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[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 

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. 

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