A photoacid generator has formula (1) wherein R is H, F, Cl, nitro, alkyl or alkoxy, n is 0 or 1, m is 1 or 2, r is an integer of 0-4, and r' is an integer of 0-5. A chemically amplified resist composition comprising the photoacid generator has advantages including a high resolution, focus latitude, long-term PED dimensional stability, and a satisfactory pattern profile shape. When the photoacid generator is combined with a resin having acid labile groups other than those of the acetal type, resolution and top loss are improved. The composition is suited for deep UV lithography. ##STR00001##