We present and discuss the fabrication process and the performance of a
flexible micro thermoelectric generator with electroplated Bi2Te3 thermocouples
in a SU-8 mold. Demonstrator devices generate 278uWcm-2 at dTmeas=40K across
the experimental set up. Based on model calculations, a temperature difference
of dTG=21.4K across the generator is assumed. Due to the flexible design and
the chosen generator materials, the performance stays high even for curved
contact surfaces. The measurement results correlate well with the model based
design optimization predictions.Comment: Submitted on behalf of EDA Publishing Association
  (http://irevues.inist.fr/handle/2042/16838

Durrer, L.

Glatz, W.

Hierold, Ch.

Schwyter, E.

English

arXiv

We present and discuss the fabrication process and the performance of a flexible micro thermoelectric generator with electroplated Bi2Te3 thermocouples in a SU-8 mold. Demonstrator devices generate 278uWcm-2 at dTmeas=40K across the experimental set up. Based on model calculations, a temperature difference of dTG=21.4K across the generator is assumed. Due to the flexible design and the chosen generator materials, the performance stays high even for curved contact surfaces. The measurement results correlate well with the model based design optimization predictions

I-Revues

9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
Flexible Micro Thermoelectric Generator 
based on Electroplated Bi2+xTe3-x 
 
Etienne Schwyter, Wulf Glatz, Lukas Durrer, Christofer Hierold 
Micro and Nanosystems, Department of Mechanical and Process Engineering, 
ETH Zurich, Switzerland 
 
 
Abstract - We present and discuss the fabrication 
process and the performance of a flexible micro 
thermoelectric generator with electroplated Bi2+xTe3-x 
thermocouples in a SU-8 mold. Demonstrator devices 
generate 278µWcm-2 at ∆Tmeas=40K across the 
experimental set up. Based on model calculations, a 
temperature difference of ∆TG=21.4K across the 
generator is assumed. Due to the flexible design and the 
chosen generator materials, the performance stays high 
even for curved contact surfaces. The measurement 
results correlate well with the model based design 
optimization predictions. 
 
I.  INTRODUCTION 
Power MEMS are proposed – among other applications – 
to provide electrical energy for autonomous microsystems 
and sensors. Micro thermoelectric generators (µTEGs) based 
on the Seebeck effect convert thermal energy, represented by 
heat flow (Q•H,Q•C) and driven by a temperature difference 
across the generator, into an electrical potential difference. 
For thermoelectric generators the material of choice has to 
have not just a high Seebeck coefficient but also a good 
electrical conductivity for a low internal resistance of the 
generator and a high thermal resistance in comparison to the 
thermal resistances of the generator’s package, which 
provides the thermal interface to the environment. Bismuth 
telluride (Bi2+xTe3-x) is the state of the art thermoelectric 
material for power harvesting and cooling applications near 
room-temperature. 
 
II.  MICRO FABRICATION PROCESS 
A.  µTEG Chip Processing 
In our previous publication [1] on the fabrication, testing 
and modelling of fully flexible µTEGs, a first proof of 
concept was presented. This fabrication concept has been 
adopted and extended for the use of Bi2+xTe3-x as 
thermoelectric material, replacing the nickel and copper 
thermolegs. The fabrication process consists of four 
deposition, photolithography and etching steps followed by 
an electrochemical deposition (ECD) of both Bi-rich and Te-
rich Bi2Te3 in 200-500µm thick SU-8 molds, respectively. 
The chips are mechanically polished and three subsequent 
deposition, photolithography and etching steps complete the 
fabrication of the micro thermoelectric generator. For the 
Bi2+xTe3-x µTEGs, the material of choice for the electrical 
interconnects and contacts is gold, since copper would have 
been oxidized in the electrolyte solution and copper ion 
diffusion into the Bi2+xTe3-x thermo legs would have 
deteriorated the generator’s performance. The SU-8 mold is 
left in place and serves as package providing flexibility and 
stability to the device. The Bi2+xTe3-x µTEG demonstrator 
device is shown in Fig. 1. Fig. 2 shows the working principle 
of a generator device. 
 
 
Fig 1 Micro thermoelectric generator with Bi2+xTe3-x thermolegs 
and gold interconnects. 
 
