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MEMS99_MicroBearing - Fabrication and Characterization of a...

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Fabrication and Characterization of a Micro Turbine/Bearing Rig Chuang-Chia Lin, Reza Ghodssi, Arturo A. Ayon, Dye-Zone Chen, Stuart Jacobson, Kenneth Breuer, Alan H. Epstein and Martin A. Schmidt Microsystems Technology Laboratories and Gas Turbine Lab Massachusetts Institute of Technology, Cambridge, MA 02139, USA e-mail: [email protected]; Tel: (617)253-0031 Abstract This paper reports on a process to build, package, and instrument a 5-level wafer-bonded micro-machined turbine/bearing rig. The process flow involves the use of 5 wafers, 16 masks, and 9 deep silicon etching steps, as well as utilizing aligned wafer bonding, double-sided deep reactive ion etching (DRIE), and Laser-Assisted-Etching (LAE). This paper also shows experimental results on flow characteristics of the hydrostatic thrust bearings and the preliminary rotational performance of the device. Introduction The first major fabrication challenge of realizing a miniature gas turbine generator is to demonstrate a baseline process capable of integrating the turbine rotor, bearings, and gas interconnects into a small package [1] [2]. This structure, called the micro-bearing rig, not only validates a process methodology for fabrication of freely-rotating high aspect ratio devices, but it is also a vehicle for research into critical micro air bearing stability issues. An exploded view of the device is shown in figure 1. The turbine rotor, located in the 3 rd wafer, is supported by two different air bearings. (Fig. 2) Two pairs of wafers which cap the center wafer from both sides provide the pair of hydrostatic thrust bearing that support the rotor axially. Each hydrostatic thrust bearing has eight flow restrictors that provide the required pressure compensation. Externally pressurized nitrogen is brought in through these restrictors and flows through the bearing gap only a few microns wide. A self-pressurized hydrodynamic journal bearing is designed to support the bearing radially [3]. Counter rotor-tilting flow channels and fluidic interconnects are also incorporated into the two wafer pairs that cap the center wafer. To simplify the nomenclature, we named the wafers in terms of functionality. As shown in figure 1, from top to bottom, they are called: forward foundation plate (FFP), forward endplate (FEP), rotor plate (RP), aft endplate (AEP), and aft foundation plate (AFP). Wafer 1 2 3 4 5 Figure 1 Exploded view of the micro bearing rig. The five layers are: 1. Forward foundation plate (FFP), 2. Forward endplate (FEP), 3. Rotor plate (RP), 4. Aft endplate (AEP), and 5. Aft foundation plate (AFP). Figure 2 Schematic cross-sectional drawing indicating the location of the bearings and the SEM of the actual device cross-section. 7KUXVW EHDULQJV -RXUQDO EHDULQJ 7XUELQH URWRU
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Process Flow The device has been successfully fabricated, and was outlined in a late news poster at Hilton Head 98 [4]. The process flow involves the use of 16 masks, and 9 deep silicon etching steps on 5 wafers. A schematic illustration of the process is shown in figure 3. The fabrication starts by
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MEMS99_MicroBearing - Fabrication and Characterization of a...

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