SELF-ASSEMBLED MONOLAYER FILM FOR ENHANCED IMAGING OF ROUGH SURFACES WITH ATOMIC-FORCE MICROSCOPY

The significance of the surface chemical state and resulting forces on the imaging fidelity of atomic force microscopy (AFM) was investigate d experimentally in ambient air. Adhesion tests were conducted on smoo th monocrystalline (100) silicon substrates with sharp silicon surface s tips of 5 nm nominal radius. Surface topography imaging of rough pol ycrystalline silicon surfaces was performed with similar sharp silicon tips. The substrates were initially subjected to different chemical t reatments to obtain either hydrophilic or hydrophobic surface behavior s. The AFM tip treatment involved the deposition of an organized close -packed octadecyltrichlorosilane (OTS) hydrophobic self-assembled mono layer. Statistical results for the surface force at the tip-sample int erface versus the separation distance and the various moments of the s urface height distribution function demonstrated that imaging artifact s due to sticking of the tip on sidewalls of steep asperities were par ticularly pronounced when imaging hydrophilic surfaces with uncoated t ips. The tip artifacts comprised spurious negative excursions or exagg erated asperity heights depending on the direction of scanning. Signif icant 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 hydro philic and hydrophobic smooth silicon surfaces), the consistency of th e calculated surface height moments, and the negligibly Small effect o f the scanning direction on the imaged topographies. The results sugge st that the present treatment of the AFM tip surface could provide an effective means for overcoming tip artifacts with other difficult-to-i mage surfaces.