Electrochemical deposition is the key to mass-producibility.
Submillimeter-sized thermoelectric devices based on thick films of Bi2–xSbxTe3 are undergoing development. As electronic circuits are designed with ever greater packaging densities and power densities, there is an increasing need for small thermoelectric devices to provide active spot cooling for power amplifiers and other heat-generating electronic components. Bi2–xSbxTe3 is the preferred thermoelectric material for the operational temperature range (2–xSbxTe3 thermoelectric devices could also be used as electric-power generators; moreover, because of the smallness of the thermoelectric legs in these devices, the numbers of legs can be of the order of 100 times those of conventional bulk thermoelectric devices, making it possible to generate higher potentials (of the order of 100 V), that are more compatible with other electronic components.
The figure depicts a representative device of this type, wherein the Bi2–xSbxTe3 thermoelectric legs have lengths, widths, and thicknesses of the order of tens of microns. The thermoelectric legs are integrated with contact, diffusion-barrier, and electrode layers, all sandwiched between two thermally conductive, electrically insulating outer layers (made of AlN or synthetic diamond substrates) that are placed in contact with the heat source and heat sink, respectively. The Bi2–xSbxTe3 legs are made by electrochemical deposition from an aqueous solution; the other parts of the device are fabricated by various conventional integrated-circuit-fabrication techniques, including photolithography and vacuum deposition.
In comparison with such conventional semiconductor-fabrication techniques as sputter deposition, vacuum evaporation, and chemical vapor deposition (CVD), electrochemical deposition offers several advantages for fabrication of the Bi2–xSbxTe3 thermoelectric legs. Growth rates achievable by electrochemical deposition are in the range of tens of microns per hour — fast enough for mass production of devices in the desired size range — whereas lower growth rates achievable by the conventional techniques are better suited to fabrication of submicron-sized devices. The conventional techniques are poorly suited to fabrication of Bi2–xSbxTe3 devices because the vapor pressures of Bi, Sb, and Te are very different; on the other hand, the compositions of films grown by electrochemical deposition can be controlled via the concentrations of the constituents of the aqueous deposition solutions.
Substrates can be coated with metal interconnection busses and contact pads and then masked, all by conventional integrated-circuit-fabrication techniques. Then Bi2–xSbxTe3 thermoelectric legs can be electrodeposited onto the contact pads through the holes in the masks. Thus, conventional integrated-circuit-fabrication techniques can readily be combined with electrochemical deposition for mass production of submillimeter Bi2–xSbxTe3 thermoelectric devices.
A significant part of the development effort thus far has been devoted to the electrochemical deposition process. The general approach is to dissolve Bi, Sb, and Te in nitric acid, then put the resulting solution in an electrochemical cell, wherein Bi2–xSbxTe3 (which is insoluble in HNO3) is deposited on a cathode. Process parameters that must be optimized include the pH of the solution; the concentrations of Bi, Sb, and Te in the solution; the temperature of the solution; stirring of the solution; the deposition voltage and current density; the surface finish of the cathode; and post-deposition annealing. One of the findings of experiments conducted thus far is that the limited solubility of Sb in the solution can be enhanced by use of tartaric acid as a buffer.
This work was done by Jean-Pierre Fleurial, Margaret A. Ryan, Alex Borshchevsky, Wayne Phillips, Elizabeth Kolawa, G. Jeffrey Snyder, and Thierry Caillat of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category. NPO-20472