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[0101] When used, the acid generator of addition type is preferably added in an amount of 0.1 to 50 parts, and more preferably 1 to 40 parts by weight per 100 parts by weight of the base polymer. 

[0102] In case the acid generator is an acid generator-and› base polymer, this acid generator is a polymer, preferably comprising recurring units derived from a compound capable of generating an acid in response to actinic light or radiation. In this case, the acid generator is preferably a base polymer to be described below, specifically comprising recurring units (f) as essential unit. 

## Base Polymer 

[0103] The chemically amplified resist composition of the invention preferably contains a base polymer. Where the resist composition is of positive tone, the base polymer comprises recurring units containing an acid labile group, preferably recurring units having the formula (al) and/or recurring units having the formula (a2). These units are simply referred to as recurring units (al) and (a2). 

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