THE DEVELOPMENT OF THE MICRO-GENERATOR ON THE SUBSTRATE BASED THIN FILM

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1 Proceedings of MNHT2008 Micro/Nanoscale Heat Transfer International Conference January 6-9, 2008, Tainan, Taiwan MNHT THE DEVELOPMENT OF THE MICRO-GENERATOR ON THE SUBSTRATE BASED THIN FILM Jun-ichiro Kurosaki Kitakyushu, Fukuoka , JAPAN Saburo Tanaka Kitakyushu, Fukuoka , JAPAN Koji Miyazaki Kitakyushu, Fukuoka , JAPAN Hiroshi Tsukamoto Kitakyushu, Fukuoka , JAPAN ABSTRACT We fabricated bismuth-telluride based thin films and their in-plane thermoelectric micro-generators (4mm 4mm) on a glass substrate by using the flash evaporation method through shadow masks. We prepared fine powders of Bi 2.0 Te 2.7 Se 0.3 (ntype) and Bi 0.4 Te 3.0 Sb 1.6 (p-type). The shadow masks are fabricated by standard micro-fabrication processes such as nitridation of silicon, dry etching and wet etching. The output voltages of micro-generators are lower than that of a thermoelectric generator based on bulk materials. The main reason is because the temperature difference between cool and hot junctions of the micro-generator is small compared to a thermoelectric generator based on bulk materials. In this study, the micro-generators were fabricated on a silicon nitride substrate based thin film. By fabricating the micro-generator on the thin film substrate, a large temperature difference between cool and hot junctions is obtained due to the thin film effect and the heat radiation to air of the thin film substrate. At the silicon nitride substrate based thin films, the thermal conductivity is significantly reduced by 1.2 W/ (m K). The thin film substrate is prepared by applying the fabrication processes used for shadow masks. The silicon nitride substrate based thin film is fabricated by nitridation of silicon and then back etching the silicon wafer. The fabricated substrate thickness is 2.5 µm and 4.5 µm (4 mm 4 mm). The temperature between cool and hot junctions is measured by using the noncontact thermometer which senses the far-infrared radiation. The output voltage of the micro-generator based thin film is measured by giving a temperature difference by heating the bottom of the silicon nitride substrate based thin film. Keywords: Thermoelectricity, MEMS, Micro-generator, Bismuth telluride, Wasted heat recovery NOMENCLATURE T Temperature, K V Output voltage, V α Seebeck coefficient, V/K π Peltier coefficent, V INTRODUCTION Recently many portable devices have been developed by advanced miniaturization technology. In such portable devices, wire-less network devices which can operate without userinteraction can achieve much which was impossible before. For example, radio frequency tags (RF-tags) exist as such wireless network devices. This device has ID information in tag, for example, this device makes the management of goods distribution easy and accurate (SCM: Supply Chain Management). In addition the RF-tags can be used for record keeping, article management and personal presence monitoring management. However, the RF-tags must receive an electricity supply from a small battery to operate. Therefore, various small 1 Copyright 2008 by ASME

2 Powder vessel Substrate Guide Tungsten boat Glass chamber the thin film devices. We evaluated the Seebeck coefficients of the micro-generators through Kelvin's law. The silicon nitride substrate based thin film is fabricated using micro-fabrication processes. This type of substrate can be given a great temperature difference and heated easily from the bottom of substrate by thermal conduction to the substrate. For example, if the micro-generator is evaporated on this substrate, it can be used for a health-meter embedded in the body as well as a TPMS (Tire Pressure Management System) by getting heat energy from the body and the brake pad of the vehicle. Vacuum Fig. 1 Schematic diagram of flash evaporation reactor. batteries for such as wireless network devices have been a focus of development. For example, micro-turbines [1] and micro-fuel-cells [2], etc. have been developed. However, these devices have problems in that they require complex machinery, fuel servicing