Optimized Fabrication of Ultra-Deep Reactive Ion Etched Silicon Components for the MEMS Rotary Engine Power System

At the core of the MEMS REPS is a Wankel-type rotary engine, which, while it lends itself well to the planar fabrication processes typically used in MEMS manufacturing, has a history of limitations in leakage performance. Specifically, the apex-seals of this type of rotary engine have been a focus of research efforts for improved performance. Macro-sized Wankel-type engines such as those implemented in automobiles, have complex sealing systems that require difficult assembly; scaling down of this type of system would make assembly of the MEMS REPS extremely difficult. Instead, to incorporate seals into the MEMS engine design, an integrated in-plane flexure system is fabricated in place via the ultra-DRIE process. While this novel seal design significantly reduces the complexity of the engine’s assembly, the structures have been the most difficult to manufacture of all of the system’s Silicon features.

The challenges in ultra-DRIE fabrication of the Silicon components were to minimize aspect ratio dependent etching (ARDE), as well as to maintain a prescribed 90º sidewall profile, and striation free surfaces. The ARDE issue had been the primary limitation of previous efforts to develop the integrated apex seals due to an etch lag effect. Photomask layouts from previous research led to a process in which the ARDE lag effect caused the small seal features to etch considerably slower than the larger engine features. The first step in process development was to modify the layout of the engines’ rotors and housings to incorporate uniform “guard” trenches of 250 μm defining the component features and to introduce a backside DRIE etch step for the features of the seals. These developments led to the first successful fabrication of through-wafer cantilever seals. The second development was to remove the backside etch step from the process by further revising the layout with all features defined by 40 μm “guard” trenches. A benefit of this single-sided DRIE approach was the avoidance of potential misalignment during backside photolithography; also, These modifications led to a simplified single-step ultra-DRIE process, but the resulting etch surfaces and profiles were in need of improvements in smoothness and straightness to maintain geometrical tolerances.

To develop an ultra-DRIE recipe capable of resolving the critical tolerances, the Taguchi Method for design-of-experiments (DOE) was applied. The results from this DOE were then used as a guide to better understand ultra-DRIE for MEMS REPS and to tune existing recipes. In the interest of time, the Taguchi Method for DOE was attractive as it produced an understanding of four factors at three levels from only nine experiments compared to 81 experiments for a full factorial DOE. The results from the DOE and fine-tuning of the DRIE recipes showed exceptional improvement in striation-free etch depth up to 400 μm.