Flexible Thin-film Planar Peltier Microcooler L.M. Goncalves 1, C. Couto 1, J.H. Correia 1, P. Alpuim 2, Gao Min 3, D.M. Rowe 3 1
University of Minho, Department of Electronics, 4800-058 Guimarães, Portugal. 2 University of Minho, Department of Physics, 4800-058 Guimarães, Portugal. 3 University of Cardiff, School of Engineering, Cardiff, United Kingdom. email:
[email protected]
Abstract The present work reports on the fabrication and characterization of a planar Peltier microcooler on flexible substrate. The microcooler was fabricated on flexible Kapton© polyimide sheet, 12 µm thick, using Bi2Te3 and Sb2Te3 thermoelectric elements, deposited by thermal coevaporation. The cold area of the device (4 mm2) is cooled using four pairs of thermoelectric elements, connected in series with aluminium / nickel contacts. Optimization of deposition parameters allowed the fabrication of films with power factors of 4.9×10-3 W.K-2.m-1 and 3.3×10-3 W.K-2.m-1 for Bi2Te3 and Sb2Te3, respectively. These values are comparable with the best published results for the same material, under various fabrication methods (thermal coevaporation, sputtering, MOCVD, flash-evaporation or ECD). The performance of the Peltier microcooler was analyzed by infrared image microscopy in still-air and vacuum. The temperature difference between the cold side and the hot side of the device was 4ºC.
configuration also allows for easier fabrication processes, since all the metal contacts are placed on the same plane, and all the structures are planar, avoiding bridge structures for top contact. A flat cold area is readily available at the centre of the device. Heat flow
Fig. 1: Conventional thermoelectric device.
Introduction Thermoelectric microcoolers with efficient cooling capacity, small area (less than a millimetre) and short response time are in high demand, especially if they lend themselves to integration with microelectronic circuits. Since Peltier devices are reversible, they can also be used as generators, converting thermal energy into electrical energy. Micro-thermoelectric generators can be used in a lot of small, low-power devices such as hearing aids or wrist watches. The conventional thermoelectric cooler (Fig.1), with the heat flux perpendicular to hot and cold areas, cannot be scalable to microchip dimensions, using the same fabrication methods used for macro scale devices. New microsystems technology must be used instead, based on thick/thin film technology [1] [2]. Deposition and pattern processes of such films (tens to hundreds of micrometers) are still under development, and a long deposition time is necessary, to achieve good materials. The fabrication of the top contact is also an issue on these devices, due to its bridge structure [3]. Böttner et al. avoided top contact constrains by using two wafers glued [1]. A planar design of device [4] [5], shown on Fig. 2, allows the fabrication of devices with smaller film thickness. Although the planar structure has lower heat-pump capability, it can still be useful for many applications. In addition, this
Fig. 2: Planar thermoelectric device. Several techniques have been used to fabricate bismuth telluride and antimony telluride thermoelectric films. These include: co-sputtering, electrochemical deposition (ECD), metal-organic chemical vapour deposition (MOCVD) or flash evaporation [6]. Some of them have higher growing rate, but with poor thermoelectric film quality. Some other require more expensive equipment, or a lengthy period to prepare the process or to fabricate the film. Co-evaporation is used in this work [7]. This technique proved to be a cost-effective process, with an excellent material quality, despite requiring a low deposition rate to obtain high quality materials. Tentative deposition of telluride films by direct evaporation of the bulk compound materials proved to be impossible due to the large differences in vapour pressure of bismuth and tellurium, resulting in a compositional gradient along the film thickness [8].
Glass, silicon and polyimide were used as substrates, with good film-to-substrate adhesion. However, for thermoelectric applications, 12 µm-thick polyimide film was chosen as substrate because of the low thermal conductivity (0.15 W.m−1.K−1) and appropriate value of thermal expansion coefficient (12×10-6 K−1) which closely matches the thermal expansion coefficient of the telluride films, thus reducing residual stress and improving adhesion. Flexible substrates (Fig. 3) add useful mechanical properties to the composite film-substrate and enable their integration with many novel devices [9].
