An onium salt is provided. The chemically amplified resist composition can be processed by DUV or EUV lithography to form a resist pattern with improved resolution, reduced LWR, and collapse resistance. A resist composition comprising an onium salt having a nitrogen-containing aliphatic heterocycle and an aromatic carboxylic acid structure as a quencher is provided. When processed by deep-UV or EUV lithography, the resist composition exhibits a high resolution and reduced LWR and prevents the resist pattern from collapsing.