Paper PS+MN-WeM3
Deep Reactive Ion Etch Process Optimization for Control of Sidewall Profile and Morphology as a Function of Aspect Ratio

Wednesday, October 20, 2010, 8:40 am, Room Galisteo

Deep reactive ion etching (DRIE) has become an enabling technology for the fabrication of many integrated microsystems, including accelerometers and gyroscopes, micro-fluidic devices, sensors, electrostatically actuated devices, and devices requiring back side optical access. The ability to etch deep Si structures with anisotropic sidewalls hundreds of microns deep has established a new set of devices in the MEMS area. Significant improvements in equipment and understanding of the process conditions have improved device yield and performance, and process reliability. Even with these improvements, several process issues are not well understood and often limit applications for the process. For example, sidewall morphology is often dominated by scalloping created by the iterative deposition-depassivation-etch cycle. Scalloping may make it difficult for deposition of materials on the sidewalls post DRIE or create non-optimal flow conditions for micro-fluidic devices. In addition, profile control of deep structures as a function of aspect ratio has not been optimized. For example, we have observed that creating positively tapered trench sidewalls often results in a trench bottom that exhibits a characteristic micromasked, grassy appearance. Conversely, eliminating the grass often results in a profile that undercuts the etch mask. Depending on the application, these phenomenon prevent the use of DRIE for device fabrication or cause the process to be optimized for specific structures thus preventing yield of other structures. In this presentation, we report on our efforts to vary DRIE process conditions to optimize sidewall profile and sidewall morphology as a function of aspect ratio. Structures considered in this study range from 10 microns to 700 microns in width, with etch depths to several hundred microns. We observe that passivation time, as well as ion energy, and ion flux in both the depassivation and etch cycles, have significant effect on the sidewall profile as a function of aspect ratio. We have also included morphing experiments in this study, where morphing is changing DRIE process parameters as a function of total process time. To optimize sidewall profile and morphology, the magnitude of the process changes during the morphing process is not necessarily linear with time. Results of these experiments will also be reported.

Sandia National Laboratories is a multi program laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.