Hybrid silicon/silicon carbide microstructures and silicon bond strength tests for the MIT Microengine

Abstract

The Gas Turbine Laboratory (GTL) and the Microsystems Technology Laboratory (MTL) at the Massachusetts Institute of Technology initiated a joint effort to develop a series MEMS-based turbine engines and turbo generators in 1995. This thesis focuses on two independent research topics: first, the use of hybrid silicon/silicon carbide structures to extend the operating envelope of the first generation microengine, and second, a testing technique to measure the toughness of silicon to silicon fusion bonds. Due to the relatively low strength of Si at high temperatures, the all-silicon demonstration device does not yet meet the design specifications. The introduction of limited amounts of SiC in the turbine disc and turbine blades can increase the temperature tolerance of the rotating structure by 150-200K. A turbine disc with a 30% SiC core, and hollow turbine blades with a 300pim tall SiC core yield significant improvements in the microengine performance when compared to the all-silicon baseline design: 30% increase in compressor pressure ratio and fourfold increase in shaft power output. However, more aggressive cooling schemes or re-design of the rotating spool is needed for further improvements. Fabrication of the hybrid structures is compatible with the current microengine process flow, although some key SiC process steps must be developed further. A testing technique has been developed to measure the toughness of Si-Si fusion bonds using bi-layer interfacial notched specimens in a four point bend fixture. The test results confirm the trade-off between annealing time and temperature to achieve similar bond strengths. The experimental results agree with theory and published data. Subsequent experiments should further investigate the effect of different annealing time, surface preparation and contacting atmosphere on bond strength. The technique could also be applied to test bond strength between dissimilar materials, for instance silicon and silicon carbide.