocouple and the arrangement of the thermocouple array have been described. A rim structure has been introduced in order to increase the temperature difference across the thermocouple junctions. The modeling of the thermocouple and the thermopile has been performed analytically and numerically. An output power of about 1 μW at an output voltage of more than 1 V is expected from the current design of thermopiles in a watch-size generator. The key material properties of the poly-SiGe have been measured. The thermopile has been fabricated and tested. Experimental results clearly demonstrate the advantage of the rim structure in increasing output voltage. In presence of forced convection, the output voltage of a non-released thermopile can increase from about 53 mV/K/cm2 to about 130 mV/K/cm2 after the rim structure is formed. A larger output voltage from the thermopile is expected upon process completion

I-Revues

 Stresa, Italy, 25-27 April 2007 
 
MICROMACHINED POLYCRYSTALLINE SIGE-BASED THERMOPILES FOR MICROPOWER 
GENERATION ON HUMAN BODY 
 
Z. Wang
1,2, V. Leonov
1, P. Fiorini
1, and C. Van Hoof
1 
1IMEC vzw, Kapeldreef 75, B-3001, Leuven, Belgium 
2Catholic University Leuven, B-3000, Leuven, Belgium 
 
ABSTRACT 
 
This paper presents a polycrystalline silicon germanium 
(poly-SiGe) thermopile specially designed for 
thermoelectric generators used on human body. Both the 
design of the single thermocouple and the arrangement of 
the thermocouple array have been described. A rim 
structure has been introduced in order to increase the 
temperature difference across the thermocouple junctions. 
The modeling of the thermocouple and the thermopile has 
been performed analytically and numerically. An output 
power of about 1 μW at an output voltage of more than 1 
V is expected from the current design of thermopiles in a 
watch-size generator. The key material properties of the 
poly-SiGe have been measured. The thermopile has been 
fabricated and tested. Experimental results clearly 
demonstrate the advantage of the rim structure in 
increasing output voltage. In presence of forced 
convection, the output voltage of a non-released 
thermopile can increase from about 53 mV/K/cm
2 to 
about 130 mV/K/cm
2 after the rim structure is formed. A 
larger output voltage from the thermopile is expected 
upon process completion. 
 
1. INTRODUCTION 
 
Body area networks, consisting of multiple sensors, 
transducers and transceivers deployed on human body for 
medical use, call for long-lasting, if possible everlasting, 
power supplies. Thermoelectric generators (TEGs) 
transforming the heat of the human body into electrical 
energy, could provide power autonomy to the nodes of 
the network and replace traditional batteries. An 
autonomous body area network powered by TEGs has 
already been reported [1]. In that application the TEG is 
based on commercial bismuth telluride (BiTe) 
thermopiles, which are fabricated by standard machining 
and serial production techniques. Therefore, their cost is 
prohibitive and their size is relatively large. 
Micromachining technology allows the 
miniaturization of the traditional thermopiles. Till now, 
several miniaturized thermopiles have been reported, 
either based on BiTe or on poly-SiGe [2-6]. Besides the 
merit of providing power autonomy, miniaturized 
thermopiles are cheaper, smaller and can be 
monolithically integrated with other components, such as 
power management circuits. 
However, till now, none of the current miniaturized 
thermopiles can provide a usable power output when 
applied on human body due to its adverse thermal 
conditions，namely the low temperature differences from 
the ambient air and the large thermal resistance of the 
body. This paper presents a type of poly-SiGe based 
micromachined thermopile specially designed for human 
body applications.  
 
2. DEVICE PRINCIPLE 
 
Fig. 1(a) shows the scheme of a typical TEG, which 
consists of a number of thermocouples connected 
thermally in parallel and electrically in series, sandwiched 
between a hot plate and a cold plate. Fig. 1(b) shows the 
equivalent thermal circuit describing a TEG placed on 
human body. 
 
(a) 
 
(b) 
Fig.1. (a). Scheme of a typical TEG; (b). Equivalent 
thermal circuit of a TEG placed on human body. 
 
