Metal-assisted Chemical Etching (MacEtch) is a novel nanofabrication method we have discovered (Appl. Phys. Lett. 77, 2572 (2000) and Patent US#6,790,785.) originally to produce porous silicon and then developed to produce extremely high aspect ratio semiconductor nanostructures including Si, Ge, GaAs, InGaAs, InP, SiC, GaN, Ga2O3 homo- and hetero-junctions. It uses noble metal (such as Au, Pt and Ag) deposited on the surface of a semiconductor (e.g. Si) as a catalyst to catalyze the hole (h+) generation from an oxidant (such as H2O2) in an acidic (or basic) solution (such as HF) to induce local oxidation (Si + 4h+ --- Si4+) and reduction (2H+ + 2e- --- H2) reactions. This results in the removal of semiconductor materials without net consumption of the metal. Under controlled consitions, the reactions occur only at the interface between metal and the semiconductor, under controlled etching conditions. As a result, metal descends into the semiconductor as the semiconductor is being etched right underneath, acting as a negative resist etch mask. When the catalyst metal is patterned in any shape and dimension, the pattern can be engraved into the semiconductor to produce micro and nanostructures including arrays of pillars for energy harvesting and storage, vias for photonic crystals, and arbitrary shape and patterns for applications including metamaterials. MacEtch is essentially a wet etching method yet produces anisotropic high aspect ratio semiconductor micro and nanostructures without incurring lattice damage.
Metal-assisted chemical etching of silicon is an electroless method that can produce porous silicon by immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias. We have been studying the metal-assisted hydrofluoric acid etching of silicon using dissolved oxygen as an oxidizing agent. Three major factors control the etching reaction and the porous silicon structure: photoillumination during etching, oxidizing agents, and metal particles. In this study, the influence of noble metal particles, silver, gold, platinum, and rhodium, on this etching is investigated under dark conditions: the absence of photogenerated charges in the silicon. The silicon dissolution is localized under the particles, and nanopores are formed whose diameters resemble the size of the metal nanoparticles. The etching rate of the silicon and the catalytic activity of the metals for the cathodic reduction of oxygen in the hydrofluoric acid solution increase in the order of silver, gold, platinum, and rhodium.
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To examine the catalytic activity of the metals for oxygen reduction, we measured the current density versus the potential curves of the metal electrodes in the oxygen-saturated HF solution (Figure 6). The cathodic current obtained in the potential range indicated in Figure 6 is for the oxygen reduction because no cathodic current was obtained for the oxygen-free HF solution with argon bubbling as the fine solid curve for the Rh electrode in Figure 6. The on-set potential of the cathodic current shifted towards the positive direction in the same order of Ag, Au, Pt, and Rh as the etching rate of Si. Also, the potential for the current density corresponding to the etching rate of Si shifted towards the positive direction in the same order. These results indicate that the catalytic activity of the metals to the cathodic oxygen reduction in the HF solution increases, and the overpotential of the local cathodic reaction of the metal-assisted HF etching of Si decreases in the order of Ag, Au, Pt, and Rh. The decrease in the overpotential of the local cathodes positively shifts the mixed potential of the local galvanic cells and increases the current density of the galvanic cells, i.e., the etching rate of Si.
Current density versus potential curves of metal electrodes in oxygen-saturated HF solution. Lines indicate the following metal electrodes: dotted, Ag; dashed, Au; dot-dashed, Pt; and thick-solid, Rh. Fine-solid line indicates Rh electrode in oxygen-free HF solution (pure argon bubbling).
In this study, we investigated the catalytic activity of noble metals, Ag, Au, Pt, and Rh, for the metal-assisted HF etching of Si using dissolved oxygen as an oxidizing agent. The catalytic activity of the noble metals for the cathodic oxygen reduction in a HF solution increases in the order of Ag, Au, Pt, and Rh. The activity controls the local cathodic reaction of the metal-assisted HF etching of Si under dark conditions. Thus, the etching rate of Si is determined by the dissolved oxygen concentration, the kind of metals, and the metal coverage of the Si surfaces.
Silicon (Si) nanostructures are important building blocks for many technological applications. In particular, silicon nanowires (NWs) find a wide array of usage in areas such as electronics1,2, photovoltaics3,4, energy storage5, optical devices6,7, catalysis8, drug delivery9, thermoelectrics10, as well as biological and chemical sensors11,12,13. The importance of fabricating reliable Si nanowires has provided the motivation for the exploration of many different fabrication methods. These include vapour-liquid-solid growth, molecular beam epitaxy, and reactive ion etching. Of the available techniques, metal-assisted chemical etching (MacEtch) remains a key approach in achieving large area and low temperature fabrication at relatively low cost. MacEtch has also the added advantage of achieving ordered crystalline Si nanostructures without damages, as opposed to those prepared using high-energy ions in reactive ion etching. As such, various reports have utilized MacEtch to yield the fabrication of well-defined and high quality semiconductor nanostructures, such as nanowires, nanoplates or nanofins and bulk microstructures14,15,16,17,18,19.
