Onium salts, photoacid generators for resist compositions, resist compositions, and patterning process

An onium salt of formula (1):

• wherein R1 is an unsubstituted, straight, branched or cyclic alkyl group of 1 to 10 carbon atoms, the group
• an unsubstituted aryl group of 6 to 14 carbon atoms, 4-methylphenyl, 4-ethylphenyl, 4-methoxyphenyl, 4-tert-butylphenyl, 2,4-dimethylphenyl, 2,4,6-trimethylphenyl, or 2,4,6-triisopropylphenyl, R2 which may be the same or different is hydrogen or a an oxo- or hydroxy-substituted or unsubstituted, straight, branched or cyclic alkyl group of 1 to 6 carbon atoms, p is an integer of 1 to 5, q is an integer of 0 to 4, p+q=5, R3 which may be the same or different is a substituted or unsubstituted, straight, branched or cyclic alkyl group of 1 to 10 carbon atoms or substituted or unsubstituted aryl group of 6 to 14 carbon atoms, M is a sulfur or iodine atom, and “a” is equal to 3 when M is sulfur and equal to 2 when M is iodine.