Normally the thermal resistances of human body and 
ambient air are much larger than that of the thermopile so 
©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3               Z. Wang
1,2, V. Leonov
1, P. Fiorini
1, and C. Van Hoof
1 
MICROMACHINED POLY SIGE-BASED THERMOPILES FOR MICROPOWER GENERATION ON HUMAN BODY 
 
that the heat flow can be considered constant and 
approximately independent of the specific design of the 
thermopile. Under this hypothesis it can be easily shown 
that the maximum power output is obtained when the heat 
flow through the thermocouples is equal to that through 
the air gap, namely when the thermal resistance of the 
thermocouples is equal to that of the air gap.  
If the thermopile of Fig. 1 is fabricated by 
micromachining techniques, both the lateral size and the 
height of the single thermocouple will be of the order of a 
few micrometers. Thermocouples can be fabricated on a 
silicon substrate and capped by a silicon top plate forming 
the structure shown in Fig. 2a. As the thermoelectric 
materials have a thermal conductivity about 100 times 
larger than the one of air, the condition of maximum 
power (matched thermal resistance of air gap and 
thermoelectric material) corresponds to the situation 
where the thermocouples occupy only about 1/100 of the 
hot plate area. (For clarity this feature is not represented 
correctly in the figure). The total thermal conductance of 
the device can be roughly evaluated as twice the one of 
the air between the plates. As they are very close (several 
micrometers) the thermal conductance is large leading to 
negligible temperature difference and output power. 
Furthermore, as the temperature difference is low, a large 
number of thermocouples connected electrically in series 
are needed to generate a usable voltage output, such as 1 
V.  
An alternative and more efficient arrangement is 
shown in Fig. 2(b). Only the part of the Si wafer 
containing the thermocouples is used and mounted 
between metal cold and hot plates. The thermal resistance 
of the air gap is now hundreds of times larger than the 
case in Fig. 2(a). Then the temperature drop across the 
thermocouples and the output voltage are effectively 
increased. The number of thermocouples necessary to 
obtain 1 V voltage output also decreases drastically. As 
the condition of optimum power corresponds to the 
equality of the thermal resistance of air gap and 
thermoelectric material, the area occupied by the 
thermoelectric material further decreases, it can become 
so small to make the chip handling inconvenient. A 
solution to this problem consists of using larger chips 
which are deeply etched where the thermocouples are not 
placed. Moreover, in order to deal with a more regular 
chip, the thermopile is arranged in a rim shape, as shown 
in Fig 2(c). For clarity, the thermocouples arranged in a 
rim shape placed on the bottom chip are illustrated in a 
3D schematic in Fig. 3. In the following we will 
concentrate on the modeling and fabrication of this 
structure, which, according to the above discussion, is the 
one which better combines ease of fabrication and higher 
performances. 
 
 
(a) 
 
(b) 
 
(c) 
Fig. 2. (a) Cross-sectional view of thermopile sandwiched 
between a hot plate and a cold; (b) A bottom chip and a 
top chip are inserted in between the hot plate and the 
cold plate; (c) The bottom chip and the top chip are 
enlarged and deeply etched and the thermopile is 
arranged in a rim shape. 
 
 
 
Fig. 3. 3D schematic of the thermocouples in a rim shape. 
 
 
3. MODELING 
 
In this section, the structure and the thermal properties of 
the single thermocouple are first discussed. Then, based 
on the result on the single thermocouple, the output 
performances of the full TEG are computed analytically.  
Each single thermocouple is composed of one p- and 
one n-type poly-SiGe legs, which are interconnected by 
aluminum pads at the junctions, as shown in Fig 4. If 
there is a temperature difference between the two 
junctions, a voltage is built up due to the Seebeck effect. 
In our design, some measures have been taken to improve 
the thermal isolation. The thermal isolation of the cold 
junctions from the hot die surface has been improved by 
etching a trench into the Si substrate below. A small 
vertical step, varying from 0.5 to 3 μm in height, has been 
©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3               Z. Wang
1,2, V. Leonov
1, P. Fiorini
1, and C. Van Hoof
1 
MICROMACHINED POLY SIGE-BASED THERMOPILES FOR MICROPOWER GENERATION ON HUMAN BODY 
 
made between the two junctions by means of an 
additional sacrificial layer. The thermocouple leg has to 
climb over this vertical step, thus increasing its length and 
hence its thermal resistance. Moreover, the width of the 
middle part of the thermocouple legs is reduced to 
increase the thermal resistance, while the width of the 
thermocouple legs at junctions remains relatively large to 
ensure a low contact resistance.  
 
