Preparation and Characterization of Bi 2 Te 3 /Sb 2 Te 3 Thermoelectric Thin-Film Devices for Power Generation

Transcription

1 Journal of ELECTRONIC MATERIALS, Vol. 43, No. 6, 2014 DOI: /s Ó 2013 TMS Preparation and Characterization of Bi 2 Te 3 /Sb 2 Te 3 Thermoelectric Thin-Film Devices for Power Generation MIN-YOUNG KIM 1 and TAE-SUNG OH 1,2 1. Department of Materials Science and Engineering, Hongik University, Seoul , Republic of Korea ohts@hongik.ac.kr A device based on a new double-layer-leg thin-film concept has been successfully fabricated by flip-chip bonding of 242 pairs of n-type Bi 2 Te 3 and p-type Sb 2 Te 3 thin-film legs electrodeposited on top substrates to those processed on bottom substrates. Based on the output voltage current curve, the internal resistance of the double-layer-leg thin-film device was measured to be 3.47 kx at an apparent temperature difference of 25.9 K across the device. The actual temperature difference across the thin-film legs was estimated to be 3.51 K, which is 13% of the apparent temperature difference DT of 25.9 K applied across the thin-film device. The double-layer-leg thin-film device exhibited an open-circuit voltage of 0.43 V and maximum output power of 13.1 lw atan apparent temperature difference DT of 25.9 K. Key words: Thermoelectric device, thin film, bismuth telluride, antimony telluride, flip chip INTRODUCTION Micro power generators based on energy harvesting are in great demand for use in wearable electronics to replace bulky batteries with limited lifetime. 1 8 As one of the potential micro power sources based on energy harvesting, thermoelectric thin-film generators have drawn significant attention because they can be operated with minor temperature differences produced by small heat sources, such as body heat. 9 Furthermore, thermoelectric thin-film generators have various advantages, such as high power density, no moving parts, long lifetime, and high reliability. 1 8 Thermoelectric thin-film devices can be classified as having either an in-plane or cross-plane configuration based on the direction of heat flow through the device. Compared with in-plane thin-film devices, the cross-plane configuration is more suitable for micro thermoelectric generators because of the low electrical resistance and lack of parasitic heat flow through the substrate. 2 However, crossplane thermoelectric thin-film devices with vertical (Received June 29, 2013; accepted November 17, 2013; published online December 7, 2013) configuration suffer from various difficulties from the viewpoint of fabrication processing. To ensure an optimum specific power density for a cross-plane device, the thickness of the p-type and n-type thinfilm legs needs to be several tens of micrometers, leading to a long process time for thin-film deposition. 2,6,7 Furthermore, bonding of the upper electrodes to the thermoelectric thin-film legs is difficult, hindering the realization of cross-plane thermoelectric thin-film devices. 6 8 To resolve the processing drawbacks of such cross-plane thermoelectric thin-film devices, we propose a new concept for a cross-plane device, i.e., a double-layer-leg thin-film device, whose schematic configuration is shown in Fig. 1a. Compared with the conventional cross-plane thin-film device with a single-layer-leg configuration shown in Fig. 1b, the double-layer-leg thin-film device illustrated in Fig. 1a requires less processing time to form the thin-film legs and the gap between the top and bottom substrate can be easily increased to maintain a large temperature difference across the thin-film device. Furthermore, in this study, the thin-film legs on the top and bottom substrates were bonded together using a flip-chip process, as used in microelectronics packaging to mount semiconductor 1933