There has been a great deal of progress made in understanding the MacEtch process and this is well summarized in a few reviews20,21. Here, we briefly describe the key steps in the currently accepted MacEtch mechanism. The metal catalyst (e.g., Au or Ag) is the key ingredient that accelerates the etching of Si. This is achieved by accelerating the generation of hole charges when hydrogen peroxide (H2O2) is reduced at the electrolyte-metal catalyst interface, according to the following reaction:
This results in accelerated etching for those Si regions that are in contact with the metal catalyst. The nature of the method allows for the creation of desired structures simply by patterning the metal catalyst on the Si substrate20,21. For example, uniform silicon nanowires can be fabricated by MacEtch using mesh-patterned metal catalyst films on Si.
While the above-mentioned model is generally well accepted, there are still a few key phenomena that lacked coherent explanations. In particular, there is still a lack of understanding in the fundamental reasons behind the stop-etch phenomenon shown by metals like chromium (Cr)22,23,24,25, and also debates about the mass transport during etching4,15,18,20,23,26,27,28. This knowledge is important to understand the basic MacEtch mechanism. Thereafter, this knowledge can enhance the versatility of design by providing choices in the selection of blocking metals or metal catalysts. In this work, we will focus on three key aspects of MacEtch related to transport/transfer processes. The first is the mass transport of the chemical species involved in the etching. The second is the transport process of hole charges to Si and finally, the oxidative charge transfer process at the metal/Si interface. We provide clear evidence and understanding on the transport pathway of the reagents and by-products, which have previously been elusive. We also show that the charge injection process at the metal/Si interface can be governed by a reaction driven transfer. Finally, we demonstrate that the charge transport process to the Si interface can be accomplished by ion transport, instead of the widely accepted electronic hole conduction. The ion transport process provides a more unified picture for MacEtch and helps to explain several phenomena: (1) Creation of porous layer in the metal catalyst, (2) failure of both work function and electronegativity in governing the charge transfer, and (3) reconciliation of metal ions etching and thin film etching.
One of the debated topics in MacEtch is the transport of the reagents and by-products during the etching process. These can be classified into three different proposed models, illustrated as Models I, II and III in Fig. 2. The first model (Model I) describes the transport of Si atoms through the metal before they are oxidized and etched at the metal/solution interface20. Models II and III are more frequently cited when describing the MacEtch process, and both involve the oxidation and etching of Si at the metal/Si interface instead. The difference lies in how the reagents and by-products are transported into and out of the metal/Si interface. Model II describes this transport via the boundary of the metal/Si interface23,26,27,28 while Model III describes the transport through the porous metal film4,15,18.
In Model I, Si atoms diffuse through the metal catalyst and are oxidized and etched at the metal/solution interface. In Model II, reagents and by-products of etching diffuse along the metal/Si interface. In Model III, reagents and by-products of etching diffuse through the porous metal catalyst.
In the current understanding of the MacEtch process, holes that are catalytically generated at the surface of the metal film are then transported to the metal/Si interface, whereby Si oxidation occurs via hole injection into Si, or electron extraction from Si26,34. Since the electronic conductivity in the metal catalyst is not expected to be an issue, the transport barrier at the interface appears to be the limiting factor in controlling the oxidation of Si. Therefore, in this section, we set out to investigate the charge transfer at the metal/Si interface in the MacEtch process by designing bilayer structures as shown in Fig. 5a(i). The top layer of the catalyst is designed to remain as Au to serve two functions. The first is to provide the same reduction catalyst, thus keeping the hole generation rate consistent at the etching solution interface. The second function is to act as a protection layer for the bottom metal layer. In doing so, we can now change the type of metal in the bottom layer to investigate how etching is affected when properties, such as the contact barrier, is altered. Our group has previously shown that such a bilayer structure using Au and Cr acted effectively as an etching barrier22. In this work, we will extend the range of metals used as the bottom layer to include titanium (Ti), silver (Ag) and nickel (Ni). The choice of metals used is dictated by both the range of their properties, such as the metal work function or electronegativity, and the stability of the metal in the etching solution35. 2ff7e9595c
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