Stresa, Italy, 26-28 April 2006

ENERGY CONVERSION USING NEW THERMOELECTRIC GENERATOR

G. Savelli1,2, M. Plissonnier1, J. Bablet1, C. Salvi1, J.M. Fournier1,2

1
CEA/LITEN/DTNM/LCH, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
2
LEG, CNRS UMR 552, BP 46, 38402 St Martin d’Hères Cedex, France
ABSTRACT (low thermal gradient) matches energy sensor’s power
need.
During recent years, microelectronics helped to develop The used thermoelectric materials (TE) are here bismuth
complex and varied technologies. It appears that many of and antimony. Both Bi and Sb are semimetals, that is,
these technologies can be applied successfully to realize there is an energy overlap between the valence and
Seebeck micro generators: photolithography and conduction bands. Near room temperature, the
deposition methods allow to elaborate thin thermoelectric thermoelectric power (or Seebeck coefficient) of both Bi
structures at the micro-scale level. and Sb are enough small: typical values are about -
Our goal is to scavenge energy by developing a miniature 70µV.K-1 for Bi and 40µV.K-1 for Sb [4]. Nevertheless,
power source for operating electronic components. First the acquired experience with these usual thermoelectric
Bi and Sb micro-devices on silicon glass substrate have materials for our devices will have been very enriching
been manufactured with an area of 1cm2 including more and helpful for our presently device improvement with
than one hundred junctions. Each step of process more competitive thermoelectric materials.
fabrication has been optimized: photolithography, Thus, in this paper we review a wafer technology
deposition process, anneals conditions and metallic approach to manufacture a thermoelectric device. Design
connections. Different device structures have been and technological process steps will be identified in order
realized with different micro-line dimensions. Each to propose a strategy to manufacture thermoelectric
devices performance will be reviewed and discussed in converters (TEC).
function of their design structure.
2. EXPERIMENTAL

2.1. Device description

1. INTRODUCTION
We use a four inches glass substrate. 42 chips and 6 test
Since the last decade, there is a growing interest of areas are distributed on it. Fig.1. shows this Bi-Sb
wireless sensor nodes with goals of monitoring human prototype.
environment. Because advances in low power VLSI
design and CMOS fabrication have dramatically
decreased power requirements of sensors, it is now
possible to consider self-powered system in sensor node
[1].
In the same time, the interest in producing micro-
electromechanical systems (MEMS) opens new
opportunity in the field of micro power generation. Micro
thermoelectric converters are a promising technology due
to the high reliability, quiet operation and are usually
environmentally friendly. The efficiency of a
thermoelectric device is generally limited to its associated Fig.1. Bi-Sb device realized by microelectronic
Carnot Cycle efficiency reduced by a factor which is technologies: PVD deposition, photolithography….
dependent upon the thermoelectric figure of merit (ZT) of
the materials [2] used in fabrication of the thermoelectric Dimension of chips are 1x1cm2, bismuth and antimony
device. Recent developments in micro-thermoelectric lines widths are 20, 30 or 40µm, spaced by 20µm.
devices using thin film deposition technology [3] have This geometry is obtained by photolithography with a
shown that energy scavenged from human environment serial of 3 masks.

©TIMA Editions/DTIP 2006 ISBN: 2-916187-03-0

 G. Savelli, M. Plissonnier, J. Bablet, C. Salvi, J.M. Fournier
Energy conversion using new thermoelectric generator

These geometries allow us to obtain 250 lines (125 in PVD, it allows to obtain grain sizes smaller than with
bismuth and 125 in antimony in alternation), i.e 125 evaporation deposition [5]. Furthermore different
junctions for the 20x20 chips, 208 lines and 104 junctions annealing conditions have been studied to reduce grain
for the 30x20 chips and 166 lines and 83 junctions for the sizes. Two kinds of annealings will be compared in
40x30 chips. section 3: annealing by furnace and by laser. Annealings
Moreover Bi and Sb lines are electrically connected in by furnace are realized at 260°C for bismuth (Tm[Bi-Ti]=
series by using Ti and Au metallic junctions. Fig.2. shows Tm[Bi]=271°C) and 355°C for antimony (Tm[Sb-
these lines more in details and an enlargement of the Au]=360°C) with Ar atmosphere for 8 hours (Tm are
metallic connections. obtained according to Binary Alloy Phase Diagrams).
Annealing by laser are realized using a Xe-Cl excimer
laser with 200ns of impulsion length and a wavelength of
308nm.
To analyse our devices performances, we use several
characterization tools: Scanning Electron Microscopy
(SEM) to check geometry dimensions and
photolithography quality, X-rays diffraction to control
crystallographic structure of bismuth and antimony, a
four probe method is used to measure electrical
resistivity, and a thermoelectric characterization tool
which provides us Seebeck coefficient and useful
electrical power.

