Multi-step plasma etching of high aspect ratio silicon nanostructures for metalens fabrication

Abstract

Inductively coupled plasma etching of silicon nanostructures for metalens applications using a continuous, multi-step C4F8/SF6 plasma was investigated to achieve high aspect ratio (HAR) features down to tens of nanometers with smooth sidewalls. In the process, the ion bombardment and the free radical transport significantly change among HAR nanostructures as the etching progresses, posing challenges to profile control. With a fixed gas ratio, a change in the profile angle occurs at a depth of approximately 400 nm, transitioning from a positive taper to a negative one. Additionally, a wave-like pillar profile is produced when using three separate (i.e., plasma turned off after each step) etching processes with varying gas ratios. To optimize passivation and etching, we adopt a three-step C4F8/SF6 plasma etching process with varying gas ratios at different etching depths. By keeping the plasma on after each step, the continuous, three-step process provides more flexibility for tuning the etching of HAR nanostructures with smooth and vertical profiles. Metalens nanostructures with 71 nm diameter and 1 μm height were created using the appropriate gas ratio. The feature size variation is less than 10 nm. This proposed continuous, multi-step process improves the controllability of silicon etching in C4F8/SF6 plasma, facilitating the nanofabrication of silicon metalens and other nanodevices.