Fluorine/fluoride etching

Analytical Methods. Fluorite is readily identified by its crystal shape, usually simple cubes or interpenetrating twins, by its prominent octahedral cleavage, its relative softness, and the production of hydrogen fluoride when treated with sulfuric acid, evidenced by etching of glass. The presence of fluorite in ore specimens, or when associated with other fluorine-containing minerals, may be deterrnined by x-ray diffraction. 

We have already inferred several methods by which a transition from etching to polymerization might be accomplished. These involved the effective lowering of the F/C ratio in the system by, for example, the addition of Hj or C2F to a CF, plasma or even the effective removal of fluorine by conversion to stable volatile fluorides in the etching process. The transition in the reverse direction (from polymerization to etching) may also be achieved by, for example, for example, the addition of Oj to a CjF plasma to effectively remove carbon as CO. In much of the work to be discussed below, CjFg is employed in the system whose F/C stoichiometry allows convenient rates of both etching and polymerization to be maintained. 

The effective removal of fluorine from the system by the etching process to form stable volatile fluorides tends to lower the effective fluorine/carbon ratio in the plasma and, thus, increase the concentration of unsaturated species and therefore the polymerization rate (as discussed in a previous section) at the anode. 

Potassium Bifluoride (Potassium Acid Fluoride), KHFa, mw 78.11 colorless, poisonous corrosive crysrs mp — decomp sol in w dil ale insol in abs ale. Can be prepd by crystallizing from an aq soln of KF in HF (Ref 1). Used for etching glass production of fluorine and of flux in metallurgy Rsfs ]) Gmel in-Kraut 3ysf Nunibcr 22(1937)... 

The importance of fluorine lies largely in the use of several compounds of that element. Calcium fluoride, or fluorspar (CaF2), is used as a flux and, like sodium fluoride (NaF), as an insecticide. Ammonium fluoride (NH4F) is used as a disinfectant hydrofluoric acid is used in the etching of glass and organic compounds are used as commercial refrigerants. 

Uses fluoride compounds are used in making steel, chemicals, ceramics, lubricants, dyes, plastics, and pesticides fluorine and hydrogen fluoride are used to make certain chemical compounds hydrofluoric acid is used for etching glass fluorides are also added to drinking water, dental products, toothpaste, and mouth rinses... 

The elemental reactions are, however, rather complicated and remain the subject of debate. For many studies on the kinetics of silicon oxide etching, identifying the fluoride species in the solution responsible for the etching reactions has been a major concern. This is because HF is a weak acid, which ionizes only partially to fluoride ions, and its aqueous solutions may contain a significant quantity of fluorine in the form of un-ionized HF and partially ionized bifluoride HFJ. 

In an earlier study, Judge found that in solutions of NH4F and NH4CI close to neutral pH, essentially all fluoride is present in the unprotonated state and the solutions exhibit a near zero rate of Si02 dissolution. Judge thus suggested that the free fluoride ion is not one of the species responsible for the dissolution. This conclusion was further supported by Kikuyama et who found that the etch rate of Si02 in KF solutions, in which all the fluorine is in the form of free fluoride ions, is very small as shown in Fig. 4.33. The etch rate in IM KF is 0.001 A/s, which is threefold smaller than that in a HF solution of the same concentration. 

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