Anodic etching, silicon

Kelly MT, Chun JKM, Bocarsly AB (1994) High efficiency chemical etchant for the formation of luminescent porous silicon. Appl Phys Lett 64 1693-1695 Kidder JN Jr, Williams PS, Pearsall TP, Schwartz DT, Nosho BZ (1992) Comparison of light emission from stain-etch and anodic-etch silicon films. Appl Phys Lett 61 2896-2898 Koker L, Wellner A, Sherratt PAJ, Neuendorf R, Kolasinski KW (2002) Laser-assisted formation of porous silicon in diverse fluoride solutions hexafluorosilicate deposition. J Phys Chem B 106 4424-4431... 

The formation of etch pits and tunnels on n-Si during anodization in HF solutions was reported in the early 1970 s. It was found that the solid surface layer is the remaining substrate silicon left after anodic dissolution. The large current observed on n-Si at an anodic potential was postulated to be due to barrier breakdown.5,6 By early 80 s7"11 it was established that the brown films formed by anodization on silicon substrate of all types are a porous material with the same single crystalline structure as the substrate. 

Aqueous electrolytes of high pH etch silicon even at open circuit potential (OCP) conditions. The etch rate can be enhanced or decreased by application of anodic or cathodic potentials respectively, as discussed in Section 4.5. The use of electrolytes of high pH in electrochemical applications is limited and mainly in the field of etch-stop techniques. At low pH silicon is quite inert because under anodic potentials a thin passivating oxide film is formed. This oxide film can only be dissolved if HF is present. The dissolution rate of bulk Si in HF at OCP, however, is negligible and an anodic bias is required for dissolution. These special properties of HF account for its prominent position among all electrolytes for silicon. Because most of the electrochemistry reported in the following chapters refers to HF electrolytes, they will be discussed in detail. 

Electrochemical properties of silicon single crystals, usually cuts of semiconductor wafers, have to be considered under two distinct respects (1) As an electrode, silicon is a source of charge carriers, electrons or positive holes, involved in electrochemical reactions, and whose surface concentration is a determining parameter for the rate of charge transfer. (2) As a chemical element, silicon material is also involved in redox transformations such as electroless deposition, oxide generation, and anodic etching, or corrosion processes. 

Electrolytic (anodic) etching of silicon occurs in HF solutions (5). Using concentrated HF, the silicon is dissolved in the divalent state ... 

A spatial distribution of Pb centres (g = 2.0029, g — 2.0086) has been revealed in porous silicon formed by anodic etching of crystalline silicon in hydrofluoric acid.144 Oxygen ions were also implanted using accelerator at 2 MeV and compared with photoluminescence. 

Corman, T., Enoksson, P., Stemme, G., Deep wet etching of borosilicate glass using an anodically bonded silicon substrate as mask J. Micromech. Microeng. 1998, 8, 84-87. 

A number of different fluoride salts have been used as electrolytes for electropolishing silicon. Not only must the fluoride salt be sufficiently water soluble but the anode reaction product, the corresponding fiuosilicate, must also be readily soluble in water. Potassium fluoride is highly water soluble but the fiuosilicate is not. At a critical anodic etch rate silicon will passivate in KF solutions due to the precipitation of SiFg on the surface. Ammonium fluoride and ammonium fiuosilicate are both sufficiently water soluble. The acids HF and SiFg are even more water soluble. 

There are essentially three different ways how to prepare nanometer sized silicon particles. The porous silicon is, as already mentioned, prepared by anodic etching of silicon wafers in an HF/ethanol/water solution [6, 7]. The microporous silicon has typically a high porosity of 60-70 vol.%, and it consists of few nm thin wires which preserve the original orientation of the wafer. The thickness of the wires varies within the PS layer and the material is very brittle. Free standing PS films can be prepared by application of a high current density after the usual etching of the desired thickness of the PS. 

Reaction Mechanism for Anodic Etching of Silicon in Fluoride Electrolytes. 104... 

C. Serre, S. Barret, and R. Herino, Characterization of the electropolishing layer during anodic etching of p-type silicon in aqueous solutions, J. Electrochem. Soc. 141, 2049, 1994. 

The mechanism of electrochemical etching to produce porous silicon has been studied by a number of researchers [11-13]. Although it is certain that several different reactions are occurring simultaneously, anodic etching of crystalline silicon ultimately leads to oxidation and dissolution of the surface to silicon hexafluoride (Scheme 16.1). Under these conditions, Si-Si bonds are electrochemically activated and react with fluoride ions to form soluble, molecular perfluoro species solvation of these silicon fluorides by the etching medium yields a physically irregular, high area porous silicon matrix. Visual indicators for the anodization are the appearance... 

Fig. 16.8. I mages of patterned silicon substrates, (a) SEM image of 100 pm wide Si02 mesas on silicon, (b) Optical image of (a) after anodic etching of the silicon surface, (c) Optical image of (b) after cathodization in...

Electroless deposition of the catalytic Pt or Pt-Ru layer was proposed for the preparation of electrodes in microdirect methanol fuel cells.53 A porous silicone substrate is prepared by the anodic etching in HF-ethanol-water (1 1 1) solution. After the etching, at the surface of porous silicon substrate, a thin film of titanium is sputtered and then a film of Pt or Pt-Ru alloy with thickness of about 150-200 nm was electroless deposited. The electrodes prepared in this way helped in minimization of the fuel cell size and increased the reactive area of the catalyst over the silicon electrode surface. 

Bailes M., Bohm S., Peter L. M., Ridley D. J. and Greef R. (1998), An electrochemical and ellipsometric study of oxide growth on silicon during anodic etching in fluoride solutions , Electrochim. Acta 43, 1757-1772. 

M.K. Andrews, G.C. Turner, Bipolar effects in the fabrication of silicon membranes by the anodic etch stop, Sens. Actuators 1991, A29, 49-57. 

Another remarkable synthetic effort has been made by the preparation of colloidal Q-particles of the technically more relevant IV-IV materials (i.e., silicon and germanium) [20-25]. Silicon nanoparticles, especially, are currently drawing a lot of attention, since it was found by Canham [26] that nanostructured silicon formed under anodic etching of silicon wafers (called porous silicon ) exhibits bright red fluorescence. Due to the indirect nature of the band transition, bulk silicon shows, by contrast, almost no fluorescence and thus cannot be utilized for optoelectronic devices. 

The samples with porous layers were fabricated by electrochemical anodic etching of p-type, 12 Ohmcm and n-type 0.01 Ohmcm monocrystalline silicon wafers in 48 % water solution of HF at the current density of 50 mA/cm2. The anodized area of 1 cm in diameter was defined by the window in a Si3N4 thin film mask deposited onto the wafers. The anodization time was chosen in the range of 15-90 min in order to get porous layers of a thickness from 30 to 180 pm. The integral porosity was estimated by gravimetry to be of about 60 %. 

The large effect of bulk doping and Fermi level on surface reactivity is seen when n-type and p-type silicon single crystals are etched. A high Fermi level in the silicon (upward curling band edges) etches with formation of clean crystal planes, while the surface of anodically etched p-type silicon is ramified and highly porous. 

Another example of a fractal surface is anodically etched p-type silicon. It has a fractal porosity and shows photoluminescence and even electroluminescence, which would be impossible in compact bulk silicon. Electrons behave differently in fractally distributed solids than in three-dimensional extended lattices they are supposed to be more localized. The density of states p in a three-dimensional box is... 

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