Direct e-beam writing of dense and high aspect ratio nanostructures in thick layers ofPMMA for electroplating

Abstract

Due to the ability of 100 keV electrons to penetrate deep into resist with little scattering, we wereable to directly write various dense and high aspect ratio nanostructures in 540 nm and 1.1 µm thick layers of poly(methyl methacrylate) (PMMA) resist. The PMMA molds produced byelectron beam lithography were developed using a high contrast developer. Themolds were used to transfer the pattern into metallic nanostructures by fillingthe developed trenches with Au by electroplating. By exposing lines narrowerthan the target width, we observed improved process latitude and line widthcontrol. The obtained aspect ratios of the dense structures are nearly 20 in 1.1 µm PMMAlayers and > 16 for structures electroplated into this PMMA mold. The fabrication method wassuccessfully applied to produce Au diffractive x-ray Fresnel zone plates ofexceptionally good quality with 50 and 70 nm outermost zones using 540 nm and 1.1 µm thick PMMA molds. In addition, we also produced regular arrays of high aspect ratio anddense Au nanorods with periods down to 100 nm and high aspect ratio split-ring resonators.