Interconnect structures possessing a non-porous (dense) low-k organosilicate glass (OSG) film utilizing a porous low-k OSG film as an etch stop layer or a porous low-k OSG film using a non-porous OSG film as a hardmask for use in semiconductor devices are provided herein. The novel interconnect structures are capable of delivering improved device performance, functionality and reliability owing to the reduced effective dielectric constant of the stack compared with that of those conventionally employed and also because of the relatively uniform line heights made feasible by these unique and seemingly counterintuitive features. The present invention also provides a fluorocarbon-based dual damascene etch process that achieves selective etching of a dense low-k OSG films relative to that of a porous low-k OSG film owing to the tunability of the gas-phase fluorine:carbon ratio (gas dissociation) and ion current below a critical threshold and given the larger carbon content of the porous film relative to that of the dense film.