2 1934 Kim and Oh chips with small solder bumps or metallic bumps to a substrate for high-density, fine-pitch interconnection. 10,11 Flip-chip technology has strong potential for use in fabrication of cross-plane thermoelectric thin-film devices. 8 In this work, we fabricated such a double-layerleg thin-film device using flip-chip bonding of 242 pairs of n-type Bi 2 Te 3 and p-type Sb 2 Te 3 thin-film legs electrodeposited on both the upper and bottom substrates and evaluated its power generation characteristics. Fig. 1. Schematic illustrations of (a) the double-layer-leg thin-film device proposed in this study and (b) a conventional cross-plane thin-film device with single-layer-leg configuration before bonding of top and bottom substrates. EXPERIMENTAL PROCEDURES Figure 2 illustrates the processing steps for the double-layer-leg thin-film device. Both the top and bottom substrates of the thin-film device were made from Si wafers, having a thermal conductivity of 148 W/m K (comparatively high among nonmetals), 12 to minimize the thermal resistance of the substrates and eliminate the detrimental effect of thermal expansion mismatch between the top and bottom substrates. As shown in Fig. 2a, 0.1-lm-thick Ti, 0.5-lm-thick Cu, and 1-lm-thick Ti were sequentially sputtered onto a Si wafer as a seed layer for electrodeposition of Bi 2 Te 3 and Sb 2 Te 3 thin films. The Ti/Cu/Ti metallization was later used to form the bottom and top electrodes of the thin-film device. As shown in Fig. 2b, an AZ 4620 positive photoresist pattern of 10 lm thickness was formed to create 100-lmdiameter holes, and the Bi 2 Te 3 thin-film legs were electrodeposited from nitric acid aqueous solution containing 50 mm Bi-Te with Bi/(Bi + Te) mole fraction of 0.5 in 1 M HNO 3 at constant current of 12.5 ma/cm 2. As illustrated in Fig. 2c, an AZ 4620 positive photoresist pattern of 15 lm thickness was again formed to create 100-lm-diameter holes, and the Sb 2 Te 3 thin-film legs were electrodeposited from aqueous solution containing 70 mm Sb-Te electrolyte with Sb(Sb + Te) mole fraction of 0.9 at constant current of 1.25 ma/cm 2. Then, mechanical polishing was performed on the overgrown Bi 2 Te 3 and Sb 2 Te 3 thin-film legs in the photoresist template to obtain a smooth, flat surface as well as a Fig. 2. Schematic illustration of the fabrication processes of the double-layer-leg thin-film device: (a) sputtering of Ti/Cu/Ti metallization and photoresist (PR) patterning, (b) electrodeposition of Bi 2 Te 3 thin-film legs, (c) electrodeposition of Sb 2 Te 3 thin-film legs, (d) planarization, (e) electrodeposition of Ni/Sn capping layer, (f) electrode patterning, and (g) flip-chip bonding of top and bottom substrates.

3 Preparation and Characterization of Bi 2 Te 3 /Sb 2 Te 3 Thermoelectric Thin-Film Devices for Power Generation 1935 uniform height of the thin-film legs, as shown in Fig. 2d. The details of the electrodeposition process for Bi 2 Te 3 and Sb 2 Te 3 thin-film legs have been reported elsewhere. 8,13,14 After photoresist patterning was performed to open the top surfaces of the Bi 2 Te 3 and Sb 2 Te 3 thin-film legs, 1-lm-thick Ni and 3-lm-thick Sn were sequentially electrodeposited on top of the thin-film legs as an adhesion layer and bonding layer, respectively, as illustrated in Fig. 2e. Subsequent photoresist patterning and metallization etching were conducted to produce the electrodes underneath the n-type and p-type thinfilm legs, as shown in Fig. 2f. Scanning electron micrographs of the top and bottom substrates, which consist of 242 pairs of 10-lm-thick Bi 2 Te 3 and Sb 2 Te 3 thin-film legs with the Ni/Sn capping layer, are shown in Fig. 3. To connect the Bi 2 Te 3 and Sb 2 Te 3 thin-film legs on the top substrate to those on the bottom substrate, a flip-chip bonding technique was adapted using an anisotropic conductive adhesive (ACA, type LS210FP3; Fujikura Kasei Co., Ltd.). After dispensing the ACA onto the bottom substrate, the Bi 2 Te 3 and Sb 2 Te 3 thin-film legs on the top substrate were aligned and bonded to those on the bottom substrate by applying a bonding pressure of 52 MPa at 160 C for 30 s to form the double-layerleg thin-film device, as illustrated in Fig. 2g. The Seebeck coefficient (a) of the electrodeposited Bi 2 Te 3 and Sb 2 Te 3 thin films of 10 lm thickness was measured at room temperature by applying a temperature difference of 20 C across the two ends of the film. The electrical resistivity (q) was