Oligomeric silsesquioxanes (Oligo-SQs) with random structures featuring phenyl (Ph) groups or both Ph and methyl (Me) groups as their substituents were synthesized by the sol–gel method. The ratio of Ph and Me groups controlled the molecular weights and branching in the Oligo-SQs. Branching increased as the molecular weight increased (when the alkaline dissolution rates of all Oligo-SQs adjusted to ∼40 nm s−1). A diazonaphthoquinone (DNQ) derivative was added to the Oligo-SQs to impart positive photosensitivity. A Ph content ratio of over 50 mol% was found to be necessary for the Oligo-SQs to exhibit DNQ compatibility and good patterning performance. Infrared absorption spectroscopy analysis and density functional theory calculations indicated the formation of novel aromatic siloxane DNQ aggregates upon hydrogen bonding between hydroxyl group domains in the Oligo-SQ (formed by hydroxyl groups bonded to Si on adjacent Ph moieties) and the DNQ derivative, which adopts only one of its possible resonance structures.

Positive photosensitive materials prepared by adding a diazonaphthoquinone (DNQ) derivative to matrices of oligomeric silsesquioxanes (oligo-SQs) with random structures, including only a phenyl (Ph) substituent or both Ph and methyl (Me) substituents (wherein the Ph group content is over 50 mol. %), exhibit a high patterning ability (i.e., a high-resolution pattern formation under 10  μm). One cause of such a high patterning ability is the particularly low solubility of unexposed areas in an alkaline developer, caused by the DNQ derivative-containing oligo-SQs having over 50 mol. % of Ph group content. However, the effects of the exposed areas on the alkaline dissolution behavior, which can be another factor of high patterning ability, have not been examined yet. In this study, the effects of the Me group content on the alkali solubility of the exposed areas on the optimum exposure dose (Eop), the resolution limit, and the pattern shape were examined for a positive photosensitive material having over 50 mol. % of Ph group content. As a result, the resolution did not change with increasing Me group content but the Eop decreased, and the pattern shape tended to change from showing a protruding defect in the central portion to having a normal taper shape. Furthermore, density functional theory calculations for the oligo-SQ model compounds suggested the formation of a new hydroxyl (OH) group domain that includes Me groups in oligo-SQs with increasing Me group content. This new type of OH group domain differs from that derived from oligo-SQs possessing only Ph groups. Moreover, the results suggested that the intramolecular hydrogen bonding was weaker in the new domain than in the domain from oligo-SQs containing only the Ph groups and that the intermolecular interaction between the domain and the DNQ was also weak. The weak intermolecular interaction seemed to be the cause of the high alkali solubility in the exposed areas.