Ultra-Sensitive Scanning Force Microscopy Utilizing a Novel Cantilever Geometry

The principle of scanning force microscopy was first demonstrated in 1986 by Binnig, Quate, and Gerber. The original Atomic Force Microscope (AFM) utilized a small, horizontally cantilevered microprobe tip. When scanned over a surface the up-and-down deflections of the cantilever due to the changing forces between the sample and the tip provided a topographical profile of the samples surface with near atomic resolution. By vibrating the tip up-and-down at the mechanical resonance frequency of the cantilever, noise is reduced by the quality factor of the mechanical resonator and by the narrow frequency range of the measurement. This dynamic horizontal cantilever technique has two primary drawbacks: the mechanical resonance is sensitive only to changes in force over its vertical motion; and large variations in force between the surface and the tip often result in uncontrolled vertical deflection (i.e. tip crashes). To reduce the incidence of tip crashes, cantilevers that are relatively rigid must be used, which limits the ultimate sensitivity of the microscope.Researchers at Arizona State University have developed a novel technique for scanning force microscopy utilizing a vertical cantilever geometry. This vertical cantilever has several advantages over previous horizontal cantilever force microscopes. (1) The vertical cantilever is sensitive to vertical forces and horizontal force gradients. (2) The vibration direction is easily altered to provide directional information about horizontal forces. (3) The vertical cantilever geometry inhibits uncontrolled vertical deflections, reducing the incidence of tip crashes, and permitting the use of ultra-small force constant cantilevers for superior force sensitivity.