Nanoscale optical assessment of photochemical changes of SU-8 photoresist induced by ultrashort near-IR optical excitation

Modern device manufacturing heavily relies on photolithographic techniques to fabricate three-dimensional (3D) nanostructures. Direct laser writing using multiphoton absorption is commonly employed. A key challenge in troubleshooting 3D structure fabrication is understanding why structures are destroyed and/or defects are created during development (collectively “fabrication errors”). In this report, we explore new approaches to avoid and assess fabrication errors. We demonstrate a novel way to expose photoresists by developing rows of SU-8 photoresist using intense (~ 10–40 TW/cm2) femtosecond pulses at much longer (1.7 µm) wavelength than conventionally used for multiphoton micromachining. We then apply atomic force microscopy-infrared absorption spectroscopy to provide chemically-specific nanoscopic imaging to distinguish photoinduced modifications of SU-8 photoresist. The focus of this report is on testing the feasibility of novel fabrication and detection methods. While no 3D structures were fabricated in this report, infrared femtosecond excitation can be combined with existing 3D patterning techniques to fabricate 3D structures. By inducing, detecting and imaging development at the nanoscale, the demonstrated new approach opens new doors for future 3D manufacturing and its quality assessment.