DEVELOPMEMNT OF FLEXIBLE THERMOELECTRIC GENERATOR USING FPCB

Jung Yup Kim1*, Kwang Eun Lee1, Seungwoo Han1

1Nano-mechanics Team, Korea Institute of Machinery and Materials, Daejeon, Republic of Korea *Presenting Author: jykim@kimm.re.kr

Abstract: We proposed the flexible thermoelectric generator (TEG) using double side FPCB (flexible printed circuit boards). The proposed flexible thermoelectric generator has the unique vertical structure. The heat of the flexible thermoelectric generator flows vertically via plated through hole (PTH). The n-type (Bi-Te) and p-type (Sb-Te) thin films are deposited by the co-evaporator and patterned using metal mask which has the 50 mm thickness. To protect the patterned thermoelectric (TE) thin film, the cover-layers are added. We have characterized the properties of TE thin film and the performance of the fabricated flexible TEG. To check the flexibility of the proposed flexible TEG, it is attached to pipes of different diameters and its internal resistance is measured. The results show that the proposed flexible TEG can be installed on non-flat surface and generate the power output. Keywords: thermoelectric, flexible, generator, FPCB INTRODUCTION

Autonomous power sources for microsystems and sensors can be substitute for the bulky battery which has a limited life time and makes their working possible without the power-line. Among many autonomous power sources, thermoelectric generators (TEG) attract public attention because these can recover waste heat into electric energy and take no chemical supply with almost permanent lifetime. The major two configurations of TEG are the vertical type and the horizontal type. The fabrication process of the horizontal type is simpler than that of the vertical type because all the metal electrodes are processed in one step. In addition, the longer length of thermoelectric (TE) legs is easily implemented.

Most of TEGs have parallel hard plates. Therefore these cannot be installed on curved surface. For the integration and miniaturization on the non-flat surface such as skin of human body and pipe, thermoelectric generator needs to be flexible. Many researchers report on flexible thermoelectric generator. Bi-Te based flexible TEG which has vertical configuration is introduced [1]. However, its top hard plate is omitted. The assembled vertical flexible TEG from horizontally fabricated unit thermopile strip is shown [2]. However, simple assembly is not ideal method. The horizontal flexible TEG using additional copper sheet to make vertical heat flow is reported [3]. However, the heat is not fully transferred to TE thin film.

Recently, thin film thermoelectric generators are developed based on MEMS technology. Major difficulties of the fabrication of TEG are in the patterning techniques on the thermoelectric legs. The lift-off method with SU-8 photoresist [4] has the temperature limit due to the maximum working temperature of SU-8 photoresist. The thermoelectric properties of the thin films are worse than those obtained in bulk materials. To avoid the temperature limits on film deposition, the thermoelectric legs are patterned with the wet-etching technique [5]. However,

the influence of the etchant composition in the etch rate and pattern quality must be investigated because it involves the hazardous acids in the process.

In this study, we proposed the flexible TEG using double side FPCB (flexible printed circuit boards). The proposed TEG has the unique horizontal configuration. The heat of the flexible thermoelectric generator flows vertically via plated through hole (PTH). The TE legs are fabricated by the stencil lithography. The stencil lithography provides high substrate temperature required to fabricate high-quality thermoelectric films. The major components of the proposed flexible TEG are p-type Sb-Te and n-type Bi-Te thin films. We have characterized the properties of TE thin film and the performance of the fabricated flexible TEG. DESIGN AND FABRICATION

Figure 1 shows the drawing of the proposed flexible TEG which has the horizontal configuration. In the proposed flexible TEG, thermoelectric (TE) legs are placed in horizontal direction and linked in series electrically. Compared to previous work, the heat of the proposed flexible TEG flows to the vertical direction with aid of cover-layer and plated through hole (PTH). Cover-layer is composed of adhesive and polyimide film which has low thermal conductivity. PTH plays a role as an electric and thermal interconnection between top and bottom electrode.

(a) Front side (b) Back side

(c) Previous work (A-A’ section in (a))

(d) This work (A-A’ section in (a))

Fig. 1: The proposed flexible TEG.