and maintenance. A thermoelectric element does not have such complex machinery and the necessity to be supplied fuel and be maintained. In this study, we fabricate a micro-generator by an in-plane thermoelectric element to operate small portable devices. Bismuth-telluride alloys are used as the thermoelectric material. Bismuth-telluride alloys are the most popular materials at room temperature thermoelectric materials because of their high thermoelectric properties [3]. In particular, thin film technology is advantageous for miniaturizing device size. However, this technology is not perfect and there are some issues. The main issue of the thin films is relativity poor transport properties compared to the bulk material. To improve the thin film properties, many thin film processes have been attempted such as flash evaporation [4, 5], MBE [6, 7], MOCVD [8, 9], PLD [10], electrodeposition [11, 12], and inkjet printing [13]. In this study, we employ the flash evaporation method for the fabrication of bismuth-telluride based thin films. This deposition method is able to have low production cost because of its simple design that consists only of a vacuum chamber with a particle holder, tungsten boat for evaporation and a substrate holder. On the other hand, the performance of the thin films is relativity poor without the annealing process. In fact, annealing processes are known to enhance the transport properties [14]. We fabricated both n-type and p-type bismuth-telluride based thin films using the flash evaporation method. The thin film thermoelectric micro-generators are fabricated by using shadow masks. The overall size of the thin film devices, which consist of 16 pairs of legs connected by copper electrodes, is 4mm by 4mm. We measure the output voltage near room temperature as a function of the YAG laser power of EXPERIMENT Flash evaporation method Both n-type and p-type bismuth-telluride based thin films are prepared by a flash evaporation method. A schematic of our deposition setup (ULVAC VPC-260) is illustrated in Fig. 1. The vacuum chamber contains a powder vessel with guide, tungsten boat for evaporation and a substrate holder. The distance between tungsten boat and substrate is 30 mm. The tungsten boat contains a slight pocket (50 mm length, 10 mm width, 2 mm depth) to prevent the powders from spilling out from the boat. The guide is made from stainless steel and covered with a Teflon thin film to help the powders pass through smoothly to the tungsten boat. For flash evaporation, we first load 5 g of powder in the vessel, and place the glass substrate (Pyrex 7740, 50 mm length, 50 mm width, 1.1 mm thick) on the holder. The chamber is evacuated to Pa. A current of 80A is then applied to the tungsten boat until the substrate temperature reaches 200 o C. Finally, by tilting the powder vessel gradually, the powders are fed to the tungsten boat which evaporates them on the glass substrate. Shadow masks for p-type, n-type and their junctions The bismuth-telluride based thin film thermoelectric devices are fabricated by the flash evaporation method through shadow masks. The shadow masks are fabricated using a standard micro-fabrication process as shown in Fig. 2. We prepared silicon wafer. The silicon nitride thin film is deposited on the silicon wafer by P-CVD (Plasma Chemical Vapor Deposition). For patterning configurations of each shadow mask, we spread photoresists on the silicon nitride thin film, then the photoresists are removed in our arbitrary pattern by exposing and developing. Silicon nitride thin films are etched by RIE(Reactive Ion Etcher) in the same pattern with developed photoresist. Finally, remaining silicones are etched by soaking in KOH, thus shadow masks are fabricated with 2 Copyright 2008 by ASME