measured using a four-point probe, and the power factor (P) was evaluated using the relation P = a 2 /q. The power generation characteristics of the double-layer-leg thin-film device were measured by placing the device between a heat source and a water-cooled copper heat sink. The temperature difference DT across the thin-film device was measured using thin K-type thermocouples placed between the thin-film device and the heat source/sink surfaces. RESULTS AND DISCUSSION The composition and room-temperature thermoelectric properties of the Bi 2 Te 3 and Sb 2 Te 3 films of 10 lm thickness are listed in Table I. Although the Bi 2 Te 3 film was electrodeposited from a solution with Bi/(Bi + Te) mole fraction of 0.5, the composition of the Bi 2 Te 3 film was close to the Bi 2 Te 3 stoichiometry with slight Bi deficiency. This result can be attributed to the fact that the reduction rate of Bi 3+ /Bi is slower compared with that of HTeO + 2 /Te at a certain electrodeposition potential. 8,15 As shown in Fig. 4a, the electrodeposited Bi 2 Te 3 film exhibited a diffraction pattern where the (110) peak was the strongest, indicating that (110) preferred orientation was formed by stacking of the Te Bi Te layers parallel to the substrate. This type of (110) preferred orientation of electrodeposited Bi 2 Te 3 films has been frequently observed by others Although the Sb 2 Te 3 film was electrodeposited from an electrolyte with Sb/(Sb + Te) mole ratio of Fig. 3. Scanning electron micrographs of the (a) top and bottom substrates consisting of Bi 2 Te 3 and Sb 2 Te 3 thin-film legs and (b) Ni/Sn capping layer on thin-film legs. Table I. Composition and thermoelectric characteristics of electrodeposited 10-lm-thick Bi 2 Te 3 and Sb 2 Te 3 films Bi or Sb Composition (at.%) Seebeck Coefficient (lv/k) Electrical Resistivity (mx cm) Power Factor ( W/m K 2 ) Bi 2 Te 3 film Sb 2 Te 3 film

4 1936 Kim and Oh Intensity (arb. unit) (101) Sb (015) (1010) θ (degree) Fig. 4. x-ray diffraction patterns of 10-lm-thick (a) Bi 2 Te 3 and (b) Sb 2 Te 3 films. 0.9, the composition of the electrodeposited Sb 2 Te 3 film was slightly Sb-rich compared with the Sb 2 Te 3 stoichiometry, which could be attributed to the fact that the SbO + /Sb reaction has a more negative redox potential than that of the HTeO + 2 /Te reaction. 8,21,22 As shown in Fig. 4b, the x-ray diffraction pattern of the 10-lm-thick Sb 2 Te 3 film revealed a strong tendency for amorphization with additional diffraction peaks of metallic Sb and an unknown phase. Although electrodeposited Bi 2 Te 3 films are crystalline, Sb 2 Te 3 films electrodeposited in directcurrent mode at room temperature are commonly amorphous or poorly crystalline. 13,21 26 However, Sb 2 Te 3 films processed by direct-current electrodeposition at elevated temperature of 100 C or by pulse electrodeposition are reported to be crystalline. 21,23,25 As listed in Table I, the Bi 2 Te 3 film of 10 lm thickness exhibited a negative Seebeck coefficient of 59.5 lv/k, which is smaller than the values of 100 lv/k to 250 lv/k reported for bulk Bi 2 Te 3. 8,14,27,28 This result could be due to the high carrier concentration of /cm 3,whichissignificantly higher than the reported optimum value of /cm 3 to /cm The Seebeck coefficients and power factors reported in literature for electrodeposited n-type Bi 2 Te 3 films lie within the ranges of 40 lv/k to 100 lv/k and W/m K 2 to W/m K 2, respectively The thermoelectric properties of bulk Bi 2 Te 3 could be remarkably enhanced by creating n-type Bi 2 (Te,Se) 3 and p-type (Bi,Sb) 2 Te 3 tertiary alloys. 16,29 However, electrodeposited n-type Bi 2 (Te,Se) 3 films have not exhibited a noticeable improvement in thermoelectric properties compared with n-type Bi 2 Te 3 films. 16 Although substantial enhancement of the thermoelectric characteristics to values close to those of the bulk is obtained (110) (0015) (205) (a) (b) by incorporation of Sb, 16 electrodeposited (Bi,Sb) 2 Te 3 film is p-type and cannot replace the n-type Bi 2 Te 3 film. Table I states that the Sb 2 Te 3 film with 10 lm thickness had a high Seebeck coefficient of lv/k, which can be attributed to the amorphization of the Sb 2 Te 3 film. Reported Seebeck coefficients of amorphous Sb 2 Te 3 films processed by radiofrequency sputtering and vacuum evaporation are as high as 750 lv/k and 350 lv/k, respectively. 