3. RESULTS
Fig.2. Bismuth and antimony lines with electric
connections. 3.1. Annealings influence

Bismuth and antimony lines, and titanium and gold In this part, we present results obtained before and after
connections are deposited by sputtering PVD (Physical annealing and compare the two methods. Here
Vapor Deposition) system. This deposition choice is cristallographic structures and electrical resistivity are
explained in the next paragraph. Sputtering process use studied. Firstly, annealings influence on bismuth lines is
six inches targets for a best thickness uniformity. Each tested.
deposition is realized in Ar atmosphere at 1.2 Pa. In As Bi layers are “granular”, resistivity is too high. Thus
addition, an rf generator provides power supplies from 0 Bi resistivity before annealing is 1600µΩ.cm.
to 300W and operates at a frequency of 13.56MHz. To decrease this value, we tested two kinds of annealing.
Also it is necessary to add adhesion layers for bismuth Fig.3. shows Bi lines before and after the two kinds of
and antimony deposition. These sublayers permit Bi and annealing (on this Fig.3. just Bi lines are deposited).
Sb to adhere to the substrate during photolithography
operations (resin addition, etching…). A flash Ti a b
adhesion layer (1nm) is used for Bi lines and flash Ti+Au
adhesion layers (1nm+1nm) are used for Sb lines. We
will check in part 3. that these added layers have no
influence on both thermoelectric results and
crystallographic structures.

2.2. Device treatment and characterization

Annealing is the crucial point for this device. Indeed in c

thermoelectrics, it is important to obtain the lowest
electrical resistivity. To succeed in these lowest values
high layers quality are necessary. But, for example,
bismuth is well known to deposite in the form of grains
[5-6] which increases resistivity. Objective is thus to
improve layers quality, and so to reduce grains size. For
that, in first, sputtering deposition is used because, in
Fig.3. Influence of annealing on Bi lines - a: without
annealing - b: annealing by furnace at 260°C - c:
annealing by laser.

©TIMA Editions/DTIP 2006 ISBN: 2-916187-03-0

 G. Savelli, M. Plissonnier, J. Bablet, C. Salvi, J.M. Fournier
Energy conversion using new thermoelectric generator

Fig.6. shows two lines, one of bismuth and one of

antimony, where each line has an annealed part and not
Fig.3.a. confirms the granular aspect of Bi lines. We see the other one.
with fig.3.b. that some parts of lines melted and can Not annealed part Annealed part
involve too important discontinuities in lines. Resistivity
after annealing by furnace (at 260°C for 8 hours)
decreases to 900µΩ.cm.
Fig.3.c. shows Bi lines after annealing by laser. The Sb
granular aspect seems to have disappeared. Fig.4. shows
the influence of the annealing by laser.

Bi

Fig.6. Influence of annealing by laser on both Bi and Sb

lines.

Fig.4. Bi line: one part is annealed by laser, not the other We always see that Bi granular aspect disappears and that
one (photo realized by SEM). there is no important visible change concerning Sb layer.
To confirm these observations, Fig.7. present bismuth and
On Fig.4. we chose voluntary to anneal just one part of antimony X-rays diffraction spectra.
the line to see the difference. We see that grains
disappeared on the annealed part. Fig.5. shows a cross-
section of the line (annealed part): absence of grains is Bi spectra
confirmed.
20000
Lin (Counts)

3
10000

1
0

20 30 40 50 60 70 80 9

2-Theta - Scale
Fig.5. Cross section of the Bi annealed line (photo
realized by SEM). Sb spectra
4000

The granular aspect of Bi layer is reduced. This is

confirmed by resistivity measurements. We obtain a
Lin (Counts)

