US 10,101,653 B2  60  59  -continued  The sulfonium salt having formula (1-1) or iodonium salt  having formula (1-2) may be synthesized, for example, by  ion exchange with a sulfonium or iodonium salt of weaker  acid than the iodinated phenoxy or iodinated phenylalkoxy› containing fluorinated sulfonic acid. Typical of the weaker  acid than the iodinated phenoxy or iodinated phenylalkoxy› containing fluorinated sulfonic acid are carboxylic acid,  hydrochloric acid and carbonic acid. Alternatively, the sul› fonium or iodonium salt may be synthesized by ion  exchange of a sodium salt of an iodinated phenoxy or  iodinated phenylalkoxy-containing fluorinated sulfonic acid  with a sulfonium or iodonium chloride.  In the resist composition, the sulfonium salt having for› mula (1-1) or iodonium salt having formula (1-2) is prefer› ably used in an amount of 0.01 to 1,000 parts, more  preferably 0.05 to 500 parts by weight per 100 parts by  weight of the base polymer, as viewed from sensitivity and  acid diffusion suppressing effect.  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) or  recurring units having the formula (a2). These units are  simply referred to as recurring units (al) and (a2).  ;)  a  ____r<O_z_~F3  o~  ,  Lso$_{3}$ - o  0  I  0  ,-Q-o  I  0  0  I  0  ,-Q-o  I  0  0  F3C~S03- I  ,-q  0  I  0  F3C~S03- I  ,-q  0  I  Base Polymer 