30,31 The electrodeposited Sb 2 Te 3 film exhibited a power factor of W/m K 2, which is significantly higher compared with that reported for crystalline Sb 2 Te 3 films, primarily due to the contribution of its large Seebeck coefficient. 8,32,33 The details of the thermoelectric properties of the electrodeposited Bi 2 Te 3 and Sb 2 Te 3 films can be found elsewhere. 8 Figure 5 shows an optical micrograph and crosssectional scanning electron micrographs of the double-layer-leg thin-film device fabricated by the flip-chip process using ACA. The gap between the top and bottom substrates was 30 lm. The conductive Ni balls in the ACA were trapped between the Ni/Sn capping layers of the top and bottom substrates, achieving electrical contact between the double-layer thin-film legs. The power generation characteristics of the double-layer-leg thin-film device consisting of 242 pairs of n-type Bi 2 Te 3 and p-type Sb 2 Te 3 thin-film legs were investigated at room temperature by applying a temperature difference (DT) of 7.4 K to 25.9 K across the top and bottom substrates. Figure 6 illustrates the output voltage current characteristics of the double-layer-leg thin-film device. The output voltage exhibited a maximum at the opencircuit condition and then linearly decreased with increasing output current, i.e., with decreasing external load. From the slopes of the output voltage current curves, the internal resistance r G of the thin-film device could be evaluated as 4.06 kx, 3.79 kx, 3.63 kx, 3.47 kx, and 3.47 kx at DT of 7.4 K, 10.6 K, 15.0 K, 21.0 K, and 25.9 K, respectively. The internal resistance r G of the thin-film device slightly decreased with increasing DT. The internal resistance r G is the sum of the material resistance r m and the interfacial resistance of the flip-chip-bonded joints r i. The material resistance r m comprises the resistances of the Bi 2 Te 3 legs, the Sb 2 Te 3 legs, and the Ni/Sn caps on the thin-film legs on the top and bottom substrates as well as the Ti/ Cu/Ti electrodes on the top and bottom substrates. The interfacial resistance r i corresponds to the resistance at the flip-chip-bonded interface between the Ni/Sn caps of the top and bottom substrates. Using the resistivities of the Bi 2 Te 3 and Sb 2 Te 3 thin films listed in Table I with values of X m, X m, X m, and X m as the resistivities of Cu, Ti, Ni, and Sn, respectively, the material resistance r m was calculated as 358 X. By subtracting the material resistance r m from the internal resistance r G of 4.06 kx to 3.47 kx, the interfacial resistance r i

5 Preparation and Characterization of Bi 2 Te 3 /Sb 2 Te 3 Thermoelectric Thin-Film Devices for Power Generation 1937 Fig. 5. (a) Optical micrograph and (b) cross-sectional scanning electron micrograph of a double-layer-leg thin-film device. Enlarged image of the flip-chip bonded interface in (b) is shown in (c). was calculated as 3.70 kx to 3.11 kx, depending on the temperature difference DT ranging from 7.4 K to 25.9 K across the thin-film device. This type of large interfacial resistance, 9 to 10 times the material s resistance, will cause a substantial temperature drop across the flip-chip bonded interface and deteriorate the performance of the thin-film device. The dependence of the open-circuit voltage on the apparent temperature difference DT applied across the thin-film device is shown in Fig. 7. The opencircuit voltage exhibited a linear increase with increasing DT and reached 0.43 V at DT of 25.9 K. The open-circuit voltage V 0 generated by a thin-film device can be expressed as V 0 = madt G, where m is the number of n p leg pairs, a is the combined Seebeck coefficient of an n p leg pair, and DT G is the actual temperature difference across the thin-film legs. 2,8 The slope of open-circuit voltage versus DT in Fig. 7 was 17.1 mv/k, which is significantly smaller than the value of mv/k estimated using the Seebeck coefficients of 59.5 lv/k and lv/k for the 10-lm-thick Bi 2 Te 3 and Sb 2 Te 3 thin films, respectively, as listed in Table I. This type of discrepancy between the measured and Fig. 6. Output voltage current curves of the double-layer-leg thinfilm device measured at various apparent temperature differences. estimated Seebeck coefficients of the thin-film device can be caused by a large difference between the apparent DT applied across the thin-film device