3000

resistivity of 800µΩ.cm after annealing by laser. 3

Annealing influence on Sb layers is also reviewed with
different annealing conditions. Deposited Sb resistivity is 2000

1100µΩ.cm. 2
After an annealing of 355°C for 8 hours, its resistivity 1000

decreases to 825µΩ.cm. There is no notable improvement

after an annealing by laser, its resistivity is 825µΩ.cm 1
too. 0

21 30 40 50 60 70 80 90

2-Theta - Scale
Fig.7. Bi and Sb X-rays spectra - 1: without annealing -
2: after annealing by laser - 3: after annealing by
furnace

©TIMA Editions/DTIP 2006 ISBN: 2-916187-03-0

 G. Savelli, M. Plissonnier, J. Bablet, C. Salvi, J.M. Fournier
Energy conversion using new thermoelectric generator

First, we can observe that we obtain a good 3.2. Thermoelectric results

crystallographic structure, i.e. a rhombohedral structure
for bismuth films. All peaks are identified by the JCPDS Thermoelectric generators are characterized by the
files. These parameters are: a=b=4.547Å, c=11.8616Å, Seebeck voltage VS and the useful electrical power Pu,
and α=β=90°, γ=120°. For antimony films, we obtain a defined by the following equations:
rhombohedral structure too. In the same way, its structure V
2
Pcc
is identified by the JCPDS files, with the following Pcc = s ; Pu = (eq.1)
parameters: a=b=4.307Å, c=11.273, and α=β=90°, Rg 4
γ=120°.  
These spectra show that Bi and Sb annealing by laser Rg = N ρ L Bi
+ρ L Sb
+2ρ L
m
 (eq.2)
don’t disturb the initial polycrystalline structure, which is 
Bi
A Bi A Sb
Sb
m
A
m

an important point, because this structure, which can be

 
considered as a slightly distorted cube, is responsible for Rg ≈ N ρ L Bi
+ρ L Sb
 with ρm << ρBi and ρSb
a minute band overlap, leading to the presence of small  A
Bi
Bi A Sb
Sb 

and equal electron and hole densities at all temperatures
[7]. Moreover, any peak reveals the presence of titanium where Pcc is the short-circuit power, Rg is the chip global
or gold, which ensures that any Bi-Ti or Sb-Au alloy electric resistance, N is the total junctions number for one
appeared during annealing. chip, ρ is the electrical resistivity, L is the line length and
To resume, Fig.8. shows a table summarizing resistivity A the line section area. Metallic - TE materials contact
results for Bi and Sb, with or without annealing. resistance is here neglected in front of global lines
resistance.
1800 Vs depends on Seebeck coefficient (or thermoelectric
a power S) and the number of electrically connected
1500 junctions in series. Thus Vs increases when lines density
increases. In the same way, useful electrical power Pu
1200 depends on Vs and Rg (eq.1). In that case, lines geometry
impacts directly to Pu. Moreover, eq.2 predicts Rg, and so
900 Vs and P. Thus using eq.2, we report on Fig.9. Rg values
for each device geometry: Rg theo. is calculated with bulk
600
material resistivity (ρBi=117µΩ.cm, ρSb=40.1µΩ.cm at
300K and ρm is the metallic junctions resistivity). Rg cal. is
300
Bi Bi-f urnace Bi-laser
calculated with experimentally measured Bi-Sb resistivity
(i.e. after annealing by laser). And finally, Rg eff. is the
1200
effective device electrical resistance after the last
b annealing. Moreover, as expected, Rg eff. depends strongly
1100
on lines geometry and lines density.
1000
20x20 30x20 40x20
900 N 125 104 83
Rg theo. (kΩ) 17,8 9,9 5,9
800 Rg cal. (kΩ) 184,7 102,4 61,3
Rg eff. (kΩ) 82 63,8 31
700
Sb Sb-f urnace Sb-laser
Fig.9. Comparison of Rg values using different
Fig.8. Influence of annealing on electrical resistivity - resistivity values.
a: for Bi lines - b: for Sb lines.
Results show that annealing process is critical in order to
decrease Rg to the minimal value Rg theo. Last annealing
Finally, we realize a last annealing on the final device at gave a decrease of 55% of the chip global resistance.
240°C for 9 hours in Ar atmosphere. This annealing Moreover in any case, Rg values decrease normally by
permits to improve interface layers and to stabilize the increasing lines width and so, decreasing lines number.
device.

