Studies of surface interactions at nanometer scale with atomic force microscopy combined with scanning electron microscopy.

摘要

The Atomic Force Microscope (AFM) is an important instrument measuring surface topography and related phenomena; and can study nanometer-scale surface interactions. Surface interactions in ambient air are complicated by surface contamination layers, which do not occur in vacuum or liquid environments. This thesis studies nanometer scale surface interactions, in ambient air, using AFM's high force sensing capability. Several experimental methods were developed and new insights into surface interactions at nanometer scale were obtained. Substantial improvement was made on the lateral resolution of AFM operation in air.; To position the force sensing tip over a specific nanometer scale area, a novel instrument combining AFM and Scanning Electron Microscopy (SEM) was designed, built and commissioned. The system is capable of analyzing AFM tip conditions, in order to study the effects of tip/surface contact. An essentially new method, which monitors the dynamic sensor signal and its fluctuation while changing the tip-sample gap, was developed for vibrating cantilever studies. Sometimes, an electrostatic force occurs in surface contamination, which can be measured with scanning Kelvin-probe force microscopy (KFM); this made it possible to develop a technique for probing surface contamination electrical properties.; A capillary force is associated with the contamination layer. Both capillary force magnitude and layer thickness were measured by fitting the approach curve with the capillary force theoretical model. Studying capillary force with cantilevers of differing spring constants, we demonstrated that capillary force can be balanced by a cantilever having a sufficiently large spring constant. With KFM, the contamination layer was found to contain a charge distribution that changes with time. A model is proposed, showing that the surface contamination layer contains a molecular layer bonded tightly to the sample surface. With KFM, dopant concentration was measured on an MBE-grown semiinsulating sample cross-section, and electrical potential scan edge effect was observed. Systematically studying tip-sample contact, two types of contact processes were identified: jump-to-contact and ramp-to-contact, and the conditions under which they occur. A new spatial region (near-contact region), minimizing tip-sample gap without tip-sample contact, was discovered. Operating in the near-contact region is the optimal operating mode of a vibrating cantilever AFM.