Dependence of silicon fracture strength and surface morphology on deep reactive ion etching parameters

The development of a high power-density micro-gas turbine engine is currently underway at MIT. The initial goal is to produce the components by deep reactive ion etching (DRIE) single crystal silicon. The capability of the silicon structures to withstand the very high stress levels within the engine limits the performance of the device. This capability is determined by the material strength and by the achievable fillet radii at the root of turbine blades and other etched features rotating at high speeds. These factors are strongly dependent on the DRIE parameters. Etching conditions that yield large fillet radii and good surface quality are desirable from a mechanical standpoint. In order to identify optimal DRIE conditions, a mechanical testing program has been implemented. The designed experiment involves a matrix of 55 silicon wafers with radiused hub flexure specimens etched under different DRIE conditions. The resulting fracture strengths were determined through mechanical testing, while SEM analysis was used to characterize the corresponding fillet radii. The test results will provide the basis for process optimization of micro-turbomachinery fabrication and play an important role in the overall engine redesign.