120  US 9,423,690 B2  119  -continued  In the resin (A) used in the composition of the present  invention, the molar ratio of respective repeating structural  units contained is appropriately set in order to control dry  etching resistance of the resist, suitability for a standard  developer, adhesion to a substrate and resist profile and  further resolution, heat resistance, sensitivity and the like  which are performances generally required for the resist.  The form of the resin (A) in the present invention may be  any form of a random type, a block type, a comb type and  a star type.  The resin (A) may be synthesized, for example, by  polymerization of radicals, cations, or anions of an unsatu› rated monomer, corresponding to each structure. In addition,  it is possible to obtain a target resin by using an unsaturated  monomer corresponding to a precursor of each structure to  polymerize, and then performing a polymerization.  Examples of a general synthesis method include a batch  polymerization method of dissolving an unsaturated mono› mer and a polymerization initiator in a solvent and heating  the solution to perform a polymerization, a dropping polym- erization method of adding dropwise a solution containing  an unsaturated monomer and a polymerization initiator to a  heated solvent over 1 to 10 hours, and the like, and a  dropping polymerization method is preferred.  Examples of the solvent used for a polymerization include  the solvent that may be used to prepare an electron beam› sensitive or extreme ultraviolet ray-sensitive resin compo› sition, which will be described later. The polymerization is  more preferably performed by using the same solvent as one  used in the photosensitive composition of the present inven- tion. Accordingly, generation of particles during storage may  be suppressed.  The polymerization reaction is preferably performed  under an inert gas atmosphere such as nitrogen or argon. As  for the polymerization initiator, the polymerization is initi- ated by using a commercially available radical initiator  (azo-based initiator, peroxide and the like). The radical  initiator is preferably an azo-based initiator, and an azo› based initiator having an ester group, a cyano group or a  carboxyl group is preferred. Preferred examples of the  initiator include azobisisobutyronitrile, azobisdimethylvale- ronitrile, dimethyl 2,2’-azobis(2-methylpropionate) and the  like. If necessary, the polymerization may be carried out in  the presence of a chain transfer agent (for example, alkyl› mercaptans, and the like).  The concentration of the reaction is 5% by mass to 70%  by mass, and preferably 10% by mass to 50% by mass. The  temperature of the reaction is usually 10°  C. to 150°  C.,  preferably 30°  C. to 120°  C., and more preferably 40°  C. to  100°  C.  After the completion of reaction, the reaction solution is  allowed to cool to room temperature and be purified. The  purification may be performed by a typical method, such as  a liquid-liquid extraction method of applying water-washing  or combining water-washing with an appropriate solvent to  remove residual monomers or oligomer components, a puri- fication method in a solution state, such as ultrafiltration of  removing only polymers having a molecular weight not  more than a specific molecular weight by virtue of extrac› tion, a reprecipitation method of adding dropwise a resin  solution in a poor solvent to solidify the resin in the poor  solvent thereby removing residual monomers and the like, a  purification method in a solid state, such as washing of the  resin slurry separated by filtration with a poor solvent, and  the like. For example, the resin is precipitated as a solid by  contacting the reaction solution with a solvent (poor solvent)  in which the resin is sparingly soluble or insoluble, in a  volumetric amount of 10 times or less and preferably 10 to  5 times the reaction solution.  The solvent used at the time of operation of precipitation  or reprecipitation from the polymer solution (precipitation or  reprecipitation solvent) may be sufficient if the solvent is a  poor solvent for the polymer, and the solvent may be  appropriately selected from a hydrocarbon, a halogenated  hydrocarbon, a nitro compound, ether, ketone, ester, carbon› ate, alcohol, carboxylic acid, water, and a mixed solvent  including these solvents, according to the kind of the poly› mer, and may be used. Among these solvents, a solvent  including at least alcohol (particularly, methanol or the like)  or water is preferred as the precipitation or reprecipitation  solvent.  The amount of the precipitation or reprecipitation solvent  used may be appropriately selected by considering the  efficiency, yield and the like, but in general, the amount is  100 parts by mass to 10,000 parts by mass, preferably 200  by parts by mass to 2,000 parts by mass, and more preferably  300 parts by mass to 1,000 parts by mass, based on 100 parts  by mass of the polymer solution.  The temperature at the time of precipitation or reprecipi- tation may be appropriately selected by considering the  efficiency or operability but is usually in the order from 0 to  50°  C., and preferably in the vicinity of room temperature  (for example, approximately from 20°  C. to 35°  C.). The  precipitation or reprecipitation operation may be performed  by a known method such as batch system and continuous  system using a commonly employed mixing vessel such as  a stirring tank.  The precipitated or reprecipitated polymer is usually  subjected to commonly employed solid-liquid separation  such as filtration and centrifugation, then dried and used.  The filtration is performed by using a solvent-resistant filter  element, and preferably under pressure. The drying is per› formed under atmospheric pressure or reduced pressure  (preferably under reduced pressure) at a temperature of 