Thermal Conductivity of Thermoelectric Thick Films Deposited by an Electrodeposition Process

Heng-Chieh Chien a, Chii-Rong Yang b, Li-Ling Liao a, Da-Jeng Yao c, Chun-Kai Liu a, Ming-Ji Dai a, Ra-Min Tain a

a Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan, ROCb Department of Mechatonic Technology, National Taiwan Normal University, Taipei 10610, Taiwan, ROCc Institute of NanoEngineering and MicroSystems, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC

ABSTRACT

Due to the lack of advanced technology and difficulties in sample preparing, direct measurement of thermal conductivity for thin-film thermoelectric material is rarely reported in literatures. In this work, there are four types of thermoelectric thin films measured for their intrinsic thermal conductivity by using a modified parallel-strip technique. These four types of thermoelectric thin films are of Bi-Te and Sb-Te compositions, resulting from electrodeposition process with different aqueous solutions. From the measurement results, three types of the films shown quite low intrinsic thermal conductivity less than 0.5 Wm-1K-1; and one type of the films can not be measured due to sample preparing problem. According to the scanning electron microscope observations, the grain boundaries in the thermoelectric thin films are probably the cause that results in their low thermal conductivity and the fragile structure is the cause of difficulty for the sample preparation.

INTRODUCTION

The practicability of thermoelectric (TE) devices is mainly limited by the available TE materials, whose performance needs to be enhanced to compete with conventional cooing and energy-generation systems. The efficiency of TE materials is directly related to a dimensionless figure of merit which is defined as kTSZT �2� , where � is the electrical conductivity, S the Seebeck coefficient, k the thermal conductivity and T is the absolute temperature. Thus, the enhancement of the TE figure of merit can be achieved by increasing the Seebeck coefficient (S) as well as the electrical conductivity (�), or decreasing the thermal conductivity (k). Among the common TE materials, bismuth telluride (Bi2Te3) which usually served as an n-type material and antimony telluride (Sb2Te3) served as a p-type material are generally reported to show optimal performance near room temperature in bulk form because having high thermoelectric figure of merit. However, compared with bulk materials, TE film structures are expected to have a significantly lower thermal conductivity because of strong phonon scattering at both film interfaces and at its denser grain boundaries1-4, and therefore show the feasibility in the applications of hot spot cooling and electric generation operated at small-to-moderate temperature differences.

Several methods have been used to fabricate the Bi2Te3 and Sb2Te3 films. Among these methods, the electrodeposition process5-8

is considerably relative simple and cost-effective compared with the dry processes such as molecular-beam epitaxy (MBE)9, MetalOrganic Chemical Vapor Deposition (MOCVD)10, pulsed laser deposition11, 12, flash evaporation13 as well as sputtering14. Furthermore, the electrodeposition technique is also easy in controlling film thickness from submicrometer to several tens micrometers that shows feasibility and suitability for fabricating the micro-TE devices.

There are many methods15-18, have been proven to measure the thermal conductivity of thin-film with a thickness in the range ���������� ������� However, methods to measure the thermal conductivity of a thick film with a thickness larger than several micrometers are rarely encountered in previous literatures due to a strong heat spreading effect in in-plane direction of thick-film. The significant lateral heat spreading in thick-film result in that we cannot simply use the Fourier’s law to calculate the thermal conductivity due to inaccurate heat flow and immeasurable film/substrate interfacial temperature.

In this study, we measured the intrinsic thermal conductivity of the TE materials, Bi2Te3 and Sb2Te3, which deposited by the electrodeposition process with the aqueous solution added ion-typed surfactant and that non-added ion-typed surfactant respectively. Here, the ion-typed surfactant is anionic sodium dihexyl sulfosuccinate (SDSS) which usually be used in the electrodeposited process to enhance the probability for fabricating a complete micro TE structure19. For samples preparing, the measured TE layer (thickness ���� �� ��� ���������� ��� �������� ���������� ���� ����� ��in-coated a layer of photosensitive epoxy ������ ����� �� ������ ������� ��� ��dielectric layer to separate the metallic heating strip from the measured TE layer. We applied the modified parallel-strip method20,21 to measure the thermal conductivity of such the thick-film samples. By measuring the TE layers with various thicknesses, the thermal resistances of the layers can yield the intrinsic thermal conductivity of the TE materials.