Fig. 2 shows the fabrication process of the proposed flexible TEG. The major fabrication process steps entail the double side FPCB fabrication process for the metal electrodes, the stencil lithography for the TE thin film deposition and the cover-layer bonding process. The 4 inch double side FPCB is used for the device fabrication. In the double side FPCB, the upper and lower copper sheet are attached to the polyimide sheet by adhesive. We used the ready-made double side FPCB which has the 1/3 oz copper per square feet. Its initial thickness of copper is 12 mm. For the more flexibility, the thinner TE material is desirable. But the thickness of copper gets thick by electroplating the PTH. In this study, the electroplating is suppressed in the contact part between the copper sheet and the TE thin film by means of dry film resist (DFR #1). So, the good step coverage is obtained in the contact part between the copper sheet and the TE thin film. After the electroplating the PTH, the copper thickness of DFR #1 area is 12 mm while that of the other area is 18 mm (Fig 2.(d)). DFR #2 is used to make the electrode pattern and the Au elecroless-plating is taken to prevent the oxidation of copper. The dummy pattern in Fig. 2(g) is used to protect the thermal mismatch between the TE thin film and the polyimide sheet. The TE thin film is deposited using co-evaporator and patterned by the stencil lithography for the thermoelectric property enhancement and the simple fabrication process. In the stencil lithography, the limitations involved by PR can be overcome and the patterned TE thin film is made in a single process step. For the good step coverage of the thermoelectric legs, the shadow mask needs to be very thin. In this stencil lithography, the metal shadow mask made by invar is used and its thickness is 50 mm. The metal shadow mask is aligned with the previously made electrode pattern by the custom-made aligner. When co-evaporating, the rotating substrate holder is heated at 200 ℃ and the base pressure is 1.0e-7 Torr. The high purity (99.999%) Bi, Te and Sb source are used. The evaporation rate of N-type TE material is 0.33 Å/s for Bi and 1 Å/s for Te and that of P-type TE material is 2 Å/s for Bi and 4 Å/s for Te. Co-evaporated TE thin film thickness was about 10.0 μm for p-type and 17 μm for n-type. Finally, the laser-cutted cover-layer is bonded

with the hot press which has 145℃ and 50 MPa condition. After the cover-layer bonding, the fabricated flexible TEG takes the annealing at 250℃ for 3 hour to enhance the thermoelectric property. Fig. 3 shows the fabricated flexible TEG.

(a) Double side FPCB

(b) Drilling and DFR #1 patterning

(c) Electroplating for PTH

(d) DFR #1 remove

(e) DFR #2 patterning

(f) Etching for Electrode patterning

(g) DFR #2 remove & Au electroless-plating

(h) TE film deposition (N-type & P-type)

(i) Cover-layer bonding

Fig. 2: Fabrication process (cross section A-A` in Fig 1(a)).

(a) Front side (b) Back side

Fig. 3: The fabricated flexible TEG.

CHARACTERIZATION TE Thin Film

Fig. 4 shows the surface SEM image of the TE thin film. P-type (Sb-Te) thin film has the larger grain size. The roughness of n-type (Bi-Te) thin film is not so good. Generally, the smooth surface is desirable in

TE thin film. We presume that the roughness is originated from adhesive. The EDS analysis data of TE thin film is shown in Table 1. Generally TE content above 60 at.% is desirable. The measured properties of the fabricated TE thin film in Table 2 show that the TE thin film’s performance is enhanced by annealing.

(a) Bi-Te (b) Sb-Te

Fig.4: SEM image of TE thin film.

Table 1: EDS analysis of TE thin film.