 
Fig. 4. 3D schematic drawing of a thermocouple with a 
2.5-μm-high step. 
 
The single thermocouple (Fig. 4) has been modeled 
numerically. A fully 3D model of a single thermocouple 
bonded to the top chip (Fig. 10(f)) has been built in FEM 
software MSC Marc for parametric optimization. The 
thermal resistance can be determined from FEM 
simulation. As boundary conditions, a known heat flow is 
forced into the bottom surface and a fixed temperature is 
applied to the top surface. As an example of the 
simulation results, the steady-state temperature 
distribution is shown in Figure 5. For clarity, only the 
temperature distribution on the bottom is shown. The 
thermal resistance of this single thermocouple can be 
calculated as the ratio between the temperature difference 
from the bottom surface to the top surface and the heat 
flow.   
 
 
Fig. 5. FEM simulation of thermocouple in MSC Marc 
under a fixed heat flux (the top chip is not shown for 
clarity). 
 
In the single thermocouple design (Fig. 4), the end 
width of thermocouple a, the middle width of 
thermocouple  b and the thermocouple height h can be 
separately optimized in order to investigate their 
influence on the thermal resistance. As shown in Fig. 6, 
the thermal resistance depends almost linearly on the 
thermocouple height h. The data are obtained with the 
end width a  fixed at 10 μm and the middle width b  fixed 
at 3 μm. 
 
 
Fig. 6. Thermal resistance of a single thermocouple 
depending on the thermocouple height. 
 
On the other hand, the thermal resistance is inversely 
proportional to the width when other parameters are fixed. 
This is shown in Fig. 7, where the thermal resistance 
decreases from 2.58×10
5 K/W to 1.29×10
5 K/W when the 
middle width b increases from 0.5 μm to 4 μm. 
 
 
Fig. 7. Thermal resistance of a single thermocouple 
depending on the thermocouple width. 
 
In the final mask layout, various types of 
thermocouples have been designed by varying the 
thermocouple widths a and b. The end width a varies 
from 3 μm to 10 μm while the middle width b varies from 
1 μm to 3 μm. The thermal resistance of each design type 
36.19 K 
29.68 K 
p-SiGe 
Al 
Si3N4
Si Substrate
Trench
a  b 
h 
n-SiGe
©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3               Z. Wang
1,2, V. Leonov
1, P. Fiorini
1, and C. Van Hoof
1 
MICROMACHINED POLY SIGE-BASED THERMOPILES FOR MICROPOWER GENERATION ON HUMAN BODY 
 
is shown in Fig. 8 for a fixed value of h equal to 0.5 μm. 
 
 
Fig. 8. Thermal resistance of the thermocouples 
implemented in the final mask. The two rows of numbers 
on the abscissa represent respectively the quantities a 
and b defined in the text. 
 
The thermal resistance of a single thermocouple 
determines the total thermal resistance of the thermopile 
which contains thousands of thermocouples thermally in 
parallel. Therefore, the thermal resistance of each 
thermocouple should be increased by reducing the width 
and increasing the height. However, these structural 
parameters in the practical design are subject to the 
technology limit, such as the depth of focus in contact 
photolithography. 
The TEG on human body has been modeled via a 
network of thermal resistors representing different 
components, such as radiator and human body. The 
output performance of a watch-size TEG on human body 
for various design types are shown in Fig. 9. Type A 
contains 2350 thermocouples while type B contains 4700 
 
 
thermocouples. An output power of about 1 μW at an 
output voltage of more than 1 V is expected from the 
current design of thermopiles in a watch-size generator. 
 
 
Fig. 9. Calculated output voltage and power from various 
types of TEGs in the current design. . The three rows of 
numbers on the abscissa represent, from top to bottom,  
the quantities a and b defined in the text and the number 
of thermocouples (A= 2350 and B=4700). 
 