By adjusting the fabrication parameters to the optimal values, the power factors of 4.9×10-3 W.K-2.m-1 and 3.3×10-3 W.K-2.m-1 were obtained for Bi2Te3 and Sb2Te3 films respectively [7]. The effect of substrate temperature and composition on film power factor is shown on Fig. 5 and Fig. 6. The best films are obtained at higher substrate temperature, with the composition near stoichiometry for Bi2Te3 films and Te rich for Sb2Te3 films. Table 1 summaries the thermoelectric properties of selected films, with Seebeck coefficient and electrical resistivity measured at room temperature. It can be seen in Fig. 7 and Fig. 8 that the crystal formation is visible and reveals grains of 500 nm approximately. XRD analysis was performed and materials identified by patterns 15-863 and 15-874. 6.0
Fig. 3: Flexible thermoelectric film.
Power Factor (x10 -3 W.K -2 .m -1 )
T sub=190ºC T sub=230ºC
5.0
T sub=270ºC 4.0 3.0 2.0 1.0 0.0 50%
55%
60%
Materials Optimization
65%
70%
75%
Te conte nt %
Fig. 5: Influence of substrate temperature and film composition on Bi2Te3 films power factor.
A thermal co-evaporation system shown in Fig. 4 was designed for fabrication of thermoelectric thin-films. Due to precise control of evaporation rate of each material and optimized substrate temperature, high quality films with high thermoelectric figure of merit are obtained.
3.0
Power Factor (x10 -3 W.K -2 .m -1 )
T sub=150ºC T sub=180ºC
2.5
T sub=220ºC 2.0 1.5 1.0 0.5 0.0 50%
55%
60%
65%
70%
75%
Te conte nt %
Fig. 6: Influence of substrate temperature and film composition on Sb2Te3 films power factor.
Table 1: Properties of selected films measured at 300K.
Fig. 4: Co-evaporation system.
80%
Temp. Tsub ºC
Comp.
Film #273
270
#320
220
(%Te)
Seebeck α µV.K-1
Resist. ρ µΩ.m
P.F. x10-3 W.K-2.m-1
62 %
-248
12.6
4.87
73 %
199
11.9
3.3
80%
Fig 7: Surface and cross-sectional images of a Bi2Te3 film deposited on glass, Tsub = 270 ºC and %Te = 62%.
The microcooler was placed on top of a metal block, with a hole in the center. The metal block serves as a heat sink to keep the hot junction temperature constant. A current of 4 mA was supplied to the device. Using an infrared microscope, a thermal image of the device was acquired (Fig. 10). The ptype leg and n-type (respectively on the left and right of Fig.10) have the cold side junction below base temperature and hot side junction above base temperature. The difference measured between hot side and cold side, under vacuum is around 4 ºC. The achieved temperature on cold side is 1.5ºC below base temperature. Fig 11 shows the temperature profiles along the thermoelectric legs. The top graph represents the p-type leg and bottom graph the n-type leg. A
metal pad
metal pad B
Sb2Te3
Bi2Te3 Fig 8: Surface and cross-sectional images of a Sb2Te3 film deposited on glass, Tsub=220ºC and %Te=73% metal pad Devices A microcooler was fabricated on a 12 µm polyimide substrate. Metal pads (1 µm of aluminium covered with 20 nm of nickel) were evaporated through a metal mask. On top of contacts, 10 µm-thick bismuth telluride and antimony telluride films were deposited, also through metal masks, resulting in eight thermoelectric legs. Seebeck coefficients of 170 µV.K-1 and 185 µV.K-1 and electrical resistivity of 18 µΩ.m and 15.5 µΩ.m was measured on n-type and p-type films respectively (these values were measured on glass substrate, fabricated on the same run of the device). Fig 9 shows a photograph of the fabricated microcooler.
A’
B’
Fig. 10: Thermal image of two legs of the microcooler (each colour represents different temperature).
A – A’
B – B’
Fig. 9: Thermoelectric microcooler on a flexible membrane.
Fig. 11: Temperature profile along p-type and n-type thermoelectric legs, respectively on top and bottom graph.
