Self-assembled monolayer film for enhanced imaging of rough surfaces with atomic force microscopy

Abstract

The significance of the surface chemical state and resulting forces on the imaging fidelity of atomic force microscopy (AFM) was investigated experimentally in ambient air. Adhesion tests were conducted on smooth monocrystalline (100) silicon substrates with sharp silicon tips of 5 nm nominal radius. Surface topography imaging of rough polycrystalline silicon surfaces was performed with similar sharp silicon tips. The substrates were initially subjected to different chemical treatments to obtain either hydrophilic or hydrophobic surface behaviors. The AFM tip treatment involved the deposition of an organized close-packed octadecyltrichlorosilane (OTS) hydrophobic self-assembled monolayer. Statistical results for the surface force at the tip-sample interface versus the separation distance and the various moments of the surface height distribution function demonstrated that imaging artifacts due to sticking of the tip on sidewalls of steep asperities were particularly pronounced when imaging hydrophilic surfaces with uncoated tips. The tip artifacts comprised spurious negative excursions or exaggerated asperity heights depending on the direction of scanning. Significant enhancement of the AFM imaging quality was accomplished with OTS-coated tips, as demonstrated by the markedly reduced magnitude of the adhesive force at the tip-sample interface (about 1 nN for both hydrophilic and hydrophobic smooth silicon surfaces), the consistency of the calculated surface height moments, and the negligibly small effect of the scanning direction on the imaged topographies. The results suggest that the present treatment of the AFM tip surface could provide an effective means for overcoming tip artifacts with other difficult-to-image surfaces.