Influence of van der Waals interactions on morphology and dynamics in ultrathin liquid films at silicon oxide interfaces

TL;DR

This study investigates how van der Waals interactions influence the morphology and molecular dynamics of ultrathin liquid films on silicon oxide, revealing substrate-dependent mobility restrictions and layering effects.

Contribution

It provides new insights into the role of van der Waals forces in ultrathin film behavior on different silicon oxide surfaces, combining tracer diffusion and ellipsometry data.

Findings

Molecular mobility is significantly slowed within 4 nm of the substrate.

Up to four liquid layers form during film thinning.

Native oxide surfaces restrict mobility more than thermal oxide.

Abstract

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.