 
Fig 2 Working principle of a µTEG connected to a heat source 
and heat sink. 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
B.  Electrochemical Deposition of Bi2+xTe3-x 
The electrochemical deposition of Bi2+xTe3-x has been 
studied intensively during the last few years [2,3]. The full 
control over the stoichiometric ratio between the compounds 
is the most challenging task for ECD of Bi2+xTe3-x. However, 
electroplating is cheap, fast and efficient, which makes this 
method interesting for mass production. 
The electrolyte solutions are prepared by dissolving 
80mmol O2Te and 20-60mmol Bi2O3 in 2M HNO3 (65%). 
Thus a 2M HNO3 solution containing Bi3+ and HTeO2+ ions 
with a measured pH-value of 0.02 is achieved. All 
depositions are performed with a three electrode 
configuration, where a Mercury Sulfate Electrode (MSE) is 
used as reference electrode. A Pt grid is used as counter 
electrode and the structured SU-8 molds were connected to 
the working electrode. It is advantageous to apply a pulsed 
plating method to homogeneously fill molds with high 
aspect ratios. Primarily the ion concentration at the reaction 
surface does not deplete and secondly the undesired reaction 
products have time to dissipate. The pulse and pause 
duration (1-200ms and 4-5s, respectively), the pulse current 
density (40-120Acm-2) or pulse potential (0.4-0.8V), the ion 
concentration (see above), the electrolyte convection 
(mechanical stirrer) and the temperature (room temperature) 
is set in order to deposit both, bismuth and telluride rich 
thermolegs [4] (Fig. 3). With shorter pulses, the material 
becomes denser, but at the same time the material’s internal 
stress increases. Hence, cracks appear and the stoichiometric 
control cannot be guaranteed over the whole deposition area. 
Due to the high aspect ratio of the thermopile holes, ion 
diffusion is dominant over convection, for which reason the 
pause duration has to be rather long. The deposition of Bi-
rich Bi2Te3 is performed in electrolyte baths with Bi2O3 
concentrations of 40-60mmol. The Te-rich Bi2Te3 is 
deposited using Bi2O3 concentrations of 20-40mmol. 
 
III.  MEASUREMENT SETUP 
The manufactured µTEGs are tested and characterized 
with the same measurement setup as used for the Cu/Ni 
generators [1]. The generators are clamped between a Peltier 
cooling element mounted on a water cooling system and 
three thermoresistors integrated in a metal bar. The 
temperature difference is measured and controlled by two 
thermoresistive sensors close to the generator contact area. 
The obtained potential difference and the internal electrical 
resistance of the µTEGs were measured in a four probe 
configuration. The matched load case maximum output 
power was subsequently calculated. Due to the limited 
thickness of the device and the non perfect thermal contact 
to heat source and sink, the temperature difference across the 
generator surfaces ∆TG is significantly lower than the 
temperature difference measured between the sensors ∆Tmeas. 
The fraction of ∆Tmeas that is lost in the interface is directly 
depending on the thermal interface resistance K, the TC 
length and the ratio of insulating to thermoactive area, AV. 
The specific electrical contact resistance ρC directly 
influences the power output by influencing the internal 
resistance Ri of the generator. 
 
IV.  ANALYTICAL MODEL 
The model based analysis [1] of the performance of the 
µTEGs is shown in Fig. 4. The characteristics are calculated 
for a ∆Tmeas of 40K and the average K=3.9KW-1 for Ni/Cu 
and Bi2Te3 generators for comparison. The optimal TC 
length for Bi2Te3 generators is around 100 - 300µm. This fits 
well to the capabilities of microsystems technology, 
applying SU-8 as photosensitive resist and flexible substrate 
material. 
 
 
Fig 3 Electrochemically deposited Bi2Te3 thermolegs with 
high aspect ratios. 
 
0.0001
0.001
0.01
0.1
1
10
100
1000
10000
100000
1 10 100 1000 10000 100000
TC length [µm]
Po
w
er
 p
er
 a
re
a 
[ µ
W
/c
m
2 ]
ρC=2.11*10
-11Ωm2
K=KH=KC=3.9WK-1
∆Tmeas=40K
Bi2Te3-type
Cu/Ni-type
 
Fig 4 Comparison of the performance of Cu/Ni and Bi2Te3 based µTEGs 
with double logarithmic axis. 
300µm 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
V.  MEASUREMENT RESULTS AND DISCUSSION 
A.  µTEG Chip Processing 
The first set of Bi2+xTe3-x µTEGs yielded 71.6±1.0µWcm-2 
for a ∆Tmeas of 40.3±0.1K (see Fig. 6). After an annealing 
step for 18 hours at 200°C, the performance increased by a 
factor of 3.9 and the generator delivered 278.5±5.4µWcm-2 
for a similar ∆Tmeas of 40.0±0.3K. In comparison with the 
Cu/Ni generator, the Bi2+xTe3-x generator is over 60 times 
better for the same operating conditions. The maximum 
output power measured so far was 344.1µWcm-2 for 
∆Tmeas=44.4K and results in an efficiency factor ZT of 
0.18µWcm-2K-2. 
 
