Interlayer-mediated catalyst engineering for ultra-high aspect ratio silicon nanostructures

TL;DR

MacEtch in gas phase enables plasma-free, anisotropic silicon etching but suffers from catalyst contamination and inconsistent pattern transfer for ultra-high aspect ratio features. The study introduces an interlayer between resist and catalyst (Cr or Al$_{2}$O$_{3}$) to physically separate contaminants and allow thorough cleaning before Pt deposition, enabling two patterning routes: interlayer etching (I-Et) and interlayer lift-off (I-Lo). These approaches yield reproducible, large-area HAR silicon structures with aspect ratios exceeding $250:1$ and feature sizes down to ~10–13 nm, including dense optical elements for X-ray optics. The interlayer strategy broadens access to plasma-free MacEtch in smaller labs, maintains pattern fidelity over 1 cm$^{2}$ areas, and supports multilayer top coatings, marking a significant advance in reliable silicon nanostructuring for photonics and X-ray applications.

Abstract

Reliable and precise etching of silicon nanostructures with ultra-high aspect ratios is required in many fields. Metal assisted chemical etching (MacEtch) in vapor is a plasma-free etching method that attracts considerable attention owing to the ability to create smooth, high aspect ratio nanostructures. MacEtch understanding and applications are limited by low fidelity and inconsistent pattern transfer from the catalyst layer to the silicon substrate. The locally constrained electrochemical interactions at the catalyst site make MacEtch particularly sensitive to catalyst contamination reducing the reaction rate and pinning the catalyst during etching. Removing contaminants is essential to improve pattern transfer for reliable processes on a larger area and higher aspect ratio. Physically separating the main source of carbon - the resist - from the catalyst with a sacrificial and functional interlayer solves this issue. The interlayer separates the resist and the catalyst and allows for thorough cleaning of the substrate before catalyst deposition. The resulting clean catalyst has improved stability, quality and reproducibility, enabling reliable fabrication of dense (50% patterned area) high aspect ratio (>250:1) nanostructures. Two different interlayer materials (Cr and Al$_{2}$O$_{3}$) and two patterning approaches are presented, showcasing etching of various high aspect ratio nanostructures, such as X-ray Optics.

Figures (10)

• Figure 1: a)-f) show the schematic process flow of the I-Et approach to patterning. The text describes the process flow. SEM images in cross-section (i-k) are given for 3 selected steps. Top-down EDX images are provided for platinum signal (g) and the Cr interlayer signal (h) after MacEtch.
• Figure 2: Schematic process flow of the I-Lo approach to patterning. Cross-sectional SEM images of the selected steps with Al$_2$O$_3$ as the interlayer material.
• Figure 3: A schematic showing the issue in pattern transfer arising from a poor interlayer profile (a-b), solved by 20 nm plasma etching of Si using the interlayer as hard mask (c-e). The supporting cross-sectional SEM images show select steps in the process.
• Figure 4: A schematic showing an advanced process using a bilayer as the interlayer to create an undercut. The scheme proceeds as I-Lo, followed by a selective lateral etch of the lower layer before the deposition of Pt. h)–j) show cross-sectional SEM images of the selected steps of the process with Cr/Al2O3 as the interlayers.
• Figure 5: Etch rate of H-bar structures with 200 nm linewidth as a function of a) number of runs (opening the pot and letting the vapor out-diffuse) and b) sample temperature. c) sweep of four sets of consecutively etched samples using the I-Et method (H-bar structures with different linewidth asa indicated). The structures were sorted according to the catalyst quality. SEM in cross-section of three examples with different qualities of the catalyst at the bottom of the etched trenches: d) a catalyst layer with good quality, e) a wavy catalyst without tears, and f) catalyst with isolated motions and breaks.