Excimer Laser Micromachining of MEMS Materials

Conventional photolithography-based microfabrication techniques are limited to two-dimensional fabrication and only particular materials can be used. Excimer laser micromachining enables to overcome those limitations and facilitates three-dimensional micromanufacturing with a variety of materials. The objective of this research is to present a comprehensive characterization study which provides guidelines to the efforts to identify optimal process parameters for excimer laser micromachining of micro-electro-mechanical systems (MEMS) materials. By using 248 nm KrF excimer laser and 193 nm ArF excimer laser with five representative MEMS materials (Si, soda-lime glass, SU-8 photoresist, poly-dimethysiloxane (PDMS) and polyimide), relations between laser parameters (fluence, frequency and number of laser pulses) and etch rates in vertical and lateral directions, aspect ratio of laser machined trenches and surface quality were investigated. Etch rate per shot was proportional to laser fluence but inversely proportional to number of laser pulses. Laser frequency did not show a notable impact on etch rates. Aspect ratio was also proportional to laser fluence and number of laser pulses but is not affected by laser frequency. Physical deformation in laser machined sites was investigated using SEM imaging. This qualitative study demonstrated that either laser frequency or laser fluence or sometimes their combined effect is the dominant factor in terms of ablation surface quality while the dominant factor varies from material to material under different wavelengths. Energy dispersive x-ray spectroscopy (EDXS) was utilized to analyze material surface before and after laser ablation. It was found that for all five materials oxygen amount increased while the amounts of other elements decreased after laser ablation. The mechanisms behind the process-feature relations are discussed based on the experimental data. This comprehensive characterization study provides guidelines to identify optimized laser ablation parameters for desired microscale structures on MEMS materials. In order to demonstrate the three-dimensional microfabrication capability of KrF and ArF excimer laser, two novel implantable biomedical microscale devices made of SU-8 and PDMS were successfully fabricated using the optimized KrF excimer laser ablation parameters obtained in the current study as well as cutting and local removal of insulation for a novel floating braided neural probe made of polyimide and nichrome using the optimized ArF excimer laser ablation parameters.