| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014) | |
| Biomaterial Interfaces | Tuesday Sessions |
| Session BI-TuM |
| Session: | Biomaterial & Wet Interface Characterization |
| Presenter: | David Castner, University of Washington, USA |
| Authors: | J. Taylor, University of Washington, USA D.J. Graham, University of Washington, USA D.G. Castner, University of Washington, USA |
| Correspondent: | Click to Email |
To process, reconstruct, and understand the 3D data from complex materials such as multi-component polymers, drug delivery scaffolds, cells and tissues, it is essential to understand the sputtering behavior of these materials. Though much is understood about sputtering characteristics of some organic materials, there is still a general lack of understanding of how organic and biological materials sputter, especially as the complexity of the materials increase. For example, in multicomponent systems each component may have a different sputter rate, resulting in differential sputtering that will distort the reconstructed depth profile. Thus, accurate reconstruction involves accounting for differential sputter rates, complex sample geometries, etc. Polystyrene and PMMA on Si were used as model systems to optimize methods for depth profile reconstruction. Depth profiling of single component and bilayer films was performed using an Ar1000+ sputter source and Bi3+ analysis beam on an ION-TOF V ToF-SIMS instrument. PMMA sputtered at a significantly higher rate than polystyrene, whilst sputtering of Si can be considered negligible.
Typically the z-axis of depth profiles is converted to depth using an average sputter rate based on measured film thickness and time to remove the film. However, this fails to account for sputter rate variations during the profile, leading to inaccurate film thickness, interfacial position and resolution, and the appearance of penetration into the Si substrate. Applying measured single component sputter rates to the bilayer films, and assuming a step change in sputter rate, yields more accurate film thickness and interface positions; noticeably sharpening the polymer-Si interface. The conversion from sputter time to depth can be further improved by applying a linear change in sputter rate between components across the interface. This further sharpens the interfaces, bringing overall film thickness and interface position more closely in line with expected values. We also have observed a gradual change in sputter rate with mixed polymer blends, possibly due to nanoscale interfacial mixing during sample preparation and storage or induced during the sputter process. Sensitivity analysis performed on variables in sputter rate measurements reveals further scenarios for inaccurate depth profile reconstruction.
This work with a simple laminar system highlights the need for both careful evaluation of component sputter rates and correct application of methods for conversion of sputter time to depth if accurate 3-D reconstructions of complex multi-component samples such as tissue engineering scaffolds are to be achieved.