Aug. 27, 2020  102  US 2020/0272048 Al  improvement in reaction contrast. It is thus possible to  improve a contrast while suppressing acid diffusion.  [0078]  In the resist composition, the cyclic ammonium salt  is preferably present in an amount of 0.001 to 50 parts by  weight, more preferably 0.01 to 40 parts by weight per 100  parts by weight of the base polymer, as viewed from  sensitivity and acid diffusion suppressing effect. The cyclic  ammonium salt may be used alone or in admixture.  [0079]  In the unexposed region or prior to acid decompo› sition, the cyclic ammonium salt is highly lipophilic and  least dissolvable in alkaline developer. After acid decompo› sition, it releases a cyclic ammonium salt having a low  molecular weight and a carboxylic acid. Thus alkaline  solubility is increased, which is particularly effective in the  case of alkaline development. That is, any pattern film  thickness loss is restrained in the unexposed region whereas  scum formation at the pattern bottom is prevented in the  exposed region. Particularly when the resist film has a  thickness of at least 100 nm, the heterocyclic amine com› pound is effective for preventing scum formation at the  pattern bottom in the exposed region.  [0080]  Base Polymer  [0081]  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).  [0082]  In formulae (al) and (a2),  RA  is each independently  hydrogen or methyl. R 11  and R 12  each are an acid labile  group. Y$^{1 }$is a single bond, phenylene or naphthylene group,  or C$_{5}$-C$_{12 }$ linking group containing at least one moiety  selected from ester bond and lactone ring. Y $^{2 }$is a single bond  or ester bond. When the base polymer contains both recur› ring units (al) and (a2). R 11  and R$^{12 }$may be the same or  different.  [0083]  Examples of the monomer from which the recur› ring units (al) are derived are shown below, but not limited  thereto.  RA  and R$^{11 }$are as defined above.  (al)  (a2)  ~  y2  OR12  0 