Morphology measurements by AFM tapping without causing surface damage: A phase shift characterization

The morphology measurement of a surface can be done by using an atomic force microscope (AFM). However, it is difficult to ensure that the measurement does not introduce any damage to the sample surface. This paper proposes that phase shift, the phase change between the original surface and scanned area, can provide a characteristic signal of the tip-surface interaction. On a poly (methyl methacrylate) thin film, the present investigation explored the relationship between phase shift and nondestructive surface morphology measurement under the tapping mode of an AFM. The study showed that when the drive amplitude was doubled, the phase shift reached from 0.47 degrees to 1.85 degrees. Under this condition, wrinkles became observable. With the tip radius in the range of 15-20 nm, no phase shift appeared between a scanned area and the original surface after multiple measurements. In this case, the tip-surface energy dissipation was in the range of 10-35 eV, showing a nondestructive interaction of the surface with the AFM tip. When the tip radius was about 55 nm, under the same tip excitation parameters, the energy dissipation per tap varied from 60 to 110 eV, and a phase shift occurred in the range of 0.02-0.64 degrees, while the surface plastic deformation was still extremely minor after multiple tip scanning. A higher phase shift was occurred on the softer surface attributed to multiple scanning under tapping mode. The study found that the phase shift characteristics was a more sensible measure to signify the transition from a nondestructive to a destructive surface morphology measurement by using the tapping mode of an AFM.