Spirothiopyran-Based Reversibly Saturable Photoresist

Super-resolution lithography holds the promise of achieving three-dimensional (3D) nanopatterning at deep subwavelength resolutions with high throughput. 3D super-resolution lithography schemes demonstrated thus far have all been serial in nature, primarily due to the lack of a photoresist chemistry that not only couples a saturable reversibly switchable reaction with a writing step but also has a low saturation threshold. Here, we demonstrate that combining the reversible photoisomerization of spirothiopyran with the thiol-Michael conjugate addition reaction achieves the necessary photochemical characteristics. Green light was found to saturate inhibition of the thiol-Michael addition writing step at very low intensity thresholds. By formulating a spirothiopyran-functionalized polyethylene glycol copolymer, we demonstrate spatial control over cross-linking using inhibition by green light. Kinetics measurements combined with photokinetic simulations show that interference lithography on a spirothiopyran maleimide-based writing system using conventional light sources (e.g., a 2 W green laser) should deliver super-resolution features (∼45 nm wide lines) in thick films (tens of microns) over large areas (hundreds of microns on a side). The unique combination of reversible photochromic switching of spirothiopyran with the thiol-Michael addition reaction marks an important step toward realizing a highly parallelized 3D super-resolution writing system.