D. Amrit, L. Majid, T. Le, T. Garcia
University of New Mexico,
United States
Keywords: bimorph cantilever, KOH etching, MEMS fabrication
Summary:
Bimorph cantilevers are essential components in high sensitivity Micro-Electromechanical Systems (MEMS) devices used for sensing and actuation applications. Cantilevers respond to thermal, electrical, or mechanical stimuli, resulting in measurable deflection. However, achieving a defect-free and consistent release during fabrication remains a key challenge due to issues of stiction, surface roughness, undercutting, and non-uniform etching. In this work, an optimized anisotropic potassium hydroxide (KOH) wet etching process was developed for the complete release of Bimorph cantilevers structure from the sacrificial silicon (Si) layer. KOH etching is an anisotropic wet etching process that is used for selectively removing silicon along specific crystal planes. Depending on the crystal orientation, anisotropic etching occurs at different rates whereas isotropic etching progresses uniformly in all directions. In particular, the {111} plane etches much more slowly than the {100} planes. The lower reaction rate of the {111} plane is caused due to larger activation energy required to break Si–Si bonds behind the etch plane, making them more resistant to the etching reaction. Additionally, the KOH etching process is sensitive to temperature, solution concentration, and silicon crystal orientation, all of which significantly influence the etch rate and surface morphology. The fabrication process was done on a silicon wafer of (100) orientation, where the standard photolithography process was performed to pattern the bimorph cantilever, followed by metal sputtering to pattern the heater elements. Anisotropic KOH-based wet etching of the underlying crystalline silicon was then employed to release the structure beams. KOH concentration (20–35%) and temperature (70–85 °C) were systematically varied to study their effects on etch rate, surface morphology, and undercut profiles. The effect of adding isopropyl alcohol (0 to 10% by volume) to KOH etchant solution was also examined to assess its influence in enhancing surface quality and release uniformity. Results show that addition of isopropyl alcohol (IPA) during KOH etching reduces hydrogen bubble adhesion, enhances surface smoothness, and minimizes undercutting, resulting in higher release yield and uniform surface morphology. The Consistent release of silicon nitride (Si3N4) cantilever’s structure is achieved through precise control of the anisotropic etch process by optimizing parameters such as temperature and KOH concentration. Using these results, a guideline was established for the efficient design and release process of MEMS bimorph cantilevers.