Reconstructing accurate ToF-SIMS depth profiles for organic materials with differential sputter rates

Abstract

To properly process and reconstruct 3D ToF-SIMS data from systems such as multi-component polymers, drug delivery scaffolds, cells and tissues, it is important to understand the sputtering behavior of the sample. Modern cluster sources enable efficient and stable sputtering of many organics materials. However, not all materials sputter at the same rate and few studies have explored how different sputter rates may distort reconstructed depth profiles of multicomponent materials. In this study spun-cast bilayer polymer films of polystyrene and PMMA are used as model systems to optimize methods for the reconstruction of depth profiles in systems exhibiting different sputter rates between components. Transforming the bilayer depth profile from sputter time to depth using a single sputter rate fails to account for sputter rate variations during the profile. This leads to inaccurate apparent layer thicknesses and interfacial positions, as well as the appearance of continued sputtering into the substrate. Applying measured single component sputter rates to the bilayer films with a step change in sputter rate at the interfaces yields more accurate film thickness and interface positions. The transformation can be further improved by applying a linear sputter rate transition across the interface, thus modeling the sputter rate changes seen in polymer blends. This more closely reflects the expected sputtering behavior. This study highlights the need for both accurate evaluation of component sputter rates and the careful conversion of sputter time to depth, if accurate 3D reconstructions of complex multi-component organic and biological samples are to be achieved. The effects of errors in sputter rate determination are also explored.

Figure 1

Representative high-resolution XPS scans of the C1s region for PMMA (bottom), polystyrene (center) and bilayer (top) spun-cast polymer films on Si wafer. Presence of the π-π* shakeup peak and absence of C-O-C and O-C=O peaks (fitted components shown in black) in bilayer sample suggests the presence of an intact polystyrene overlayer atop initially deposited PMMA film.

Figure 2

(A) Representative ToF-SIMS depth profile through a polystyrene-PMMA bilayer spun-cast film on Si wafer plotted against sputter time. (B-D) The same representative ToF-SIMS depth profile transformed to plot depth using different sputter rate assignment methods. (B) Transformation by constant sputter rate defined from measured bilayer thickness and sputter time to polymer-Si interface. (C) Transformation using step change between measured component sputter rates at polystyrene-PMMA and PMMA-Si interfaces. (D) Linear transition between component sputter rates across depth-resolution of polystyrene-PMMA and PMMA-Si interfaces. Intensities of characteristic peaks of each component (Silicon: Si⁺, m/z28; PMMA: C₄H₅O⁺, m/z 69; polystyrene: C₇H₇⁺, m/z 91) are plotted on a log₁₀ intensity scale against sputter time (a) or depth from surface (b-d). Expected position of polystyrene-PMMA and PMMA-Si interfaces from AFM measurements of single-component and bilayer films are shown as vertical line (mean) and shaded area (± 1 SD).

Figure 3

Characterization of polystyrene-PMMA blend films. (A) XPS peak intensities for blends. (B) Sputter rates of blends relative to pure polystyrene. Linear trend lines shown.

Figure 4

Enlargements from Figure 2, showing the polymer-Si interface regions of representative depth profiles reconstructed with sharp (top) and linear (bottom) sputter rate transitions. Intensities of characteristic peaks of each component (Silicon: Si⁺, m/z 28; PMMA: C₄H₅O⁺, m/z 69; polystyrene: C₇H₇⁺, m/z 91) are plotted on a log₁₀ intensity scale against depth from surface.

Figure 5

(A) Representative AFM height maps of PS, PMMA and blended films and crater bases after sputtering for 0 (surface), 3, 9, or 15 (crater bottom) seconds. Images show 1 x 1 μm area. (B) Roughness parameter (R_q) of film surface and sputter crater bases in PS, PMMA and blended films. Linear trend fitted for 5–15 s of sputter time.

Figure 6

(A) Depth profiles (n=20) reconstructed from simulated sputter rates, normally distributed around the mean of the measured sputter rates. (B) Depth profiles reconstructed with sputter rates selected from the 5^th (dashed line) or 95^th (dot-dashed line) percentile of sputter rates for both polystyrene and PMMA. (C). Depth profiles reconstructed with sputter rates selected from the 5^th and 95^th percentile (dot-dashed line), or 95^th and 5^th percentile (dashed line) for polystyrene and PMMA respectively. Profiles reconstructed from mean sputter rates is shown with a solid line.