MEASUREMENT METHOD

In this study, we used a modified method, originated from the parallel-strip technique20,21 to determine the thermal conductivity of a TE electrodeposition thick-film. Fig. 1 shows the schematic cross-section diagram of the measured sample used for the modified method. The typical spacing (r) between the heating strip and the ������������������� !�the widths of the heating strip and the sensing ������������� ������� "������������#�$���� ���������������measured temperature of the sensing strip, Tf (x=r), to deduce the unmeasured average temperature at the interface between the TE film and substrate, Tav-s, by using a simple empirical correlation which expressed as,

� �lkr/rln

alq)rx(TT

s

ossav

��

����

2 , for 50m % r % 120m (1)

where k and T denote the thermal conductivity and the temperature respectively with indices “s” for the substrate, “f” for the film and “av-s” for the substrate over the range of –���������. The parameters, q, a and l, are the generated Joule heat flux, the half width and length of the heating strip, respectively. Due to negligible heat flux in cross-plane direction, Ts (x=r) is equal to the measured sensing strip temperature, Tf (x=r).

FIGURE 1. Schematic diagram of the laminated film structure used for the parallel-strip method.

In Eq.(1), ro is a fitting parameter. After data analyzing with the analytical solution20, we find ro can be expressed as,

5331661014638000985000050

� �:ofunit ( 43810

2

1

12

.k.C.k.C

t.tCar

fint,

fint,

Cr

ro

o

o

�����

��

��

��

(2)

Because that the unmeasured temperature, Tav-s, can be calculated, we hence propose a method for measuring the thick-film thermal conductivity as follows. The average temperature on top side of the film (Tav-f) is measured by using the heating strip electrical resistance-temperature relation. The average interface temperature (Tav-s) between the thin film and the substrate is not measurable, and can be deduced by using Eq. (1), which involves the measurement of the sensing strip temperature Ts(x=r) and the use of the semi-empirical relation ar

oro ��� . The thick-film intrinsic thermal

conductivity (kint,f ) is then calculated using the Fourier’s law,

��

����

���

�

� ��i

fint,Q

savfav Rk

talQ

TT21

� (3)

where Q=q·2al denotes the generated Joule heat by the heating strip. Ri is the total interfacial thermal resistance involved the ones at the interfaces between the metal strip and the film, and between thin film and the substrate. The factor (�Q) is introduced to compensate for the inaccuracy induced by the thickness-dependence of lateral heat spreading within the thick-film. The factor is thus modeled herein as the fraction of the Joule heat crossing the region of -��������� to the total Joule heat and gives

047338000009208995210000710

):ofunit ( 110

2

1

21

.k.D.k.D

��t.)tDexp(D

fint,

fint,

Q

��������� (4)

The suitable ranges of Eq. (2) and Eq. (4) are limited in 0.1���int,f���2W m-1K-1 and 2������10m.

EXPERIMENTS

The TE thick-film material systems of Bi-Te (n-type) and Sb-Te (p-type) fabricated by the electrodeposition process were prepared for this measurement. Each of them was deposited with the electrolyte added an ion-typed surfactant and with that non-added the surfactant respectively. Thus, there were four types of TE materials would be measured, namely;

Type A: Bi-Te material deposited by non-added surfactant solution, Type B: Sb-Te material by non-added surfactant solution, Type C: Bi-Te material by added surfactant solution, and Type D: Sb-Te material by solution added surfactant.

Here, the surfactant is anionic sodium dihexyl sulfosuccinate (SDSS) (Cytec Industries, Inc., USA). SDSS is an ion-typed surfactant and used to improve the completeness for fabricating a micro TE structure by enhancing the wettability of electrolyte. The electrolyte compositions for Type A material incorporate 1.0 M (mol/L) of nitride acid, 7.5×10-3 M of Bi2O3 powder and 1.0×10-2 M of TeO2powder. And that for Type B incorporate 1.0 M nitride acid, 7.5×10-3

M of Sb2O3 powder, 1.0×10-2 M of TeO2 powder and 0.34 M sodium citrate (served as a complexing agent). For depositing Type C and D materials, the electrolytes, as same as Type A and B respectively, were added additionally with the SDSS surfactant of concentration 5mL/L. Energy Dispersive X-ray Analysis (EDX) provides the atomic composition of the deposited TE thick-film materials, the atomic proportion of the thick films is agreeable to that of the bulk materials (Bi:Te and Sb:Te = 2:3).

