1. Field of the Invention
The invention relates to a thermoelectric module device and fabrication thereof, and more particularly relates to a thin film thermoelectric module device.
2. Description of the Related Art
A thermoelectric module device is a device capable of transforming electricity into heat and/or heat into electricity, and can be applied in cooling/heating and generating electric power. When electricity is applied to a thermoelectric module device, heat absorption and heat release are generated on opposite ends of the device, thus allowing for cooling or heating. When two ends of a thermoelectric module device have temperature differences, the device can output direct current and can thus be used to generate electric power.
FIG. 1 shows a cross section of a conventional thermoelectric module device. As shown in this figure, a conventional thermoelectric module device typically comprises a bulk-shaped p-type thermoelectric material 108, a bulk-shaped n-type thermoelectric material 106, a bottom conductive metal layer 112, a top conductive metal layer 110, a top insulating substrate 104 and a bottom insulating substrate 102. As shown in FIG. 1, the p-type thermoelectric material 108 and the n-type thermoelectric material 106 are typically arranged vertically and both are connected by the bottom conductive metal layer 112 and the top conductive metal layer 110. Taking the application of inputting electricity for cooling for example, direction of the current transference in the p-type thermoelectric material 108 and the n-type thermoelectric material 106 is parallel to the direction of heat transference in the thermoelectric module device and heat absorption and release are generated at the top and bottom ends. Taking the application of generating electric power with temperature differences as an example, direction of the thermal flow in the p-type thermoelectric material and the n-type thermoelectric material is also parallel to the direction of electricity transference. However, this device does not have high efficiency because of the limitation of the figure of merit (ZT) of bulk-shaped thermoelectric material. Typically, the greatest cooling capacity is only 3˜5 W/cm2 and the electrical power generating efficiency with temperature difference of 200° C. is only 2˜3%. Therefore, it is desired for a thermoelectric module device to use high ZT thermoelectric material in order to improve thermoelectric transforming efficiency.
Professors Hick and Dresselhaus of Massachusetts Institute of Technology in year 1993 disclosed that ZT can be greatly improved when size of the thermoelectric material is reduced to nano-scale. Venkatasubramanian et al. of RTI research institute in year 2001 disclosed that the p-type Bi2Te3/Sb2Te3 super lattice thin film can have a ZT value of about 2.4 at room temperature, which breaks through the bottleneck of ZT values of less than 1. American Hi-Z company researched a p-type B4C/B9C and an n-type Si/SiGe quantum well thin film and estimated the thin film to have ZT value larger than 3. According to the research results above, thin film thermoelectric material has the advantage of a high ZT value and can break through bottlenecks associated with the conventional buck-shaped thermoelectric material. In addition, because a thermoelectric module device with a small size is easy to be formed, less material is used for thin film thermoelectric material. Also, thermoelectric material is used widely for cooling a micro electronic device micro or producing a high efficiency thermoelectric generator.
However, the thin film thermoelectric material is not highly efficient when directly used in conventional devices. Referring to FIG. 2, which shows a cross section of a conventional thermoelectric module device comprising thin film thermoelectric material. A p-type thermoelectric material thin film 216 and an n-type thermoelectric material thin film 210 are interposed between a top substrate 204 and a bottom substrate 202, and the p-type thermoelectric material thin film 216 and the n-type thermoelectric material thin film 210 are disposed on a metal column 206 and bottom conductive metal layer 212, and below the top conductive metal layer 208. The p-type thermoelectric material thin film 216 and the n-type thermoelectric material thin film 210 can be attached to the top substrate 204 by the top solder layer 214, and the metal column 206 is attached to the bottom substrate 202 by the bottom solder layer 218.
As shown in FIG. 2, when the thermoelectric thin film materials 216, 210 having high ZT are directly used in the conventional thermoelectric module device, the cooling/heating or electricity generating efficiency is not good, because the thermoelectric thin film materials 216, 210 are too thin (about tens of nanometers to tens of micrometers thick) and the heating and cooling sources of the thermoelectric module device are too close to generate heat flow transference back easily. Further, because the thermoelectric thin film materials 216, 210 are very thin, electricity and heat resistance between the thermoelectric thin film materials 216, 210 and the metal layer 208, 212 greatly affects the device, and joule's heating also reduces efficiency of the device. Hence, the device directly applied with the thermoelectric thin film materials 216, 210 having high ZT does not perform as well as expected.