The deposition and mechanical properties of alkylsiloxane films deposited on Si(111)

Abstract: Thin organic films of octadecyltrichlorosilane (OTS) and aminopropyltriethoxysilane (APTES) were deposited on oxidized Si(111) substrates and characterised using atomic force microscopy (AFM) and nanoindentation methods. Films derived from a novel alkylsilane, 4-[N,N-(2-pyridyl)aniline]triethoxysilane (NDTS), which has properties similar to related electroluminescent and fluorescent compounds, were also examined. This latter compound could be used to directly couple a fluorescent or electroluminescent centre to a semiconductor surface, with applications in microfluidics or other microelectronic-based sensors. AFM images showed that when OTS or APTES was deposited from ethanol solution onto the oxidized Si surface, an inhomogeneous film was formed, containing both smooth regions and polymerised particles of the alkylsilanes up to 200nm in diameter. Nanoindentation force/displacement curves could clearly distinguish these two regions of the film, and they showed quite different mechanical properties. When deposited from a solution of ethanol and CH2Cl2, OTS films were smooth and completely covered the surface. Discontinuities in the force/displacement curves, corresponding to a breakthrough by the indentor through the film into the substrate showed that these films were some 2nm in thickness. When OTS, APTES or NDTS were deposited from CH2Cl2 solution, similar discontinuities could be observed, and showed that the film thickness increased, and roughness decreased, as the concentration of alkylsilane in solution was decreased. The results are discussed in the context of using nanoindentation methods as a means of characterising organic thin films. [Copyright &y& Elsevier]