Molecular Depth Profiling of Multilayer Polymer Films Using Time of Flight Secondary Ion Mass Spectrometry
M S. Wagner
The low penetration depth and high sputter rates obtained using polyatomic primary ions have facilitated their use for the molecular depth profiling of some spin-cast polymer films by secondary ion mass spectrometry (SIMS). In this study, dual-beam time-of-flight (ToF) SIMS (sputter ion = 5 keV SF5+, analysis ion = 10 keV Ar+) was used to depth profile spin-cast multilayers of poly(methyl methacrylate), PMMA, poly(2-hydroxyethyl methacrylate), PHEMA, and trifluoroacetic anhydride-derivatized poly(2-hydroxyethyl methacrylate), TFAA-PHEMA on silicon substrates. Characteristic positive and negative secondary ions were monitored as a function of depth using SF5+ primary ion doses necessary to sputter through the polymer layer and uncover the silicon substrate (> 5 x 1014 ions/cm2). The sputter rates of the polymers in the multilayers were typically less than for corresponding single layer films, and the order of the polymers in the multilayer affected the sputter rates of the polymers. Multilayer samples with PHEMA as the outermost layer resulted in lowered sputter rates for the underlying polymer layer due to increased ion-induced damage accumulation rates in PHEMA. Additionally, the presence of a PMMA or PHEMA overlayer significantly decreased the sputter rate of TFAA-PHEMA underlayers due to ion-induced damage accumulation in the overlayer. Typical interface widths between adjacent polymer layers were 10 nm to 15 nm for bilayer polymer films and increased with depth to 35 nm for the trilayer polymer films. The increase in interface width and observations using optical microscopy showed the formation of sputter-induced surface roughness during the depth profiles of the trilayer polymer films. This study shows that polyatomic primary ions can be used for the successful molecular depth profiling of multilayer polymer films and presents new opportunities for the analysis of thin organic films using ToF-SIMS.