(Meth)acrylate derivative, polymer and photoresist composition having lactone structure, and method for forming pattern by using it

A photoresist material for lithography using a light of 220 nm or less which comprises at least a polymer represented by the following formula (2) and a photo-acid generator for generating an acid by exposure: [structure] wherein R 1 , R 2 , R 3 and R 5 are each a hydrogen atom or a methyl group; R 4 is an acid-labile group or one of a specified subset of alicyclic hydrocarbon groups, alicyclic hydrocarbon groups, or hydrocarbon groups; R 6 is a hydrogen atom or one of a specified subset of hydrocarbon groups or alicyclic hydrocarbon groups; x, y and z are optional values which meet x+y+z=1, 0<x≦1, 0≦y<1 and 0≦z<1. Also disclosed is a resin having a (meth)acrylate unit of an alicyclic lactone structure represented by the formula (3): [structure] wherein R 8 is a hydrogen atom or a methyl group, and R 9 is one of a specified subset of hydrocarbon groups.