Carbon-13 labeled polymers: an alternative tracer for depth profiling of polymer films and multilayers using secondary ion mass spectrometry

[sup.13]C labeling is introduced as a tracer for depth profiling of polymer films and multilayers using secondary ion mass spectrometry (SIMS). Deuterium substitution has traditionally been used in depth profiling of polymers but can affect the phase behavior of the polymer constituents with reported changes in both bulk-phase behavior and surface and interracial interactions. SIMS can provide contrast by examining various functional groups, chemical moieties, or isotopic labels, [sup.C]-Labeled PS ([sup.13]C-PS) and unlabeled PS ([sup.12]C-PS) and PMMA were synthesized using atom-transfer radical polymerization and assembled in several model thin-film systems. Depth profiles were recorded using a Cameca IMS-6f magnetic sector mass spectrometer using both 6.0-keV impact energy [Cs.sup.+] and 5.5-keV impact energy [O.sub.2.sup.+] primary ion bombardment with detection of negative and positive secondary ions, respectively. Although complete separation of [sup.12][C.sup.1]H from [sup.13]C is achieved using both primary ion species, 6.0-keV [Cs.sup.+] clearly shows improved detection sensitivity and signal-to-noise ratio for detection of [sup.12]C, [sup.12][C.sup.1]H, and [sup.13]C secondary ions. The use of [Cs.sup.+] primary ion bombardment results in somewhat anomalous, nonmonotonic changes in the [sup.12]C, [sup.12][C.sup.1]H, and [sup.13]C secondary ion yields through the PS/PMMA interface; however, it is shown that this behavior is not due to sample charging. Through normalization of the [sup.13]C secondary ion yield to the total C ([sup.12]C + [sup.13]C) ion yield, the observed effects through the PS/PMMA interface can be greatly minimized, thereby significantly improving analysis of polymer films and multilayers using SIMS. Mass spectra of [sup.13]C-PS and [sup.12]C-PS were also analyzed using a PHI TRIFF I time-of-flight mass spectrometer, with 15-keV [Ga.sup.+] primary ion bombardment and detection of positive secondary ions. The [sup.12][C.sub.7.sup.1][H.sub.7] ion fragment and its [sup.13]C-enriched analogues have significant secondary ion yields with negligible mass interferences, providing an early indication of the potential for future use of this technique for cluster probe depth profiling of high molecular weight [sup.13]C-labeled fragments.