Fabrication of HAR-nano structures using a nano X-ray shadow mask

Abstract

This thesis describes a novel method for the fabrication of high-aspect-ratio nano structures (HAR-nano structures) using a nano X-ray shadow mask and deep X-ray lithography (DXRL). The nano X-ray shadow mask is fabricated by depositing an X-ray absorber layer $(Au, 3 \mum)$ onto the back side of a nano shadow mask. The nano shadow mask is produced with nano-sized apertures whose dimensions are reduced to several tens of nanometers by the accumulation of low-stress silicon nitride $(Si_xN_y)$ using the LPCVD (Low Pressure Chemical Vapor Deposition) process on the shadow mask. The shadow mask containing apertures with a size of 1\mum is fabricated on a bulk micromachined $Si_xN_y$ membrane. The thickness of an absorber layer must be in the range of several tens of micrometers in order to obtain a contrast of more than 100 for the conventional DXRL process at the Pohang Light Source (PLS). However, an absorber layer with a thickness of $3 \mu m$ can provide a sufficient contrast if the modified DXRL of the central beam stop method is used, which blocks high-energy X-rays. The nano shadow mask with 30 nm-sized apertures is fabricated and the nano X-ray shadow mask with 250 nm-sized apertures is fabricated by depositing a $3-\mu m-$ thick-absorber layer on the nano shadow mask with 500 nm sized apertures. HAR-nano structures(circles with a diameter of 420 nm and lines with a width of 274 nm) with aspect-ratios of over 10:1 on a $3.2 \mu m$ SU-8 are successfully fabricated by using the nano X-ray shadow mask and the central beam stop method. In this experiment, since the adhesion between the fabricated nano SU-8 structures and the substrate is weak, a nano structure is separated from a substrate or becomes inclined due to the high-aspect-ratio structures during the developing and drying processes. To minimize these phenomena, agitation was not utilized in the developing process, while D.I water was evaporated by using an natural drying process.