6 1938 Kim and Oh and the actual DT G across the thin-film legs, because most of the electrical resistance and thermal resistance of the thin-film device occurs at the flipchip joints. 8 Using the relation V 0 = madt G, the actual temperature difference DT G across the thinfilm legs can be estimated as 0.93 K, 1.43 K, 2.0 K, 2.82 K, and 3.51 K for an apparent temperature difference DT of 7.4 K, 10.6 K, 15.0 K, 21.0 K, and 25.9 K, respectively. The actual temperature difference DT G was 13% of the apparent temperature difference DT and ranged from 7.4 K to 25.9 K. Fig. 7. Open-circuit voltage of the double-layer-leg thin-film device as a function of the apparent temperature difference applied across the device. Fig. 8. Output power current characteristics of the double-layer-leg thin-film device measured at various apparent temperature differences. The output power current characteristics of the double-layer-leg thin-film device at various apparent temperature differences DT are plotted in Fig. 8. The maximum output power, obtained at the matched-load condition, increased with increasing DT and reached 13.1 lw atdt of 25.9 K, yielding a power density of 45.8 lw/cm 2. The maximum output power P 0 of the thermoelectric device under the matched-load condition can be expressed by Eq. 1. P 0 ¼ m2 a 2 TG 2 4r ; (1) G where m is the number of n p pairs, a is the summed Seebeck coefficient for an n p couple, r G is the internal resistance of the device, and DT G is the actual temperature difference across the n p pairs. 2 Substituting an a value of 501 lv/k, calculated from the Seebeck coefficients of the Bi 2 Te 3 and Sb 2 Te 3 films in Table I, anr G value of 3.47 kx, and DT G of 3.51 K into Eq. 1, the maximum power P 0 can be estimated as 13.0 lw at an apparent temperature difference DT of 25.9 K, which is in excellent agreement with the measured value of 13.1 lw. As reported for single-layer-leg thin-film devices, the thermoelectric performance of the double-layer-leg thin-film device could be substantially improved by reducing the interfacial resistance and thereby minimizing the temperature drop at the flip-chipbonded interface. CONCLUSIONS A double-layer-leg thin-film device was successfully fabricated using flip-chip bonding of 242 pairs of n-type Bi 2 Te 3 and p-type Sb 2 Te 3 thin-film legs electrodeposited on the top substrate to those processed on the bottom substrate. Based on the output voltage current curve, the internal resistance of the double-layer-leg thin-film device was determined to be 4.06 kx to 3.47 kx at apparent temperature differences of 7.4 K to 25.9 K. Approximately 90% of the internal resistance was attributed to the interfacial resistance of the flip-chip joints. Although the reason was not clear, the internal resistance decreased with increasing apparent temperature difference. Comparison of the measured Seebeck coefficient of the thin-film device with the value estimated using the Seebeck coefficients of the Bi 2 Te 3 and Sb 2 Te 3 thin films suggested that the actual temperature difference DT G across the thinfilm legs was 13% of the apparent temperature difference DT applied across the thin-film device. An open-circuit voltage of 0.43 V and maximum output power of 13.1 lw were obtained at an apparent temperature difference DT of 25.9 K. The maximum output power of the thin-film device could be substantially improved by reducing the interfacial resistance and thereby minimizing the temperature drop at the flip-chip-bonded interface. The thermoelectric performance of cross-plane-type thin-film devices is strongly dependent on the temperature

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