Spirothiopyran based super-resolution interference lithography

Abstract

In this thesis, we develop a new super-resolution writing system with the desired low light thresholds for parallel nanopatterning by combining the reversibly saturable isomerization of photochromic spirothiopyran with the thiol-Michael addition reaction. To study the kinetics and optimize the minimum feature sizes realizable with the spirothiopyran writing chemistry, a 1D super-resolution patterning prototype system was designed for self-assembled monolayers on glass substrates. The switchable photoresponsive surfaces fabricated are capable of direct writing various maleimide functionalized molecules in solution. The kinetics of spirothiopyran photoswitching in the monolayer is found to be highly sensitive to the chemical microenvironment. After optimizing the monolayer composition, we experimentally demonstrate large area 1D nanopatterning with 90 nm feature size and using a 2-color interference lithography setup. The lateral feature size of the written patterns is shown to be tunable by controlling the relative intensity of the initiation and inhibition wavelengths. The resultant nanopatterns formed are characterized using super-resolution microscopy. The kinetics of the writing system were examined in detail with the aid of a numerical model, and the factors affecting the overall resolution and contrast were identified. We further adapt this material system to pattern photoresponsive polymers for bulk volume nanopatterning. These results mark important steps toward realizing a highly parallelized fabrication technique with nanoscale resolution, over large volumes in three dimensions.

Rapid, high throughput patterning in bulk polymeric systems with nanoscale resolution in three dimensions has long remained a coveted target for materials scientists. State of the art fabrication techniques capable of nanoscale resolution like electron beam lithography are only capable of serial point-by-point writing, besides being associated with high setup and usage costs. Optical interference lithography has long been an attractive technique to cheaply and rapidly pattern three dimensional features in polymer photoresists despite both the resolution and feature size being limited by diffraction. In the past few years, Stimulated Emission Depletion Microscopy (STED) inspired lithography schemes have shown the ability to direct-write features well below the diffraction limit using visible light. However, the high light thresholds required for effective photoinhibition renders them unsuitable to be used for interference lithography and limits their use to point by point writing.