O2 vs. Ar Gas Cluster Ion Beam Sources for ToF-SIMS Depth Profiling of Thick Polymer and Metal Film Samples.

Gas cluster ion beam sources (GCIB) are very important tools at Corning and have many uses ranging from sputter cleaning of glass surfaces to remove surface contamination to depth profiling of organics, glass and glass coatings. Recently, we have been utilizing both Ar GCIB and O2 GCIB sources to probe the chemistries of glass and polymer substrates. Here we describe our most recent work in the development of depth-profiling protocols for thick (>500 nm) polymeric films and metal film characterization under different ion beam conditions. Two important polymer systems were interrogated: 1) Parylene C films (∼4 µm), and 2) plasma polymerized hexa(methyldisiloxane) (ppHMDSO), a silicone-based film with an irregular structure (∼500 nm). Both films represent classes of materials that can be used for making hydrophobic conformal coatings. The literature regarding the surfaces and in-depth analysis of these materials is sparse. The overall effect of ion beam impact energy (5 keV, 10 keV and 20 keV), as well as the chemistry of the ion beam (O2 vs Ar GCIB) on the resulting depth profiles were investigated. As an example, the resulting depth profiles in Figure 1 show that in Parylene C, O2 gas clusters (Figure 1b) are better than Ar gas clusters (Figure 1a) for depth profiling, as they provide a means of free radical inhibition in a material that tends to crosslink. Specifically note that in Figure 1a, the Parylene C crater turns black during the sputter process due to accumulation of damage and topography during the sputtering process. Sputtering essentially stopped after 1 µm of material was removed. The rise in C with respect to the molecular Parylene C signal is evidence that significant crosslinking of the Parylene C is occurring. Alternatively, the 10 keV O2 GCIB can depth profile through the entire film with little to no damage accumulation as can be observed in Figure 1b. ToF-SIMS depth profiling of Parylene C (∼4 µm) with (a) 10 keV Ar1400+ vs. (b) 10 keV O2(1400)+ GCIB. Images shown next to each profile are the instrument microscope images of the crater after profiling. In the case of the ppHMDSO (data not shown), low energy Ar clusters (≤5 keV) are best as other conditions can transform this silicone type material into silica and/or SiC. The chemistry and morphology of the polymer sputtered crater bottoms were further characterized in detail with a combination of X-Ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM) to give insight into the chemical changes occurring during sputtering. These aspects will be discussed further in the presentation. O2 GCIB was also used to demonstrate the application of depth profiling in metal layers. Figure 2 shows the resulting comparison between Transmission Electron Microscopy (TEM) data and associated ToF-SIMS data acquired using 20 keV O2(900)+. The results are superior as compared to what is obtained with Cs, or even Ar clusters (data not shown), and is the only beam that was able to resolve the Cr layer (∼3 nm) located at the interface between the metal layers and the Si substrate, despite the inherent roughness of the standard. Characterization of NIST SRM-2135C Cr/Ni multilayers (a) TEM HAADF (high-angle annular dark field) image. (b) ToF-SIMS depth profile using 20 keV O2(900)+ GCIB.