Fabrication of a Fully-Integrated Multiwatt µTurboGenerator

here is a need for compact, high-performance power sources that can outperform the energy density of modern batteries for use in portable electronics, autonomous sensors, robotics, and other applications. The current research is aimed at fabricating a fully-integrated, multiwatt micro turbogenerator on silicon that canproduce 10 W DC output power One of the main challenges involves the seamless integration between silicon and the magnetic components required to generate power. The generator requires a NiFe soft magnetic back iron and laminated stator for flux redirection as well as NdFeB permanent magnet pieces to serve as flux sources. In addition, copper windings must be fabricated above the laminated stator to couple to the alternating flux in order to extract electrical power from the machine.

Great strides have been made in the past year to quantify the requirements on the magnet pieces that will go into the rotor housing. Manufacturing accuracy of the pieces is critical because variations in the magnet geometries create an overall rotor imbalance, which can cause the rotor to crash during transcritical operation. A procedure in which the gaps around the magnet pieces are filled with solder and then polished back using chemical-mechanical planarization has been developed; this process can reduce the effective imbalance of the rotor by an order of magnitude.

The assembly and packaging procedure for the turbogenerator is also critical because the embedded permanent magnets cannot withstand temperatures much above 150 oC. This temperature restriction rules out the use of fusion bonding for the final die-level assembly after rotor insertion. Based on earlier results, an eutectic In-Sn bonding scheme that requires only 140 oC has been researched. In this scheme, Cr/Au is deposited on one bonding surface and Cr/Sn/In/Au is deposited on the other surface; both depositions are done using an e-beam evaporator without breaking vacuum. By painting no-clean flux on both surfaces and compressing the dies together on a hot plate, we form the bond.