Confinement Effects of Ion Tracks in Ultrathin Polymer Films

We show direct experimental evidence that radiation effects produced by single MeV heavy ions on a polymer surface are weakened when the length of the ion track in the material is confined into layers of a few tens of nanometers. Deviation from the bulk (thick film) behavior of ion-induced craters starts at a critical thickness as large as $\sim 40\mathrm{nm}$, due to suppression of long-range additive effects of excited atoms along the track. Good agreement was found between the experimental results, molecular dynamic simulations, and an analytical model.

Figure 1

SFM images of PMMA thin films of different thicknesses bombarded by (a)–(e) 923 MeV ${\mathrm{Pb}}^{\mathrm{eq}+}$ ions at normal incidence and by (f)–(j) 2.2 GeV ${\mathrm{Au}}^{\mathrm{eq}+}$ ions at grazing incidence. The thickness of the layer is given in each image. The scale bar of 100 nm is for images (a)–(e) and the 200 nm bar is for images (f)–(j). The height scale given in (f) is valid for all images, except in (e).

Figure 2

Averaged crater dimensions produced by single heavy ions on PMMA as a function of film thickness $h$ for normal incidence: (a) crater diameter, (b) crater depth, and (c) volume of the protruded rim above the unperturbed surface position. In (b) a data point for $h=100\mathrm{nm}$ measured with a blunt tip was also included. Solid lines show the results for the analytical model and open circles are simulation data (details in Supplemental Material [16]).

Figure 3

Final snapshot ($t=160\mathrm{ps}$) from one MD simulation ($h=20\mathrm{nm}$), including displacement vectors from initial positions. Only a thin slice through the center of the crater is shown. PP indicates collective displacements due to the pressure pulse and M the melt flow contributing to the rim volume. The inset shows the number of ejected particles as a function of depth (98% comes from $z<h/2$).