The copper/plasma-polymerized octofluorocyclobutane interface

Abstract

To satisfy the demand for increased signal transmission speed and device density in the next generation of multilevel integrated circuits, a material with a permittivity less than 3 is needed, that has compatibility with copper and copper processing. The authors found that plasma-polymerized octofluorocyclobutane (PPOFCB) is a smooth, transparent, film that adheres strongly to the substrate and has a relatively high fluorine to carbon (F/C) ratio, indicative of a low permittivity. This paper considers the copper/PPOFCB interface by investigating the effects of an OFCB plasma both on copper and on the deposited PPOFCB. X-ray photoelectron spectroscopy (XPS) of copper surfaces exposed to an OFCB plasma, for times as short as 1 s and as long as 30 min, was used to characterize the copper, the PPOFCB and the interface between them. During a 1 s OFCB plasma exposure, copper fluorination occurs, producing CuF (with a F ion peak appearing in the F 1s spectrum) as well as the deposition of fluorocarbon species (with a CF n peak in the F 1s spectrum). During the next few seconds of OFCB plasma exposure, the amount of fluorocarbon deposited begins to dominate the surface, although oxidized copper and oxidized hydrocarbon contaminant are still observed. The copper substrate beneath a 15 nm PPOFCB film is not detectable via XPS. The F/C ratio for PPOFCB can be as high as 1.68 after brief deposition times (30 s), reaching a plateau value of 1.5 after several more minutes of plasma exposure; this reduction in F/C to a plateau is associated with ion bombardment and vacuum UV defluorination. PPOFCB is a smooth, planarizing film, with plasma polymerization occurring predominantly on the surface and reducing the RMS roughness from 0.97 to 0.46 nm for the copper substrate.