Highly ordered hexagonally arranged nanostructures on silicon through a self-assembled silicon-integrated porous anodic alumina masking layer

Abstract

A combined process of electrochemical formation of self-assembled porous anodic alumina thin films on a Si substrate and Si etching through the pores was used to fabricate ideally ordered nanostructures on the silicon surface with a long-range, two-dimensional arrangement in a hexagonal close-packed lattice. Pore arrangement in the alumina film was achieved without any pre-patterning of the film surface before anodization. Perfect pattern transfer was achieved by an initial dry etching step, followed by wet or electrochemical etching of Si at the pore bottoms. Anisotropic wet etching using tetramethyl ammonium hydroxide (TMAH) solution resulted in pits in the form of inverted pyramids, while electrochemical etching using a hydrofluoric acid (HF) solution resulted in concave nanopits in the form of semi-spheres. Nanopatterns with lateral size in the range 12-200 nm, depth in the range 50-300 nm and periodicity in the range 30-200 nm were achieved either on large Si areas or on pre-selected confined areas on the Si substrate. The pore size and periodicity were tuned by changing the electrolyte for porous anodic alumina formation and the alumina pore widening time. This parallel large-area nanopatterning technique shows significant potential for use in Si technology and devices.

Porous Silicon—A Versatile Host Material

“…Recently porous anodic alumina was used for pre-patterning of the silicon surface to promote the formation of ordered porous silicon structures [63,64]. Generally porous silicon is formed by anodization of a silicon wafer in an HF-containing electrolyte.…”

“…On the one hand for example porous anodic alumina is used as mask [63,64] as mentioned above and on the other hand the silicon wafer is pre-structured by lithography to realize a regular arranged “lattice” of pores. Especially macroporous silicon is often fabricated by pre-patterning of the wafer.…”

Polygonal pits on silicon surfaces that are created by laser-assisted chemical etching

“…2 A hexagonal array of square pits was also created on a Si plate by using an alkaline etching method in combination with the anodization and dry-etching processes. 39 Laser irradiation during the etching process is useful to control the shape and arrangement of the etch pit. It was reported that argon laser irradiation realized a selective etching on Si plates owing to the spot heating that caused local enhancement of the etching rate.…”

A template-based method for preparing ordered porous silicon

“…However, previous experiments paid more attention to improving the depth of the holes and the porosity of the PS, while questions pertaining to uniform pore size and the regular ordering of the holes were not resolved. Typically, ordered PS is obtained by Lithography [11] and Anodized Aluminum Oxide templates [12]. In this work, a new method, modifying silica nanospheres with the addition of Ag nanopaticles, is reported for the fabrication of ordered PS.…”