Static Time-of-Flight Secondary Ion Mass Spectrometry of Monolayers on Scribed Silicon Derived from 1-Alkenes, 1-Alkynes, and 1-Haloalkanes

Abstract

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was performed on monolayers prepared by scribing silicon under a homologous series of 1-alkenes, 1-alkynes, and 1-haloalkanes: CH2CH(CH2)nCH3 (n = 2, 5, 9), HC⋮C(CH2)nCH3 (n = 2, 5, 9), Cl(CH2)nCH3 (n = 4, 7, 9), Br(CH2)nCH3 (n = 4, 7, 11), I(CH2)nCH3 (n = 0, 1, 2, 4, 7, 11), and I13CH3. Numerous SiCxHy+ and CxHy+ fragments and adduct ions were observed. The results support a proposed binding model that 1-haloalkanes bind to the silicon surface through one C−Si bond and that 1-alkenes and 1-alkynes generally bind through two C−Si bonds. For instance, silicon surfaces scribed under 1-haloalkanes show less carbon by X-ray photoelectron spectroscopy (XPS) than silicon scribed under 1-alkenes and 1-alkynes with the same number of carbon atoms, but they show more intense SiCxHy+ fragments by ToF-SIMS. Above a certain chain length, the relative intensities of the fragment and adduct ions for a homologous series generally increase with increasing alkyl chain length, which is in agreement with carbon surface coverages measured by XPS and the proposed binding models. Anomalously strong SiCH3+ and SiC2H5+ fragments observed in silicon scribed under CH3I and CH3CH2I suggest formation of methyl- and ethyl-terminated silicon, respectively. An isotopic study of silicon scribed under 13CH3I and CH3I provides additional evidence for formation of methyl-terminated silicon and suggests sputter-induced decomposition of the near-surface region by ToF-SIMS. Ab initio calculations of a few SiCxHy+ type fragments are shown to verify assignments of structure. We also note an alternative explanation for some of the results based on the density of alkyl chains on the surfaces.