Triphenylsulfonium hexafluoroarsenate

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. 

Ito has also extended this type of photochemistry to the electron-beam-induced catalytic acidolysis of acid-labile main chain acetal linkages in polyphthaldehyde. These polymers, like the poly(2-methylpentene-l-suIfone) (PMPS) sensitizer in NPR resist described earlier have ceiling temperatures on the order of -40 °C. As normally used, the polyaldehydes are end-capped by acylation or alkylation and are thus quite stable. The main chain bonds are very sensitive to acid-catalyzed cleavage which in turn allows the whole chain to revert to monomer in an unzipping sequence similar to that occuring in irradiated PMPS. Irradiation of polyphthaldehyde containing 10% of a suitable sensitizer such as triphenylsulfonium hexafluoroarsenate with either deep UV... 

PBOCST is thermally stable to ca. 200°C. Above 200°C, the polymer loses about 45% of its weight as carbon dioxide and isobutene (15). Diphenyliodonium and triphenylsulfonium hexafluoroarsenates are thermally stable to ca. 250° and 300°, respectively. (16,17) Consequently, the resist formulated from PBOCST and these salts is stable to the baking conditions required for formation of high quality spin coated films, and the formulations have a long shelf life when stored at room temperature under yellow light. 

Figure 2. In ared spectra of poly(2,3-ciiaceto -5-norbomene) containing 12 mol% triphenylsulfonium hexafluoroarsenate a) not irradiated b) after UV irradiation c) heated to 150 °C for 2 min d) heated to 200 °C for 10 min e) heated to 350 °C for 30 min...

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). 

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. 

The UV sensitivity of the polyphthalaldehyde-onium salt system is dependent on the concentration and structure of the onium salts. At 10 wt% loading the sensitivity to narrow band width 254 nm radiation is 1-7 mJ/cm2 and is insensitive to the structure of the salts. However, at 2 wt% loading of triphenylsulfonium or diphenyliodonium hexafluoroarsenates or -hexyloxybenzenediazonium tetrafluoroborate much higher doses are required to achieve self-development and the sensitivity decreases with the salt structures in the sequence listed. 

Dudgeon (25) describes systems suitable for curing in deep sections (e,g., as auto body solders) which may be adapted for heat or cationic UV curing (or a combination of both), Fast curing (2-3 min) systems with full depth cure are based on CTBN or HTBN modification of an epoxycyclohexyl ether and mixtures of triphenylsulfonium hexafluoro-antimonate (50% in propylene carbonate) and diphenyl-iodonium hexafluoroarsenate (50% in MEK) with low levels of copper naphthenate. These systems operate at high talc loadings to form deep-section curable putties. Examples suffice where a cluster of two infrared lamps and two ultraviolet lamps operate efficiently to effect cure. 

---