Controlling the Nature of Etched Si Nanostructures: High- versus Low-Load Metal-Assisted Catalytic Etching (MACE) of Si Powders

Deep Etching of Silicon Based on Metal-Assisted Chemical Etching

A deep etching method for silicon "micro"structures was successfully developed. This wet etching process is based on metal-assisted chemical etching (MACE), which was previously mainly utilized to etch the features that have lateral dimensions of "nanometers." In this novel MACE, the critical improvement was to promote the "out-of-plane" mass transfer at the metal/Si interface with an ultrathin metal film. This enabled us to etch micrometer-wide holes, which was previously challenging due to the mass transport limitation. In addition, it was found that when ethanol was used as a solvent instead of water, the formation of porous defects was suppressed. Under the optimized etch conditions, deep (>200 μm) and vertical (>88°) holes could be carved out at a fast etch rate (>0.4 μm/min). This novel deep MACE will find utility in applications such as microelectromechanical systems (MEMS) devices or biosensors.

Metal-Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

Electroless etching of semiconductors has been elevated to an advanced micromachining process by the addition of a structured metal catalyst. Patterning of the catalyst by lithographic techniques facilitated the patterning of crystalline and polycrystalline wafer substrates. Galvanic deposition of metals on semiconductors has a natural tendency to produce nanoparticles rather than flat uniform films. This characteristic makes possible the etching of wafers and particles with arbitrary shape and size. While it has been widely recognized that spontaneous deposition of metal nanoparticles can be used in connection with etching to porosify wafers, it is also possible to produced nanostructured powders. Metal-assisted catalytic etching (MACE) can be controlled to produce (1) etch track pores with shapes and sizes closely related to the shape and size of the metal nanoparticle, (2) hierarchically porosified substrates exhibiting combinations of large etch track pores and mesopores, and (3) nanowires with either solid or mesoporous cores. This review discussed the mechanisms of porosification, processing advances, and the properties of the etch product with special emphasis on the etching of silicon powders.

A Vacuum Ultraviolet-Enhanced Oxidation Mechanism for Pd: Near-Surface Oxidation for Atomic Layer Etching

Density functional theory (DFT) is used to better understand the oxidation of Pd metal using vacuum ultraviolet (VUV) light co-exposed with O₂, which is known to produce O and O₃. The oxidation of Pd metal arising from O, O₂, and O₃ is assessed on bare Pd, Pd with a 0.25 monolayer of adsorbed atomic O, and Pd with increasing O incorporation into the substrate. DFT calculations are complemented experimentally by co-exposing 20 nm Pd films to 1 Torr of O₂ and VUV photons (6.5 < hν < 11.3 eV) from a D₂ lamp at temperatures ranging from 50 to 200 °C and times from 30 s to 40 min. Oxidation of Pd is characterized using in situ X-ray photoelectron spectroscopy. Co-exposures at 50 °C and 1 Torr O₂ are performed with the Pd illuminated by the VUV light and shadowed from the VUV light in attempting to select for the oxidant that impinges on the Pd surface and causes oxidation. Results suggest that atomic O incident from the gas phase is responsible for oxidation of Pd, as no PdO_x formation is observed for the same time period with the sample shadowed. Growth of PdO_x via O diffusion is studied with the nudged elastic band method. Atomic O diffusion through Pd has an activation energy barrier of ∼2.87 eV with respect to a surface O. This decreases to ∼1.80 eV once the 0.25 monolayer of O occupies the surface. The extent of Pd oxidation is limited to the near-surface Pd region for all times and temperatures investigated. PdO_x formation does not appear to exceed one to two atomic layers of Pd for conditions explored herein.

Low-Load Metal-Assisted Catalytic Etching Produces Scalable Porosity in Si Powders

The recently discovered low-load metal-assisted catalytic etching (LL-MACE) creates nanostructured Si with controllable and variable characteristics that distinguish this technique from the conventional high-load variant. LL-MACE employs 150 times less metal catalyst and produces porous Si instead of Si nanowires. In this work, we demonstrate that some of the features of LL-MACE cannot be explained by the present understanding of MACE. With mechanistic insight derived from extensive experimentation, it is demonstrated that (1) the method allows the use of not only Ag, Pd, Pt, and Au as metal catalysts but also Cu and (2) judicious combinations of process parameters such as the type of metal, Si doping levels, and etching temperatures facilitate control over yield (0.065-88%), pore size (3-100 nm), specific surface area (20-310 m²·g^-1), and specific pore volume (0.05-1.05 cm³·g^-1). The porous structure of the product depends on the space-charge layer, which is controlled by the Si doping and the chemical identity of the deposited metal. The porous structure was also dependent on the dynamic structure of the deposited metal. A distinctive comet-like structure of metal nanoparticles was observed after etching with Cu, Ag, Pd, and, in some cases, Pt; this structure consisted of 10-50 nm main particles surrounded by smaller (<5 nm) nanoparticles. With good scalability and precise control of structural properties, LL-MACE facilitates Si applications in photovoltaics, energy storage, biomedicine, and water purification.