3 Silicon nitride Silicon Plasma Chemical Vapor Deposition Photoresist spreading (a) (b) (c Fig. 3 Photo of shadow masks: (a) For p-type bismuth-tellurides, (b) For n-type bismuth-tellurides, (c) For electrodes. development Reactive Ion Etching (a) (b) KOH Etching Fig. 2 Micro-fabrication processes for making a shadow mask of silicon with silicon nitride. arbitrary pattern. Photographs of fabricated shadow masks are shown in Fig. 3. (c) (d) 5mm Micro-generator on glass substrates The bismuth-telluride based thin film thermoelectric generators are fabricated by the flash evaporation method. The thin film generators are deposited on glass substrates (Pyrex 7059) using patterned shadow masks for the p- and n-type thin films and their junctions. Fig. 4 shows a photograph of the thin film micro-generators. The dimension of the p- and n-type legs is mm (length) 0.2 mm (width) 1.0 µm (thickness) and the spacing between the legs is 0.2 mm. The overall size of the thin film generator on the glass substrate is 4 mm 4 mm, and 1.0 mm thick, consisting of 16 (or 8) p/n couples. We measure the output voltages of the thin film thermoelectric generators while heating at the center of the micro- generators by using a YAG laser through a microscope (Fig. 5). Measurement of the thermoelectric properties of the micro-generator We measure the output voltages of the thin film thermoelectric devices at room temperature while heating at the center of the devices by a YAG laser instead of measurements of Seebeck coefficients, α. The Seebeck coefficients can be evaluated by Kelvin's law α = V / T. The output voltages of thin film Fig. 4 Photo of fabricated micro-coolers with leg length: (a) 0.5mm (16 p/n pairs), (b) 1.5mm(8 p/n pairs) and 0.5mm(8 p/n pairs), (c) 3mm (4 p/n pairs) and 1.5mm (8 p/n pairs) 0.5 (4 p/n pairs), (d) 1.5 (8 p/n pairs). thermoelectric devices are measured as a function of the heating power of the YAG laser as shown in Fig. 5. The output voltages of the micro-generators are proportional to output power of the YAG laser. The maximum output voltage is 6.7 mv measured by Model c. The imposed temperature gradient is parallel to the length of the thermoelectric legs. In order to measure the temperatures at the surface of glass substrate, thermocouples (chromel alumel) are attached near the focused YAG laser spot by pressing the thermocouple bead into an indium ball on the substrate. The temperature difference between cool and hot junctions is measured at 2.5 K through thermocouple measurements. Therefore the Seebeck coefficient is evaluated to be 168 µv/k (=6,700 µv / 2.5 K/ 16 junctions) per pair. If the temperature of a silicon nitride substrate based thin film is measured by the same method as the glass substrate in the same conditions, the thin film must be broken. However, the temperature difference estimated with its thermal resistivity is calculated to be approximately 410K, neglecting heat losses 3 Copyright 2008 by ASME

4 Monitor CCD Camera YAG Laser Optical Microscope Sample Fig. 7 Photo of micro-generator on the silicon nitride substrate based thin film. e Fig. 5 Experimental setup for thermoelectric voltage measurements of the micro-devices. Thermo meter & Thermo couple Radiation thermometer Sample Soft contact arm & Precise probe Heating 5mm Fig. 6 Photo of silicon nitride substrate based thin flilm. by radiation and convection, because the thermal resistivity of the silicon nitride substrate based thin film is 208 times as large as the glass substrate. Micro-generator on the silicon nitride substrate based thin film. We fabricate a silicon nitride substrate based thin film as shown Fig. 6. The substrate is made up by a silicon nitride based thin film on the outside and a silicon wafer sandwiched in between. The thermal resistance of silicon nitride based thin films is very large, therefore a large temperature difference between thermoelectric legs can be obtained. If we heat the bottom of the substrate, the thermal conduction will run through the silicon nitride based thin film from the silicon. For this reason, a temperature difference between cool and hot junctions of micro-generators can be obtained by planing the substrate on any heating source. Digital multi meter Fig. 8 Experimental setup for the thermoelectric voltage measurements of the micro-generator on the silicon nitride substrate based thin film. The silicon nitride substrate based thin film is fabricated by applying same micro-fabrication processes of shadow masks. The silicon nitride thin films are deposited on