User:Azeredo1/Metal Assisted Chemical Etching

Metal Assisted Chemical Etching

Definition

Metal Assisted Chemical Etching is a novel wet-etching* technique used to prepare porous and crystalline features – e.g. nanowires, micro-nano channels - in bulk semiconductors. It relies on an electrochemical reaction between a semiconductor surface and a solution of hydrofluoric acid and hydrogen peroxide catalyzed by a noble metal architecture. With applications in electronics, optics, plasmonics, energy storage and conversion, this technique is an economical, scalable and, sometimes, self-assembled alternative for the semiconductor industry to manufacture advanced materials for devices such as battery anodes, solar cells, laser cavities, and light emitting diodes.

Background

Origins, uses, HAgNO3 comparison

Principles

Overview

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Figure 1: Metal Assisted Chemical Etching of Si

Similar to any electrochemical process, the mechanism of MacEtch encompasses the existence of a local cathode and anode, and its reactions. In such model, the metal catalyst and semiconductor interface represent the cathode and anode, respectively. Through the metal catalyst, charge injection is sustained from solution to substrate and charge is balanced by the cathodic and anodic reactions. The presence of a local site for reaction defines the selectivity of this etching mechanism; bulk Si, for example, etches at a 10nm per hour rate in MacEtch, while the regions at the vicinity of the metal catalyst etch at much faster rates in the order of 1-10nm/min. Several parameters have a significant role during this process. Among which these parameters, researchers have documented the effect of UV illumination, temperature, wafer orientation, catalyst type and shape, and HF to H₂O₂ molar ratio. It has also been reported the effect of these properties on the morphology of the etched semiconductor surface as well as its resultant crystallographic structure. Most of this characterization work has mostly been done for Silicon.

Theory

Figure 2: Metal Assisted Chemical Etching of Si

At the cathode, it is generally accepted in literature that the reduction of hydrogen peroxide takes place resulting in the injections of holes from solution into the substrate (e.g. Si) through the catalyst. Other cathodic reactions are suggested in literature**. At the anode, three mechanisms for Silicon dissolution have been proposed** some of which are shown in the Figure 2.

The Effect of Etching Parameters on Etch Rate and Side-Wall Characteristics

In light of the proposed mechanisms for MacEtch, it is expected that dopant concentration, temperature, illumination and the molar ratio of HF:H2O2 have a significant effect on the kinetics of the aforementioned mechanisms. Moreover, such parameters may also affect the side-wall orientation from being straight, slanted. Temperature, Illumination, Rho, Doping

Crystallographic Properties of Silicon Treated with MacEtch

Origins, uses, HAgNO3 comparison

Materials

For Silicon etching, MacEtch has been demonstrated with Pt, Au, Ag, Al and others... Si Wafers with doping concentrations anywhere from p- to n- have been shown to etch straight down.

Integration with Silicon Manufacturing Methods

Once the anisotropic regime was established, a cascade of researchers have demonstrated the integration of MacEtch with planar lithographical processes – interference lithography, solid-state ionic imprinting, nanoimprint lithography, e-beamlithography – and self-assembled metallic patterning processes (Anodic Alumina Oxide). The latter methods represent ways to define a 2-D metallic architecture on top of a wafer and the former procedure to sink down the metal catalyst and form 3-D cavities in Silicon.

Caption1

Applications

Lithium-ion Battery

Single-crystal Silicon is theoretically the ideal material for lithium-ion battery anodes with a charging/discharging rates and storage capacity 4-10 times* greater than conventional graphite. However, Silicon undergoes a volumetric expansion of 400%* during the battery lithiation cycle, causing the mechanical stresses to destroy the anode's material continuity. Researchers** currently propose the use of porous Si and Si nanowires as mechanical architectures that can better accommodate for such observed expansion. It is also proposed the use of MacEtch as an economical alternative process for fabricating such high-performance anodes.