Paper PS2+MS-MoM6
Surface Roughening Mechanisms during Porous SiOCH Etching Processes

Monday, October 15, 2007, 9:40 am, Room 607

Introducing dual damascene structures for the interconnections has been a means of improving their electrical performances. However, lowering the effective dielectric constant remains a major stake. Increase the porosity of the dielectric material or remove the trench bottom etch stop layer are some solutions. As a result, the trench etch process is stopped into the porous material which may lead to a tricky trench bottom roughness. In addition, sidewall metal diffusion barriers have to be thinned down to keep the copper line resistance low. In this context, the trench bottom roughness may also affect metal barrier coverage. In this study, roughness of dielectric materials is characterized by SEM and AFM after partial etching. Dielectric etching is known to be controlled by the thickness and composition of a fluorocarbon overlayer which depends on the plasma characteristics (etch chemistry…) and on the materials properties (composition, porosity,…). Thereby, in order to understand the mechanisms controlling the porous SiOCH roughening, different etch plasmas have been performed on materials with different percentages of porosity (7, 25 and 30%). For a high polymerizing (CF₄/Ar/CH₂F₂), a low polymerizing (CF₄/Ar) and a pure physical sputtering plasma (Ar), surface composition has been characterized by quasi in situ XPS and the roughness has been studied as a function of the etched thickness. Those experiments highlight different trends. Firstly, the 7 % porous SiOCH does not exhibit any significant roughness whatever the etching plasma (rms roughness = 0.5nm). Secondly, porous SiOCH with a higher porosity (25 and 30%) is roughened when exposed to fluorocarbon based plasmas. The resulting roughness increases linearly versus the etched thickness in the range of a tenth of nanometers. This increase is fast when the concentration of fluorocarboned species at the etched surface is low, while a higher amount of fluorocarboned species limits it. At last, sputtering of porous SiOCH using a pure Ar plasma, namely the absence of fluorocarboned species at the etched surface, leads to a surface as smooth as the pristine material (rms roughness = 0.2 nm). Those results highlight the critical role of porosity and the presence of fluorocarboned species on the dielectric surface roughening. On the basis of those observations, a hypothesis will be proposed for the initiation and maintaining of the dielectric roughness.