To measure the intrinsic thermal conductivity of TE films, we conducted measurements in two batches. For the first batch, we spin-coated the photosensitive epoxy resin layers (trade name: WPR-1201,product of JSR Micro, Inc.) with varied thickness on a silicon substrate (P/Boron, 20-25 �� � "� ����&����� ��'�� � �"� ���� �����deposited the Cr/Au (thickness~20nm/200nm) heating strip and sensing strip upon the resin layer. Because the resin, WPR-1201, served as a dielectric layer, is a negative photoresist, therefore, we used a positive photoresist, AZ P4620 (Clariant Inc.) to pattern and lift-off the heating and sensing strip that deposited on the resin. In ����� ���&"� ���� ������ �*� ���� �������� ������ ��� ��� � ���� ����� �*� ����������������������� !������������������������ ���������������� "����� �������������*� ��� ������ �+�������� ������<������#�'>00) supplies a dc current to the heating strip for generating a Joule heat, and the measured electrical resistance is used to convert into the heating strip temperature. A digital multimeter (Keithley 2700) measures the electrical resistance of the sensing strip, which is also used to convert into its temperature (shown in Fig. 2).

FIGURE 2. Schematic diagram of the sample chip and the measurement system.

In measurements for this batch we determined the total thermal resistance of the resin thick film that includes the thermal resistance of the resin layer and the interfacial thermal resistance of both Si-resin and Cr-resin. The total thermal resistance of the resin film, R*, depends on its film thickness (t), expressed as

sinreCrsinreint,sinreSi R)t(RR)t(R *�� � (m2KW-1) (5)

For measurements in the second batch, we made a laminated film structure to measure its thermal resistance. The laminated film structure is fabricated by depositing the electrodeposition TE film on a silicon substrate and then the epoxy resin layer (WPR-1201) is

spin-coated upon the TE film. Finally, we used lithography and lift-off processes to deposit the Cr/Au heating and sensing strip pattern upon the resin layer (shown in Fig.1). For this batch, we measured the total thermal resistance of the laminated structure (R**), expressed as

sinreCrsinreint,sinreTETEint,TESi R)t(RR)t(RR)t(R **��� � (m2KW-1) (6)

Eq. (7) is the obtained equation by subtracting Eq. (5) from Eq. (6), and can be written as,

)RRR()t(RRR sinreSisinreTETESiTEint,***

��� ��� ( 7 )

The left term of Eq. (7), *** RR � , is linearly proportional to the TE film thickness, t, because of the thermal resistance expression,

TEint,TEint, k/t)t(R � . Thus, the slope gives the reciprocal thickness-independent intrinsic thermal conductivity, 1/ kint,TE. The right-second term is a sum of the interfacial resistances which can be obtained by the intersection of fitting curve to y-axis, and independent of film thickness.

RESULT AND DISCISSION

As mentioned previously, two batches of measurements were conducted to measure and extract the intrinsic thermal conductivity of TE films. In the first batch, we measured the thermal resistances of the photosensitive epoxy resin layers (trade name: WPR-1201) with varied thickness on a silicon substrate, the measured thermal resistance (shown in Fig. 3) is expressed as

)KW(m 105744.15952.3 -126* ���� tR (8)

Figure 3. The observed thermal resistance of the epoxy resin layer.

According to Eq. (5) and the thermal resistance expression(

sinreint,sinreint, k/t)t(R � ), the total interfacial thermal resistance can be obtained by the intersection of the fitted curve to y-axis; the intrinsic thermal conductivity (

sinreint,k ) can be calculated as the fitted curve slope reciprocal. To consider the uncertainties, the results of the first batch for the instinct thermal conductivity is

(5.84%)016202780 ..k sinreint, �� (Wm-1K-1).

