Soft and hard trimming of imprint resist masks to fabricate silicon nanodisk arrays with different edge roughness

Abstract

To investigate the formation mechanism of wrinkle structures in imprinted resist masks generated by oxygen reactive ion etching (O2 RIE), we compared UV/ozone exposure (soft trimming) and O2 RIE (hard trimming) as oxidative trimming methods to tune the diameters of disk resist masks in ultraviolet nanoimprint lithography of Mie-resonant silicon nanodisks (Si NDs). Variations in the residual layer thicknesses of the imprinted resin patterns demonstrated that the wrinkle structures around the disk resist masks increased after the residual layer was removed completely. A comparison between soft and hard trimming indicated that the UV/ozone exposure maintained a relatively small edge roughness of the disk resist masks during a reduction in diameter from 370 to 160 nm, whereas O2 RIE caused a large edge roughness owing to wrinkle structures with diameters below 300 nm. The wrinkled structures are likely to have originated from the wrinkling instability caused by the formation of an ion-damaged layer near the resist surfaces via O2 RIE involving ion bombardment, which could be transferred to a monocrystalline Si layer on a synthetic quartz substrate. Microscopic optical measurements revealed that 230 nm-diameter Si-ND arrays with small and large edge roughnesses exhibited almost identical reflection spectra at visible wavelengths.