Sub-5nm fabrication on free-standing structure enabled by helium ion microscope

Abstract

Sub-5 nm nano-structure with sub-nanometer accuracy and sub-nanometer line edge roughness (LER) is critical in nano-optics and nano-electronics especially for integrated circuits. However, the challenge is partially resolved by electron beam lithography (EBL) with critical dimension of 2 nm, LER down to sub-nanometer value is still a long outstanding problem. We systematically study scanning helium ion beam lithography (SHIBL) on suspended structures from point-exposure to raster scanning, sub-5 nm nano-structure with sub-nanometer accuracy and sub-nanometer LER is achieved by our proposed strategies. We first study the visualization of the interaction volume which is a basic unit of direct beam lithography at sub-10 nm scale by a spot irradiation of sub-nanometer helium ion beam into an approximately free-standing resist. The visualized interaction volume suggests helium ion beam has an excellent capability in nanofabrication. Specifically, helium ion beam lithography is 1000 times more efficient than electron beam lithography, owns a sub-4 nm resolution, can achieve a large pattern aspect ratio (greater than 8), and does not suffer from backscattering effect at a normal exposure dose. Furthermore, the interaction volume has been theoretically studied by considering the spatial distribution of energy deposited in the resist, and eventually lead to a model for pattern prediction and proximity effect corrections. Based on the data of sub-nanometer level accuracy spatial distribution of helium ion beam point exposure energy, we use SHIBL to generate aerial image with a profile of sub-nanometer LER. We use SHIBL to expose hydrogen silsesquioxane (HSQ) resist on SiNx membrane and present the 0.16 nm spatial imaging resolution based on the geometric construction with stranding-free thin membrane. The free-standing membrane serves as an energy filter of helium ion beam. Thanks to the elimination of backscattering induced secondary electrons, we can systematically study the LER variation with the relevant parameters including step size, designed exposure linewidth (DEL), delivered dosage and resist thickness, and selected high contrast developer, thus offering the process window that enables the fabrication of lines with LER down to 0.2 nm. Furthermore, around 33 nm nanogap structures with sub-nanometer LER are patterned on suspended SiNx thin film using SHIBL. Atomic layer deposition (ALD) of aluminum oxide is utilized to shrink the nanogap from 33 nm to 4 nm layer by layer conformally with sub-nanometer accuracy without sacrificing LER. Electron beam evaporation and scanning helium ion beam milling (SHIBM) is utilized to metallize and functionalize the nanogap. Nanoimprint combined with the strategy to fabricate sub-5 nm nanostructure with sub-nanometer accuracy/LER enabled by helium ion microscope (HIM) has the potential to make up for the poor LER nanofabrication by EBL and extreme ultraviolet (EUV) lithography when the processing scale is down to 10 nm. Moreover, the ability to demo sub-5 nm nano-electronics/optics device offers more possibilities in the future. (456 words)