Piljoong Kang

Fabrication and high-speed characterization of SU-8 shrouded two-dimensional microimpellers

In this study, the performance of shrouded two-dimensional microimpellers was measured for application to a micro centrifugal pump used in a portable fuel cell system. Two types of microimpellers were designed, and fabricated by a multi-layer photolithography process using an ultrathick photoresist (SU-8 3000). Microimpellers with a diameter of 10 mm were tested using an air spindle up to maximum rotation speeds determined by the destruction of the microimpellers, which are 350 000 rpm for a purely-radial-outlet blade impeller and 450 000 rpm for a backward blade impeller. The purely-radial-outlet blade impeller showed a higher pressure rise—2.8 kPa at 150 000 rpm, 5.1 kPa at 200 000 rpm and 12.5 kPa at 300 000 rpm. The measured performance satisfies the requirement of the micro centrifugal pump. However, the measured pressure rises are roughly half of the simulated values. The influence of the radial clearance and height difference between the impeller and the diffuser was investigated using an electromagnetic motor and impellers with a diameter of 16 mm. Against expectation, a better pumping performance was obtained with a larger tip clearance, and the height difference did not affect the pumping performance. These unexpected results suggest that the tip clearance and the clearance between the impeller shroud and the stationary housing act as a diffuser.

Demonstration of a MEMS-based turbocharger on a single rotor

A MEMS-based turbocharger with a novel configuration was developed and demonstrated. The compressor and turbine are installed on the same plane of the rotor. The advantages of this configuration include structural simplicity and good rotor balance. For the fabrication, we developed a special deep RIE condition to realize spike-free, uniform etching. And, we found a low wafer warp condition of five times of the anodic bonding. The device rotated at 50?000 rpm. The tip speed of the compressor approached 25 m s?1 and it is only a twentieth of the aimed tip speed. The device could not pass the natural frequency of the rotor. From the test results, we found some issues to be solved for high speed rotation. Concerning fabrication, the accurate manufacturing of bearing orifices, and a flat, uniform etched surface are required for the thrust bearing. A straight journal bearing wall is required for the hydrostatic journal bearing. Concerning bearing design, the journal bearing does not have sufficient stiffness or load capacity to support the centrifugal force of the rotor because of the remarkably short bearing length.

Micro Turbocharger on a Single Silicon Rotor

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.Copyright

Cavity-Through Deep Reactive Ion Etching of Directly-Bonded Silicon Wafers

Etched silicon wafers were directly bonded, and then the bonded silicon wafers were deeply etched by Bosch process through etched cavities. This cavity-through DRIE (deep reactive ion etching) was investigated after systematic study on plasma-assisted silicon direct bonding. The first half of this paper mainly reports the generation of voids in wafers directly bonded with N2 or O2 plasma treatment due to H2O release from hydroxy groups in low and high temperature annealing. The second half reports etch profiles and etch rates for cavity-through DRIE using different recipes. Cavity-through DRIE in conjunction with silicon direct bonding is practical and useful for multi-layer MEMS.