Interlayer‐Catalyst Method for Metal‐Assisted Chemical Etching of Ultra‐High Aspect Ratio Silicon Nanostructures

Reliable and precise etching of silicon nanostructures with ultra‐high aspect ratios is required in many fields. Metal‐assisted chemical etching (MacEtch) in HF vapor and O 2 is a plasma‐free etching method that attracts considerable attention owing to its 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, which reduces the reaction rate and pins the catalyst during etching. Removing contaminants is essential to improve pattern transfer for reliable processes over a larger area and with a 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 thorough cleaning of the substrate before catalyst deposition. The resulting clean catalyst enables the fabrication of dense (50% patterned area) high aspect ratio (>250:1) nanostructures. Different interlayer materials (Cr, Al 2 O 3 , and SiO 2 ) and two patterning approaches are presented, showcasing etching of various high aspect ratio nanostructures, such as X‐ray optics.