Etching fluoride electrolytes

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

Hydrogen fluoride Catalyst in some petroleum refining, etching glass, silicate extraction by-product in electrolytic production of aluminum Petroleum, primary metals, aluminum Strong irritant and corrosive action on all body tissue damage to citrus plants, effect on teeth and bones of cattle from eating plants... 

A passivating oxide is formed under sufficiently anodic potentials in HF, too. However, there are decisive differences to the case of alkaline and fluoride-free acidic electrolytes. For the latter electrolyte the steady-state current density prior to passivation is zero and it is below 1 mA cnT2 for alkaline ones, while it ranges from mA cm-2 to A cm-2 in HF. Furthermore, in HF silicon oxide formation does not lead to passivation, because the anodic oxide is readily etched in HF. This gives rise to an anodic I-V curve specific to HF, it shows two current maxima and two minima and an oscillatory regime, as for example shown in Fig. 4.7. 

If the hole concent ration in the semiconductor is relatively low, as in low resistivity n-type germanium or silicon, the available holes in the surface region are used up at low current densities and the etch rate is slow. The anodic current under these conditions can be increased by providing additional holes at the surface. Holes produced as a result of illuminating the semiconductor give uniform electrolytic etching on n-type semiconductors. Germanium is electro-lytically etched in several electrolytes while silicon can only be dissolved anodically in fluoride solutions. A thick film of amorphous silicon forms on silicon anodes in acid fluoride solutions below a critical current density. 

Whereas germanium may be electrolytically etched in a large number of electrolytes, silicon has only been dissolved anodically in fluoride solutions. Strong alkaline solutions chemically attack silicon, forming a soluble silicate and hydrogen gas, and the rate of attack increases rapidly with temperature. However if a piece of silicon is made anodic in a hot strong alkaline solution such as IN KOH, the chemical attack stops when the anode potential is greater than a critical value. 

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. 

J. C. Hake, M. M. Rieger, G. M. Schmid, and P. A. Kohl, Electrochemical etching of silicon in non-aqueous electrolytes containing hydrogen fluoride or fluoroborate, J. Electrochem. Soc. 146, 1960, 1999. 

Three processes are believed to compete during the anodisation process field-assisted oxidation of Ti to TiOi, field assisted dissolution of Ti metal ions in the electrolyte and the chemical dissolution of Ti and TiOi due to etching by fluoride ions, enhanced by the presence of (Equations 3.27-3.29). ° The chemical steps are not thought to... 

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