B.  Electrochemical Deposition of Bi2+xTe3-x 
As depicted in Fig. 4 and 5, thermolegs with constant 
stoichiometry and a thickness of over 300µm are deposited 
at a rate of 20µm/h. All the stoichiometric analysis have 
been made by Energy Dispersive X-ray Detection (EDX) 
and the measured stoichiometric ratios covering all the 
fabricated generators did range from 0.8 (Bi rich) to 2.1 (Te 
rich). Subsequent qualitative Seebeck measurements 
have reconfirmed that both, Bi-rich and Te-rich thermo legs 
with positive and negative Seebeck coefficients are 
achieved. 
 
VI.  CONCLUSION 
The fabrication of vertical, flexible Bi2+xTe3-x µTEGs 
could be shown, by using a novel, fast and low cost ECD 
approach with deposition rates exceeding 20µm/h. This 
novel ECD method further more allows full control of the 
stoichiometry over the whole deposit. The deposition of n- 
and p-rich Bi2+xTe3-x with a thickness of over 300µm and an 
aspect ratio of at least 1.5 could be proven. According to our 
analytical model, the stated process enables µTEG 
fabrication in the ideal thermoleg thickness regime. The 
µTEG performance could further be improved by a factor of 
3.9, if thermal annealing was performed for 18h at 200°C. 
In addition to annealing, we see several routes to 
optimization: adjust the stoichiometry of Te-rich and Bi-rich 
thermolegs for higher Seebeck coefficients by optimizing the 
process parameters during electroplating, reduce grain 
boundaries by adjusting the deposition speed, and reduce the 
internal resistance by improving contact and interconnect 
resistances. Electroplating allows increasing the thermo legs’ 
length towards 200 - 300µm, which is the simulated 
optimum for the given material combination (see Fig. 3). 
Electroplating in general offers several advantages above 
sputtering of Bi2+xTe3-x as thermoelectric material, which has 
yielded the current state-of-the-art µTEGs with an efficiency 
factor of 2.4µWcm-2K-2 [5]. These advantages are: high 
deposition speed and thicker layers, low material and energy 
consumption and low costs on large substrate sizes. 
 
Fig 5 SEM image with overlaid EDX line scan of an electrochemically 
deposited Bi2+xTe3-x thermoleg measured in its growth direction. 
 
0
50
100
150
200
250
300
350
400
0 10 20 30 40 50
∆Tmeas [K]
ou
tp
ut
 p
ow
er
 [ µ
W
/c
m
2 ]
Bi2Te3 annealed
model of Bi2Te3 annealed
Bi2Te3 as deposited
model of Bi2Te3 as deposited
Cu/Ni
model of Cu/Ni
 
Fig 6 Comparison of the measured matched output power for Cu/Ni, Bi2Te3 
and annealed Bi2Te3 micro thermoelectric generators in comparison to the 
model based calculations. 
 
REFERENCES 
[1] W. Glatz, S. Muntwyler, C. Hierold, „Optimization and Fabrication of 
Thick Flexible Polymer Based Micro Thermoelectric Generator“, 
Sensors and Actuators A-Physical, Vol. 132, pp. 337-345, 2006 
[2] M. Takahashi, M. Kojima, S. Sato, N. Ohnisi, A. Nishiwaki, K. 
Wakita, T. Miyuki, S. Ikeda, and Y. Muramatsu, „Electric and 
thermoelectric properties of electrodeposited bismuth telluride 
(Bi2Te3) films“, Journal of Applied Physics, Vol. 96, 
pp. 5582-5587, 2004 
[3] M. Martin-Gonzalez, A. Prieto, R. Gronsky, T. Sands, and A. Stacy, 
„Insights into the Electrodeposition of Bi2Te3“, Journal of the 
Electrochemical Society, Vol. 149 (11), pp. C546-C554, 2002 
[4] W. Glatz, L. Durrer, E. Schwyter, C. Hierold, „Novel mixed method 
for the electrochemical deposition of thick layers of Bi2+xTe3-x with 
controlled stoichiometry“, submitted to Chemistry of Materials, ACS 
Publications, November 2007 
[5] H. Bottner, J. Nurnus, A. Gavrikov, G. Kuhner, M. Jagle, C. Kunzel, 
D. Eberhard, G. Plescher, A. Schubert, K.H. Schlereth, „New 
thermoelectric components using microsystem technologies“, J. 
Microelectromech. Syst., Vol. 13, pp. 414-420, 2004 
scan line 
scan line 
2M HNO3, 80mmol O2Te, 40mmol Bi2O3, 
-0.47V, 100ms
0
0.5
1
1.5
2
0 50 100 150 200 250 300
distance [um]
ra
tio
 T
e/
B
i


Flexible Micro Thermoelectric Generator based on Electroplated Bi2Te3

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