N type P type

Bi Te Sb Te As-deposit 37.21 62.79 49.13 50.87

After anneal 37.89 62.11 48.51 51.49 (unit: atomic ratio)

Table 2: The measured properties of TE thin films.

n/p

(1019cm-3) μ

(cm2/vs) ρ

(μΩm) α

(μV/K) α2/ρ

(mW/K2m)

N type As-deposit -5.64 10 110 -129 0.15 After anneal -3.70 11 158 -128 0.10

P type As-deposit 7.52 20 40 117 0.33 After anneal 2.26 106 26 146 0.82

TEG Performance

Fig. 5 shows the experimental set-up of flexible TEG. Two soucemeter™ (Keithley 2400) are used for the current and voltage measurement and two jigs are placed for connecting hot plate and heat sink with the flexible TEG. The flexible TEG is heated by hot plate and the temperature difference is checked by measuring the temperature of hot site jig and cold site jig via thermocouple (T/C). The initial internal electrical resistance of the flexible TEG is 465 ohm. Fig. 6 shows the generated power output at a given temperature difference. At the matched load case, maximum output power is generated. It shows the maximum power output 1.5 mW at temperature difference of 137 K.

Fig. 5: Experimental set-up.

0 10 20 30 40 50 600.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Powe

r Out

put(m

W)

Voltage(mV)

DT=37 K DT=62 K DT=87 K DT=113 K DT=137 K

Fig.6: Generated power output.

Flexibility

The flexibility of the proposed flexible TEG has been experimentally investigated. To proof the status of flexible TEG in a non-planar state, the flexible TEG is attached to the pipes of different radii and their internal electrical resistance is measured (Fig. 7(b)). The results of the experiment are given in Fig. 8. No significant change of electrical resistance is observed at more than 20 mm diameter. Full functionality may be assumed since any damage to the flexible TEG would result in a abrupt increase (breaking of contact or crack growth in TE thin film) or decrease (electrical short of flexible TEG) of the electrical resistance. The achieved flexibility of TEG allows for installation onto non-plat surfaces. The small bending diameter of TEG will even permit attachment to the thin pipes and the human finger.

(a) Bended flexible TEG (b) The flexible TEG on pipe

Fig. 7: Flexibility test.

0 10 20 30 40 50 60

0.6

0.8

1.0

1.2

1.4

1.6

Elec

tric

Resis

tanc

e(kW

)

Pipe Diameter(mm) Fig.8: Flexibility test result.

CONCLUSION

The flexible TEG has the portability and can be installed on the non-flat surface. For the implementation of the flexible TEG, the soft substrate using FPCB is adopted. The proposed flexible TEG using double side FPCB which are in horizontal configuration has the plated through hole (PTH) and the cutted cover-layer. PTH and cover-layer make the vertical heat flow even at the horizontal configuration. By making vertical heat flow, the proposed flexible TEG takes the advantage of the vertical TEG configuration. TEG in vertical configuration is easily installed on the various application places because the cold site and hot site is not on the same plane in most cases.

We have characterized the properties of TE thin film and the performance of the proposed flexible TEG. The maximum power output is 1.5 mW at temperature difference of 137 K. No significant change of electrical resistance is observed at more than 20 mm diameter in flexibility test. The proposed flexible TEG shows the feasibility of installation on non-flat surface and application to the various fields.

REFERENCES [1] Wulf Glatz, Etienne Schwyter, Lukas Durrer,

and Christofer Hierold, Bi2Te3-Based Flexible Micro Thermoelectric Generator With Optimized Design, Journal of Microelecromechanical Systems, vol 18, no. 3, June 2009.

[2] Dzung Viet Dao, Akohiro Miyaoka, Susumu Sugiyama, Hiroshi Ueno, and Kouichi Itoigawa, Development of a Flexible Thermopile Power Generator Utilizing BiTe-Cu Thin Films, IEEJ Trans. SM, vol. 128, no. 9, 2008.

[3] Yasuhiro Iwasaki and Masatoshi Takeda, Development of Flexible Thermoelectric Device: improvement of Device Performance, International Conference on Thermoelectrics, 2006.

[4] da Silva, Fabrication and measured performance of a first-generation microthermoelectric cooler, J. Microelectromech. Syst. v14 issue 5, p1110-1117 (2005).

[5] L.M. Goncalves, Thermoelectric micro converters for cooling and energy-scavenging systems, J. Micromech. Microeng.,18 (2008).