There are four material types of TE samples, deposited on a silicon substrate and covered with a WPR-1201 epoxy resin layer, were measured in the second batch measurements. Subtracting the observed thermal resistance of the first batch ( *R ) from that of the second batch ( **R ), we can obtain the apparent thermal resistance of the TE layers which expressed as mathematical form of Eq. (7). The

calculated thermal resistance and the intrinsic thermal conductivity of the TE films (shown in Fig. 4) are listed as follows:

Type A: 7,

** 105.4269.4 ����� sumiRtR (m2KW-1)

and (11.95%)0279.0234.0int, ��TEk (Wm-1K-1) suited to the film

����&�����*�� �>Q� �����'>� !�

Type B: 7,

** 105.4394.2 ����� sumiRtR (m2KW-1)

and (13.15%)0549.0418.0int, ��TEk (Wm-1K-1) suited to thickness from

[�� ����'� !�

Type C: 7,

** 105.418.2 ����� sumiRtR (m2KW-1) and

(9.24%)0424.0459.0int, ��TEk (Wm-1K-1) suited to thickness from

\� �����>� �

Here, sumiR ,

denotes the sum of the interfacial resistances which has

been appeared on Eq. (7). As for Type D--Sb-Te material which deposited by solution added surfactant, the test failed and did not yield any credible results of thermal conductivity because we could not find a reasonable linear correlation between the thermal resistance and film thickness even though quantities of sample were measured for this type material.

Figure 4. The observed thermal resistance of the TE films is shown as a linear function of thickness for Type A, B and C materials.

Directly measured thermal conductivities of both Bi-Te and Sb-Te materials are rarely encountered in previous literatures; most of literatures obtain the value by adding up the calculated lattice thermal conductivity (kl) and the electron thermal conductivity (ke) which converted from measured electrical conductivity with Wiedemann-Franz law23-25. M. Takaishi26 shows the measured thermal conductivity of n-type Bi-Te thin-film, annealed at 200• •, is 1.20 (m2KW-1). S. Miura27 gives k=1.75 (m2KW-1) for hot-extruded Bi2Te3 bulk compound. The book edited by D. M. Rowe28 shows the measured thermal conductivity of n-type bulk Bi2Te3 is 0.8 (m2KW-

1) and that of p-type bulk Bi2Te3 is 0.963 (m2KW-1), also another chapter29 of the same book lists measured values of single crystal bulk TE materials: k=2.0 (m2KW-1) for Bi2Te3 (in-plane); k=5.6 (m2KW-1) for Sb2Te3 (in-plane); and 1.6 (m2KW-1) for Sb2Te3 (cross-plane). The measured values of this work are at least a half smaller than that of previous articles.

Fig. 5 shows the SEM images of Type A, B and C samples. Observing all SEM pictures we had taken, the film morphology of each type material is coincident within the measured film thickness range. The significantly low intrinsic thermal conductivity is probably resulted from their obvious grain boundaries. As for Type D material, as mentioned previously, the measurements were not successful because hardly find a reasonable linear correlation between the thermal resistance and film thickness. By reviewing the SEM pictures of this type material, we found the interface between TE layer and resin layer is blurred (shown in Fig. 6), the cause probably come from the resin layer deposition process: namely, for spin-coating an appropriately thin resin layer on the TE film, a high spin-speed of 5000 rpm and 50 seconds duration was applied for the process. Such a high spin-speed makes the colloidal resin grind the bumpy surface of the fragile TE film and therefore a thin mixed layer is formed between the resin and the TE film. The mixed layer, has an uncertain thickness and undetermined thermal properties, is probably a cause result in that we cannot measure a certain intrinsic thermal conductivity by using these samples.

Figure 5. SEM images of Type A, B and C samples under the filmthickness 1.009• •m, 1.034• •m and 1.462• •m, respectively.

Figure 6. TEM images of Type D samples under the film thickness 0.231• •m and 0.520• •m.

CONCLUSION

The thermal conductivity of TE thin-film materials can be rarely encountered in previous scientific literatures due to difficulty in test samples preparation and lack of appropriate measurement methods. In this paper, we demonstrate a modified parallel-strip technique, suitable for the ranges of 0.1���kint,f���2 W m-1K-1 and 2���t���10m, to measure the intrinsic thermal conductivity of Bi-Te and Sb-Te thermoelectric thin films; the measured results are much lower than that of the previous literatures. We argue the lower thermal conductivity is probably resulted in by the fragmental grains structure of the films which made by electrodeposition process. In the future, based on the same method and sample preparation

processes, we plan to measure more thermoelectric thin-film materials, deposited by different process such as sputtering and screen printing; and deposited by electrodeposition with different controlled parameters, to explore their distinctive physical characteristics.

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