Results from the research and development of an ultra-deep reactive ion etching (Ultra-DRIE) process are presented here. The driving motivation of this process development is the manufacturing of high aspect ratio (HAR) Silicon microelectromechanical systems (MEMS), and specifically components for the MEMS Rotary Engine Power System (MEMS REPS). The ultimate goal of the MEMS REPS project is to develop a system capable of producing portable electrical power from a small device. By using a liquid hydrocarbon as the source of the MEMS REPS, the system would offer a higher power density than the battery. To accomplish this feat, the components of the engine must be fabricated with a high-tolerance manufacturing process. Previous development of the ultra-DRIE process for MEMS REPS has led to production of HAR structures but not within the required tolerance of 1 μm. The research presented here is focused on improving the manufacturing tolerance of the ultra-DRIE process.
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.
December 31, 2005
Martinez, F. C. (2005). Optimized Fabrication of Ultra-deep Reactive Ion Etched Silicon Components for the MEMS Rotary Engine Power System. (n.p.): University of California, Berkeley.