Investigation of surface roughening of low-k films during etching using fluorocarbon plasma beams

The surface roughness evolution of solid organosilicate glass (OSG) and methylsilsesquioxane spin-on porous low-k films after etching in C₂F₆/Ar plasmas was characterized as a function of ion bombardment energy, ion fluence reaching the surface (or, equivalently, the etching time), ion impingement angle, and plasma polymerization propensity in a newly designed plasma beam system in which the plasma chemistry, ion energy, ion flux, and ion incident angle can be adjusted independently. A polymerization-induced micromasking mechanism was proposed to explain the surface roughening of these low-k films. The porous structure in the substrate plays a critical role in the film roughening evolution. This effect can be understood using the concept of pore filling with polymeric deposits that etch more slowly under fluorocarbon plasma exposure. Upon exposure to etching, the polymer forms micromasks that induce roughening. Under the same etching conditions, the solid OSG film remains smooth during etching because only a fairly thin and uniform layer of polymer deposits on the surface of solid OSG substrates during the etching in C₂F₆/Ar plasmas. Consequently, the inhomogeneity caused by the polymer deposition is not sufficient to induce micromasking in the absence of surface inhomogeneities. Additionally, the roughness level of the porous low-k film etched in C₂F₆/Ar plasma is primarily related to the film thickness removed, although it also increases with the ion bombardment energy; no similar trend was observed on nonporous OSG films under the same etching conditions. Striations formed when etching porous low-k films at grazing angles. The striatio- - n formation is mainly due to shadowing effects, although conditions of net polymer deposition quenched the striation formation.