Tatsuya Kawase

Metal-assisted chemical etching of Ge(100) surfaces in water toward nanoscale patterning

We propose the metal-assisted chemical etching of Ge surfaces in water mediated by dissolved oxygen molecules (O2). First, we demonstrate that Ge surfaces around deposited metallic particles (Ag and Pt) are preferentially etched in water. When a Ge(100) surface is used, most etch pits are in the shape of inverted pyramids. The mechanism of this anisotropic etching is proposed to be the enhanced formation of soluble oxide (GeO2) around metals by the catalytic activity of metallic particles, reducing dissolved O2 in water to H2O molecules. Secondly, we apply this metal-assisted chemical etching to the nanoscale patterning of Ge in water using a cantilever probe in an atomic force microscopy setup. We investigate the dependences of probe material, dissolved oxygen concentration, and pressing force in water on the etched depth of Ge(100) surfaces. We find that the enhanced etching of Ge surfaces occurs only when both a metal-coated probe and saturated-dissolved-oxygen water are used. In this study, we present the possibility of a novel lithography method for Ge in which neither chemical solutions nor resist resins are needed.

Catalytic behavior of metallic particles in anisotropic etching of Ge(100) surfaces in water mediated by dissolved oxygen

The authors demonstrate that Ge(100) surfaces containing metallic particles are etched anisotropically in water. This originates from the catalytic reduction of dissolved oxygen (O2) in water to water molecules (H2O) on the metallic particles, which is followed by the enhanced oxidation of Ge around the particles. The soluble nature of Ge oxide (GeO2) in water promotes the formation of inverted pyramidal etch pits composed of (111) microfacets. On the basis of the results, the authors propose strategies for avoiding unwanted surface roughening during the wet cleaning of Ge.

Photoetching of Silicon by N-Fluoropyridinium Salt

A photoetching method with -fluoropyridinium salts is proposed in this study. Si is etched by applying -fluoropyridinium salts to its surface and exposing the surface to light. The etched surface that uses liquid -fluoropyridinium salts is smoother than when solid -fluoropyridinium salts are used. The etching depth increases with exposure time. H-terminated hydrophobic Si is easier to etch than OH-terminated hydrophilic Si. is produced by photoetching. This suggests that -fluoropiridinium salts receive excited electrons from Si and supply Si with active F species.

Formation of Pyramidal Etch Pits Induced by Metallic Particles on Ge(100) Surfaces in Water

We demonstrate that pyramidal etch pits composed of (111) micro facets are formed on single-crystalline Ge(100) surfaces when the Ge samples with metallic particles are immersed into water. The mechanism behind this anisotropic etching is the enhanced oxidation of Ge around metallic particles by the catalytic activity of noble metals to reduce dissolved oxygen (O2) to water molecules, and the subsequent removal of the oxide in water due to the soluble nature of Ge oxides. This enhanced etching of Ge surfaces is observed not only with metallic particles but also with a metallic thin film. The results insist that remaining metallic contaminants can roughen Ge surfaces during a rinse process with water, which will be important to design wet cleaning processes for Ge wafers.

Pit Formation, Patterning and Flattening of Ge Surfaces in O2-Containing Water by Metal-Assisted Chemical Etching

Metal-assisted chemical etching is a novel method of etching a Ge surface in contact with a noble metal in water. Its basic mechanism involves the catalytic activity of metals to reduce dissolved O2 molecules in water, which accompanies the formation of a soluble oxide (GeO2) on the Ge surface around the metal. Here, we apply this electroless etching to the pit formation, nanoscale patterning and surface flattening of Ge. The fundamental etching properties for these three processes are also presented.