MEMS/ECD Method for Making Bi<sub>2–x</sub>Sb<sub>x</sub>Te<sub>3</sub> Thermoelectric Devices
- Monday, 23 June 2008
Devices containing diverse materials in complex three-dimensional shapes can be fabricated.
A method of fabricating Bi2–xSbxTe3-based thermoelectric microdevices involves a combination of (1) techniques used previously in the fabrication of integrated circuits and of microelectromechanical systems (MEMS) and (2) a relatively inexpensive MEMS-oriented electrochemical- deposition (ECD) technique. The devices and the method of fabrication at an earlier stage of development were reported in “Sub milli meter-Sized Bi2–xSbxTe3 Thermoelectric Devices” (NPO-20472), NASA Tech Briefs, Vol. 24, No. 5 (May 2000), page 44. To recapitulate: A device of this type generally contains multiple pairs of n- and p-type Bi2–xSbxTe3 legs connected in series electrically and in parallel thermally. The Bi2–xSbxTe3 legs have typical dimensions of the order of tens of microns. Metal contact pads and other non-thermoelectric parts of the devices are fabricated by conventional integrated-circuit and MEMS fabrication techniques. The Bi2–xSbxTe3 thermoelectric legs are formed by electrodeposition, through holes in photoresist masks, onto the contact pads.
Heretofore, most MEMS have been made from materials compatible with silicon integrated-circuit processing, such as silicon, silicon dioxide, and silicon nitride. Moreover, commercial MEMS fabrication techniques have been mostly limited to structures of a substantially two-dimensional character. However, to be useful, thermoelectric microdevices must consist of a variety of materials (including metals and semiconductors), and complex, three-dimensional shapes are needed to effect the required series electrical and parallel thermal connections. In a typical case, the n- and p-type Bi2–xSbxTe3 thermoelectric legs must be connected electrically in series with bridging metal interconnections on top and bottom surfaces (see Figure 1). These interconnections are required to be have low contact resistance and high mechanical strength and must be capable of withstanding large current densities and temperature gradients. Most challenging is the requirement to form the p- and n-type tall, heavily doped compound semiconductor legs protruding from the same surface.
The present method overcomes the limitations of prior MEMS fabrication techniques and makes it possible to satisfy the aforementioned requirements. The method is implemented in a process (see Figure 2) that includes the following main steps:
- In a subprocess that includes sputtering, electrodeposition, photolithography, and etching, a bottom interconnection pattern of Au contact pads (typically 3 μm thick) is formed on a layer of Cr (typically 0.1 μm thick) that has been sputtered onto the oxidized upper surface of an Si substrate.
- A photoresist having a thickness corresponding to the desired height of the thermoelectric legs is deposited, exposed, and developed to form through-the- thickness holes ending at those portions of the Au contact pads to which the p-type thermoelectric legs are to be bonded.
- Sb2Te3 (p-type) thermoelectric legs are electrodeposited in the holes.
- A thin layer of photoresist is deposited to cover the tops of the p-type thermoelectric legs.
- Step 2 is repeated to form holes at the intended locations of Bi2Te3 (ntype) thermoelectric legs.
- The n-type thermoelectric legs are deposited in the holes.
- The thin top photoresist layer deposited in step 4 is removed.
- A partly sacrificial layer of gold <0.01 μm thick is deposited to ensure uniform ECD in step 10.
- A thicker top photoresist layer is deposited and patterned to form a mold for a top interconnection pattern of Ni contact pads.
- The Ni contact pads, typically 3 μm thick, are electrodeposited in the holes in the mold. The electrodeposition parameters are chosen to keep stresses in the pads low so that the pads do not pull off the thermoelectric legs.
- In a series of etches, the excess Cr between the bottom contact pads, the excess Au between the top contact pads, and the photoresist layers are removed.
A device containing 63 Bi2Te3 (n-type) and 63 Sb2Te3 (p-type) thermoelectric legs, each 20 μm tall and 60 μm in diameter, was fabricated by this method and demonstrated to be capable of thermoelectric cooling. This device can be considered a prototype of future devices for exerting precise thermal control in microscopic regions and for extracting small amounts of electric power from temperature gradients.