©TIMA Editions/DTIP 2006 ISBN: 2-916187-03-0

 G. Savelli, M. Plissonnier, J. Bablet, C. Salvi, J.M. Fournier
Energy conversion using new thermoelectric generator

Fig.10. shows the useful electric power Pu as a function of

temperature gradient and using these different values of
Rg for the 40x20 device.
8
40x20 theo
For the Seebeck voltage, as explained before, we obtain
7
40x20 eff the most important value (535mV for ∆T=100K) for the
6 40x20 cal device which has the highest junctions numbers (i.e.
5 20x20). On the contrary and as shown in Fig.9.,
4 concerning Pu, we obtain the highest value (1.2µW for
3 ∆T=100K) for the device which has the lowest junctions
2 numbers (i.e. 40x20). These power values are enough
1 low, mainly due to global electrical resistances, and so
0
due to device geometry.
0 10 20 30 40 50 60 70 80 90 100 110
∆T (°C) 4. CONCLUSION
Fig.10. Useful electric power Pu as a function of
Design and technological process steps were reviewed
temperature gradient and Rg.
and thermoelectric device performances have been
discussed. Different annealing conditions demonstrated
This graph shows that using bulk material resistivity their strong influence on improving Bi-Sb electrical
values, we can obtain 7.2µW for ∆T=100K. Without the resistivity and consequently increasing thermoelectric
last annealing we obtain 0.65µW and with this last generator performances. Seebeck voltage and electrical
annealing, we obtain 1.2µW for ∆T=100K, which power have shown a high voltage and low power density
confirm a decrease of nearly 50%. for each device geometry. This means that these
It is interesting to compare the Seebeck voltage configurations are not enough well adapted to supply
(Fig.11.a.) and useful electrical power (Fig.11.b.) as a high power but are interesting for temperature sensors
function of lines geometry. thanks to a good sensitiveness in voltage.
Nevertheless, we currently study 3D device geometry
600 better adapted for higher powers including our process
20x20
a steps optimization performed during this work.
500 30x20
40x20
New high performance thermoelectric materials,
400 environmentally friendly as doped silicon and silicon
germanium alloys, will be process taking design
300
optimization, process steps and electrical characterization
200 into account.
100
5. REFERENCES
0
0 10 20 30 40 50 60 70 80 90 100 110
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1,2
20x20 b [2] S.B. Schaevitz, “A MEMS thermoelectric generator”, M.
30x20 Eng. In Electrical and computer Science, Massachusetts
40x20
1 Institute of technology, Cambridge, MA, (2000).
0,8 [3] H. Böttner et al., “New thermoelectric Components using
Microsystem technologies”, Journal of micro-electromechanical
0,6
system, Vol. 13, No. 3, (2004).
0,4 [4] G.S. Nolas, J. Sharp and H.J. Goldsmid, “Thermoelectrics –
0,2
Basic Principles and New Materials Developments”, Springer,
(2001).
0
[5] D. E. Beutler, N. Giordano, “Localization and electron-
0 10 20 30 40 50 60 70 80 90 100 110
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∆T (°C)
Review B, Vol. 38, No. 1, pp. 8-19, (1988).
[6] F. Y. Yang, K. Liu, K. Hong, D. H. Reich, P. C. Searson, C.
Fig.11. Seebeck voltage Vs evolution (a) and useful L. Chien, “Large Magnetoresistance of Electrodeposited Single-
electrical power Pu evolution (b) as a function of
temperature gradient and geometry.

©TIMA Editions/DTIP 2006 ISBN: 2-916187-03-0

 G. Savelli, M. Plissonnier, J. Bablet, C. Salvi, J.M. Fournier
Energy conversion using new thermoelectric generator

Crystal Bismuth Thin Films”, Science, Vol. 284, pp. 1335-1337,

(1999).
[7] D.M. Rowe, “Thermoelectrics Handbook – Macro to Nano”,
CRC Press, (2006).

©TIMA Editions/DTIP 2006 ISBN: 2-916187-03-0