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[0103] Under the action of acid, a polymer comprising recurring units of formula (a) is decomposed to generate carboxylic acid, turning to be alkali soluble. 

[0104] The acid labile group XA may be selected from a variety of such groups. Examples of the acid labile group include groups of the following formulae (Ll) to (L4), tertiary alkyl groups of 4 to 20 carbon atoms, preferably 4 to 15 carbon atoms, trialkylsilyl groups in which each alkyl moiety has 1 to 6 carbon atoms, and oxoalkyl groups of 4 to 20 carbon atoms. 

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[0105] In formula (Ll), RLoi and RL $^{02 }$each are hydrogen or a straight, branched or cyclic alky 1 group of 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms. Exemplary alkyl groups include methyl, ethyl, propyl, isopropyl, n-butyl, s-butyl, t-butyl, cyclopentyl, cyclohexyl, 2-ethylhexyl, n-oc› tyl, norbomyl, tricyclodecanyl, tetracyclododecanyl, and adamantyl. 

[0106] RL $^{03 }$is a monovalent hydrocarbon group of 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms, which may contain a heteroatom such as oxygen, examples of which include unsubstituted straight, branched or cyclic alkyl groups and substituted forms of such alkyl groups in which some hydrogen atoms are replaced by hydroxyl, alkoxy, oxo, amino, alkylamino or the like, or in which at least one carbon atom is replaced by a heteroatom such as oxygen. 

