Investigation of electron induced damaging of molecular overlayers by imaging static secondary ion mass spectroscopy

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Abstract
We applied time of flight-secondary ion mass spectroscopy (SIMS) imaging under static conditions to investigate the interaction of electrons with a covalently bonded silane self-assembled layer of pentaflourphenyldimethylchlorosilane on Si and a physisorbed Langmuir–Blodgett film of a phospholipide dipalmitoylphosphatidylcholine on Au. Well defined surface areas of the respective layer system were irradiated with electron beams of different energies (0.5–25 keV) and different fluences (up to 3×1014 electrons/cm2). The prebombarded surface areas were analyzed by imaging static SIMS, providing characteristic secondary ion intensities as a function of electron energies and fluences. From these results energy dependent electron induced damage cross sections σe(E) could be determined. We found a strong energy dependence of σe(E), very similar to that well known for electron induced secondary electron yields. For both investigated layers we found very similar damage cross sections σe of about 10−15 cm2 for 1 keV electrons. This is in contrast to the strong influence of the binding energy between the molecular overlayer and the substrate, on ion induced damage cross sections σi. Our results demonstrate the general capabilities of imaging static SIMS to investigate quantitatively damaging effects in molecular overlayers and surfaces.