both surfaces of the silicon wafer by P-CVD. For patterning one surface of the silicon wafer, photoresist is spread on the silicon nitride thin films. The photoresist is removed as our arbitrary pattern by exposing and developing. Then, the silicon nitride thin films are etched by RIE(Reactive Ion Etcher) in the same pattern of developed photoresist at only one surface, finally, we etch the remaining silicon at the patterned surface by soaking in KOH. The dimension of the fabricated silicon nitride based thin film is 4 mm (length) 4 mm (width) 4 µm (thickness). A photograph of the silicon nitride substrate based thin film is shown in Fig. 6, the silicon nitride based thin films are allowed to show. We fabricate micro-generators on silicon nitride substrate based thin films by the flush evaporation method through the shadow masks. Figure 7 shows a micro-generator on silicon nitride substrate based thin films. The Area of dark green is 4 Copyright 2008 by ASME

5 V, mv Model a Model b Model c Model d 1 2 P, W Fig. 9 Themoelectric voltage of the micro-devices heated by YAG laser. thick silicon and thin silicon nitride layers of 304 µm thickness, and the area of yellow substrate is only the thin silicon nitride film of 4 µm. Thermoelectric elements are placed in area of yellow silicon nitride substrate based thin film. Measurement of thermoelectric voltage at the microgenerator on the silicon nitride substrate based thin film We measure the thermoelectric voltage of the thin film thermoelectric generators on the silicon nitride substrate based thin film while heating at the bottom of the substrate (Fig. 8). The output voltages are measured by connecting precise probes at copper pads of micro-generator. The output voltages of the micro-generator is 2 mv or above during the temperatures of substrate are o C. RESULTS AND DISCUSSIONS We measured the output voltages of the thin film thermoelectric devices at room temperature while heating at the center of the devices by a YAG laser since direct measurement of the Seebeck coefficients, α is not possible. The Seebeck coefficients can often be evaluated by Kelvin's law α = V/ T. The output voltages of thin film thermoelectric devices are measured as a function of the heating power of the YAG laser as shown in Fig. 9. The output voltages of the micro-generators are proportional to output power of the YAG laser. In principle, the output voltage of a micro-generator is greatly affected by the temperature distribution of the substrate due to the slenderness of micro-generators. These results show that if the thermal conduction of the substrate can be manipulated, the output voltages will increase. The maximum output voltage is 6.7mV measured by Model c(fig. 4). The higher output voltages always mean higher density of thermoelectric elements in micro-generator. We can consider that the output 3 4 V, mv T, K Fig. 10 Thermoelectric voltage by heating from bottom of substrate. voltages are related to the density of thermoelectric elements in the micro-generator. The temperature difference between cool and hot junctions is measured as 2.5 K through thermocouple measurements. Therefore the Seebeck coefficient is evaluated to be 168 µv/ K (=6,700 µv/ 2.5 K/ 16 junctions) per pair. The measured Seebeck coefficient is less than half that of bulk materials because the film is not annealed. Takashiri et al. describe that the annealing in hydrogen improves the Seebeck coefficient of bismuth-telluride based thin film to µv/k (p-type) and µv/k (n-type) [5,14,15]. We measured the output voltages of the thin film thermoelectric devices on the fabricated substrate at room temperature while heating at the bottom of substrate. The measurement result is shown in Fig. 10. The output voltages of the micro-generator is 2 mv at a temperature difference of 3.4 K with the heating temperature of 34 o C, and then microgenerator maintained a constant output voltage while heating at a constant temperature. However, the measurement of temperatures is not precise. The temperature of thin film substrate can be evaluated by applying one-dimensional heat transfer analysis in a fin-style model. Therefore, the