Tilt-Rotate Evaporation Colloidal Lithography for Large-Area Nearly-Monodisperse Metasurfaces

Colloidal lithography (CL) has become a key fabrication technique for metasurfaces due to the method’s low cost and scalability. Multiple types of structures are fabricated from CL, such as nanohole and nanodot arrays. Controllability of shape, pitch, and size of the features is necessary to tune the optical response of the metasurface. CL allows this control by utilizing assembled colloids as an ordered mask that can be plasma etched to achieve the desired features. Although CL allows for large-scale fabrication the resulting structures are limited in their minimal feature size and uniformity. Reducing the diameter of the polystyrene sphere mask by plasma etching unavoidably increases their coefficient of variation (CV) and deforms their shape, thereby limiting the pitch-to-hole-diameter ratio of the resulting nanohole array (NHA) to less than 3:1 and the minimum hole size to 200 nm with a 10% or better CV. While there has been extensive research on improving the average diameter and defect density in CL derived NHAs, little work has been done in reducing the polydispersity of the hole diameters in the NHAs, especially in the sub-200 nm diameter range.<br/>We show that a modified CL method, tilt-rotate evaporation colloidal lithography (TRE-CL), breaks the trade-off between hole diameter and polydispersity by leveraging glancing angle evaporation, not plasma etching, to adjust the hole size. [1] TRE-CL allows pitch-to-hole-diameter ratios as high as 7:1 and nanohole diameters down to 60 nm while maintaining a nearly constant CV below 10% and hole circularity above 91%. Furthermore, we transfer these hole arrays into ultrathin Si₃N₄ films to fabricate adhesion-layer-free plasmonic Au nanodot arrays down to 70 nm in diameter with 10% CV embedded in Si₃N₄. TRE-CL allows for large-scale fabrication of uniform metasurfaces quickly and at a low cost while achieving periodic features in the critical 50 – 200 nm regime.<br/><u>References:</u><br/>[1] M. J. Caso, M. G. Benton, and K. M. McPeak, JVST A 40 (4), 10.1116/6.0001874, (2022). (cover image)<br/><u>Contact Information:</u><br/>MaCayla J. Caso: [email protected]<br/>Michael G. Benton: [email protected]<br/>Kevin M. McPeak: [email protected]