2. Thermal contact resistance between thermoelectric material and metal pad is higher than expected. The effect of this thermal contact resistance is visible in the large temperature difference between the thermoelectric elements and surround metal pad areas, mainly on hot side junction. 3. The electrical contact resistance is higher than that used in simulation. A resistance value of 2 Ω was measured between Bi2Te3 and metal pad and less than 0.2 Ω between Sb2Te3 and metal pad. These values correspond to a contact resistivity of 10-2 Ω.cm2 and 10-3 Ω.cm2 on a contact area of 0.5 mm2. Values between 10-3 Ω.cm2 and 10-6 Ω.cm2 have been reported [1] [3]. The resistance value of the entire device (eight thermoelectric legs) is 46 Ω, i.e. 11.5 Ω in each of the four pairs of elements. A resistance of 1 Ω was measured on the metal pads; 2 × (2 + 0.2) = 4.4 Ω was measured for contact resistance between thermoelectric elements and pads and 3 Ω was measured in each thermoelectric leg. The contact resistance was measured using a four probe method. A current was applied through the metal-thermoelectric element junction. The voltage was measured between a fixed point on metal side and consecutive points along the direction of current flow. It consists of two terms: an IR drop and the junction built-in voltage. By reversing the current sign it was possible to evaluate both terms. Fig. 12 shows the resistance, R, calculated from the IR drop, as a function of position and Fig. 13 shows the voltage drop after subtraction of the IR drop as a function of position. It can be seen that the IR drop largely supersedes the built-in voltage. Different materials are being studied for contacts in an attempt to reduce the contact resistance. A thin layer of nickel, chromium, gold, titanium or bismuth will be deposited on top of aluminium or copper pad and the corresponding contact resistivities and voltages will be compared.
10 Resistance from X=0 (O hm )
1. The power factor of the thermoelectric films incorporated in the microcooler (1.6×10-3 W.K-2.m-1 for n-type and 2.2×10-3 W.K-2.m-1 for p-type) were significantly smaller that the best values obtainable for these materials, e.g., 4.9×10-3 W.K-2.m-1 for n-type [7].
12
Material with higher contact resistance
8
Metal TE Pad element
6
4
Material with lower contact resistance
2
0 0
1
2
3 Pos ition X (m m )
4
5
6
Fig. 12: Calculated resistance along metal pad and thermoelectric leg. A contact with high resistance (7 Ω) and a contact with low resistance (0.5 Ω) are shown. 100 80
Material with positive contact voltage
60 40 Voltag e (uV)
The achieved temperature difference and the minimum achieved temperature on cold side are far from expected values calculated on simulations. A hot-side-to-cold-side temperature difference of 10 ºC and a cold side to base temperature difference of 5 ºC were expected. These differences are due to three main reasons explained bellow:
Metal TE Pad element
20 0 0 -20 -40 -60
1
2
3
4
5
6
Material with negative contact voltage
Pos ition (m m )
Fig. 13: Measured voltage (after subtraction of the IR drop) along metal pad and thermoelectric leg. Different materials used for contact give different junction built-in voltages. Conclusions Thermoelectric films (1 – 10 µm thick) with high figure of merit were fabricated and patterned using co-evaporation method. Power factors of 4.9 ×10-3 W.K-2.m-1 and 3.3 ×10-3 W.K-2.m-1 have been obtained in Bi2Te3 and Sb2Te3 films, respectively. These values compare well with the best published results for the same materials. Thin-film planar technology was employed for the fabrication of thermoelectric devices. A microcooler was fabricated and a temperature difference of 4 ºC was achieved, between the hot and cold side of the device. Apart from the figures of merit of materials, the thermal and electrical contact resistances affect significantly the performance of thermoelectric microcoolers. This work also demonstrated that the compatibility of flexible electronics with thermoelectric microcooler, which use a 12 µm-thick polyimide foil as substrate, due to its low thermal conductivity and high upper working temperature.
Acknowledgments
References
This work was supported by POCTI MPYROM and Portuguese Foundation for Science and Technology (SFRH/BD/18142/2004). The authors would like to thank Ralph Jones (Cardiff University) for performing EDX analysis, Helin Zou (Imperial College, London) for help in preparing co-evaporation system and F. Volklein (University of Wiesbaden) for performing thermal conductivity measurements.
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