between the exposed and unexposed regions, patterns of  good profile having improved resolution, LWR and CDU are  obtained.  [0078]  The betaine type onium compound may be pre› pared, for example, according to the following scheme.  Although reference is made to the synthesis of a sulfonium  compound of cyclic acetal structure having the formula  (A-a), the synthesis method is not limited thereto.  [0077]  The molecular resist composition of the invention  is characterized by comprising the betaine type onium  compound and a solvent to be described later, but not a base  resin. The term "base resin" refers to a resin or polymer  which is commonly used in conventional resist composi› tions and present in the largest amount among other com› ponents exclusive of the solvent, that is, as the main com› ponent. In a resist composition comprising abase resin, the  base resin is adapted to change its solubility in developer in  response to exposure. In the resist composition of the  invention, because of the absence of a base resin, the betaine  type onium compound is uniformly dissolved and dispersed  in the solvent without agglomeration. In the exposed region,  the betaine type onium compound is photo-decomposed into  a corresponding sulfonic acid having more affinity to alka› line developer. Since the sulfonic acid after photo-decom› position retains the cation moiety bonded thereto even after  decomposition, it is believed that the acid is least diffusive.  In the unexposed region, no photo-decomposition reaction  takes place and the betaine structure is maintained. Since the  cation and anion moieties are present in a common mol› ecule, the covalent bond is predominant over the ionic bond,  which indicates a poor affinity to the alkaline developer. The  consequence is an enhancement of dissolution contrast  between the exposed and unexposed regions. Due to the  synergistic effect of the uniform dispersion of the betaine  type onium compound and the high dissolution contrast  Dec. 3, 2020  72  US 2020/0379345 Al  -continued  Reactant A  Q (Re)e  "’  .,,::9  [OJ  )-((  (Rd)d  Intermediate A  "’  .,,::9  ------- Q (R"Je  )-((  .. 0  HO~OH  (Rd)d  Intermediate B  Intermediate C  Intermediate D  (Rljf  ---0- F  MgX  TMSCl  HOmSO; M’  Intermediate E 