This paper explains the origin of our previous observation that, when a silsesquioxane-based low-k film is etched in fluorocarbon plasmas, the thickness of a surface modified layer, in which cage-like Si-O bonds are dissociated to extents greater than a specified level, changes linearly with log[F]²/[CF₂], where [F] and [CF₂] denote concentrations of F and CF₂ radicals in the bulk plasma. During the etching process, the substrate consists of three distinct layers: a fluorocarbon layer, a modified surface layer, and an unmodified layer. F density at the interface between the fluorocarbon and the modified surface layers, denoted as F₀ in this study, is determined in proportional to [F]2/[CF2], and the density decreases exponentially with the film depth. As a result, the thickness of the modified surface layer changes in proportion to a parameter, log[F]²/[CF₂].