performances of the micro-generator on the thin film substrate can be estimated by evaluated temperatures. CONCLUSION Flash evaporated bismuth-telluride based thin film microgenerators were fabricated using shadow masks. The thin film generators consist of 16 p/n(or 8 p/n) couples of legs connected by copper electrodes. The dimension of p- and n-type legs is mm (length) 0.2 mm (width) 1.0 µm (thickness) and the spacing between legs is 0.2 mm. The maximum output voltage of 6.7 mv is obtained through a 2.5K temperature difference in Model c, and the thermoelectric voltage corresponds to 168 µv/k Seebeck coefficient per p/n leg. That is half the value of the bulk material value because 5 Copyright 2008 by ASME

6 the annealing of the thin film is not carried out this time. The performance of the micro-devices will be improved to 400 µv/k per pair which is larger than that of bulk by the annealing process. The silicon nitride substrate based thin film is prepared by using micro-fabrication processes. The micro-generators are fabricated on the thin film substrate. For giving the temperature difference in the micro-generator, the bottom of substrate is heated by a heating source without special equipments such as YAG laser. The output voltages of the micro-generator are 2 mv with the temperature difference of 4K and a heating temperature of 34 o C. ACKNOWLEDGMENTS The authors would like to thank Semiconductor center in Kitakyusyu Science and Research Park regarding the microfabrication processes. REFERENCES [1] Spadaccini, C,M., Zhang, X., Cadou, C,P., Miki, N., Waitz, I,A., 2003, "Preliminary development of a hydrocarbon-fueledcatalytic micro-combustor" Sensors and Actuators A, Vol. 103, pp [2] Aravamudhan, S., Rahman, A,R,A., Bhansali, S., 2005, "Porous silicon based orientation independent, selfprimingmicro direct ethanol fuel cell " Sensors and Actuators A, [3] Rowe, D.M., 1995, "CRC Handbook of Thermoelectrics," Florida: CRC press. [4] Foucaran, A., Sackda, A., Giani, A., Pascal Delannoy, F., Boyer, A., 1998, "Flash evaporated layers of (Bi 2 Te 3 - Bi 2 Se 3 )(N) and (Bi 2 Te 3 -Sb 2 Te 3 )(P)," Materials Science and B, Vol. 52, pp [5] Takashiri, M., Shirakawa, T., Miyazaki, K., Tsukamoto, H., in press, "Fabrication and characterization of Bi 0.4 Te 3.0 Sb 1.6 thin films by flash evaporation method," Journal of Alloys and Compounds. [6] Shafai, C., Brett, M.J., 1997, "Optimization of Bi 2 Te 3 thin films for microintegrated Peltier heat pumps," Journal of Vacuum Science and Technology A, Vol. 15, pp [7] Inoue, T., Miyazaki, K., 1999, "Molecular deposition and thermoelectric evaluation of bismuth telluride films," Thermal Science &, Vol. 7, pp [8] Giani, A., Pascal-Delannoy, F., Boyer, A., Foucaran, A., Gshwind, M., Ancey, P., 1997, "Elaboration Bi 2 Te 3 by metal organic chemical vapor deposition," Thin Solid Films, Vol. 303, pp.1-3. [9] Giani, A., Boulouz, A., Pascal-Delannoy, F., Foucaran, A., Boyer, A., 1998, "MOCVD growth of Bi 2 Te 3 layers using diethyl tellurium as a precursor," Thin Solid Films, Vol. 315, pp [10] Dauscher, A., Thomy, A., Scherrer, H., 1996, "Pulsed laser deposition of Bi 2 Te 3 thin films," Thin Solid Films, Vol. 280, pp [11] Takahashi, M., Katou, Y., Nagata, K., Furuta, S., 1994, "The composition and conductivity of electrodeposited Bi- Te alloy films," Thin Solid Films, Vol.240, pp [12] Miyazaki, Y., Kajitani, T., 2001, "Preparation of Bi 2 Te 3 films by electrodeposition," Journal of Crystal Growth, Vol. 229, pp [13] Miyazaki, K., Iida, T., Tsukamoto, H., 2003, "Microfabrication of Bi 2 Te 3 by using micro-jet," Proceedings of 22th International Conference on Thermoelectrics, pp [14] Takashiri, M., Shirakawa, T., Miyazaki, K., Tsukamoto, H., 2006, "Fabrication of n-type bismuth-telluride thin films by flash evaporation method," Transactions of the Japan Society of Mechanical Engineers A, Vol. 72, pp (in Japanese). [15] Takashiri, M., Shirakawa, T., Miyazaki, K., Tsukamoto, H., 2007, "Fabrication and characterization of bismuthtelluride-based alloy thin film thermoelectric generator by flash evaporation method," sensor and actuators A, Vol. 138, pp Copyright 2008 by ASME