Hexafluoroarsenate

Submitted by ANDREJ MALC and KAREL LUTAR Checked by SCOTT A. KINKEADf 

Dioxygenyl hexafluoroarsenate can be prepared by the reaction between arsenic pentafluoride and dioxygen difluoride or by heating a mixture of arsenic pentafluoride, fluorine, and oxygen. However, photosynthesis, using this latter mixture, appears to be the most convenient method for the preparation of dioxygenyl hexafluoroarsenate. 

The same photochemical reactor and apparatus is used for this synthesis as described previously (see procedure for xenon difluoride). About 280 mbar of 

for [OJ [AsFg] F, 51.60% As, 33.91% O, 14.49%. Found F, 50.8% As, 33.7%. Infrared spectra show only the band associated with AsFJ. 

Dioxygenyl hexafluoroarsenate is a white, loose, nonvolatile powder. It is stable at room temperature, but above 100°C it decomposes rapidly. It hydrolyzes readily, and therefore should be handled in a dry box. 

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Arsenic forms the binary compounds arsenous triduoride and arsenic pentafluoride, as well as a series of compounds and the acid of the very stable hexafluoroarsenate ion. 

It is used as a fluorinating reagent in semiconductor doping, to synthesi2e some hexafluoroarsenate compounds, and in the manufacture of graphite intercalated compounds (10) (see Semiconductors). AsF has been used to achieve >8% total area simulated air-mass 1 power conversion efficiencies in Si p-n junction solar cells (11) (see Solarenergy). It is commercially produced, but usage is estimated to be less than 100 kg/yr. 

Hexafluoroarsenic acid [17068-85-8] can be prepared by the reaction of arsenic acid with hydrofluoric acid or calcium fluorosulfate (29) and with alkaH or alkaline-earth metal fluorides or fluorosulfonates (18). The hexafluoroarsenates can be prepared directly from arsenates and hydrofluoric acid, or by neutrali2ation of HAsF. The reaction of 48% HF with potassium dihydrogen arsenate(V), KH2ASO4, gives potassium hydroxypentafluoroarsenate(V)... 

Because of the special stabiHty of the hexafluoroarsenate ion, there are a number of appHcations of hexafluoroarsenates. For example, onium hexafluoroarsenates (33) have been described as photoinitiators in the hardening of epoxy resins (qv). Lithium hexafluoroarsenate [29935-35-1] has been used as an electrolyte in lithium batteries (qv). Hexafluoroarsenates, especially alkaH and alkaline-earth metal salts or substituted ammonium salts, have been reported (34) to be effective as herbicides (qv). Potassium hexafluoroarsenate [17029-22-0] has been reported (35) to be particularly effective against prickly pear. However, environmental and regulatory concerns have severely limited these appHcations. 

Arsenic trifluoride (arsenic(III) fluoride), AsF, can be prepared by reaction of arsenic trioxide with a mixture of sulfuric acid and calcium fluoride or even better with fluorosulfonic acid. Chlorine reacts with ice-cold arsenic trifluoride to produce a hygroscopic soHd compound, arsenic dichloride trifluoride [14933-43-8] ASCI2F35 consisting of AsQ. and AsF ions (21). Arsenic trifluoride forms a stable adduct, 2AsF2 SSO, with sulfur trioxide and reacts with nitrosyl fluoride to give nitrosonium hexafluoroarsenate(V) [18535-07-4] [NO][AsFg]. 

Arsenates are oxidizing agents and are reduced by concentrated hydrochloric acid or sulfur dioxide. Treatment of a solution of orthoarsenate with silver nitrate in neutral solution results in the formation of a chocolate-brown precipitate of silver orthoarsenate, Ag AsO, which may be used as a test to distinguish arsenates from phosphates. With hydrofluoric acid, orthoarsenate solutions yield hexafluoroarsenates, eg, potassium hexafluoroarsenate [17029-22-0] (KAsFg)2 H2O. Arsenates of calcium or lead are used as insecticides sodium arsenate is used in printing inks and as a mordant. 

