L. Majid, T. Le, A. Dahal, T. Garcia
University of New Mexico,
United States
Keywords: bimorph cantilever, XeF₂ release etching, isotropic silicon etch, release-hole optimization
Summary:
In micro-electromechanical Systems (MEMS), thermal actuators (bimorph cantilevers) are used for precision motion in micro-scale applications. This work demonstrates a fabrication process for electro-thermal bimorph cantilevers using vapor-phase xenon difluoride (XeF₂) etching for the essential release step. The XeF₂ process was chosen for its high selectivity and residue-free behavior, which enables stiction-free release through strategically placed etch holes. XeF₂ is highly selective towards Silicon (Si), Molybdenum (Mo), and Germanium (Ge) and compatible with standard MEMS processes, making it a superior alternative to the anisotropic silicon (Si) etch methods by reducing cost and prototyping. This etch process is a purely chemical, isotropic reaction that removes silicon without ion bombardment or plasma assistance. While the isotropic etch mostly produces a rough silicon surface, this has little impact on the performance of the released cantilever. Previous studies on XeF₂ etching primarily address the fundamental chemical mechanisms and isotropic etch behavior; this study focuses on optimizing the release step for practical MEMS fabrication. Specifically, we investigate how release hole geometry influences the efficiency and yield of cantilever release. This fabrication process demonstrates a more robust method for achieving a significantly higher release yield compared to traditional KOH wet-etching. The developed structures were made through reactive ion etch (RIE) patterning of the silicon nitride (Si₃N₄) cantilever on a (100) Si wafer, followed by sputter metal deposition and lift-off process for the heater element. The release of the cantilever was done through a plasma-free, isotropic XeF2 silicon etching that removes the underlying silicon, freeing the cantilever. The result indicates a systematic optimization of the release process, detailing the critical correlation between cantilever geometry, etch area, and perforation design. Data from initial test wafers, characterizing the XeF₂ etch rate and selectivity, is used to refine the final controlled, multi-cycle release recipe. The combination of isotropic etching and selective material stopping provides a direct route to efficient release of thermal actuators. A statistical evaluation of process consistency and actuator performance, including different metrics such as release yield and tip deflection, will be presented to fully validate the fabrication method. Overall, this work establishes a systematic framework for designing and optimizing release holes in XeF₂-based MEMS fabrication. The proposed approach enables higher yield, reduced stiction, and reproducible performance in thermal actuator devices, offering a scalable pathway for industrial MEMS prototyping and production.