Three-dimensional surface sculpting of freestanding metal-composite nanomembranes

Abstract

In this work we report structural functionalization of metal-composite nanomembranes through formation of micrometer-sized three-dimensional hollow self-supported shapes with tailorable ordering and dimensions. The basic idea is to use double-sided, two-stage etching of the sacrificial layer, first to define the preform for the nanomembrane sculpting, then to fully remove the sacrificial structure and release the nanomembrane. In our experiments we utilized 15–20 μm thick ‹100› Si diaphragms as sacrificial layers. Conventional KOH-based bulk micromachining was used to sculpt pyramidal holes within the diaphragms and modified radiofrequent sputtering to deposit a 5–20 nm Cr-based nanomembrane precursors. After the removal of sacrificial membranes the released freestanding nanomembranes had an aspect ratio up to 1000,000 and contained hollow micrometer-sized pyramidal shapes which do not collapse in spite of their large wall-to-side aspect ratio (>1000). Although the geometry of the sculpted shapes in our experiments is defined by the Si bulk micromachining, one may readily fabricate a number of other shapes, including hemispherical, ellipsoidal, etc., the sizes and shapes being limited by the particular microfabrication technique chosen for the diaphragm pre-forming. Some potential applications of 3D sculpted nanomembranes can be found in subwavelength diffractive optics, plasmonics, energy harvesting, microfluidics, biointerfaces, to name just a few.