Chemical effects in cluster SIMS depth profiling of polymer-based materials

Abstract

Aim of this Ph.D. thesis was that of investigating the role played by chemistry in the in-depth analysis of polymer-based materials, the importance of which has become relevant with the introduction, in secondary ion mass spectrometry, of cluster ion sources. While the introduction of cluster-SIMS represented a real breakthrough in the possibility of obtaining molecular information along the depth of polymer materials, however it turned out that this possibility was dependent on the chemical behaviour, under cluster beam irradiation, of the particular system under investigation. During this PhD work the effect of the introduction nitric oxide, a well known radical scavenger, in the residual atmosphere of the SIMS spectrometer (NO-dosing) during cluster beam depth profiling of polymer based films, either single- or multicomponent systems, was studied. The general idea underlying the thesis project was that of developing a method for interfering with the ion-beam initiated reaction pathways that lead to accumulation of damage in the subsurface of polymers, and that in many cases prevents the possibility of following, along the sample depth, secondary ion signals that are related with the original structure of the polymer. In other words, the idea was that of extending, by exploiting chemical methods, the molecular depth profiling capabilities of cluster ion beams (and fullerene beams in particular, that in the recent years constituted a major breakthrough for the in-depth investigation of organic and polymer films. With our nitric-oxide assisted C60-SIMS methodology, successful C60 molecular depth profiles of PS, PAMS, PC were obtained, while the same polymers cannot be profiled successfully in the standard conditions of C60-SIMS, i.e. in the absence of NO, because of their tendency to accumulate damage by increasing accumulation of crosslinks (type I behaviour). At the same time, polymers that can be profiled in standard conditions are not affected by nitric oxide. The investigation was extended to more complex systems in which polymers with different behaviour (type I or II) under fullerene beam irradiation were co-existing, namely multilayered systems, including hybrid polymer/metal multilayers, immiscible polymer blends, properly chosen random copolymers and additive-containing systems. In all the examined cases the general observation is that the presence of nitric oxide during C60 has a positive uniforming effect, not only on the possibility of obtaining molecular in-depth information also in the presence of difficult polymers, but also on the erosion rates, even in the presence of inorganic interlayers. This makes the nitric oxide-assisted C60 depth profiling a very promising tool for spatially resolved investigation of complex polymer-based systems, with potential applications in those technological fields exploiting polymer-based micro- and nano-structured systems.