Onium salts photochemistry

Onium Salts, Photochemistry and Photophysics of (DeVoe, Olofson, and... 

One final example of the application of onium salt photochemistry in positive resist materials should be mentioned, because it does not include any postexposure acid-catalyzed processes and therefore does not encompass the principle of chemical amplification (79). Interestingly, Newman (79) has determined that onium salts themselves can inhibit the dissolution of novolac in aqueous base and that irradiation of such an onium salt-novolac resist restores the solubility of the resin in developer and leads to a positive-tone image. In this application, the onium salt behaves like diazonaphthoquinone in a typical positive resist. Recently, Ito (80) has reported also the use of onium salts as novolac dissolution inhibitors. 

Important mechanistic insights have been obtained in the last 5 years, particularly owing to the availability of such modern techniques as laser-flash time-resolved transient absorption spectroscopy [22,23], photo-CIDNP [21c, 24], and so on. It is thus the particular purpose of this review to analyze this recent work on mechanisms for photodecomposition of iodonium and sulfonium salts, and to evaluate its implications for practical applications of onium salt photochemistry. 

Recently, nonionic acid precursors based on nitrobenzyl ester photochemistry have been developed for chemically amplified resist processes (78-80). These ester based materials (Figure 8) exhibit a number of advantages over the onium salt systems. Specifically, the esters are easily synthesized, are soluble in a variety organic solvents, are nonionic in character, and contain no potential device contaminants such as arsenic or antimony. In addition, their absorption characteristics are well suited for deep-UV exposure. 

A recent volume in the Advances in Photochemistry series contains a chapter devoted to the photochemistry and photophysics of onium salts, including aryliodonium salts486. 

Chart 3.2. Photochemistry of onium salt cationic photoiniUators. 

The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

MXn = bf4, PF6, AsF6,SbF6, etc. Scheme 1. Photochemistry of onium salts. 

The larger portion of the current literature on photochemistry of onium salts deals either with fundamental mechanisms of onium salt photodecomposition (see above) or application of this chemistry to generation of catalysts for cationic polymerizations (see below). Application to the photoinitiation of radically polymerizable monomer systems developed in parallel with applications to cationic polymerizations [17,96, 111]. 

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