Iodonium salt, chemically amplified positive resist composition and resist pattern forming process

A iodonium salt having the formula (A):

• wherein n1 is 0 or 1, n2 is 0, 1, 2, 3 or 4, n3 is 0 or 1, n4 is 0, 1, 2, 3 or 4,
• LA is a single bond, ether bond, ester bond, sulfonic ester bond, carbonate bond or carbamate bond,
• Ralk is a C6-C18 alkyl group or C4-C18 fluorinated alkyl group;
• when R1 is a C6-C18 alkyl group, the alkyl group has at least one straight chain structure of 6 or more carbon atoms;
• when Ralk is a C4-C18 fluorinated alkyl group, the fluorinated alkyl group has at least two groups selected from —CF2— and —CF3;
• the alkyl or fluorinated alkyl group may have some —CH2— replaced by an ether bond or carbonyl group and may contain a cyclic structure selected from cyclopentane, cyclohexane, adamantane, norbornyl and benzene rings at its end or in a carbon-carbon bond,
• R1 is halogen, nitro, cyano, hydroxy, a C1-C20 hydrocarbyl group which may contain a heteroatom, C1-C20 hydrocarbyloxy group which may contain a heteroatom, or C1-C20 hydrocarbylthio group which may contain a heteroatom;
• when n2 is 2, 3 or 4, a plurality of R1 may be identical or different and may bond together to form a ring with the carbon atoms to which they are attached,
• R2 is halogen, nitro, cyano, hydroxy, a C1-C20 hydrocarbyl group which may contain a heteroatom, C1-C20 hydrocarbyloxy group which may contain a heteroatom, or C1-C20 hydrocarbylthio group which may contain a heteroatom;
• when n4 is 2, 3 or 4, a plurality of R2 may be identical or different and may bond together to form a ring with the carbon atoms to which they are attached,
• two aromatic rings attached to I+ in the formula may bond together to form a ring with the iodine atom to which they are attached.