 
5. FABRICATION 
 
The thermopile is fabricated using surface 
micromachining technology, as shown in Fig. 10. The Si 
substrate is firstly patterned and etched to form the 
trenches (Fig. 10(a)). Then the trenches, which are 2.5 
μm deep, are filled with the first layer of sacrificial TEOS 
and the wafer is planarized by chemical mechanical 
polishing (CMP). In the next step, an additional layer of 
TEOS, which is 0.5 μm high, is deposited on top and 
patterned in order to form the step between cold and hot 
junctions. A thin layer of Si3N4 is then deposited for 
electrical insulation (Fig. 10(b)). P- and n-type poly-SiGe 
are deposited by LPCVD and patterned using dry etch to 
form thermocouples (Fig. 10(c)). An aluminum layer is 
sputtered and patterned to interconnect the two types of 
poly-SiGe legs (Fig. 10(d)). In order to form ohmic 
contact, an argon plasma clean is done before the 
sputtering and a sintering in forming gas atmosphere is 
performed afterwards. The rim is then formed by dry etch 
as deep as 250 μm (not shown) and the top die is bonded 
to the thermocouples (Fig. 10(e)). Finally, the device is 
released in a mixture of HF and glycerol and diced (Fig. 
10(f)).  
 
 
 
©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3               Z. Wang
1,2, V. Leonov
1, P. Fiorini
1, and C. Van Hoof
1 
MICROMACHINED POLY SIGE-BASED THERMOPILES FOR MICROPOWER GENERATION ON HUMAN BODY 
 
 
 
 
 
 
Fig. 10. Key process steps for the thermocouple array (a) 
pattern trench in a Si substrate; (b) deposit and pattern 
sacrificial TEOS layer; (c) deposit and pattern p- and n-
type SiGe; (d) deposit and pattern Al; (e) bonding with 
top die; (f) final vapor HF release. 
The fabricated thermopile placed on the rim structure 
on the bottom chip is shown in Fig. 11. 
 
Fig. 11. SEM picture of the micromachined thermopile on 
the rim. 
The relevant material properties of p- and n-type poly-
SiGe used in the thermopile have been characterized. The 
electrical resistivity is 1.05 mΩcm and 5.87 mΩcm for p- 
and n-type poly-SiGe respectively. The specific contact 
resistance between poly-SiGe and aluminum has been 
measured by cross bridge Kelvin structures. The specific 
contact resistance for p-type poly-SiGe is 86 Ωμm
2 and 
that for n-type poly-SiGe is 40 Ωμm
2. The Seebeck 
coefficient has been measured by a specially designed 
experimental set-up. The average Seebeck coefficient for 
p-type poly-SiGe is 69 μV/K and that for n-type poly-
SiGe is -248 μV/K. 
Although thermal conductivity has not been measured, 
a value of 3 W/K/m can be assumed, as measured on 
poly-SiGe deposited in the same conditions and in the 
same system before. Therefore, the thermoelectric figure-
of-merit at room temperature, which characterizes the 
overall thermoelectric properties of materials, is 0.025 
and 0.096 for p- and n-type poly-SiGe respectively. The 
improvement of the figure-of-merit of p-type poly-SiGe, 
which is relatively low, would lead to higher TEG output 
performance. 
 
5. MEASUREMENT 
 
The thermopile on the rim (Fig. 11) and the top die have 
been fabricated separately. Their assembling is ongoing at 
this moment. The functionality of the thermopile has been 
demonstrated successfully before the HF release. The 
thermopile has been heated up on a thermal chuck. The 
output voltage is measured as a function of the chuck 
temperature. In such conditions, the temperature drop on 
the thermopile would be a small fraction of 1 K. In order 
to increase the temperature gradient between the hot and 
cold junctions, a rudimental radiator, made of a bare 
silicon die, is positioned onto the thermopile touching the 
cold junctions only. The temperature difference can be 
further increased by flushing the measurement chamber 
with nitrogen.  
The experimental results shown in Fig. 12 were 
obtained on a thermopile of the type 10-3-A. The output 
voltage increases with the temperature difference between 
the thermal chuck and the ambient air. Before the rim is 
made, the output voltage is about 13 mV/K/cm
2 without 
nitrogen flush. With the nitrogen flush, the output voltage 
increases by a factor of 4 and is as large as 53 mV/K/cm
2. 
After the rim is formed, the output voltage is then 
increased to about 30 mV/K/cm
2  and 130 mV/K/cm
2 
respectively for the conditions without and with nitrogen 
flush.   This improvement illustrates the advantage of the 
rim structure in increasing the temperature difference 
across thermocouple junctions. 
 
