  Single molecule tracer diffusion studies of evaporating (thinning) ultrathin
tetrakis-2-ethyl-hexoxysilane (TEHOS) films on silicon with 100 nm thermal
oxide reveal a considerable slowdown of the molecular mobility within less than
4 nm above the substrate (corresponding to a few molecular TEHOS layers). This
is related to restricted mobility and structure formation of the liquid in this
region, in agreement with information obtained from a long-time ellipsometric
study of thinning TEHOS films on silicon substrates with 100 nm thermal or 2 nm
native oxide. Both show evidence for the formation of up to four layers.
Additionally, on thermal oxide, a lateral flow of the liquid is observed, while
the film on the native oxide forms an almost flat surface and shows negligible
flow. Thus, on the 2 nm native oxide the liquid mobility is even more
restricted in close vicinity to the substrate as compared to the 100 nm thermal
oxide. In addition, we found a significantly smaller initial film thickness in
case of the native oxide under similar dipcoating conditions. We ascribe these
differences to van der Waals interactions with the underlying silicon in case
of the native oxide, whereas the thermal oxide suffices to shield those
interactions.
