T. Le, A. Dahal, T. Garcia, L. Majid
University of New Mexico,
United States
Keywords: Xenon difluoride, potassium hydroxide, release etch, thermal actuators, bimorph cantilevers
Summary:
In micro-electromechanical systems (MEMS), the release of suspended structures such as cantilevers and beams is a crucial fabrication step that directly affects device performance. This study compares two scalable release processes for bimorph cantilever fabrication: dry, isotropic xenon difluoride (XeF₂) etching and wet, anisotropic potassium hydroxide (KOH) etching. By applying both methods to identical cantilever designs, parameters such as undercut rate, release quality, and actuation performance are quantified to evaluate wet versus dry release etching. The study provides design rules and optimized process parameters for the successful release of MEMS structures on silicon (Si). Vapor-phase XeF₂ is a gaseous etchant with high selectivity towards Si, fast etch rate, and isotropic etch profile. These characteristics make XeF₂ an attractive solution for releasing suspended structures, such as cantilevers and beams, from Si layers. Since XeF₂ is a gaseous etchant, its etch characteristics are dependent on gas-phase diffusion and surface reactions. Thus, a portion of this study focuses on characterizing XeF₂ etching of Si for the release of microstructures. In contrast, KOH is an aqueous etchant with a high etch rate, anisotropic etch profile, and moderate selectivity toward silicon dioxide (SiO₂) and silicon nitride (Si₃N₄). KOH is often used to remove large volumes of silicon, specifically in bulk micromachining to form cavities or release structures from Si substrates. As a liquid-phase etchant, the KOH etch rate is highly dependent on temperature, concentration, and crystal orientation, as well as diffusion and reaction kinetics at the solid–liquid interface. In this study, several bimorph cantilever arrays are fabricated on a (100) Si wafer with a Si₃N₄ layer grown on the surface. The Si3N4 layers is patterned using reactive ion etch (RIE), followed by metal sputtering and lift-off, and then etched using KOH or XeF2. For each etchant, etch parameters are studied to quantify their effects on release quality and release time. The Si3N4 cantilevers are also tested using direct current (DC) power to quantify cantilever performance. To evaluate fabrication consistency and manufacturability, Statistical Process Control (SPC) and Measurement System Analysis (MSA) are applied to quantify variations introduced during both XeF₂ and KOH release processes. Etch depth and surface morphology are measured using profilometry and microscopy, while an abbreviated Gage R&R study evaluates measurement precision and operator influence. These analyses help distinguish true process variation from measurement error and identify factors affecting undercut uniformity and release completeness. By establishing process capability indices (Cp, Cpk) for key release metrics, this work provides a quantitative basis for refining process parameters and improving yield consistency across MEMS bimorph cantilever fabrication. This work provides a comparative review of two release processes and provides mask design rules with optimized release parameters to maximize yield and process capability. The results will provide a framework for optimizing design and etch parameters to improve device yield and process uniformity.