T. Le, A. Dahal, T. Garcia, L. Majid
University of New Mexico,
United States
Keywords: XeF2, bimorph cantilever, characterization, optimization, release etch
Summary:
In micro-electromechanical systems (MEMS) fabrication, the release of suspended structures is a critical process requiring etch release methods with high selectivity, isotropy, and repeatable etch rate to fully free the moving structure. These suspended structures are often built on a silicon (Si) wafer or layer that serves as a sacrificial layer to be removed later. This can be done using aqueous etchants such as potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH). Although these etchants effectively remove Si beneath the structural layers, they follow crystalline planes and can cause stiction—adhesion between the released structure and substrate from surface tension of liquid trapped during etching—which can hinder motion or collapse the structure. Stiction can be mitigated by critical point drying, structural design modification, or by using non-aqueous etchants such as plasma- or vapor-phase etchants. Vapor-phase xenon difluoride (XeF₂) is a particularly attractive solution for releasing suspended structures from Si sacrificial layers. XeF₂ is a gaseous etchant with high selectivity towards Si, reasonable etch rate, and an isotropic etch profile. Due to its gaseous nature, suspended structures being released from Si sacrificial layers harbor no liquid, reducing the need for an extra critical point drying step. Additionally, XeF₂ exhibits high selectivity for Si, molybdenum (Mo), and germanium (Ge) over other MEMS and semiconductor materials, reducing the need for a protective mask layer over existing structures and electrical circuits. Despite these advantages, XeF₂ can still be challenging to use, particularly for complex geometries. Due to its gaseous nature, XeF₂ relies on gas-phase diffusion and surface reaction to remove Si, making the etch rate and uniformity highly dependent on structural geometry and access to sacrificial regions. Thus, openings or perforations (release holes) are often added to increase etch surface area and access to the sacrificial layer under the structural element to be released. These design elements reduce etch time to completely release the moving structure, while poorly optimized processes and designs can cause excessive XeF₂ consumption, incomplete release, or non-uniform undercutting resulting in low yield and increased cost. This research characterized XeF₂ etching behaviors for releasing suspended microstructures from Si layers and established design guidelines linking release hole geometry and distribution to release etch performance. Experimental cantilever (bimorph thermal actuator) arrays with varying opening sizes, shapes, and distributions were fabricated and released to quantify etch rates, uniformity, and undercut profiles. Leveraging the isotropic nature of XeF₂, experimental results demonstrated that well-optimized and properly distributed openings enable complete structural release while minimizing XeF₂ consumption. From these findings, a qualitative framework is presented for the optimal design and distribution of release hole openings to achieve effective, uniform, and material-conscious XeF₂ release of suspended MEMS structures. This study provides experimentally validated insights and design principles for optimizing perforation geometry and process parameters to achieve complete XeF₂ release for suspended microstructures with improved uniformity and reduced etchant use. The results contribute to a more reliable and predictable understanding of XeF₂ behavior in MEMS fabrication and establish a foundation for integrating vapor-phase release processes into advanced microfabrication workflows.