Perylene-3,4,9,10-bis(dicarbonamide) electrical conductivity, 1, 358 Perylene hexafluoroarsenate conductors, 1, 355 Perylene vat dyes, 1, 336-337 Pestalotin synthesis, 3, 841 Pethidine... 

Hexa/luorobenzene, perfluoromethylbcnzene, and octafluoronaphthalene are oxidized to the corresponding cation salts by dioxygenyl hexafluoroarsenate [45] (equation 38) Salts of monocyclic perlluoroaromatics are unstable above 15 °C, whereas that of octafluoronaphthalene is indefmitely stable at room temperatures... 

Lithium hexafluoroarsenate is thermally stable [54, 55] but shows environmental risks due to possible degradation products [56-58], even though it is itself not very toxic. Its LD 50 value is similar to that of lithium perchlorate [55]. Just like lithium hexafluorophosphate, it can initiate the polymerization of cyclic ethers. Polymerization may be inhibited by tertiary amines [59], or 2-methylfuran [60], yielding highly stable electrolytes. 

Lithium glasses, advantage of, 14 28-29 Lithium halides, 15 134, 138-140 Lithium hexafluoroarsenate, in lithium cells, 3 459... 

For comparison, the solution quantum yield was determined by the merocyanine dye technique. Acetonitrile solutions of triphenylsulfonium hexafluoroantimonate were irradiated with a 5 m. /cm2 dose. Dye solution was added and the acid content was determined by changes in dye absorption. The quantum yield for acid production was determined to be 0.8, which agrees reasonably well with the value (0.71) determined for the hexafluoroarsenate salt (8). 

Much less work has been focused on the effect of polymer structure on the resist performance in these systems. This paper will describe and evaluate the chemistry and resist performance of several systems based on three matrix polymers poly(4-t-butoxycarbonyloxy-a-methylstyrene) (TBMS) (12), poly(4-t-butoxycarbonyloxystyrene-sulfone) (TBSS) (13) and TBS (14) when used in conjunction with the dinitrobenzyl tosylate (Ts), triphenylsulfonium hexafluoroarsenate (As) and triphenylsulfonium triflate (Tf) acid generators. Gas chromatography coupled with mass spectroscopy (GC/MS) has been used to study the detailed chemical reactions of these systems in both solution and the solid-state. These results are used to understand the lithographic performance of several systems. 

The dinitrobenzyl tosylate, (15) triphenylsulfonium hexafluoroarsenate (16), and triphenylsulfonium triflate (17) were prepared as described in the literature. The monomers, 4-t-butoxycarbonyloxy-a-methylstyene (t-BOC-a-methylstyrene), and 4-t-butoxycarbonyloxystyrene (t-BOC-styrene) and their respective homopolymers, TBS and TBMS were prepared as described in the literature (12,14). TBSS was prepared by conventional, free-radical methods (13,18). The composition of this polymer (ratio of SO2 to t-BOC styrene) is controlled by changing the polymerization temperature and/or initiator concentration (Table II). 

The triphenylsulfonium trifluoromethanesulfonate (Tf) photoactive acid generator affords the highest sensitivity (3-5 mJ cm-2) for all polymer systems studied. The contrast for these systems ranged between 2 and 6 and sub-micron resolution was obtained with all the materials. Resist systems using the triphenylsulfonium hexafluoroarsenate (Ar) precursor exhibited slightly lower sensitivities (16-20 mJ cm-2) while contrast values were similar, i.e., 2-6. Upon formulation with 5 wt% 2,6-dinitrobenzyl tosylate (Ts) the substituted styrenes exhibited still lower sensitivities (65-170 mJ cm2) and contrast remained in the range of 2-6. 

As mentioned above, the conventional diazonium salts have good optical properties as CEL dyes and negative working sensitizers for the two-layer resist system. However, almost all diazonium salts are stabilized with metal-containing compounds such as zinc chloride, tetrafluoroborate, hexafluoro-antimonate, hexafluoroarsenate, or hexafluorophosphate, which may not be desirable in semiconductor fabrication because of potential device contamination. To alleviate the potential problem, new metal-free materials have been sought for. 

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