Microsphere Photolithography (MPL) uses an array of self-assembled microspheres as optical elements. Flood illumination is focused to a photonic jet by each microsphere. Simulation and experiments show that photonic jet can be as small as λ/3, with collimation of more than a wavelength. This provides significant potential for pattern transfer of sub-micron patterns over large-areas and offers an inexpensive alternative to direct-write techniques such as e-beam lithography or two-photon absorption. This has applications such as SERS and SEIRA templates as well as metasurfaces to control radiation heat transfer. For these applications, the underlying substrate is important for the device performance and often presents a considerable index-contrast with the photoresist. The substrate significantly affects the behavior of the photonic jet and changes the necessary dose, minimum feature size, and morphology of the exposed area.
This paper explores the effects of the substrate on the process. Numerical models using commercial (HFSS) frequency-domain Finite Element Method (FEM) is used to simulate the interaction of light with the microsphere/photoresist/substrate. The distribution of the electric field is used to predict the exposure curve for the process. In general, metals and high index materials cause significant standing waves in the photoresist which modifies the hole morphology and ultimate feature size. These predictions are compared to i-line illuminated experiments with SEM measured hole dimensions for aluminum, germanium, and glass substrates. The objective of the paper is to establish design rules for the process which can be incorporated into the device design.
Chuang Qu and Edward C. Kinzel, "Substrate effects on the microsphere photolithography process (Conference Presentation)," Proc. SPIE 10544, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics XI, 105440L (Presented at SPIE OPTO: January 29, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291399.5751531131001.
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