Rim 
Thermocouples 
©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3               Z. Wang
1,2, V. Leonov
1, P. Fiorini
1, and C. Van Hoof
1 
MICROMACHINED POLY SIGE-BASED THERMOPILES FOR MICROPOWER GENERATION ON HUMAN BODY 
 
 
Fig. 12. Dependence of the output voltage on the 
temperature difference between the chuck and the 
ambient air. 
 
A larger output voltage is expected after release of 
sacrificial TEOS because its removal improves the 
thermal isolation between the cold and hot junctions. 
 
6. CONCLUSION 
 
A micromachined poly-SiGe-based thermopile for human 
body applications has been designed and modeled. In 
order to improve the output performance, a rim structure 
has been introduced. The influence of structure 
parameters on the thermal resistance of single 
thermocouple has been investigated by FEM simulation. 
Analytical models show that about 1-2 μW at a voltage of 
more than 1 V can be obtained with the current 
thermopile design in a watch-size TEG when placed on 
human body. The thermopile has been fabricated and 
tested. Poly-SiGe used as the thermoelectric material has 
been characterized. The functionality of the thermopile 
has been demonstrated in thermal measurements before 
the final release. The increase in output voltage due to the 
rim structure has been observed. In presence of forced 
convection, the measured output voltage is increased 
from about 53 mV/K/cm
2 to about 130 mV/K/cm
2 after 
the rim structure is formed. A further increase in output 
voltage is expected upon completing the full device. 
 
 
 
 
 
 
 
 
 
 
 
 
 
7. REFERENCES 
 
 [1] V. Leonov, T. Torfs, P. Fiorini and C. Van Hoof, 
“Thermoelectric Converters of Human Warmth for Self-
powered Wireless Sensor Nodes”, IEEE Sensor Journal,  
Special Issue on Intelligent Sensors, 2007 (in press). 
 
 [2] H. Böttner, J. Nurnus, A. Gavrikov, G. Kühner, M. 
Jägle, C., Künzel, D. Eberhard, G. Plescher, A. Schubert 
and K.-H. Schlereth, “New Thermoelectric Components 
Using Microsystem Technologies”, in IEEE  Journal of 
MicroElectroMechanical Systems, Vol.13, No.3, pp.414-
420, Jun. 2004. 
 
[3] J. P. Fleurial, G. J. Snyder, J. A. Herman, P. H. 
Giauque and W. M. Philips, “Thick-film Thermoelectric 
Microdevices”,  18th International Conference on 
Thermoelectrics, pp. 294-300, Baltimore, U.S., Aug. 
1999. 
 
[4] S. Hasebe, J. Ogawa, M. Shiozaki, T. Toriyama, S. 
Sugiyama, H. Ueno and K. Itoigawa, “Polymer Based 
Smart Flexible Thermopile for Power Generation”, 17th 
International Conference on MicroElectroMechanical 
Systems, pp. 689-692, Maastricht, the Netherlands, Jan. 
2004. 
 
[5] W. Glatz and C. Hierold, “Flexible Micro 
Thermoelectric Generator”, 20th International 
Conference on MicroElectroMechanical Systems, pp. 89-
92, Kobe, Japan, Jan. 2007. 
 
[6] M. Strasser, R. Aigner, C. Lauterbach, T. F. Sturm, 
M. Franosch and G. Wachutka, “Micromachined CMOS 
Thermoelectric Generator As On-chip Power Supply”, 
Sensors and Actuators A, Vol.114, pp.362-370, 2004. 
©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3               

Flexible Micro Thermoelectric Generator”,

New Thermoelectric Components Using Microsystem Technologies”,

Polymer Based Smart Flexible Thermopile for Power Generation”,

Thermoelectric Converters of Human Warmth for Selfpowered Wireless Sensor Nodes”,

MICROMACHINED POLYCRYSTALLINE SIGE-BASED THERMOPILES FOR MICROPOWER GENERATION ON HUMAN BODY

Micromachined Polycrystalline Sige-Based Thermopiles for Micropower
  Generation on Human Body

Micromachined Polycrystalline Sige-Based Thermopiles for Micropower Generation on Human Body

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