US 10,180,626 B2  59  60  Exemplary structures for the inventive sulfonium salt  include arbitrary combinations of cations with anions, both  as exemplified above.  The resist composition comprising the inventive sulfo› nium salt is improved in lithography properties such as EL,  MEF and LWR for the following reason. The nitrogen atom  in the triarylamine structure contained in the inventive  sulfonium salt has a very low basicity due to the conjugated  effect with the three aryl rings and the inductive effect of  sulfonium ion and aryl groups. The conjugated acid exhibits  an acidity as demonstrated by a pKa value of about -7 to -3.  However, since the sulfonium ion is decomposed in the  course of acid generation upon exposure, the inductive effect  is lost, and the nitrogen atom has a pKa value of about -5  to -2. This acidity is at an equivalent level as compared with  an a,a-difluorosulfonic acid generating PAG which is com› mon as the PAG in ArF resist compositions. For example,  the PAG having 2-acyloxy-1,1,3,3,3-pentafluoropropane-l- -continued  oroJ(,  S-0  o-7’11  0  CN-~--+-1--+--1  -+--1  s~  0  F  F  F  ,--{:N-~-1  lf-------+--1 s~  ~  0  F  F  F  0  /\  II  0  N-S  \_J  II  0  0  0- 0-~  II  0 