Micro Turbocharger on a Single Silicon Rotor

Abstract

This paper describes the design, fabrication and preliminary test of a MEMS-based turbocharger. In this device, a compressor and a turbine are formed on the same side of the rotor to escape miss-alignment during bonding process. The rotor is supported by hydrostatic journal and thrust bearings. The device rotated only at low rotation speed up to 2500 rpm, and the rotation speed did not remarkably changed by adjusting air supplies to the hydrostatic bearings. From the test results, we found important issues to be solved for high speed rotation. Concerning to fabrication, flat and smooth bearing surfaces is difficult to fabricate, and bearing/tip clearances are difficult to precisely control due to spikes on etched surfaces and the warp of the devices induced by anodic bonding. We developed special deep RIE recipe to realize spike-free, uniform etching. And, we found that the wafer warp became negligible at a bonding temperature of 320 °C and an applied voltage of 400–600 V, when a 1 mm thick Pyrex glass substrate was used. Even after 5 times anodic bonding to stack 6 wafers, the warp was below 2 μm. The other concern is on bearing design. The journal bearing has a very low L/D number (length divided by diameter). As a result, the journal bearing has a small journal surface, and it is difficult to install orifices on the journal bearing. Additionally, there is interference between the journal and thrust bearing due to air leakage.