Multistage Anisotropic Wet Etching by KOH for MEMS/NEMS Structures

Adaptive Optics (AO) systems add value in applications that involve light collection through turbulent media. Examples include Earth-based telescopes, microscopy, optical communication, and high-energy lasers, where solutions for addressing distorted images are needed. A key challenge in building AO systems involves achieving sufficient correction of distorted images through optical elements that can change shape with enough vertical displacement, spatial resolution, and frequency to adequately correct images. One such AO element is a deformable mirror that is bonded to an array of MEMs-based actuators that is made to operate using the Lorentz force. These elements can operate with very low power and are known as Low-Voltage Deformable Mirrors (LVDM). This study delves into a multi-step Potassium Hydroxide (KOH) etching process, encompassing surface preparation, mask deposition, lithography, and etching. KOH bulk micromachining proves critical in precision crafting of detailed 3D microstructures through anisotropic wet etching, offering unparalleled control over structural depth and morphology. LVDM Lorentz actuators are systematically arrayed in 20 x 20 geometries, interconnected with a flexible, reflective membrane mirror. Fabrication of the actuator arrays using Silicon on Insulator (SOI) wafers utilizes a multi-step KOH bulk process, with a design that integrates a crossbar, pillar, and springs, with a need for a crucial depth of up to 35 mu m. This study provides a detailed account of the two-step KOH etching process, emphasizing critical parameters for successful Lorentz actuator fabrication on SOI wafers. Exploration of process variables like temperature, etchant concentration, and etching time is undertaken, considering their impact on etch rate and selectivity. Understanding these variables proves vital for achieving high-quality and reproducible MEMS structures in micro-/nanofabrication. The study also tackles challenges associated with multistage anisotropic wet etching, including etch pit formation, crystallographic defects, and surface roughness. Strategies to mitigate these challenges, such as surfactant use, additives, and post-etch treatments, are discussed, emphasizing their effectiveness in improving final device performance and reliability.