Fabrication of hierarchical nanostructures using free-standing trilayer membrane

Hierarchical nanostructures are typically fabricated with multiple lithography steps on prepatterned micro- or nanostructures. However, such conventional multiple lithography steps result in poor coverage and uniformity, especially when patterning on high-aspect-ratio micro- or nanostructures. In this work, the authors present a new fabrication method which can make hierarchical nanostructures even on high-aspect-ratio prepatterns, using a free-standing trilayer membrane. The nanoporous trilayer composite films, consisting of metal/photoresist/antireflective coating, is created on a planar silicon substrate by using laser interference lithography, reactive ion etching, and e-beam deposition. Then, a customized solution of ammonia (NH₃), hydrogen peroxide (H₂O₂), and water (H₂O) is used to release the composite film from the supporting substrate. The free-standing composite membrane is then transferred on a prepatterned silicon substrate. Showing no major defects on the membrane surface, the flexible and soft trilayer membrane allows a good, uniform contact with the top surface of the prepatterns and is utilized as a robust etch mask in the deep reactive ion etching (DRIE) process of silicon to fabricate hierarchical nanopore structures. Since the membrane covers the top surfaces of the prepatterns seamlessly, the nanopore patterns of the membrane are transferred onto the top surfaces with great fidelity during the etching process. The membrane can also be used as a stencil for deposition processes. For example, metal is deposited through the nanoporous membrane by a lift-off process. Then, the transferred metal nanodot patterns are used as an etch mask in the DRIE process to achieve high-aspect-ratio hierarchical nanopillar structures. Compared to conventional methods to fabricate hierarchical nanostructure, the introduced new approach is more reliable and versatile to realize large-area hierarchical nanostructures with a good uniformity, in both nanopore and nanopillar array by using the same single membrane.